Synthesis of a Sulfonated Two-Dimensional Covalent Organic Framework as an Efficient Solid Acid Catalyst for Biobased Chemical Conversion

Article abstract of DOI:10.1002/cssc.201500755

Because of limited framework stability tolerance, de novo synthesis of sulfonated covalent organic frameworks (COFs) remains challenging and unexplored. Herein, a sulfonated two-dimensional crystalline COF, termed TFP-DABA, was synthesized directly from 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid through a previously reported Schiff base condensation reaction, followed by irreversible enol-to-keto tautomerization, which strengthened its structural stability. TFP-DABA is a highly efficient solid acid catalyst for fructose conversion with remarkable yields (97% for 5-hydroxymethylfurfural and 65% for 2,5-diformylfuran), good chemoselectivity, and good recyclability. The present study sheds light on the de novo synthesis of sulfonated COFs as novel solid acid catalysts for biobased chemical conversion.

Full text of DOI:10.1002/cssc.201500755

DOI: 10.1002/cssc.201500755
Communications
Synthesis of a Sulfonated Two-Dimensional Covalent
Organic Framework as an Efficient Solid Acid Catalyst for
Biobased Chemical Conversion
[
a]
[a]
[a]
[b]
[a]
[a]
Yongwu Peng, Zhigang Hu, Yongjun Gao, Daqiang Yuan, Zixi Kang, Yuhong Qian,
[a]
[a]
Ning Yan, and Dan Zhao*
Because of limited framework stability tolerance, de novo syn-
thesis of sulfonated covalent organic frameworks (COFs) re-
mains challenging and unexplored. Herein, a sulfonated two-
dimensional crystalline COF, termed TFP-DABA, was synthe-
sized directly from 1,3,5-triformylphloroglucinol and 2,5-diami-
nobenzenesulfonic acid through a previously reported Schiff
base condensation reaction, followed by irreversible enol-to-
keto tautomerization, which strengthened its structural stabili-
ty. TFP-DABA is a highly efficient solid acid catalyst for fructose
conversion with remarkable yields (97% for 5-hydroxymethyl-
furfural and 65% for 2,5-diformylfuran), good chemoselectivity,
and good recyclability. The present study sheds light on the
de novo synthesis of sulfonated COFs as novel solid acid cata-
lysts for biobased chemical conversion.
by using molecular catalysts (precatalysts) as building blocks
[
3g,h]
to prepare catalytically active COFs remains limited.
The use of renewable biomass resources (e.g., fructose, glu-
cose, cellulose, starch, etc.) in biobased chemical synthesis
through chemical conversion is of great significance, not only
for basic science, but also to relieve growing concerns with
regard to the exhaustion of fossil fuels and deterioration of our
[
6]
environment. As a versatile and key scaffold for biorefineries,
5-hydroxymethylfurfural (HMF) has a huge potential in down-
[
7]
stream applications, such as fuels and fine chemicals.
Currently, HMF is mainly synthesized from biobased hexose
[
8]
sugars through acid-promoted dehydration. Several sulfonic
acid derived homo- and heterogeneous catalysts have been
[
9]
developed for this process. Challenges to homogeneous cata-
lysts include product separation and equipment corrosion,
whereas heterogeneous catalysts normally suffer from prob-
lems of low catalytic performance and long reaction times.
Therefore, the design of highly efficient solid acid catalysts is
still of paramount importance for this conversion.
Covalent organic frameworks (COFs) are porous crystalline
polymers constructed from organic building blocks by using
[
1]
reticular chemistry. Because of their features, such as periodic
architectures, low densities, and permanent porosities, they
have been widely investigated for various applications, such as
Banerjee et al. demonstrated remarkable chemical stabilities
of COFs obtained through irreversible enol-to-keto tautomeri-
[2]
[3]
[4]
[10]
gas storage/separation,
catalysis, and optoelectronics.
zation.
Inspired by their findings, herein we report the
COFs can serve as ideal heterogeneous catalysts due to their
uniform yet tunable porosity and rich functionality, and are ca-
pable of reaching excellent catalytic activity and selectivity
values that are superior to their homogeneous analogues. In
principle, catalytically active COFs can be constructed from
direct or indirect strategies that are widely employed in pre-
de novo synthesis of a sulfonated 2D COF named TFP-DABA.
