Enhanced catalytic activity of a hierarchical porous metal-organic framework CuBTC

Article abstract of DOI:10.1039/c5ce00938c

A previously reported mixed ligand strategy was used to synthesize a classic metal-organic framework CuBTC, which exhibited a hierarchical porous structure including micropores and around 3.9 nm. Thanks to the facile mass transfer and denser open metal si

Full text of DOI:10.1039/c5ce00938c

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DOI: 10.1039/C5CE00938C
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Enhanced Catalytic Activity of a Hierarchical Porous Metal-Organic
Framework CuBTC
Received 00th January 20xx,
Accepted 00th January 20xx
Zhigang Hu, Yongwu Peng, Kai Min Tan, and Dan Zhao*
A previously reported mixed ligand strategy was used to synthesize a classic metal-organic framework CuBTC, which
exhibited a hierarchical porous structure including micropores and mesopores of around 3.9 nm. Thanks to facile mass
transfer and denser open metal sites induced by mixed ligand strategy, the hierarchical porous CuBTC demonstrates
enhanced catalytic activity towards both ring-opening reaction of styrene oxide to 2-methoxy-2-phenylethanol and
cyanosilylation reaction of benzaldehyde to cyanohydrins.
DOI: 10.1039/x0xx00000x
www.rsc.org/
strategy was used by Barin et al. in creating defects in CuBTC.45
In their study, increased pore volume and surface area were
observed in CuBTC upon fragment ligand incorporation, but
the pore size remained more or less unchanged. Marx et al.
used the mixed ligand strategy to introduce catalytically active
functional groups into CuBTC.46 In another study done by Fang
et al., extra open Cu sites as well as hierarchical porosity
including micropores and mesopores were introduced into
CuBTC frameworks by mixing BTC ligand with defective
linkers.47
In view of the above results, we herein report a facile
synthesis of hierarchical porous CuBTC (Scheme 1), which was
achieved similarly by introducing an asymmetric ligand
(isophthalic acid, IPA) but with a different synthetic condition
compared to the work of Barin et al.45 The resultant
hierarchical porous CuBTC exhibited microporous structure as
well as mesoporous texture with pore sizes as large as 3.9 nm.
In addition, the effect of hierarchical porosity on catalytic
performance was studied. Compared to pristine CuBTC, the
hierarchical porous one demonstrates a much higher catalytic
activity towards both ring opening reaction of styrene oxide to
2-methoxy-2-phenylethanol and cyanosilylation reaction of
benzaldehyde to cyanohydrins, which can be attributed to
facilitated mass transfer in mesopores and denser Lewis acid
sites.
Introduction
Metal-organic frameworks (MOFs), also known as porous
coordination polymers (PCPs), have become one of the most
exciting porous crystalline materials nowadays.1-3 Possessing
tunable porosity and rich chemical functionalities, MOFs have
been widely explored in applications such as storage,4,
5
separation,6, 7 sensing,8-10 catalysis,11, 12 etc. To date, most of
the reported MOFs are microporous, with pore sizes less than
2 nm.13 The relatively small pore size of MOFs has limited their
applications in heterogeneous catalysis, where mesopores
(pore size between 2 and 50 nm) are normally preferred for
increased mass transfer.14 For example, CuBTC (aka HKUST-1)
is a classic MOF composed of benzenetricarboxylate (BTC)
ligand with dicopper paddlewheel secondary building units
(SBUs) that has been extensively studied due to its facile
synthesis and potential commercialization.15-17 The solvent
molecules binding to the SBUs of CuBTC can be removed
affording open metal sites that can serve as hard Lewis acid for
heterogeneous catalysis.18-23 However, its catalytic ability
toward large substrates is greatly limited by its microporous
structure with window openings of only ca. 6 Å in diameter
that prevent the diffusion of large substrates. Several
approaches have been reported to introduce mesoporosity
into MOFs,24-26 such as crystal engineering,27-30 ligand
extension,31-33 template-assisted synthesis,34-38 post-synthetic
modification,39,
mechanochemical synthesis,41 etc. In
40
principle, the relatively week coordination bonds in MOFs can
be twisted, broken, and reconstructed leading to MOFs of
interesting and useful properties.42 Recently, Zhou et al.
reported a truncated ligand strategy by mixing symmetric
ligands with reduced symmetric ligands to synthesize
44
isostructural MOFs with enlarged pore size.43,
A similar
Scheme 1. Synthetic scheme of hierarchical porous CuBTC
using truncated mixed ligand strategy.
