Catalytic conversion of glucose to 5-hydroxymethylfurfural using zirconium-containing metal-organic frameworks using microwave heating

Article abstract of DOI:10.1039/c8ra06021e

5-Hydroxymethylfurfural (5-HMF) can be prepared by the catalytic dehydration of glucose or fructose using a range of homogeneous or heterogeneous catalysts. For our research, a selection of closely related Metal-Organic Frameworks (MOFs) were used as catalysts in the conversion of glucose to 5-HMF due to their chemical and thermal stability as well as the Lewis acidity of zirconium. Our initial study focused on the use of UiO-66-X (X = H, NH₂ and SO₃H), optimization of the dehydration reaction conditions, and correlation of the catalytic activity with the MOF's properties, in particular, their surface area. The highest yield of 5-HMF (28%) could be obtained using UiO-66 under optimal reaction conditions in dimethylsulfoxide and this could be increased to 37% in the presence of water. In catalyst recycling tests, we found the efficiency of UiO-66 was maintained across five runs (23%, 19%, 21%, 20%, 22.5%). The post-catalysis MOF, UiO-66-humin, was characterized using a range of techniques including PXRD, FT-IR, ¹³C Solid State NMR and N₂ gas adsorption. We continued to optimize the reaction using MOF 808 as the catalyst. Notably, MOF 808 afforded higher yields of 5-HMF under the same conditions compared with the three UiO-66-X compounds. We propose that this might be attributed to the larger pores of MOF 808 or the more accessible zirconium centres.

Full text of DOI:10.1039/c8ra06021e

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Catalytic conversion of glucose to 5-
hydroxymethylfurfural using zirconium-containing
metal–organic frameworks using microwave
heating†
Cite this: RSC Adv., 2018, 8, 31618
*
*
Jue Gong, Michael J. Katz and Francesca M. Kerton
5-Hydroxymethylfurfural (5-HMF) can be prepared by the catalytic dehydration of glucose or fructose using
a range of homogeneous or heterogeneous catalysts. For our research, a selection of closely related Metal–
Organic Frameworks (MOFs) were used as catalysts in the conversion of glucose to 5-HMF due to their
chemical and thermal stability as well as the Lewis acidity of zirconium. Our initial study focused on the
use of UiO-66–X (X ¼ H, NH2 and SO3H), optimization of the dehydration reaction conditions, and
correlation of the catalytic activity with the MOF's properties, in particular, their surface area. The highest
yield of 5-HMF (28%) could be obtained using UiO-66 under optimal reaction conditions in
dimethylsulfoxide and this could be increased to 37% in the presence of water. In catalyst recycling tests,
we found the efficiency of UiO-66 was maintained across five runs (23%, 19%, 21%, 20%, 22.5%). The
post-catalysis MOF, UiO-66–humin, was characterized using a range of techniques including PXRD, FT-
IR, 13C Solid State NMR and N2 gas adsorption. We continued to optimize the reaction using MOF 808 as
the catalyst. Notably, MOF 808 afforded higher yields of 5-HMF under the same conditions compared
with the three UiO-66–X compounds. We propose that this might be attributed to the larger pores of
MOF 808 or the more accessible zirconium centres.
Received 15th July 2018
Accepted 4th September 2018
DOI: 10.1039/c8ra06021e
rsc.li/rsc-advances
2,5-dimethylfuran).5,6 The latter two compounds can be further
converted (Fig. 1) to a broad range of valuable products
Introduction
including fuel additives and the bio-derived polymer poly-
ethylene furanoate (PEF) that is a possible alternative to the
widely-used petroleum-derived polymer polyethylene tere-
phthalate (PET).7
Generally, 5-HMF can be synthesized via dehydration reac-
tions of sugars such as glucose, fructose and sucrose with
different homogeneous or heterogeneous catalysts.5 Several
homogeneous catalysts (e.g., H2SO4, HCl and CrCl3) have been
investigated. However, difficulties in post-reaction catalyst
separation prevents the further development of some of these
Production of CO2 from the combustion of fossil fuels and the
resulting rise in atmospheric levels of CO2 is contributing to
climate change.1 Thus, biomass is being explored as a renew-
able alternative to fossilized resources in a range of applica-
tions.1,2 Biomass from land plants is typically lignocellulosic
biomass, which consists of cellulose, hemicellulose and lignin.3
Cellulose is a carbohydrate polymer containing hundreds to
thousands of glucose units.4 Conversion of cellulose to valuable
products, such as the platform chemicals 5-hydrox-
ymethylfurfural and levulinic acid, is of particular interest.
5-Hydroxymethylfurfural (5-HMF) is an important platform
chemical derived from glucose,5,6 and can be obtained in near
quantitative yields using superior catalytic systems. It is
a heterocyclic organic compound, containing aldehyde and
alcohol functional groups in the 2,5 positions of a furan ring
(Fig. 1). As further illustrated in Fig. 1, 5-HMF can be used as
a building block for other compounds such as adipic acid (the
precursor of nylon), 2,5-furandicarboxylic acid and p-xylene (via
Department of Chemistry, Memorial University of Newfoundland, 230 Elizabeth
Avenue, St. John's, NL A1B 3X7, Canada. E-mail: erton@mun.ca; mkatz@mun.ca
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c8ra06021e
Fig. 1 A range of applications for 5-HMF as a platform chemical.
