As featured in:

Featuring work from the research group of Dr Lin-Bing Sun,

State Key Laboratory of Materials-Oriented Chemical

Engineering, Nanjing Tech University, Nanjing, China

Design and fabrication of mesoporous heterogeneous basic catalysts

Mesoporous solid bases are extremely desirable in green catalytic processes. Recent advances in mesoporous solid bases were reviewed, and fundamental principles on how

As featured in:

See Lin-Bing Sun,

Hong-Cai Zhou et al .,

Chem. Soc. Rev., 2015, 44 , 5092.

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Cite this: Chem. Soc. Rev., 2015,

44 , 5092

Received 31st January 2015

DOI: 10.1039/c5cs00090d www.rsc.org/chemsocrev

Design and fabrication of mesoporous heterogeneous basic catalysts

Lin-Bing Sun,*

a

Xiao-Qin Liu

a

and Hong-Cai Zhou*

b

Mesoporous solid bases are extremely desirable in green catalytic processes, due to their advantages of accelerated mass transport, negligible corrosion, and easy separation. Great progress has been made in mesoporous solid bases in the last decade. In addition to their wide applications in the catalytic synthesis of organics and fine chemicals, mesoporous solid bases have also been used in the field of energy and environmental catalysis. Development of mesoporous solid bases is therefore of significant importance from both academic and practical points of view. In this review, we provide an overview of the recent advances in mesoporous solid bases, which is basically grouped by the support type and each category is illustrated with typical examples. Cooperative catalysts derived from the incorporation of additional functionalities ( i.e.

acid and metal) into mesoporous solid bases are also included. The fundamental principles of how to design and fabricate basic materials with mesostructure are highlighted. The mechanism of the formation of basic sites in different mesoporous systems is discussed as well.

1. Introduction

For the demands of sustainable development and green chemistry, the use of heterogeneous catalysts instead of conventional homogeneous ones has received increasing attention.

1–5 Among various heterogeneous catalysts, solid bases are of great interest because they have many advantages over their liquid counterparts.

a

State Key Laboratory of Materials-Oriented Chemical Engineering,

College of Chemistry and Chemical Engineering, Nanjing Tech University,

Nanjing 210009, China. E-mail: lbsun@njtech.edu.cn

b

Department of Chemistry, Texas A&M University, College Station,

Texas 77842-3012, USA. E-mail: zhou@chem.tamu.edu

They are much less corrosive and cause fewer disposal problems.

The reactions catalyzed by solid bases usually show higher selectivity to target products and the formation of tar as a by-product can be hindered. Furthermore, solid bases allow quite easier separation of catalysts and recovery of products from reaction systems, and subsequently make it facile to recycle the catalysts. Hence, solid bases offer an environmentally benign and economical pathway for the synthesis of chemicals.

The investigation of solid bases can be traced back to 1958, when Pines and Haag first reported the dispersion of metal

Na on Al

2

O

3

.

6

The resultant material was used as a catalyst for double bond migration of 1-butene, and the ratio of cis - to trans -

2-butene was fifteen times higher than that of the thermodynamic

Lin-Bing Sun

Lin-Bing Sun obtained his PhD in

2008 from Nanjing University under the supervision of Professor

Jian Hua Zhu and Professor Yuan

Chun. He joined the faculty of

Nanjing Tech University in 2008, and became an associate professor in 2011. He was a postdoctoral research associate at Texas A&M

University with Professor Hong-Cai

Zhou in 2011–2012. His current research interests focus on fabrication of porous materials and their applications in adsorption and heterogeneous catalysis.

Xiao-Qin Liu

Xiao-Qin Liu obtained her PhD in

1999 from Nanjing Tech University under the supervision of Professor

Jun Shi and Professor Hu-Qing

Yao. She joined the faculty of

Nanjing Tech University in 1982, and became a professor in 1999.

Her current research interests focus on design, synthesis, and applications of porous functional materials, with emphasis on adsorbents and catalysts.

5092 | Chem. Soc. Rev., 2015, 44 , 5092--5147 This journal is © The Royal Society of Chemistry 2015

Review Article


Chem Soc Rev equilibrium mixture. Taking account of the strong tendency of Na to donate electrons, it is easy to understand that Al

2

O

3

supported Na acts as an effective basic catalyst. With the development of material chemistry and catalysis chemistry, a large number of solid bases have been reported since then.

These solid bases vary from single-component metal oxides

( e.g.

MgO)

7–9 to multicomponent metal oxides ( e.g.

Mg–Al hydrotalcite)

10–12 and salts ( e.g.

K

3

PO

4

)

13 with different pore structures including micropores, mesopores, and macropores.

14–17

A well-known application of the solid base is the synthesis of

4-methylthiazole, which is an intermediate in the preparation of a systemic fungicide thiabendazole.

18

The traditional synthetic route towards 4-methylthiazole consists of five steps that involve several hazardous chemicals (see eqn (1)–(5) in

Scheme 1). Interestingly, by the use of a solid base as a catalyst, the process can be reduced to two steps (see eqn (6) and (7) in

Scheme 1). The key point of this new route is the direct synthesis of 4-methylthiazole from SO

2 and the imine catalyzed by a solid base. In this reaction, the Cs-loaded zeolite was employed as a base catalyst. In addition to this renowned process, various solid bases were also prepared based on zeolites due to their capacity in recognizing and organizing molecules with excellent precision.

19–23

However, the small pore openings of zeolites result in slow diffusion of substrates, and prevent bulky molecules from reaching the active sites in micropores. Of course, the size constraints of basic zeolites are advantageous in terms of shape selectivity even if the diffusion of substrates is slow. In addition, the amount of basic species that can be introduced is limited by the small pore volume, which restricts the strength of basic sites formed on zeolites. It is therefore understandable that the reports regarding the generation of superbasic sites on zeolites are very scarce.

New opportunities have been opened up in many areas of chemistry and materials science since the discovery of mesoporous silica M41S.

24 A collection of mesoporous materials with various pore symmetry ( e.g.

hexagonal, cubic, and lamellar) 25–27

Hong-Cai Zhou

Hong-Cai ‘‘Joe’’ Zhou obtained his

PhD in 2000 from Texas A&M

University under the supervision of

F. A. Cotton. After a postdoctoral stint at Harvard University with

R. H. Holm, he joined the faculty of Miami University, Oxford in

2002. He rose to the rank of a full professor in Texas A&M University in 2008 and was promoted to

Davidson Professor of Science in

2014. His research focuses on the discovery of synthetic methods to obtain robust framework materials with unique catalytic activities or desirable properties for clean-energyrelated applications, taking advantage of the confinement effect in a microscopic or mesoscopic cavity.

Scheme 1 Synthesis of a fungicide intermediate 4-methylthiazole through the traditional route (eqn (1)–(5)) and a new route catalyzed by the solid base (eqn (6) and (7)). [Adapted with permission from ref. 18.

Copyright 1993 American Chemical Society.] and pore wall composition ( e.g.

silica, alumina, and zirconia)

28–30 was successfully synthesized by the use of templating approaches.

These mesoporous materials possess high surface area, large pore volume, and tunable pore size ranging from several to scores of nanometers, which is of great interest for adsorption, catalysis, and sensing.

31–36

The emergence of mesoporous materials gives fresh impetus to the development of solid bases. Their large pore volume provides a good platform for accommodating basic species. The pore size on the nanoscale is beneficial to mass transport. Furthermore, the reactions involving bulky substrates and/or products become possible under the catalysis of mesoporous solid bases. As a result, extensive attention has been paid to the design and fabrication of mesoporous solid bases. Basic catalysts with a mesostructure are of great importance from both practical and academic points of view. They are fascinating not only because of the high potential in industrial applications but also for fundamental research regarding the type, role, and structure of basic active sites. So far a great deal of mesoporous solid bases have been reported. The basic sites range from inorganic species to organic ones that can be introduced by an assortment of methods such as impregnation, ion exchange, and grafting. These solid bases are capable of catalyzing a series of organic reactions including double bond migration, Knoevenagel condensation, Michael addition, transesterification, hydrogenation, amination, alkylation, to name just a few (Fig. 1). In recent years, the application of solid bases has also been extended to the field of energy and environment, for instance, heterogeneous synthesis of biodiesel

37–40 and catalytic conversion of CO

2

.

41–43

The last few decades have witnessed incredible advances in mesoporous solid bases. To the best of our knowledge, however, the development of mesoporous solid bases has never been systematically reviewed. In this paper, the recent advances in mesoporous solid bases from

This journal is © The Royal Society of Chemistry 2015 Chem. Soc. Rev., 2015, 44 , 5092--5147 | 5093

Chem Soc Rev


Review Article with basic frameworks. The frameworks consist of basic species and are intrinsically basic. A typical case is MgO synthesized via a templating method. The second one is mesoporous hosts modified by basic guests. No obvious basic characteristics can be observed for the frameworks themselves. Guests need to be introduced to form basic sites on mesoporous hosts. For instance, the introduction of MgO (here as a guest) into mesoporous silica SBA-15 (the host) produces a mesoporous solid base,

MgO-modified SBA-15. This section focuses on mesoporous materials with basic frameworks.

Fig. 1 Some typical reactions catalyzed by solid bases.

various research groups as well as ours are summarized. These materials are in principle organized by the support type and each category is described with typical examples. Cooperative catalysts originating from the incorporation of additional acid or metal functionalities into mesoporous solid bases are also involved. The fundamental principles of how to design and fabricate basic materials with a mesostructure are featured.

2. Mesoporous materials with basic frameworks

In general, mesoporous solid bases can be classified into two subgroups (Fig. 2). The first one is the mesoporous materials

2.1.

Mesoporous metal oxides

2.1.1.

Mesoporous MgO.

In comparison with mesoporous silica, the preparation of nonsiliceous mesoporous materials is much more challenging. It is known that the hydrolysis and condensation of silica precursors ( e.g.

tetraethyl orthosilicate,

TEOS) can proceed in a controllable way, and the resultant silica is thermally stable during calcination. Nevertheless, the hydrolysis and condensation of nonsiliceous precursors

( e.g.

metal alkoxides) are commonly difficult to control. With great effort researchers have succeeded in the synthesis of mesoporous alumina, zirconia, titania, carbon, etc.

,

52,53 while most of these materials are either neutral or acidic. To date only a few literature concern the preparation of MgO with a mesostructure,

44,45,47,48,51 and the reports on other alkali metal and alkaline earth metal oxides are very scarce. This subsection will summarize the methods used for the fabrication of mesoporous MgO.

2.1.1.1. Soft templating method.

Inspired by the fabrication of mesoporous silica, amphiphilic copolymers have been employed as templates to direct the formation of a mesostructure for MgO. By using a comb-like copolymer, poly(dimethylsiloxaneethyleneoxide) (PDMS-PEO), as a structure-directing agent,

Kim’s group

44 successfully prepared mesoporous MgO via a

Fig. 2 Two subgroups of mesoporous solid bases and some general strategies for the generation of basicity on mesoporous supports.


Review Article


Chem Soc Rev

Fig. 3 Schematic illustration of the synthesis of mesoporous MgO using

PDMS-PEO comb-like copolymer as a template. [Adapted with permission from ref. 44. Copyright 2013 Elsevier.] sol–gel process. In aqueous solution, the PDMS chains aggregate to form micelle cores whereas the PEO chains form coronas outside the cores (Fig. 3). Due to favorable interactions, the hydrophilic Mg(NO

3

)

2 precursor preferentially incorporates into the hydrophilic PEO domains, whilst the PDMS domains generate mesopores. After calcination, mesoporous MgO with a pore size of 13.2 nm and Brunauer–Emmett–Teller (BET) surface area of

80 m

2 g

1 was produced (Table 1). A reference sample that was prepared in the absence of a template gave a surface area of only 33 m

2 g

1

, indicating the role played by the template in the synthesis. The obtained mesoporous MgO was used as a heterogeneous solid catalyst to produce biodiesel from canola oil. The mesoporous MgO can catalyze the conversion of canola oil into biodiesel with 98.2% of the product (methyl ester) content, which was greater than the reference sample prepared without a template

(82.8%). The rapid transport of reactants/products in mesoporous

MgO is believed to be responsible for the high activity.

A triblock copolymer (Pluronic P123) that is known for the synthesis of mesoporous silica SBA-15 has also used to direct the generation of mesopores for MgO. By adopting a dissolution– recrystallization strategy, single-crystalline MgO with 3D wormholelike mesopores can be constructed.

45

Instead of magnesium salts, ordinary MgO powders were employed as the starting material.

In low-angle X-ray diffraction (XRD) patterns, a single peak at about 2 y of 1 1 was observed. This corresponds to the mesostructure lacked long-range packing order and can be confirmed by transmission electron microscopy (TEM) images. It is worth noting that the obtained mesoporous MgO has a quite high surface area of 298 m

2 g

1 and a large pore volume of 0.45 cm

3 g

1

. The sample should provide a promising candidate for use as a heterogeneous basic catalyst.

2.1.1.2. Hard templating method.

In the abovementioned cases, the copolymers work as soft templates and play a structuredirecting role. As an alternative, the hard-templating method attracts considerable attention for the synthesis of nonsiliceous mesoporous materials. In a hard-templating route, the voids of a template, typically porous silica or carbon, are impregnated with solutions of desired composition. The subsequent solidification and removal of the template may result in a negative replica. It should be stated that silica has never been reported to template the synthesis of mesoporous MgO. The introduction of MgO into pores of silica is obviously feasible. However, it is rather difficult to remove the silica template selectively while leaving the MgO replica intact. Carbon-related porous materials thus become the template of choice.

¨th and coworkers

46 reported the fabrication of mesoporous MgO through a hard-templating method using the mesoporous carbon aerogel as a template and Mg(NO

3

)

2 solution as a precursor. The mesoporous carbon aerogel was synthesized by drying a resorcinol/formaldehyde polymer under ambient pressure conditions in place of the generally used supercritical drying approach, which could be beneficial to a scale-up of the synthetic route. After removal of the carbon template by combustion in air, the obtained MgO primary particles are close to spherical shapes, which are connected to form a 3D network structure. The surface area and pore volume can reach 154 m

2 g

1 and 0.66 cm

3 g

1

, respectively. A carbon aerogel template with a larger pore size also results in MgO with increasing pore size.

This method thus provides a possibility to control the pore structure of MgO. Because the long-range order is absent in the structure of a carbon aerogel template, the resultant MgO shows a disordered mesostructure.

Further opportunities arise from the utilization of mesoscopically ordered carbons ( e.g.

CMK-3

54

) as hard templates.

These carbon materials are themselves prepared by replication using mesoporous silica as a matrix. In other words, the resulting mesoporous MgO is synthesized by double replication (Fig. 4).

47,55

It is the negative replica of the parent carbon phase and the positive replica of the original silica phase. Through such a double replication process, periodically ordered mesoporous MgO can be obtained. The TEM image presents an intact long-range periodic order with hexagonal symmetry, which corresponds to that of

Table 1 Summary of reported mesoporous MgO, corresponding synthetic methods, and their properties

Method

Soft template

Template

Poly(dimethylsiloxane-ethyleneoxide) (PDMS-PEO)

Poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (Pluronic P123)

Mesopore structure

Wormhole-like

Wormhole-like

Surface area

(m

2 g

1

)

80

298

Hard template Carbon aerogel

CMK-3

Cotton fibers

No template Decomposition of acetate a

Solvothermal annealing a

Precipitation a a

Synthetic methods for mesoporous MgO.

Disordered

Hexagonal

Slit-like

Wormhole-like

Disordered

Disordered

154

306

213

131

373

190

Pore volume

(cm

0.22

0.45

0.66

0.51

0.27

0.24

0.71

0.22

3 g

1

)

Pore size (nm)

13.2

7.6

12.6

5.6

3.5

3.6

2.0

11.4

46

47

48

49

50

51

Ref.

44

45


Chem Soc Rev


Review Article

Fig. 4 Schematic representation (view along the pore axis) of the double replication process from (a) SBA-15 silica over (b) CMK-3 carbon to (c) mesoporous MgO. [Reprinted with permission from ref. 55. Copyright

2006 American Chemical Society.] the original SBA-15 silica. The N

2 adsorption–desorption isotherm shows the characteristic type IV shape, comparable to those of

SBA-15 silica and CMK-3 carbon. The obtained mesoporous MgO exhibits a high surface area of 306 m

2 g

1 and more importantly, a narrow pore size distribution at 5.6 nm. It is the first report concerning a mesoporous MgO with a long-range ordered pore arrangement.

In addition to artificial carbon materials, natural plant materials have also been used as hard templates because of their advantages in cheapness and diversity.

56 A biomorphic mesoporous MgO was synthesized by using cotton fibers as a hard template and Mg(OAc)

2 as a precursor.

48 The mesoporous

MgO materials replicate the zonal morphology of cotton, and their frameworks are constructed by many uniform MgO nanocrystals, as shown in scanning electron microscopy (SEM) images (Fig. 5). The mesoporous structure with slit-like pores can be formed by judicious choice of the amount of precursor.

Taking account of the application of the natural plant as a template, it is reasonable that no long-range ordered mesostructure can be obtained for the material. The biomorphic mesoporous MgO was used to catalyze the decomposition of

2-propanol.

48

It is known that 2-proponal undergoes dehydrogenation to form acetone over basic sites, whereas acidic sites favor the dehydration of 2-proponal to propene. In comparison with the reference sample prepared in the absence of a cotton template, the biomorphic mesoporous MgO presents a higher yield of acetone. This suggests the stronger basicity of the biomorphic mesoporous MgO. It is noticeable that the selectivity of acetone is as high as 90% on the biomorphic mesoporous

MgO at 550

1

C, in contrast to 55% on the reference MgO. Such a high selectivity of acetone is scarcely observed on MgO catalysts prepared using other methods. These results indicate that the biomorphic mesoporous MgO is a promising candidate for application in base-catalyzed reactions.

2.1.1.3. Synthesis without a template.

Attempts have also been made to prepare mesoporous MgO in the absence of any templates. Grassian’s group

49 reported a template-free synthetic method for mesoporous MgO via the thermal decomposition of anhydrous Mg(OAc)

2

. The resultant material had a surface area

Fig. 5 SEM images of (a) the cotton fiber and (b and c) mesoporous MgO synthesized using cotton as the template at different levels of magnification. [Adapted with permission from ref. 48. Copyright 2008 Elsevier.] of 131 m

2 g

1 and a pore size of 3.6 nm. The formation of mesopores is owing to the aggregation of plenty of small primary MgO nanoparticles with interparticle connections.

Cao et al.

50 developed a solvothermal annealing route for the synthesis of mesoporous MgO, in which the Mg(OH)

2 nanoplate was firstly prepared via a solvothermal method. The mesoporous structure could be formed by the dehydration of the Mg(OH)

2 precursor during the annealing process. The solvothermal reaction temperature, solvothermal reaction solvent, and annealing temperature had important effects on the mesostructure of MgO samples. Under optimum conditions, the surface area reached as high as 373 m

2 g

1

. Jung and coworkers

51 synthesized thermally stable single crystalline mesoporous MgO using a simple precipitation method. They found that the precipitation temperature played a decisive role in the fabrication of a mesoporous structure.

The MgO samples precipitated at room temperature showed polycrystals with macropores, while those at 100 1 C gave single crystals with mesopores. A single diffraction line was observable in the low-angle XRD pattern, which demonstrated the formation of a mesoporous structure but the lack of a long-range periodic order.

2.1.2.

Mesoporous mixed oxides.

The templating methods can be extended to the fabrication of mixed oxides with a mesostructure as well. The basicity of resulting materials generally originates from MgO or hydrotalcite-like compounds.

57

By using the cationic surfactant, cetyltrimethylammonium bromide (CTAB),


Review Article


Chem Soc Rev as a structure-directing agent, mesoporous Mg–Al mixed oxides with Mg/Al ratios varying from 0.125 to 8 could be synthesized.

The phases of mesoporous frameworks were the mixture of g -Al

2

O

3 and periclase (MgO) or the mixture of hydrotalcite and periclase depending on the Mg/Al ratio. The mixed oxides possessed wormhole-like mesopores with a surface area of

120–266 m

2 g

1

. With the increase of Mg content the porosity of mixed oxides tended to decrease. The utilization of an anionic surfactant, lauric acid, could also direct the formation of mesopores in Mg–Al mixed oxides.

58

In the hydrothermal process, long chain fatty acid molecules restrict the rapid growth of the individual nanoparticles composed of Mg–Al mixed oxide and thus keep them in a nano-dimension. Mesoporosity in the material is generated because of the interparticle voids. The mixed oxide showed a surface area of 244 m

2 g

1 and considerable mobility of the interlayer exchangeable ions. In addition to Mg–Al mixed oxides, Mg–Fe

59 and Ca–Al

60 mixed oxides with mesoporosity were also reported, and the formation of strong basicity was demonstrated by further modification with KF.

In summary, both soft and hard templating methods have been successfully adopted for the construction of mesoporous basic oxides. Generally speaking, the soft templating method is simple and time-saving. Mesopores can be directly generated in basic oxides via a sol–gel process in the presence of surfactants.

Nevertheless, the obtained materials always show the absence of a mesostructure of a long-range periodic order (typically wormhole-like pores). The key points for the formation of a well-ordered mesostructure should be the rate of hydrolysis and condensation of precursors as well as the interaction between surfactants and precursors. Some possible approaches are the choice of proper solvent systems and the addition of chelating agents. For a hard templating method, the sacrificial template, mesoporous carbon, is compulsory, while other templates such as mesoporous silica cannot work. This is due to the characteristics of basic oxides ( e.g.

MgO), which can only survive during calcination rather than etching with acid or base. Hence, presynthesis of mesoporous carbon is required either by replication of mesoporous silica or possibly, a supramolecular templating approach. This makes the process kind of complicated and timeconsuming. However, the mesoporous structure in theory can be accurately tailored by appropriate selection of carbon templates and the infiltration conditions. Moreover, it is the unique strategy that is able to produce mesoporous MgO with a highly ordered mesostructure at present.

By using silica nanospheres as hard templates, 3,3

0

-diaminobenzidine, diphenyl isophthalate, and/or benzene-1,3,5-tricarboxylic acid triphenyl ester as building blocks, highly cross-linked mesoporous poly(benzimidazole) can be prepared (Fig. 6a).

61

The etching of silica yielded a solid with a well-defined mesostructure as evidenced by TEM and N

2 adsorption–desorption measurements. The mesoporous polymer gave a narrow distributed pore diameter of about 13 nm, which was consistent with the size of the template. Taking account of the N-containing groups in frameworks, the mesoporous polymer was active in

Knoevenagel condensation of various aldehydes with malonic derivatives.

62

For the reaction of benzaldehyde with malononitrile, the yield of the Knoevenagel product reached 100% at room temperature for 20 h. Nonetheless, when poly(benzimidazole) synthesized in the absence of porogen was employed as a catalyst, only traces of products were detected under the same reaction conditions. That means, the mesoporosity plays a vital role besides the basic frameworks.

An interesting mesoporous polymer was synthesized through aqueous-phase radical polymerization of triallylamine using the supramolecular assembly of the anionic surfactant

2.2.

Mesoporous organic polymers

Mesoporous organic polymers are a rapidly expanding class of materials that combine mesoporosity with organic functionalities. They are prepared by direct linking of organic building blocks (or monomers) or through templating routes similar to inorganic silica. Unlike inorganic silica counterparts, however, the structure of mesoporous organic polymers can be tailored through judicious choice of organic building blocks with diverse geometrical and chemical variations. By directly using building blocks containing basic catalytic centers, mesoporous polymers with intrinsically basic frameworks can be fabricated.

Fig. 6 Idealized structure of (a) cross-linked poly(benzimidazole)

[Adapted with permission from ref. 61. Copyright 2007 American Chemical

Society.] and (b) the covalent triazine framework CTF-1. [Reprinted with permission from ref. 64. Copyright 2012 Wiley-VCH.]


Chem Soc Rev


Review Article sodium lauryl sulfate (SDS) as a soft template.

63

Basic sites were introduced by ion exchange of the polymer with NaOH solution, in which Cl ions were replaced by OH ions. Low-angle

XRD and TEM analysis suggests the presence of mesophase and disordered wormhole-like mesopores. The results from N

2 adsorption showed that the polytriallylamine has a mesopore size of ca.

3.1 nm, while the surface area was relatively low

(18 m

2 g

1

). The mesoporous polytriallylamine acted as a very efficient heterogeneous basic catalyst for direct aldol reactions of various aromatic aldehydes with acetone. Moreover, the polymer could be recycled more than five times without an appreciable loss in activity.

A new kind of polymer, covalent triazine framework (CTF), can be synthesized by trimerization of the building block dicyanocompounds.

64–66

To enable the formation of extended and periodic networks, the reaction has to be carried out not only in solution but also at high temperatures ( i.e.

400 1 C).

A salt melt, namely ZnCl

2

, was thus applied as a solvent. In addition, the salt acted as a polymerization catalyst and also as a template, since crystalline frameworks only formed in certain monomer/ZnCl

2 ratios. Hence, careful adjustment of the reaction conditions with 1,4-dicyanobenzene as a monomer produced a crystalline framework denoted CTF-1 (Fig. 6b). The high number of basic nitrogen sites makes CTF-1 efficient in catalytic synthesis of cyclic carbonates via the cycloaddition of

CO

2

.

64

Enhanced surface area and the presence of mesopores dramatically promote the activity of a polymer. The chemical composition was also found to affect the reaction, as proven by increased activity at lower reaction temperatures, when a more basic, pyridine-based, framework was used as catalyst.

In recent years, an immense amount of polymers with various functionality and porosity have been synthesized.

67–76

A substantial portion of porous polymers are used as adsorbents for CO

2 capture due to the basicity.

77–84 Actually, the intrinsic basicity of frameworks makes the polymers extremely potential in reactions catalyzed by bases. Unfortunately, in contrast to extensive studies on CO

2 capture, much less attention has been paid to the applications of these polymers as basic catalysts. There are at least three advantages for mesoporous basic polymers, that is, the mesoporous structure, the diversity of building units, and the hydrophobicity of frameworks. It is obvious that mesopores can accelerate mass transport and allow bulky molecules to access active sites, while the diversity of building units makes it possible to construct polymers with certain base strength that demanded for different reactions. In contrast, the influence of hydrophobicity of frameworks is not that well-known. In the reactions involving water as a byproduct, the hydrophilic frameworks apparently compromise the catalytic activity.

85,86 The adsorption of water near the active centers can lead to partial deactivation of catalysts owing to competition with the reactant species. It is worthy of note that polymers generally exhibit hydrophobic features, which is different from inorganic materials ( e.g.

mesoporous silica) that are hydrophilic in nature. These properties make mesoporous basic polymers highly promising for use as heterogeneous catalysts. Following the widespread applications of porous polymers in adsorption, a rapid growth in catalysis is expected in the coming future.

2.3.

Periodic mesoporous organosilica

Periodic mesoporous organosilicas (PMOs) are a kind of inorganic– organic hybrid material that bridges inorganic mesoporous silica and organic mesoporous polymers. They are synthesized by the use of organic-bridged silane precursors, that is R–[Si(X)

3

] n

, where n is larger than 2, R is the organic group, and X is OMe,

OEt, Oi-Pr, or allyl. In contrast to organic functional groups grafted onto the pore surface in mesoporous silica, in PMOs organic groups are incorporated into frameworks without plugging the pore space, which makes PMOs highly potential in catalysis.

87–90

To date, a range of organic groups have been incorporated into the frameworks of PMOs, leading to the formation of a series of interesting materials.

91–93

It is easy to understand that the incorporation of basic groups can endow

PMOs with basicity.

By using organosilane (EtO)

3

Si–C

2

H

2

–C

5

H

3

N–C

2

H

2

–Si(OEt)

3 and (EtO)

3

Si–C

6

H

4

–C

5

H

3

N–Si(OEt)

3

PMOs containing divinylpyridine 94 as precursors, crystal-like and phenylpyridine groups 95 were synthesized, respectively, by Inagaki’s group. The pyridinederived PMOs show a crystal-like pore wall structure in which the pyridine moieties are densely and regularly arranged. As a result, these PMOs are basic in nature. Recently, the same group also reported the preparation of bipyridine-containing PMO from 100% organosilane precursor, namely (i-PrO)

3

Si–C

10

H

6

N

2

–Si(Oi-Pr)

3

.

96

The obtained material presented well-resolved diffraction lines in low-angle XRD patterns, indicative of a 2D hexagonal lattice.

The nitrogen adsorption–desorption isotherms gave a type-IV isotherm, which was characteristic of ordered mesoporous materials. Further calculation showed that the PMO had a

BET surface area of 739 m

2 g

1

, a pore volume of 0.41 cm

3 g

1

, and a density functional theory (DFT) pore diameter of 3.8 nm.

