Development of a SO3H-functionalized UIO-66 metal-organic framework by postsynthetic modification and studies of its catalytic activities

Article abstract of DOI:10.1002/ejic.201402509

A novel metal-organic framework UiO-66-NH₂-derived Br?nsted acid catalyst was synthesized on a gram scale by employing a postsynthetic modification strategy under mild conditions. The nanomorphology of the catalyst was designed and developed to enhance its catalytic performance. Acetalization and benzimidazole formation were evaluated to demonstrate the high reactivity and selectivity of the nanoscaled UiO-66-NH-RSO₃H catalyst, which were found to be comparable to the reactivity and selectivity of the strong homogeneous Br?nsted acid catalyst. Furthermore, the UiO-66-NH-RSO₃H catalyst was recycled several times without compromising the yield and selectivity. A novel metal-organic framework UiO-66-NH₂-derived Br?nsted acid catalyst is synthesized by employing a postsynthetic modification strategy under mild conditions. Acetalization and benzimidazole formation are evaluated to demonstrate the high reactivity and selectivity of the nanoscaled UiO-66-NH-RSO₃H catalyst.

Full text of DOI:10.1002/ejic.201402509

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DOI:10.1002/ejic.201402509
Development of a SO3H-Functionalized UiO-66 Metal–
Organic Framework by Postsynthetic Modification and
Studies of Its Catalytic Activities
Yi Luan,[a] Nannan Zheng,[a] Yue Qi,[a] Jie Yu,[a] and Ge Wang*[a]
Keywords: Metal–organic frameworks / Heterogeneous catalysis / Nanostructures / Brønsted acids / Size selectivity
A
novel metal–organic framework UiO-66-NH2-derived
to demonstrate the high reactivity and selectivity of the nano-
scaled UiO-66-NH-RSO3H catalyst, which were found to be
comparable to the reactivity and selectivity of the strong
homogeneous Brønsted acid catalyst. Furthermore, the UiO-
66-NH-RSO3H catalyst was recycled several times without
compromising the yield and selectivity.
Brønsted acid catalyst was synthesized on a gram scale by
employing a postsynthetic modification strategy under mild
conditions. The nanomorphology of the catalyst was de-
signed and developed to enhance its catalytic performance.
Acetalization and benzimidazole formation were evaluated
MOF structures that are highly acid stable; therefore, a
large portion of available MOF materials are excluded.
Yaghi developed a PSM method to synthesize strongly
Brønsted acidic IRMOF-3-RSO3H from Zn-based IR-
MOF-3.[9] Despite the attention paid to IRMOF-3-RSO3H,
this material tends to self-decompose because of the acid-
sensitive nature of the IRMOF-3 support and the strong
acidity of the sulfonic acid group. To address this issue, we
developed a novel MOF UiO-66-NH2-derived strong
Brønsted acid catalyst through a PSM strategy and investi-
gated its catalytic activities. The synthesis of the desired
UiO-66-NH-RSO3H MOF catalyst is highly convenient
and can be performed on a gram scale under extremely mild
conditions. Acetalization was chosen as a model reaction to
test the catalytic properties of the newly synthesized MOF
catalyst. The utility of UiO-66-NH-RSO3H was further ex-
tended to the generation of 1,2-disubstituted benzimid-
azoles and benzothiazoles, the derivatives of which exhibit
antiviral activity against HIV, herpes (HSV-1), influenza,
and human cytomegalovirus (HCMV).[10]
Introduction
Brønsted acids are useful catalysts in various modern or-
ganic syntheses owing to their excellent selectivities and re-
activities in catalysis.[1] However, it is well known that
homogeneous Brønsted acids have problems, such as diffi-
culties in their separation from the product, severe equip-
ment corrosion, and environmental pollution.[2] In recent