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systems.8 Thus, different heterogeneous catalysts such as State NMR). Furthermore, with optimized reaction conditions,
metal–organic frameworks, metal oxides e.g. zirconias, zeolites we examined how larger pores within similar Zr-containing
and related porous materials e.g. mesoporous zirconium materials (MOF 80823) affect the catalytic conversion of
phosphates have begun to be investigated in recent years.5,6 In glucose to 5-HMF.
this way, it has become clear that a balance is needed between
Lewis acidic sites to isomerize glucose to fructose and Brønsted
acidic sites to dehydrate fructose to 5-HMF. Furthermore, the
Lewis acidic sites can also lead to the formation of humin as
a by-product.6a,d
Experimental
Instrumentation
FT-IR spectra (400–4000 cmꢀ1) were recorded at room temper-
ature on a Bruker Alpha FT-IR Spectrometer with a single-
bounce diamond ATR accessory at a resolution of 4 cmꢀ1
using 36 scans. Powder X-ray diffraction (PXRD) patterns were
recorded with a Rigaku Ultima IV diffractometer equipped with
a copper sealed-tube operated at 40 kV and 44 mA ltered to
Metal–organic frameworks (MOFs) are a class of crystalline
materials, built up from metal cations/clusters (nodes) and
bridging organic ligands (linkers).9 With judicious choice of
node and linker, MOFs having a wide range of pore-sizes and
pore functionalities have been formed.10 Additionally, pre/post
synthetic functionalization of MOFs can be used to introduce
different linkers or functionalities into the frameworks.9b,11
Given the toolbox available for MOF synthesis, different struc-
tures and properties can be designed. The combination of the
facile synthesis as well as the large pore sizes, low density, and
their thermal and chemical stability have made these materials
ideal in many different elds such as gas storage and separa-
tion,12 catalytic reactions,9b,13 proton-, and ion-conduction.14
Currently, glucose conversion to 5-HMF with MOFs is in its
infancy. Li, Hensen, and co-workers reported the rst applica-
tion of MOF (MIL-101) in dehydration of fructose and glucose.
However, the yield was just 2% in dehydration of glucose to 5-
HMF.15 Kitagawa's research group investigated isomerization
from glucose to fructose using different MIL-101(Cr) derivatives
and demonstrated that MIL-101(Cr)–SO3H had higher conver-
sion and better selectivity.16 Bao and co-workers reported a 5-
HMF yield of 44.9% for the dehydration of glucose to 5-HMF
with MIL-101(Cr)–SO3H.17 Whilst Herbst and Janiak reported
a 29% yield of 5-HMF with MIL-101(Cr)–SO3H in a solvent
mixture, THF/H2O (v/v 39 : 1).18 Most recently, Katz, Farha and
co-workers showed phosphate-modication in NU-1000
enabled the catalytic reaction of glucose to 5-HMF.19 In
related research, the Zhao research group developed a new
MOF, NUS-6, built from zirconium (Zr) or hafnium (Hf) clusters
and sulfonated organic linkers.20 They examined these catalysts
for dehydration of fructose to 5-HMF and found high yields
(98%) and selectivity (98%) using NUS-6(Hf).20 However, it
should be noted that they performed these reactions in DMSO
and it is known that this solvent can catalyze fructose conver-
sion to 5-HMF.21
˚
1.54 A using a graphite monochromator. Simulated powder
diffractograms were obtained using the Mercury 3.8 soware
suite. N2 gas adsorption isotherms were collected on a Micro-
metrics Tristar II 3020 instrument with the sample maintained
at 77 K using N2(l). Before measurements, samples were acti-
vated on a Micrometrics Smart VacPrep by rst heating at 353 K
until a pressure <5 mmHg was achieved. Subsequently, the
sample was heated under vacuum at 423 K for 10 h. Data was
analyzed via the MicroActive Soware suite. Thermogravimetric
analysis (TGA) was carried out using a TA Instruments Q500.
Samples ꢀw1ere placed in a platinum pan and heated at a rate of
7 ꢁC min under a N2 atmosphere from 25–600 ꢁC under a N2
atmosphere with ow rate of 50 mL minꢀ1. Solid-state NMR
spectra were obtained at 298 K using a Bruker Avance II 600
spectrometer, equipped with a SB Bruker 3.2 mm MAS triple-
1
tuned probe operating at 600.33 MHz for H and 150.97 MHz
for 13C. Chemical shis were referenced to tetramethylsilane
(TMS) using adamantane as an intermediate standard for 13C.
The samples were spun at 20 kHz. 13C{1H} cross-polarization
spectra were collected with a Hartmann–Hahn match at 62.5
1
kHz and H decoupling at 100 kHz. The recycle delay was 3 s
and the contact time was 2000 ms. Solution 1H NMR experi-
ments were performed on a Bruker AVANCE III 300.
Synthesis of MOFs
UiO-66–X (X ¼ H, NH2, SO3H) and MOF 808 were synthesized
according to reported literature methods.9d,23,24 See the ESI† for
details.