TFP-DABA was prepared by the Schiff base condensation reac-
tion between 1,3,5-triformylphloroglucinol (TFP) and 2,5-diami-
nobenzenesulfonic acid (DABA), followed by irreversible enol-
to-keto tautomerization, which locked it into a more stable iso-
morph that was compatible with the sulfonic acid groups
(Scheme 1). TFP-DABA was studied as a solid acid catalyst in
fructose dehydration to HMF, or 2,5-diformylfuran (DFF), when
KBr was used as a cocatalyst; it exhibited remarkable yields
(97% for HMF and 65% for DFF), good chemoselectivity, and
good recyclability.
[5]
paring heterogeneous catalysts. Most of the reported catalyti-
cally active COFs are prepared through indirect strategies, such
as postmodification or immobilization with other catalytically
[
3a–f]
active species.
Because of the limited framework stability
tolerance towards catalytically active functional groups (e.g.,
acid or basic groups), the direct strategy (or de novo synthesis)
A suspension of TFP and DABA in a 10:1 (v/v) mixture of me-
sitylene/dioxane (1:4) and aqueous acetic acid (3m) was
heated at 1008C for 3 days to give TFP-DABA as a red crystal-
line solid in 86% yield (see the Experimental Section). TFP-
DABA was insoluble in water and common organic solvents
[
a] Dr. Y. Peng, Z. Hu, Dr. Y. Gao, Dr. Z. Kang, Y. Qian, Prof. N. Yan,
Prof. D. Zhao
Department of Chemical and Biomolecular Engineering
National University of Singapore
and was formulated to be C H N O S based on elemental
4
Engineering Drive 4, Singapore 117585 (Singapore)
12
8
2
5
E-mail: chezhao@nus.edu.sg
analysis performed on a guest-free sample. FTIR spectra of
TFP-DABA indicated the disappearance of the carbonyl stretch-
[
b] Prof. D. Yuan
À1
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
ing band of TFP ( n˜ =1643 cm ) accompanied by a series of
new characteristic stretching bands observed at n˜ =1578 and
À1
1
238 cm arising from the C=C and CÀN stretching bands, re-
Fuzhou, 350002 (P.R. China)
spectively (Figure S1a in the Supporting Information). The
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500755.
À1
bands observed at n˜ =1026 and 1080 cm , along with
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Thermogravimetric
analysis
(
TGA) reveals that TFP-DABA is
thermally stable up to 2508C
Figure S4 in the Supporting In-
(
formation). Field-emission (FE)
SEM images show that TFP-
DABA crystallized in a uniform
block-shaped morphology with
dimensions ranging from 50 to
1
00 nm (Figure S5 in the Sup-
porting Information). The perma-
nent porosity of TFP-DABA was
confirmed by N2 sorption iso-
therms at 77 K (Figure 2). The ac-
tivated sample exhibits a typical
type IV isotherm, with a pore
3
À1
volume of 0.48 cm g
and
of
a
1
BET
surface
area
2
À1
2
À1
58.6 m g (272.7 m g based
on the Langmuir model). The
hysteresis between adsorption
Scheme 1. Synthetic scheme for the preparation of TFP-DABA by Schiff base condensation and enol-to-keto tauto- and desorption branches may
merization (cavity size considering van der Waals radius is indicated).
come from the interstitial voids
[
12]
among the COF nanoparticles.
The smallest pore size of TFP-
DABA calculated by using nonlocal density functional theory
(NLDFT) was 13.8 Š, which was quite close to the pore size of
À1
a shoulder at n˜ =1438 cm , can be attributed to the O=S=O
symmetric and asymmetric stretching bands, which indicate
the existence of the sulfonic acid group. The amount of dan-
gling sulfonic acid groups in TFP-DABA was estimated to be
À1
3
.15 mmolg by acid–base titration, which was comparable to
À1
the theoretical value of 3.42 mmolg and confirmed almost
full accessibility of these groups. The structure of TFP-DABA
1
3
was further confirmed by C cross-polarization magic-angle-
spinning nuclear magnetic resonance (CP/MAS NMR) spectros-
copy; the characteristic chemical shift at d=182.1 ppm was at-
tributed to the carbonyl carbon (Figure S2 in the Supporting
Information). The results of FTIR and CP/MAS NMR spectrosco-
py are similar to previous studies, which confirms enol-to-keto
[10]
tautomerization in TFP-DABA.