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CuBTC (15 mg) were carried out at a 1:2 molar ratio of
benzaldehyde (0.3 mmol, 30 μl) and trimethylsilyl cyanide
Experimental Section
Materials and methods
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DOI: 10.1039/C5CE00938C
(TMSCN, 0.6 mmol, 80 μl) in dichloromethane (3 mL) at room
temperature and 40 °C, respectively. A certain amount of the
reaction media (0.5 mL) was taken after 12, 24, 36, and 48 h of
reaction time and filtered, respectively. The filtrates were left
dry and analyzed by NMR.
All of the reagents used were obtained from commercial
suppliers and used without further purification. Powder X-ray
diffraction (PXRD) patterns were obtained on a Bruker D8
Advance X-ray powder diffractometer equipped with a Cu
sealed tube (λ = 1.54178 Å) at a scan rate of 0.02 deg s-1. NMR
data were collected on a Bruker Avance 500 MHz NMR
spectrometer (DRX500). Field-emission scanning electron
microscope (FE-SEM) analyses were conducted on an FEI
Quanta 600 SEM (20 kV) equipped with an energy dispersive
spectrometer (EDS, Oxford Instruments, 80 mm2 detector).
Samples were treated via Pt sputtering before observation.
Synthetic procedures
Results and Discussion
Structural characterization
(a)
HP-CuBTC-0.10
HP-CuBTC-0.25
The hierarchical porous CuBTC (referred to as HP-CuBTC)
samples were synthesized by revising a reported method.45
Briefly, mixtures of benzene-tricarboxylic acid (BTC, 100 mg,
0.51 mmol) and copper nitrate trihydrate [Cu(NO3)2·3H2O, 200
mg, 0.82 mmol] together with different ratios of isophthalic
acid (IPA, 0.5, 1, 2, and 4 equivalent mole of BTC, namely 40,
80, 160, and 320 mg) were dissolved in 20 ml of mixed solvents
containing dimethylformamide (DMF)/ethanol/water (1:1:1
v/v/v). The solutions were sonicated for 30 min and then
heated at 85 °C for 20 h. The solid products were recovered by
centrifuge, washed with DMF for three times, and then soaked
in DMF at 80 °C for 24 h to completely remove any un-reacted
ligands trapped inside the frameworks. The samples then
underwent solvent exchange with methanol for three days,
and were fully activated at 120 °C under vacuum for 24 h.
Gas sorption measurements
HP-CuBTC-0.11
HP-CuBTC-0.08
CuBTC
CuBTC, simulated
4
8
12 16 20 24 28 32 36 40
2  / degree
(b)
H1
HP-CuBTC-0.10
H2
H3
H4
H1
HP-CuBTC-0.25
H3
H2
H4
Gas sorption isotherms were measured up to 1 bar using a
Micromeritics ASAP 2020 surface area and pore size analyzer.
Before the measurements, samples (ca. 100 mg) were
degassed under reduced pressure (< 10-2 Pa) at 150 °C for 12 h.
UHP grade He and N2 were used for all the measurements. Oil-
free vacuum pumps and oil-free pressure regulators were used
to prevent contamination of the samples during degassing
process and isotherm measurements. The temperature of 77 K
was maintained with a liquid nitrogen bath. Pore size
distribution data were calculated from the N2 sorption
isotherms at 77 K based on non-local density functional theory
(NLDFT) model (assuming slit pore geometry) and Barrett-
Joyner-Halenda (BJH) desorption model in the Micromeritics
ASAP 2020 software package.
H1
HP-CuBTC-0.11
H3
H2
H4
H1
H1
HP-CuBTC-0.08
H3
H2
H4
CuBTC
H1
BTC+IPA
H3
H2
H4
8.4 8.2 8.0 7.8 7.6 7.4 7.2 7.0
 / ppm
Figure 1. (a) PXRD patterns of simulated CuBTC, as-synthesized
CuBTC, and HP-CuBTC samples obtained through mixed ligand
strategy; (b) H-NMR spectra of mixed ligands, CuBTC, and HP-
CuBTC samples digested in D2O/NaOH.