years, heterogeneous Brønsted acid catalysts, which can be
achieved through the incorporation of sulfonic acid groups
on a solid support (e.g., organic polymers, MCM-41, etc.),
have exhibited advantages over homogeneous catalysts such
as easy recovery.[3]
Recently, metal–organic frameworks (MOFs)[4] have
emerged as a new class of microporous supports in hetero-
geneous acid catalysis as a result of their highly tailorable
nature, porous structure, and large surface area.[5] A prein-
stalled sulfonic group is generally required to prepare a
Brønsted acid MOF, which results in very few successful
MOF–acid catalysts.[6] Postsynthetic modification (PSM) of
porous MOFs could generate a wide variety of function-
alized MOF scaffolds through stable covalent bonds; this is
highly desirable, as it could introduce Brønsted acidic moie-
ties easily and rapidly.[7] However, the introduction of
Brønsted acidic functionalities onto a MOF support is gen-
Results and Discussion
Beginning with the 2-amino-1,4-benzenedicarboxylate
erally conducted under harsh condition with the use of cor- (NH2–H2BDC) linker, amino-functionalized UiO-66-NH2
rosive and toxic reagents, such as chlorosulfonic acid and that is topologically identical to UiO-66 (Scheme 1, left)
H2SO4.[8] These strategies can only be applied on certain was obtained. Gram-scale amounts of UiO-66-NH2 were
treated with 1,3-propanesultone to afford the UiO-66-NH-
[a] School of Materials Science and Engineering, University of
RSO3H catalyst after 12 h at 25 °C (Scheme 1). The nano-
morphology of UiO-66-NH2 was achieved successfully by
employing acetic acid as an additive.[11] UiO-66-NH2 crys-
tals appear to adopt an octahedral morphology with dia-
meters of 150 nm. The structural morphology was main-
tained after PSM with 1,3-propanesultone, as evidenced by
Science and Technology Beijing,
30 Xueyuan Road, Haidian District, 100083 Beijing, P. R.
China
E-mail: gewang@mater.ustb.edu.cn
http://www.nmclyjs-ustb.com/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402509.
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Scheme 1. The generic procedure for postsynthetic modification of UiO-66-NH2 by using 1,3-propanesultone.
scanning electron microscopy (SEM, Figure 1). The UiO- high similarity of the two crystalline structures. Also, the
66-NH-RSO3H catalyst demonstrated excellent thermal PXRD patterns showed that the UiO-66-NH-RSO3H
and acidic stability over the course of our investigation, material possessed the same reflections as its unmodified
which is a significant improvement over its analog, IR- precursor UiO-66-NH2, which indicated that the structural
MOF-3-RSO3H.
topology throughout the reaction was maintained (Fig-
ure S1).
The modification ratio of the alkyl sulfonic moiety was
analyzed by 1H NMR spectroscopy after acid digestion
(Figure 2). These studies indicated that 49% of the amine
groups were functionalized with alkylsulfonate groups (see
the Supporting Information for the full NMR spectra). Ad-
ditional evidence for modification was obtained by analysis
of UiO-66-NH2 and UiO-66-NH-RSO3H by FTIR spec-
troscopy (Figure 2, see also Figure S2). The strength of the
band at ν = 1255 cm–1, which was attributed to free amine
Figure 1. SEM images of nanoscaled (a) UiO-66-NH2 and (b) UiO-
66-NH-RSO3H.
˜
group, was significantly reduced (Figure 2, top). The ob-
–
served SO3 stretching band of the UiO-66-NH-RSO3H
The powder X-ray diffraction (PXRD) pattern of nano-
UiO-66-NH2 was similar to that of regular UiO-66-NH2
(Figure S1, Supporting Information).[12] This indicated the
catalyst at ν = 1043 cm–1 represents the UiO-66-NH-
˜
RSO3H network (Figure 2, bottom). The electrospray ion-
Figure 2. (a) NMR and (b) FTIR spectra of UiO-66-NH2 (top) and UiO-66-NH-RSO3H (bottom).