Catalytic conversion of glucose to 5-HMF
Herein, a series of Zr-cluster-based MOFs were prepared,
characterized and studied as catalysts for the conversion of Reactions were performed in triplicate, to assess reproduc-
glucose to 5-HMF. Initially, we performed catalytic reactions ibility, using a Biotage microwave synthesizer. In a typical run,
using UiO-66 and its analogues (UiO-66–NH2 and UiO-66– glucose (100 mg) and UiO-66 (20 mg) were weighed in a 2 mL
SO3H). We determined the optimal reaction conditions by microwave reaction vial. Subsequently, 2 mL DMSO-d6 was
varying different parameters such as time, temperature and added. The vial was sealed and heated in the microwave at
ꢁ
catalyst loading. During this work, we found that the activity of 160 C for 30 min. Aer tꢁhe reaction, the mixture was cooled
UiO-66 was possibly inhibited due to the formation of humin. with pressurized air to 50 C. Then, the vial was removed from
Humin, which is formed from acid-catalyzed dehydration of the synthesizer, allowed to cool to room temperature, opened,
sugars, is a furanic, branched polymer and is oen poorly and 15 mL of 1-naphthaldehyde (internal standard) was added
characterized due to its intractable nature and limited solu- into the reaction mixture for quantitative 1H NMR analysis.
bility.22 Thus, a focus of our work was to characterize the humin Quantitative 1H NMR data was in agreement with data from GC
formed using different methods (PXRD, FT-IR and 13C Solid analysis using a quantitation method previously reported.24
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For the recycling test, a reaction was performed on a larger
scale. Glucose (1000 mg), UiO-66 (200 mg), DMSO-d6 (15 mL)
were added to a vial. The mixture was heated in the microwave
at 160 ꢁC for 30 min. Aer the reaction, the mixture was
centrifuged and decanted out for quantitative 1H NMR analysis.
The solid was washed with 15 mL DMSO (protio) three times
and separated by vacuum ltration. Then, the solid was dried in
a vacuum oven (10ꢀ2 mbar) at 80 C overnight and reused for
ꢁ
the next run. Alternatively, the solid was dried in a conventional
ꢁ
oven at 200 C overnight. Similar yields were obtained by each
drying method.
For characterization of humin on the MOF surface, the
reaction conditions and the treatment procedure of the used
solid catalyst were the same as in the recycling test.
Results and discussion
Synthesis and characterization of MOFs
UiO-66–X (X ¼ H, NH2, SO3H) were synthesized via a sol-
vothermal method.9d,25 UiO-66–X consists of Zr6O4(OH)4 octa-
hedral nodes with 2-X-1,4-benzenedicarboxylate (BDC-X)
organic linkers (Fig. 2).9b,25
A FT-IR study of UiO-66 and its analogues reveals that FT-IR
spectra of activated forms of UiO-66–X are similar (Fig. S1†).
Two strong absorption bands in the region of 1560–1600 cmꢀ1
and 1380–1415 cmꢀ1 are attributed to carboxylate asymmetric
and symmetric stretching.25,27 An intense absorption band is
observed in the region of 1653–1665 cmꢀ1 due to the C]O
stretch of DMF within the pores of the MOF.26,28 Another
medium absorption band in the region of 1495–1507 cmꢀ1 is
the results of CC ring symmetric stretches within the linkers.28
For UiO-66–NH2, in the high frequency region, two absorption
bands can be found at 3350 cmꢀ1 and 3451 cmꢀ1 as the result of
asymmetric and symmetric stretches of a primary amino group
(–NH2).29,30 In the low frequency region, a weak N–H bending
vibration at 1617 cmꢀ1 and a strong C–N stretching absorption
located at 1257 cmꢀ1 also conrm the presence of the amino
group.29 For UiO-66–SO3H, the O]S]O asymmetric and
symmetric stretching bands appear at 1076 cmꢀ1 and
Fig. 3 PXRD patterns of UiO-66–X (X ¼ H, NH2, and SO3H) including
the simulated pattern for UiO-66 (top) and MOF 808 including the
simulated pattern of MOF 808 (bottom).
1024 cmꢀ1 25 PXRD patterns of UiO-66–X are shown in Fig. 3. All
.
of these three solids retain their crystallinity and their corre-
sponding patterns are almost same as compared with simulated
UiO-66.
As with UiO-66, MOF 80823 is built up from octahedral
[Zr6O4(OH)4]12+ cationic nodes and trimesic acid (H3BTC)
organic linkers (Fig. 4). The secondary building units are con-
nected with six BTC linkers. Six formate ligands cap the node
and provide for the anions necessary for charge balancing.23
Compared with UiO-66–X, MOF 808 has a larger pore size with
23,26
˚
˚
the internal pore diameter of 18.4 A vs. 6 A for UiO-66.
The
FT-IR spectra of MOF 808 is almost the same as those reported
by others (Fig. S2†).31,32 Two intense absorption bands at
1606 cmꢀ1 and 1378 cmꢀ1 are attributed to carboxylate asym-
metric and symmetric stretching. PXRD study shows that the
experimental pattern of MOF 808 used in the present study is
almost the same as the simulated pattern (Fig. 3).
Fig. 2 (a) The fundamental cornerstones of UiO-66 are Zr6O4(OH)4 Conversion of glucose into 5-HMF
octahedral clusters with twelve carboxylate groups coordinated to the
Zr-cluster MOFs are potential candidates for conversion of
glucose due to their chemical stability and exceptionally high
surface areas. Also, we hypothesized that the Lewis acidic Zr(IV)
zirconium cations (top) and 2-X-1,4-benzenedicarboxylate (BDC-X)
organic ligands, X ¼ H, NH2, or SO3H (bottom). (b) The cubic unit cell of
UiO-66. Zr, blue; O, red; C, gray; H, white.