The pore wall thickness was estimated to be 1.4 nm, which corresponds to three layers of bipyridine moieties in the pore walls. The particle morphology with a diameter of 200–500 nm was observed from the SEM image (Fig. 7). The TEM images showed 1D channels throughout the particles, suggesting the single crystal-like structure. A structural model of the PMO is displayed in

Fig. 7. The bipyridine and silica belt-like layers in the pore walls are arranged alternatively with a periodicity of 1.16 nm in the channel direction. These PMOs possessing pyridine units in frameworks and an ordered mesostructure are promising for applications as heterogeneous basic catalysts in various reactions.

In addition to pyridine, other basic organic units such as amine can also be incorporated into frameworks, resulting in the formation of mesoporous solid bases with an excellent catalytic performance. By surfactant ( i.e.

CTAB)-directed co-condensation of bis[3-(triethoxysilyl)propyl]amine (BTEA) with TEOS, an aminobridged PMO in uniform nanospheres ( ca.

85 nm) was synthesized.

97

The resultant material exhibits short and straight mesopore channels with amino groups integrally incorporated into frameworks (Fig. 8a). The PMO was applied to catalyze watermedium Knoevenagel condensation.

97

It is worth noting that water-mediated organic reactions should be carefully conducted


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Fig. 7 (a) SEM and (b–d) TEM images as well as (e) structure model of bipyridine-containing PMO. Silicon, yellow; oxygen, red; carbon, gray; nitrogen, pink; hydrogen, white. Hydrogen on bipyridine is omitted. [Reprinted with permission from ref. 96. Copyright 2014 American Chemical Society.]

Fig. 8 Schematic illustration of the preparation of (a) PMOs with amines in frameworks and (b) mesoporous silica with amino groups grafted onto the surface. [Adapted with permission from ref. 97. Copyright 2012 Elsevier.] since they contained three phases. The Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate presented nearly

100% selectivity toward the target product. The conversion of benzaldehyde increased progressively with the reaction time.

Nonetheless, it could not reach 100% benzaldehyde conversion even after reaction for a very long period, which is possibly caused by the poor solubility of organic substrates in water. The effect of the water amount was also studied. It was also found that with the increase of the water amount, the conversion of benzaldehyde first increased and then declined. The protic water might activate benzaldehyde with weak acidity, which favors the base-catalysis. However, an excess amount of water is harmful for the reaction since the amine groups could be covered by water molecules. In addition to Knoevenagel condensation, the obtained PMO was also employed to catalyze

Henry reactions.

97

It showed much higher activity and selectivity as compared with amino-functionalized MCM-41 prepared by grafting amino groups onto the pore surface in both reactions.

In terms of various structural characterization and kinetic investigations, the high catalytic activity and selectivity can be attributed to the short and straight pore channels in PMO nanospheres, making the amine sites highly accessible to organic reactant molecules. The accessible basic catalytic sites may facilitate the diffusion, and hence promote the adsorption of reactant molecules, resulting in the enhanced activity and selectivity. In the case of amino-grafted MCM-41, the pendant amino groups may block the mesopore channels as illustrated in Fig. 8b, which compromises the catalytic activity. Moreover, the amino-containing PMO showed the catalytic efficiency comparable to the homogeneous catalyst diethylamine. The catalytic activity, in combination with the recyclability, makes the amino-containing PMO promising in substitution of traditional homogeneous catalysts.

Various organic basic groups can be incorporated into the frameworks of PMOs by using the surfactant-directed approach, which leads to the fabrication of mesoporous solid bases with different basicity. Three organosilanes, namely (EtO)

3

Si–

(CH

2

)

3

–NH

2

, (MeO)

3

Si–(CH

2

)

3

–NH–(CH

2

)

2

–NH

2

, and (MeO)

3

Si–

(CH

2

)

3

–C

5

H

5

N, were employed to fabricate basic PMOs by co-condensation with 1,2-bis(triethoxysilyl)ethane. The resultant materials containing amine, diamine, and pyridine groups were denoted PMO-N, PMO-DN, and PMO-Py, respectively.

98

The amount of basic sites for PMO-N, PMO-DN, and PMO-Py was measured to be 0.611, 0.832, and 0.491 mmol g 1 , respectively. It should be stated that the same molar amount of organosilane precursors was used in the synthetic process; thus, the different amount of basic sites should be caused by the different hydrolysis and condensation rate of organosilanes. According to the nature of basic sites, the strength of basic sites in the materials declines in the order of PMO-Py 4 PMO-DN 4 PMO-N. The esterification of oleic acid with methanol was employed to evaluate these PMO catalysts. The conversion of oleic acid over different catalysts decreased in the order of PMO-DN 4 PMO-Py 4

PMO-N. These results clearly show that the activity is dependent on the basicity of catalysts with regard to both the strength and amount of basic sites.

In summary, basic PMOs are an interesting kind of material with potential in reactions catalyzed by bases. It should be noted that the fabrication of basic PMOs relies on the appropriate organosilanes, which may limit the type of basic PMOs.

Because the basic sites are incorporated into frameworks, the diffusivity of mesoporous channels in PMOs is evidently better than that in amino-grafted materials. In the meanwhile, some basic sites may be blocked in the frameworks, making the access of reactants difficult.


Chem Soc Rev


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3. Generation of basic sites on mesoporous supports

Extensive attention has been paid to the generation of basic sites on mesoporous supports. Because these mesoporous hosts seldom show any basic properties, the introduction of basic guests is required. From the viewpoint of hosts, their synthesis ( e.g.

mesoporous silica) is much easier as compared with mesoporous materials with intrinsic basic frameworks ( e.g.

mesoporous magnesia). From the viewpoint of guests, there are more choices of basic species derived from various guests. The thoughtful combination of basic guests with mesoporous hosts allows the fabrication of new types of materials with a range of basic nature

(strength and amount of basic sites) and mesopore symmetry.

3.1.

Mesoporous silica

Among various candidates with mesostructure, mesoporous silica is the best choice of support in theory. They are readily synthesized and have good stability. So far an incredible degree of control can be achieved on mesoporous silica with various pore structures. Hence, mesoporous silica should be the appropriate starting materials for preparation of mesoporous solid bases.

A series of methods have been developed to generate basic sites on mesoporous silica as described below.

3.1.1.

Basicity of as-synthesized mesoporous silica.

It is known that mesoporous silica is synthesized by using surfactants as structure directing agents. The as-synthesized samples present low surface area, because their pores are occluded by the surfactants. Usually, these samples are subjected to calcination

(or extraction) to remove the surfactants. Guest species can then be introduced to the supports with open mesopores. Interestingly, the as-synthesized mesoporous silica MCM-41 was found be to an efficient basic catalyst for the Knoevenagel condensation reaction.

99 This indicates the presence of basic sites in surfactantcontaining samples. A collection of experiments were designed to examine the origin of basicity.

100 First, subsequent to synthesis, mesoporous silica was extensively washed with water to remove any eventually adsorbed raw materials (salts, hydroxides or amines), to ensure that the activity was not caused by these impurities. Second, the as-synthesized MCM-41 was calcined to remove the surfactants. For the obtained material only made of silica walls, it did not present marked catalytic activity under the same reaction conditions. Third, the surfactants themselves

(CTAB) were directly used to catalyze the Knoevenagel condensation, while the conversion was negligible. Based on these results, it is clear that the basicity originates from the as-synthesized mesoporous silica and not solely from the presence of the organic surfactants or inorganic silica walls.

Through a deep analysis of experimental results, Oliveira et al.

100 found that catalytic performance was mainly dependent on the amount of framework silicon in as-synthesized samples. They suggested that the basic sites were the siloxy anions (

R

SiO ), interacting with the CTA

+ cations. X-ray photoelectron spectroscopy (XPS) was further employed to explore the basic sites by

Cardoso et al.

101

It is known that O 1s XPS is a way of directly examining Lewis basicity in aluminosilicates and that small changes in oxygen binding energy mirror a great difference in structural oxygen basicity. The measurements of as-synthesized and calcined MCM-41 showed that O 1s binding energy was obviously lower in the presence of the voluminous CTA + cations. That means, the Lewis basicity of the Si–O oxygen atom in as-synthesized MCM-41 is much higher. Taking into consideration that CTA

+ is a voluminous cation, its interaction with

R

SiO sites is weak, moving the chemical balance

SiOCTA 2

R

SiO + CTA

+ to the right and increasing the basicity of

R

SiO . The measurements of solid-state

29

Si magicangle spinning (MAS) nuclear magnetic resonance (NMR) confirmed the existence of

R

SiO , which was the basic site itself.

101

Because the channels of as-synthesized sample are occupied by the surfactants, the high activity in Knoevenagel condensation is mostly due to the basic sites present in the pore-mouth.

99,101

The effect of surfactant chain length on basic catalytic properties of MCM-41 was investigated by Martins and

Cardoso.

102

A series of quaternary ammonium surfactants,

C n

H

2 n +1

N(CH

3

)

3

+ (C n

TA + ), with different alkyl chain lengths

( n = 10, 12, 14, and 16) were used, producing MCM-41 materials with different hexagonal spacing and pore diameter. These materials were employed to catalyze the Knoevenagel condensation of butyraldehyde and ethyl cyanoacetate. The results showed that MCM-41 containing C

16

TA

+ and C

14

TA

+ cations had almost the same activity of about 59.0%. However, for a lower chain length, i.e.

C

12 and C

10

, the yield of the product reduced to 50.3% and 40.8%, respectively. The difference of activity can be ascribed to two characteristics of these materials relating to their ordering degree. The first characteristic is the amount of surfactants in the samples. In terms of thermogravimetric (TG) analysis, the sample with a higher ordering degree has a larger amount of CTA

+ cations, which indicates more

R

SiO basic sites. As a result, a higher activity of MCM-41 samples is a consequence of the higher amount of siloxy anions. The second characteristic is the number of pore mouths in the external surface. Disordered samples have thicker silica walls, which suggests a reduced number of pore mouths in the external surface.

102

This also indicates a lower amount of siloxy anions involved in the Knoevenagel reaction.

Therefore, the as-synthesized MCM-41 suitable for a base catalyzed reaction should have a high ordering degree that is prepared with long carbon chain surfactants C n

TA

+

( n 4 14).

The basicity of as-synthesized mesoporous silica MCM-48 and MCM-50 has also been studied besides MCM-41. The adsorption of CO

2 monitored by calorimetry was employed to examine the basicity.

103

These as-synthesized samples showed weak basicity according to low differential heat of CO

2 tion between 70 and 90 kJ mol adsorp-

1

. Nonetheless, these samples were found to be highly active in the transesterification of rapeseed oil with ethanol under mild temperature conditions

(79

1

C). Total oil conversion was achieved in 1 h under the catalysis of MCM-48. Among the catalysts, the following ranking was observed as regards their activity in transesterification: MCM-48 4

MCM-50 4 MCM-41. This ranking is in line with the total amount of sites probed by CO

2 differential heat of CO

2 adsorption.

adsorption as well as the


11

12

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Chem Soc Rev

5

6

Table 2 Summary of types of basic species and supports, synthetic methods, and reactions catalyzed by the resultant mesoporous bases

No.

1

2

3

4

Basic species Support

MCM-41

SBA-15

HMS

SBA-1

SBA-15

FSM-16

MCM-41

MCM-41

SBA-15

MCM-41

MCM-41

Synthetic method

Post-synthetic grafting

Direct synthesis

Vapor phase deposition


Direct synthesis


Direct synthesis

Direct synthesis



Direct synthesis

Direct synthesis


Direct synthesis

Base-catalyzed reaction

Isomerization; nitroaldol condensation

Knoevenagel condensation; nitroaldol condensation

—

Knoevenagel condensation;

Michael addition

Claisen–Schmidt condensation; Knoevenagel condensation; Michael addition


Michael addition

—


1,4-Conjugate addition

Isomerization

Nitroaldol condensation


Isomerization

Nitroaldol condensation

Ref.

108 and 109

110 and 111

112

113 and 114

57, 115 and 116

114

117

116

118

109

111

116

109

111

7

Polyethyleneimine SBA-15

MCM-41

MCM-41

Polymerization



—

Knoevenagel condensation; ring-opening reaction


119

120 and 121

120–122

8 SBA-15 Post-synthetic grafting Nitroaldol condensation 123

9 MCM-41

SBA-15




Interesterification reaction

120 and 121

124

10 MCM-41 Post-synthetic grafting Knoevenagel condensation; ring-opening reaction

120–122

MCM-41 Post-synthetic grafting Michael addition 125

MCM-41 Post-synthetic grafting

MCM-41

SBA-15



Aldol condensation;


Michael addition

Knoevenagel condensation

Knoevenagel condensation

126

127

127

13


Chem Soc Rev

Table 2 ( continued )

No.

Basic species

14


Review Article

Support

SBA-15

Synthetic method


Base-catalyzed reaction

Diels–Alder reaction

Ref.

128

15 SBA-15 Post-synthetic grafting Transesterification reaction

129

In conclusion, this kind of base should be limited to mesoporous silica prepared by using cationic surfactants rather than anionic or nonionic ones, according to the origin of basicity. Whilst the as-synthesized mesoporous silica shows remarkable base catalytic activity, their deactivation takes place to a greater or lesser extent.

100,103

This is due to the leaching of surfactants during reactions, which is difficult to avoid taking account of the solubility of surfactants in reaction systems. As a result of the thermal stability of organic surfactants, this kind of catalyst should be only used at relatively low temperatures in contrast to inorganic basic oxides. This also excludes the possibility of regeneration of active sites at high temperatures.

In any case, as-synthesized mesoporous silica is a type of interesting solid base; it can be directly used as base catalyst without any extra modification or pretreatment.

3.1.2.

Basic species immobilization.

Immobilization is a general strategy that has been utilized to attach organic groups to the silica surface via the formation of covalent bonds.

104–106

Basic sites can be generated on mesoporous silica by immobilizing organic bases. This typically involves reactions between silane compounds and the silica host. Due to the formation of a covalent bond between organic guest species and inorganic hosts, high stability against leaching is aimed at. Various basic species have been introduced to mesoporous silica as summarized in Table 2.

3.1.2.1. Immobilization methods.

and polymerization.

(Fig. 9).

107

So far two main methods have been developed to immobilize basic species, which are post-synthetic grafting and direct synthesis. For the method of post-synthetic grafting, several specific approaches are also used, including liquid phase grafting, vapor phase deposition,

Both post-synthetic grafting and direct synthesis are widely used methods for the introduction of organic basic species

In the case of post-synthetic grafting, mesoporous silica is first fabricated, leaving silanol groups on the surface of mesopores. Organic basic species are then grafted through the silylation of organoalkoxysilane, R

1 n

–Si–(R

2

O)

4 n

( n = 1–3), with silanol groups. On the other hand, the direct synthesis method

(also called one-pot synthesis) is based on the co-condensation of tetraalkoxysilanes ( e.g.

TEOS) and organoalkoxysilanes in the

Fig. 9 Introduction of organic bases to mesoporous silica by (a) postsynthetic grafting and (b) direct synthesis using a silane compound. [Adapted with permission from ref. 107. Copyright 2012 Elsevier.] aqueous solution containing a surfactant, leading to the production of basic species-containing networks through sol–gel chemistry.

It is worth noting that quite different properties are observed for mesoporous silica containing basic species derived from postsynthetic grafting and direct synthesis.

Among organic basic species, aminopropyl is the most popular one and has been introduced to different silica hosts including MCM-41,

108,109

SBA-15,

113 and hexagonal mesoporous silica (HMS).

114

In these cases, 3-aminopropyltrimethoxysilane

[H

2

N–(CH

2

)

3

–Si–(OCH

3

)

3

, APTMS] and 3-aminopropyltriethoxysilane [H

2

N–(CH

2

)

3

–Si–(OEt)

3

, APTES] are regularly employed as precursors. A detailed comparison of aminopropyl-containing

MCM-41 prepared by direct synthesis and post-synthetic grafting was made by Kubota’s group.

107

For the material from direct synthesis, the intensity of diffraction lines in low-angle XRD patterns greatly depends on the amount of aminopropyl groups.

When the proportion of APTMS in silica sources was higher than

0.2, only a single diffraction line can be observed, suggesting that the array of mesopore channels became disordered.

107

Further increasing such a proportion to 0.7, no precipitate could be formed even after aging for 4 days. In addition, with the increase of APTMS proportion, the d

100 spacing of the material is raised. This variation is inconsistent to the observation of the functionalization of MCM-41 with vinyltriethoxysilane by Ozin’s group, in which the d

100 spacing of the material decreases.

130


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They considered that the reduction of the unit cell was caused by the strong interaction between nonpolar organic groups and the tails of surfactant molecules. This hydrophobic interaction may further draw the organic precursors into the micelles, leading to the decrease in the d

100 spacing. In the synthetic system of aminopropyl-containing MCM-41, Kubota’s group proposed that the hydrophobic interaction between the terminal organic moiety of the organoalkoxysilane and the long alkyl chain of the surfactant could be interrupted by the existence of hydrophilic amino groups.

107

As a result, the organic moiety would not be drawn into the micelles, leading to the increase in d

100 spacing. Unlike the material from direct synthesis, the aminopropyl-containing material from post-synthetic grafting showed identical diffraction lines with regard to intensity and position as compared with pristine mesoporous silica.

114,121

This reveals that the pore symmetry is scarcely affected by grafting amine through post synthesis.

In the case of direct synthesis, amino groups are introduced during the formation of pore walls. It is thus possible that some amino groups are blocked in the pore walls instead of exposed on the surface. The amount of amino groups in the materials can be determined through two approaches, namely nitrogen elemental analysis

111 and argentometric titration (titrated with

AgNO

3 followed by neutralization with HCl).

131

The nitrogen elemental analysis gives the total amount of amino groups in the material, whereas the argentometric titration presents the amount of amino groups on the surface. When the proportion of APTMS in silica sources was 0.2, the resulting aminopropylcontaining MCM-41 showed an amine content of 0.83 mmol g

1 from nitrogen elemental analysis, while 0.76 mmol g argentometric titration.

107

1 from

The difference implies that about 8% of total amino groups in the material are not exposed on the surface but blocked in the walls. The percentage of blocked amino groups is also affected by the APTMS proportion in silica sources and the molecular size of amino-containing precursors.

107

When the APTMS proportion in silica sources increased to 0.4, the unexposed amino groups became 18%. Instead of

APTMS, two other precursors with large size 6-amino-4azahexyltrimethoxysilane [H

2

N–(CH

2

)

2

–NH–(CH

2

)

3

–Si–(OCH

3

)

3

,

AATMS] and 9-amino-4,7-diazanonyltrimethoxysilane [H

2

N–(CH

2

)

2

–

NH–(CH

2

)

2

–NH–(CH

2

)

3

–Si–(OCH

3

)

3

, ADTMS] were also used, while 35% and 41% of amino groups were blocked in the resultant materials. Further calculation indicated that the respective surface density of APTMS, AATMS, and ADTMS silanes was 1.38, 0.82 and 0.80 nm

2

. The lower surface density of larger amino-containing moieties indicates that the size of precursors greatly influences the functionalization of silica by direct synthesis. Unlike direct synthesis, the method of postsynthetic grafting produces materials with amino groups well exposed on the surface. This is because the pore walls have been already fabricated before the introduction of organoalkoxysilanes, and the incorporation of amino groups into pore walls is eluded. For the materials derived from post-synthetic grafting, the surface density of APTMS, AATMS, and ADTMS silanes was 1.8, 1.6 and 2.1 nm

2

, respectively, suggesting that the amount of silanes grafted onto the surface was quite similar regardless of the size of organoalkoxysilanes.

107

This is obviously different from the materials prepared by direct synthesis, in which the surface density of silanes is significantly dependent on the size of organoalkoxysilanes.

Mesoporous silica can be synthesized under neutral, basic, and acidic conditions. Being different under neutral and basic conditions, acidic conditions are considered to be unfavorable for the introduction of amino groups through direct synthesis.

In some cases only materials with a disordered mesostructure were obtained by co-condensation of organoalkoxysilanes and silica sources. It is reported that the protonated amino groups may interfere with the self-assembly of structure-directing agents and silica sources under strongly acidic conditions.

132

Additionally, protonated amino groups are produced in acidic synthetic systems, so that the obtained materials are not basic at all. To solve these problems, the strategies of prehydrolysis and deprotonation were developed.

113

For the introduction of aminopropyl-functionalized SBA-15, the precursor APTES was used. Prior to the addition of APTES, the prehydrolysis of

TEOS was conducted and played a crucial role in the formation of an ordered mesostructure.

115 For the materials without prehydrolysis of TEOS, no diffraction lines could be observed in low-angle XRD patterns. When the prehydrolysis of TEOS was conducted for longer than 1 h, three diffraction lines ascribed to 2D hexagonal pore regularity became visible. Moreover, the diffraction peaks became intense with an increase in prehydrolysis time, suggesting that a more ordered mesostructure was constructed. Before the use of materials in base-catalytic reactions, a deprotonation process was employed to neutralize the protonated amino groups. The materials were usually treated with tetramethylammonium hydroxide (TMAOH), leading to the formation of active amino groups. It should be stated that compounds with too strong basicity are not proper for deprotonation, due to the possible damage of siliceous frameworks by strongly basic compounds.

133 The deprotonated materials possessed an ordered mesostructure and basicity; they were demonstrated to be good basic catalysts in Knoevenagel condensation reactions.

113

The large mesopores of SBA-15 are beneficial to the diffusion of substrates, products, and solvents encountered in the liquid-phase reactions. The reactions of various aldehyde or ketones with ethyl cyanoacetate were studied. It was found that aldehyde and cycloketone displayed the highest yield, while aromatic ketone exhibited the lowest yield. The reaction activity decreased in the order of aldehyde 4 cycloketone 4 aliphatic ketone 4 aromatic ketone. That is caused by the steric hindrance around the carbonyl group in the reactions. In comparison with that over MCM-41 of similar amine loading, amine-containing

SBA-15 showed higher activity in the reactions, which could be attributed to the fast diffusion of substrates/products in SBA-15 with a larger mesopore size.

In the case of post synthesis, the reactions of organoalkoxysilanes with surface silanol groups are commonly conducted by refluxing in toluene, which is called liquid phase grafting.

Because of the quite different accessibility of the sites on the external particle surface, the pore surface close to the entrances, and the internal pore surface, the distribution of immobilized


Chem Soc Rev


Review Article amino groups in the final products is normally inhomogeneous.

Moreover, the method of liquid phase grafting is hard to elude hydrolysis and cross-linking when introducing organoalkoxysilanes from a solvent. Even when proceeding in dry solvents, trace water adsorbed on the surface of silica is unavoidable so that hydrolysis becomes possible. To solve these problems, a technique of vapor phase deposition is proposed instead of liquid phase grafting. Vapor phase deposition is able to facilitate the elimination of silane cross-linking, and subsequently resulting in a more and coworkers

112 systematically compared the immobilization of aminopropyl groups on MCM-41 through liquid phase grafting and vapor phase deposition. Vapor phase deposition was performed in an atomic layer deposition (ALD) reactor at a reduced pressure. The precursor APTMS was vaporized and deposited onto MCM-41 at a prescribed temperature. The maximum amine loading was 1.2 mmol per gram of pristine

MCM-41, which was the same as the material prepared from liquid phase grafting. The nitrogen adsorption results showed that the material derived from vapor phase deposition exhibited a narrower pore size distribution (full width at half maximum

FWHM = 0.30 nm) as compared with that from liquid phase grafting (FWHM = 0.42 nm). Because the materials contain an identical amount of amino groups, the narrow pore size distribution is attributed to the uniform distribution of functional groups.

134

The wide pore size distribution is most likely a consequence of pore blocking caused by cross-linking of APTMS.

If cross-linking of APTMS takes place close to the entrance of a given pore, diffusion of additional APTMS molecules into this pore is obstructed, which leaves part of the pore surface unmodified. Accordingly, such a partially blocked pore corresponds to a relatively wide pore size distribution.

In addition to liquid phase grafting and vapor phase deposition, a polymerization method was also developed to produce reactive primary amine groups on the surface of mesoporous silica in the form of surface-grown polyethyleneimine.

119 The polymer was grown directly from the surface silanol groups utilizing a highly reactive, non-bulky monomer, aziridine (Fig. 10).

By use of the polymerization method, the amount of introduced amino groups thus does not depend on the initial concentration of silanol groups on the surface. Correspondingly, the loading of amino groups reached as high as 6.5 mmol g

1 on SBA-15 which was hard to achieve through other methods including liquid phase grafting and vapor phase deposition.

119

Because the polymerization reactions were catalyzed by acid, SBA-15 was first functionalized with carboxylic acid. With the catalytic groups distributed homogeneously on the surface, the monomers could be transported to the catalytically active sites before the polymerization reactions occurred. This enables a kinetic control of the polymerization and results in a more sharp increase in amine surface concentration as to the pure silica SBA-15. The nitrogen sorption isotherms displayed almost parallel adsorption and desorption branches,

119 suggesting that no pore blocking took place and the mesopores were fully accessible after the introduction of large amounts of functional groups. This is believed to be of great importance for applications in catalysis.

To summarize, basic amino groups can be introduced to mesoporous silica by using either post-synthetic grafting or direct synthesis. The two methods possess certain merits as well as drawbacks. By using the method of post-synthetic grafting, the structure of mesoporous silica can be well preserved, since the pore walls have been already fabricated. The type of amino functional groups that can be introduced by post-synthetic grafting is quite abundant, as can be seen from Table 2. Nonetheless, the loading of amino groups is limited by the amount of silanols on the surface. Also, the homogeneous distribution of organic functionalities is difficult, and the post-synthetic grafting results in the condensation of organic groups close to the pore entrances causing the blockage of channels. On the other hand, the direct synthesis method offers a more homogeneous distribution of the organic functional groups. A higher loading of amino groups can be achieved without blocking the pore channels. However, direct synthesis has a tendency to decrease the ordering of mesostructure as a result of the incorporation of functional groups. It should also be noted that direct synthesis may make some amino groups blocked in the walls instead of exposed on the surface, since the functional groups are introduced during the formation of pore walls. Researchers may select a proper immobilization method in terms of the needs of specific applications.

Fig. 10 Introduction of amino groups into mesoporous silica through one-step hyperbranching polymerization of surface-grown polyethyleneimine. [Adapted with permission from ref. 119. Copyright 2006 Royal

Society of Chemistry.]

3.1.2.2. Types of amines.

Various types of amino groups have been immobilized on mesoporous silica, leading to the formation of a series of mesoporous basic materials as listed in Table 2. The type of amino groups has an important effect on the basicity and catalytic performance of materials. To examine the influence of amine types, three organoalkoxysilanes, that is APTMS,

[3-(2-aminoethylamino)propyl]trimethoxysilane (MeO)

3

Si–(CH

2

)

3

–

NH–(CH

2

)

2

–NH

2

, and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (MeO)

3

Si–(CH

2

)

3

–NH–(CH

2

)

2

–NH–(CH

2

)

2

–NH

2

, were used as precursors and grafted onto Al-containing MCM-41, leading to the formation of materials labelled APMS/AlMCM-41,

2APMS/AlMCM-41, 3APMS/AlMCM-41, respectively (Fig. 11).

135

The nitrogen content of APMS/AlMCM-41 was measured to be

1.8 mmol g

1

, and that of 3APMS/AlMCM-41 is 4.5 mmol g

1

.

Low-angle XRD results reflected that the ordered mesoporous structure was maintained despite the high loading of nitrogen.

By combing the TG and infrared (IR) techniques, it can be


Review Article


Chem Soc Rev

Fig. 12 Immobilization of organic base through the reaction of free amine TBD with chloropropylsilyl pre-grafted onto mesoporous silica.

Fig. 11 Models of different types of amine immobilized on mesoporous silica. [Adapted with permission from ref. 135. Copyright 2010 Elsevier.] found that the decomposition of amino groups on 2APMS/

AlMCM-41 and 3APMS/AlMCM-41 took place at higher temperatures as compared with that on APMS/AlMCM-41. This indicates the better thermal stability of materials with larger moieties, which is related to the existing form of different types of amine.

It is known that the immobilization of amines proceeds through the reactions of alkoxy groups from precursors with silanols from silica. Detailed IR analysis exhibited that alkoxy species were well detected in 2APMS/AlMCM-41 and 3APMS/AlMCM-41, suggesting that only one alkoxy group seems to be involved in the reactions. Also, IR spectra in hydroxyl region gave rise to hydrogen bonded species in 2APMS/AlMCM-41 and 3APMS/

AlMCM-41. According to these IR results, Blasco-Jime et al.

proposed the model of materials grafted with different types of amine (Fig. 11).

135

The existing form of amine involving hydrogen bonds is responsible for the higher thermal stability of 2APMS/AlMCM-41 and 3APMS/AlMCM-41.

The resultant materials were employed to catalyze liquid phase Knoevenagel reactions of benzaldehyde and ethyl cyanoacetate and diethyl malonate in the absence of any solvent. The activity decreased in the following order: APMS/AlMCM-41 4

2APMS/AlMCM-41 4 3APMS/AlMCM-41.