The crystallinity of TFP-DABA was determined by powder X-
ray diffraction (PXRD) analysis (Figure 1). Based on the hexago-
nal P 6¯ (no.174) space group, two possible structural models
with eclipsed and staggered 2D stacking were built by using
[11]
the Materials Studio software package.
The experimental
PXRD pattern of TFP-DABA matches well with the simulated
pattern obtained by using the eclipsed 2D stacking model and
contains no diffraction peaks that could be attributed to start-
ing materials (Figure S3 in the Supporting Information). Two
sharp peaks at 4.7 and 16.38 were observed in the experimen-
tal PXRD pattern of TFP-DABA; these corresponded to the re-
flection from the (100) and (001) planes, respectively. The
slightly broad peak at higher 2q angles may be attributed to
[
10c]
defects in the p–p stacking between successive COF layers.
After geometrical energy minimization by using the universal
force field, the unit cell parameters of TFP-DABA were calculat-
ed to be a=b=22.90 Š and c=5.47 Š.
Figure 1. a) Crystal structure of TFP-DABA, assuming 2D eclipsed stacking
viewed through [001] (left) and [100] (right) directions along with the inter-
layer distance; b) simulated and experimental PXRD patterns of TFP-DABA.
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could still be achieved, but with only 54% yield of HMF
(
Table 1, entry 7); this indicated that TFP-DABA was more
active and chemoselective than its homogeneous counterpart.
A control experiment without catalyst revealed that only 5%
fructose conversion was achieved, without any detectable HMF
as the product (Table 1, entry 8). Furthermore, when a scaled-
up amount of fructose (tenfold) was employed with TFP-DABA
as the catalyst, full fructose conversion with 95% yield of HMF
were still attained (Table 1, entry 9). These results reveal that
TFP-DABA is a highly efficient catalyst for the dehydration of
fructose into HMF. To the best of our knowledge, TFP-DABA is
among the best solid acid catalysts for this transformation re-
[
8c]
ported so far. It needs to be pointed out that, although
water has been proposed as the green solvent of choice for
Figure 2. Nitrogen sorption isotherms (77 K) and pore size distribution
inset) of TFP-DABA.
[
7a,8b]
(
many biomass-conversion reactions,
no catalytic activity
was observed in our study with water as the solvent. This is
[
13]
probably because of the instability of HMF in water.
1
.4 nm measured from the predicted crystal structure
We then examined the heterogeneity and recyclability of
TFP-DABA. The supernatant of the reaction media obtained by
using a regular filter during the dehydration reaction process
did not afford any additional product; this indicates the role of
heterogeneous catalyst played by TFP-DABA. Upon completion
of the reaction, TFP-DABA could be recovered in quantitative
yield and used repeatedly without significant loss of its catalyt-
ic activity for the following three runs (full fructose conversions
with 94, 95, and 94% yield of HMF for 1–3 runs, respectively;
Table 1, entry 10). Moreover, to avoid the excess use of TFP-
DABA, we also recycled and reused the catalyst before equilib-
rium conversion (Table 1, entry 2); 66, 64, and 61% fructose
conversions with 64, 61, and 59% yield of HMF could be ob-
tained for 1–3 runs, respectively (Table 1, entry 11). The FTIR
spectrum of recycled TFP-DABA remained the same as that of
the original one (Figure S1b in the Supporting Information).
Moreover, the supernatant of the reaction media did not con-
tain any monomers (TFP or DABA) of TFP-DABA, which indicat-
ed its structural integrity during catalysis. Recycled TFP-DABA
lost its crystallinity after three runs, as revealed by the PXRD
patterns (Figure S6 in the Supporting Information). This might
due to the partial exfoliation of 2D COF material; this was also
(
Scheme 1).
The presence of 1D microporous channels and easily accessi-
ble sulfonic acid groups in TFP-DABA prompted us to explore
its application as a solid acid catalyst for fructose dehydration.
