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Research Article
Substantial Turnover Frequency Enhancement of MOF Catalysts by
Crystallite Downsizing Combined with Surface Anchoring
A. Lisa Semrau,§ Philip M. Stanley,§ Alexander Urstoeger, Michael Schuster, Mirza Cokoja,
and Roland A. Fischer*
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sı Supporting Information
*
ABSTRACT: We report on the preparation of surface-anchored nanoparticles of the
metal−organic framework (MOF) UiO-66 (Universitet i Oslo; Zr6O4(OH)4(bdc)6; bdc2−
= 1,4-benzene dicarboxylate). The surface-anchored nano-MOFs (SA-NMOFs) were
prepared by covalent anchoring of a presynthesized, functionalized UiO-66 nano-MOF
(NMOF) on surface-modified poly(dimethylsiloxane). The SA-NMOFs exhibit discrete
NMOFs (<30 nm) which do not aggregate. The SA-NMOFs retain a high surface area,
rendering them interesting catalysts. We compared the catalytic activities of SA-NMOFs in
the cyanosilylation of benzaldehyde with those of the bulk UiO-66 and colloidal-dispersed
UiO-66 NMOFs (size: 22 ± 3 nm). The SA-NMOFs exhibit a boost in activity by a factor of 100,000−1,000,000 owing to (a) the
generally larger surface area of NMOFs and (b) the suppressed aggregation of the nanoparticles by surface immobilization. In
contrast, colloidal NMOFs rapidly aggregate, as shown by dynamic light scattering. The general applicability of our approach for
other Lewis acid-catalyzed reactions is demonstrated by comparing the activities of the three catalyst systems for the cycloaddition of
CO2 and propylene oxide to propylene carbonate, where SA-NMOFs by far outperform the bulk MOFs and defect-engineered
MOFs, respectively. This discovery paves the way for application of SA-NMOFs as efficient catalyst materials.
KEYWORDS: metal−organic frameworks, surface anchoring, lewis acid catalysis, heterogeneous catalysis, composites, downsizing,
nanoparticles
■
INTRODUCTION
Metal−organic frameworks (MOFs) are attractive catalyst
materials because of the modular building principle, which
allows for designing a wide range of properties of relevance for
catalytic transformations. Targeted reactions reach from base/
acid catalyzed1 and oxidation reactions2 to photochemical
reactions.3 Zr-MOFs such as UiO-664 (Universitet i Oslo;
Zr6O4(OH)4(bdc)6; bdc2− = 1,4-benzene dicarboxylate) are
known for their Lewis acidic properties in catalysis,5−7 which
have been investigated in the past years. Because of their
excellent thermal and chemical stability, UiO-66 is a model
system for defect engineering8−10 and catalysis.5−7 General
drawbacks that restrict the use of MOFs as catalysts include
various issues such as comparably expensive synthesis, limited
chemical stability, and difficult handling of the MOF powders.
Despite the fact that MOFs are heterogeneous catalysts by
nature, they still suffer from challenging processing, for
example, pellet formation for application in catalytic reactors.
The inferior catalytic activity of MOFs in most reactions as
compared to existing benchmarks can be ascribed to the
diffusion limitation in the typically small pore aperture. MOF
defect engineering with respect to hierarchical porosity is one
possible solution. Alternatively, reducing the typical diameter
of MOF particles to the nanoscale can reduce the diffusion
limitation effect in comparison with bulk MOF materials.