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ization mass spectrometry (ESI-MS) results also showed the NH-RSO3H is also significantly higher than those of MOF
expected molecular ion peak of m/z = 302 for the modified sulfonic acid catalysts reported in the literature.[16]
component in the UiO-66-NH-RSO3H framework (Fig-
Table 1. Acetalization of benzaldehyde with different catalysts.[a]
ure S3). The high porosity of the parent UiO-66-NH2 struc-
ture was retained after reaction, as evidenced by nitrogen
adsorption isotherms collected for the reaction products.
Furthermore, the alkyl sulfonic group occupies a large por-
tion of the pore structure. As a result, UiO-66-NH-RSO3H
has a calculated surface area of 434.76 m2 g–1, as deter-
Entry Catalyst[b]
Time [h] Yield[c] [%]
TON
mined by the Brunauer–Emmett–Teller (BET) method,
which is reduced from 1263.58 m2 g–1 for the parent UiO-
66-NH2 catalyst (Figure S4). However, the reduction in the
surface area caused by occupation of the pores had no
negative impact on the excellent catalytic activity of the
UiO-66-NH-RSO3H catalyst. This is due to the high flexi-
bility of the alkyl sulfonic group, which allows entry and
exit of substrate molecules. Nanoscale UiO-66-NH2 and
UiO-66-NH-RSO3H were examined by thermogravimetric
analysis (TGA) to confirm the thermal and structural sta-
bility of the heterogeneous sulfonic acid catalyst. The nano-
MOF catalyst showed good thermal stability, which was
similar to UiO-66-NH2. A weight loss at 372 °C was ob-
served according to TGA, which agrees with the literature
data. The weight loss of the modified UiO-66-NH-RSO3H
1
2
3
4
5
6
–
12
12
12
12
12
12
12
10
10
0
–
71
24
13
9
4
2
pTsOH
H-USY
99
34
18
12
6
3
82
98
Cu3(BTC)2
Fe(BTC)
IRMOF-3
UiO-66-NH2
UiO-66-NH-RSO3H
UiO-66-NH-RSO3H
7
8[d]
9
59
71
[a] Reaction conditions: Benzaldehyde (1.0 mmol), catalyst
(10.0 mg for UiO-66-NH-RSO3H, 1.4 mol-%), ethanol (0.5 mL) at
25 °C. [b] MOF catalysts were used on the nanoscale except for
entry 7. [c] Determined by GC–MS by using nitrobenzene as an
internal standard. [d] Bulk size.
We performed a kinetic profile of the reaction between
sample started at 256 °C, which is significantly lower. This benzaldehyde and ethanol (Figure S6) to study the conver-
indicates the successful incorporation of new alkyl sulfonic sion as a function of time in the presence of the UiO-66-
groups, which results in reduced stability (Figure S5).
NH-RSO3H catalyst. Full conversion of the substrate was
We initiated our investigation by evaluating the acetaliza- achieved at 10 h, and the catalytic reaction did not proceed
tion reaction of benzaldehyde (1a) at room temperature further if the catalyst was removed from the system.