135

In addition, isomerization of safrole to prepare isosafrole was utilized to evaluate the basicity of materials. The conversion of safrole over APMS/

AlMCM-41, 2APMS/AlMCM-41, and 3APMS/AlMCM-41 was 95%,

81%, and 69%, respectively.

109

The catalytic activity in safrole isomerization is in good agreement with that in Knoevenagel reactions. Apparently, there are an optimum number of amino groups to enhance the basicity of these materials for catalysis, and amino groups with short chain length are more active. In

2APMS/AlMCM-41 and 3APMS/AlMCM-41, the hydrogen bonding between amino groups and silanols can compromise the base strength of materials. Furthermore, the stabilization of amines presented in the model (Fig. 11) hampers the accessibility of active sites for reactant molecules, thus decreasing the catalytic activity.

By using amino-containing organoalkoxysilanes as precursors, organic bases can be directly introduced to mesoporous silica.

Alternatively, mesoporous silica can be pre-functionalized with chloropropylsilyl groups, followed by the reaction of chloropropylsilyl groups with free amines. A collection of free amines were tried, namely piperidine, pyrolidine, triazabicyclo[4,4,0]dec-

5-ene (TBD), and 2,4,6-tri-aminopyrimidine, leading to the fabrication of various interesting mesoporous solid bases denoted

Pip/MCM-41, Pyr/MCM-41, TBD/MCM-41, and PM/MCM-41, respectively (Fig. 12).

120

The basicity of these materials was assessed by the synthesis of monoglycerides from fatty acids and glycidol. Kinetic analysis showed that the initial rate constant for the reactions over the used TBD/MCM-41 catalyst was the highest followed by Pip/MCM-41, Pyr/MCM-41, and

PM/MCM-41.

120

The catalytic activity approximately correlated with the base strength of free organic bases, except for Pyr/MCM-41. The low activity of Pyr/MCM-41 is associated with the high chloride content in the material (0.45 mmol g

1

). This is because some

HCl liberated in the grafting reactions are adsorbed on the newly formed basic sites, which thus compromises the basicity of resulting materials. Aiming to solve such a problem, two methods have been adopted, that is post-treatment to remove adsorbed

HCl and pre-functionalization with chlorine-free linkages.

To remove adsorbed HCl, the post-treatment of aminocontaining materials with base was conducted. Take Sachdev and Dubey’s work as an example, piperazine was introduced into mesoporous silica SBA-15 through chloropropylsilyl linkages, and thus basic sites were contaminated by HCl produced in the synthetic process.

123

The sample was treated with triethylamine for 24 h to neutralize HCl. The removal of HCl and the formation of piperazine can be confirmed by the ninhydrin test, in which the color of solid was changed from white to pink.

The presence of piperazine can be confirmed by the IR band at

3500 cm

1 ascribed to N–H stretching of secondary amine. The basicity of the material was also determined by the use of

Hammett indicators

136 and titration.

137

The base strength was found in the range of 8.3

o H o 11.0. Hammett indicators have been widely used for the measurement of base strength of a solid. When an electrically neutral acid indicator is adsorbed on a solid base from a nonpolar solution, the color of the acid indicator is changed to that of its conjugate base, provided that the solid has the necessary basic strength to impart electron pairs to the acid. Hence, it is possible to determine the basic strength by observing the color changes of acid indicators over a range of values, as listed in Table 3.

138–144

The resulting material was applied to catalyze nitroaldol condensation for the synthesis of b -nitroalkanols, which are important intermediates in various organic transformations.


Chem Soc Rev


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Table 3 Hammett indicators used for the measurement of base strength

Indicator

Original color Basic color

Base strength ( H )

Bromthymol blue m -Cresol purple

Phenolphthalein

Alizarin yellow R

Colorless Blue

Yellow Purple

Colorless Red

Yellow Red

2,3,4-Trinitroaniline

2,4-Dinitroaniline

Yellow

Yellow

4-Chloro-2-nitroaniline Yellow

4-Nitroaniline Yellow

Reddish-orange

Violet

Orange

Yellow-orange

7.2

8.3

9.3

11.0

12.2

15.0

17.2

18.4

Benzidine

4-Chloroaniline

Aniline

Triphenylmethane

Xylene

Toluene

Colorless Purple

Colorless Peach

Colorless Mauve

Colorless Red

Colorless Yellow

Colorless Olivine

22.5

26.5

27.0

35.0

39.0

41.0

The amino-grafted SBA-15 showed 70% conversion and 85% selectivity for the desired product in the nitroaldol condensation of p -nitrobenzaldehyde and nitroethane.

123 It is worthy of note that only 63% conversion and 72% selectivity were obtained under the catalysis of pure piperazine in the homogeneous system. These results thus demonstrated that after post-treatment, amino-functional mesoporous silica were rather active in base-catalyzed reactions. It should be stated that, however, the structure of mesoporous silica might be damaged during the post-treatment with triethylamine. The surface area and pore volume of amino-containing SBA-15 are only 76 m

2 g

1 and 0.16 cm

3 g

1

, which are much lower than those of parent

SBA-15 (surface area: 678 m

2 g

1

; pore volume: 1.2 cm

3 g

1

).

123

Therefore, it is of great significance to select a base with appropriate strength for post-treatment, aiming to remove adsorbed HCl and maintain the structure of mesoporous silica simultaneously.

Instead of chloropropyl groups, chlorine-free linkages were utilized to avoid the release of HCl that can poison the basic sites. By the reactions of silanols with 3-trimethoxylsilylpropoxymethyloxirane, MCM-41 was functionalized with oxirane groups as shown in Fig. 13.

125

The base precursor, TBD, can react with oxirane groups under mild conditions without the formation of any byproducts including HCl. The newly formed basic sites are thus kept intact and active in a collection of reactions.

In Michael reactions, the obtained material accelerated only the desired l,4-addition, and bypassed side reactions such as dimerizations or rearrangements. The material exhibited good

Fig. 13 Grafting organic base onto mesoporous silica by using the silylation reagent without chlorine.

selectivity for the reactions of typical reactants. For example, the yield of the 1,4-adduct was 52% with the selectivity of 100% for the reaction of ethyl cyanoacetate with cyclopentenone.

125

The activity of TBD-functionalized MCM-41 was superior to that of Cs-modified MCM-41. Moreover, the material was applied to catalyze Knoevenagel condensations of aromatic aldehydes.

The reactions were able to proceed smoothly at room temperature with an excellent selectivity of 90–100%.

125

Obviously, the structure of mesoporous silica can be well preserved by the use of chlorine-free linkages. This is different from the posttreatment approach in which the destruction of the mesostructure takes place.

In summary, a variety of amines can be anchored on mesoporous silica, producing plenty of fascinating heterogeneous basic catalysts. The base strength of these heterogeneous catalysts is generally dependent on the strength of free amines; meanwhile, the amount of basic sites relies on the amount of amines introduced. In order to fully understand the properties of theses catalysts, however, some other factors should also be considered such as the size of amines. Hydrogen bonds can be formed between surface silanols and amines with a large size, which weakens the basicity of catalysts. By pre-functionalizing mesoporous silica with chloropropylsilyl groups, a number of different amines can be introduced by the reactions of free amines with chloropropylsilyl groups. Nevertheless, HCl released in the grafting process can be adsorbed on newly formed basic sites, which poisons the basic sites of obtained materials. It is therefore necessary to treat the materials to remove adsorbed HCl or pre-functionalize with chlorine-free linkages.

3.1.2.3. Effects of support properties.

By using surfactanttemplating methods, mesoporous silica with different structural parameters (such as pore size, channel length, and particle diameter) can be synthesized. Also, the surface acidity can be tailored through the incorporation of metal ( e.g.

Al and Nb) into siliceous frameworks. These properties have an imperative effect on the catalytic performance of amino-functionalized mesoporous silica despite that the same type and amount of amino groups are introduced.

The effect of pore size on base catalysis was examined using aminopropyl-functionalized mesoporous silica spheres.

108

By changing the type of surfactant and synthetic conditions, mesoporous silica spheres with a similar particle diameter and amine content were obtained, while the pore size varied with the chain length of the surfactant (alkyltrimethylammonium halide).

The material NH

2

-C

14

-MS with a pore size of 1.34 nm can be synthesized in the presence of the template tetradecyltrimethylammonium (C

14

TMACl) and subsequently grafted with aminopropyl groups. Similarly, the materials NH

2

-C

16

-MS (1.53 nm),

NH

2

-C

18

-MS (1.89 nm), and NH

2

-C

18

PS-MS (2.66 nm, aminopropyl grafted onto pore-expanded mesoporous silica) with different pore sizes were produced. It is worthy of note that the pore size of NH

2

-C

10

-MS is around 0 nm since most of the mesopores are filled with amino-moieties. By judicious choice of synthetic methods, a series of aminopropyl-functionalized mesoporous silica spheres with the pore size varying from B 0 to 2.66


Review Article


Chem Soc Rev were successfully constructed. Because these materials have identical morphologies, particle diameters, and mesopore alignments, the differences in the catalytic performance should be attributed to the differences in the pore size.

To assess the effect of the pore size on catalytic activity, nitroaldol condensation of benzaldehyde derivatives with nitromethane was employed, and the yield and turnover number

(TON) were determined.

108

The yield increased with the increase in pore size, and reached a maximum of 86% for NH

2

-C

16

-MS.

The activity of NH

2

-C

10

-MS was quite low with a yield of 2% and a

TON of 1. Under the catalysis of NH

2

-C

14

-MS (pore size: 1.34 nm) and NH

2

-C

16

-MS (1.53 nm), the TON increased to 29 or 48, respectively. Nevertheless, when NH

2

-C

18

-MS (pore size: 1.89 nm) and NH

2

-C

18

PS-MS (2.66 nm) were used as catalysts, the TON declined to 43 and 36, respectively. The TON for nitroaldol condensation as a function of pore size of catalysts is displayed in Fig. 14. Apparently, there is an optimal pore size at 1.53 nm for the reaction. Moreover, the tendency kept the same when different para -substituted benzaldehyde derivatives were used as reactants. To further study the effect of molecular size of the reactant on catalytic activity, multi-substituted aldehydes such as 2,4,6-trimethoxybenzaldehyde and 3,4,5-trimethoxybenzaldehyde were also used. The results showed that the optimum pore size for the reaction shifted to a larger pore size.

108

Based on these results, it is clear that the pore size of aminopropylfunctionalized mesoporous silica influences the catalytic activity; furthermore, the optimal pore size depends on the molecular size of reactants.

With the assistance of a proper amount of zirconium ions and NaCl, aminopropyl-functionalized mesoporous silica SBA-15

(NH

2

-SBA-15) with different morphologies can be synthesized.

115

Different morphologies, namely platelets, rods, and fibers, correspond to different channel lengths, and thus the effect of the channel length on catalytic performance can be investigated.

Fig. 15 SEM images of NH

2

-SBA-15 with (a–c) platelet, (d–f) rod, and (g and h) fiber morphologies synthesized with an NaCl/TEOS molar ratio of 2,

3.5, and 6, respectively. [Reprinted with permission from ref. 115. Copyright

2012 Royal Society of Chemistry.]

Fig. 14 Effect of pore size of aminopropyl-functionalized mesoporous silica spheres on catalytic activity in nitroaldol condensation. [Adapted with permission from ref. 108. Copyright 2007 Elsevier.]

As presented in Fig. 15, the NH

2

-SBA-15 material synthesized with an NaCl/TEOS molar ratio of 2 gave rise to a hexagonal platelet morphology with a channel length of 200–300 nm. The short and straight mesoporous channels can be observed from the side view of the platelet. For the material synthesized with an NaCl/TEOS molar ratio of 3.5, the rod-like particles piled up one on the top of another were seen. It is noticeable that a number of channels at the centers of interconnected rods were joined, leading to the channel length ranging from

B

600 nm to

10 m m. When the NaCl/TEOS molar ratio is 6, the material is markedly fiber-like aggregates with the channel length around

10 m m. Table 4 summarizes the morphologies and channel length of NH

2

-SBA-15 prepared with different NaCl/TEOS molar ratios.

115 It is worthy of note that the N loading of materials with different channel lengths is almost the same. Other structural parameters including the unit cell ( a

0

), BET surface area, and pore volume are also quite similar. Hence, the different catalytic performance of these materials could be ascribed to the difference in the channel length.

The aminopropyl-functionalized SBA-15 with different channel lengths was applied to catalyze the synthesis of flavanone.


Chem Soc Rev


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Table 4 Physicochemical and catalytic properties of NH

2

-SBA-15 materials with different morphology and channel length [adapted with permission from ref. 115. Copyright 2012 Royal Society of Chemistry]

NaCl/TEOS

(mol mol

1

2

3.5

6

) Morphology

Platelet

Rod

Fiber

Channel length

N loading

(mmol g

1

) a

0

(nm)

200–300 nm 1.8

600 nm to 10 m m 1.6

4 10 m m 1.7

12.4

12.2

12.2

S

BET

(m

2

442

429

446 g

1

) V p

(cm

3

0.74

0.80

0.74

g

1

) BA conv.

100

91

89 a

(%)

Flavanone yield

72

57

48 a

(%) TOF b

8.3

1.4

0.2

(h

1

) a

Reaction conditions: N

2

, 140

1

C, 10 h, 150 mg catalyst, 10 mmol benzaldehyde (BA), 15 mmol 2 yield per amino site in the first hour.

0

-hydroxyacetophenone.

b

Based on the flavanone

Flavanone is a useful intermediate in lots of pharmaceutical syntheses.

145,146

Members of the flavanoid family were documented to possess various pharmacological activity. Normally, flavanones are synthesized through Claisen–Schmidt condensation of benzaldehyde and 2

0

-hydroxyacetophenone followed by intramolecular Michael addition of 2

0

-hydroxychalcone.

Under the catalysis of the platelet NH

2

-SBA-15 material, the conversion of benzaldehyde was 100%, which was higher than that catalyzed by the rod (91%) and fiber materials (89%).

115

Likewise, the yield of target product flavanone over the platelet material (72%) was higher than that over the rod (57%) and fiber materials (48%). More importantly, the benzaldehyde conversion and flavanone yield over the platelet material increased much quicker in the reaction periods, indicating the fast diffusion of reactant/product molecules in the mesopores with a short channel length. What is more, the platelet material presented an initial turnover frequency (TOF) of 8.3 h

1

, which is apparently superior to the rod material (1.4 h as the fiber one (0.2 h

1

) as well

1

). In terms of these results, it is clear that the platelet material exhibits a better catalytic performance in flavanone synthesis in contrast to the rod and fiber analogues.

Taking account of their comparable properties ( e.g.

amine loading, surface area, and pore volume), the excellent performance is attributed to the short channeling pores that can greatly facilitate the molecular diffusion in the liquid phase reaction.

The effect of particle diameter on basicity was investigated based on aminopropyl-functionalized mesoporous silica spheres

(NH

2

-MS), which were synthesized directly through co-condensation of amino-containing precursor APTMS and silica source

TEOS.

111

By changing the synthetic conditions or adding an extra silica source, NH

2

-MS with different particle diameters

(310–780 nm) and the same mesopore size could be obtained

(Fig. 16). Nitroaldol reactions of 4-hydroxybenzaldehyde with nitromethane were applied to evaluate the catalytic activity of these materials. The TON is 167, 152, and 135 when the

NH

2

-MS materials with 490, 680, and 780 nm were used to catalyze the reactions, respectively.

111

This means that the smaller the diameter, the higher the activity. This is due to that most of the surfaces of the particles consist of mesopores, then the smaller the particle, the larger the number of pores that are accessible per unit weight. In the case of the material with the smallest particle diameter of 310 nm, the TON is only 137. This is because the aggregation of particles becomes a noteworthy factor with the smaller diameter. The use of sonification prior to the reaction can prevent aggregation; as a result, the TON over the material with a particle diameter of 310 nm was

Fig. 16 SEM images of aminopropyl-functionalized mesoporous silica spheres with the particle diameter of (a) 310 nm, (b) 490 nm, (c) 680 nm, and (d) 780 nm. Scale bars represent 1.1

m m. [Reprinted with permission from ref. 111. Copyright 2008 Elsevier.] recovered to 164, which is close to that of the material with a particle diameter of 490 nm.

The amino group, APTMS, was also immobilized on the

MCM-41-type support with different chemical compositions, namely pure silica MCM-41, aluminosilicate (AlMCM-41; Si/Al =

64), and niobosilicate (NbMCM-41; Si/Nb = 64). The evaluation of these materials was performed by Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate and diethyl malonate.

135

For amino groups immobilized on different supports, the conversion of benzaldehyde decreases in the order of AlMCM-41 4

NbMCM-41 4 MCM-41 in both Knoevenagel reactions. Martıń-

Aranda’s group proposed that the formation of imine salt (the reaction route via activation of benzaldehyde) required the abstraction of an OH group from the intermediate.

135

The acidity of the support facilitates such a process, and hence enhances the total rate of Knoevenagel reactions. These materials were also utilized to catalyze the isomerization of safrole by the same group.

109

An identical activity sequence was observed for safrole isomerization. In other words, the presence of Al or Nb in supports favors the reactions catalyzed by amino-functionalized mesoporous silica.

To summarize, physicochemical properties of supports

( i.e.

pore size, channel length, particle diameter, and chemical composition) have an essential impact on the catalytic performance of amino-functionalized mesoporous silica. It should be stated that, when one factor is examined, other factors should


Review Article


Chem Soc Rev be kept as identical as possible. There is an optimum pore size for the reactions catalyzed by amino-functionalized mesoporous silica, while the optimum pore size is dependent on the molecular size of reactants. The short channeling mesopores and the small particle diameters are beneficial to the diffusion of reactant molecules, which usually result in a high catalytic activity and a fast reaction rate. In addition, the incorporation of some metal atoms (such as Al and Nb) can improve the catalytic performance of amino-functionalized mesoporous silica.

These investigations are helpful for the selection of appropriate supports aiming to construct high-efficient amino-functionalized catalysts.

3.1.2.4. Immobilization of basic species other than amines.

Besides various types of traditional amines, some other basic species, including ionic liquid, proazaphosphatranes, and lithium tert -butoxide, were also immobilized on mesoporous silica, leading to the formation of some interesting heterogeneous basic catalysts.

Owing to their high thermal stability, low vapor pressure, and ease of handling, ionic liquids have been considered green reaction media for replacing conventional organic solvents in various catalytic processes. Besides reaction media, ionic liquids play an increasing significant role in some reactions as basic catalysts. Recently, a concept of ionic liquid heterogenization has been recognized to combine the merits of heterogeneous catalysts with those of basic ionic liquids.

147,148

The ionic liquid,

N -(3-aminopropyl), N (3)-(3-triethoxysilylpropyl)-4,5-dihydroimidazolium bromide hydrobromide, was prepared and immobilized on MCM-41 and SBA-15 by Zhao et al.

(Fig. 17)

127

The protonated amino groups in the obtained materials were neutralized by treating with KHCO

3

, which results in the fabrication of mesoporous silica-immobilized amino-containing ionic liquid

(NH

2

-IL-MS). As a comparison, aminopropyl-functionalized mesoporous silica (NH

2

-MS) were also prepared. In the Knoevenagel reaction of benzaldehyde with malononitrile, NH

2

-IL-MS presented higher activity than pure ionic liquid as well as NH

2

-MS.

The pure ionic liquid exhibited progressive deactivation in the

Fig. 17 Immobilization of ionic liquid on mesoporous silica. [Adapted with permission from ref. 127. Copyright 2010 Elsevier.] reuse process; the yield of benzylidene malononitrile declined from 97% to 74% after six recycles.

127

Similarly, the aminofunctionalized mesoporous silica exhibited a fairly poor performance in the process of recycle. The yield was 74% over

NH

2

-MS reused five times when MCM-41 was the support. In contrast, at least 85% yield was obtained over NH

2

-IL-MS after reusing ten times. The deactivation of heterogeneous catalysts can be ascribed to the leaching of active species. In terms of the data of elemental analysis, the leaching levels of NH

2

-IL-MS

( o 5%) were evidently lower than that of NH

2

-MS (16–18%), even recycle times are more for NH

2

-IL-MS. Therefore, the immobilized basic ionic liquid possessed much better reusability in contrast to the amino-functionalized mesoporous silica. The excellent catalytic performance of NH

2

-IL-MS is because of the depressed reaction between amino groups and the hydrogen at C2 of imidazolium derivative cations in the immobilized ionic liquid. It is known that amino groups in aminopropylcontaining mesoporous silica are able to attack the Si–O–Si bonds, resulting in significant leaching of aminopropyl moieties for NH

2

-MS during the recycle. For an immobilized ionic liquid, the presence of dihydroimidazolium cations in the organic structure enlarges the distance between amino groups and the pore surface and as a consequence, the amino groups can hardly attack the Si–O–Si bonds. This should be responsible for the excellent stability of immobilized ionic liquid in heterogeneous catalysis.

Proazaphosphatranes that are known as Verkade’s superbases have been utilized as efficient basic catalysts in a variety of organic reactions.

149

Three siloxane-containing azido derivatives of the proazaphosphatranes ( 1a–c ) were immobilized on SBA-15, leading to the construction of heterogeneous basic catalysts 1a–c @SBA-15 with different basic and steric properties (Fig. 18).

128

TG results showed that the amount of functional groups was 0.8, 0.55, and 0.47 mmol g 1 of silica for 1a @SBA-15, 1b @SBA-15, and

1c @SBA-15, respectively. This is also in good agreement with phosphorus elemental analysis, pointing out that a lower loading (around 50%) for the more sterically hindered neopentyl and methoxybenzyl bases. The resulting mesoporous solid bases were used to catalyze the Diels–Alder reactions of 3-hydroxy-2pyrone and anthrone with two electron-deficient dienophiles, dimethyl fumarate and N -methylmaleimide. These materials,

1a–c @SBA-15, showed considerable activity in the Diels–Alder reactions, whereas the activity depended on the nature of the starting diene and dienophile. In all reactions, the activity of materials decreases in the order of methoxybenzyl 4 neopentyl

4 methyl azidophosphatrane moiety. This catalytic activity trend was mirrored for their molecular counterparts, 1a–c , which generally exhibited higher activity than their heterogeneous analogues. Reusability was studied for the most active catalyst, the methoxybenzyl derivative 1c @SBA-15. The catalyst showed high reusability until the 3rd run. Nevertheless, deactivation was observed in the 4th and 5th cycles going from full conversion to 70% and 45% conversion, respectively.

128

The deactivation may be caused by leaching of some active species and/or plugging of some pores with reactants and products.


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Fig. 18 The synthetic strategy to siloxane-containing azidophosphatranes and their immobilization on mesoporous silica SBA-15. [Adapted with permission from ref. 128. Copyright 2010 Wiley-VCH.]

Although a large number of basic species have been successfully immobilized on mesoporous silica, most of the reported basic species are N-containing organic moieties. The introduction of basic metal species to mesoporous silica through immobilization are rather scarce. Recently, Li et al.

developed a molecular precursor approach to anchor Li sites on SBA-15 via the reaction of lithium tert -butoxide (LTB) with silanol groups at room temperature (Fig. 19).

129 The obtained material was denoted LTB/SBA-15. The maximum loading is dependent on the number of silanols on the surface of SBA-15 and can reach

6.5 mmol g

1

, which is obviously higher than the amount of amine that could be introduced. A reference material with the same Li content (6.5 mmol g

1

), denoted LiNO

3

/SBA-15, was prepared using the conventional method, namely incorporation

Fig. 19 Generating basic sites on mesoporous silica by immobilization of

LTB and incorporation of LiNO

3

. [Reprinted with permission from ref. 129.

Copyright 2012 Royal Society of Chemistry.] of the precursor LiNO

3 and calcination (Fig. 19). In the case of

LTB/SBA-15, the ordered mesostructure could be well preserved as demonstrated by the results of low-angle XRD, N

2 adsorption–desorption, and TEM. For the reference material LiNO

3

/

SBA-15, however, no diffraction lines could be observed in the low-angle XRD pattern. Moreover, quite low BET surface area

(9 m

2 g

1

) and pore volume (0.04 cm

3 g

1

) were observed for the reference material. These results clearly suggested that the mesostructure of LiNO

3

/SBA-15 was completely damaged in the synthetic process. Further investigations based on IR and

TG indicated that part of Li were still bonded with tert -butoxy groups in the final materials.

150

More specifically, the reaction of LTB with silanols of SBA-15 took place with the loss of three equiv. of tert -butanol per [LiO t

Bu]

4 tetramer, leaving one tert butoxy group in the resultant material as displayed in Fig. 19.

Because each silanol group is isolated on the surface of mesoporous silica, the basic sites, resulted from point-to-point molecular grafting, could be well dispersed and highly active as heterogeneous catalysts.

150,151

The catalytic performance of LTB/SBA-15 was evaluated by the synthesis of dimethyl carbonate (DMC) through the transesterification of ethylene carbonate and methanol. DMC is a useful green chemical and has been proposed as a methylating agent, a polar solvent, and a fuel additive.

152,153

Traditionally,

DMC is synthesized in the presence of homogeneous strong bases. Under the catalysis of LTB/SBA-15, the yield of DMC could reach 41.3% at 65 1 C for 4 h.

129

Nonetheless, only 12.2% of DMC is yielded over the reference material LiNO

3

/SBA-15 under the same reaction conditions. The catalytic performance was also compared with some typical solid bases. When MgO was used to catalyze the reaction, only 7.6% of DMC was produced. The CsX zeolite with strong basicity gave the DMC yield of 6.1%. Since alumina is a well-known support with good alkali-resistance, Li

2

O/Al

2

O

3 was prepared via the method of metal salt incorporation. The obtained material with strong basicity exhibited considerable activity in the synthesis of DMC with 18.8% yield. These results demonstrated the excellent catalytic activity of LTB/SBA-15 in the transesterification reaction, which was higher than that of the material prepared by traditional metal salt incorporation and some typical heterogeneous strong bases.

In conclusion, a wide range of basic species can be introduced to mesoporous silica with various pore symmetry and pore size by using the immobilization strategy, leading to the formation of many interesting mesoporous solid bases. These mesoporous solid bases are active in various organic reactions, and Knoevenagel condensation is a well-known reaction frequently reported in the literature. Table 5 summarizes the Knoevenagel condensation of typical substrates, namely benzaldehyde with malononitrile or ethyl cyanoacetate. It should be stated that the reaction conditions ( e.g.

solvent, catalyst dosage, temperature, and time) have a great effect on the activity of a catalyst, which should be thus fully considered when comparing different catalysts. As shown in Table 5, grafting of the same basic group aminopropyl (AP) onto different supports results in the formation of catalysts with different activity. There is a trend that


Review Article


Chem Soc Rev

Table 5 Knoevenagel condensation of benzaldehyde (A) with malononitrile (B) or ethyl cyanoacetate (C) under the catalysis of different solid bases

Catalyst

Mesoporous poly(benzimidazole)

NH

2

-bridged PMO

CTAB-MCM-41

AP-MCM-41

AP-SBA-15

AP-SBA-1

MAP-MCM-41

Preparation method

Polymerization of 3,3

0

-iaminobenzidine, diphenyl isophthalate, and benzene-1,3,5tricarboxylic acid triphenyl ester

Co-condensation of BTEA with TEOS

As prepared MCM-41 containing template

CTAB

Grafting of aminopropyl (AP) onto MCM-41

Grafting of AP onto SBA-15

Substrates, solvent, and ratio (substrate1 : substrate2 : solvent : catalyst/ mmol : mmol : mL : mg)

A, B, acetonitrile, 1 : 1 : 5 : 50

A, C, acetonitrile, 1 : 1 : 5 : 50

A, C, H

2

O, 1 : 1.2 : 6 : 38

A, B, benzene, 1 : 1.04 : 0.8 : 80

A, C, benzene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1.04 : 0.8 : 80

Reaction temperature, time

RT, 6 h

RT, 20 h

100

1

C, 2 h

100

1

C, 6 h

RT, 20 h

40

1

C, 4 h

RT, 6 h

RT, 6 h

RT, 0.5 h

RT, 1 h

RT, 0.5 h

RT, 1 h

RT, 0.5 h

RT, 1 h

RT, 1 h

Yield (%)

62

100

90

100

95

93

94

97

81

99

86

99

43

96

70

Ref.