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DMSO itself is an efficient catalyst for the conversion of fructose
for 5-HMF, as observed by Amarasekara et al. in 2008.21 There-
fore, our catalytic studies focused solely on glucose as the
substrate.
Our initial study attempted to determine the most active of
the three MOFs (UiO-66, UiO-66–NH2 and UiO-66–SO3H) in the
conversion of glucose to 5-HMF (Fig. 5). The initial microwave
reaction was performed with a catalyst loading of 10 mg for
100 mg of glucose (i.e., 10% w/w). Under identical conditions,
the highest yield of 5-HMF (20%) was obtained using UiO-66
(Fig. 5).
Fig. 4 (a) Illustration of Zr6O4(OH)4 secondary building units (top) and
trimesic acid (H3BTC) organic linkers (bottom). (b) Structural repre-
sentation of MOF 808. Zr, blue; O, red; C, gray; H, white. m3-O and H
atoms are omitted for clarity.
Somewhat surprisingly, the yield of 5-HMF obtained using
UiO-66–NH2 was 4% lower than that obtained using UiO-66
and only 5% yield was obtained using UiO-66–SO3H. This
observation is contrary to our initial hypotheses and some-
what surprising given the similar number of accessible Lewis
metal nodes could facilitate dehydration of glucose. We thought acidic sites available in each of these materials. We propose
that the Brønsted acidic SO3H-functionalized UiO-66 could that the surface area of the UiO-66 materials is the critical
improve the yield of 5-HMF via bifunctional acid catalysis. In parameter in determining reactivity and that the presence of
the case of UiO-66–NH2, we postulated that the presence of the –NH2 and –SO3H groups results in much lower surface areas
amino group might assist in the glucose isomerization process (1045 m2 gꢀ1 and 515 m2 gꢀ1 respectively compared with 1650
due to its Lewis basic properties. Thus, UiO-66 and its m2 gꢀ1 for UiO-66) and hence, lower yields (Table S1 and
analogues were considered as our primary catalyst targets in the Fig. S3†). This also implies that the Lewis acidic centres within
dehydration of glucose. Reactions were performed in DMSO-d6 the pores (rather than only surface Lewis acid sites) are playing
to allow monitoring of reactions using 1H NMR spectroscopy.21 a key role in the isomerization step (glucose to fructose) in this
Identical results were obtained in reactions performed in reaction.
protio-DMSO. Fructose, a potential intermediate,21 was not
observed in the NMR spectra of any of the reaction mixtures. In
a control reaction, 5-HMF was formed in 2% yield in the
Optimization of reaction conditions
As the initial screening showed that the highest yield of 5-HMF
was obtained using UiO-66, we then tried to optimize reaction
conditions to improve the yield of 5-HMF. The results for
catalytic conversion of glucose to 5-HMF with different catalyst
loadings at various temperatures and times are reported in
Table 1 and Fig. S4.†
absence of a MOF catalyst. We also conrmed that Zr salts or
other soluble leachates were not responsible for the catalysis by
heating UiO-66 in DMSO-d6 (160 ꢁC, 20 min), ltering the
solution and using this solution to perform a second control
reaction. This also gave a yield of 2% – identical to that obtained
using fresh DMSO-d6. We note that under the conditions pre-
sented in Fig. 5, a control (blank) reaction using fructose in the
place of glucose gave a 61% yield of 5-HMF. This shows that
Table 1 Optimization of temperature, time and amount of UiO-66 for
the conversion of glucose to 5-HMFa,b
Entry UiO-66 loading (mg) Temperature (ꢁC) Time (min) Yield (%)
1
2
3
4
5
6
7
8
10
10
10
10
20
20
20
20
20
20
30
160
160
160
160
150
160
160
170
180
190
160
20
30
40
50
30
30
30
30
30
30
30
20
21
16
15
9
28
37c
26
26
16
25
9
10
11
a
Unless stated otherwise, the dehydration reaction of glucose to 5-HMF
was conducted in the presence of 100 mg glucose and 2 mL DMSO-d6.
b
Aer reaction, 15 mL 1-naphthaldehyde was added into the reaction
mixture for quantitative 1H NMR analysis. No glucose was observed in
the spectra of any reaction mixtures (monitored by C(2) OH d 6.2),
Fig. S5 and S6. Reaction was conducted in 2 mL solvent mixture of
DMSO-d6/water (2.5% v/v water).
c
Fig. 5 Yield of 5-HMF in the catalytic conversion of glucose to 5-HMF.
Conditions: microwave, 100 mg glucose, 10 mg UiO-66–X (X ¼ H,
NH2, SO3H), 2 mL DMSO-d6, 160 ꢁC, 20 min.
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(Table 1, Entry 6). However, the yield of 5-HMF dropped slightly
to 25% when 30 mg UiO-66 was used. Dehydration of glucose is
accelerated in an acidic environment.5b Thus, more UiO-66
added to the reaction system, and consequently a greater
number of Lewis acidic Zr(IV) sites, should speed up the
conversion of glucose to 5-HMF and increase the yield of 5-
HMF. Unfortunately, in this study the yield of 5-HMF decreases
at higher catalyst loadings possibly due to side reactions
(including humin formation) between glucose and 5-HMF i.e.
the MOF-catalysed glucose isomerization to the intermediate
fructose may have been sped up but so have Lewis-acid cata-
lyzed side reactions.
