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This article is part of the
Metal-organic frameworks
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Guest editors: Neil Champness, Christian Serre and
Seth Cohen
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A versatile metal–organic framework for carbon dioxide capture and
cooperative catalysiswz
Jinhee Park,a Jian-Rong Li,a Ying-Pin Chen,a Jiamei Yu,b Andrey A. Yakovenko,a
Zhiyong U. Wang,a Lin-Bing Sun,a Perla B. Balbuenab and Hong-Cai Zhou*a
Received 28th June 2012, Accepted 21st August 2012
DOI: 10.1039/c2cc34622b
A multi-functional MOF PCN-124 was constructed from Cu
paddlewheel motifs and a judiciously designed novel ligand
bearing carboxylate, pyridine, and amide groups. PCN-124
exhibits selective adsorption of CO2 over CH4 and excellent
catalytic activity in a tandem one-pot deacetalization–Knoevenagel
condensation reaction as a cooperative catalyst.
Over the last two decades, metal–organic frameworks (MOFs)
as a class of well-ordered porous crystalline materials have
attracted continuous interests owing to their structural and
functional diversity and tailorability, as well as high porosity
and large surface area.1 Depending on the specific needs of
an application, the overall structure, porosity, and surface
functions of a MOF can be designed and fine-tuned through
the judicious choice of metal or metal-containing building
nodes and organic bridging ligands.2 The designable potential
of MOFs leads to their diverse applications, such as in gas
storage,3 separation,4 heterogeneous catalysis,5 drug delivery,6 and
molecular recognition.7 In this report, we wish to demonstrate the
construction of a multi-functional MOF with selective CO2 uptake
over CH4, and interestingly cooperative catalytic functions
through the design of a novel amide-bearing ligand.
Well-defined crystalline structures of MOFs provide a great
platform to develop heterogeneous catalysts. Active catalytic
centers can be directly embedded in the ligands or metal nodes
prior to MOF synthesis or postsynthetically installed.5 MOFs are
unique among heterogeneous catalysts because of the potential
to precisely install different, even functionally incompatible,
catalytic centers such as acidic and basic groups at separate
desired positions within their crystalline framework structure.
Toward this direction some multi-functional MOFs have been
developed as heterogeneous catalysts capable of catalyzing
tandem reactions.8 It should be pointed out that while homogeneous catalysts such as soluble enzymes have shown the
successful combination of chemically antagonistic functions,
controlling the separation of these groups on the heterogeneous catalyst is not an easy task.9
Because MOFs possess tunable structures which are stable
and crystalline, we perceived it feasible to integrate multiple
functions within one single MOF through careful design. We
sought to develop a versatile MOF which would integrate
both selective gas adsorption and catalytic power for tandem
reactions. A novel ligand, 5,5 0 -((pyridine-3,5-dicarbonyl)bis(azanediyl))diisophthalate (PDAI, Fig. 1a), with two isophthalate
and one pyridine groups connected through amide bonds, was
thus designed to construct a copper paddlewheel-based MOF
PCN-124 (PCN stands for ‘‘porous coordination network’’).
The open metal sites and amide groups were expected to
increase the interaction between adsorbed CO2 and the pore
surface as demonstrated in MOF literature10 and could serve
a
Department of Chemistry, Texas A&M University, College Station,
TX 77842-3012, USA. E-mail: zhou@chem.tamu.edu;
Fax: +1 979-845-1595
b
Artie McFerrin Department of Chemical Engineering, Texas A&M
University, College Station, TX 77842, USA
w This article is part of the ChemComm ‘Metal-organic frameworks’
web themed issue.
z Electronic supplementary information (ESI) available: Full experimental details, crystallographic data, GCMC simulation, TGA, IR,
additional gas adsorption isotherms of PCN-124, and NMR spectra of
products from the tandem deacetalization–Knoevenagel condensation
reaction. CCDC 858820. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2cc34622b
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The Royal Society of Chemistry 2012
Fig. 1 (a) 5,50 -((Pyridine-3,5-dicarbonyl)bis(azanediyl))diisophthalate,
PDAI. (b) Coordination environment of the dinuclear Cu paddlewheel
motifs and ligands in PCN-124. (c) A cuboctahedral cage in PCN-124.
(d) A single cubic framework of PCN-124 constructed by ligand
skeletons bridging cage molecular building blocks (Cu atoms are shown
in cyan, O atoms in red, N atoms in blue, C atoms in green). (e) The selfinterpenetrated structure of PCN-124.