Various studies of the catalytic activity dependence of nano© 2020 American Chemical Society
MOFs (NMOFs) with respect to their particle size have been
reported to date,11−14 showing that a smaller particle size
makes MOFs more catalytically active, which is considered to
be a consequence of greater external surface area and lower
diffusion barriers. However, it is known that colloidal dispersed
NPs tend to agglomerate and aggregate during catalysis,15
which leads to a reduction in the activity. In general, capping
agents,16 which interact with the nanoparticles by covalent17 or
ionic interactions,18 can prevent agglomeration by steric or
electrostatic interactions; however, they may interfere with the
catalytic reaction as well.18 Thus, anchoring catalytically active
species on support materials is a common approach to
compromise between catalytic reactivity on the one hand and
separation issues on the other hand.19 In principle, there are
different possibilities that realize this approach for MOFs, such
as immobilizing corresponding NMOFs20,21 on surfaces or
fabricating MOF thin films.22−25 Although the latter have been
studied in catalytic reactions such as phosphonester degradation23−25 or Knoevenagel condensation,22 the catalytic properReceived: February 1, 2020
Revised: February 6, 2020
Published: February 7, 2020
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the hydrodynamic diameter was found to be 164 ± 24.4 nm
(NMOF 1) and 91.2 ± 37.7 nm (NMOF 2) (see the
Supporting Information, Chapter S6, Figures S6 and S7),
which corresponds to the size distributions that we determined
in SEM images (see the Supporting Information, Chapter S2,
Figures S1−S4). After storing the colloidal solutions for one
month under ambient conditions, the DLS spectra reveal
agglomeration of the primary particles to 531 ± 76 nm for
NMOF 1. In contrast, NMOF 2 shows no tendency for
agglomeration, and a characteristic hydrodynamic diameter of
78.1 ± 24.4 nm was measured (see Figure 2d). This illustrates
that ADA can be used as a capping agent in order to effectively
stabilize the particles in colloidal solution. The degree of
functionalization was determined by 1H NMR spectroscopy
after sample digestion. Specifically, a dried powder sample of
NMOF 2 was dispersed in DMSO/DCl (4/1 vol%) and
digested by heating. The mass fraction of ADA was found to be
11.0 ± 1.3 wt % (see the Supporting Information, Chapter S5).
In the next step, the NMOFs 1 and 2 were utilized as
catalysts for the cyanosilylation of benzaldehyde. The reaction
was carried out with 3 mol % Zr loading in toluene. All
reactions were carried out at least three times. The time−yield
plots show the average of these experiments, with error bars
marking the deviation. The yield equals the conversion because
no side products were detected (for more details, see the
Supporting Information, Chapter S6).
The measured kinetic curves (see Figure 3a) show a strong
initial period with high catalytic activity and a low
reproducibility. Afterward, a rapid de-activation of the catalyst
can be observed. The strong pronounced catalyst de-activation
after the initial period can be explained by aggregation of the
nanoparticles in solution. We investigated this phenomenon by
DLS (see Figure 3b). The hydrodynamic diameter of the
particles in toluene solution changes drastically from 164 nm
(NMOF 1) to 396 nm after 1 h and 1110 nm after 8 h of
reaction time. We found a similar trend for the hydrodynamic
diameter of NMOF 2, which grows with longer reaction time.
This shows that the introduced capping agent (ADA) cannot
effectively prevent the particle aggregation during the catalytic
reaction. In a previous report, we have studied the
cyanosilylation with defect-engineered UiO-66 samples as
catalysts.6 We reproduced these samples (Supporting Information, Chapter S7, Figure S11) and repeated the
performed experiments in dichloromethane (DCM) and
toluene (Supporting Information, Chapter S8, Figures S12
and S13). The reaction kinetics are highly dependent on the
solvent, and the reaction is significantly faster in DCM (see the
Supporting Information, Chapter S8, Figure S13). In
comparison with the defect-engineered UiO-66-10TFA, the
NMOFs 1 and 2 exhibit a higher catalytic activity and full
conversion after 72 h (see Figure 3). Both NMOFs feature a
comparably low missing linker defect content (see the
Supporting Information, Chapter S10), which was calculated
from TGA data according to established procedures.8 The
defect content corresponds to the formula Zr6O5.80(bdc)6.20 of
the activated sample. Their overall enhanced activity is in
contrast to the comparably low defect content and thus must
be related to the reduced characteristic particle size. In
summary, the slightly enhanced activity of both catalysts
NMOF 1 and 2 as compared to the benchmark system UiO66-10TFA can be explained by a combination of agglomeration and low defect content of the synthesized particles.
ties of covalently surface-anchored NMOFs (SA-NMOFs)
have not been investigated.
Therefore, we examined the immobilization of NMOFs and
the effect of the immobilization on the tendency of the
NMOFs to aggregate and hence on the catalytic activity.
Specifically, we present the synthesis and characterization of
UiO-66 nanoparticles (NMOF 1), their colloidal stabilization
by 12-azido-dodecan-1-amine (NMOF 2) as a capping agent
and the immobilization on polydimethylsiloxane (PDMS) and
Si/SiO2 substrates. For the evaluation of the catalytic
properties of these supported catalysts, we chose the Lewis
acid-catalyzed cyanosilylation of benzaldehyde and the cycloaddition of CO2 with propylene oxide as model reactions. The
catalysis revealed a massive enhancement of activity of the
immobilized catalysts by a factor of 100,000−1,000,000.