(c) Effect of temperature on yield of 5-HMF. High temper-
atures are essential for the dehydration of glucose.5b,33,34 Thus,
temperature was varied from 150 ꢁC to 190 ꢁC. As shown in
Table 1 and Fig. S4,† the yield of 5-HMF increases signicantly
initially from 150 ꢁC to 160 ꢁC and reaches 28% yield of 5-HMF
Fig.
conversion.
6 Pathways for decomposition of 5-HMF during glucose
ꢁ
at 160 C. This is a similar temperature to that reported to be
(a) Effect of reaction time on yield of 5-HMF. Reaction
optimum for the DMSO-catalyzed conversion of fructose to 5-
HMF.21 In the current study, the yield of 5-HMF decreases at
reaction temperatures above 160 ꢁC and drops to 16% at 190 ꢁC
due to decomposition of 5-HMF, as has been reported by
others.33,34 A small contribution to the decreased yield was also
observed, via 1H NMR, from the rehydration of 5-HMF at higher
temperatures. Around 8% formic acid was obtained above
160 C but no levulinic acid was seen via H NMR. Therefore,
the main cause of such low yields is the polymerization of 5-
HMF to form insoluble humin by-products.
In a previous study, an optimum yield of 5-HMF could be
obtained in the solvent mixture of THF/water (v/v 39 : 1) in the
catalytic conversion of glucose/cellulose.18,38 Thus, for our
reaction, the solvent mixture of DMSO-d6/water (v/v 39 : 1) was
also investigated. The maximum yield (37% of 5-HMF) was
obtained (Table 1, Entry 7), which was nearly 10% higher than
the yield (28%) using pure DMSO-d6, under the same reaction
conditions. Our results are consistent with those obtained by
others, which provides further evidence that just a small
amount of water can facilitate the conversion of glucose to 5-
HMF, due to increased solubility of the substrate.18,38
times of 20 to 50 min were tested. The results in Table 1 and
Fig. S4† demonstrate that the yield of 5-HMF increases initially
and then decreases with increasing reaction time. We suspect
that this is due to the decomposition of 5-HMF aer 30 min;
this trend has been observed by others.33,34 Whilst keeping UiO-
66 loading and temperature constant, the highest yield (21%) of
5-HMF was obtained at 30 min (Table 1, Entry 2). As illustrated
in Fig. 6, there are three typical pathways for the decomposition
of 5-HMF (the rehydration of 5-HMF to levulinic acid and formic
acid, as well as self- or cross-polymerization of 5-HMF).33,35–37 In
our study, the rehydration of 5-HMF was inhibited by the
presence of DMSO since less than 4% formic acid was evident
by 1H NMR in the reaction mixtures. A similar observation was
reported by Qi et al. in 2009.33 Levulinic acid was not seen in the
1H NMR of reaction mixtures and therefore must have reacted
to form an insoluble by-product. We propose that the reduction
in yield aer an optimum time has passed is most likely due to
polymerization of 5-HMF to form humin. A more detailed
discussion of humin formation is presented below.
1
ꢁ
(b) Effect of catalyst loading on yield of 5-HMF. To assess
the effect of different catalyst loadings on yields of 5-HMF, the
reaction was conducted in the presence of 100 mg glucose at
160 ꢁC for 30 min using 10, 20 or 30 mg of UiO-66. The yield of 5-
HMF in a control blank reaction was 3% but increased to 21%
when 10 mg UiO-66 was used and 28% when 20 mg was used
It is important to compare our results with those already
performed using different MOFs. Table 2 summarizes the yield
of 5-HMF in the catalytic conversion of glucose to 5-HMF with
three different typical MOFs. Unlike UiO-66–SO3H catalyst
Table 2 Comparison of glucose conversion to 5-HMF using UiO-66 and literature reports using three different MOFs
Reaction conditions
Entry (Ref.)
Glucose loading (mmol)
Solvent
Catalyst
Temp. (ꢁC)
Time
5-HMF yield (%)
1 (this work)
2 (this work)
3 (ref. 18)
4 (ref. 18)
5 (ref. 19)
6 (ref. 19 and 38)
7 (ref. 19 and 38)
8 (ref. 19 and 38)
0.56
0.56
1.24
1.24
0.10
0.10
DMSO-d6
UiO-66
UiO-66
MIL-101
MIL-SO3H
NU-1000
PO4/NU(half)
PO4/NU(half)
PO4/NU(half)
160
160
130
130
140
140
140
140
30 min
30 min
24 h
24 h
5 h
5 h
5 h
5 h
28
37
2.0
29
2.3
15
20
64
39 : 1 (v/v) DMSO-d6/water
39 : 1 (v/v) THF/water
39 : 1 (v/v) THF/water
Water
Water
9 : 1 (v/v) 2-PrOH/water
9 : 1 (v/v) 2-PrOH/water
0.10
0.10 ꢂ 10ꢀ2
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studied herein, the SO3H-functionalized MIL catalyst gave
During the course of reactions, we observed that the color of
a good yield (29%) of 5-HMF but only 2% 5-HMF was formed the solid catalyst changed from white to dark brown (Fig. S7†).
