Exceptionally Efficient and Recyclable Heterogeneous Metal–Organic Framework Catalyst for Glucose Isomerization in Water

Article abstract of DOI:10.1002/cctc.201701825

Heterogeneous catalysts are desired for the conversion of glucose, the most abundant sugar in renewable biomass, but presently their synthesis requires highly toxic chemicals with long synthesis times. We report the conversion of glucose into fructose and 5-hydroxymethylfurfural on a heterogeneous catalyst that is stable and selective and operates in the most environmentally benign solvent, water. We used a bifunctional solid with Lewis and Br?nsted acid sites by partially replacing the organic linker of the zirconium organic framework UiO-66 with 2-monosulfo-benzene-1,4-dicarboxylate. This catalyst showed high product selectivity (90 %) to 5-hydroxymethylfurfural and fructose at 140 °C in water after a reaction time of 3 h. It was recyclable and showed only a minor loss in activity after the third recycle, offering a realistic solution for the bottleneck glucose isomerization reaction for scale-up and industrial application of biomass utilization.

Full text of DOI:10.1002/cctc.201701825

Accepted Article
Title: Exceptionally Efficient and Recyclable Heterogeneous Metal-
Organic Framework Catalyst for Glucose Isomerization in Water
Authors: Ryan Oozeerally, David L Burnett, Thomas W Chamberlain,
Richard I Walton, and Volkan Degirmenci
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Link to VoR: http://dx.doi.org/10.1002/cctc.201701825
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ChemCatChem
10.1002/cctc.201701825
COMMUNICATION
Exceptionally Efficient and Recyclable Heterogeneous Metal-
Organic Framework Catalyst for Glucose Isomerization in Water
[
a]
[b]
[b]
[b]
Ryan Oozeerally, David L. Burnett, Thomas W. Chamberlain, Richard I. Walton,* and Volkan
Degirmenci*[a]
Abstract: Heterogeneous catalysts are desired for the conversion of
glucose, the most abundant sugar in renewable biomass, but
presently their synthesis requires highly toxic chemicals with long
synthesis times. We report the conversion of glucose to fructose and
same time via simple synthesis protocols; in this case without
highly toxic and corrosive HF,[12] in less than 24 h at 120°C.
5-hydroxymethyl furfural on a heterogeneous catalyst that is stable
and selective and operates in most environmentally benign solvent,
water. We used a bi-functional solid with Lewis and Brønsted acid
sites by partially replacing the organic linker of the zirconium organic
framework UiO-66 with 2-mono-sulfo-benzene-1,4-dicarboxylate.
This catalyst shows high product selectivity (90 %) of 5-
hydroxymethylfurfural and fructose at 140°C in water after 3 h
reaction. It is recyclable and shows only minor loss of activity after a
rd
3
recycle, offering a realistic solution for the bottleneck reaction of
glucose isomerization for scale up and industrial application of
biomass utilization.
Sustainable production of chemicals requires the utilization of
renewable resources, one of the most promising of which is
lignocellulosic biomass.[1-2] Biomass derived sugars (e.g.,
glucose or fructose) can be converted into platform molecules,
e.g. 5-hydroxymethyl furfural (HMF), which can be further
processed into monomers, fuel additives, paints and a variety of
fine chemicals envisaged in a future biorefinery[3-4]. Although
[
5]
fructose can be converted into HMF easily , glucose is the main
building block of lignocellulosic biomass and its conversion
remains challenging.[4] The best performing heterogeneous
catalyst for this conversion is tin-incorporated beta zeolite (Sn-
4
+
beta) with Sn occupying a fraction of tetrahedral sites in the
zeolite framework.[6-8] Sn-beta can effect the isomerization of
glucose to fructose in water with high selectivity (> 50%).[7]
However, Sn-beta requires long crystallization times, up to 40
days, which is industrially unviable, at high temperatures, 140°C,
and, moreover, requires the use of hydrofluoric acid, an acute
poison and extremely corrosive.[7] In this work, we present a
recyclable catalyst for glucose isomerization. It is based on
modified UiO-66 (Figure 1a),[9] a thermally and hydrothermally
robust metal-organic framework (MOF), which we show matches
the conversion and product selectivity of Sn-beta.
Figure 1. a) Schematic representation of UiO-66 framework. b) Glucose
conversion to HMF through isomerization into fructose; c) Isomerization of
glucose in water on metal organic framework catalysts; UiO-66, UiO-66-
MSBDC(10) and UiO-66-MSBDC(20). Reaction conditions: 140 °C, 3 hours,
stock solution of 10 wt. % glucose in deionized water.
The advantage of using MOFs as heterogeneous catalysts is
the potential for tuning the solids’ properties by inclusion of
desired functional ligands[10]
,
such as acid sites,[11] and at the
The challenge in the HMF production from glucose is to
achieve high product selectivity. The reaction proceeds through
isomerization of glucose to fructose (Figure 1b)[13] which is the
limiting step to achieve high selectivity. It is proposed in the
literature that the reaction is catalysed by Lewis acids[ , which
enable a hydride shift between carbon atoms of glucose[ , at
the same time, proximal silanol groups or Brønsted acid sites
[
[
a]
b]
Mr R. Oozeerally, Dr V. Degirmenci
School of Engineering
University of Warwick
Coventry, CV4 7AL, UK
E-mail: v.degirmenci@warwick.ac.uk
Dr D. L. Burnett, Mr T. W. Chamberlain, Prof R. I. Walton
Department of Chemistry
13]
14]
form
a hydrogen-bonding network, facilitating the proton
[
15]
mobility . UiO-66 is a zirconium-based MOF with benzene-1,4-
dicarboxylate (BDC) linkers, showing high stability in air up to
University of Warwick
Coventry, CV4 7AL, UK
E-mail: r.i.walton@warwick.ac.uk
[9]
500°C as well as hydrothermal inertness . Defects in the form
of coordinatively unsaturated Zr4+ sites provide Lewis acidity.[16]
We find that UiO-66 itself is active in glucose conversion (Figure
Supporting information for this article is given via a link at the end of
the document
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ChemCatChem
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COMMUNICATION
1
c) showing 16 % conversion accompanied with 10 % product
values (See SI Table S1 and S2). Thermogravimetric analysis
(TGA) shows an extensive loss in mass at around 510°C for
both the standard and functionalized UiO-66 materials (See SI
Figure S4). This is consistent with the reported decomposition
temperature of 540°C for UiO-66 and approximately 500°C for
sulfonic UiO-66 materials reported in literature.[9, 18] Mass loss
indicates an MSBDC linker content of 14.6 % and 24.7 % for
UiO-66-MSBDC(10) and UiO-66-MSBDC(20), respectively,
close to the expected values. As a result, the ratio of the
zirconium:linker in UiO-66, UiO-66-MSBDC(10) and UiO-66-
MSBDC(20) were found as 5.51, 5.11 and 5.63 respectively and
thus coordinatively unsaturated Zr4+ sites are present (See SI
Table S3 and S4).
yield at 140°C in 3 h. However, it lacks Brønsted acid sites.
Therefore, we used a catalyst synthesized by partially replacing
the BDC linker with 2-mono sulfonated benzene-1,4-dicarboxylic
acid (MSBDC)[17-18] which shows 31 % glucose conversion under
same reaction conditions with 28 % product yield (Figure 1c).
This corresponds to an exceptional product selectivity of around
9
0 %, which is similar to previously reported Sn-beta zeolite.[7]
Powder X-ray diffraction (PXRD) analysis of the catalysts
shows the formation of crystalline MOF structures (Figure 3a).
Indeed, the addition of the MSBDC did not alter the average
structure of UiO-66. The lattice parameter of the fresh UiO-66
was determined as 20.7516(2) Å (See SI Figure S5). This value
compares well with the reported literate value of 20.7551(5) Å,[
while the lattice parameter of UiO-66-MSBDC(20) was
determined as 20.7431(13) Å (Figure 3a) and a similar result
was obtained for UiO-66-MSBDC(10) (See SI Figure S5).
9]
Figure 2. SEM Image (a) and zirconium EDX mapping of UiO-66. SEM Image
The significant increase in fructose yields, combined with
marginal increases in HMF yields, suggests that the modification
of UiO-66 with MSBDC could affect the Lewis acidity in two
ways. First, more defective materials are formed; this is
supported by the increase in the mesopore volume of the UiO-
(c), zirconium (d) and sulfur (e) EDX mapping of UiO-66-MSBDC(20).
The ratio between BDC and MSBDC linkers is critical for the
successful synthesis of a stable functionalized UiO-66 material.
Higher ratios of MSBDC within the framework have already been
shown to decrease the stability UiO-66.[9, 18] Indeed, we find that
6
6-MSBDC catalysts (See SI Table S5 and Figure S6). Second,
the Lewis acidity of Zr4 is known to be enhanced significantly by
the presence of a nearby electron-withdrawing group that has
been extensively studied in sulfated zirconia catalysts.[ This
effect has recently been reported in MOFs in the presence of
+
if only MSBDC is used as the ligand then the material
subsequently collapses on hydrothermal treatment (Supporting
Information, Figure S1). As such, materials containing 10 and 20
percent functionalized linker were synthesized (UiO-66-
MSBDC(y); where y represents the mol. % of MSBDC linker in
total linker content). SEM images (Figure 2a, c) show the
particle morphology of UiO-66 and UiO-66-MSBDC(20).
Zirconium EDX mapping (Figure 2b, d) demonstrates the
uniform distribution of zirconium atoms in both MOF structures,
while, sulfur EDX mapping of the UiO-66-BDC(20) catalyst
21]
electron withdrawing fictional groups such as -NO
2
on the
organic linker;[22] indeed, we find that -NO
modified UiO-66
2
shows improved conversion over the parent material (Figure S8)
and so it is conceivable that the sulfonyl acid groups have a
similar effect. Clearly, further work is needed to understand fully
the interplay of the acid functionalities.
The recyclability of the catalysts is crucial for scale up and
industrial application: we studied this by recovering the solid
catalysts using a centrifuge and washing with water after each
reaction cycle. It was observed that full recovery of the catalysts
was not possible due to the presence of small catalyst particles
that remained dispersed in the reaction medium. These
nanocrystalline materials have intrinsically higher activity (Figure
S8). However, once the small particles are filtered out after the
first run, all the catalyst is recoverable in the consecutive
reaction cycles (See SI Table S6). Therefore, although a
decrease in glucose conversion was observed after the first run,
no loss of activity was observed in the following 3 recycles
(
Figure 2e) indicates a similar distribution of modified linker
across the MOF crystal. Although EDXA mapping does not give
information on the 3-dimensional distribution, it clearly implies
the uniform distribution of Brønsted acid sites with some
evidence for enrichment at the crystal surface UiO-66-
MSBDC(20) catalyst (See SI Figure S2 for all catalysts). Further,
EDX analysis of the MSBDC containing materials reveals an
absence of sodium, supported by bulk ICP-OES analysis,
consistent with the displacement of sodium ions during synthesis
3
to yield Brønsted acidic SO H sites.
The incorporation of sulfonic acid groups was also confirmed
through FT-IR spectroscopy. New peaks appear in the UiO-66-
MSBDC catalysts at 620, 1078, 1180 and 1223 cm-1 and their
intensity increases with the increasing linker content (See SI
Figure S3). These bands are attributed to the characteristic
asymmetric bending and symmetric and asymmetric stretching
of S=O double bonds and S-O bonds.[19-20] Elemental analyses
(
Figure 3b), particularly for the UiO-66-MSBDC(20) catalyst (See
SI Table S9 for product yields). The PXRD pattern of the UiO-
6-MSBDC(20), which was recovered after four runs shows that
6
the integrity of the MOF lattice is maintained (Figure 3a).
Zirconium and sulfur EDX mapping of the catalysts after four
reaction cycles further confirmed the integrity of the recycled
catalysts (See SI Figure S2). The recycling of UiO-66 and UiO-
of fresh catalysts also show S:Zr ratios close to the expected
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6
6-MSBDC(10) catalysts show minor loss of activity after the 4th
It is important to note, however, that elemental analysis of
the reaction solution after first reaction cycle (3 h reaction at
140°C) showed only trace amounts of sulfur and zirconium
present showing the stability of the catalyst with negligible
leaching during the reaction (See SI Table S4). Finally, the
performance of the UiO-66 materials were compared to Sn-beta.
In the literature, Sn-beta is used as a glucose isomerization
catalyst with a Sn to glucose ratio of 1:50 and catalyst weight of
Sn-beta far exceeds the amount of MOF catalyst used in this
study under similar reaction conditions, where Sn-beta shows
run. This loss in activity could be in part due to the formation of
undesired side products, such as humins. These are poorly
characterized oligomeric species, which are known to be the
main side product of this reaction . These insoluble products
can accumulate on the catalyst surface and block the active
sites. Indeed, the recovered catalyst mass in recycle tests
increased due to the collection of inseparable side products
[
3]
(
See SI Table S3), would explain the lower sulfur counts in EDX
analysis of the recycled catalyst as compared to fresh catalysts.
[
7]
54 % glucose conversion with 30 % fructose yield . Similar
a)
conversion (48 %) and product yield (34 %, See SI Figure S7)
were obtained when 40 mg of catalyst (UiO-66-MSBDC(20))
was used, which is still less than the quarter of the amount of
Sn-beta catalyst (200 mg).
a = 20.7431 (13) Å
Tailor-made MOFs with desired functionalities have made it
possible to achieve exceptionally efficient catalysts for glucose
isomerization in water. UiO-66-MSBDC catalysts containing dual
acidity, Lewis and Brønsted, provide exceptional product
selectivity of around 90 % for glucose conversion into fructose
and HMF reaching the performance of Sn-beta zeolite. Other
MOF catalysts reported in the literature for glucose isomerization
use either frameworks constructed from toxic metals (e.g.,
chromium[23-26]) and/or have been used in non-aqueous solvents
that are toxic or flammable (e.g. DMSO or THF[27]). Our results
show that UiO-66-MSBDC(y) catalysts are highly promising for
scale-up because as well as operating in aqueous conditions
and being recyclable, their synthesis does not require the toxic
and corrosive conditions with a simple protocol and short
duration. Scale-up of MOF synthesis using continuous flow
reactors, often using water as a reaction medium, makes this a
realistic prospect.[28] Enzymes including metal centres and basic
histidine moieties possessing multi-functional capabilities are
Nature’s catalysts providing high selectivity at the expense of
slow reactions and sensitive operational systems. Future work
on the MOF catalysts will be devoted towards the better
understanding of the active sites of this catalyst and the
mechanism of their activity to optimize the product distribution,
and their long-term stability in industrially relevant flow chemistry
conditions.
2
0
40
60
a = 20.7953 (18) Å
2
0
40
60
10
20
30
40
50
60
70
2
 (Degrees)
b)
Run 1
Run 2
Run 3
Run 4
40
35
30
25
20
15
10
5
0
Experimental Section
Synthesis of catalysts: UiO-66 was prepared by mixing 2.481 g zirconium
chloride (Alfa Aesar), 3.54 g 1,4-benzenedicarboxylicacid (Sigma Aldrich),
100 ml N,N-dimethylformamide (Fisher Scientific) and 20 ml hydrochloric
acid (37 %, VWR). The synthesis mixture was then transferred to a
PTFE-lined autoclave and heated to 120 °C for 24 h. Afterwards,
materials were filtered, washed with methanol and dried in air at 70°C.
UiO-66-MSBDC(y) catalysts were prepared by substituting the benzene-
)
)
(10
(20
DC
UiO-66
DC
1,4-dicarboxylic
acid
with
monosodium
2-sulfo-benzene-1,4-
UiO-66-MSB
UiO-66-MSB
dicarboxylate (TCI Chemicals). Catalytic activity tests: Catalyst (10 mg)
was placed in a reaction vial (4 ml) with a magnetic stirring bar and 10
wt. % aqueous glucose solution was added. The vial was closed and
placed in a preheated oil bath at 140 °C for 3 h. The reaction was
quenched at 0°C and the product mixture analysed by HPLC.
Characterisation of catalysts: Powder XRD data were collected using a
Panalytical X’Pert Pro MPD equipped with monochromatic Cu Kα1
radiation and a PIXcel solidstate detector. Micrographs and elemental
Figure 3. a) PXRD patterns of the UiO-66-MSBDC(20) as fresh catalyst
th
(above) and after the 4 run (below). Insets show the 2 theta region between
10 and 70 degrees. The green lines are the fitted profile, black dots are
observed data and the blue line is the difference in the two patterns. The ticks
represent positions of allowed Bragg peaks: pink for UiO-66 and pale blue,1,4-
benzenedicarboxylic acid. b) Glucose conversion after recycle tests.
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maps were obtained using a Zeiss Gemini scanning electron microscope
with a large are SDD EDX detector, operating at 5 keV. Nitrogen
adsorption isotherms were measured at -196°C on a Micromeritics
ASAP2020 system. The samples were outgassed at 150°C for 12 h prior
to the sorption measurements. Infra-red spectra were recorded using a
Perkin Elmer Paragon 1000 FT-IR Spectrometer in attenuated total
reflection mode. Thermogravimetric analysis (TGA) was performed using
a Mettler Toledo Systems TGA/DSC 1 instrument under a constant flow
of air (50 mL/min). Elemental analysis was performed by Medac Ltd (UK)
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Entry for the Table of Contents
Highly selective glucose conversion into fructose and 5-hydroxymehtyl furfural over metal-organic
framework catalyst (UiO-66-MSBDC). It operates in water at little over 100 °C, providing benign
conditions with non-toxic reagents. It is recyclabile and constructed from a readily available and
inexpensive organic ligands.
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