Our initial studies identified TFP-DABA as an effective catalyst
for the dehydration of fructose into HMF and a variety of reac-
tion conditions were then screened. The screening of reaction
temperature was conducted by using dimethyl sulfoxide
(
DMSO) as solvent with 5 mol% loading of TFP-DABA as a cata-
lyst. We found that 1008C was the most suitable temperature
for this reaction to afford a high yield of HMF (65%) and good
chemoselectivity (97%; Table 1, entries 1–4). Increasing the cat-
alyst loading from 5 to 10 mol% led to better yields of HMF
(
up to 97%) with similar chemoselectivity (Table 1, entries 2, 5,
and 6). For comparison, when the monomer DABA was used
as a catalyst under similar conditions, full fructose conversion
Table 1. Synthesis of HMF from fructose catalyzed by TFP-DABA.
[
3c]
observed in a previous study.
However, the short-range
chemical bonding and composition of TFP-DABA should
remain unchanged. We found that the crystallinity of recycled
TFP-DABA could be easily reconverted by subjecting it to the
initial synthetic conditions, as confirmed by the PXRD and BET
profiles of recrystallized TFP-DABA (Figures S6 and S7 in the
Supporting Information). Although without crystallinity after
three runs, TFP-DABA still remained a solid that could be re-
covered through centrifugation.
[
a]
Entry
TFP-DABA
T
Fructose
HMF
loading [mol%] [8C] conversion [%] selectivity [%] yield [%]
1
2
3
4
5
6
5
5
5
5
7.5
10
–
–
10
10
5
80
100
120
140 >99
100 84
100 >99
100 >99
100
42
67
88
97
97
90
80
97
97
54
41
65
79
80
81
97
54
n.d.
95
94, 95, 94
64, 61, 59
To further study the catalytic performance of TFP-DABA and
its homogeneous counterpart DABA, kinetic studies for fruc-
tose dehydration were carried out by using a 10 mol% loading
of TFP-DABA or DABA in DMSO at 1008C (Figure 3). When
DABA was used as the catalyst, full fructose conversion was
obtained within 10 min, although the desired yield of HMF
was moderate (70%) and then decreased to 54% after 1 h due
to side reactions. In contrast, TFP-DABA displayed remarkably
enhanced catalytic activity and chemoselectivity, with the fruc-
[b]
7
8
9
0
[
c]
[c]
5
n.d.
95
94, 95, 94
97, 95, 97
[d]
[e]
[e]
100 >99
100 >99
100
1
11
66, 64, 61
[
[
[
a] On a 0.28 mmol scale, see the Experimental Section for details.
b] 10 mol% DABA was used as the catalyst. [c] N.d.=not determined.
d] On a 3 mmol scale. [e] Recycled TFP-DABA was used as the catalyst for
one to three cycles, respectively.
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transformation should occur through the formation of 5-(bro-
momethyl)furan-2-carbaldehyde, followed by a Kornblum-type
reaction, with the help of the association of TFP-DABA with
bromide salts, which was similar to the previous study dis-
[
16d]
closed by Laugel et al.
As far as we know, this catalytic per-
formance is comparable to the best results reported so far for
[
16c]
such a conversion.
In conclusion, we presented the de novo synthesis of a sulfo-
nated COF TFP-DABA, which exhibited excellent catalytic activi-
ty and chemoselectivity for the conversion of fructose into
HMF and DFF. Our study paves a solid way towards the
de novo synthesis of catalytically active COFs and their applica-
tions in biobased chemical conversion.
Figure 3. Kinetic study of fructose dehydration catalyzed by TFP-DABA (blue)
and DABA (red). Reaction conditions: fructose (50 mg), catalyst (10 mol%),
DMSO (1 mL), 1008C.