■
RESULTS AND DISCUSSION
Synthesis and Characterization of 1 and 2. UiO-66
nanoparticles, denoted as NMOF 1, were prepared by a
solvothermal reaction of H2bdc, acetic acid, and ZrOCl2 in
dimethylformamide (DMF) at 90 °C for 18 h.26 The exterior
surface of NMOF 1 was subsequently functionalized at the bdc
linkers.27 For this, an ethanol solution of NMOF 1 and 12azidododecan-1-amine (ADA) was treated with N,N-diisopropylcarbodiimide and N-hydroxysuccinimide for 4 h at room
temperature, resulting in alkylazide-functionalized NMOF 2
(see Figure 1). The obtained materials NMOF 1 and 2 were
Figure 1. Reaction of the NMOF 1 with ADA to form the azideterminated NMOF 2.
investigated by powder X-ray diffraction (PXRD), infrared
spectroscopy (IR), dynamic light scattering (DLS), porosity
evaluation by N2 adsorption, and scanning electron microscopy
(SEM) (see the Supporting Information, Chapter S2, Figures
S1−S4). PXRD (see Figure 2a) confirms that the unfunctionalized NMOF 1 exhibits the structure of bulk UiO-66 and that
the structure is retained during the azide functionalization of
the particle exterior of NMOF 2. Expectedly, the IR spectra of
the NMOF 1 and 2 and ADA (see Figure 2b) show that
NMOF 2 reveals additional peaks for the C−H stretching
vibrations at 2920 and 2863 cm−1 and an azide-stretching
vibration (2111 cm−1) in contrast to NMOF 1. These
characteristic stretching vibrations also occur in the spectrum
of ADA, which corroborates the successful functionalization.
Moreover, by the reduction in the intensity of the OH
bending vibration of the free carboxylic acid at the exterior
surface at 1664 cm−1, comparing the spectra of before and after
alkylazide functionalization, we can deduce that NMOF 1
reacted with ADA forming the amide linkage. Standard
nitrogen adsorption studies [Brunauer-Emmett-Teller (BET)
analysis] show the expected moderate reduction of the BET
surface area from 918 m2/g (NMOF 1) to 777 m2/g (NMOF
2) (see Figure 2c). This reduction in the mass specific surface
area by 15% matches nicely with the amount of alkylazide
functionalization covalently attached to the NMOF particles
(for calculations, see the Supporting Information, Chapter S5).
DLS was used to determine the hydrodynamic diameter of
the NMOF particles in solution. Directly after the synthesis,
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Figure 2. Characterization of UiO-66(NP) (NMOF 1, blue curves) and the alkylazide-functionalized N3-UiO-66(NP) (NMOF 2, green curves).
(a) Powder X-ray diffractograms of NMOF 1 and 2. The reference pattern was calculated with the program mercury from the single crystal
structure of UiO-66 (CCDC 733458). (b) Infrared spectrum of NMOF 1, NMOF 2, and ADA (yellow) recorded after activation in an argon
atmosphere (full range spectrum can be found in the Supporting Information, Chapter S3, Figure S5). (c) N2 adsorption isotherms of NMOF 1
and 2 after activation in vacuo for 100 °C. (d) DLS spectra (number distribution) of NMOF 1 and 2 measured 4 weeks after their synthesis in
ethanol. DLS spectra of the freshly synthesized samples can be found in the Supporting Information, Chapter S4, Figures S6 and S7.
Figure 3. Cyanosilylation of benzaldehyde catalyzed by NMOF 1 and 2. (a) Time−yield plot of the cyanohydrin (2-phenyl-2-((trimethylsilyl)oxy)
acetonitrile) produced by cyanosilylation of benzaldehyde and 2.0 equiv trimethylsilyl cyanide (TMSCN) in toluene catalyzed by 3 mol % Zr of
NMOF 1 (blue), NMOF 2 (green), UiO-66-HCl (orange), UiO-66-10TFA (red), and without any catalyst (blank, black). (b) DLS spectra
(number distribution) of NMOF 1 and 2 recorded before catalysis (0 h) and after 1 h and after 8 h of catalysis.
Substrate Anchoring and Immobilization of NMOFs 1
and 2. To prevent the deactivation of the catalyst by
aggregation in colloidal solution, we chemically immobilized
(surface anchoring, SA) the nanoparticles on different
substrates such as silicon and PDMS. Therefore, we used
two different approaches such as amide bond chemistry (SANMOF 3) and click chemistry (SA-NMOF 4) (see Figure 4).