using bare, unsulfontated MIL-101.18 The possible explanation The existence of humin on the surface or inside the pores of
is that MIL-SO3H has more Brønsted acidic sites and maintains UiO-66 would cause a decrease in the surface area, which would
a fairly high surface area (1333 m2 gꢀ1) as compared with bare affect the yield of 5-HMF. Thus, we examined the surface area of
MIL-101.18 Katz, Farha and co-workers found that the UiO-66 before and aer reaction – the latter material we refer to
phosphate-modied NU-1000 gave much higher yield of 5-HMF, as UiO-66–humin from herein. From N2 adsorption isotherms,
comparing with the yield using bare NU-1000.19 Moreover, the the BET surface area for UiO-66 and UiO-66–humin was seen to
yield of 5-HMF increased in the solvent mixture of 2-PrOH/water decrease signicantly from 1650 m2 gꢀ1 to 598 m2 gꢀ1 respec-
(v/v 9 : 1) with PO4/NU(half).19,38 They also demonstrated that tively (Fig. 8). Also, TGA data conrmed the presence of
a lower concentration of glucose could reduce humin forma- a signicant amount of organic matter within UiO-66–humin.
tion, which enhanced the yield of 5-HMF (Table 2, Entry 8).19
Between 100 and 450 ꢁC a weight loss of 36 wt% is observed for
UiO-66–humin whereas no weight loss occurs between these
temperatures for UiO-66. This organic matter would account for
10 wt% of the original glucose used.
Recycling test
In order to evaluate the stability of UiO-66, catalyst recycling
experiments were performed, and the results are reported in
Fig. 7. Overall, the yield of 5-HMF decreased very slightly on rst
re-use (run 2, 19%) but gave a similar yield to run 1 upon its
fourth re-use (run 1, 23%; run 5, 22.5%). The mean yield across
runs 2–5 was 20.6%, standard deviation ꢃ 1.5%.
Characterization of humin on MOF surface
Humin is an unavoidable byproduct in the catalytic conversion
of biomass,39 and its formation can be catalyzed by Lewis acidic
centres.6a,d The structure of humin has not been well-studied
until recently. Sumerskii proposed that humin is composed of
60% furan rings together with 20% ether or acetal aliphatic
linkers.22b Furthermore, the mechanism of humin formation is
not fully understood. In the acidic dehydration of glucose using
MOFs, the formation of humin has been observed aer reaction
by our group and others.18,19,40 However, as far as we are aware
although papers have reported the formation of humin on MOF
surfaces indirectly, e.g., through surface area measurements,
the characterization of humin on the surface of a MOF has not
been reported previously. Herein, we tried to investigate the
existence of humin on UiO-66 using different characterization
techniques.
Due to the formation of humin, the color of UiO-66 changed
to dark brown aer glucose reactions (Fig. S5†). We assume that
the existence of the humin on the surface signicantly affects
the yield of 5-HMF obtained. Then, we examined the surface
area of UiO-66 and UiO-66–humin. Its adsorption ability
decreased signicantly aer reaction (Fig. 8). The BET surface
area of UiO-66–humin was 598 m2 gꢀ1, which was considerably
lower than that of UiO-66 before the reaction (1653 m2 gꢀ1).
These differences in the N2 adsorption isotherms indicate the
formation of humin would inevitably affect the efficiency of
UiO-66 in catalysis, assuming that the reaction proceeds via
a heterogeneous mechanism wherein surface area will be
a critical parameter for catalytic activity. We note in studies by
others that similar decreases were observed.18 Herbst and Jan-
iak reported that the surface area of MIL-SO3H was reduced
from 1333 m2 gꢀ1 to 443 m2 gꢀ1 aer reaction in the THF/water
mixture of 39 : 1.18
Fig.
7 Recycling experiment with UiO-66. Reaction conditions:
microwave, 1000 mg glucose, 200 mg UiO-66, 15 mL DMSO-d6,
160 ꢁC, 30 min.
PXRD patterns of simulated UiO-66, synthesized UiO-66 and
UiO-66–humin show no signicant changes in peak positions
(Fig. 9). However, the crystallinity of UiO-66–humin is dimin-
ished with respect to UiO-66 since the two main peaks at 2q 7.4ꢁ
and 8.5ꢁ have become broader, and likely less intense (as
evidence by the signal-to-noise ratio) aer reaction. We
hypothesize that the occurrence of an additional small peak at
Fig. 8 Nitrogen adsorption isotherms at 77 K of UiO-66 and UiO-66–
humin.
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Fig. 9 PXRD patterns of UiO-66 simulated (black), UiO-66 synthe-
sized (red) and UiO-66–humin (blue).