1
Catalytic reactions
Ring-opening reactions of styrene oxide with methanol
catalyzed by either pristine CuBTC or HP-CuBTC were
performed in closed vials heated at 40 °C under stirring. In
specific, styrene oxide (150 mg, 1.25 mmol) was dissolved in
methanol (5 ml), toward which 50 mg of activated MOFs
(either pristine CuBTC or HP-CuBTC) was added. A certain
amount of the reaction media (0.5 mL) was taken after 1, 2,
and 3 h of reaction time and filtered, respectively. The filtrates
were left dry and analyzed by NMR. Cyanosilylation reactions
of benzaldehyde catalyzed with either pristine CuBTC or HP-
It is possible that the solvothermal reaction between
copper nitrate and a mixed ligand of BTC and IPA can lead to
phase impurities in resultant CuBTC because IPA alone can
react with copper salt to give MOF products.48 This possibility
was ruled out in this study by checking the PXRD patterns of
obtained HP-CuBTC samples. As can be seen from Figure 1a, all
the HP-CuBTC samples exhibit similar PXRD patterns identical
to that of pristine CuBTC and the one simulated from single
crystal model of CuBTC, indicating that the resultant HP-CuBTC
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samples have a framework connection identical to that of H2-type hysteresis between adsorption andViewdeArstioclrepOtniolinne
pristine CuBTC. This is probably because of the geometry branches at P/P0 > 0.45 (Type IV).13 TDyOpeI: 10IV.10i3s9o/tCh5eCrEm00s93a8rCe
similarity between IPA and BTC so that the overall CuBTC normally associated with mesoporosity confirming the
framework topology can still be maintained even after increased pore size caused by IPA incorporation, and H2-type
incorporation of IPA moieties.43 FE-SEM images also revealed hysteresis indicates a presence of “bottleneck” pores with
that HP-CuBTC crystals have a similar octahedral shape to that large cages accessible via small openings.13 Indeed, theoretical
of pristine CuBTC (Figure 2).
study has predicted similar isotherms for hierarchical porous
The successful incorporation of IPA into HP-CuBTC was CuBTC.53
further confirmed by NMR spectra of digested MOFs (Figure
1b). The incorporated amount of IPA can be adjusted by
applying different feed molar ratios of IPA to BTC. According to
the integration of assigned peaks, the incorporated ratios are
(a)
400
0.08, 0.11, 0.25, and 0.10, for feed ratios of 0.5, 1, 2, and 4,
respectively. Therefore, the resultant MOFs were named as
HP-CuBTC_0.08, HP-CuBTC_0.11, HP-CuBTC_0.25, and HP-
CuBTC_0.10, respectively. Surprisingly, the incorporated molar
ratio of IPA didn’t increase linearly with the increase of feed
molar ratio, and there is a maximal incorporated ratio of 0.25
at a feed molar ratio of 2. A reduced incorporated ratio at
higher feed ratio may be attributed to the role of crystal
growth inhibitor played by IPA at high concentrations.49, 50 It is
worth noting that the NMR signal peaks from BTC in the
digested MOF samples are split, possibly due to the different
chemical environment of neutralized carboxylate moieties.
300
200
100
0
400
300
200
100
0
CuBTC
HP-CuBTC-0.08
HP-CuBTC-0.11
HP-CuBTC-0.25
HP-CuBTC-0.10
1E-61E-51E-41E-30.01 0.1
Relative Pressure / P P0-1
1
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure / P P0-1
(b)
700
600
500
400
300
200
100
0
2.5
HP-CuBTC-0.10
HP-CuBTC-0.25
HP-CuBTC-0.11
HP-CuBTC-0.08
CuBTC
2.0
1.5
1.0
0.5
0.0
20
30
40
50
60
Pore Width / Å
0
10
20
30
40
50
60
Pore Width / Å
Figure 3. (a) N2 sorption isotherms of CuBTC and HP-CuBTC. Closed
and open symbols represent adsorption and desorption,
respectively. The BET surface areas are also provided; (b) Pore size
distribution data of CuBTC and HP-CuBTC calculated using NLDFT
method (imbedded: pore size distribution data calculated using BJH
desorption model).
Figure 2. FE-SEM images of (a) CuBTC, (b) HP-CuBTC-0.08, (c)
HP-CuBTC-0.11, and (d) HP-CuBTC-0.25. Scale bars represent
10 μm.