(Table 1). Strong Brønsted acids are known to promote the
The aforementioned conditions were successfully applied
acetalization of benzaldehyde (Table 1, entry 2), and H- to a range of acetalization reactions by using the UiO-66-
USY (Si/Al ratio = 6) was tested as a zeolite Brønsted acid NH-RSO3H (1.0 mol-%) solid catalyst (Table 2). The proto-
catalyst. Only low yields were obtained upon using H-USY col was validated for a series of aromatic aldehydes and was
with 835 μmolg–1 acid sites and the turnover number used to convert them into commercially important acetals
(TON) was calculated to be 24 (Table 1, entry 3). Acetal in good yields without the need for water removal. Ethane-
formation can also be promoted by certain MOF structures, 1,2-diol (2aa) also acted as a hydroxy nucleophile to form
such as Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate),[13] an acetal with a five-membered ring (Table 2, entry 2). For
FeBTC,[14] and InIII–MOF[15] through a Lewis acid acti- substituted aromatic aldehydes, fluorinated aldehydes 1b
vation mechanism by using a high loading of the catalyst. and 1c were tolerated, which gave the products in yields of
However, of the MOFs tested, Cu3(BTC)2 and FeBTC 84% and 85%, respectively (Table 2, entries 3 and 4). Elec-
functioned poorly under our low catalyst loading condi- tron-rich aldehyde 1d, which is generally inert under acetali-
tions (1.4 mol-% catalyst loading, Table 1, entries 4 and 5). zation conditions, gave a moderate yield of the product un-
IRMOF-3 and UiO-66-NH2 also showed low reactivity at der our UiO-66-NH-RSO3H catalyzed conditions (Table 2,
room temperature after 12 h (Table 1, entries 6 and 7). Re- entry 5).[13,17] This observation further proves the outstand-
markably, UiO-66-NH-RSO3H gave the desired benzalde- ing activity of the UiO-66-NH-RSO3H catalyst for acid ca-
hyde acetal product in quantitative yield after 10 h (Table 1, talysis. Heteroaromatic aldehyde 1e also gave the desired
entry 9). It can be concluded that the alkyl sulfonic acid product in good yield (Table 2, entry 6).
moiety is responsible for acetalization reactivity. At 49%
To demonstrate that the catalytic reactions occur mostly
modified ratio of NH2, nanosized UiO-66-NH-RSO3H in the cavities of the catalyst rather than on the external
showed better performance than its corresponding bulk- surface, the size selectivity of UiO-66-NH-RSO3H was in-
sized material, with a TON up to 71 (Table 1, entries 8 and vestigated (Table 2, entries 1, 7 and 8). The window opening
9). The enhanced activity of the nanosized UiO-66-NH- of UiO-66-NH-RSO3H was measured from the pore-size
RSO3H catalyst suggests a higher utilization rate of the distribution, and it was found to be 8.5 Å.[18] The well-de-
MOF surface and the pores, which efficiently reduces diffu- fined pores of UiO-66-NH-RSO3H allow free entry and exit
sional limitations for the movement of the catalyst and the of benzaldehyde, which has dimensions of 6.0ϫ4.3 Å. As
substrate in liquid-phase catalysis. The high activity of the size of the aromatic aldehydes increased (e.g., 1a, 1f,
nanosized UiO-66-NH-RSO3H is comparable to that of 1g), the yield of the acetalization product decreased corre-
pTsOH, as shown in Table 1, entry 2. The TON of UiO-66- spondingly. Under the homogeneous reaction conditions
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Table 2. Acetalization promoted by UiO-66-NH-RSO3H.[a]
(Table 3, entries 3 and 4). Furthermore, the same size selec-
tivity as that previously observed for large aromatic alde-
hyde 1g was obtained (Table 3, entry 5).
Table 3. Synthesis of benzimidazoles and benzothiazoles.[a]
Entry Substrate
X
Product Conv.[b] Selectivity[c]
[%]
[%]
1
2
3
4
5
1a (R = H)
1a (R = H)
1b (R = p-F)
1d (R = p-OMe)
9-anthraldehyde (1g) NH
NH
S
NH
NH
5a
5b
5c
5d
5f
99
99
99
99
Ͻ5
92
99
90
95
n.d.
[a] Reaction conditions: Benzaldehyde (1.0 mmol), nucleophile
(1.2 mmol), and catalyst (0.5 mol-%) at 25 °C for 1 h. [b] Deter-
mined by analysis of the crud material by NMR spectroscopy.
[c] Yield of isolated product; n.d.: not determined.