Fig. 10 13C solid state NMR spectrums of UiO-66 (bottom) and UiO-
66–humin (top).
a 2q of 6.2ꢁ in UiO-66–humin represents ‘forbidden’ reections
for the topological space group due to diffuse scattering by the
humin.41
peaks, that are not found in the spectrum of UiO-66, strongly
conrm the formation of humin on UiO-66 during glucose
dehydration reactions. It should be noted that the peaks at
ꢄ130, 138 and 170 ppm are broad and although these are in
a similar place to the carbon atoms of the 1,4-benzene-
dicarboxylate (BDC) linker in the spectrum of UiO-66, they
might be coincident with substituted carbon atoms of furan
rings and carbon atoms from carboxyl or ester groups in the
humin.22d
Using FT-IR spectroscopy, we nd that the bands in the
spectrum for UiO-66–humin are much broader than those in
the spectrum of UiO-66 (Fig. S8†). In the spectrum of UiO-66–
humin, a broad peak around 3352 cmꢀ1 can be attributed to
C–O stretch from alcohols.22b,42 A weak absorption band at
2919 cmꢀ1 is from aliphatic C–H stretches.22b,42 Some differ-
ences between the spectra of these materials might be attrib-
uted to the existence of furan rings, such as the broader C]C
We also set up three comparative reactions for studying the
effect of humin formation on the yield of 5-HMF and to assess
inhibitory affect that the humin has by blocking access to pores
within the MOF. Initially, we ran a microwave reaction with
20 mg UiO-66 at 160 ꢁC for 30 min – this reaction affords a yield
of 28%. Aer that, we explored two different routes. Either
20 mg of fresh UiO-66 or 100 mg of glucose was added into the
reaction system and the vial heated under the same optimal
reaction conditions for 30 min. We found that the yield of 5-
HMF increased from 28% to 35% when 20 mg of fresh UiO-66
was added – this suggests that some unreacted glucose and
intermediate dehydration products are still present in the initial
reaction mixture but unable to react to form 5-HMF once UiO-
66–humin has formed. In contrast, the overall yield of 5-HMF
decreased from 28% to 23% when an additional 100 mg glucose
was added aer the initial 30 min reaction. We think the
additional 100 mg glucose could possibly cross-polymerize with
the 5-HMF to produce insoluble humins and the used UiO-66
with a diminishing surface area may not catalyze the isomeri-
zation of the additional glucose effectively. In a third study,
a reaction was performed using 40 mg UiO-66 for 2 ꢂ 30 min at
160 ꢁC and a 5-HMF yield of only 26% obtained. This indicates
that humin formation reduces the amount of 5-HMF that can
form and that this is likely due to the humin blocking the pores
of the MOF. This provides further evidence for the Lewis cata-
lyzed reaction occurring within the pores of the MOF, which was
previously suggested by the lower yields obtained using UiO-66–
SO3H that had a lower surface area.
stretching absorption at 1583 cmꢀ1 and the C–O stretching
absorption at 1017 cmꢀ1
,
with the latter band being signi-
42
cantly more intense than a weak absorption in the same region
for UiO-66. Below 1000 cmꢀ1 in the ngerprint region, peaks at
952 cmꢀ1 and 746 cmꢀ1 might also indicate the presence of
substituted furan rings.22c
To further investigation the formation of humin on UiO-66,
13C solid-state NMR spectroscopy was applied to examine the
UiO-66 samples before and aer reaction (Fig. 10). The 13C
solid-state NMR spectrum of UiO-66 contains three character-
istic peaks at chemical shis of 128.9, 137.1 and 170.8 ppm.
Based on the study by Devautour-Vinot and Martineau et al.,43
the peak at 128.9 ppm is ascribed to the –CH group of the
aromatic rings. The peak at 137.1 ppm is from the quaternary
aromatic carbon atoms. The peak at 170.8 ppm is assigned to
carbon atoms from the carboxylate groups.43 The appearance of
a low-intensity peak at 167.6 ppm is due to the C]O group of
DMF solvent molecules present in the pores of the MOF.
Comparing the 13C Solid State NMR spectrum of UiO-66–humin
with UiO-66, signicant peak broadening is observed in the
spectrum, which indicates the formation of a material that is
less crystalline, corroborating the PXRD data (Fig. 9).
Moreover, an intense peak located around 39 ppm repre-
sents the presence of tertiary and/or quaternary aliphatic car-
bons.22d A broad signal between 60 and 90 ppm is the result of
many different C–O groups from alcohol or ether functionalities
within the humin structure.22c,d These two mid-to-high eld
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Unfortunately, in attempts to re-use MOF 808, recycling was
less successful than with UiO-66. Upon re-use of MOF 808 for
dehydration of glucose, in a similar fashion to that described
above using UiO-66, the yield for run 2 was only 14%
(compared with 38% in run 1, and 19–22.5% in runs using
recycled UiO-66). This implies that this higher surface area
MOF is less stable to the hydrolytic conditions present in this
transformation – hydrolytic instability has been studied
previously for other zirconium-containing MOFs.46
We also attempted the conversion of sucrose to 5-HMF with
MOF 808. The reaction conditions were the same as those used
in the glucose conversion to 5-HMF with MOF 808. The yield of
5-HMF is 46%, which is 8% higher than that formed in sucrose
conversion in a control reaction (38%) without added catalyst
wherein DMSO would catalyze the conversion. This implies
that the added MOF catalyst is playing a minor role in
affording the good yield of 5-HMF from sucrose. Also, in
sucrose conversion to 5-HMF, it is interesting that there is no
signicant difference in the yield of 5-HMF using UiO-66 (44%)
and MOF 808 (46%). Based on the higher yields in these
reactions compared with those of glucose, we hypothesize that
intermediates particular to glucose conversion (either from
glucose itself or from hydrolysis of sucrose) rather than fruc-
tose conversion are responsible for humin formation on the
surfaces of the MOF catalyst and inhibit its reactivity. During
the course of our research, it has been reported that UiO-66 is
an efficient catalyst for isomerizing glucose to fructose in
alcoholic media.47 Therefore, solvent choice is critical to the
outcome of these reactions too and the presence of alcohol
would likely suppress formation of humin on the catalyst
surface.