62

62

97

99

99

99

99

99

99

PZP-MCM-41

DMAPP-MCM-41

TBD-MCM-41

NH

NH

2

2

-IL-MCM-41

-IL-SBA-15

N-MCM-48

N-KCC-1

MgO/MCM-41

N-MgO/MCM-41

Mesoporous C

3

Mesoporous C

3

Mesoporous C

3

N

4

N

4

N

4

Mesoporous C

3

N

4

MIL-101(Cr)

AP-MIL-101(Cr)

DETA-MIL-101(Cr)

ED-MIL-101(Cr)

BD-MIL-101(Cr)

DD-MIL-101(Cr)

PD-MIL-101(Cr)

NH2-MIL-101(Al)

NH2-MIL-101(Fe)

-CO

-OH

t

Bu

3

Grafting of AP onto SBA-1

Grafting of N -methylaminopropylsilyl (MAP) onto MCM-41

Grafting of 3-piperazinopropylsilyl (PZP) onto MCM-41

Grafting of N , N -dimethylaminopropylsilyl

(DMAP) onto MCM-41

Grafting of triazabicyclo[4,4,0]dec-5-ene

(TBD) onto MCM-41

Immobilization of amine-containing ionic liquid on MCM-41

Immobilization of amine-containing ionic liquid on SBA-15

Nitridation of MCM-48 with NH

3 for 20 h at 900

1

C

Nitridation of fibrous nanosilica at 500

1

C for 12 h

Impregnation of Mg(CH

3

COO)

2 into

MCM-41 followed by calcination at 550

1

C

Nitridation of MgO/MCM-41 with NH

3

800

1

C for 12 h at

Polymerization of cyanamide template by silica

Deprotonation of mesoporous C

3

N

4

K

2

CO

3


3

K

2

OH

N

4


3

N

4 t

BuOK with with with

Hydrothermal reaction of terephthalic acid with Cr(NO

3

)

3

9H

2

O

Grafting of AP onto MIL-101(Cr)

Grafting of diethylenetriamine (DETA) onto

MIL-101(Cr)

Grafting of ethylenediamine (ED) onto

MIL-101(Cr)

Grafting of butane-1,4-diamine (BD) onto

MIL-101(Cr)

Grafting of decane-1,10-diamine (DD) onto

MIL-101(Cr)

Grafting of benzene-1,4-diamine (PD) onto

MIL-101(Cr)

Hydrothermal reaction of 2aminoterephthalic acid with AlCl

3

6H

2

O

Hydrothermal reaction of 2aminoterephthalic acid with FeCl

3

A, C, toluene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1.04 : 0.8 : 80

A, C, toluene, 1 : 1 : 2 : 5

A, B, H

A, B, H

2

A, C, H

2

2

O, 1 : 1 : 2.5 : 5

O, 1 : 1 : 2.5 : 5

O, 1 : 1 : 2.5 : 5

A, C, H

2

O, 1 : 1 : 2.5 : 15

A, B, toluene, 1 : 1 : 4 : 5

A, B, ethanol, 1 : 1.2 : 2 : 6

A, C, ethanol, 1 : 1.2 : 2 : 6

A, B, toluene, 1 : 1 : 2.5 : 3.75

A, B, toluene, 1 : 1 : 2.5 : 3.75





A, C, cyclohexane, 1 : 1 : 25 : 20

A, C, cyclohexane, 1 : 1 : 25 : 20

A, B, toluene, 1 : 2 : 2.6 : 10.5

A, C, cyclohexane, 1 : 1 : 25 : 20

A, C, toluene, 1 : 1 : 25 : 20

A, C, cyclohexane, 1 : 1 : 25 : 20

A, C, toluene, 1 : 1 : 25 : 20

A, C, toluene, 1 : 1 : 25 : 20

A, C, toluene, 1 : 1 : 25 : 20

A, B, toluene, 1 : 1 : 2.5 : 17.5

A, C, toluene, 1 : 0.88 : 0.63 : 5.7

A, C, DMF, 1 : 0.88 : 0.63 : 5.7

A, B, toluene, 1 : 1 : 2.5 : 17.5

80

80

20

70

25

80

70

70

70

70

80

80

60

60

80

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

C, 6 h

C, 6 h

C, 24 h

C, 2 h

C, 1 h

25

1

C, 1 h

25

1

C, 1 h

25

1

C, 1 h

50

1

C, 1 h

60

1

C, 3 h

RT, 3 h

RT, 3 h

80

1

C, 4 h

C, 4 h

C, 2 h

C, 2 h

C, 2 h

C, 2 h

C, 19 h

C, 19 h

RT, 0.5 h

RT, 1 h

RT, 2 h

80

1

C, 19 h

60

1

C, 15 h

80

1

C, 19 h

60

1

C, 15 h

C, 15 h

C, 15 h

C, 0.25 h

80

1

C, 3 h

40

1

C, 4 h

40

1

C, 3 h

80

1

C, 0.25 h

4

13

0

41

54

99

4 99

11

62

94

96

99

89

93

99

24

61

66

77

31

96

48

68

98

97

89

97

98

82

45

61

90

31

78

78

99

99

125

127

127

127

127

157

158

158

159

159

160

160

160

160

161

161

162

161

163

161

163

163

163

164

165

165

164


Chem Soc Rev


Review Article the mesoporous silica with larger pores exhibits higher activity as well as a faster reaction rate ( e.g.

AP-SBA-15 4 AP-MCM-41 4

AP-SBA-1). It is believed that this is one of the molecular recognition phenomena. The effect of primary, secondary, and tertiary amine moieties on the catalytic activity can also be found.

The secondary amino groups (MAP and PZP) present obviously lower activity than the primary group (AP). Under the same conditions, no activity is detected at all when tertiary amino groups such as DMAP groups were used. In addition to their applications in heterogeneous catalysis, amino-functionalized mesoporous silica materials are also widely utilized in adsorptive removal of CO

2 and coordination with metal ions.

154–156

It is therefore possible that some strategies for the design and fabrication of mesoporous solid bases can also be applied to other functional materials, and vice versa . Whilst wide applications are proposed, these organic–inorganic mesoporous bases need to be improved from two aspects. First, these materials are usually prepared in refluxing toluene for a quite long time, which is fairly expensive and complicated. Second, the presence of organic moieties limits the applications of these materials at relatively low temperatures ( o 170

1

C); otherwise, the degradation of organic moieties may take place. On the other hand, immobilization is supposed to endow the resulting materials with high stability against leaching, since covalent bonds between basic species and mesoporous silica are established.

3.1.3.

Nitrogen incorporation.

In comparison with plenty of research on cation-substituted mesoporous silica, relatively less attention has been paid to the substitution of anions in the frameworks. Since the first report of silica treated with ammonia by

Morrow et al.

a few decades ago,

166 anion-substituted frameworks have been widely investigated for an assortment of materials.

Typically, the oxygen-containing frameworks are subjected to nitridation, namely treatment with ammonia at high temperatures. The oxygen in frameworks is then partially displaced by nitrogen, leading to the formation of oxynitride frameworks which show basic catalytic activity. For crystalline materials such as zeolites, the nitridation is rather difficult due to the high structure stability of crystallinity, and thus the nitrogen content of resultant materials is low.

167

In contrast, mesoporous silica possesses amorphous frameworks, which makes the substitution of nitrogen for oxygen easy to occur. So far the nitridation of various mesoporous silica such as MCM-41,

168

MCM-48,

157 and SBA-15

169 has been carried out, producing a series of mesoporous solid bases with oxynitride frameworks.

A high temperature and a long time are generally required for the nitridation process. The nitrogen content of resultant materials relies on the nitridation temperature and duration.

When mesoporous silica was treated with ammonia at 900 1 C for about 20 h, the nitrogen content of oxynitrides was ca.

10 wt% despite the type of mesoporous silica.

157,170 Increasing the nitridation temperature to 1100

1

C, about 20 wt% of nitrogen content could be obtained.

170,171

The effect of nitridation duration was also examined. By prolonging the nitridation duration from

20 to 40 h, the nitrogen content of MCM-41 treated at the same temperature of 950 1 C increased from 16 wt% to 19 wt%.

171

Further calculation revealed that for SBA-15 treated at 1100 1 C,

Scheme 2 Proposed processes for nitridation of mesoporous silica using ammonia.

57% of oxygen atoms could be substituted by nitrogen atoms.

This corresponds to the composition formula of SiN

0.76

O

0.87

instead of SiO

2 before nitrogen incorporation.

170

Although a higher density of silanol groups on the surface is beneficial to the incorporation of a larger amount of nitrogen,

157 the substitution of nitrogen for oxygen should not be limited to the surface silanols. If so, it is impossible that the 57% of oxygen atoms were replaced by nitrogen atoms. Hence, ammonia reacts not only with silanol groups (eqn (1)) but also with siloxane bonds

(eqn (2) and (3)) as illustrated in Scheme 2.

169,170 The formation of

–NH

2 and –NH– groups originating from the reactions between ammonia and siloxane bonds is predominant at elevated temperatures. In the meanwhile, the obtained –NH

2 groups can condense to produce nitrides (N

3 and –NH–

) as shown in eqn (4) and (5) at higher temperatures (Scheme 2).

Because of the high temperatures used for nitridation, much attention has been paid to the structure of mesoporous silica.

A typical example was depicted in Fig. 20.

170

After treating SBA-15 at 1100 1 C, the ordered mesostructure could be generally preserved as the three reflections (100), (110), and (200) were still observable. In comparison with pristine SBA-15, nevertheless, the (100) reflection became slightly weak, while the intensity

Fig. 20 Low-angle XRD patterns of (a) pristine SBA-15 and nitrogenincorporated SBA-15 obtained through treating SBA-15 with ammonia at

(b) 900

1

C, (c) 950

1

C, (d) 1000

1

C, (e) 1050

1

C, and (f) 1100

1

C for 18 h.

[Reprinted with permission from ref. 170. Copyright 2005 Elsevier.]


Review Article


Chem Soc Rev of (110) and (200) reflections decreased evidently. That means, the long-range ordering of the mesoporous structure got worse to some extent after nitridation. It can be seen from Fig. 20 that the (100) reflection shifted to a higher angle after nitridation, which corresponds to the decrease in the d

100 spacing. Similar results were also observed on mesoporous silica other than

SBA-15, upon exposure to high temperatures the contraction of the lattice and reduction in the pore size occurred.

168,171

It is noticeable that when mesoporous silica was subjected to thermal treatment under the same conditions ( e.g.

900

1

C, 20 h) in air instead of ammonia, the obtained materials showed a completely destroyed structure with a quite low surface area and a pore volume.

171

These results suggest that the ammonia atmosphere is of great importance for the maintenance of structural ordering in the process of nitridation.

Knoevenagel condensation was used to probe the basicity of nitrogen-incorporated mesoporous silica. Nitridated MCM-48 were active catalysts for the Knoevenagel reactions between benzaldehyde and malononitrile. Under the catalysis of the optimum material, the conversion achieved 96% with 100% selectivity of 1,1-dicyanophenylethylene after reaction at 60

1

C for 3 h.

157

This activity was higher than that of the typical solid base MgO, which showed the conversion of about 70% under the same conditions. Michael addition reaction is known to need stronger basic sites than Knoevenagel condensation. Over a catalyst with strong basicity, the Michael addition reaction of malononitrile with the double bond of 1,1-dicyanophenylethylene can proceed. However, no Michael addition products were observed over nitridated MCM-48, suggesting that the basicity is not strong enough to catalyze the Michael addition under the reaction conditions employed.

157

In order to enhance the basicity of nitrogen-incorporated mesoporous silica, a methylation method was employed. By reacting with methyl iodide, the hydrogen connected to nitrogen in nitridated SBA-15 was substituted by a methyl group, leading to the formation of N -methylated SBA-15.

172 The methyl group donates an electron to the nitrogen atom in silica frameworks, and thus enhances the basicity of nitridated SBA-15. The catalytic performance of nitrogen-incorporated mesoporous silica was usually evaluated by Knoevenagel reactions in which benzaldehyde is reacted with active methylene compounds such as malononitrile or ethyl cyanoacetate. Interestingly, the methylated material can catalyze the Knoevenagel condensation of benzaldehyde with diethyl malonate (15% conversion at 24 h) that cannot be catalyzed by the material before methylation.

172

These results clearly reveal that the basicity of nitridated mesoporous silica can be improved by methylation.

The support morphology also has an important effect on the basicity of nitridated silica. By replacing traditional mesoporous silica supports with fibrous nanosilica (namely KCC-1), the multifold increase in the catalytic activity of the nitridated materials for

Knoevenagel reactions was reported recently.

158

The improvement of catalytic activity is caused by amine accessibility, which is excellent in fibrous nanosilica because of the open and flexible fibrous structure, which is believed to facilitate penetration and interaction with basic amine sites. The catalytic performance of this material was compared with a collection of catalysts. In the

Knoevenagel condensation of benzaldehyde with diethyl malonate, nitridated SBA-15 gave no activity.

N -Methylated SBA-15 was able to convert 15% of the substrate as described above.

172

Remarkably, nitridated KCC-1 possessed conversion as high as

52% under the same reaction conditions, and was obviously superior to the aforementioned materials.

158

To summarize, nitrogen incorporation provides a curious alternative to generate basicity on mesoporous silica. Because basic sites are formed by the substitution of nitrogen for oxygen, the blockage of pores caused by the introduction of bulky moieties can be avoided. Nevertheless, quite high temperatures are obligatory for the nitridation process, which is energyintensive and harmful to the structure of mesoporous silica.

It should be stated that the basicity of this kind of material is generally weak.

3.1.4.

Introduction of inorganic basic species.

It is known that the introduction of inorganic basic species ( e.g.

K

2

O and

CaO) into porous metal oxide supports ( e.g.

Al

2

O

3 can produce solid bases with a high strength.

173–176 and ZrO

2

Inspired by

) this sophisticated method, inorganic basic species were used to modify mesoporous silica, aiming to obtain mesoporous solid bases with a high strength. To date, both direct synthesis and post-synthetic modification have been attempted, that is, base precursors are introduced, respectively, before and after the formation of siliceous frameworks. The synthetic methods have an important effect on the properties of resulting materials. Also, the basicity and structure of obtained materials are strongly dependent on the basic species employed.

3.1.4.1. Direct synthesis.

The alkaline earth metal oxide, MgO, is the first one introduced to mesoporous silica through direct synthesis. In the synthetic process, the precursor Mg(OAc)

2 was added to the synthetic system of SBA-15 containing a silica source and a template.

177 After the formation of siliceous frameworks, the liquid phase was removed by evaporation instead of filtering, so that the leaching of Mg species could be avoided.

After calcination, solid bases MgO/SBA-15 were obtained. These materials presented typical XRD patterns of 2D hexagonal pore ordering identical to pure SBA-15, suggesting the existence of an ordered mesoporous structure. Interestingly, the diffraction lines of materials derived from direct synthesis were much intenser than that of materials originated from the conventional impregnation method. This is caused by the salt effect of the precursor Mg(OAc)

2

.

177

The addition of inorganic salts led to a mesostructure with a better order in non-ionic surfactant templating systems, due to the specific interaction between surfactants and metal ions.

178

The obtained MgO/SBA-15 materials showed wheat-like morphologies consisting of rope-like domains in SEM images, which was also similar to pure SBA-15. The base strength was detected by the use of Hammett indicators. The MgO/

SBA-15 materials gave base strength ( H ) of 22.5, which was comparable to bulk MgO. For MgO/SBA-15 containing 20 wt% of

MgO, the amount of basic sites was measured to be 6.69 mmol g

1

, which was 67% of the theoretical value (10 mmol g

1

).

177

That means some Mg species were not converted to basic sites,


Chem Soc Rev


Review Article which might exist in the form of MgCl

2

. The presence of chlorine was demonstrated by XPS, which should come from HCl that was used as a catalyst in the synthesis of mesoporous silica.

179–182

Another typical alkaline earth metal oxide, CaO, has also been introduced to mesoporous silica SBA-15 through the method of direct synthesis. By using Ca(NO

3

)

2 as a precursor,

Sun et al.

synthesized a series of CaO/SBA-15 materials with the

Ca/Si ratio ranging from 0.1 to 0.5.

183

Based on low-angle XRD patterns, they also found that the ordered mesopores were well preserved in the process of direct synthesis, and the structure was better as compared with the sample prepared by impregnation. Temperature-programmed desorption of CO

2

(CO

2

-TPD) was employed to characterize the basicity of resultant materials.

For the material prepared by direct synthesis, the predominant peak centered at 220 1 C, corresponding to medium basic sites derived from the adsorption of CO

2 on the surface of CaSiO

3 species. For the control material prepared by impregnation, a desorption peak at about 660 1 C due to strong basic sites was also observed, which was caused by isolated CaO species on the surface. Despite the difference in CO

2

-TPD patterns, both materials exhibited similar catalytic activity in the production of biodiesel through transesterification of sunflower oil with methanol.

183

This indicates that medium basic sites are sufficient for biodiesel production. It is interesting to note that the stability of two catalysts is quite different. No deactivation was found on the CaO/SBA-15 material prepared through direct synthesis in five cycles, while a dramatic decrease of catalytic activity (from B 100% to B 65% biodiesel yield) was found on the impregnated material. These results can be explained by the better distribution of Ca species and enhanced interaction of

Ca species with silica walls for the material prepared by direct synthesis. In addition to SBA-15, CaO can also be introduced into mesoporous silica MCM-41 through the same method of direct synthesis.

184

In summary, the reports regarding the introduction of inorganic basic species through direct synthesis are relatively limited. The basic species are focused on alkaline earth metal oxides MgO and CaO, and no alkali metal oxides have been introduced until now. Because the base precursors are added to the synthetic systems, the reactions of these precursors with catalysts for condensation of silica sources (typically strong acids and bases) may take place. This may lead to the formation of compounds ( e.g.

metal chlorides) that are difficult to convert to basic sites. In addition, the as-synthesized samples comprise organic templates as well as base precursors, and both decomposition of templates and conversion of precursors occur in a single calcination process. The decomposition of templates produces a large number of CO

2

, which may contaminate some strongly basic sites generated in situ . Moreover, such a process has a stronger effect on the basic species with a higher strength. As a result, the base strength of Ca-containing materials derived from direct synthesis is much lower in contrast to pure CaO.

3.1.4.2. Post-synthetic modification.

Post-synthetic modification is widely used for the introduction of inorganic basic species into mesoporous silica. The basic species vary from single- to multi-component metal oxides.

185–187

The utilization of alkali metal salts as basic species was reported as well.

188

Besides, alkaline earth metal oxides-modified mesoporous silica were further subjected to nitridation to tailor the surface properties of solid bases.

159,189 As a result, a series of mesoporous solid bases were fabricated by post-synthetic modification.

For the introduction of single-component alkali metal oxides, their acetates and nitrates are usually employed as the precursors.

187,190

Through impregnation and calcination, alkali metal (Li, Na, K, Rb, and Cs) oxide modified mesoporous silica could be obtained. It should be noted that the presence of alkali metal oxides degraded the structure of mesoporous supports although the loading amount was only 0.5 mmol g

1

. Low-angle

XRD patterns and N

2 adsorption isotherms indicated partial disorder in the arrangement of mesopores in all alkali metal modified materials.

190

This effect increased from Li to Cs impregnated materials. The catalytic activity was evaluated by the isomerization of eugenol.

190

Rb-MCM-41 presented maximum conversion around 72%, which was obviously higher than that on Cs-MCM-41

(53%). Li- and Na-MCM-41 afforded the lowest conversion

(16 and 23%), while K-MCM-41 exhibited moderate conversion of 46%. It is noticeable that the chemical interaction of alkali metals with siliceous frameworks can destruct the hexagonally ordered structure of mesoporous silica. Rb and Cs compounds interact less or in a different way with mesoporous supports. On the other hand, the basicity of alkali metal oxides increases from

Li to Cs. Hence, a volcanic curve for the isomerization of eugenol was observed with a maximum for the Rb-containing sample.

The modification of mesoporous silica with multi-component metal oxides is also an alternative approach to generate basic sites.

186,191

One important mixed oxide that is worthy to be mentioned is Mg–Al hydrotalcite (also called layered double hydroxide). Hydrotalcite, with a positively charged layeredbrucite type of structure, is uncommon in nature but can be easily synthesized in laboratory.

192–194 Positive charges are generated in the hydrotalcite structure, through the replacement of Mg by Al, which are neutralized by the incorporation of exchangeable anions in the hydrated interlayer region.

195–197

It is reported that the actual active sites participating in catalysis are situated at the edges of the platelets. Thus, nano-crystalline hydrotalcite was constructed in the nanosized pore channels of mesoporous silica SBA-15, aiming to create more active edge sites.

198

By immersing in the mixed solution, the precursors

Mg(NO

3

)

2 and Al(NO

3

)

3 were first introduced to SBA-15, which had been pre-modified with Si–CH

3 groups on the outer surface.

The hydrophobic nature of the outer surface and the hydrophilic nature of the inner surface made it possible for the nitrate to be loaded only in the pore channels of the mesoporous support.

After calcination at 600

1

C the nitrates were converted to oxides.

And then, through hydrothermal treatment, Mg–Al hydrotalcite that confined in the pore channels of SBA-15 was formed

(Fig. 21). The hexagonal ordered structure of SBA-15 was well preserved after the formation of hydrotalcite.

The catalytic performance of the mesopore-confined hydrotalcite nanocrystals was studied with aldol condensation reactions.

198

Unlike supported or unsupported hydrotalcite platelets synthesized


Review Article


Chem Soc Rev

Fig. 21 Schematic illustration of synthetic pathways for mesopore-confined hydrotalcite. (a) SBA-15 pre-modified with Si–CH

3 on the outer surface;

(b) nitrates loaded in mesopores; (c) oxide incorporated composites;

(d) formation of hydrotalcite within the pore channels. [Adapted with permission from ref. 198. Copyright 2008 Royal Society of Chemistry.] by other methods, this kind of material showed high catalytic activity without any activation because its original hydroxide forms a hydrotalcite structure. In the self-condensation reaction of acetone, the catalytic activity of hydrotalcite-containing

SBA-15 based on the weight of hydrotalcite was quite high.

It was about eight times that of unsupported hydrotalcite platelets. The thermodynamic equilibrium (23%) was achieved after reaction for 1.5 h. To understand the reason for such a high activity, CO

2 adsorption was adopted to determine the total number of accessible active sites in the catalysts. A remarkably larger number of active sites of hydrotalcite-containing SBA-15 were found in comparison with unsupported hydrotalcite platelets.

As a result, the high reactivity could be attributed to the mesoporeconfined hydrotalcite crystallites with a particularly small size in the lateral dimension ( o 9 nm), which implies a great amount of active edge sites. The reusability of hydrotalcite-containing SBA-15 was examined as well. Before reuse, the solid was separated from the reaction medium by filtration, reactivated by heat treatment and rehydration. The results showed that the immobilized catalyst can be repeatedly used without obvious loss in activity.

Besides basic oxides, alkali metal salts have also been used to modify mesoporous silica. Recently, solid bases derived from

K salts (namely, K

2

CO

3

, K

2

SiO

3 porous silica were reported.

188

, and KOAc) supported on meso-

The K salts were introduced to

SBA-15 by impregnation, which led to the obvious improvement of basicity and catalytic activity. The obtained materials were utilized to catalyze the production of biodiesel from Jatropha oil.

The activity of the K

2

SiO

3 that derived from K

2

CO

3 impregnated catalyst was superior to and KOAc. It is worth noting that the base strength of these K salt modified samples was lower than

15.0.

188

That means, medium or weak bases can also catalyze the production of biodiesel through transesterification.

The modification of mesoporous silica with alkaline earth metal oxides can produce mesoporous basic materials. However, the base strength of these materials may decrease due to the interaction of basic oxides with siliceous supports as reported by Guan’s group.

189

Furthermore, the abundant hydroxyl groups on the surface of mesoporous silica will bring many by-products in base-catalyzed reactions. Further nitridation of

MgO-loaded MCM-41 was thus conducted.

159 In general, the mesoporous structure could be preserved after MgO impregnation and nitridation, while the surface area decreased from

953 m

2 g

1

(parent MCM-41) to 191 m

2 g

1

(nitridized MgO-

MCM-41). IR spectra displayed that the bridging –NH– groups and terminal –NH

2 groups were incorporated into the frameworks of silica by nitridation. This is expected to enhance the base strength owing to the replacement of oxygen by nitrogen with lower electronegativity. The characterization of surface properties revealed that the acidic hydroxyl groups in MgO-MCM-41 were significantly reduced by nitridation.

159

In contrast to

MgO-MCM-41, the nitridized MgO-MCM-41 material showed an improved basic catalytic performance in a collection of reactions including Knoevenagel condensation, Claisen–Schmidt reaction, and dehydrogenation of 2-propanol.

Apparently, post-synthetic modification is a more flexible method for the introduction of inorganic basic species as compared with direct synthesis. Various inorganic basic species have been successfully introduced to mesoporous silica through post-synthetic modification. This leads to the formation of mesoporous solid bases with different basicity. It is worthy of note that the basicity of these materials is not well consistent with the basicity of inorganic basic species. For instance, K

2

O is a typical alkali metal oxide with strong basicity, and the base strength of K

2

O/Al

2

O

3 is as high as 27.0.

133

Nevertheless,

K-modified MCM-41 materials exhibit base strength lower than

15.0. That means, strength of basic species is not the unique factor affecting the preparation of mesoporous solid bases, some other factors should also be considered as described in the following section.

3.1.4.3. Effects of basic species.

Different basic species possess quite different properties (such as mobility and electronegativity) apart from base strength. These properties have an important effect on both basicity and structure of materials resulted from basic species-modified mesoporous silica. To examine the effect of basic species, a series of alkali and alkaline earth metal nitrates were employed to modify mesoporous silica SBA-15

(Table 6).

199

An identical loading amount (namely 2.1 cations nm

2 or 2.6 mmol g

1

) was used for all of the base precursors.

The low-angle XRD patterns gave quite different results for different nitrates (Fig. 22). No diffraction peaks were identified from the samples modified by alkali metal nitrates, reflecting that the ordered mesostructure of the siliceous support was destroyed during activation. These data are inconsistent with

Calvino-Casilda’s results mentioned above, 190 which may be caused by the different loading amounts of alkali metal oxides.

Conversely, after modification with alkaline earth metal nitrates, the samples showed three diffraction peaks indexed as (100), (110), and (200) reflections corresponding to p 6 mm hexagonal symmetry, which was identical to parent SBA-15 and indicated the preservation of the ordered mesostructure.


Chem Soc Rev


Review Article

Table 6 Physicochemical properties of different nitrates modified SBA-15 and corresponding oxides [adapted with permission from ref. 199. Copyright 2008 American Chemical Society]

Base precursor

Properties of corresponding oxide

Melting point (

1

C)

Tammann temperature

(

1

C)

Electronegativity of metal ion ( w i

)

NaNO

KNO

3

Ba(NO

3

CsNO

3

Mg(NO

3

Ca(NO

3

Sr(NO

3

3

)

2

)

)

)

2

2

2

1132

4 763

490

2800

2900

2430

1973

430

4 245

109

1264

1314

1079

850

2.79

2.46

2.37

6.55

5.00

4.75

4.45

Base strength of resulting materials ( H )

9.3

15.0

15.0

22.5

27.0

27.0

27.0

Fig. 23 Different structure and basicity of mesoporous silica SBA-15 modified by alkali and alkaline earth metal nitrates. [Adapted with permission from ref. 199. Copyright 2008 American Chemical Society.]

Fig. 22 Low-angle XRD patterns of mesoporous silica SBA-15 modified by alkali and alkaline earth metal oxides using nitrates as precursors.

(a) NaNO

3

, (b) KNO

3

, (c) CsNO

3

, (d) Mg(NO

3

)

2

, (e) Ca(NO

3

)

2

, (f) Sr(NO

3

)

2

, and (g) Ba(NO

3

)

2

. [Adapted with permission from ref. 199. Copyright 2008

American Chemical Society.]

Further investigations revealed that these materials exhibited rather different base strength. The basic sites formed on alkali metal nitrates modified samples possessed low strength varied from 9.3 to 15.0 (Table 6). Interestingly, a high base strength of

22.5 was obtained on Mg-containing samples and superbasicity with a strength of 27.0 was formed on the samples containing

Ca, Sr, and Ba. Based on these results, it is clear that basic species have an essential effect on both basicity and structure of obtained materials, as summarized in Fig. 23.

To explore the factors affecting the mesostructure of the composites, the host–guest interaction is firstly taken into consideration, since different guest species may possess quite different interaction with the host. The interaction of the host– guest must be moderate because too weak host–guest interaction may lead to the difficulty in both dispersion and decomposition of base precursors, whereas too strong interaction can cause the destruction of the host structure. The electronegativity of metal ions ( w i

) in oxide is demonstrated to correlate with the interaction between the oxide and the support. The electronegativity of metal ions in alkali and alkaline earth metal oxides was thus calculated and listed in Table 6.

199

With the decrease in electronegativity, the metal–oxygen contact in the oxide becomes weak, leading to a stronger interaction of the oxide with the support.

200 To take one example, the cation in

Cs

2

O has the lowest electronegativity amongst the oxides studied, which leads to the strongest interaction of Cs

2

O with the support silica. In contrast, the highest electronegativity of cations in MgO causes the weakest host–guest interaction.

As displayed in Table 6, the electronegativity of alkali metal ions (2.37–2.79) is much lower as compared with that of alkaline earth metal ions (4.45–6.55). Hence, the interaction between alkali metal oxides and the support silica should be much stronger. Such a strong interaction may lead to the reaction of basic oxides with the siliceous support, producing some silicatelike compounds during activation. Therefore, the mesoporous structure of materials modified by alkali metal nitrates is inevitably damaged. In contrast, the high electronegativity of cations in alkaline earth metal oxides should be responsible for the maintenance of an ordered structure of resultant materials.