In the first step, PDMS or silicon substrates were reacted
with different silanes (3-aminopropyl)triethoxysilane (APTES)
or (3-chloroproyl)trimethoxysilane (CPTES) to functionalize
the substrate surface. The Cl-terminated substrates were
additionally treated with sodium acetylide at 130 °C for 5 h
to generate free alkyne groups for click chemistry. The
successful functionalization of the PDMS substrates was
confirmed by contact angle measurements, IR spectroscopy,
and staining experiments (see the Supporting Information,
Chapter S11−S13, Figures S17−S26). In the second step, the
PDMS substrates were treated with NMOF 1 or NMOF 2 in
order to generate different SA-NMOF composites 1/2@
PDMS. In the first approach to yield (SA-NMOF 3), the
PDMS substrate was immersed in an ethanol dispersion of
NMOF 1, N,N-diisopropylcarbodiimide and N-hydroxysuccinimide. For the second approach (SA-NMOF 4), NMOF 2,
CuSO4, and ascorbic acid (to generate the Cu(I) catalyst for
click chemistry) were dispersed in ethanol, and the PDMS
substrates were added.
SEM images were recorded for SA-NMOF 3 and 4 (see
Figure 5). Additional images can be found in the Supporting
Information (see Chapter S14, Figures S27−S30). The images
confirm a dense coating of the substrate surface with NMOF 1
and 2. The nonideal dispersion of nanocrystallites on the
surface is not fully elucidated but presumably stems from the
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Figure 4. Surface anchoring and immobilization of NMOF 1 and 2 to PDMS. In the first step, the PDMS surface is functionalized to yield either
amine (−NH2) or alkyne (−CCH) surface functionalization. In the second step, these PDMS substrates are treated with NMOF 1 and 2 to yield
the samples NMOF 1@PDMS (SA-NMOF 3) and NMOF 2@PDMS (SA-NMOF 4).
successful, yielding a dense structure with high surface
coverage.
Catalytic Tests with Surface-Anchored NMOFs 3 and
4. After a short activation for 30 min at 80 °C under air in an
oven, the SA-NMOF 3 and 4 were immersed into a mixture of
liquid benzaldehyde and trimethylsilyl cyanide in 4 mL of
toluene as the solvent. The time−yield plots, as shown in
Figure 6a, reveal a remarkably high catalytic activity of these
samples. After the catalytic reaction (100% conversion, 1.5 h),
the samples were washed with copious amounts of toluene and
ethanol and then placed in an oven at 80 °C for 12 h and the
catalysis experiment was repeated (see Figure 6b).
The hot filtration test and the unchanged catalytic
performance in the second cycle point to negligible leaching
of the catalyst species into the solution. Additionally, SEM
images after the catalysis (see the Supporting Information,
Chapter S16, Figures S31 and S32) show that the microstructure of the composite is retained.
In order to allow comparison of the catalyst performance
with the literature-known catalysts, we calculated the initial
reaction rates and turnover frequencies (TOFs) for the
investigated systems (see Table 1 and the Supporting
Information, Chapter S17).
Figure 5. SEM images recorded for NMOF 1@Si (100) (a) and
NMOF 2@Si (100) (b).
silane self-condensation during functionalization.28 Surface
microscopic imaging such as atomic force microscopy and
SEM proved difficult and nonspecific for the PDMS substrates.
More significant data were obtained for the silicon substrates.
Inductively coupled plasma mass spectrometry measurements were performed to determine the amount of Zr
deposited on the roughly 1 cm2 PDMS substrates. The exact
values can be found in the Supporting Information, Chapter
S15. The obtained data are 4.05 ± 0.53 nmol for SA-NMOF 3
and 4.09 ± 0.51 nmol for SA-NMOF 4 (Supporting
Information, Chapter S15). In summary, the immobilization
of both NMOFs (1, 2) on PDMS or silicon substrates was
Figure 6. Cyanosilylation of benzaldehyde catalyzed by SA-NMOF 3 and 4. The time−yield plot of cyanohydrin (2-phenyl-2((trimethylsilyl)oxy)acetonitrile) produced by cyanosilylation of benzaldehyde and 2.0 equiv TMSCN in toluene catalyzed by 1 cm2 of the
composites (a) SA-NMOF 3 (blue), SA-NMOF 4 (green), blank PDMS (gray), and without any catalyst (blank, black). (b) Additionally, the
time−yield plot under the same reaction conditions for the first (◆) and the second catalytic cycle (▲) of SA-NMOF 3 and 4 and for the hot
filtration of the reaction solution after 15 min (▼).
