Early Transition Metal Doped Tungstite as an Effective Catalyst for Glucose Upgrading to 5-Hydroxymethylfurfural

Article abstract of DOI:10.1007/s10562-018-2483-4

Glucose valorization to 5-hydroxymethylfurfural (HMF) remains challenging in the transition towards renewable chemistry. Lewis acidic tungstite is a viable, moderately active catalyst for glucose dehydration to HMF. Literature reports a multistep mechanism involving Lewis acid catalyzed isomerization to fructose, which is then dehydrated to HMF by Br?nsted acid sites. Doping tungstite with titanium and niobium improves activity by optimizing the ratio between Lewis and Br?nsted acid sites. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2483-4

Catalysis Letters
https://doi.org/10.1007/s10562-018-2483-4
Early Transition Metal Doped Tungstite as an Effective Catalyst
for Glucose Upgrading to 5-Hydroxymethylfurfural
Jan J. Wiesfeld1 · Nico A. J. M. Sommerdijk2 · Emiel J. M. Hensen1
Received: 9 June 2018 / Accepted: 4 July 2018
© The Author(s) 2018
Abstract
Glucose valorization to 5-hydroxymethylfurfural (HMF) remains challenging in the transition towards renewable chemistry.
Lewis acidic tungstite is a viable, moderately active catalyst for glucose dehydration to HMF. Literature reports a multistep
mechanism involving Lewis acid catalyzed isomerization to fructose, which is then dehydrated to HMF by Brønsted acid sites.
Doping tungstite with titanium and niobium improves activity by optimizing the ratio between Lewis and Brønsted acid sites.
Graphical Abstract
Keywords Biomass · Glucose · 5-Hydroxymethylfurfural · Tungsten oxide · Doping
1 Introduction
Continuous consumption of ever-diminishing fossil
resources and the negative impact on the environment asso-
ciated with their combustion calls for their replacement with
* Emiel J. M. Hensen
alternative renewable feedstocks. Lignocellulosic biomass
is a promising starting material for the production of sus-
tainable fuels and chemicals [1, 2]. Upgrading of biomass
will likely revolve around a select number of platform mol-
ecules with a wide variety of downstream applications [3,
4]. 5-Hydroxymethylfurfural (HMF) is regarded as one of
the most versatile platform chemicals, its derivatives having
potential applications as fuel additives, solvents and mono-
mers for the plastics industry [5, 6].
e.j.m.hensen@tue.nl
Jan J. Wiesfeld
j.j.wiesfeld@tue.nl
Nico A. J. M. Sommerdijk
n.a.j.m.sommerdijk@tue.nl
1
2
Laboratory of Inorganic Materials Chemistry, Schuit
Institute of Catalysis, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Laboratory of Materials and Interface Chemistry,
Eindhoven University of Technology, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
HMF can essentially be obtained from both fructose and
glucose. While fructose is easier to upgrade to HMF, it is
Vol.:(0123456789)
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J. J. Wiesfeld et al.
also costly and scarce compared to glucose [7–9]. Glucose
can be obtained from lignocellulosic biomass in relatively
pure form, making it cheap and abundant compared to fruc-
tose. These aspects make glucose an attractive feedstock for
HMF [10–13]. However, the upgrading of glucose to HMF is
much more challenging than that of fructose [14–16]. While
mineral acids such as HCl and H2SO4 can be used as cata-
lysts for the conversion of either sugar to HMF in aqueous
solution at elevated temperatures, especially glucose suffers
from severe degradation to humins under these conditions
[17–19]. Also, HMF can be rehydrated to levulinic acid in
water of low pH [15, 18]. Sustainable HMF production from
glucose thus requires specialized and atom-efficient cata-
lysts, as simple Brønsted acids alone are ill-equipped for
this purpose.
catalysts including Lewis acidic metal chlorides in ionic liq-
uids are also unattractive because of the difficulties associ-
ated with separation [29, 30].
A different branch of catalysts capable of these transfor-
mations are transition metal oxides such as TiO2, Nb2O5 and
WO3. These materials are abundant and cheap, and express
water-tolerant Lewis acid sites and tunable acid–base prop-
erties with promising glucose-upgrading abilities [31, 32].
Recently, we have shown that tungstite (WO3·H2O) is also
a viable contender [33]. The material comprises distorted
WO5·H2O octahedra, which share 4 equatorial corner oxy-
gens to form a stacked sheet-like material with a low surface