Taking into account that the electron transfer can well reflect the host–guest interaction, the semi-empirical electron transfer calculation was carried out using the AM1 method in the

Gaussian 98 program.

201

Typical models of alkali and alkaline earth metal oxides supported on silica are illustrated in Fig. 24. If a guest oxide has a strong interaction with the host, a quantity of electrons should be transferred from metal to neighboring oxygen atom in the host. Otherwise, such an electron transfer should be rare. Table 7 lists the electron transfer from metal to oxygen atoms in the guest (ET1) and to that in the host (ET2).

Apparently, the ET2/ET1 ratio of alkali metal oxide loaded SBA-15 is much higher than that of alkaline earth metal oxide containing samples. This indicates the stronger interaction between alkali metal oxides and the siliceous support, which is in agreement with the foregoing electronegativity analysis.

The mobility of inorganic species is considered to be another factor influencing the mesoporous structure. If the supported oxide is easy to diffuse or transfer in the pores of silica at elevated temperatures, the potential reactions of these basic oxides with the siliceous support could be accelerated during activation, which is harmful to the maintenance of the mesoporous structure. Tammann temperature, defined as half the


Review Article


Chem Soc Rev

Fig. 24 Typical models of (A) alkali and (B) alkaline earth metal oxides supported on mesoporous silica.

Table 7 Charge number and electron transfer in different oxide loaded

SBA-15 samples a

Metal oxide

Mulliken charge number

M2 O1 M1 O2

Electron transfer

M1

-

O1

(ET1)

M1

-

(ET2)

O2

ET2/ET1

Na

2

O 1.000

1.596 1.000

0.856 0.596

K

2

O 1.000

1.359 1.000

0.809 0.359

MgO — 1.988 2.000

0.698 1.988

CaO —

SrO —

1.993 2.000

1.995 2.000

0.690 1.993

0.691 1.995

0.404

0.641

0.012

0.007

0.005

a

Please refer Fig. 24 for the explanation of M1, M2, O1, and O2.

0.678

1.786

0.006

0.004

0.003

However, it is hard to build strongly basic sites on mesoporous silica through direct modification with alkali metal oxides.

Direct modification usually leads to the degradation of mesostructure and the formation of weakly basic sites. Through an in-depth analysis, it is found that there are two factors that hinder the generation of strong basicity on mesoporous silica through modification with alkali metal oxides.

138

The first factor is the difficulty in the decomposition of base precursors to basic species on mesoporous silica. For example, high temperatures of around 700

1

C are required for the decomposition of KNO

3 on silica.

139

The second factor is the poor alkali-resistance of mesoporous silica. The strongly basic species generated after activation can corrode siliceous frameworks, leading to the collapse of the mesoporous structure. Aiming to generate strong basicity on mesoporous silica, both weaknesses should be overcome. Recently, a dualcoating strategy was developed to construct strong basicity on mesoporous silica.

137–139,208–211

Mesoporous silica is precoated with an interlayer prior to the introduction of base precursors.

Such an interlayer plays a double role by promoting the lowtemperature decomposition of base precursors and improving the alkali-resistance of the siliceous support. The type of the interlayer varies from different metal oxides to carbon.

3.1.5.1. Metal oxide interlayers.

The metal oxide Al

2

O

3 is extensively utilized as a support for the preparation of catalysts and adsorbents, and strong basic sites and even superbasic sites have been successfully generated on Al

2

O

3

.

212–214 As a result, Al

2

O

3 should be a good choice for the formation of a layer on the surface. As reported by Sun et al.

, 139 Al

2

O

3 was introduced to mesoporous silica SBA-15 by both direct and post synthesis, followed by the loading of base precursor KNO

3

; the resultant materials were denoted KAS(d) and KAS(p), respectively (Fig. 25). The base precursor KNO

3 was also directly loaded on SBA-15 for comparison, and the obtained material melting point in Kelvin, is related to the mobility of metal ions or atoms in a metal oxide.

202–204

The mobility of metal ions in oxide raises rapidly in the vicinity of its Tammann temperature.

As shown in Table 6, alkali metal oxides have much lower

Tammann temperature compared to alkaline earth metal oxides, indicating the higher mobility of alkali metal cations in oxides than that of alkaline earth metal cations. Typically, the activation of samples is performed at 500–600 1 C to decompose nitrates to corresponding basic oxides. This temperature is higher than the Tammann temperature of alkali metal oxides

( o 430

1

C) but lower than that of alkaline earth metal oxides

( 4 850

1

C), suggesting that the cations in alkali metal oxides are very ‘‘active’’ during the activation while the cations in alkaline earth metal oxides are relatively ‘‘stable’’. Therefore, the high mobility may promote the interaction of alkali metal oxides, which are strongly basic in nature, with siliceous support, leading to the destruction of the mesoporous structure of resultant materials.

To summarize, the loading of alkaline earth metal oxides can form strong basicity on mesoporous silica. However, strongly basic sites are difficult to generate through the introduction of alkali metal oxides, and the collapse of the mesostructure occurs frequently. It can be tentatively concluded that the maintenance of the mesostructure is related to the interaction between metal oxides and the siliceous support together with the mobility of cations in metal oxides, which can be, respectively, characterized by the electronegativity of metal ions and the Tammann temperature of metal oxides. Obviously, the selection of proper basic species is of great significance for the fabrication of strong basicity on mesoporous silica.

3.1.5.

Dualcoating strategy.

Alkali metal oxides are very useful basic species and can catalyze a variety of organic reactions.

205–207

Fig. 25 Comparison of K species loaded on SBA-15 as well as on Al

2

O

3

modified SBA-15 derived from direct and post synthesis. [Reprinted with permission from ref. 139. Copyright 2010 American Chemical Society.]


Chem Soc Rev


Review Article was represented as KS. The decomposition behavior of KNO

3 different supports was investigated by TG.

139 on

As displayed in

Fig. 26, all materials gave a weight loss centered at about 100

1

C originating from the desorption of physically adsorbed water.

The weight loss at this stage for KAS(d) and KAS(p) was apparently larger than that for KS, indicating that the presence of Al

2

O

3 promoted the adsorption of water. The subsequent gradual weight loss up to 760

1

C for the materials without Al

2

O

3 and up to 650

1

C for Al

2

O

3

-containing materials can be assigned to the decomposition of KNO

3

. The DTG curves showed an evident peak at 690

1

C for KNO

3 supported on SBA-15, while at 460 1 C for KNO

3 supported on Al

2

O

3

-containing SBA-15. From these results, it is clear that the presence of Al

2

O

3 of KNO

3 at low temperatures.

promotes the decomposition

The mesoporous structure of KS, KAS(d), and KAS(p) was studied by various techniques.

139

In the absence of Al

2

O

3

, the mesostructure of KS was seriously destroyed. For KAS(d), the existence of an Al

2

O

3 interlayer from post synthesis avoided the contact of strongly basic K species with siliceous frameworks, and the mesostructure was well maintained. However, introducing Al

2

O

3 by direct synthesis failed to protect the mesoporous structure in the case of KAS(p). These results revealed that the effect of Al

2

O

3 on the protection of the mesostructure is closely related to the location of Al. Through direct synthesis, a number of Al species could enter the frameworks of SBA-15; nonetheless, most Al species may locate on the surface of pores and form a smooth layer for the post-synthetic sample

(Fig. 25).

139,215–217

Accordingly, the resultant materials exhibit quite different basicity.

139

A base strength of less than 9.3 was detected on the materials KS and KAS(d), suggesting the weak basicity of materials. It is worthy of note that basic sites with a high strength of 27.0 emerged on KAS(p), which can thus be regarded as a solid superbase.

Apart from Al

2

O

3

, ZrO

2 is also widely used as support with good alkali-resistant ability.

218–220 As a result, mesoporous silica

SBA-15 was precoated with ZrO

2 prior to the introduction of a base precursor. Similar to that happened on Al

2

O

3

-modified

SBA-15, the approach employed for the loading of ZrO

2 has an essential effect. Three approaches, namely impregnation, grinding, and ammonia/water-induced hydrolysis (AIH), were reported for the loading of ZrO

2

(Fig. 27).

138

Both the amount and dispersion degree of ZrO

2 play a crucial role in the generation of strong basicity. An intact layer could be formed on SBA-15 if the content of ZrO

2 was higher than 30 wt% and the AIH method was used. After introducing K, the obtained material exhibited a well-ordered mesostructure and a strong basicity.

The mesostructure cannot be preserved provided that the content of ZrO

2 was lower than 30 wt% or that conventional coating methods (namely impregnation and grinding) were employed. The resultant materials were used to catalyze the synthesis of DMC through the transesterification of ethylene carbonate and methanol.

138

The yield of DMC reached 28.4% at a reaction time of 4 h over the ZrO

2

-containing material.

Such an activity is apparently higher than that over the material without a ZrO

2 interlayer, which can only yield 9.8% of DMC under the same conditions. The method for the introduction of

Fig. 26 (A) TG and (B) DTG curves of (a) KS, (b) KAS(p), and (c) KAS(d) samples before activation. Curves in (B) are offset for clarity. [Reprinted with permission from ref. 139. Copyright 2010 American Chemical Society.]

Fig. 27 Comparison of K species loaded on SBA-15 as well as on ZrO

2

modified SBA-15 prepared through different approaches. [Reprinted with permission from ref. 138. Copyright 2013 Royal Society of Chemistry.]

ZrO

2 interlayers also has an important effect on the activity of resulting materials. For the materials containing ZrO

2 interlayers prepared through impregnation and grinding, the yield of DMC is 22.5% and 15.7%, respectively. Based on the structural characterization and catalytic results, it is conclusive that the catalytic performance of materials is strongly dependent on their mesostructure and basicity.

Recently, the use of different metal oxides as interlayers was systematically studied by Sun et al.

211

Before the loading of

KNO

3

, metal oxides Al

2

O

3

, MgO, ZrO

2

, and CeO

2 were introduced to SBA-15 with an identical molar amount and the same grinding method. The introduction of all metal oxides tailored the properties of mesoporous silica greatly. In contrast to

SBA-15, the decomposition of KNO

3 on metal oxide-containing

SBA-15 occurred at much lower temperatures regardless of the type of oxide. Although the decomposition of KNO

3 was accelerated in the presence of all metal oxides, the structure and basicity strongly depended on the type of metal oxide. The ordered mesostructure was well maintained for the material coated with

Al

2

O

3

, while the mesoporous structure only partially survived for the material containing MgO. In the case of materials containing ZrO

2 and CeO

2

, however, the mesoporous structure was seriously damaged, which was comparable to the material


Review Article


Chem Soc Rev without any metal oxides ( i.e.

KNO

3

-modified SBA-15). Superbasic sites are generated on the materials coated with Al

2

O

3 and MgO, whereas only weak basicity was detected on the materials containing ZrO

2 and CeO

2

. The great difference in the mesostructure and basicity can be ascribed to the dispersion degree of metal oxides. Al

2

O

3 and MgO are able to form smooth layers on the surface of SBA-15, which prevents the host from corroding by resultant strongly basic species. Nonetheless, ZrO

2 and CeO

2 tend to aggregate and produce large particles, and thus the reaction of basic species with siliceous frameworks is still unavoidable.

Apparently, the loading of metal oxides on mesoporous silica can promote the decomposition of a base precursor, which is independent of the type of metal oxide investigated until now.

However, the structure and basicity of resultant materials are strongly dependent on the metal oxides used. An ideal metal oxide should be capable of forming a smooth layer coated on the surface of mesoporous silica. Of course, the dispersion degree of the same metal oxide can also be improved by using optimum loading methods.

3.1.5.2. Carbon interlayers.

Alkali metal nitrates are frequently used precursors for the synthesis of solid bases.

221–223

It is worth noting that nitrates are well-known oxidizing agents.

The conversion of nitrates to oxides should be accelerated through redox reactions. In this regard, a layer of carbon was precoated on mesoporous silica SBA-15 prior to the introduction of base precursor KNO

3

, in which furfuryl alcohol was applied as a carbon source (Fig. 28). The presence of a carbon interlayer endows the host with reducibility.

137

Hence, the formation of basic sites was realized by the redox interaction between KNO

3 and the carbon interlayer. Such a redox process could be completed at a temperature of B 400 1 C (Fig. 29), which was obviously lower than the conventional thermally induced decomposition of KNO

3

Al

2

O

3

-coated SBA-15.

139 and also lower than that on

Furthermore, the residual carbon layer prevents the siliceous frameworks from corroding by newly formed basic species. Thus, solid bases possessing both the ordered mesostructure and high base strength were prepared.

To examine the pathway on carbon-promoted conversion of

KNO

3

, gaseous products formed in the process were monitored using a mass spectrometer (MS). As shown in Fig. 29, four signals with m / z values of 30, 32, 44, and 46 were detected and can be, respectively, ascribed to NO, O

2

, CO

2

, and NO

2

.

Fig. 29 (A) TG and MS analysis of KNO

3 supported on (A) SBA-15 and

(B) carbon-coated SBA-15. [Reprinted with permission from ref. 137.

Copyright 2013 Royal Society of Chemistry.]

The decomposition of KNO

3 on SBA-15 mainly produced NO and O

2 together with a small amount of NO

2

. In the presence of a carbon layer, nevertheless, the conversion of KNO

3 proceeded via a rather different way and the main products were NO and

CO

2

. Consequently, the conversion of KNO

3 to basic species

K

2

O on SBA-15 and carbon-containing supports can be tentatively described by eqn (1) and (2) in Scheme 3.

137 In addition to KNO

3

, other alkali metal nitrates such as LiNO

3 can also be converted to corresponding basic oxides through the redox reactions with carbon layers, leading to the fabrication of various mesoporous solid strong bases.

208

It is known that carbon interlayers can be prepared through a range of organic precursors. In previous studies, furfuryl alcohol was used as a carbon source.

137,208

Actually, organic templates involved in the synthesis of mesoporous silica are also alternative carbon sources.

224–226

Traditionally, as-prepared mesoporous silica is calcined in an oxygen-containing atmosphere to remove the template prior to the introduction of base precursors (Fig. 30).

Instead of calcination, the treatment of as-prepared SBA-15 in a nitrogen atmosphere was adopted by Sun et al.

209

For the samples carbonized at different temperatures of 500, 700, and 900 1 C, the carbon content decreased from 4.0 to 3.3 and 1.8 wt%. In theory,

Fig. 28 Promoting low-temperature conversion of KNO

3 and strong basicity generation on mesoporous silica by coating a carbon interlayer. [Reprinted with permission from ref. 137. Copyright 2013 Royal Society of Chemistry.]

Scheme 3 Conversion of KNO

3 to basic species K

2

O on SBA-15 and carbon-coated SBA-15. [Reprinted with permission from ref. 137. Copyright 2013 Royal Society of Chemistry.]


Chem Soc Rev


Review Article

Fig. 30 Low-temperature conversion of LiNO

3 and creation of strongly basic sites on mesoporous silica promoted by template-derived carbon. [Reprinted with permission from ref. 209. Copyright 2014 Royal Society of Chemistry.]

2.6 wt% of carbon is required for the conversion of all supported

LiNO

3

(20 wt%) according to eqn (2) (Scheme 3). TG analysis indicated that the carbon-containing support itself gave a weight loss of 1.1 wt% during heating. Thus, a minimum carbon content of 3.7 wt% is compulsory, which consists of 2.6 wt% of carbon that reacted with LiNO

3 and 1.1 wt% of carbon that was consumed in the process. It is therefore reasonable that

LiNO

3 can be completely converted on the support treated at

500

1

C with a carbon content of 4.0 wt%. This can also explain that only part of LiNO

3 is converted on the supports treated at

700 and 900 1 C, since their carbon contents are lower than

3.7 wt%. The carbon endows the support with reducibility, which makes the conversion of LiNO

3 take place at a temperature of about 400 1 C. This is analogous to the carbon derived from furfuryl alcohol.

208

Accordingly, strong basicity is constructed on mesoporous silica SBA-15 at low temperatures, and the ordered mesostructure is well maintained.

In summary, carbon is a more efficient interlayer than metal oxides from the viewpoint of conversion of base precursors to basic species. This is caused by the reducing ability of carbon, which is absent for metal oxides. The effect of carbon interlayers on protection of siliceous frameworks is equal to that of metal oxide interlayers. Moreover, carbon is inexpensive and easily available; a variety of organics can be used as carbon sources. The catalytic performance of materials containing carbon interlayers is worth noting. Table 8 summarizes the transesterification reactions of ethylene carbonate with methanol over different catalysts, which generally require catalysts with strong basicity. Under the catalysis of K

2

O/SBA-15 without interlayers, only 3.2% of DMC was yielded after reaction for

4 h. Under the same reaction conditions, however, the yield increased to 28.4% over the catalyst K

2

O/ZrO

2

/SBA-15 containing a ZrO

2 interlayer. Surprisingly, the yield of DMC reached as high as 38.6% over K

2

O/C/SBA-15 containing a carbon interlayer. This indicates that carbon is more efficient as an interlayer for the generation of strong basicity as compared with metal oxide ZrO

2

.

It is worth noting that the method for the introduction of carbon interlayers also plays an essential role. The sample Li

2

O/C/

SBA-15 containing carbon from a template (43.4%) was more active than that from furfuryl alcohol (32.4%). Therefore, it is a promising choice for generation of strong basicity on mesoporous silica through coating carbon, and the use of proper carbon sources should be considered.

3.1.6.

Ion exchange.

One of the most attractive properties of zeolites is their ability to exchange cations reversibly. The composition of a zeolite can therefore be tailored by simply contacting the solid with a solution of the corresponding metal salt.

229–232

A large number of alkali cations ( e.g.

K

+ and Cs

+

) have been introduced to the cavities of zeolites without alteration of

Table 8 Transesterification of ethylene carbonate with methanol over different catalysts a

Catalyst Preparation conditions DMC yield (%)

Li

2

O/SBA-15

LTB/SBA-15

Li

2

Na

Na

K

K

K

K

2

2

2

2

O/C/SBA-15

2

2

CsX

O/mesoporous ZrO

O/mesoporous CeO

O/SBA-15

O/ZrO

MgO

2

/SBA-15

O/C/SBA-15

O/mesoporous CeO

CaO/SBA-15

2

2

2

Impregnation of LiNO

Immobilization of tert


400

1

C; furfuryl alcohol was used as carbon source


3

400

1

C; template was used as carbon source

In situ functionalization of mesoporous ZrO

2

In situ functionalization of mesoporous CeO

2

Impregnation of KNO

Impregnation of KNO

3 at 550

1

C

Ion exchange of NaX with CsNO

Calcination of Mg(NO

3

3

3

3

) into SBA-15 followed by calcination at 600

-butoxide (LTB) on SBA-15 into carbon-coated SBA-15 followed by calcination at into carbon-coated SBA-15 followed by calcination at in the hard template process in the hard template process into SBA-15 followed by calcination at 600 into ZrO

2

Impregnation of Ca(NO

3

6H

)

2

2

2

3

1

1

C

C

-modified SBA-15 followed by calcination

Impregnation of KNO

3 into carbon-coated SBA-15 followed by calcination at

400

1

C; furfuryl alcohol was used as carbon source

In situ functionalization of mesoporous CeO

2 for 3 times

O at 500

1

C in the hard template process into SBA-15 followed by calcination at 550

1

C

6.4 (1 h)

12.2 (4 h)

34.2 (1 h)

41.3 (4 h)

24.8 (1 h)

32.4 (4 h)

28.3 (1 h)

43.4 (4 h)

66.5 (1 h)

62.6 (1 h)

64.6 (4 h)

0.7 (1 h)

3.2 (4 h)

19.4 (1 h)

28.4 (4 h)

31.1 (1 h)

38.6 (4 h)

35.9 (1 h)

43.5 (4 h)

6.1 (4 h)

7.6 (4 h)

5.4 (1 h)

13.4 (4 h) a

Reaction conditions: methanol, 0.5 mol; ethylene carbonate, 0.1 mol; catalyst, 0.5 wt% of methanol, temperature, 65

1

C.

Ref.

129

129

208

209

227

228

137

138

137

228

129

129

129


Review Article


Chem Soc Rev the crystallinity, which endows the zeolites with basic properties. Inspired by zeolites, the method of ion exchange was also attempted for the generation of strong basicity on mesoporous silica. In principle, mesoporous silica may fulfill these conditions, provided that they contain a sufficiently high amount of Al. Aluminum tetrahedra create charge defects in the silicate frameworks which are compensated by cations; thus, alkali cations can be ion exchanged onto mesoporous silica. For this purpose, mesoporous silica MCM-41 with a Si/Al molar ratio of

1.25 was synthesized in the basic medium containing sodium ions.

233

Both as-prepared MCM-41 and the calcined sample were used for ion exchange, and quite different results were obtained. For calcined MCM-41, the standard procedures to exchange Na

+ for other cations led to gradual collapse of the mesoporous structure. The degradation of the mesoporous structure was accompanied by the formation of octahedrally coordinated Al instead of original tetrahedral Al species. In the case of as-prepared MCM-41 in the presence of surfactant molecules, however, various metal ions (K + , Cs + , and Ca 2+ ) could be obtained by completely exchanging Na + . These molecules prevent dealumination of frameworks during exchange, and thus preserving the long-range mesoscopic ordering of the parent MCM-41. As a result, mesoporous materials containing alkali cations were synthesized.

It should be noted that mesoporous materials built with amorphous frameworks have a drawback of low hydrothermal stability. The replacement of amorphous frameworks by crystalline zeolitic ones is thus explored. A series of mesoporous zeolites have been successfully synthesized.

234–240

Alkali and alkaline earth cations can be introduced to these mesoporous zeolites by ion exchange, basic sites are thus generated. A case in point is Ryoo’s report, they prepared mesoporous sodalite with high Al content and crystalline zeolitic walls using an organosilane surfactant.

241 Basic sites were generated through the following ion exchange with K + . The basicity of the resultant material was stronger than that of CsNaX or KAlMCM-41 when compared at a similar Al concentration. The mesoporous sodalite exhibited high catalytic activity in liquid phase Knoevenagel condensation and Claisen–Schmidt condensation reactions. The material also showed high activity and stability toward deactivation in a vapor phase acetonylacetone condensation reaction and thus could be utilized for longer reaction time, while

CsNaX deactivated rapidly because of coke formation.

Although ion exchange is a well-known method for the generation of basic sites on zeolites, the application of this method on mesoporous silica is relatively scarce. It is known that tetrahedral Al species can generate charge defects in frameworks, which endows mesoporous materials with capacity of ion exchange. However, the preparation of mesoporous silica with a high content of tetrahedral

Al is difficult, which should be one of the reasons for the scarce reports. Another reason is the poor hydrothermal stability of mesoporous silica with amorphous frameworks, and the ordered mesostructure can be degraded in the process of ion exchange.

In this regard, mesoporous zeolites with crystalline frameworks may be promising candidates for the fabrication of mesoporous solid bases via ion exchange.

3.2.

Mesoporous alumina

The templating method has been shown to organize silica into a variety of mesoporous forms. This method to mesostructured materials has also been extended, with continuing success, to non-silica oxides. Alumina is one of the most popular components for mesoporous walls other than silica. So far mesoporous alumina has been synthesized using both soft templates

(including cationic

242 and anionic surfactants,

243 as well as nonionic block copolymers

244–249

) and hard templates.

250

Templatefree synthesis of mesoporous alumina was reported as well.

53

The emergence of mesoporous alumina opens up new opportunities for the preparation of new solid bases, since alumina is a good support for strongly basic species.

3.2.1.

Basicity of pure mesoporous alumina.

It is generally accepted that Al

2

O

3

Al

2

O

3 is a metal oxide with acidity, the utility of as a solid base catalyst and its surface basic properties have been scarcely elucidated. CO

2

-TPD was performed to evaluate the strength and the number of base sites on pure mesoporous Al

2

O

3 by Seki and Onaka, and some interesting results were reported.

251

Non-mesoporous g -Al

2

O

3 and MgO were also employed for comparison. The amount of CO

2 from MgO was far greater than those from g -Al

2

O

3 desorbed and mesoporous Al

2

O

3

. However, a further study in the high temperature region gave some different results. As displayed in Fig. 31, although both g -Al

2

O

3 and mesoporous Al

2

O

3 showed the peak in the region 800–1000 K, only mesoporous Al

2

O

3 gave the peak centered at a higher temperature of about 1100 K. These results unambiguously suggest that mesoporous Al

2

O

3 has the basic sites with supreme strength that are stronger than the basic sites on g -Al

2

O

3

.

The catalytic performance of mesoporous Al

2

O

3 on the

Knoevenagel reaction of benzaldehyde with ethyl cyanoacetate was studied. Normally, the Knoevenagel reactions are performed under solvent-free conditions or in traditional organic

Fig. 31 TPD plots of CO

2 desorbed from g -Al

2

O

3

(gray line) and mesoporous

Al

2

O

3

(black line) in the temperature range 800–1273 K. [Adapted with permission from ref. 251. Copyright 2007 Elsevier.]


Chem Soc Rev


Review Article solvents such as toluene and methanol, while acidic supercritical CO

2 medium was used.

251

Although MgO could yield the Knoevenagel product under solvent-free conditions, the yield did not increase after supercritical CO

2 was introduced.

This is caused by the rapid formation of the inactive species

(namely MgCO

3

) on the surface, and implies that the classic solid base MgO is unsuitable for base-catalyzed reactions in supercritical CO

2

. In contrast, g -Al

2

O

3 and mesoporous Al

2

O

3 can catalyze the reaction under the supercritical CO

2

Because the p K a condition.

value of ethyl cyanoacetate is 9, the basic sites stronger than H = 9 functioned in supercritical CO

2 to promote the reaction. It is noteworthy that the activity of mesoporous

Al

2

O

3 was much higher than that of g -Al

2

O

3 at the initial stage of the reaction, which demonstrated that mesoporous Al

2

O

3 was the most appropriate catalyst for the Knoevenagel reaction in supercritical CO

2

.

3.2.2.

Sulfation.

The basicity on the surface of mesoporous alumina can be tuned by sulfation. Mesoporous Al

2

O

3 ing SO

4

2 in the frameworks (designated as meso -Al

2

O contain-

3

/SO

4

2 ) was synthesized by the hydrolysis of Al(Osec -Bu)

3 in the presence of different sulfate sources, namely H

2

SO

4 and (NH the SO

4

2

4

)

2

SO

4

.

252 ions exist in the Al

2

O

3

, Al

2

(SO

4

)

3

IR spectra were used to characterize whether frameworks or on the surface.

,

Since no distinctive absorption bands originated from the covalent S

Q

O bonds of SO

4

2 species on the surface were detected, it can be deduced that SO

4

2 ions were not on the surface but incorporated into the Al

2

O

3 frameworks, resulting in weakening characteristic S

Q

O stretching vibrations. It should be stated that surface SO

4

2 species with S

Q

O bonds increase the surface acidity of Al

2

O

3 convert Al

2

O

3 remarkably and in some cases into solid superacids.

253,254

In contrast, bulk

SO

4

2 ions with partial double bond character hardly affect the surface acidity.

255

This is similar to the case in sulfated ZrO

2 that surface SO

4

2 species on ZrO

2 remarkably increase the surface acidity of ZrO

2 and make the oxide into a solid superacid, while

SO

4

2 ions inside the ZrO

2 acidity of ZrO

2

.

256 frameworks scarcely raise the surface

IR spectra of pyrrole adsorbed on samples revealed that the SO

4

2 species in the frameworks slightly suppressed the average strength of basic sites (O

2

) on meso -

Al

2

O

3

/SO

4

2

, but gave rise of a small amount of strongly basic sites that promoted the Tishchenko reaction in supercritical

CO

2

.

252

Under the catalysis of an optimum meso -Al

2

O

3

/SO

4

2 catalyst, the yield of phthalide in the Tishchenko reaction of phthalaldehyde can reach 73%. This activity is obviously higher than nonsulfated mesoporous Al

2

O

3

(51–62% yield). Under the same conditions, the yield over CaO was quite low (1%), which was due to the instantaneous formation of inactive CaCO

3 on the CaO surface in the supercritical CO

2 medium.

3.2.3.

Sodium metal deposition.

The enhancement of surface basicity of metal oxides through blowing of alkaline metal vapor against the activated surface is a well-established method.

257

The resulting materials commonly possess basic sites with unusually strong basicity. Several attempts have been made on the preparation of alkali metal-doped g -Al

2

O

3

. Instead of conventional g -Al

2

O

3 composed of macro, meso, and micropores in irregular size, mesoporous Al

2

O

3 with a narrow pore-size distribution in the mesopore region was used as a support for Na deposition.

258

In a typical synthesis, mesoporous Al

2

O

3 was charged into a Pyrex glass tube bearing a side tube in which sodium azide (NaN

3

) was placed. After activation, mesoporous Al

2

O

3 was mixed with

NaN

3 introduced from the side tube. The resulting solid mixture was then heated under vacuum to disperse metallic

Na generated by the thermal decomposition of NaN

3 onto the surface of activated mesoporous Al

2

O

3

. Sodium-doped mesoporous Al

2

O

3

Al

2

O

3

.