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Table 1. Calculated Initial Reaction Rate (r′) and TOFs for the Cyanosilylation of Benzaldehyde with 2.0 equiv TMSCN
Catalyzed by Colloidal Solutions of NMOF 1 and 2, SA-NMOF 3 and 4, and UiO-66-10TFA and UiO-66-HCl in Toluene
bulk UiO-66 samples
colloidal solution
UiO-66
-10TFA
r′ [mmol h−1]
TOF [h−1]
2.13 × 10−2
± 3.3 × 10−3
7.29 × 10−1
± 1.6 × 10−1
NMOF
-HCl
1.89 × 10−1
± 4.3 × 10−2
6.45 ± 1.87
1
2.87 × 10−1
± 1.3 × 10−1
9.79 ± 5.14
Table 2. Comparison of Previously Reported TOF Values
for the Cyanosilylation of Benzaldehydea,b
catalyst
equiv TMSCN
T/°C
TOF
2
2
2
2
1.5
2
1
40
40
40
40
80
25
25
1.96 × 107
2.82 × 107
9.6
1.02
2158
990
582
ref
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29
30
31
1.97 × 10−1
± 3.9 × 10−2
6.74 ± 1.76
3
4
79.0 ± 5.3
116 ± 19
1.96 × 107
± 3.8 × 106
2.82 × 107
± 8.3 × 106
when compared to the well-established defect-engineered UiO66 (see the Supporting Information, Chapter S8, Figures S12
and 13). To further improve our knowledge of the prepared
catalysts, we additionally tested their catalytic activity
supported on silicon substrates (see the Supporting Information, Chapter S19, Figure S34). The catalysts (SA-NMOF 5
and 6) show an equally strong initial activity compared to SANMOF 3 and 4 but do not reach full conversion. Presumably,
this can be attributed to the degradation of TMSCN during
the reaction (see the Supporting Information, Chapter S19).
Full conversion can be observed by the addition of 10 equiv
TMSCN, instead of 2 equiv, ruling out catalyst de-activation
phenomena. Additionally, we prepared a UiO-66 thin film37 on
both Si and PDMS and supported the UiO-66-HCl particles
on PDMS and Si substrates. As catalysts, UiO-66-HCl
particles, which were anchored to PDMS via the amide
approach showed a similar catalytic activity compared to SANMOF 3 (see the Supporting Information, Chapter S19).
When supported on Si, the activity was similar to SA-NMOF
5. In comparison, the catalytic activity of the synthesized UiO66 film was lower than that of the deposited particles on both
substrates (see the Supporting Information, Chapter S19,
Figure S35) as expected because of the higher diffusion barrier
in films as opposed to single particles.
To prove the versatility and general applicability of our
concept, we applied SA-NMOF 3 in a second model reaction,
the cycloaddition of carbon dioxide with propylene oxide,
which is one of the best studied reactions involving CO2.38−40
Time−yield plots (see Figure 7) show that 2 cm2 of SANMOF 3, corresponding to a catalyst mass loading of 2.25 μg
of UiO-66 (8.08 ng Zr), is more active than 43 mg of UiO-66
powder synthesized without and with 10 equiv trifluoroacetic
acid as a modulator.
The investigated composite systems SA-NMOF 3 and 4
show remarkably high TOFs in the range of 20,000,000 h−1.
To put this into perspective, the industrial benchmark for
catalysts is usually at least in the medium to high ten thousands
or low hundred thousands for selected processes.32,33
However, MOF catalysts typically have a significant lower
TOFs, with most compounds ranging up to hundreds and
sometimes low thousands.34−36 For this specific reaction, we
compared our values with those reported in the literature for
MOFs (Table 2, a full list of the literature, see the Supporting
SA-NMOF 3
SA-NMOF 4
UiO-66-HCl
UiO-66-10TFA
InPF-15
Mg(ABTC)(DMI)b
InPF-16
SA-NMOF
2
work
work
work
work
a
A full overview of the literature can be found in the Supporting
Information, Chapter S25. b3,3′,5,5′-azobenzene-tetracarboxylate
(ABTC4−), 1,3-dimethy-2-imidazolidinone (DMI).