The recyclability of the UiO-66-NH-RSO3H catalyst was
examined by isolating it from the reaction mixture (centri-
fugation or filtration, then washing with chloroform and
dichloromethane, and drying). The strong covalent bond
between the alkyl sulfonic acid moiety and the amino group
on UiO-66-NH-RSO3H ensures the stability of the catalytic
functional groups, and thus a yield of 95% is maintained
after six runs of acetalization (Figure S7). The supernatant
liquid of the ethanol suspension showed no reactivity
towards acetalization of the benzaldehyde substrate, which
indicates no leakage of the UiO-66-NH-RSO3H catalyst.
The X-ray powder diffraction pattern and FTIR spectrum
of the UiO-66-NH-RSO3H catalyst after six runs were also
indistinguishable from those of the fresh catalyst. A similar
result was observed for the catalyst promoted synthesis of
benzimidazoles (Figure S7). These facts clearly demonstrate
that there was no leaching or catalyst decomposition over
the catalytic process.
[a] Reaction conditions: Benzaldehyde (1.0 mmol), ethanol
(0.5 mL), and UiO-66-NH-RSO3H (1.0 mol-%) at 25 °C for 12 h.
[b] Determined by GC–MS.
with the use of pTsOH (1 mol-%) as the catalyst, the desired
acetal products were obtained from 4-tert-butylbenzalde-
hyde (1f) and 9-anthraldehyde (1g) in 92 and 88% yield,
respectively. However, 1f reacted much slower under UiO-
66-NH-RSO3H catalysis because the 8.3 Å size of 1f is close
to the window opening of this catalyst, which results in the
sluggish movement of the substrate. Furthermore, 1g can-
not freely move in or out of the pores of UiO-66-NH-
RSO3H as a result of its large diameter, which leads to the
lowest yield among aldehydes 1a, 1f, and 1g. A small
amount of product may be formed through active sites on
the surface of the catalyst (Table 2, entry 8).
The utility of the new UiO-66-NH-RSO3H catalyst was
further extended to generate 1,2-disubstituted benzimid-
azoles and benzothiazoles, which are widely found in natu-
ral products. Enhanced yields for 1,2-disubstituted benz-
imidazole 5a and benzothiazole 5b were obtained with a
Conclusions
In conclusion, a simple and rapid one-step approach to
catalyst loading of only 0.5 mol-%, which is significantly prepare alkyl sulfonic acid functionalized UiO-66-NH2
lower than that of previous reports (Table 3, entries 1 and through postsynthetic modification was developed. The
2).[19] The Brønsted acid sites are fixed on the MOF struc- structure and nanomorphology of UiO-66-NH-RSO3H
ture to prevent catalyst self-aggregation through hydrogen were retained after postsynthetic modification. The new
bonding, which results in higher catalytic efficiency. Substi- nanoscaled UiO-66-NH-RSO3H showed high activity and
tuted benzaldehydes 1b and 1d underwent the condensation selectivity in acetalization reactions and benzimidazole for-
reaction and gave desired products 5c and 5d in good yields mation at very low catalyst loadings. The catalyst did not
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SHORT COMMUNICATION
suffer from a leaching problem during catalysis and could
be recycled over six runs without any significant loss in ac-
tivity or selectivity. Future work, including the further ex-
tension of the utility of UiO-66-NH-RSO3H and the incor-
poration of a chiral sulfonic acid moiety, is on going.
Acknowledgments
The authors thank the Innovative Foundation of the China
National Petroleum Corporation (grant number 2012D-5006-0504)
and Beijing Natural Science Foundation (grant number 2144052)
for financial support. We also thank Dr. P. N. Moquist (Inter-
national Knowledge Editing) for helpful discussions and English
language editing.