Surface area and pore size distribution
N2 sorption data collected at 77 K and a pressure up to 1
bar were adopted to evaluate the surface areas of these
samples using Brunauer–Emmett–Teller (BET) model (Figure
3a, Table 1). The pristine CuBTC synthesized in this study has a
Type I isotherm indicating its microporous structure, with a
BET specific surface area of 1690 m2 g-1 which is comparable to
the reported values.51, 52 With the increase of IPA incorporated
into the frameworks, the isotherm of the products exhibited
an increasingly distinct hybrid Type I/IV behavior featured by
sharp increase of gas uptake at low pressures (Type I) and a
In order to further confirm the hierarchical porosity, both
NLDFT and BJH desorption models were applied to calculate
the pore size distribution. As can be seen from the result of
NLDFT calculation (Figure 3b), the pore size distribution of
CuBTC is featured by two sizes of 8.6 Å (major) and 6.8 Å
(minor), which matches quite well with the crystal structure. It
is interesting to note that HP-CuBTC samples exhibit smaller
pore sizes of ca. 5.7 Å (major) and ca. 8.0 Å (minor), possibly
because of the introduction of truncated IPA moieties into the
frameworks that make some of the pocket (e.g. the closed
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tetrahedron defined by four BTC moieties in CuBTC adopted herein for convenient compare with tVhieew AlirttieclreaOtnulirnee
58
framework) accessible to probe gas molecules. BJH desorption value.18,
Incubation of benzaldehyde and TMSCN with
DOI: 10.1039/C5CE00938C
models reveal that mesopores with pore size of ca. 3.9 nm pristine CuBTC as the catalyst at 313 K for 48 h led to a
were readily developed in HP-CuBTC samples, confirming their conversion of 56.4% (Figure 4b), which is similar to the
58
hierarchical porous structure containing both micropores and reported value of ca. 50%.18,
However, this reaction is
mesopores. Micropores are intrinsic for CuBTC-like crystal relatively sluggish, with a conversion of only 8.4% within the
structure, while mesopores may come from ligand first 12 h of reaction time. When HP-CuBTC-0.25 was used as
truncation43, 44 and/or missing linker defects.47 Considering the the catalyst, a substantially increased conversion of 45.6% was
identical PXRD patterns of HP-CuBTC to pristine CuBTC, the observed under the same reaction time, indicating much faster
mesopores in HP-CuBTC should be randomly distributed within reaction kinetics. After 48 h, 64.2 % of benzaldehyde was
the frameworks instead of being oriented in an ordered way.36, converted to cyanohydrins catalyzed by HP-CuBTC-0.25, which
54
is also higher than the reaction catalyzed by pristine CuBTC
(56.4 %). However, we have to point out that the MOF
Table 1. Summary of BET surface area (BET S.A.), micropore catalysts in this study survived after 36 h of reaction time, but
volume (micro. vol.) and mesopore volume (meso. vol.) of completely dissolved after 48 h indicating their instability
CuBTC and HP-CuBTC MOFs
MOFs
CuBTC
under this reaction condition.18 Similar finding was reported by
Opanasenko et al.21, 22 In order to evaluate MOFs’ catalytic
BET S.A.a)
1690
970
Micro. Vol.b)
0.55
0.39
0.39
0.30
0.39
Meso. Vol.c)
0.01
0.03
0.04
0.07
0.02
performance under
reactions were carried out at a lower temperature of 298 K.
Once again, HP-CuBTC-0.25 exhibited much higher
a milder condition, similar catalytic
HP-CuBTC-0.08
HP-CuBTC-0.11
HP-CuBTC-0.25
HP-CuBTC-0.10
a
990
930
1000
conversion (34.5%) than that of pristine CuBTC (10.8%) after
20 h of reaction time.
a) m2 g-1; b) dpore < 20 Å, cm3 g-1, NLDFT; c) 20 Å < dpore < 1000 Å,
cm3 g-1, NLDFT.
(a)40
Catalytic performance study
Larger pore sizes are supposed to facilitate the mass
30
transfer of substrates within porous structures which is highly
55
beneficial for heterogeneous catalysis.14,
Theoretical
calculation also reveals that molecular diffusion in mesopores
is faster than in micropores of MOFs.53 Considering that HP-
CuBTC_0.25 has open copper sites as Lewis acid with
mesopores of about 3.9 nm, we chose two model reactions
that can be catalyzed by Lewis acid to study the impact of
hierarchical porosity on the catalytic performance of MOFs.