Information, Chapter 26). The TOFs that we measured are
four orders of magnitude higher than those that were
previously reported. We attribute the observed enhancement
to two different but synergetic factors: the downsizing effect
and the prevention of particle aggregation during the catalysis.
Another benefit of the SA-NMOFs is the retained high activity
in different solvents such as DCM, DMF, and toluene (see the
Supporting Information, Chapter S18, Figure S33) especially
Figure 7. Cycloaddition of CO2 with propylene oxide catalyzed by SA-NMOF 3 and 4. Time−yield plot of propylene carbonate formed by the
insertion of CO2 into propylene oxide. For all drawn through lines, the general conditions of p(CO2) = 1 bar, 31.05 mmol propylene oxide, and
2.49 mmol Bu4NBr (as a cocatalyst) were applied. The reaction was catalyzed by either 2 cm2 of SA-NMOF 3 (blue), 0.5 mol % Zr of UiO-66-HCl
(orange), 0.5 mol % Zr of UiO-66-10TFA (red) applied as microcrystalline powders, or without any catalyst (black). The dotted line was recorded
with p(CO2) = 3 bar, T = 100 °C, and 2 cm2 of SA-NMOF 3 (blue, dotted) as a catalyst. The respective blank text without any catalyst is shown as
the continuous blue curve. The other substance amounts were kept constant.
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Table 3. Comparison of previously reported TOF values for the cycloaddition of CO2 to POa
catalyst
p(CO2)/bar
T/°C
TOF
SA-NMOF 3
SA-NMOF 3
UiO-66-HCl
UiO-66-10TFA
Int-MOF-5
IL-[In2(dpa)3(1,10phen)2]b
DMAP-Zr-BDC-MOF
3
1
1
1
1
100 W2
10
100
50
50
50
50
12
50
7.74 × 106
1.24 × 106
5.2
6.4
5400
3100
1095
ref
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41
42
43
work
work
work
work
a
A full overview of the literature can be found in the Supporting Information, Chapter S27. bmicrowave assisted; 1,10-phenanthroline (phen),
diphenic acid (H2dpa), and 4-dimethylamino-pyridine (DMAP).
(21 mg, 0.066 mmol) and benzene dicarboxylic acid (50 mg,
0.30 mmol, 4.5 equiv) and were dissolved by sonication in 3
and 1 mL of DMF, respectively. Afterward, the H2bdc and
ZrOCl2 solutions were mixed and 0.35 mL of acetic acid was
added. The solution was heated to 90 °C for 12 h. The
resulting colloidal solution was washed three times with 4 mL
of DMF and then three times with 4 mL of ethanol each. In
between the washing steps, a centrifuge was used to separate
the particles from the solution, yielding a colloidal solution of
NMOF 1 in ethanol.
Synthesis of NMOF 2. NMOF 1 ethanol solution (4 mL)
was mixed with 18 mg of 12-azidododecane-1-amine (0.08
mmol), 22.3 μL of N,N-diisopropylcarbodiimide (18 mg, 0.14
mmol), and catalytic amounts of N-hydroxysuccinimide. The
reaction mixture was stirred at room temperature for 4 h and
subsequently washed two times with 4 mL of ethanol and then
two times with 4 mL of toluene each.
Surface Anchoring NMOF 1 and 2 and Fabrication of
SA-NMOF 3 and 4. Two different anchoring methods based
on amide and click chemistry (triazol formation) were
employed resulting in four kinds of samples depending on
the substrate (PDMS or silicon) or the anchoring chemistry.
The samples are denoted as NMOF 1@PDMS (SA-NMOF
3), NMOF 2@PDMS (SA-NMOF 4), NMOF 1@Si (SANMOF 5), and NMOF 2@Si (SA-NMOF 6). For the amide
bond formation (SA-NMOF 3 and SA-NMOF 5), amineterminated substrates (PDMS or silicon) were placed in a
reaction mixture consisting of 3 mL of colloidal NMOF 1
solution in EtOH, 1 mL of additional EtOH, 22.3 μL of N,Ndiisopropylcarbodiimide (18 mg, 0.14 mmol), and catalytic
amounts of N-hydroxysuccinimide for 4 h at room temperature. Afterward, the substrates were washed thoroughly with
EtOH and were sonicated for 10 min in EtOH. For the triazol
formation via click chemistry, SA-NMOF 4, 5 mg of CuSO4,
20 mg of ascorbic acid, and 2 mL of the azide-functionalized
NMOF 2 colloidal solution in EtOH were mixed with
additional 1 mL of ethanol. The alkyne-terminated substrates
were placed in this solution for 24 h at room temperature.