Experimental Section
Synthesis of TFP-DABA
tose conversion and HMF yield increasing steadily towards 100
and 97%, respectively, in 1 h. It is clear that, compared with its
homogeneous analogue, TFP-DABA exhibited slower reaction
kinetics possibly due to slow mass diffusion within the porous
framework. The enhanced catalytic activity of TFP-DABA may
originate from the increased acidity of sulfonic acid groups
A 10 mL Pyrex tube was charged with TFP (21.0 mg, 0.1 mmol),
DABA (28.2 mg, 0.15 mmol), mesitylene/dioxane (1:4 v/v, 1 mL),
and aqueous acetic acid (0.1 mL, 3m). The mixture was stirred for
10 min to give a homogenous suspension. The tube was then flash
frozen at 77 K by using a liquid nitrogen bath and degassed by
three freeze–pump–thaw cycles, sealed under vacuum, and then
heated at 1008C for 3 days. A red precipitate formed and was col-
lected by centrifugation, washed with anhydrous DMF, anhydrous
CH Cl , anhydrous THF, and anhydrous acetone. The washed
[14]
owing to the p-extended network of TFP-DABA.
Next, we further expanded TFP-DABA to another biobased
chemical synthesis. Although HMF has been widely explored
as a versatile synthetic intermediate for fine chemicals, its ap-
plication is largely limited by the low stability. This shortcom-
ing can be overcome by oxidizing HMF to DFF, which is anoth-
2
2
sample was then solvent-exchanged with anhydrous methanol 2–
times and dried at 1508C under vacuum for 12 h to give a deep-
3
red powder in 86% yield. Elemental analysis calcd (%) for
C H N O S: C 49.31, H 2.76, N 9.58, S 10.97; found: C 49.94, H 2.70,
12
8
2
5
[6,7,15]
er important and more stable platform molecule.
Great ef-
N 9.06, S 11.26.
forts have been made for this transformation, although only
[
16]
low yields of DFF are obtained that rarely exceed 60%.
Fructose conversion into HMF
Recently, Laugel et al. reported a one-pot synthesis of DFF
from fructose with NaBr as a catalyst in DMSO under a high
In a typical run, fructose (50 mg), TFP-DABA (8 mg, 10 mol%), and
DMSO (1 mL) were added to a 10 mL Pyrex tube. The tube was
purged with argon, sealed, and then placed in a preheated oil
bath at 1008C with vigorous stirring for 1 h. The reaction was
stopped by quenching the tube in an ice bath at 08C. The mixture
was filtered and then decanted into a volumetric flask with water
as diluent. The products were analyzed by HPLC.
[16d]
temperature of 1508C.
To demonstrate the applicability of
TFP-DABA as a solid acid catalyst, we adopted a similar proto-
col for the one-pot synthesis of DFF from fructose directly
under the catalysis of TFP-DABA and KBr. As shown in
Scheme 2, with the help of a 10 mol% loading of TFP-DABA
Fructose conversion into DFF
In
a typical procedure, fructose (50 mg), TFP-DABA (8 mg,
1
0 mol%), KBr (7 mg, 20 mol%), and DMSO (1 mL) were added to
a 10 mL Pyrex tube. After sealing, the tube was placed in a preheat-
ed oil bath at 1008C with vigorous stirring for 12 h. The reaction
was stopped by quenching the tube in an ice bath at 08C. The
mixture was filtered and then decanted into a volumetric flask
with water as the diluent. The products were analyzed by HPLC.
Scheme 2. One-pot synthesis of DFF from fructose catalyzed by TFP-DABA
and KBr.
and 20 mol% loading of KBr, fructose could be directly con-
verted into DFF in 65% yield with full conversion after 12 h
under a lower temperature of 1008C at atmospheric pressure.
Without TFP-DABA and only a 20 mol% loading of KBr, no DFF
was observed under the identified conditions. Moreover, there
was a minimal impact on DFF yield when this transformation
Acknowledgements
This work is supported by the National University of Singapore
(
NUS Start-up Funding R-279-000-369-133, CENGas R-261-508-
01-646) and the Singapore Ministry of Education (MOE AcRF
Tier 1 R-279-000-410-112, AcRF Tier 2 R-279-000-429-112).
0
was conducted under a high-pressure oxygen environment
5
(
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Keywords: biocatalysis
heterocycles · heterogeneous catalysis · synthesis design
· covalent organic frameworks ·
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Received: June 4, 2015
Revised: July 11, 2015
Published online on September 3, 2015
ChemSusChem 2015, 8, 3208 – 3212
www.chemsuschem.org
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