258 was obtained and represented as Na/

The mesoscopic ordering of mesoporous Al

2

O

3 meso was

well preserved after the addition of Na. Also, the loading of Na did not result in the occurrence of new peaks as well as the disappearance of original peaks in wide-angle XRD patterns, indicating that Na presented predominantly on the surface, not altering the original frameworks of mesoporous Al

2

O

3

.

Na/ meso -Al

2

O

3 can catalyze the isomerization of a -pinene to b -pinene at room temperature, implying the presence of superbasic sites.

258

The selectivity of b -pinene was 93–96%, while parent mesoporous Al

2

O

3 and Na-doped conventional g -Al

2

O

3 showed lower selectivity of 58% and 89%, respectively. The mechanism for the generation of basicity was also studied. It is reported that the metallic Na generated by the pyrolysis of NaN

3 reacts with a metal oxide surface to give an electron to a single oxygen vacancy on the subsurface.

259

The electrons that incorporated into the frameworks activate the neighboring surface

O

2 ions, converting the ions to the active superbasic sites.

3.2.4.

Basic oxide modification.

Modification with basic oxides was carried out to generate basicity on mesoporous Al

2

O

3

. Both alkali metal oxides

260 and alkaline earth metal oxides

261–263 have been introduced to mesoporous Al

2

O

3

. In contrast to post-synthetic modification, direct synthesis is preferred for the introduction of basic oxides. This is because the stability of mesoporous

Al

2

O

3 is not as high as its analogue mesoporous SiO

2

, and structural damage in the process of post-synthetic modification is possible to take place.

To date at least seven different transition alumina phases have been claimed.

264

Among these, g -Al

2

O

3 is perhaps the most important for catalytic and adsorptive applications. Due to the transition phase that has a spinel structure with a specific population of Al

3+ defects in the tetrahedral sites, g -Al

2

O

3 is a well-known support for a variety of solid strong bases. As a result, the introduction of K

2

O to mesoporous g -Al

2

O

3 the method of direct synthesis.

260 was reported via

It is interesting to note that

K

2

CO

3 was used to adjust the pH value of the reaction system such that the base precursor KNO

3 was produced in situ by hydrolysis of Al(NO

3

)

3

. More specially, KNO

3

-coated mesoporous boehmite (AlOOH) with a K/Al ratio ( n ) ranging from 0.05 to 0.27

was obtained initially (denoted as KMA( n )-B). The formation of both KNO

3 and boehmite is essential for subsequent generation of basic materials since they are precursors of K

2

O and g -Al respectively.

264

The conversion of KNO

3

2

O

3 and boehmite took

, place in the subsequent calcination step, in which KNO

3 decomposed to strongly basic species K

2

O on g -Al

2

O

3 was formed in situ . The resultant basic materials were represented as

KMA( n )g . The mesoporosity of materials can be demonstrated by low-angle XRD patterns as well as N

2 adsorption–desorption results.


Review Article


Chem Soc Rev

Table 9 Physicochemical characteristics of MAg and KMAg samples [adapted with permission from ref. 260. Copyright 2008 Wiley-VCH]

Sample S

BET a

(m

2 g

1

) V p a

(cm

3 g

1

) D p a

(nm)

MAg

KMA (0.05)g

KMA (0.11)g

KMA (0.18)g

KMA (0.27)g

334

281

186

154

121

0.393

0.354

0.266

0.241

0.191

3.8

3.8

4.1

4.3

4.5

a

S

BET

: BET specific surface area, V p

: pore volume, D p

: average pore diameter.

Base strength ( H ) o 9.3

22.5

27.0

27.0

27.0

Basicity

(mmol g

0.30

1.03

1.78

2.44

3.25

1

)

1-Hexene isomerization

Con. (%) cis / trans ratio

1.6

2.0

7.6

20.0

34.8

0.8

1.2

2.0

4.4

4.0

The base strength was determined by Hammett indicators. Basic sites with a high base strength of 27.0 were observed for the materials with a K/Al ratio of between 0.11 and 0.27 (Table 9). That means the obtained materials are solid superbases. The amount of basic sites increased with the increase of K content, and that of

KMA(0.27)g reached 3.25 mmol g

1

. Double-bond isomerization of 1-hexene was performed to characterize the basicity.

260

The conversion of 1-hexene is low (1.6%) on MAg and the cis / trans ratio is less than one (Table 9). The introduction of K species improves both the 1-hexene conversion and the cis / trans ratio, and the highest cis / trans ratio of 4.4 is obtained on KMA(0.18)g . These results confirm the presence of unusually strongly basic sites.

In addition to K

2

O, the basic oxide MgO was also incorporated into mesoporous g -Al

2

O

3 via the self-assembly of inexpensive inorganic salts, in which the surfactant templating method was utilized.

262

The obtained materials showed a mesoporous structure with no long-range order in the pore arrangement.

Interestingly, in comparison with pure mesoporous g -Al

2

O

3

, the lattice parameters of Mg-containing materials become larger from wide-angle XRD patterns.

262

It is known that g -Al

2

O

3 has a spinel structure, and cations are able to insert into the vacancies on its surface. Because the ionic radius of Mg is larger than that of Al, the insertion of Mg results in lattice expansion. This gives rise to the decrease in the cationic deficiency of g -Al

2

O

3

, thus stabilizing the structure. These results can also explain the water-resistance of the solid bases as shown in Fig. 32.

As a comparison, MgO-modified mesoporous g -Al

2

O

3 prepared by conventional wet impregnation.

262 was

The results indicated that the strategy of direct synthesis allowed the mesoporous g -Al

2

O

3 to maintain its structure, while wet impregnation led to considerable structural damage. The successful fabrication of MgO-incorporated mesoporous g -Al

2

O

3 is associated with the preparation of proper precursors. The mesoporous structure of precursors was well preserved during their transformation into target materials. Based on these studies, a one-pot, two-step strategy was developed to synthesize different metal ( e.g.

Fe, Cr, and Pb) oxide-functionalized mesoporous g -Al contents (up to 39 wt%) due to their usefulness.

2

O

3

29,262 with various

By utilizing triblock copolymer as a structure directing agent, metal oxide precursor-modified mesoporous boehmite was firstly prepared through hydrolysis and self-assembly of inexpensive inorganic sources (see Step 1 in Fig. 33). In the second calcination step, the guest metal oxide was formed and in situ coated on newly formed mesoporous g -Al

2

O

3

. This strategy permits the synthesis and modification of mesoporous alumina in a one-pot process, and avoids structural damage in post-treatment. Such a strategy may provide a promising method for the fabrication of mesoporous g -Al

2

O

3 functionalized with various active species.

In summary, mesoporous Al

2

O

3 should be a good candidate for the fabrication of solid bases, since Al

2

O

3 is a well-known support for solid strong bases and even superbases. Although the presence of basic sites on pure mesoporous Al

2

O

3 has been demonstrated, the basicity is relatively weak. The deposition of metallic Na can create superbasicity on mesoporous Al

2

O

3

, whereas the synthetic process is rather complicated. In contrast, the introduction of alkali metal oxides or alkaline earth

Fig. 32 Total and aqueous-soluble amount of basic sites in the MgOincorporated mesoporous g -Al

2

O

3 with different Mg/Al atomic ratios.

[Reprinted with permission from ref. 262. Copyright 2009 American

Chemical Society.]

Fig. 33 One-pot, two-step strategy for the synthesis of mesoporous g -Al

2

O

3 functionalized with metal oxides. Step 1: formation of metal oxide precursor (MeO x -pre) and AlOOH through hydrolysis and self-assembly of inorganic sources; Step 2: generation of guest MeO x which was in situ coated on newly formed g -Al

2

O

3 frameworks through calcination. [Reprinted with permission from ref. 263. Copyright 2012 Springer.]


Chem Soc Rev


Review Article metal oxides is a more potential alternative. It should be stated that mesoporous Al

2

O

3 porous Al

2

O

3 is less stable as compared with its counterpart mesoporous SiO

2

, which limits the use of mesoas a support. Further applications of mesoporous

Al

2

O

3 in the preparation of solid bases as well as other functional materials are dependent on the enhancement of stability.

3.3.

Mesoporous zirconia

Owing to its high melting point, low thermal conductivity, and high corrosion resistance, ZrO

2 is highly attractive for use as a catalyst support. By loading basic oxides on ZrO

2

, strong basicity can be generated, which is similar to what happened on Al

2

O

3

.

Inspired by the preparation of mesoporous SiO

2 and Al

2

O

3

, lots of research work have been published on the preparation of mesoporous ZrO

2 by using either soft

265–268 or hard templates.

227

The appearance of mesoporous ZrO

2 offers new chances for the production of mesostructured solid bases. Through the incorporation of alkali metal oxides

269,270 or alkaline earth metal oxides, 271–273 strong basicity and even superbasicity have been successfully formed on mesoporous ZrO

2

, producing a series of new mesoporous solid bases with fascinating properties.

3.3.1.

Incorporation with alkali metal oxides.

By using triblock copolymer P123 as a template and zirconium(

IV

) n -propoxide as a precursor, mesoporous ZrO

2 gel process.

270 was constructed through a sol–

The obtained solid was then refluxed in aqueous

NaOH solution. After calcination to remove the template, Na

2

Oincorporated mesoporous ZrO

2

(denoted Nameso -ZrO

2

) was produced. In addition to the introduction of basic Na species, the post-treatment with aqueous NaOH solution played an important role in improving the thermal stability of mesoporous ZrO

2

.

For the materials without any post-treatment, no diffraction peaks were detected in the low-angle XRD patterns.

270

This indicated that the mesoporous structure was completely collapsed. In the case of materials post-treated with NaOH, diffraction peaks located at about 2 y of 1

1 were observed, demonstrating that mesoscopic frameworks were well preserved. Moreover, the

Nameso -ZrO

2 material showed much higher surface area after high-temperature calcination than the analogue that did not undergo any post-treatment. The BET surface area of Nameso -

ZrO

2 remained 198 m

2 g

1 even after calcination at 700 1 C, while the material without any post-treatment gave the surface area of 120 m

2 g

1 upon calcination at 400 1 C and as low as

29 m

2 g

1 at 600 1 C. Actually, Na should act as the dopant that could inhibit the crystallization and the growth of ZrO

2 walls at high temperatures, thus enhancing the structural stability of the mesoporous ZrO

2

.

CO

2

-TPD profiles of Nameso -ZrO

2 exhibited a desorption peak at 680 1 C besides the one at 150 1 C, which was absent in pure mesoporous ZrO

2 basicity.

270 and indicated the presence of strong

It is assumed that the basicity of the Nameso -ZrO

2 principally originated from the highly dispersed Na

2

O in the

ZrO

2 substrate and partially from the oxygen vacancy on the ZrO

2 surface, despite that the size of Na

2

O nanoparticles was quite tiny and even undetectable by XRD. The catalytic performance of Nameso -ZrO

2 was assessed.

270 on the conversion of propylene carbonate

It was found that the material Nameso -ZrO

2

Fig. 34 An in situ functionalization strategy in a hard template process for the synthesis of Na-modified mesoporous ZrO

2

. [Reprinted with permission from ref. 227. Copyright 2011 American Chemical Society.] exhibited rather high activity, which was remarkably higher than that over the zeolite NaY and CaO/ZrO

2

. The high activity can be ascribed to the strong basicity as well as the high surface area of Nameso -ZrO

2

.

Two typical crystalline phases are often observed for ZrO

2

, and monoclinic rather than the tetragonal phase is preferentially formed in mesoporous frameworks. This is caused by the metastability of the tetragonal phase, but monoclinic ZrO

2 lacks the vacancies and active sites demanded for use as catalyst supports.

133

This drawback hinders the application of mesoporous ZrO

2 in preparing solid bases as well as other functional materials. To construct mesoporous ZrO

2 with tetragonal frameworks, an interesting in situ functionalization strategy in a hard template process was developed (Fig. 34).

227

The key point for this strategy is the flexible utilization of mesoporous SiO

2 and basic solution. In addition to use as a hard template, mesoporous SiO

2 also offers a silicon source for the formation of Si–O–Zr linkages, which is vital to the stabilization of tetragonal ZrO

2 and the generation of superbasicity. The basic solution played a double role by removing the SiO

2 template and functioning as the guest. The Na species was thus coated onto the mesoporous ZrO

2 formed in situ . Different concentration of aqueous NaOH solution was employed to remove the siliceous template. It is noticeable that the obtained materials exhibited a quite similar Si/Zr ratio of about 0.31.

227

It was also attempted to prolong the reaction time and use NaOH solution with different concentrations, whereas the Si/Zr ratio remained constant. This implies that the form of silicon existing in resultant materials is different from that in the SBA-15 template, which has good alkali-resistant capacity and can survive in strongly basic solution.

The results of low-angle XRD, along with the TEM images, reflect that the obtained Na-modified mesoporous ZrO

2

2D hexagonal pore regularity.

227 has a

This suggests that the structure of parent SBA-15 is well replicated. The frameworks of mesoporous ZrO

2 show a tetragonal crystalline phase. The stabilization of the tetragonal phase at 600

1

C is exciting, considering the usefulness of this crystalline phase but its instability at elevated temperatures.

268

The crystalline phase transformation of unsupported ZrOCl

2 was also studied. As displayed in Fig. 35, intense diffraction peaks of tetragonal ZrO

2 the sample calcined at 400 1 C.

227 were observed for

Nonetheless, the monoclinic


Review Article


Chem Soc Rev

Fig. 35 Wide-angle XRD patterns of (A) ZrOCl

2

8H

2

O and (B) ZrOCl

2

8H

2

O/SBA-15 calcined at (a) 400, (b) 500, (c) 600, and (d) 700

1

C.

[Reprinted with permission from ref. 227. Copyright 2011 American

Chemical Society.] phase appeared at 500 1 C and increased with increasing temperature. At a temperature of 700

1

C, the monoclinic phase became predominant. This revealed that the formation of the monoclinic phase was inevitable at temperatures higher than

500

1

C for bulk ZrO

2

. However, only the tetragonal phase existed for ZrOCl

2 confined in the mesopores of SBA-15 in spite of different calcination temperatures. The confined space of mesopores in the siliceous template is considered the first factor for the production of tetragonal ZrO

2 frameworks. The second factor should be the formation of Si–O–Zr linkages. The substitution of Zr with impurities, such as Y, Mg, and Ca, is proved to be effective in stabilizing tetragonal ZrO

2

.

274

The impurities induce reduced kinetics of ZrO

2 crystallite growth, which corresponds to an increase of the transformation temperature from the tetragonal to the monoclinic phase. The

Si–O–Zr linkages can be viewed as chemical impurities that can stabilize the tetragonal phase. The stability of the tetragonal phase at high temperatures is of great importance for the fabrication of strong basicity on mesoporous ZrO

2

.

Base strength of only 9.3 was detected for mesoporous

ZrO

2

.

227

Nevertheless, Na-modified mesoporous ZrO

2 showed base strength as high as 27.0, which provided the evidence for the creation of superbasicity. To elucidate whether the superbasicity was resulted from some unusual oxides of Na, the material was also activated in different atmospheres including

H

2

, N

2

, or O

2 before the detection of base strength. Interestingly, all of the materials kept the high base strength of 27.0, which excludes the possibility that the superbasicity originates from some special oxides such as Na m

O ( m 4 2) or Na

2

O n

( n 4 1). Otherwise, the base strength should be changed because the compounds Na m

O ( m 4 2) and Na

2

O n

( n 4 1) would be destroyed during activation in oxidative and reductive atmospheres. According to the results of titration, X-ray fluorescence (XRF), IR, and XRD, Na

2

O formed in high-temperature activation should be the main basic species on Na-modified mesoporous ZrO cult to identify.

2

, even though alkaline metal oxides are diffi-

208,260

The resultant material Na-modified mesoporous ZrO

2 was applied to catalyze the synthesis of DMC from the transesterification of ethylene carbonate and methanol.

227

This material was quite active and the TOF value reached 95.0 h

1

, which was even far higher than that over the traditional homogeneous catalyst CH

3

ONa (45.8 h 1 ).

It is conclusive that incorporation with alkali metal oxides can form strong basicity on mesoporous ZrO

2

. The presence of alkali metal is beneficial to the stability of mesoporous structure and tetragonal frameworks, despite the synthetic methods

(soft and hard templates). It should be stated that the research is mainly focused on Na

2

O, and the reports concerning other alkali metal oxides are relatively limited. Further studies regarding the incorporation with alkali metal oxides other than

Na

2

O are expected, since basic sites with higher strength are possible to create by using K

2

O, Rb

2

O, and Cs

2

O.

3.3.2.

Incorporation with alkaline earth metal oxides.

Alkaline earth metal oxides MgO and CaO have been incorporated into mesoporous ZrO

2

, producing mesoporous solid bases with different properties. Unlike alkali metal oxides, alkaline earth metal oxides can form solid solutions with ZrO

2

. By adding

Ca(NO

3

)

2 to the synthetic system containing the Zr precursor and template, CaO-incorporated mesoporous ZrO

2 obtained.

272 could be

Through adjusting the dosage of precursors, the

Ca/Zr molar ratio of resultant materials can be tuned from 0 to 1.

With the increase of Ca/Zr, the diffraction peak in low-angle

XRD patterns became weaker and broader, and no diffraction peaks were observed for the material with a Ca/Zr ratio of 1.

Apparently, the excessive Ca precursor added to the system showed a strong impact on the hydrolysis/condensation of the

Zr precursor and the self-assembly of the gel, which affected and even damaged the formation of mesoporous frameworks.

In wide-angle XRD patterns, no diffraction peaks ascribed to

CaO were detected when the Ca/Zr molar ratio ranged from

0 to 0.5. The absence of CaO crystalline peaks suggested the formation of homogeneous CaO–ZrO

2 solid solution.

271

When excessive Ca was added to the system, free CaO nanoparticles were formed on the surface of mesoporous ZrO

2

.

CO

2

-TPD was utilized to characterize the basic properties of

CaO-incorporated mesoporous ZrO

2

. There were three main desorption peaks at about 120, 300, and 600

1

C, representing three types of adsorption sites with different basicity coexisting on the surface of solid bases.

272

With the increase of Ca content, the high-temperature desorption peak became intenser, while the low-temperature desorption peaks tended to be weaker. This reflected that the incorporation of CaO created stronger basic sites and part of weakly basic sites pertaining to the ZrO

2 surface were covered. Calculation from the CO

2

-TPD peak area suggested that CaO-incorporated mesoporous ZrO

2 displayed much higher CO

2 with pure CaO.

272 adsorption capacity as compared

This indicates the obtained solid bases possess high basic density, which should be ascribed to their mesoporous frameworks and high surface area. Because that more basic sites were exposed in the pore surface, the catalytic activity of these solid bases was greatly improved.

272

By using a similar sol–gel process to CaO, MgO can be introduced to mesoporous ZrO

2

, producing new mesoporous solid bases.

275

The obtained materials displayed relatively high surface area and thermal stability. Their mesoporous structure


Chem Soc Rev


Review Article was well preserved even after treatment at 700 1 C. To evaluate the basicity of obtained solid bases, synthesis of propylene glycol methyl ether from propylene oxide and methanol was employed.

275 The catalytic performance was also compared with MgO/ZrO

2 resulting from impregnation. The activity of mesoporous MgO–ZrO

2 reached to a maximum value at ca.

10 h and then became stable. Nevertheless, the impregnated solid base MgO/ZrO

2 showed inferior performance and its activity dropped rapidly after 50 h of time-on-stream. The high activity and stability of mesoporous MgO–ZrO

2 can be ascribed to the mesoporous structure and specific basic sites.

In conclusion, the incorporation of alkaline earth metal oxides can generate strong basicity on mesoporous ZrO

2

. It is interesting to note that alkaline earth metal cations can substitute Zr

4+ ions in the lattice, resulting in the formation of homogeneous solid solutions, which is barely reported in other mesoporous supports. As a result, the thermal stability of the mesoporous structure and tetragonal frameworks of obtained materials can be greatly improved. Also, the leaching of active species is obviously retarded, which makes the mesoporous

ZrO

2

-derived solid bases quite stable in catalytic processes.

3.4.

Mesoporous ceria

Ceria is one of the most important rare earth oxides. Among the rare earth family, Ce is the most abundant element. Cerium is even more abundant in the Earth’s crust (66.5 ppm) than that of Cu (60.0 ppm). Ceria-based materials have found applications in a wide range of fields, such as catalysis,

276–280 adsorption,

281 fuel cells,

282–284 and sensing.

285–287

It is known that

CeO

2 has a fluorite crystal structure, which is similar to that of

ZrO

2

(slightly distorted fluorite structure). The existence of vacancies on the surface of CeO

2

, that is beneficial for the generation of strong basicity, has been demonstrated by both theoretical and experimental approaches. Hence, CeO

2 should be an ideal candidate for the preparation of solid strong bases.

Recently, an in situ functionalization strategy based on the hard-templating synthetic system of mesoporous CeO

2 reported.

228 was

Mesoporous silica SBA-15 was used as the hard template, and the precursor Ce(NO

3

)

3 was introduced to the pores of the silica template by incipient wetness impregnation.

After calcination, the intermediate CeO

2

/SBA-15 was formed.

Aqueous NaOH or KOH solutions were then used to dissolve the silica template. After filtrating and drying in a vacuum, Na

2

O and K

2

O-functionalized mesoporous CeO

2 were prepared. In the synthetic process, the basic solutions play a double role by removing the silica template and functioning as the guests.

Hence, Na and K species can be coated onto mesoporous CeO

2 generated in situ . This strategy allows the fabrication and functionalization of mesoporous CeO

2 in a one step, thus avoiding the damage of mesostructure in post-synthetic modification.

The obtained materials displayed two diffraction peaks indexed as (100) and (110) reflections in low-angle XRD patterns, indicating that the pore regularity of the mesoporous silica template was replicated. For the material prepared via post-synthetic modification, however, no diffraction peaks were detectable after loading Na species. This implies that the in situ functionalization strategy is favorable to the conservation of ordered mesostructure.

In wide-angle XRD patterns, the diffraction peaks originated from cubic fluorite structured CeO

2 were detected, and no peaks from Na and K species appeared.

228 That means, Na and K compounds were well dispersed on mesoporous CeO

2

.

Further calculation exhibited that the crystallite size of CeO

2 was around 8.3 nm. It is worthy of note that the silica template has a pore size of 9.2 nm, which may offer confined space for the growth of CeO

2

,

288 resulting in the formation of a uniform crystallite size within mesopores.

The basicity of the mesoporous CeO

2 support was fairly weak, and the base strength was less than 9.3. The functionalization with Na

2

O or K

2

O resulted in a dramatic enhancement of base strength. Base strength as high as 27.0 was detected in both materials, which suggested the presence of superbasicity.

228

The obtained materials were applied to catalyze the synthesis of DMC via the transesterification of ethylene carbonate and methanol. Only a tiny amount of DMC (0.4%) was produced over the mesoporous CeO

2 support. It is worth noting that the yield of DMC reached 64.6% under the catalysis of Na

2

Ofunctionalized mesoporous CeO

2

. As a comparison, a classic solid base MgO was used to catalyze the transesterification reaction. Under the same reaction conditions, only 7.6% of

DMC was yielded. These results proved the excellent basic catalytic performance of obtained materials.

To summarize, CeO

2 is an appropriate support for the synthesis of solid bases, due to its surface properties as described above.

Though CeO

2 has been widely used in various catalytic systems, few reports concern the generation of strong basicity on CeO

2

, and much less studies on mesoporous CeO

2

. It is believed that some interesting solid bases could be produced based on mesoporous CeO

2

, provided that considerable attention was given. Of course, this also depends on the development of facile methods for the synthesis of mesoporous CeO

2

. By comparing the same basic sites dispersed on different supports, it is easy to find the role of support played in the generation of basicity.

As shown in Table 8, the catalytic activity of K

2

O/mesoporous

CeO

2 in transesterification is higher than that of K

2

O dispersed on SBA-15 regardless of interlayers. This demonstrates that metal oxides are more efficient as supports for strong basicity generation in contrast to silica. It is noticeable that Na

2

O dispersed on mesoporous ZrO

2 or CeO

2 exhibits quite high activity in transesterification, which is even higher than their

K

2

O counterparts. These results may provide a clue for the preparation of efficient basic catalysts.

3.5.

Mesoporous carbon materials

Much attention has been paid to mesoporous carbon materials due to their properties of uniform pore architecture, high surface area, electrical conductivity, thermal stability, to name just a few. These properties make mesoporous carbon potential in various applications such as adsorption, catalysis, energy storage, and biomedical engineering.

289–291

So far different methods including carbonization of polymers and sacrificial inorganic oxide templating have been used for the synthesis of mesoporous carbon.

292–294

This leads to the formation of


Review Article


Chem Soc Rev mesoporous carbon with a range of mesopore structure and pore size, which thus provides valuable candidates for the fabrication of mesoporous solid bases. To date three main approaches have been reported for the generation of basic sites on mesoporous carbon, that is, nitrogen doping, amines immobilization, as well as basic oxides modification.

3.5.1.

Nitrogen doping.

Through nitrogen doping, one of the most important carbon-derived materials, carbon nitride, can be produced. The incorporation of nitrogen atoms into the carbon nanostructure is able to enhance the mechanical, conducting, and energy-storage capacity. Because of these properties, mesoporous carbon nitride becomes a well-known and interesting material that has attracted worldwide attention.

295–300

In the meanwhile, the incorporation of nitrogen atoms into the texture of mesoporous carbon gives rise to the basic function, which results in a new family of mesoporous solid bases with interesting catalytic performance. Usually, mesoporous carbon nitride is constructed by replication from silica templates; 160,301,302 meanwhile, direct polymerization of monomers in the absence of templates is also reported.

303

A case in point for the template-free synthesis of mesoporous carbon nitride is Jun’s report.

303

Molecular, hydrogen-bonded supramolecular aggregates were produced by precipitation from two monomers, namely melamine and cyanuric acid, in dimethyl sulfoxide (DMSO). Three peaks at 10.7

1 , 18.5

1 , and

21.4

1 can be clearly identified in the XRD pattern, and are indexed as (100), (110), and (200) reflections, respectively, which indicates the in-plane hexagonal pattern of channels.

The intense peak at 27.9

1 and a d -spacing of 0.320 nm can be ascribed to graphite-like stacking of individual 2D sheets. In

IR spectra, a shift of the C

Q

O stretching vibration of cyanuric acid to a higher frequency and the triazine ring vibration of melamine to a lower frequency were observed, demonstrating the hydrogen bonding of N–H O and N–H N linkages between melamine and cyanuric acid.

304 To obtain desired carbon nitride, the supramolecular aggregates were thermally treated at different temperatures.

303

At temperatures above 325

1

C, cyanuric acid reacted with ammonia and gave rise to ammelide, ammeline, and finally melamine. Since ammonia was yielded during the condensation of melamine, thermal treatment of the precipitate under a protective gas was able to produce carbon nitride. The materials resulting from polycondensation indeed displayed the typical yellow color of graphitic carbon nitride (g-CN). Thermal polycondensation of supramolecular aggregates under nitrogen at 550 1 C can form mesoporous hollow spheres comprised of g-CN nanosheets. These carbon nitride materials are of great interest for catalytic applications involving solid bases.

In contrast to the template-free approach, more attention has been paid to the fabrication of mesoporous carbon nitride via replication from silica templates. In a typical synthesis, the mixture of precursors, for example ethylenediamine (ED) and carbon tetrachloride (CCl

4

), was introduced into the mesopores of a silica template.

301

After polymerization at low temperatures

( e.g.

100 1 C), the polymer/silica intermediate was yielded and subjected to carbonization at high temperatures ( e.g.

600 1 C) under an inert atmosphere.

SBA-16.

306,307

305

Mesoporous carbon nitride was obtained by dissolution of the silica template.

It is known that basic character and basic catalytic performance of carbon nitride originated from nitrogen atoms, which exist in the form of amine or imine groups in the frameworks.

Hence, controlling the nitrogen content in the mesoporous matrix is quite important. Vinu’s group

302 reported the construction of mesoporous carbon nitride nanoparticles with the size smaller than 150 nm and a high nitrogen content (C

4

N

2

(100), (110), and (200) reflections of p 6 mm symmetry.

302

) by using ultrasmall mesoporous silica IBN-4 nanoparticles as a template. The nitrogen content of resultant carbon nitride was twice that of their counterparts synthesized from SBA-15 and

The small size of nanoparticles was demonstrated to prevent the loss of nitrogen from frameworks during the carbonization process. The low-angle XRD pattern showed three well-resolved diffraction lines, which could be indexed as

This indicated that ordered mesostructure of silica template ISN-4 was well replicated. In the wide-angle XRD pattern, a diffraction line at 25.3

1 with a d -spacing of 0.351 was observed, which suggested that the frameworks were composed of carbon and nitrogen that were arranged in a turbostratic form. The mesoporous carbon nitride material was used as a basic catalyst for the transesterification of b -keto esters, which are useful synthons prepared by the reactions of diketene with various alcohols.