Dehydration of glucose to 5-HMF using MOF 808
Another zirconium-containing MOF, MOF 808, has emerged as
a potential candidate for gas adsorption44 and catalysis45 due to
its unique features. Compared with 12-connected or 8-con-
nected MOFs,23 MOF 808 possesses lower connectivity (6-con-
nected) and therefore it allows greater pore access to more
reactant or adsorbent molecules. Therefore, owing to its larger
surface area and wider pore size, we speculated that MOF 808 (6-
connected) may enhance the yield of 5-HMF obtained from
glucose compared with UiO-66.
A sample of MOF 808 was examined by PXRD (Fig. 3) and N2
adsorption (Fig. S3†) before use. The PXRD pattern of synthe-
sized MOF 808 shows good crystallinity as compared to the
pattern of simulated MOF 808. The BET surface area of MOF 808
was 1970 m2 gꢀ1 (Table S1†). The catalytic reaction was per-
formed at 160 ꢁC for 30 min. MOF 808 affords the highest yield
of 5-HMF (31%) of the four MOF catalysts studied herein under
our standardized conditions. This yield is 10% higher than that
produced by UiO-66 (21%) under identical conditions. We
propose that the higher yield produced by MOF 808 can be
attributed to its large pore size and high surface area. However,
it may also be due to lower coordination numbers of the
zirconium centres, which means they are can interact more
readily with substrates in the catalytic cycle. The yield of 5-HMF
was 28% in the presence of the solvent mixture of DMSO-d6/
water (v/v 39 : 1), which is slightly lower than the yield obtained
using pure DMSO-d6. We hypothesize that the aggregation of
water molecules in the pores of MOF 808 might block the
channels of MOF 808 and thereby decrease the yield of 5-HMF.23
This is more likely to occur in MOF 808 compared with UiO-66,
as the Zr centres in MOF 808 are coordinatively unsaturated
compared with those in UiO-66 and this means the water
molecules are more likely to coordinate at the Zr centres and
block the pores or prevent coordination of the glucose to the
Lewis acid sites. Further studies are therefore needed to fully
understand the interplay of surface area and coordination site
availability in such catalyst systems. We considered studying
the related zirconium-containing MOF, UiO-67, which contains
biphenyldicarboxylate linker units in place of terephthalate
units.9d However, due to the known poor hydrolytic stability of
the metal–carboxylate bonds in UiO-67,46 we did not pursue
this.
Conclusions
UiO-66 provides the highest yield of 5-HMF among UiO-66–X
compounds (X ¼ H, NH2, SO3H). Time, temperature and cata-
lyst loading were varied to determine the optimal reaction
conditions. Recycling tests show that the catalytic efficiency of
UiO-66, although only moderate, can be maintained over 5 runs.
Moreover, we were able to compare UiO-66 and UiO-66–humin
using different analytical techniques, which proved the exis-
tence of humin on UiO-66. The related MOF, MOF 808, which
has a higher surface area and more accessible zirconium
centres, gave a signicantly higher yield of 5-HMF in compar-
ison with UiO-66–X (X ¼ H, NH2, SO3H) under identical
conditions. These data suggest that, although Lewis acidic
centres are critical to the glucose isomerization step of this
process, surface area is a critical parameter for efficient MOF-
catalyzed dehydration of glucose to yield 5-HMF, and addi-
tional functionality does not enhance catalysis as the presence
of –SO3H or –NH2 functional groups leads to lower surface area
materials. Further studies would be needed to fully understand
the role of solvent, the interplay of Lewis acid and Brønsted
acidic sites in such catalysts and conrm the importance of
surface area and pore-accessibility in MOF-catalysts for this and
related reactions.
Unfortunately, MOF 808 turned dark brown aer reaction,
which indicated humin formation. Herein, MOF 808 aer
reaction is named as MOF 808-humin. Using FT-IR spectros-
copy, we compare spectrums of MOF 808 and MOF 808-humin
(Fig. S9†). In the spectrum of MOF 808-humin, a broad peak
around 3340 cmꢀ1 is due to C–O stretch from alcohols.22b,42
Other differences between these two also indicate the formation
of furan rings, for instance, the C]C stretching absorption at
1577 cmꢀ1 and the C–O stretching absorption at 1023 cmꢀ1
.
42
Moreover, below 1000 cmꢀ1 in the ngerprint region, peaks at
990 cmꢀ1 and 758 cmꢀ1 are attributed to the substituted furan
rings.22c We nd that some similarities between UiO-66–humin
and MOF 808-humin demonstrate that the structure of humin
contains furan rings with alcohol functional groups (Fig S10†).
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
We would like to acknowledge the Natural Sciences and Engi-
neering Research Council of Canada (NSERC), Canada Foun-
dation for Innovation (CFI) and Memorial University of
Newfoundland for funding. We thank Dr C. M. Schneider (C-
CART, Memorial University) for acquiring solid-state NMR
spectra and K. Andrea for TGA data.
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