Afterward, the substrates were washed with EtOH and
sonicated in EtOH for 10 min. For the synthesis of NMOF
2@Si(SA-NMOF 6), a slightly different reaction procedure
was applied that can be found in the Supporting Information,
Chapter S23.
Catalysis: Cyanosilylation of Benzaldehyde to 2Phenyl-2-((trimethylsilyl)oxy)-acetonitrile. The catalyst
(either 8.2 mg of NMOF 1 or 2, UiO-66-HCl or UiO-6610TFA, or 1 cm2 of SA-NMOF 3 or SA-NMOF 4) is
immersed in a solution of benzaldehyde (100 μL, 104 mg, 980
μmol, 1.00 equiv), dodecane (223 μL, 167 mg, 980 μmol, 1.00
equiv), trimethylsilyl cyanide (248 μL, 197 mg, 1.98 mmol,
By increasing both the CO2 pressure (3 bar) and the
temperature (100 °C), full conversion was reached after 24 h
with SA-NMOF 3 as the catalyst. In analogy to the
cyanosilylation, we observe extraordinarily high TOFs of 1.24
× 106 ± 5.16 × 105 h−1 (p(CO2) = 1 bar, 50 °C) and 7.74 ×
106 ± 1.01 × 106 h−1, (p(CO2) = 3 bar, 100 °C). All calculated
reaction rates and TOFs and also a variation of the catalyst
amount can be found in the Supporting Information (see
Chapter S20 and S21). Additionally, we compared the
obtained TOF values with those reported in the literature
(see Table 3, a full overview can be found in the Supporting
Information, Chapter S26). The measured TOF values are two
orders of magnitude higher than the ones reported in the
literature.
■
EXPERIMENTAL SECTION
Molding of PDMS. The Sylgard 184 silicone elastomer
base (15 g) and Sylgard 184 silicone elastomer curing agent
(1.5 g, 10:1 wt %) are thoroughly mixed for 3 min. Afterward,
the mixture is poured into a Petri dish and degassed in a
desiccator under moderate vacuum. For the curing procedure,
the dish is placed in an oven for 1 h at 70 °C. After cooling to
room temperature, PDMS was cut into 1 cm × 1 cm pieces.
Functionalization of PDMS Substrates. Prior to
functionalization, PDMS substrates are cleaned and etched
with a solution of H2O/H2O2/HCl (5:1:1 vol%) for 30 min.
Afterward, the substrates were washed with de-ionized water
and dried with pressurized air. Amino-terminated surfaces were
fabricated by immersion of silicon substrates in neat (3aminopropyl)-triethoxysilane for 30 min at room temperature.
Then the samples are washed with copious amounts of ethanol
and de-ionized water and placed in de-ionized water overnight.
Alkyne-terminated substrates were synthesized in a two-step
procedure, creating a chlorine-terminated surface in the first
step and replacing the chlorine in a second step by an alkyne
group. In the first step, after activation, the PDMS substrates
are immersed in neat (3-chloropropyl)triethoxysilane for 30
min at room temperature, followed by a thorough rinse of the
substrate with ethanol and de-ionized water. In the second
step, the chlorine-functionalized substrates are immersed in an
18 wt % slurry of sodium acetylene in xylene/mineral oil for 5
h at 150 °C. The cooled substrates are washed with a mixture
of tetrahydrofuran (THF) and methanol (MeOH) and
sonicated in MeOH for 10 min.
Synthesis of 1-Azido-dodecan-1-amine (ADA). 1Azido-dodecan-1-amine was synthesized from 1,12-dibromdodecane. The synthesis procedure can be found in the
Supporting Information, Chapter S22.