The first model reaction is ring-opening reaction of styrene
oxide with methanol to 2-methoxy-2-phenylethanol, with the
product being a very important organic solvent and reaction
intermediate in pharmaceutical industry.56 Reactions were
carried out at 40 °C according to the literature with either HP-
CuBTC-0.25 or pristine CuBTC as the catalyst.36 After 4 h of
reaction, HP-CuBTC-0.25 showed a much higher conversion
(35.6 %) than that of pristine CuBTC (1.97 %) (Figure 4a). This
result strongly demonstrates the benefits brought by
hierarchical porosity in heterogeneous catalysis. However, we
noticed that the crystallinity of HP-CuBTC-0.25 was retained
after first catalysis reaction but was lost during second
catalytic cycle. The reason is that CuBTC is an intrinsically
unstable MOF composed of Cu-O bonds that are susceptible to
the attack of guest molecules, which has been demonstrated
previously.18 The second model reaction we have attempted is
cyanosilylation of benzaldehyde into cyanohydrins. Reactions
were carried out at a 1:2 molar ratio of benzaldehyde to
trimethylsilyl cyanide (TMSCN) in dichloromethane (DCM, 3
20
HP-CuBTC-0.25
CuBTC
10
0
0
1
2
3
4
Reaction Time / h
(b)80
70
60
50
40
30
20
10
0
HP-CuBTC-313K
HP-CuBTC-298K
CuBTC-313K
CuBTC-298K
0
10
20
30
40
50
Reaction Time / h
Figure 4. (a) Ring-opening reaction of styrene oxide with
methanol to 2-methoxy-2-phenylethanol catalyzed by CuBTC
or HP-CuBTC-0.25 at 313 K; (b) cyanosilylation of benzaldehyde
into cyanohydrins catalyzed by CuBTC or HP-CuBTC-0.25 at 298
and 313 K, respectively.
mL) at either 298 K or 313 K according to the literature.57
A
loading of 5 mol% MOF catalyst and reaction time of 48 h was
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DOI: 10.1039/C5CE00938C
The above two model reactions have clearly indicated
enhanced catalytic activities of HP-CuBTC compared to pristine
CuBTC. One reason could be the mesopores introduced within
HP-CuBTC that facilitate the substrate transport and make it
easier to get access to catalytically active open Cu sites. In
addition, several studies have confirmed that extra open metal
sites can be generated through missing linker defects.47, 59, 60 It
is possible that the truncated ligand strategy used in this study
may generate extra open Cu sites which also help the catalysis.
Eventually, it might be a synergetic effect involving hierarchical
porosity and denser open metal sites that contributes to the
enhanced catalytic activities. Nevertheless, we have to point
out that due to a lack of quantitative measurement of
decomposition of Cu-MOFs and solid evidence to prove the
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reaction is completely heterogeneous,
a homogeneous
mechanism involving leached copper ions could also be
possible for the enhanced catalytic performance.
Conclusions
In summary, we have applied truncated mixed ligand
strategy to synthesize hierarchical porous CuBTC exhibiting
both micropores and mesopores up to 3.9 nm. Compared to
pristine CuBTC, HP-CuBTC-0.25 demonstrates a much better
catalytic performance towards both ring-opening reaction of
20.
21.
22.
styrene
oxide
to
2-methoxy-2-phenylethanol
and
cyanosilylation reaction of benzaldehyde to cyanohydrins. The
strategy demonstrated in this study may be applied to other
MOFs towards mesoporous MOFs for heterogeneous catalysis
applications.
23.
24.
25.
Acknowledgements
This work is supported by National University of Singapore
(NUS Start-up Funding R-279-000-369-133, CENGas R-261-508-
001-646) and Singapore Ministry of Education (MOE AcRF Tier
1 R-279-000-410-112, AcRF Tier 2 R-279-000-429-112).
26.
27.
28.
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DOI: 10.1039/C5CE00938C
A hierarchical porous metal-organic framework CuBTC with mesopores of 3.9 nm has been facilely
obtained as Lewis acid catalysts.