Chem. Commun., 2012, 48, 9995–9997
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as Lewis acids, Lewis bases, or hydrogen bond donors for
tandem catalysis.11
PCN-124 was successfully synthesized by reacting H4PDAI
with copper nitrate in a mixed solvent of 80% dimethylacetamide (DMA) and 20% water under solvothermal conditions.
PCN-124 possesses a self-interpenetrated (3,36)-connected
three-dimensional (3D) structure and crystallizes in the space
group Im3% m. It resembles an isostructural MOF, PMOF-3,
which was recently synthesized from 3,5-bis(3,5-dicarboxylphenylethynyl)-pyridine by the Zhang group.12 Similar to
other MOFs consisting of isophthalate moieties, twenty-four
isophthalate moieties and twelve Cu paddlewheel motifs assemble
into a cuboctahedral building unit (Fig. 1c). These cuboctahedral
cages are further linked by the ligand to generate a 3D cubic
framework (Fig. 1d). One axial site of the Cu paddlewheel motif
in the molecular cage coordinates to the pyridine group of
the ligand from another identical framework to form a selfinterpenetrated structure (Fig. 1b and e). Thus each cuboctahedral
structural building unit becomes a 36-connected node, leading to a
highly connected framework with excellent chemical and thermal
stability (vide infra). As expected, uncoordinated open Cu sites are
indeed formed in PCN-124.
A powder X-ray diffraction (PXRD) study confirmed the
phase purity of PCN-124 bulk sample and preservation of its
crystallinity after the removal of coordinating solvents (Fig. S4,
ESIz). It was thermally stable up to 300 1C, as confirmed by
thermogravimetric analysis (Fig. S6, ESIz). Interestingly, PCN-124
also showed the moderate hydrostability, particularly relevant for
potential applications in CO2 capture technologies and catalysts
(Fig. S5, ESIz). The relatively high thermal stability and the
3D porous structure prompted us to test the gas adsorption
performance of PCN-124.
The permanent porosity of PCN-124 was confirmed by
various gas adsorption measurements. A solvent-exchanged
sample was activated at 100 1C for 10 h under reduced
pressure. The N2 adsorption isotherm of the activated sample
at 77 K exhibited reversible type I behavior revealing the
microporous nature of PCN-124 (IUPAC classification). The
calculated Langmuir and Brunauer–Emmett–Teller (BET)
surface areas are 2002 m2 g1 and 1372 m2 g1, respectively.
The total pore volume is 0.579 cm3 g1, close to the specific
pore volume of 0.629 cm3 g1 (55.5%) calculated by PLATON
based on the single crystal X-ray diffraction data. Two different
kinds of micropores with diameters of 7.6 Å and 9.5 Å were
calculated based on the N2 isotherm (Fig. S8, ESIz), again in
good agreement with the single crystal data.
Development of effective methods to reduce the massive
emission of anthropogenic CO2, the major greenhouse gas, is
becoming increasingly critical due to the enormous environmental impact.13 The linearly arranged open metal sites and
amide groups in the framework of PCN-124 provided a
favorable environment for CO2 adsorption. As shown in
Fig. 2a, it was found that the CO2 uptake was as high as
114 cm3 g1 (18.3 wt%) at 295 K, 160 cm3 g1 (23.8 wt%) at
283 K, and 204 cm3 g1 (28.6 wt%) at 273 K and 760 mmHg.
The heat of adsorption at zero CO2 loading was calculated to
be 26.3 kJ mol1.14 The CO2 uptake capacity of PCN-124 is
among the highest for MOFs based on Cu paddlewheel
motifs.15 In comparison, the isostructural PMOF-3, consisting
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Chem. Commun., 2012, 48, 9995–9997
Fig. 2 (a) CO2 and CH4 adsorption isotherms of PCN-124 (black
circle, CO2 at 273 K; blue circle, CO2 at 283 K; red circle, CO2 at
273 K; black triangle, CH4 at 273 K; red triangle, CH4 at 295 K).
(b) IAST-predicted adsorption selectivity of the equimolar mixture of
CO2 and CH4 in PCN-124 at 273 and 295 K.
of bridging ethynyl bonds instead of amide ones, showed lower
CO2 uptake (12.5 wt%) at 298 K despite higher N2 adsorption
(581 cm3 g1) at 77 K.12
Another gas that is often evaluated for MOFs is methane. CH4
uptake of PCN-124 at ambient pressure was just 15.1 cm3 g1
(1.06 wt%) at 295 K and 33.4 cm3 g1 (2.33 wt%) at 273 K
(Fig. 2a), and the heat of CH4 adsorption was 15.7 kJ mol1.