302

Mesoporous carbon nitride was active and afforded high alcohol conversion in a short time. Even the less reactive long-chain primary alcohols gave the corresponding keto esters. This is due to the fact that the catalyst has a large number of basic sites originating from nitrogen functional groups in the frameworks.

The fabrication of mesoporous carbon nitride using a silica template and a cyandiamine precursor was also reported.

160

In the process of template removal, surface defects of terminal amino groups and of bridging nitrogens were created, which are believed to function as basic sites (Fig. 36). It should be stated that in perfect carbon nitride sheets, there is no electron localization in the p state, and thus no active basicity. As a result, the surface defects could promote electron relocalization on the surface, which induces

Lewis-base character on mesoporous carbon nitride towards metal-free coordination chemistry and catalysis.

308

To enhance

Fig. 36 Multifunctional properties of carbon nitride. [Reprinted with permission from ref. 160. Copyright 2012 Royal Society of Chemistry.]


4

5

Chem Soc Rev


Review Article the basicity, deprotonation of mesoporous carbon nitride was performed by treatment of as-prepared materials with basic solutions ( i.e.

K

2

CO

3

, KOH, and t BuOK).

160 The resulting materials exhibited evidently enhanced basicity, while the original mesostructure was well preserved. Knoevenagel condensation between benzaldehyde and malononitrile was employed to assess the basic catalytic performance of obtained materials. In the presence of mesoporous carbon nitride, the conversion of 33% with selectivity of 74% was achieved.

160

After the base treatment, the obvious increase in catalytic activity could be observed, in the order t

BuOK 4 KOH 4 K

2

CO

3

. Under the catalysis of mesoporous carbon nitride treated with t

BuOK, the conversion of benzaldehyde reached as high as 80% with the selectivity of 96% towards the target product.

To study the versatility of mesoporous carbon nitride for

Knoevenagel condensations, a series of aldehydes to malononitrile or ethyl cyanoacetate were employed.

160

The reaction data in the presence of t

BuOK-treated mesoporous carbon nitride were listed in Table 10. The reactions of aromatic aldehydes ( e.g.

benzaldehyde, 4-methyl-, 4-methoxy-, and 2-chlorobenzaldehyde) with malononitrile afforded high yield of corresponding

Knoevenagel condensation products (entries 1 and 4–7). The condensation reaction between benzaldehyde and malononitrile could also occur at room temperature, despite that much longer reaction time was demanded to realize the high conversion (entry 3). Aliphatic aldehydes can also undergo

Table 10 Various Knoevenagel condensations catalyzed by mesoporous carbon nitride treated with t

BuOK a

[Adapted with permission from ref. 160.

Copyright 2012 Royal Society of Chemistry.]

Entry Aldehydes Nitriles t (h) Con./sel. (%)

1

2

4

4

99/97

—/— condensation with malononitrile to yield the target products.

A dramatic drop in terms of conversion was observed when ethyl cyanoacetate instead of malononitrile was employed (entry 5), while diethyl malonate did not even react with benzaldehyde

(entry 2). These results suggest that the basicity of mesoporous carbon nitride is restively weak even after deprotonation with t BuOK.

In summary, an assortment of mesoporous carbon nitride can be produced by judicious choice of carbon/nitrogen sources and synthetic methods, thus yielding an interesting group of materials with mesostructure and basicity. The basic properties can be tailored by adjustment of nitrogen contents and deprotonation. Though the basicity of mesoporous carbon nitride is generally weak, the leaching of basic active sites is barely reported due to the incorporation of nitrogen into frameworks.

In comparison with the extensive investigations into photocatalytic and electronic characters, the basic properties are much less explored. These primary results provide evidences of basicity of mesoporous carbon nitride, which demonstrate the multifunctional catalytic nature. It is anticipated that mesoporous carbon nitride could be further used as a multifunctional basic catalyst in a range of organic transformations.

3.5.2.

Amine immobilization.

There has been great interest in chemical functionalization of carbon materials, in particular, single walled (SWCNTs) and multi-walled carbon nanotubes

(MWCNTs), with a focus on enhancing their solubility and improving the compatibility.

309–313

Inspired by this work, chemical functionalization of mesoporous carbon was also tried, which is believed to provide specific functional sites along with high surface area, large pore volume, and uniform pore size distribution.

314–316

Recently, the functionalization of mesoporous carbon CMK-3 by amines via a simple C–N coupling process was reported, and basic sites were thus created on mesoporous carbon.

317

The protocol is based on chemical modification of the carboxylic acid modified mesoporous carbon with different diamines including aliphatic, cyclic, and aromatic through an amide bond formation (Fig. 37). After amine functionalization, the

3 b

18 93/96

4.5

85/100

4 5/100

6 5 71/100

7 5 95/100

8

9

10

5

5

19

96/100

72/91

30/100 a

Reaction performed at 70

1

C using CH

3

CN (10 mL) as a solvent.

1 mmol aldehyde (ketone), 1 mmol nitrile, 50 mg catalyst.

b

Room temperature.

Fig. 37 Functionalization of mesoporous carbon with different diamines.

[Adapted with permission from ref. 317. Copyright 2014 Wiley-VCH.]


Review Article


Chem Soc Rev ordered mesostructure can be well demonstrated. It is worth noting that the unit cell constant of materials functionalized with amines was larger than that of the pristine mesoporous carbon. This should be caused by either the removal of the outer layer of carbon walls after oxidation or the transformation of the surface of carbon from hydrophobic into hydrophilic, which may enhance the adsorption of water molecules.

For the mesoporous carbon CMK-3 functionalized with different amines ( i.e.

1,2-diaminopropane, trans -1,2-diaminocyclohexane, and o -phenylenediamine), the basic properties are quite different.

317

Among the three catalysts, 1,2-diaminopropane functionalized

CMK-3 was the most active in catalyzing the aza-Michael reaction with a high conversion and product yield. However, o -phenylenediamine functionalized CMK-3 was found to be the least active but could still give good yield of the product after longer reaction time. This phenomenon can be explained by the formation of a p -stacked structure of aromatic rings for the material containing o -phenylenediamine; the lone pair of electrons on the nitrogen atom may not be available for participating in the reaction.

Under the catalysis of pristine CMK-3, only a trace amount of product was yielded even after reaction for 24 h. This demonstrates that the basicity induced by the functionalization of

CMK-3 is definitely essential for catalyzing the reaction within the confined pore channels of mesoporous carbon. In order to investigate the effectiveness of 1,2-diaminopropane functionalized mesoporous carbon, a diverse range of reactants were employed (Table 11).

317

The reactions of most aliphatic amines could take place with excellent yield in short time. For aromatic amines like aniline, however, the reaction did not proceed at all. These results can be rationalized in the line of enhanced nucleophilicity of aliphatic amines, which enables them to undergo a very facile reaction with activated alkenes to form corresponding aza-Michael products. To examine the stability of the catalyst, recycling experiments were performed.

317 A slight gradual deactivation was observed in the recycling experiments

(Table 11, entry 1). The deactivation of the catalyst could be ascribed to a small number of leaching of bonded amines into the reaction mixture.

To summarize, immobilization of amines is able to create basic sites on mesoporous carbon. As described above, a variety of organic amines can be introduced to mesoporous silica through the method of immobilization. However, the reports concerning the use of this method to generate basicity on mesoporous carbon are very occasional. In theory, most of the organic amines that were immobilized on silica could be introduced to the carbon surface, leading to the formation of carbon-based mesoporous bases with various properties. Further investigations into this aspect are thus significantly demanded. In comparison with mesoporous carbon nitride, organic amines functionalized mesoporous carbon should be less stable under both thermal and chemical circumstances. However, the method of immobilization offers a wide variety of basic species that can be introduced to mesoporous carbon, which is impossible to realize by nitrogen doping.

3.5.3.

Basic oxide modification.

To form strong basicity on mesoporous carbon, a typical alkaline earth metal oxide, MgO, has been introduced through both direct synthesis

318 and post-synthetic

Table 11 Catalytic performance of 1,2-diaminopropane functionalized mesoporous carbon on the aza-Michael addition of amines with activated alkenes a

[Adapted with permission from ref. 317. Copyright 2014 Wiley-VCH.]

Entry Amine Acceptor t (h) Product Yield b

(%)

1

2

3

4

5

6

7

8

9

10

1.0

1.5

2.0

5.0

5.5

5.0

1.0

1.5

5.0

1.5

90, 82 c

, 80 d

88

84

75

80

75

86

80

88

90 a c

Reaction conditions: amin (1 mmol), a , b -unsaturated compound

(1,2 mmol), catalyst (0.0050 g), neat, room temperature.

Isolated yield after second cycle.

d b

Isolated yields.

Isolated yield after third cycle.

modification.

319

This leads to the fabrication of a new type of mesoporous solid base, namely MgO-modified mesoporous carbon, which exhibits motivating basic and catalytic properties.

Through direct synthesis, mesoporous MgO–carbon composites were prepared by using phenolic resol as a carbon source, inorganic salt Mg(NO

3 template.

318

)

2 as a precursor and Pluronic F127 as a

After pyrolysis at 600

1

C, the mesostructure can be observed from the results of small angle X-ray scattering (SAXS) and

TEM. With the increase of MgO contents in the composites, two resolved diffraction peaks in wide-angle XRD patterns were observed, which could be attributed to (200) and (220) reflections of the periclase phase.

320–322

The CO

2

-TPD technique was employed to characterize the basicity of mesoporous MgO–carbon composites.

318

The profile of pristine mesoporous carbon displayed one desorption peak at around 100 1 C, implying the presence of weakly basic sites. Dissimilarly, mesoporous MgO–carbon composites exhibited two desorption peaks in CO

2

-TPD profiles. In contrast to pristine mesoporous carbon, the peak at a low temperature of around 100

1

C shifted a little to higher temperature and became more intense. Moreover, a new desorption peak became visible at a relatively high temperature of 220–240

1

C. These results clearly show that some basic sites can be generated through the introduction of MgO into mesoporous carbon.

In addition to direct synthesis, the strategy of post-synthetic modification was also used for the fabrication of MgO-modified


Chem Soc Rev


Review Article mesoporous carbon. The process is generally similar to the synthesis of MgO-modified mesoporous oxides. Mesoporous carbon CMK-3 with different pore diameters were synthesized and used as supports to accommodate basic species.

319 Magnesia with different contents was encapsulated inside the mesochannels of

CMK-3. All of the MgO-loaded CMK-3 materials showed an ordered mesostructure even after introducing more than 50 wt% of MgO.

Crystalline cubic MgO without any impure phase were obtained in the pore channels of CMK-3. Among the supports used, mesoporous carbon with the largest pore diameter and high pore volume offered highly crystalline small-size MgO nanoparticles with a high dispersion degree. These mesoporous solid bases were used as heterogeneous catalysts for selective synthesis of sulfinamides.

319

The support with large pore diameter and high content of MgO exhibited the highest activity with an excellent yield of sulfinamides. The catalyst also displayed much higher activity as compared with pristine MgO. The catalyst was found to be highly stable, showing good activity even after the third cycle of reaction.

To summarize, modification with basic oxides is a wellknown method for the generation of basicity on porous supports. Apparently, this method also works well in the case of mesoporous carbon supports. By thoughtful choice of basic oxides, strong and even superbasic sites are supposed to form on mesoporous carbon. The basicity of these materials is obviously stronger than their analogues prepared by nitrogen doping and amine immobilization. Besides MgO, other alkaline earth metal oxides as well as alkali metal oxides can also be introduced into mesoporous carbon, producing mesoporous solid bases that are potential in various applications.

3.6.

Mesoporous metal–organic frameworks

Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs) or porous coordination networks

(PCNs), have emerged as an extensive class of crystalline materials in recent years. They are assembled from metal ions

(or cluster nodes) and organic linkers; their structure can be tuned by the judicious choice of metal-based building blocks and organic linkers with almost endless geometrical and chemical variations.

323–325 Because of their diverse structure, adjustable functionality, and large surface area, MOFs are of great interest for potential applications including gas adsorption, catalysis, sensing, etc.

326–330

As a result of the combination of metal ions and relatively small organic linkers, the majority of MOFs are classified as microporous with a pore size less than 2 nm.

However, there are significant advantages to extending the porosity of MOFs into the mesoporous regime. The pores in the range 2–50 nm can meet the growing demands in applications involving large organic molecules as well as efficient mass transport for energy storage and biomedical catalysis. One method to increase the size of MOFs is the use of prolonged linkers and metal clusters to push pore metrics into the mesoporous regime.

331–335 Alternatively, mesopores in MOFs can be formed by using the surfactant-templating method, which has been widely utilized to synthesize mesoporous silica.

336–339 The emergence of mesoporous MOFs offers new possibilities for the fabrication of new solid bases with mesoporosity.

In general, the introduction of basic groups into MOFs can proceed via either coordinatively unsaturated metal sites

(CUSs) or organic linkers (Fig. 38A).

340

Through proper treatment, solvents and/or water coordinated on metal sites can be removed, leaving metal sites suitable for grafting amino groups.

341

Another alternative approach consists of covalent modification of amino groups attached to linker molecules.

342

It should be stated that amino-functionalized MOFs can also be produced by replacing the unfunctionalized linkers by corresponding functionalized ones.

343–350

In addition, amino groups

Fig. 38 Introduction of basic sites to MOFs by functionalization of (A) CUSs and (B) organic linkers.


Review Article


Chem Soc Rev

Fig. 39 Site-selective functionalization of MIL-101(Cr) with CUSs: (A) perspective view of the mesoporous cage of MIL-101(Cr) with hexagonal windows; (B, C) evolution of CUSs from chromium trimers in mesoporous cages of MIL-101(Cr) after vacuum treatment at 423 K; (D) surface functionalization of the dehydrated MIL-101(Cr) through selective grafting of amine molecules ( i.e.

EA) onto CUSs. Chromium atoms/octahedra yellow, carbon atoms pale gray, oxygen atoms red. [Adapted with permission from ref. 161. Copyright 2008 Wiley-VCH.] attached to organic linkers provide new sites for the grafting of basic groups (Fig. 38B).

351

3.6.1.

Functionalization via CUSs.

One of the most famous

MOFs is MIL-101(Cr) or Cr

3

(F,OH)–(H

2

O)

2

O[(O

2

C)–C

6

H

4

–(CO

2

)]

3 n H

2

O ( n E 25).

352

It has two types of zeotypic mesoporous pores with free diameters of about 2.9 and 3.4 nm accessible through two microporous windows of about 1.2 and 1.6 nm (Fig. 39A).

The large pores, in combination with robust frameworks, large

BET surface area ( 4 3000 m

2 g

1

), and high concentration of unsaturated chromium sites (up to 3.0 mmol g

1

), make

MIL-101(Cr) highly promising for fabrication of mesoporous solid bases. It was found that trimeric chromium( III ) octahedral clusters of MIL-101(Cr) have terminal water molecules, which are removable from the frameworks after vacuum treatment at

423 K for 12 h. The obtained sample offers the CUSs in the structure, which are usable for surface functionalization (Fig. 39B and C). Basic species ED with multifunctional chelating groups can thus be grafted onto the CUSs (Fig. 39D).

161

If one amino group of ED is linked to a CUS of MIL-101(Cr) by direct ligation, the other amino group is able to play the role of an immobilized basic site. It is interesting to note that this concept does not apply to the surface functionalization of mesoporous silica due to the lack of unsaturated surface sites. The solid base ED-MIL-101(Cr) was prepared by coordination of ED to the dehydrated MIL-101(Cr), which was conducted in toluene by heating to reflux.

161

In a similar process, other amines such as diethylenetriamine (DETA) and

APTMS can also be grafted onto MIL-101(Cr). The XRD results demonstrated that no apparent loss of crystallinity was observed after ED grafting. IR spectra showed that the aliphatic C–H stretching vibrations shifted to larger wavenumbers, which indicated that the molecule was coordinated to a Lewis acid center. Moreover, the concentration of Cr(

III

) CUSs determined by CO adsorption definitely declined upon increasing the amount of coordinated ED. These results undoubtedly proved that the ED molecules were coordinated to Cr(

III

) CUSs in mesoporous cages.

The basic catalytic performance of ED-MIL-101(Cr) was evaluated by using the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate and also compared with APTMS-grafted mesoporous silica SBA-15.

161 It is worth noting that the catalytic activity of ED-MIL-101(Cr) was remarkably higher than that of

APTMS-SBA-15, even though the content of free amino groups in

ED-MIL-101(Cr) (1.04 mmol g

1

) is lower than that of APTMS-

SBA-15 (2.89 mmol g

1

). Under the catalysis of ED-MIL-101(Cr), the conversion of Knoevenagel condensation was 97.1%, with a high selectivity of 99.1%. In contrast, the catalyst APTMS-SBA-15 exhibited only 74.8% conversion with 93.5% selectivity. Furthermore, the TOF over ED-MIL-101(Cr) was about 10 times higher than that over APTMS-SBA-15. The lower activity of APTMS-SBA-15 might be attributed to the actual loss of catalytically active sites by the formation of hydrogen bonds between functional groups,

353 which is absent in ED-MIL-101(Cr).

The catalytic performance of DETA-grafted MIL-101(Cr) was also tested by using Knoevenagel condensation reaction of benzaldehyde and malononitrile.

162 At room temperature, the conversion reached 98% in the presence of DETA-MIL-101(Cr)

(1.5 mol%). The conversion could further increase to 100% by enhancing the amount of the catalyst. To assess the reusability of DETA-MIL-101(Cr), a catalyst recycling test was conducted.

The catalyst could be reused three times without loss in activity.

The results of powder XRD and N

2 adsorption indicated no changes in the fresh and used DETA-MIL-101(Cr) catalyst. The elemental analysis also confirmed that there was no significant leaching of basic sites (the N content of the fresh and used catalyst was 9.86 wt% and 9.83 wt%, respectively).

162

A catalyst filtering experiment in toluene showed that the reaction did not take place at all without DETA-MIL-101(Cr), which thus suggested a truly heterogeneous reaction.

A series of diamines with different p K a values were grafted onto the CUSs of MIL-101(Cr), leading to the formation of various solid bases with mesoporosity.

163 The diamine compounds involved in the functionalization include ED (p K a

(BD, p K a

= 8.66), butane-1,4-diamine

= 10.40), decane-1,10-diamine (DD, p K a

= 11.00), and benzene-1,4-diamine (PD, p K a

= 6.08). The powder XRD patterns gave evidence of the high-crystalline frameworks and porous structure after functionalization, while the intensity of diffraction lines decreased. The pore size also declined, as number of hydrocarbons of grafted amines enhanced. These results revealed that different sizes of diamines were successfully grafted onto

CUSs, which blocked the pores partially.

To summarize, the presence of CUSs in MOFs with mesoporosity offers an intrinsic chelating property with electron-rich functional groups, amines with different basicity can thus be grafted selectively onto the unsaturated sites. This leads to the fabrication of a series of mesoporous solid bases with high activity in Knoevenagel condensation reactions. As shown in

Table 5, for the Knoevenagel condensation of benzaldehyde and malononitrile catalyzed by DETA-MIL-101(Cr), the yield can reach 98% at room temperature for 2 h. Due to the lower reactivity of ethyl cyanoacetate, much harsher conditions are required for its reaction with benzaldehyde. However, a high yield of 97% can also be obtained over DETA-MIL-101(Cr) or


Chem Soc Rev


Review Article

ED-MIL-101(Cr) at a higher temperature (80 1 C) for longer reaction time (19 h). For the catalysts grafted with different diamines, the activity displays correlation with the p K a values of diamines. BD-MIL-101(Cr) shows the highest activity and could convert 98% of benzaldehyde. The high activity can be ascribed to the high p K a value of the functional group. Accordingly, the conversion is only 89% and 45% over ED-MIL-101(Cr) and PD-

MIL-101(Cr), respectively, owing to the low p K a values of corresponding diamines. Nevertheless, DD-MIL-101(Cr) with a higher p K a value exhibits lower conversion of 82%. This should be caused by the less number of basic sites for the reaction as well as the longest chain length that hindered the diffusion of reactant molecules. It is noticeable that diamines should be used as the functional species. In the synthetic process, one amino group is linked to a CUS of MOFs by direct ligation, while the other amino group can play the role of the immobilized basic site. This synthetic concept is obviously different from the surface functionalization of mesoporous silica owing to the absence of unsaturated sites. By use of this concept, a variety of mesoporous solid bases are expected to synthesize through thoughtful selection of proper diamines and MOFs with CUSs. Although the stability has been demonstrated in some cases, deactivation of these catalysts derived from leaching of active sites seems to be an issue using this method. In contrast, the introduction of basic species to linkers through covalent boning should possess high stability against leaching, as discussed below.

3.6.2.

Functionalization via organic linkers.

Functionality can be covalently attached to organic linkers in MOFs through either direct synthesis or post-synthetic modification. The strategy of direct synthesis involves the utilization of prefunctionalized organic linkers in the synthesis of MOFs. Direct functionalization exhibits the merit of a simple one-pot synthesis that enables straightforward introduction of the desired functionality. In the case of post-synthetic modification, MOFs are first constructed followed by the attachment of functionality to organic linkers. The insertion of new functional groups allows the modification of properties while retaining the crystallinity of MOFs. To date, both direct synthesis and post-synthetic modification have been used for the introduction of basic groups into MOFs.

Direct synthesis of amino-functionalized MIL-101(Al) was reported by using AlCl

3

6H

2

O, 2-aminoterephthalic acid, and

N , N -dimethylformamide (DMF) as starting materials.

165

The pure phase NH

2

-MIL-101(Al) can only be produced under very specific synthetic conditions, where both the metal source and the solvent play a key role. Similar synthetic composition using

Al(NO

3

)

3 led to the formation of NH

2

-MIL-53(Al) rather than

NH

2

-MIL-101(Al).

354 IR spectra showed the presence of two different types of amine moieties, which are ascribed to the amines in the supertetrahedra and in the windows. The amines present in the supertetrahedra are closer to each other and to pending hydroxyl groups, which might lead to interactions via hydrogen bonding. The results of TG indicated that the decomposition of NH

2

-MIL-101(Al) took place at temperatures above 650 K in air. The obtained material was applied to catalyze the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate.

165

By using DMF as a solvent at a reaction temperature of 313 K, the yield of the target product was about

80% after reaction for 200 min. The yield decreased to around

30% when toluene was used as a solvent. In both cases the selectivity was close to 100%. The catalytic activity of NH

2

-MIL-

101(Al) was compared to the well-known homogeneous basic catalyst TBD. Similar activity was observed for the two catalysts.

The stability of the NH

2

-MIL-101(Al) catalyst was investigated as well. No deactivation was observed in the catalyst after several reuses, indicating the stability of basic sites covalently attached on organic linkers.

By using FeCl

3

6H

2

O, 2-aminoterephthalic acid, and DMF as starting reactants, another amino-functionalized MOF, namely

NH

2

-MIL-101(Fe), can be synthesized.

355

It should be stated that the stability of Fe-containing MOF is relatively low, and all post-synthetic steps were carried out under an inert atmosphere in order to avoid degradation.

164

In contrast to unfunctionalized MIL-101(Fe), the amino-functionalized counterpart showed decreased surface area and pore volume due to the presence of amino groups in pores. The N

2 adsorption isotherm gave noticeable uptake in the microporous region up to a relative pressure of 0.15; moreover, for the second step a significant discrepancy was observed, which derives from the mesoporous cavities. IR spectra demonstrated that amino groups predominantly existed in their free, unassociated form, which is expected for catalysis. The stability of NH

2

-MIL-101(Fe) was evaluated and compared with its Al analogue.

164

After contact with moisture air for 96 h at room temperature, no significant decrease of porosity was observed for NH

2

-MIL-101(Al), while NH

2

-MIL-101(Fe) entirely decomposed within several minutes. The catalytic performance of NH

2

-MIL-101(Fe) on the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was tested.

164

The catalyst NH

2

-MIL-101(Fe) showed activity comparable to

NH

2

-MIL-101(Al). This activity was also apparently higher than that of classical solid bases including MgO and hydrotalcite.

The fabrication of MOFs with mesoporosity can also be achieved by the use of elongated ligands. By utilizing Zn

4

O(CO

2

)

6 as secondary building units (SBUs) and two extended linkers containing amino functional groups, TATAB (TATAB = 4,4 0 ,4 00 s triazine-1,3,5-triyltrip -aminobenzoate) and BTATB (BTATB =

4,4

0

,4

00

-(benzene-1,3,5-triyltris(azanediyl))tribenzoate), two isostructural mesoporous MOFs with cavities up to 2.73 nm were synthesized and denoted PCN-100 and PCN-101 (PCN represents porous coordination network), respectively.

356

As shown in Fig. 40, both ligands possess amino groups that are not involved in coordination bonds for fabrication of the frameworks. Hence, the functional groups are left inside the frameworks and act as catalytically active sites. In the frameworks of

PCN-100, six Zn

4

O(CO

2

)

6 clusters as SBUs and eight TATAB ligands formed a mesoporous cavity with the internal diameter of around 2.73 nm; the size of windows is approximately

1.32 nm 1.82 nm (Fig. 40). These mesoporous cavities are further interconnected through TATAB ligands to generate a

3D non-interpenetrating extended open network. The basic catalytic performance of two mesoporous MOFs was tested by


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Chem Soc Rev

Fig. 40 (A) Elongated ligands TATAB and BTATB for the fabrication of

PCN-100 and PCN-101. (B) Mesoporous cavity of PCN-100 with an internal diameter of 2.73 nm. (C) A window of the cavity with about

1.32 nm 1.83 nm. [Adapted with permission from ref. 356. Copyright

2010 American Chemical Society.]

Knoevenagel reaction of butyl cyanoacetate and three other substrates with different molecular shape and size, namely benzaldehyde (linear; 0.61 nm 0.87 nm), 4-phenylbenzaldehyde

(linear; 0.61 nm 1.33 nm), and benzophenone (angular; 0.66 nm

1.14 nm), respectively.

356

In the case of benzaldehyde, the conversion was 93% over PCN-100. For the slightly elongated substrate, 4-phenylbenzaldehyde, the conversion decreased to

58%. As for the angular molecule benzophenone, it was not converted at all in the presence of PCN-100. A similar tendency was also observed for the reactions catalyzed by PCN-101. These results revealed the shape and size selectivity of the catalyst, and demonstrated that the catalytic reactions occurred within the mesopores of MOFs. Moreover, the catalysts were easily isolated from the reaction suspension by a simple filtration and could be recycled with no detectable loss in activity. Powder

XRD patterns of recovered catalysts showed that both MOFs well preserved their crystallinity.

In addition to direct synthesis, post-synthetic modification is a good alternative for the introduction of basic functionality into organic linkers in MOFs. Especially for MOFs that are synthesized at high temperatures, amino-containing groups cannot be incorporated due to the decomposition of starting materials during synthesis. A case in point is the synthesis of amino-functionalized MIL-101(Cr), the decomposition of

2-aminoterephthalic acid takes place in the synthetic process, since MIL-101(Cr) is prepared under hydrothermal conditions above 200

1

C. Hence, post-synthetic modification was attempted for the introduction of amino groups into MIL-101(Cr) by electrophilic aromatic substitution, that is, nitration of MIL-

101(Cr) to NO

2

-MIL-101(Cr) and subsequent reduction to NH

2

-

MIL-101(Cr).

357

The nitration of MIL-101(Cr) was conducted using nitrating acid for five hours under ice cooling. Subsequently, the reduction of nitro groups was performed using

SnCl

2 and ethanol for 6 h at 70

1

C to produce NH

2

-MIL-101(Cr).

Despite the harsh synthetic conditions involving nitrating acid, the crystallinity of MOF was barely destroyed. IR spectra showed the characteristic stretching vibration of nitro groups for the sample after nitration.

357

After reduction, the IR signals of nitro stretching vibration disappeared, while the characteristic amine vibrations became visible. After digestion of NO

2

-

MIL-101(Cr) using NaOH, only nitroterephthalic acid was observed if the nitration reaction was conducted for five hours.

Also, only aminoterephthalic acid was detected for the sample

NH

2

-MIL-101(Cr). These results thus gave evidence of the successful post-synthetic modification of organic linkers with nitro and subsequent amino groups. The reactivity of the amino groups in NH

2

-MIL-101(Cr) was demonstrated by the reaction with ethyl isocyanate.

357

NMR results showed that the desirable urea derivative was yielded. The conversion of amino groups was nearly complete, since there are no additional signals in NMR spectra. This demonstrated the accessibility of amines in mesopores of MOFs and the potential for the further introduction of functionality through amines in

NH

2

-MIL-101(Cr).