Synthesis of NMOF 1. The synthesis was performed
according to Mirkin et al.26 zirconium oxychloride octahydrate
3208
https://dx.doi.org/10.1021/acscatal.0c00550
ACS Catal. 2020, 10, 3203−3211
ACS Catalysis
pubs.acs.org/acscatalysis
Center, Technical University of Munich, D-85748 Garching,
Germany; orcid.org/0000-0001-7087-932X
Philip M. Stanley − Chair of Inorganic and Metal-Organic
Chemistry, Department of Chemistry and Catalysis Research
Center, Technical University of Munich, D-85748 Garching,
Germany
Alexander Urstoeger − Division of Analytical Chemistry,
Department of Chemistry, Technical University of Munich,
85787 Garching, Germany
Michael Schuster − Division of Analytical Chemistry,
Department of Chemistry, Technical University of Munich,
85787 Garching, Germany
Mirza Cokoja − Chair of Inorganic and Metal-Organic
Chemistry, Department of Chemistry and Catalysis Research
Center, Technical University of Munich, D-85748 Garching,
Germany; orcid.org/0000-0003-3144-4678
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c00550
2.02 equiv), and 4 mL of toluene. The resulting mixture is
stirred at 40 °C and 500 rpm. After certain time intervals, an
aliquot of 50 μL is taken, diluted with the solvent (1.5 mL),
centrifuged for 30 min, and analyzed via GC. Further
information about the analysis of the GC data can be found
in the Supporting Information, Chapter S6. The catalyst
samples SA-NMOF 3 or SA-NMOF 4 were placed in an oven
at 80 °C under air for 30 min prior to the catalysis.
Cycloaddition of CO2 with Propylene Oxide To Yield
4-Methyl-1,3-dioxolan-2-one (Propylene Carbonate).
The catalyst (either 43 mg of UiO-66-HCl or UiO-6610TFA or 2 cm2 of SA-NMOF 3) is immersed in propylene
oxide (2.1 mL, 1.804 g, 31.06 mmol, 1.00 equiv) and Bu4NBr
as the cocatalyst (805 mg, 2.49 mmol, 0.08 equiv) and
pressurized with 1 bar (3 bar) CO2 in a Fischer−Porter bottle.
The resulting mixture is stirred at 50 °C and 500 rpm. After
defined time intervals, aliquots of 0.05 mL are taken, diluted
with the DCCl3, and analyzed via 1H NMR. Further
information about the analysis of the 1H NMR data can be
found in the Supporting Information, Chapter S6. The catalyst
sample 3 was placed in an oven at 100 °C under air for 12 h
prior to the reaction.
Author Contributions
§
A.L.S. and P.M.S. contributed equally.
■
Notes
The authors declare no competing financial interest.
■
CONCLUSIONS
Surface anchoring and immobilization of nanosized UiO-66
crystallite particles result in a drastic enhancement of the
heterogeneous Lewis-acid catalyst performance as indicated by
superior TOFs up to 2.82 × 107 h−1 (cyanosilylation of
benzaldehyde) to 7.74 × 106 h−1 (CO2 cycloaddition with
propylene oxide) as compared to both microcrystalline powder
and colloidal dispersions of nanoparticles. Combining our
findings with related results of Tang et al.44 and Fischer et al.45
implies that the concept of NMOF immobilization on
substrates may offer a new strategy to overcome drawbacks
in the utilization of MOFs as molecular precisely defined, but
heterogeneous, catalysts.46 Recently, we are exploring the
integration of SA-NMOF catalysts with microfluidic devices.
■
ACKNOWLEDGMENTS
A.L.S. and P.M.S. thank the Chemical Industry Fonds (FCI)
for a PhD fellowship. This work was supported by the German
Research Foundation (DFG) Priority Program 1928 “Coordination Networks: Building Blocks for Functional Systems”. We
are grateful for the help of Katja Rodewald for measuring SEM
pictures. Additionally, we would like to thank Prof. Dr. J.
Buchner (TU Munich) for allowing us access to his
fluorescence imager. The support by the Central Analytics
Facility of the TUM Catalysis Research Center, especially from
Jürgen Kudermann, is gratefully acknowledged.
■
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ASSOCIATED CONTENT
sı Supporting Information
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c00550.
■
Research Article
Further details regarding surface and material characterization and catalysis experiments; detailed instruction
about the performed calculations; general information
about the instrumentation; comprehensive comparison
of the catalysis performances of MOFs in the
cyanosilylation of benzaldehyde and CO2 cycloaddition
to propylene oxide reported to date (PDF)
AUTHOR INFORMATION
Corresponding Author
Roland A. Fischer − Chair of Inorganic and Metal-Organic
Chemistry, Department of Chemistry and Catalysis Research
Center, Technical University of Munich, D-85748 Garching,
Germany; orcid.org/0000-0002-7532-5286; Phone: +4989-289-13081; Email: roland.fischer@tum.de; Fax: +49-89289-13194
Authors
A. Lisa Semrau − Chair of Inorganic and Metal-Organic
Chemistry, Department of Chemistry and Catalysis Research
3209
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