Experimental Section
Synthesis of Nanoscaled UiO-66-NH2: ZrCl4 (1.6 g, 6.8 mmol) in
DMF (150 mL) was stirred with acetic acid (11.4 mL, 3.4 mol) at
55 °C. A DMF solution (50 mL) of 2-aminoterephthalic acid (1.2 g,
6.8 mmol) was added to the DMF solution. Then, water (0.5 mL,
0.028 mmol) was added to the DMF solution, and the solution was
sonicated at 60 °C. The mixture was further kept in a bath at
120 °C for 24 h. After 24 h, the solution was cooled to room tem-
perature and the precipitate was collected by centrifugation. The
obtained solid was washed with DMF (2ϫ 20 mL) and ethanol
(3ϫ 20 mL) and dried under reduced pressure (80 °C, 3 h).
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Postsynthetic Modification of UiO-66-NH2: Nanoscaled UiO-66-
NH2 (1.5 g, 0.85 mmol based on MW of 1766 gmol–1) was sus-
pended in CHCl3 (20 mL), and then 1,3-propanesultone (207 mg,
1.7 mmol, 2 equiv.) was added. The mixture was stirred slowly at
25 °C for 12 h, after which the solvent was decanted. Fresh DMF
(10 mL) was added once a day for three consecutive days and then
CHCl3 (10 mL) was used to rinse the crystals once a day for two
days. The UiO-66-NH2 crystals were dried under vacuum at 40 °C
before use. Careful consideration of safety related to the operation
of 1,3-propanesultone is recommended because of its toxicity. The
international agency for research on cancer (IARC) has classified
1,3-propanesultone as a Group 2B, possible human carcinogen.
General Procedure for Acetalization of Aldehydes by using UiO-66-
NH-RSO3H: A 10 mL reaction vessel (oven dried) was charged
with UiO-66-NH-RSO3H (based on 49 mol-% alkyl sulfonic acid,
0.014 mmol, 10.0 mg or 0.010 mmol, 7.1 mg), dry ethanol (0.5 mL),
and the benzaldehyde substrate (106 mg, 1.0 mmol). The mixture
was stirred under a N2 atmosphere at room temperature (25 °C)
for 10–12 h. Upon completion of the reaction, the mixture was cen-
trifuged. The yield of the desired product was determined by GC–
MS by using nitrobenzene as the internal standard.
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General Procedure for the Synthesis of 1,2-Disubstituted Benzimid-
azole by using UiO-66-NH-RSO3H: A 10 mL reaction vessel (oven
dried) was charged with UiO-66-NH-RSO3H (based on 49 mol-%
alkyl sulfonic acid, 0.5 mol-%, 3.6 mg) and chloroform (2.0 mL).
Benzaldehyde (106 mg, 1.0 mmol) and o-phenylenediamine
(130 mg, 1.2 mmol) were added, and the mixture was then stirred
for 1 h at 25 °C. Upon completion of the reaction, the mixture
was centrifuged. The conversion of this reaction was analyzed by
1H NMR spectroscopy. The selectivity and yield were based on the
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Supporting Information (see footnote on the first page of this arti-
cle): General procedures, experimental details, PXRD pattern,
FTIR spectra, mass spectra (ESI), N2 adsorption isotherms,
1H NMR spectra, plot of conversion vs. time of catalysis, and re-
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Received: June 6, 2014
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I42509
/KAP1
Date: 20-08-14 17:16:53
Pages: 6
www.eurjic.org
SHORT COMMUNICATION
Metal–Organic Frameworks
Y. Luan, N. Zheng, Y. Qi, J. Yu,
G. Wang* .......................................... 1–6
A novel metal–organic framework UiO-66-
NH2-derived Brønsted acid catalyst is syn-
thesized by employing
a postsynthetic
modification strategy under mild con-
ditions. Acetalization and benzimidazole
formation are evaluated to demonstrate the
high reactivity and selectivity of the nano-
scaled UiO-66-NH-RSO3H catalyst.
Development of a SO3H-Functionalized
UiO-66 Metal–Organic Framework by
Postsynthetic Modification and Studies of
Its Catalytic Activities
Keywords: Metal–organic frameworks
/
Heterogeneous catalysis / Nanostructures /
Brønsted acids / Size selectivity
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim