Incorporation of Al3+ Sites on Br?nsted Acid Metal–Organic Frameworks for Glucose-to-Hydroxylmethylfurfural Transformation

Article abstract of DOI:10.1002/smll.202006541

5-hydroxylmethylfurfural (HMF) is a bio-based chemical that can be prepared from natural abundant glucose by using combined Br?nsted–Lewis acid catalysts. In this work, Al³⁺ catalytic site has been grafted on Br?nsted metal–organic frameworks (MOFs) to enhance Br?nsted–Lewis acidity of MOF catalysts for a one-pot glucose-to-HMF transformation. The uniform porous structure of zirconium-based MOFs allows the optimization of both acid strength and density of acid sites in MOF-based catalysts by incorporating the desired amount of Al³⁺ catalytic sites at the organic linker. Al³⁺ sites generated via a post-synthetic modification act as Lewis acid sites located adjacent to the Br?nsted sulfonated sites of MOF structure. The local structure of the Al³⁺ sites incorporated in MOFs has been elucidated by X-ray absorption near-edge structure (XANES) combined with density functional theory (DFT) calculations. The cooperative effect from Br?nsted and Lewis acids located in close proximity and the high acid density is demonstrated as an important factor to achieve high yield of HMF.

Full text of DOI:10.1002/smll.202006541

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Incorporation of Al3+ Sites on Brønsted Acid Metal–
Organic Frameworks for Glucose-to-Hydroxylmethylfurfural
Transformation
Vitsarut Tangsermvit, Taweesak Pila, Bundet Boekfa, Vetiga Somjit, Wantana Klysubun,
Jumras Limtrakul, Satoshi Horike,* and Kanokwan Kongpatpanich*
low cost. Glucose-to-HMF transformation
is scientifically challenging because it con-
5-hydroxylmethylfurfural (HMF) is a bio-based chemical that can be prepared
from natural abundant glucose by using combined Brønsted–Lewis acid
catalysts. In this work, Al3+ catalytic site has been grafted on Brønsted metal–
organic frameworks (MOFs) to enhance Brønsted–Lewis acidity of MOF
catalysts for a one-pot glucose-to-HMF transformation. The uniform porous
structure of zirconium-based MOFs allows the optimization of both acid
strength and density of acid sites in MOF-based catalysts by incorporating
the desired amount of Al3+ catalytic sites at the organic linker. Al3+ sites gen-
erated via a post-synthetic modification act as Lewis acid sites located adja-
cent to the Brønsted sulfonated sites of MOF structure. The local structure of
the Al3+ sites incorporated in MOFs has been elucidated by X-ray absorption
near-edge structure (XANES) combined with density functional theory (DFT)
calculations. The cooperative effect from Brønsted and Lewis acids located
in close proximity and the high acid density is demonstrated as an important
factor to achieve high yield of HMF.
tains two consecutive steps requiring both
Brønsted and Lewis acid sites.[2] Lewis
acid site promotes the isomerization of
glucose to fructose, which is subsequently
dehydrated to HMF at Brønsted acid site
of the catalysts. Several heterogeneous
catalysts including sulfate alumina,[3] zir-
conium phosphates,[4] silica,[5] and zeolites
have been developed for glucose-to-HMF
transformation.[6] However, the aforemen-
tioned catalysts possess only one acid type
in their structure due to the structural
limitations resulting in high conversion
but low HMF yield. There is still limited
number of heterogeneous catalysts with
both Brønsted and Lewis acid sites in a
single catalyst, which are expected to coop-
eratively catalyze the tandem reaction to
produce HMF from glucose. Therefore,
heterogeneous catalysts with co-existing two types of acid sites
are essential to achieve high yield of HMF from glucose at mild
conditions.
Metal–organic frameworks (MOFs) or porous coordination
polymers (PCPs) are crystalline porous materials composed of
metal clusters and organic linkers.[7] MOFs/PCPs have tunable
structure and ability to incorporate several functional groups
through their organic linkers for specific functions, which is
1. Introduction
5-hydroxylmethylfurfural (HMF) is listed in the top platform
chemicals from biorefinery for synthesizing a broad range of
chemicals and liquid fuels.[1] HMF could be principally syn-
thesized from a wide range of sugars including fructose and
glucose by using acid catalysts. Glucose is the most promising
sugar to produce HMF owning to its natural abundance and
V. Tangsermvit, T. Pila, V. Somjit, Prof. J. Limtrakul, Prof. S. Horike,
Dr. K. Kongpatpanich
Department of Materials Science and Engineering
School of Molecular Science and Engineering
Vidyasirimedhi Institute of Science and Technology
Rayong 21210, Thailand
E-mail: horike@icems.kyoto-u.ac.jp; kanokwan.k@vistec.ac.th
V. Tangsermvit, T. Pila
Department of Chemical and Biomolecular Engineering
School of Energy Science and Engineering
Vidyasirimedhi Institute of Science and Technology
Rayong 21210, Thailand
Dr. B. Boekfa
Faculty of Liberal Arts and Science
Kasetsart University
Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand
Dr. W. Klysubun
Synchrotron Light Research Institute (Public Organization)
Nakhon Ratchasima 30000, Thailand
Prof. S. Horike
Institute for Integrated Cell-Material Science
Institute for Advanced Study
Kyoto University
Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Dr. K. Kongpatpanich
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202006541.
Research Network of NANOTEC-VISTEC on Nanotechnology for Energy
Vidyasirimedhi Institute of Science and Technology
Wangchan, Rayong 21210, Thailand
DOI: 10.1002/smll.202006541
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unique feature difficult to achieve in other porous materials.
Therefore, MOFs/PCPs are interesting porous platforms for
incorporation of both Brønsted and Lewis acid sites in a single
material for glucose-to-HMF transformation.[8] Several reports
have been demonstrated on functionalization of sulfonic
groups at organic linkers of MOF materials to enhance Brøn-
sted acidity for various acid-catalyzed reactions.[9] Sulfonated
MOFs including MIL-101-SO3 and UiO-66-SO3 have been dem-
onstrated on the production of HMF from both fructose and
glucose owning to the chemical stability of these high valent
MOFs under acidic conditions.[10] In most cases, the syntheses
of HMF from fructose provided HMF yields over 80%, while
the use of these sulfonated MOFs for the preparation of HMF
from glucose suffered from yield less than 20% under the
same catalytic conditions due to insufficient Lewis acidity to
promote the isomerization of glucose.[11] The key challenge is
to develop the catalytic system to overcome the equilibrium
limitations of glucose isomerization to fructose. The incor-
poration of additional Lewis acid sites located adjacent to the
Brønsted acid sites of sulfonated MOFs could facilitate glucose
isomerization resulting in the improved yield of HMF. AlCl3
is one of common non-toxic homogeneous catalysts in sev-
eral acid-catalyzed reactions including glucose isomerization
to fructose.[10a,12] Nature of the active Al3+ species and detailed
mechanisms of AlCl3-catalyzed glucose isomerization have
been studied by advanced mass spectrometry and kinetic iso-
tope labeling technique. Several mechanistic studies point out
that a cationic species, [Al(OH)(aq)]2+ or [Al(OH)2(aq)]+ is the
catalytically active species in glucose isomerization.[12a,13] The
active Al3+ species contribute to the glucose isomerization by
accelerating glucose ring-opening and hydride transfer pro-
cesses. Although the use of Al3+ catalytic site in MOF materials
for glucose-to-HMF transformation is still largely uncovered,
Al3+ catalytic site is promising according to its strong Lewis
acidity and environmentally friendly nature. We propose that
the incorporation of the active Al3+ species into sulfonated
MOFs would be a promising route to enhance Lewis acidity
for glucose isomerization to fructose, which could be further
be dehydrated to HMF at the Brønsted acid sites of sulfonated
MOFs resulting in the overall improvement of HMF yield.
In this work, the [Al(OH)2(aq)]+ species was incorporated
to sulfonated MOFs to improve glucose isomerization, which
is an important step for the one-pot synthesis of HMF from
glucose. The local structure of the Al3+ catalytic site has been
elucidated by X-ray absorption near edge structure (XANES)
combined with density functional theory (DFT) calculations to
understand the local structure of acid sites and the influence
of the incorporated acid sites to the catalytic reaction. The acid
strength and acid density of MOF-based catalysts have been
the Al3+ sites owning to its acid- and water-resistant nature.[14]
From crystal structure of UiO-66, each Zr6O4(OH)4 cluster is
connected by twelve 1,4-benzenedicarboxylate (BDC) linkers to
build the 3D framework. Sulfonated UiO-66 (U66S) was syn-
thesized by partially replacing the original BDC linker with
monosodium 2,5-dicarboxybenzenesulfonate (BDC-SO3Na)
linker.[15] Simply reacting sulfonated UiO-66 with AlCl3 in
methanol led to the grafting of Al3+ sites on sulfonated UiO-66
(here after denoted as U66SA). The crystal structure of U66SA
remains intact as the powder X-ray diffraction (PXRD) pattern
of U66SA matches well with the simulated pattern obtained
from the crystal structure data of UiO-66[16] (Figure 1a). How-
ever, U66SA has lower crystallinity than UiO-66, which is
presumably caused by the presence of labile acidic groups in
the structure. The introduction of acidic groups also disrupts
crystal growth resulting in the morphology change in U66SA
as observed by SEM images (Figure S1, Supporting Informa-
tion). Sulfonate groups in U66SA were qualitatively and quan-
titatively evaluated by Fourier-transform infrared spectroscopy
1
(FTIR) and H NMR. Two characteristic FTIR peaks at 1078
and 1225 cm−1 were observed in U66SA, which attributed to

the S O stretching modes of sulfonate groups in the sample
(Figure S2, Supporting Information). The precise percentages
of sulfonate groups in U66SA were further calculated from the
1
integral signal of H NMR data (Figure 1b). It was determined
that 13% of the organic linker was functionalized by sulfonate
groups. The amount of sulfonate groups found in U66SA was
similar to the percentage of BDC-SO3 linker used in the syn-
thesis (18%). The number of Al3+ species observed in U66SA
according to inductively coupled plasma atomic emission spec-
troscopy (ICP-OES) and energy dispersive X-ray (EDX) map-
ping analyses are proportional to the number of S species in
the structure with 1:1 ratio of S:Al, suggesting the existence
of one Al3+ species on every unit of the sulfonate group. The
decrease in Brunauer–Emmett–Teller (BET) surface area and
pore volume of U66SA compared to the pristine UiO-66 (≈15%
decreasing) suggests the acidic groups are located inside the
pore rather than physically adsorbed on the surface of MOF
crystals (Figure 1c; Table S1, Supporting Information). The
grafting of Al3+ sites inside the pore of U66SA was further con-
firmed by the change in pore size distribution, which revealed
the decreasing in pore size of U66SA (1.77 nm) compared to the
pore size of the pristine UiO-66 (1.90 nm) (Figure 1d).
Acid properties contributed by the Al3+ sulfonate sites in
U66SA were evaluated by NH3-TPD. NH3-TPD profiles of UiO-
66, sulfonated UiO-66 (U66S) and UiO-66 grafted by the Al3+
sulfonate sites (U66SA) are illustrated in Figure 2a,b, while the
detailed acid properties of the samples are listed in Table S2,
Supporting Information. Details on acid types can be obtained
by deconvolution of the TCD peaks above 100 °C.[17] Desorption
of NH3 in each sample occur with two major peaks, which can
be ascribed to two types of acid sites. The first NH3 desorp-
tion peak at temperature range of 100–200 °C was assigned to
weaker acid sites corresponding to the adsorption of NH3 on the
acid sites. The first NH3 desorption peak can be deconvoluted
into two Gaussian curves suggesting the two different level of
acid strength. The other peak around 300 °C associated with
the adsorption of NH3 on the stronger acid sites. For U66S,
the amount of weaker acid sites calculated from the first two
1
evaluated by H-NMR and NH3 temperature-programmed des-
orption (NH3-TPD). The cooperative effect from both types
of acids in MOF has been demonstrated on glucose-to-HMF
transformation with high yield.
2. Results and Discussion
Zirconium-based MOF (Zr6O4(OH)4(1,4-benzenedicarboxylate)6
or UiO-66) has been used as a platform for incorporation of
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1
Figure 1. a) Powder X-ray diffraction (PXRD), b) liquid-state H NMR spectra of the digested samples, c) N2 adsorption isotherms, d) pore size dis-
tribution of UiO-66, U66S, and U66SA.
deconvoluted peaks was increased, while the amount of stronger
acid sites estimated from the third deconvoluted peak was sim-
ilar to UiO-66. Both weaker and stronger acid sites were found
to be increased in case of U66SA. It can be seen from NH3-
TPD result of U66S that the Brønsted acid density significantly
increased when sulfonated groups were introduced to the struc-
ture (Table S2, Supporting Information). For U66SA, grafting of
the Al3+ sulfonate sites could improve both Lewis and Brønsted
sites when compared to the pristine UiO-66. Total acid densities
of UiO-66 and sulfonated UiO-66 (U66S) were calculated from
peak area of NH3-TPD profiles to be 0.877 and 1.783 mmol g−1,
respectively. For U66SA, the introduction of the Al3+ sulfonate
sites to the structure resulted in the significant improvement
of acid density in U66SA (2.483 mmol g−1). The high acid den-
sity obtained in U66SA is considerably improved from the
reported sulfonated or Al-based solid acid catalysts.[11a,18] We
performed FTIR measurements of adsorbed pyridine to gain
more details on the acidity. However, it is difficult to discuss
the acidity from FTIR of adsorbed pyridine because the vibra-
tion band of pyridine (1450 cm−1) typically assigned for the
Lewis acid sites overlaps with the OH bending of carboxylate
group in the organic linker. Evaluation of the acidity from TPD
techniques using pure NH3 as a probe molecule discussed
previously may not account for the effect from water, which is
undoubtedly involved in the catalytic glucose-to-HMF transfor-
mation. Therefore, we studied the relationship between acidity
and catalytic activity in the presence of water by potentiometric
titration. Although we could not distinguish each type of acids,
the total acidity can be estimated from the amount of NaOH
consumed during the potentiometric titration (Table S1, Sup-
porting Information). In the presence of water, U66SA provides
higher acidity than U66S and UiO-66, following the trend in
acidity obtained from TPD measurements. As both Lewis and
Brønsted sites play a crucial role in glucose-to-fructose transfor-
mation, U66SA with high acid density is promising to serve as
the catalyst to enhance catalytic performance.
To gain deeper insight into the location and structure of Al3+
species grafted on U66SA, the chemical environment around
Al3+ ion was investigated using X-ray photoelectron spectros-
copy (XPS) (Figure 2c). A signal associated with the Al 2p state
at a binding energy of 74.5 eV is accounted for the trivalent Al
species with an octahedral coordination geometry.[19] Although
it is widely accepted that the defect sites at Zr node of UiO-66
are available for metal grafting, the coordination environment
of Zr4+ in U66SA are similar to Zr4+ in UiO-66. XPS spectra
associated with the Zr 3d state of UiO-66 and U66SA have
the same binding energy suggesting the absence of Al3+ spe-
cies bind to the Zr node of U66SA (Figure 2d). Elucidation on
a local structure of acid sites in MOFs generated via post-syn-
thetic modification poses a scientific challenge, as the disorder
acidic sites are unable to be visualized by single-crystal and
PXRD techniques for crystalline materials. Current methods
to study acidic sites at organic linkers of MOFs rely on either
advanced solid-state NMR or extended X-Ray absorption fine
structure (EXAFS) interpretation.[20] In contrast, XANES is a
promising tool to indicate the local chemical environments of
acid sites, but reference materials with similar chemical envi-
ronments are required. Alternatively, computational modeling
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Figure 2. a) Temperature-programmed desorption of NH3 (NH3-TPD) profile of UiO-66, b) NH3-TPD profile of U66SA, c) Al 2p XPS spectra, and
d) Zr 3d XPS spectra.
using DFT calculation is capable to understand the local
structure of the acid sites without the use of reference mate-
rials.[21] We therefore propose the combination of XANES and
DFT modeling to elucidate the local structure of Al3+ species
grafted on the sulfonated organic linkers. Three models rep-
resenting the possible coordination environment of the Al3+
species were prepared from DFT structural optimization based
on the findings from XPS as shown in Figure 3. We propose
that Al3+ species could bind to the sulfonate sites of Brønsted
UiO-66 as either dinuclear (Model 2Al) or mononuclear com-
plex (Models 1Al-H and 1Al-C). Model 2Al is the binding of
the dinuclear [Al2(OH)5(H2O)]+ to the sulfonate group of Brøn-
sted UiO-66 (Figure 3a). The binding mode of Al3+ in model
2Al is similar to the grafting of Al3+ in MIL-101.[22] However,
Al3+ species in U66SA tends to be a mononuclear Al3+ complex
(1Al-H or 1Al-C) rather than the dinuclear complex (2Al) due
to the smaller pore size of UiO-66 topology compared to MIL-
101. Models 1Al-H and 1Al-C are proposed as the mononuclear
Al3+ complex with different interaction between Al3+ center and
the sulfonate group of the organic linker. In both 1Al-H and
1Al-C models, Al3+ site exists in the form of [Al(OH)2(H2O)4]+,
which is previously reported as the dominant species in
dilute Al3+ condition.[13a,23] Model 1Al-H, [Al(OH)2(H2O)4]+ is
weakly bound to the sulfonate group via H-bonds (Figure 3b).
Model 1Al-C, a sulfonate group at the organic linker acts as a
chelating ligand that replaces two coordinated water molecules
in [Al(OH)2(H2O)4]+ species (Figure 3c). The simulated Al K-
edge XANES spectrum of each model was generated by using
the FEFF program code and compared with the experimental
XANE spectrum (Figure 4).[24]
configuration in terms of energy obtained from DFT calcula-
tions. The relative energies were measured to be +1.31 and
0.00 kcal mol−1 for models 1Al-H and 1Al-C respectively. The
most stable model (model 1Al-C) is the octahedron Al3+ ion sur-
rounded by six O atoms from two hydroxyl ligands, two coor-
dinated water molecules and two bridging Ob atoms from a
sulfonate group of the organic linker. The Al3+ clusters formed
in U66SA can be considered as the cationic [Al(OH)2(H2O)2]+
species bound to O atoms of the anionic sulfonate site of the

organic linker. The S O bond distance in sulfonate group is

1.45 Å, while the S Ob bond distances increases to 1.51–1.52 Å.

The Al Ob bond distances are 2.01–2.02 Å. The Mullikan
charges on Al and S atoms are 1.143|e| and 1.559|e|, respectively.
The positive charge of Al atom suggests the Lewis property of
Al3+ ion in U66SA. There is no chloride species from AlCl3 pre-
cursor remaining in U66SA according to XPS and ICP-OES
analyses indicating the complete ligand exchange of chloride
ions by either OH− or H2O in the system. We noted that the
structure of Al3+ sites was similar to the active [Al(OH)2(aq)]+
species generated when using homogenous AlCl3 to catalyze
glucose isomerization to fructose.[13a] Therefore, the grafted
Al3+ sites on U66SA would be promising to facilitate glucose
isomerization, which is the key step in glucose-to-HMF trans-
formation. The catalytic performance of U66SA and other
related catalysts for glucose-to-HMF transformation is shown
in Table 1.
By performing the batch reactions at 120 °C in DMSO/water
(9:1) mixed solvent (Entries 1–3, Table 1), UiO-66 and acid-
functionalized UiO-66 could catalyze glucose-to-HMF transfor-
mation. Among UiO-66 and its acid-functionalized derivatives
(U66S and U66SA), U66SA provides the highest catalytic per-
formance with complete conversion and 63% yield of HMF.
Although Zr4+ nodes in U66SA could partially act as Lewis acid
XANES spectrum simulated from model 1Al-C fits well with
the experimental XANES spectrum of U66SA, in turn verifying
the DFT-optimized model. Model 1Al-C is also the most stable
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reaction temperature on the catalytic performance (Entries 6
and 7, Table 1). Increasing and decreasing reaction tempera-
ture from 120 °C results in the lower glucose conversion and
HMF yield. Although reaction at higher temperature generally
provides better catalytic performance, the temperature beyond
120 °C would also destroy glucose and fructose resulting in
the poor catalytic performance. The catalytic performance
obtained from U66SA has significantly improved from ZrO2
and Al(OH)3 (Entries 8, 9, Table 1). Yield of HMF obtained
from U66SA is significantly improved from sulfonated MOFs,
which typically provide yield less than 20%, owning to the
increasing of glucose isomerization.[11a] To the best of our
knowledge, the complete conversion with 63% yield to HMF
obtained from U66SA is one of the highest performances of
MOF-based catalysts for glucose-to-HMF transformation at
the same temperature range (80–140 °C).[11a,26] To demonstrate
the importance of acid number in the catalyst, we calculate
the catalytic performance of U66SA and other related catalysts
in terms of TON by normalizing the HMF yield with the acid
number (Table S3, Supporting Information).
The heterogeneous catalytic system in U66SA was evalu-
ated by filtration and leaching tests. The catalyst was sepa-
rated from the reaction solution and the remaining solution
was continued in the absence of the solid catalyst. The amount
of HMF in the filtrate kept constant indicating the role of
U66SA as the heterogeneous catalyst for glucose-to-HMF
transformation. For leaching test, Al3+ species in the filtrate
was not detected under the limit of detection of ICP-OES. The
robustness of U66SA for glucose-to-HMF transformation was
studied by recycling test. U66SA maintained the catalytic per-
formance after the 5th run of spent catalyst (Figure S3, Sup-
porting Information), while retaining its structure according
to XRD (Figure S4, Supporting Information) and Al/Zr atomic
ratio. The leaching and catalyst recycling tests suggest the
robustness of acid sites and the potential use of U66SA as a
heterogeneous catalyst for glucose-to-HMF transformation and
other acid-catalyzed reactions.
Glucose-to-HMF transformation pathway under U66SA
was proposed in Scheme 1. The adsorptions of glucose, fruc-
tose and HMF on U66SA catalyst were calculated with DFT
calculations to understand the reaction pathway of glucose-
to-HMF catalyzed by U66SA. First, glucose adsorbed on the
surface of U66SA with adsorption energy of −27.7 kcal mol−1.
The molecular bond distance were 1.75 and 1.82 Å, suggesting
the hydrogen bond between glucose and U66SA (Figure 5a).
[Al(OH)2(H2O)2]+ site of U66SA acts as an electrophile reacting
with β-glucopyranose form of glucose through its C4 position
with the relative energy of −12.7 kcal mol−1 (Figure 5b). The
reaction intermediate is converted to keto form through an
intramolecular hydride transfer. The resulting keto form under-
goes tautomerization into enol form and cyclization to fructose,
which is adsorbed on the surface of U66SA via its C4 position
with the adsorption energy of −21.3 kcal mol−1 (Figure 5c).
Brønsted acid sites located near the Al3+ sites subsequently
facilitate the dehydration of fructose to HMF. The overall reac-
tion was found to be exothermic reaction with the relative
energy of −56.7 kcal mol−1 (Figure 5d). The key intermediates
could be further studied by using the deuterium labeling tech-
niques to understand the catalytic mechanisms in details.
Figure 3. Proposed structures of Al – sulfonate cluster on U66SA from
DFT calculations: a) 2Al, b) 1Al-H, c) 1Al-C.
sites for the reaction, the catalytic performance of U66S, which
has the same Zr node as U66SA, was poorer than U66SA con-
taining additional [Al(OH)2(H2O)2]+ sites. The result indicates
the insufficient Lewis acid strength of the Zr4+ nodes and the
necessity to have Lewis acid sites located adjacent to the Brøn-
sted acid sites. DMSO/water mixture was chosen as a solvent
system because of the ability to enhance HMF stability by pre-
venting both HMF rehydration and the formation of undesired
humin via aldol condensation.[25] Moreover, the use of water as
a co-solvent demonstrates the flexibility to conduct the catalytic
reaction under moisture conditions without decreasing the
catalytic efficiency. It should be noted that the use of mixed
solvent provides better catalytic activity than the use of pure
DMSO as a solvent (Entry 4, Table 1). However, increasing
percentage of water in the mixed solvent to 50% significantly
decrease both glucose conversion and HMF yield (Entry 5,
Table 1). Glucose-to-HMF transformation on U66SA was fur-
ther performed at 90 and 150 °C to investigate the effect of
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Figure 4. Normalized Al K-edge XANES spectrum of U66SA and simulated Al K-edge XANE spectra of DFT-optimized models.
UiO-66 catalysts in terms of acid strength and acid density were
controlled by the synthetic conditions, and both factors provide
significant contribution to the catalytic performance of glucose-
to-HMF transformation. MOF catalyst functionalized by the
Al3+ sulfonate sites promoted the catalytic reaction thanks to
the presence of the adjacent Brønsted and Lewis acid sites. The
work suggests a systematic way to incorporate different types
of acidic groups in MOFs, which could be extended for several
acid-catalyzed reactions.
3. Conclusion
In summary, we proposed the incorporation of catalytic Al3+
sites to the pore surface of sulfonated UiO-66 to create Brønsted
and Lewis acid sites for glucose-to-HMF transformation. Local
structure of the Al3+ site on sulfonated UiO-66 was elucidated
by XANES combined with DFT calculations, and it revealed the
binding of [Al(OH)2(H2O)2]+ species to the anionic sulfonate
site of UiO-66. The acidity properties of acid-functionalized
Table 1. Catalytic performance of the catalysts for glucose-to-HMF transformation.
Entry
Catalysts
UiO-66
U66S
Time [h]
24
Solvent
Conversion [%]
71.00
HMF yield [%]
2.70
Temp. [°C]
120
1
DMSO:H2O (9:1)
DMSO:H2O (9:1)
DMSO:H2O (9:1)
DMSO
2
3
4
5
6
7
8
9
120
24
43.47
14.06
U66SA
U66SA
U66SA
U66SA
U66SA
ZrO2
120
24
99.00
63.00
120
24
23.04
40.79
120
24
DMSO:H2O (1:1)
DMSO:H2O (9:1)
DMSO:H2O (9:1)
DMSO:H2O (9:1)
DMSO:H2O (9:1)
32.14
20.79
90
24
51.84
11.68
150
24
22.67
34.85
120
24
62.98
Trace
Al(OH)3
120
24
28.98
15.25
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Scheme 1. Reaction pathway of glucose-to-HMF transformation on U66SA obtained from DFT calculations.
performance liquid chromatography (HPLC) analysis was performed
on Shimadzu Prominence-I (LC-2030 3D). The C18 analytical column
(2.1 mm × 150 mm, 3 µm, Dionex Acclaimed Polar Avantage II, Thermo
Scientific) was used for the analysis of HMF, while HILIC amino HPLC
column (4.6 mm × 250 mm, 3 µm, Inertsustain, Shimadzu) was
employed for the separation of glucose and fructose using refractive
index detector (RID) for analysis. Ultrapure water used for HPLC analysis
is 18.2 MΩ cm in resistivity at 25 °C.
Synthesis of MOF Catalysts: UiO-66 was synthesized by mixing of
ZrCl4 (3.45 mmol) and 1,4-benzendicarboxylic acid (H2BDC, 3.39 mmol),
acetic acid (15 mL) and DMF (135 mL) in a 250-mL glass vial and heated
at 150 °C for 48 h. The overall procedure to synthesize sulfonated UiO-66
(U66S) is same as the synthesis of UiO-66, except for the replacement
of 20% of H2BDC by monosodium 2-sulfoterephthalate (BDC-SO3Na).
U66S was synthesized by using BDC-SO3Na (0.61 mmol) and BDC
(2.78 mmol) as organic linkers. The obtained U66S was dried at 150 °C
overnight for further use. Synthesis of U66SA was conducted under
air-free conditions due to the hygroscopic nature of AlCl3 precursor.
U66S (500 mg) and anhydrous AlCl3 (300 mg) were mixed in a round
bottom flask and added with anhydrous ethanol (100 mL). The mixture
was heated at 90 °C for 24 h. The solids of U66SA were obtained after
washing by centrifugation with methanol and dried in an oven. All MOF
catalysts were activated at 150 °C under vacuum for 12 h before catalytic
test.
4. Experimental Section
All chemicals were purchased and used without further purification.
PXRD data were collected on a Bruker D8 ADVANCE X-ray diffractometer
with a Cu Kα anode. FTIR spectra were measured by a PerkinElmer
frontier FT-IR in attenuated total reflectance (ATR) mode with KBr
1
window. H NMR spectra were recorded on a Bruker D8 AVANCE III
HD (600 MHz) at room temperature with D2O as an NMR solvent.