The much lower adsorption of CH4 compared to CO2
prompted us to investigate the CO2/CH4 selectivity. We
adopted the ideal adsorbed solution theory (IAST), which is
commonly used to predict binary mixture adsorption from
experimental pure-gas isotherms.16 In the presence of CH4 the
adsorption of CO2 does not significantly decrease, but CH4
adsorption is lower in the mixtures than in the pure state
due to competition between CO2 and CH4. The adsorption
selectivity for equimolar mixtures of CO2 and CH4 as a
function of bulk pressure is shown in Fig. 2b. The good
selectivity of CO2/CH4 in the 8–20 range at low pressure
makes PCN-124 potentially useful for natural gas upgrading.
In addition to their function of boosting CO2 adsorption
capacity and selectivity, the weakly Lewis acidic open Cu2+
centers and Lewis basic pyridine and amide groups in PCN-124
made this MOF useful for one-pot tandem reactions. As a
demonstration, we utilized PCN-124 as a heterogeneous catalyst
for the deacetalization–Knoevenagel condensation (Scheme 1).
The first step of the reaction is the acid-catalyzed deacetalization
of dimethoxymethylbenzene to give benzaldehyde. Acetals and
ketals are valuable in organic synthesis because they can serve
as protecting groups for reactive aldehydes and ketones,
one of the most important classes of organic compounds
widely used in industry and biology. The second step gave
benzylidene malononitrile, an inhibitor for the protein
tyrosine kinase,17 through the Knoevenagel reaction between
benzaldehyde and malononitrile. The Knoevenagel reaction is
typically catalyzed by base, and amide groups in a MOF as a
weak base for the reaction have been reported by Kitagawa’s
group.11a
Scheme 1 The reaction scheme of deacetalization–Knoevenagel
condensation.
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Table 1 Tandem deacetalization–Knoevenagel condensation reactions
monitored by NMR
Entry
Conversion (%) 1
Yielda (%) 2
Yielda (%) 3
1st round
2nd recycle
3rd recycle
4th recycle
Acidic cond.b
Basic cond.c
B100
B100
99.6
99.9
97.3
B0
Trace
3.37
8.06
3.29
81.3
Trace
B100
96.6
91.9
96.7
16.0
Trace
2
3
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a
The yields were calculated by integration of benzylic protons at the
end of the reaction. b Control reaction in the presence of p-toluenesulfonic acid. c Control reaction in the presence of ethylenediamine to
block the open Cu sites.
4
5
All reactions were performed with 2 mmol substrates in
DMSO-d6 using 0.5 mol% (0.01 mmol) PCN-124 for 12 h.
1
H NMR spectroscopy was used to monitor the progress of the
reaction and to determine the reaction yield. The starting material
(dimethoxymethylbenzene), the intermediate (benzaldehyde), and
the final product (benzylidene malononitrile) have distinct
1
H NMR resonances for their benzylic protons (as denoted
by different colors in Scheme 1), making it straightforward to
monitor the reaction.
Table 1 summarizes the reaction results. PCN-124 was
found to be a highly active cooperative catalyst for this
tandem one-pot reaction with a turnover number of more
than 190. Control experiments proved that both open Cu2+
sites and amide groups are essential for the reaction, and they
work cooperatively. In the presence of p-toluenesulfonic acid,
the deacetalization reaction worked well but the Knoevenagel
condensation was inhibited. In the presence of ethylenediamine,
which alone could catalyze the second step Knoevenagel
condensation, the open Cu2+ sites in PCN-124 were blocked
and neither 2 nor 3 could be formed. Moreover, PCN-124 could be
easily recovered from the reaction medium by centrifugal separation and reused at least for three cycles without significant loss of
activity. The PXRD patterns of the fourth recycled PCN-124 were
still consistent with the as-synthesized one, indicating its excellent
chemical stability (Fig. S4, ESIz).
In conclusion, we have presented a multi-functional MOF
PCN-124 with embedded chemically antagonistic functional
groups. On one hand, the micro-porosity, open Cu2+ sites,
and amide groups in this MOF enable selective adsorption of
CO2 over CH4 as well as a cooperative catalyst for a tandem
deacetalization–Knoevenagel reaction. Our work demonstrates
that through judicious design various functional groups could
be integrated into one MOF to realize distinct functions.
This work was supported by the U.S. Department of Energy
(DOE DE-SC0001015 and DE-FC36-07GO17033), the National
Science Foundation (NSF CBET-0930079), and the Welch
Foundation (A-1725).
6
7
8
9
10
11
12
13
14
15
16
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