In summary, both direct synthesis and post-synthetic modification have been employed for the introduction of basic sites into MOFs via organic linkers. A series of new mesoporous solid bases, namely amino-functionalized mesoporous MOFs, are thus produced. These mesoporous bases are active in

Knoevenagel condensation reactions, while the catalytic performance is strongly dependent on catalysts. For the NH

2

-MIL-101 catalysts derived from direct synthesis, NH

2

-MIL-101(Fe) is more active than NH

2

-MIL-101(Al) in Knoevenagel condensation of benzaldehyde and malononitrile, indicating the importance of metal sites (Table 5). The catalyst NH

2

-MIL-101(Al) can convert 90% of benzaldehyde (at 80

1

C for 3 h), which is comparable to that catalyzed by MgO/MCM-41 (93% at 80

1

C for 4 h). Interestingly, the catalyst prepared from post-synthetic grafting, DETA-MIL-101(Cr), shows quite high activity, and the yield reaches 98% at room temperature for 2 h. That means, the methods for the synthesis of catalysts are of great importance.

Through rational design and synthesis, some MOFs-based catalysts can perform better than classic solid bases ( e.g.

MgO) and even better than homogeneous catalysts. However, it should be stated that the reactions tested are predominantly limited to Knoevenagel condensations, and other basecatalyzed reactions are seldom reported. It is known that mesoporous MOFs can also be synthesized by using the surfactant-templating method, while this kind of MOF has never been applied to the fabrication of mesoporous solid bases. Actually, the use of mesoporous MOFs for preparing solid bases is still in its infancy. The reports regarding the synthesis of mesoporous MOFs are enhancing from year to year, which offers great possibilities for new solid bases.

Further investigations into the fabrication and applications of mesoporous solid bases derived from MOFs are therefore highly expected.


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4. Mesoporous solid bases with bifunctional active sites

Nature provides us with efficient enzyme catalysts which are able to achieve complicated reactions through sophisticated strategies. The highly efficient catalytic capacity of enzymes is caused by the cooperative interaction between accurately positioned functions, such as acid–base and metal–base functions. Great attention has been paid to the mimicking of such a capacity. The fabrication of cooperative catalysts can be realized through the multi-functionalization of proper porous supports ( e.g.

mesoporous silica). The porous supports immobilize two and even more functionalities separated at appropriate distances, thus performing multistep or consecutive reactions in cooperative ways (Fig. 41).

358–364

Among various functionalities, basicity is one of the most important active sites involved in cooperative catalytic systems. The incorporation of other active sites ( e.g.

acid and metal) into mesoporous solid bases produces a range of cooperative catalysts, which exhibit a superior performance as compared with catalysts with sole active sites.

4.1.

Acid–base bifunctional catalysts

Acid and base are two antagonistic functions that are difficult to coexist in homogeneous catalytic systems. By spatial isolation in mesoporous supports, the coexistence of incompatible active sites becomes possible. A large number of mesoporous acid–base bifunctional catalysts have been developed, in which acid and base functions, with different strength, are incorporated into the matrix. The acidic sites include silanols,

365,366 ureas,

367 sulfonic acids,

368–370 carboxylic acids,

371 phosphotungstic acids,

372 and Lewis acids originating from Al-doped silica frameworks;

373–375

Fig. 41 Schematic diagram for the consecutive reactions achieved by catalysts with bifunctional active sites (S1 and S2).

whereas the basic sites vary from amines (primary,

376–378 secondary,

379–381 tertiary amine,

382 and chiral amines

383,384

) to alkaline earth metal oxides 385 and layered double oxides.

386

A variety of mesoporous supports, including silica, MOFs, carbon, and phosphonate, have been employed for the fabrication of acid–base bifunctional catalysts. The preparation and applications of silica-supported acid–base bifunctional catalysts have been well reviewed in ref. 387–391. This subsection will deal with the mesoporous supports other than silica.

Due to the composition and structure, MOFs can incorporate different types of sites into both metal nodes and organic linkers, leading to the fabrication of various acid–base bifunctional catalysts.

392,393

A typical example is the post-synthetic modification of the mesoporous MOF MIL-101(Cr).

394

A three-step procedure was employed as shown in Fig. 42. First, mono-BOCethylenediamine was immobilized on CUSs through a primary amino group to form amino group protected NHBOC-MIL-101(Cr).

Second, the yielded NHBOC-MIL-101(Cr) was sulfonated at the backbone phenylene units by treatment with chlorosulfonic acid, producing NHBOC-SO

3

H-MIL-101(Cr). Third, deprotection of the amino groups by thermal treatment gave the bifunctional catalyst NH

2

-SO

3

H-MIL-101(Cr). The results of XRD and N

2 adsorption showed that the crystalline structure of MOF was well maintained during the functionalization process. The results of IR and XPS could monitor the functionalization process and demonstrated the successful incorporation of both acidic and basic sites. Elemental analysis data suggested that about 0.85 ED and

1.08 SO

3

2 per formula unit were immobilized on CUSs and ligands, respectively.

The catalytic performance of bifunctional catalyst NH

2

-SO

3

H-

MIL-101(Cr) was evaluated in a tandem reaction involving the hydrolysis of an acetal and a subsequent Henry reaction.

394

As shown in Table 12, the bifunctional NH

2

-SO

3

H-MIL-101(Cr) could convert benzaldehyde dimethyl acetal ( 2 ) into 2-nitrovinyl benzene ( 4 ) in an almost quantitative yield. Under the catalysis of

NHBOC-SO

3

H-MIL-101(Cr) and SO

3

H-MIL-101(Cr), benzaldehyde

( 3 ) was mainly produced and only a trace amount of 4 was yielded owing to the absence of free amines. Similarly, over monofunctional NH

2

-MIL-101(Cr), the yield of 3 and 4 was negligible.

The introduction of free acid ( p -toluene sulfonic acid) or base

(ethylamine) into NH

2

-SO

3

H-MIL-101(Cr) could terminate the reactions, which might be caused by the formation of ion pairs.

Also, the homogeneous mixture of free acid and base exhibited no activity. These results thus demonstrated the cooperative catalytic performance of the acid–base bifunctional catalyst.

Fig. 42 Formation of acid–base bifunctional active sites on MOFs. [Adapted with permission from ref. 394. Copyright 2012 Royal Society of Chemistry.]


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Chem Soc Rev

Further investigations indicated the good reusability of the bifunctional catalyst NH

2

-SO

3

H-MIL-101(Cr).

394

Almost no loss in activity was observed for the catalyst after three cycles. The preservation of the structure could be proven by XRD and elemental analysis of the recovered catalyst.

It is known that basic functionality can be introduced into

MOFs through replacing the unfunctionalized linkers by corresponding amino-functionalized ones in the synthetic process.

Also, CUSs within MOFs can act as Lewis acid catalytic sites in various organic reactions. Therefore, it is possible to obtain bifunctional MOF catalysts possessing both acidic and basic sites through direct synthesis without any post-synthetic modification. By utilizing AlCl

3 and 2-aminoterephthalate as starting materials, the amino-functionalized MOF, NH

2

-MIL-101(Al), was synthesized under solvothermal conditions.

395

Amino moieties in the organic linker act as Brønsted basic sites while Al

3+ metal centers function as Lewis acid, which results in acid–base bifunctional catalysts. The cooperativity of NH

2

-MIL-101(Al) was demonstrated by catalyzing deacetalization/Knoevenagel condensation 395 and Meinwald rearrangement/Knoevenagel condensation 396 cascade reactions. Of course, either acidic or basic sites in NH

2

-MIL-101(Al) can be separately used as monofunctional catalysts. The separate basic catalytic performance of NH

2

-MIL-

101(Al) has been described in Section 3.6.2. Based on the reactions catalyzed by monofunctional active sites, the bifunctional catalytic performance was studied. For example, NH

2

-MIL-101(Al) could promote the formation of benzylidenemalononitrile between benzaldehyde dimethylacetal and malononitrile that took place through consecutive deacetalization and Knoevenagel condensation reactions.

395

Inspection of the time course of the process showed that the intermediate benzaldehyde was produced from benzaldehyde dimethylacetal over the acidic sites, followed by the formation of ultimate product benzylidenemalononitrile through the reaction of benzaldehyde with malononitrile over the basic sites. After 3 h, the yield of benzylidenemalononitrile could reach 94% over NH

2

-MIL-101(Al). These results evidently proved the acid–base bifunctional catalytic performance of

NH

2

-MIL-101(Al).

Mesoporous nitrogen-rich carbon was embedded with tungsten oxide (WO x

) nanoclusters, which produced bifunctional catalysts possessing both acidic and basic sites.

397

The Lewis basicity of nitrogen-doped carbon was achieved by pyridine-type N atoms in the frameworks, while the basicity resulted from the tungsten oxide species. The catalyst was synthesized using WO x

-containing SBA-15 as the hard template and ED and carbon tetrachloride as the precursors. After polymerization and pyrolysis, the WO x

embedded carbon nitride network was formed in the mesopores of SBA-15. The removal of the silica template was realized by washing with NaOH solution, and the bifunctional catalyst

WO x

-embedded carbon nitride was produced. The catalyst showed a 2D hexagonal pore structure as demonstrated by lowangle XRD and N

2 adsorption results. Energy-dispersive X-ray spectroscopy (EDS) indicated the presence of four elements, namely W, O, C, and N. The presence of WO x with a particle size of 2–3 nm can be demonstrated by TEM images. The bifunctional activity of the catalyst was studied for the one-pot conversion of dimethoxymethylbenzene into benzylidene ethyl cyanoacetate, which involved acid-catalyzed hydrolysis followed by basecatalyzed acetal Knoevenagel condensation.

397 The reaction took place via deacetalization of dimethoxymethylbenzene to benzaldehyde followed by reaction with the methylene compound to yield the deacetalization Knoevenagel condensation product.

The time-on-stream experiment clearly suggested that the reaction was almost complete after 22 h.

Mesoporous metal phosphonates are an important class of nonsiliceous hybrid materials; they are nanocomposites of organic and inorganic components mixed on a molecular level.

398–400

A great number of phosphonic acids and their salt or ester derivatives can be utilized to fabricate metal phosphonates through sol–gel methods. Recently, an acid–base bifunctional catalyst, namely mesoporous titanium phosphonate, was constructed by a facile one-pot hydrothermal approach.

401

Specifically, the synthesis of mesoporous titanium phosphonates involved the condensation of TiCl

4 and amino-containing alendronate sodium trihydrate in the presence of oligomeric template Brij 56, the hydrothermal aging process, and the removal of surfactants. Without any post-synthetic modification, both acidic P–OH and basic

–NH

2 sites were incorporated into the resultant hybrid material.

Also, the hybrid material displayed a periodic mesostructure with a surface area of 540 m

2 g

1 and a pore volume of 0.43 cm

3 g

1

, which favored the mass transport of reactants and products during the catalytic reactions. Due to the large amount of accessible acidic P–OH and basic –NH

2 sites, the mesoporous titanium phosphonate could activate both aziridine and CO

2 efficiently, and catalyze cycloaddition reaction to produce oxazolidinones in a cooperative way. The yield of oxazolidinones reached 98% with a high regioselectivity (98 : 2).

401

The catalytic performance of mesoporous titanium phosphonate was much better than that of its nonporous analogue as well as titanium phosphonates without basic sites or with insufficient acidic sites.

In conclusion, mesoporous acid–base bifunctional materials have been demonstrated to be highly efficient in catalyzing one-pot tandem reactions, which are hard to realize by catalysts with sole acidic or basic sites. In comparison with the extensive studies on acidic sites for the fabrication of bifunctional catalysts, basic sites are mainly focused on amines and less attention has been given to other basic species such as basic oxides. Aiming to develop new bifunctional catalysts and extend their applications, further investigations concerning the incorporation of various basic species are demanded. From the standpoint of designing catalysts with high efficiency, the cooperativity of acid–base interactions is quite important. Weaker acidic sites sometimes perform better than the stronger acidic sites. For example, silanol groups are more useful partners than silica-supported stronger acids ( e.g.

carboxylic groups) in some reactions. In terms of the specific reactions, the acid–base bifunctional catalysts should be designed by tailoring the acidity and basicity as well as controlling the relative spatial positioning of the acidic and basic sites.

4.2.

Metal–base bifunctional catalysts

The incorporation of metals into mesoporous supports with basicity leads to the fabrication of metal–base bifunctional catalysts.


Chem Soc Rev


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Table 12 One pot deacetalization-nitroaldol reaction over different catalysts a

[Adapted with permission from ref. 394. Copyright 2012 Royal

Society of Chemistry.]

Entry Catalyst

Conv.

of 2 (%)

Yield of 3 (%)

Yield of 4 (%)

3

4

5

1

2

6

7

NH

2

-SO

3

H-MIL-101(Cr)

NH

2

-MIL-101(Cr)

100

NHBOC-SO

3

H-MIL-101(Cr) 100

Trace

100 SO

3

H-MIL-101(Cr)

NH

2

-SO

3

H-MIL-101(Cr) + p -toluene sulfonic acid

100

Trace NH

2

-SO

3

H-MIL-101(Cr) + ethylamine p -Toluene sulfonic acid + Trace ethylamine

3

100

Trace

100

95.5

Trace

Trace

97

Trace

Trace

0

4.5

Trace

Trace a

Reaction conditions: benzaldehyde dimethyl acetal (1 mmol), CH

3

NO

2

(5 mL), 90

1

C, 24 h.

So far the metals incorporated have been mainly transition metals including Pd, 402 Pt, 403 Co, 404 basic sites range from amines

407

Ni, 405 and Cu, 406 to alkali metal oxides whereas

408 and alkaline earth metal oxides.

406

The cooperation of metal and base makes the bifunctional catalysts active in various one-pot cascade reactions.

Due to the high activity in a lot of reactions, such as hydrogenation, oxidation, and carbon–carbon coupling, Pd is the most studied metal in metal–base bifunctional catalysis.

409–412

It is known that the performance of a catalyst is strongly dependent on the dispersion degree and stability of active species.

288,413,414

To help disperse and stabilize Pd nanoparticles, amino-functionalized mesoporous silica is usually used as a support. The use of mesoporous silica as a support is understandable owing to its high surface area and large pore size that is beneficial to the dispersion of guest species. It should be stated that amino groups can provide strong anchoring sites for attaching Pd nanoparticles. Moreover, the remaining amino groups offer additional basic sites to cooperate with the Pd nanoparticles for bifunctional catalysis.

By incorporating Pd nanoparticles into amino-functionalized mesoporous silica SBA-15, a bifunctional catalyst, that is Pd/

NH

2

-SBA-15, was synthesized.

415

The synthesis of a bifunctional catalyst with platelet morphology was begun with a P123-directed co-condensation between TEOS and APTMS in the presence of

ZrOCl

2 and NaCl. Then, Pd nanoparticles were introduced into

NH

2

-SBA-15 through adsorption and reduction of H

2

PdCl

4

. The obtained bifunctional catalyst showed an ordered mesoporous structure, as demonstrated by low-angle XRD, N

2 adsorption, and TEM results. In the high-magnification TEM images, almost no Pd particles were observed because of the highly uniform dispersion of Pd, which is in line with the wide-angle

XRD data. Elemental and inductively coupled plasma (ICP) analysis revealed that the N and Pd content were 1.30 and

3.60 wt%, respectively. XPS results indicated that Pd particles existed in the elemental Pd form and that nitrogen existed as amino groups. The metal–base bifunctional Pd/NH

2

-SBA-15 was employed to catalyze one-pot multistep synthesis of a -alkylated nitriles.

415

The reaction sequence consisted of two steps, namely

Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate on the basic sites and hydrogenation of a , b -unsaturated nitriles on the Pd nanoparticles. The bifunctional catalyze can convert benzaldehyde to the target product in 100% yield under optimum conditions. This suggests the excellent catalytic activity and selectivity of Pd/NH

2

-SBA-15. Under the same conditions, no target product was yielded at all over the monofunctional catalysts

( i.e.

NH

2

-SBA-15 and Pd/SBA-15). Furthermore, a series of substrates, including aromatic aldehydes with both electron-rich and electron-deficient substituents as well as aliphatic aldehydes and ketones, can be converted to corresponding target products.

This means that Pd/NH

2

-SBA-15 served as an efficient bifunctional catalyst for cascade reactions.

Instead of metallic Pd nanoparticles, Pd

II complexes were also immobilized on mesoporous silica to form metal–base bifunctional catalysts with supported basic sites.

416,417

The synthesis involved the reactions of APTMSl, followed by the Pd II -[ N -(2aminoethyl)-3-aminopropyltrimethoxysilane] complex.

416 Mesoporous bifunctional catalysts by immobilization of two catalytic groups, a Pd

II

–diamine complex and a primary amine (–NH

2

), on mesoporous silica, were thus fabricated. For the bifunctional catalysts derived from different supports, namely MCM-41 and

SBA-15, a similar catalytic performance was observed. The bifunctional catalysts were active in Sonogashira and Henry tandem reactions. They gave about 60% yield for the Sonogashira–Henry tandem product in 5 h by the two-step tandem reaction in one pot. Both amine and Pd

II

–diamine grafted monofunctional catalysts can catalyze the respective individual reactions, but not both. Recently, mesoporous bifunctional metal–base catalysts bearing Pd

II

-complexes and additional basic sites were fabricated by post-synthetic modification using click chemistry.

417

The

‘‘mother’’ particles bearing two chemical orthogonal functionalities, namely, azides and alkoxyamines were first prepared (Fig. 43).

The two functionalities were addressed selectively by orthogonal click-chemistry, which allows for the fabrication of a catalyst library containing different functionalities as cooperative active moieties. The azide–alkyne cycloaddition is the standard click reaction and the alkoxyamine moiety is readily chemically modified by thermal nitroxide exchange. After nitroxide exchange with various nitroxides containing amino groups and treatment with

PdCl

2

, a series of metal–base bifunctional catalysts were obtained.

The cooperativity of catalysts was proven by the Tsuji–Trost allylation of ethyl acetoacetate.

417

In addition to amino groups with relatively weak basicity, metal oxides with strong basicity were employed to fabricate bifunctional catalysts as well. A recent report is the use of mesoporous spinel magnesium aluminate (MgAl

2

O

4

) as a support.

406

Mesoporous spinel with strong basicity was prepared by a hardtemplating method using magnesium and aluminum nitrates as precursors and activated carbon as a template. The bifunctional

Cu/MgAl

2

O

4 catalyst was prepared by impregnation of Cu(NO

3

)

2 on mesoporous spinel followed by reduction in H

2

. The resultant catalyst was highly active in the aldol condensation of

5-hydroxymethylfurfural and acetone as well as in the subsequent


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Chem Soc Rev

Fig. 43 Fabrication of mesoporous metal–base bifunctional catalysts bearing Pd

II

-complexes and additional basic sites by post-synthetic modification using click chemistry. [Adapted with permission from ref. 417. Copyright 2013 Royal Society of Chemistry.] hydrogenation of the condensation products. In addition, a mesoporous solid strong base was applied to support Co

3

O

4

, producing a new metal–base bifunctional catalyst.

408

The support was synthesized by loading K

2

O on ZrO

2

/SBA-15.

138

Cobalt oxide was introduced as the metal sites through a wet impregnation approach. The cooperativity of metal sites Co

3

O

4 and basic sites K

2

O makes the bifunctional catalyst highly active in the oxidation of ethylbenzene. As a comparison, various supports including weakly acidic supports (zeolites HY and H b ), weakly basic supports (MgO, CeO

2

, and hydroxyapatite), and neutral supports (MCM-41 and ZrSBA-15) were employed.

408

Although these catalysts were synthesized through the identical procedure, their catalytic activity was much lower than that of the bifunctional catalyst. This demonstrates that the strongly basic sites play an important role in cooperating with the metal sites to catalyze the conversion of ethylbenzene.

To summarize, in contrast to acid–base bifunctional catalysts, much less attention has been paid to metal–base bifunctional catalysts. Among metal sites, Pd is the most studied, and the use of other metals for the fabrication of metal–base bifunctional catalysts is still in its infancy. As a result, further investigations regarding the fabrication of new metal–base bifunctional catalysts are desirable. Anyway, these preliminary studies have shown that bifunctional materials are able to catalyze tandem reactions in one-pot. This avoids unnecessary use of solvents and other chemicals required for the purification of intermediates in either individual reaction and subsequently, lowers the cost for the synthesis of target products.

5. Storage and activation of mesoporous solid bases

Unlike their acidic counterparts, basic materials are sensitive to atmospheric CO

2 and H

2

O and even O

2 in some cases. The active sites can be seriously poisoned if storage or activation is not well performed. As a result, great attention should be paid to storage and activation of mesoporous solid bases, which have


Chem Soc Rev


Review Article an important effect on the subsequent catalytic processes. In order to avoid contamination by atmospheric components, solid bases should be stored in the absence of air, typically under an inert atmosphere ( e.g.

N

2

) and alternatively under vacuum. It is known that basic sites with higher strength are easier to be poisoned by contaminants. In the case of solid superbases, even a tiny amount of contaminants is able to deactivate the superbasic sites. This may lead to a sharp decrease of activity in some reactions merely catalyzed by superbasic sites. Hence, rigorous storage conditions are required for the materials with strong basicity.

Activation is a necessary process for most of the solid catalysts prior to reactions. This is particularly important for solid basic catalysts. The basic sites may be covered by CO

2 and H

2

O, thus exhibiting poor activity in base-catalyzed reactions. Some materials that are known as strongly basic catalysts used to be considered as inert materials due to improper activation methods. From the viewpoint of activation, mesoporous solid bases generally fall into two categories. For the first category, organic groups ( e.g.

amines) act as the basic sites. The basicity of these materials is weak, and activation at relatively low temperatures (usually lower than 200

1

C) is sufficient to expose the active sites. In the meanwhile, low activation temperatures are beneficial to the preservation of organic moieties owing to their poor thermal stability.

For the second category, inorganic metal oxides ( e.g.

MgO) are the basic sites of materials. High-temperature activation is normally required to completely remove CO

2 and H

2

O on the

( basic sites. Take MgO as an example, liberation of CO

2 persists up to 500 1 C.

14,210 and H

2

O

For CaO, even higher temperatures ca.

600 1 C) are demanded to remove CO

2 on the basic sites.

199

Liberation of such contaminants leads to the exposure of basic sites on materials, which function as active sites for basecatalyzed reactions. It should be stated that the nature of basic sites formed by activation at different temperatures is different.

In addition to evolution of contaminants, rearrangement of surface and bulk atoms also takes place in the process of activation.

It is thus easy to understand that different activation conditions are required for different types of reactions. A case in point is

MgO, the activity maximum for 1-butene isomerization appears at an activation temperature of B 530

1

C, while that for amination of 1,3-butadiene occurs at B 700 1 C.

14

For the selection of activation temperatures, one should also keep the stability of mesoporous frameworks in mind, since destruction of the mesostructure may occur at elevated temperatures.

6. Summary and perspectives

Mesoporous solid bases have attracted extensive attention owing to their intriguing nature and versatile catalytic performance in heterogeneous reaction systems. Substantial achievements have been made with respect to their design and fabrication particularly in the last decade, which has become a vibrant area of chemical research. By judicious choice of basic species and supports, a great variety of mesoporous solid bases can be fabricated. There are obvious advantages to increasing the pore size of solid bases into the mesoporous regime, where the larger pores allow accelerated mass transport. Moreover, mesoporosity plays a significant role in catalysis involving bulky reactants and products. It should be stated that mesoporous heterogeneous basic catalysts are not simple immobilization of homogeneous basic catalysts on mesoporous supports.

Some organic reactions exclusively take place in the presence of heterogeneous basic catalysts. The special microenvironment in mesopores provides many possibilities for the introduction of various basic functionalities. Multifunctional active sites and even two hostile functionalities can coexist in the mesopores, which is impossible to realize for homogeneous catalysts. This makes it possible for researchers to design basic catalysts precisely in terms of specific needs for different reactions.

Although remarkable progress in the fabrication of mesoporous solid bases has been made, a general strategy for their production is lacking. The strategies are dependent on the types of basic species and supports. From the viewpoint of basic species, organic basic groups ( e.g.

amines) are commonly introduced by grafting, while the introduction of inorganic basic oxides ( e.g.

MgO) is mainly realized through impregnation. From the viewpoint of supports, the alkali-resistance and stability should be fully taken into account. Strong bases

(typically alkali metal oxides) can be directly loaded on mesoporous metal oxides ( e.g.

Al

2

O

3 and ZrO

2

) and carbon, but not suitable for mesoporous silica due to the poor alkali-resistant ability. As for mesoporous MOFs, the introduction of basic species requires a careful consideration of the synthetic conditions to prevent the degradation of frameworks. One should thus bear in mind that for the fabrication of mesoporous solid bases, the protocols must be meticulously established by considering the properties of both basic species and supports.

It should be stated that practical applications are far behind the fabrication of mesoporous heterogeneous basic catalysts.

Two predominant factors are believed to be responsible for this phenomenon. One factor is that the scale-up problem is scarcely explored. On the one hand, some general difficulties existing in large scale production of mesoporous materials, such as high price of templates, should be overcome. This is also essential for the practical applications of all mesoporous functional materials. On the other hand, there are individual difficulties in mesoporous materials with basicity. Strong bases are sensitive to atmospheric CO

2 and moisture, and deactivation of basic sites occurs to a greater or lesser extent when the catalysts are handled under atmospheric conditions. Therefore, the preparation and storage of catalysts should be performed in the absence of air. If this care is taken, mesoporous solid bases are able to catalyze a large amount of reactions. Alternatively, appropriate base precursors can be involved in the preparation, and strong basicity only forms in the in situ activation prior to reactions. The contamination with atmospheric poisons can thus be avoided so that the catalysts are highly efficient in catalytic processes. Another factor is that limited attention has been paid to the reactions aimed at practical applications, although mesoporous solid bases are highly potential for a lot of industrially important reactions. Many reports merely deal with proof-of-concept studies. From the standpoint of practical


Review Article applications, further investigations regarding the large scale production of mesoporous solid bases as well as their applications beyond proof-of-concept examinations are desirable.

Surface chemistry of mesoporous solid bases is actually quite complicated. The incorporation of single basic species may lead to the formation of basic sites with various strength and numbers. In most cases weak and strong bases coexist in a catalyst. Some catalysts do exhibit high activity in reactions, while the origination of activity is not frequently well disclosed. Hence, it is of great importance to systematically characterize the surface sites and subsequently, clarify the reaction mechanisms. So far much effort has been dedicated to the characterization, and the normally used techniques include titration, CO

2

-TPD, and probing reactions. It is noticeable that these techniques essentially provide overall information on the catalysts. Some information at the molecular level

( e.g.

the spatial positioning and distance of different active sites) is of more importance, especially for multifunctional sites that work in a cooperative way. It is therefore expected to develop ingenious characterization techniques to deeply understand the surface chemistry of mesoporous solid bases. Based on these results, the relationship between the nature and performance of catalysts can be well established, which is crucial for the design of new and efficient catalysts. It is also worth noting that rapid progress has been made on computational chemistry in the past decade. The combination of computational and experimental approaches can benefit the elucidation of structure–activity relationship and the development of hitherto inaccessible mesoporous solid bases.

The last decade has witnessed rapid advances in mesoporous solid bases. This review attempts to cover the state of the art in this exciting field. Because of the great variety of materials and synthetic parameters, it is hard to be entirely comprehensive. Nonetheless, we have made efforts to address the prototypical examples for particular strategies in some more detail. It is anticipated that this review will provoke interest in acquiring a basic understanding of preparing mesoporous basic materials as well as assist in the rational design and fabrication of novel mesoporous solid bases.

Acknowledgements

We acknowledge financial support of this work by the Distinguished Youth Foundation of Jiangsu Province (BK20130045), the Fok Ying-Tong Education Foundation (141069), the National

Natural Science Foundation of China (21006048), the National

High Technology Research and Development Program of China

(863 Program, 2013AA032003), the National Basic Research Program of China (973 Program, 2013CB733504), and the Project of

Priority Academic Program Development of Jiangsu Higher Education Institutions.

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As featured in:

As featured in:

www.rsc.org/chemsocrev

Chem Soc Rev

REVIEW ARTICLE

Design and fabrication of mesoporous heterogeneous basic catalysts

Lin-Bing Sun,*

Xiao-Qin Liu

and Hong-Cai Zhou*

1. Introduction

2. Mesoporous materials with basic frameworks

3. Generation of basic sites on mesoporous supports

4. Mesoporous solid bases with bifunctional active sites

5. Storage and activation of mesoporous solid bases

6. Summary and perspectives

Acknowledgements

Notes and references

Related documents

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As featured in:

As featured in:

www.rsc.org/chemsocrev

Chem Soc Rev

REVIEW ARTICLE

Design and fabrication of mesoporous heterogeneous basic catalysts

Lin-Bing Sun,*

Xiao-Qin Liu

and Hong-Cai Zhou*

1. Introduction

2. Mesoporous materials with basic frameworks

3. Generation of basic sites on mesoporous supports

4. Mesoporous solid bases with bifunctional active sites

5. Storage and activation of mesoporous solid bases

6. Summary and perspectives

Acknowledgements

Notes and references

Related documents

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