Sample for NMR analysis was prepared by digestion of the sample
(10 mg) in 2 m NaOH (20 mL) at 100 °C for 30 min in microwave reactor
(MARS6, CEM). Scanning electron microscope (SEM) equipped with
energy-dispersive X-ray spectroscopy (EDX) measurement was operated
at 1.5 kV on a JEOL JSM-7610F. XPS data were recorded on JEOL JPS-
9010MC with a Mg Kα source at 12 kV, 25 mA. C1s peak at 284.70 eV was
used as a reference to correct all binding energy values. N2 adsorption
experiments were conducted on MicrotracBEL BELSORP-mini II at
−196 °C. Each sample was activated at 150 °C under vacuum (10−3 kPa)
for 18 h before gas adsorption experiments. Surface area values were
calculated based on the BET method. ICP-OES data were obtained from
Agilent Technologies 700 series using Ar as a plasma source. NH3-TPD
was conducted by MicrotracBEL BELCAT-II. The samples were charged
in a quartz tube and heated at 300 °C for 2 h with helium flow as a
carrier gas, and after cooling down to 40 °C, ammonia adsorption was
performed. Physically adsorbed ammonia was removed by purging with
helium at 40 °C for 30 min before NH3-TPD. The NH3-TPD was carried
out by increasing the cell temperature from 40 to 350 °C. Potentiometric
titration was performed on Titrator Excellence T7 (Mettler Toledo). MOF
suspension for each titration was prepared by dispersing ground MOF
sample (50 mg) in 0.01 m NaNO3 (20 mL). The obtained suspension
was titrated with 0.1 m HCl to reach pH = 3 and followed by the titration
with 0.1 m NaOH to reach pH = 1. Each titrant was dosed with the
injection rate of 0.01 mL min−1. UV–Visible spectra were collected on
a PerkinElmer Lambda 1050 with the wavelength of 200–800 nm. High
Catalytic Test: Glucose (0.56 mmol), MOF catalyst (0.04 mmol),
DMSO (2.7 mL) and water (0.3 mL) were mixed in a 20-mL glass vial and
heated under stirring at 120 °C for 24 h. Each reaction vial was placed in
an ice bath to stop the reaction. The reaction mixture was centrifuged to
separate the supernatant from the catalyst. The supernatant liquid from
each reaction was analyzed by HPLC for the conversion and yield of
HMF from the reaction. For filtration and leaching test, the solid catalyst
was separated from the host solution, and the remaining filtrate was
further heated in the absence of the solid catalyst for additional 6 h. For
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Figure 5. a) Quantum cluster model representing the diffusion and physisorption of glucose in the gravity of U66SA calculated with M06L on the
structure of ONIOM(M06L:PM6) approach, b) Glucose adsorption on Al3+ site of U66SA, c) Fructose adsorption on U66SA, d) HMF adsorption on
U66SA. Distances are in Å.
recyclability test, the catalyst was recycled for five times under the same
reaction conditions. The catalyst did not show significant loss of the
activity after five cycles.
of BL8: XAS at Synchrotron Light Research Institute for their assistance
and support throughout X-ray absorption experiments.
Elucidation of Acid Sites: Al K-edge XANES spectrum of U66SA was
collected in the fluorescent mode at the Beamline 8 of the Synchrotron
Light Research Institute (SLRI), Thailand. The data were collected in
the range of 1500–1800 eV, covering the XANES region of Al K-edge. The
sample was calibrated with the Al foil standard (1559 eV) to correct the
energy value. The proposed model of an acid site in U66SA was prepared
by DFT with M06-L functional.[27] The 6-31G(d,p) basis set was chosen
to describe C, H, O, N, S and Al atoms while LANL2DZ was applied
for Zr atoms. U66SA model was represented by the C30H38O43SAl2Zr6
cluster. The cluster of UiO-66 was truncated from crystallographic
data.[28]Dangling bonds were terminated by H atoms and kept fixed
during a geometry optimization. All DFT calculations were performed on
Gaussian 16 program,[29] whereas molecular structures were visualized
using Vesta.[30] The atomic coordinates for XANES simulation were taken
from the DFT-optimized model (C30H38O43SAl2Zr6). Simulated Al K-edge
XANES spectrum obtained from the FEFF program code.[24c,31]
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
Research data are not shared.
Keywords
acid catalysts, biomass conversion, metal–organic frameworks, sugar
conversion
Received: October 20, 2020
Revised: December 23, 2020
Published online:
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
This work was supported by Thailand Research Fund and the Office of
the Higher Education Commission (Research grant: MRG6080278 to
K.K.). This work has been partially supported by the NANOTEC, NSTDA,
the Ministry of Science and Technology, Thailand, through its program
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