area. Perpendicular to each sheet, every W site is terminated
alternatingly by a double bonded oxygen and coordinated
water. Similarly to TiO2 and Nb2O5, it contains water-tol-
erant Lewis acid functionalities, but its Brønsted acidity is
relatively weak, which limits fructose dehydration rates.
Doping the material with Nb proved a key factor in enhanc-
ing its activity [33].
More efficient routes for glucose to HMF conversion
employ a bifunctional system to isomerize glucose to the
more reactive fructose prior to its dehydration. Enzymes can
facilitate the isomerization, but are incompatible with the
acidic conditions necessary for the dehydration. Basic and
Lewis acid catalysts can also carry out the isomerization, but
Lewis acids are preferred for their stability in acidic envi-
ronments [20]. While such two-step approaches are viable,
one-pot strategies have attracted much interest recently.
Mechanistically, it is commonly assumed that the conversion
of glucose proceeds via fructose as an intermediate. Lewis
acids play an important role in isomerizing glucose to the
more reactive fructose by stabilizing glucose in its acyclic
form, facilitating a 1,2-hydride transfer and releasing fructo-
furanose (fructose) [21–24]. Subsequent dehydration then
removes three water molecules to form HMF (Scheme 1).
To optimize the selectivity, the reaction can be carried out
in a biphasic or non-aqueous system, thereby shielding HMF
from the acidic aqueous environment and preventing rehy-
dration to levulinic acid [8, 16, 25].
In this work, we explored the effect of titanium as a
dopant on the activity of tungstite, based on the results found
by DFT calculations [34]. The materials were characterized
in detail with the aim to judge the dispersion of the dopant
in the bulk and at the surface of tungstite.
2 Experimental Methods
2.1 Chemicals
Tungsten hexachloride (WCl6, 99%, Alfa Aesar), niobium
pentachloride (NbCl5, 99.9%, Alfa Aesar), titanium tet-
rachloride (TiCl4, 99%, Sigma Aldrich) were kept in an
MBraun glovebox under an inert argon atmosphere.
Titanium(IV) oxide (TiO2, Degussa P25) and niobium(V)
oxide (Nb2O5, 99.5% Alfa Aesar) were used as commercial
standards.
Zeolites have been shown capable of isomerizing glucose
to fructose, but require addition of a Brønsted acid to dehy-
drate the fructose to HMF [14]. Additionally, the synthesis
of the most effective zeolite for this purpose, Sn-modified
Beta, is cumbersome and requires toxic reagents such as
HF [20, 26]. Additionally, zeolites are prone to framework
damage when exposed to the high temperatures and aque-
ous media the reaction requires to proceed, diminishing its
activity during prolonged reactions [27, 28]. Homogeneous
d-(-)-glucose (99%, Sigma Aldrich), 5-hydroxymethyl-
furfural (HMF, 97%, Sigma Aldrich), tetrahydrofuran (THF,
99%, Biosolve), triethylamine (TEA, 99.9%, VWR) and
acetonitrile (MeCN, HPLC-S grade, Biosolve) were used
as received. Pyridine (Sigma Aldrich, 99%) was stored on
freshly activated 3 Å molsieves (Merck).
Scheme 1 Reaction intermediates in the conversion of glucose to HMF
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Early Transition Metal Doped Tungstite as an Effective Catalyst for Glucose Upgrading to…
Deionized water (15 and 18.2 MΩ) was obtained from an
Elgastat Water Purifier present in our laboratory.
used to calculate the specific surface area (SBET) from the
adsorption data (p/p0 =0.05–0.25).
Transmission electron microscopy (TEM) images were
obtained in bright field mode from a FEI Tecnai 20 (type
Sphera) operating with a LaB6 filament at 200 kV and a
bottom mounted 1024×1024 Gatan msc 794™ CCD cam-
era and elemental mapping was performed on a probe Cs
corrected Titan2 (FEI) operating at 300 kV in ADF-STEM
mode using an Oxford Instruments X-MaxN 100TLE EDX
detector. Suitable samples were prepared by dropping a sus-
pension of finely ground material in analytical grade abso-
lute ethanol onto Quantifoil R 1.2/1.3 holey carbon films
supported on a copper grid.
2.2 Catalyst Preparation
Preparation of WO3:WCl6 (3.97 g; 10.0 mmol) was dis-
persed in 160 mL deionized water in an open beaker in con-
tact with air to initiate hydrolysis. The suspension was kept
at 50 °C overnight while stirred magnetically. The yellow
precipitate was collected by filtration and washed several
times with deionized water until neutral pH of the filtrate
was reached. The residue was then dried at 60 °C in vacuo
overnight. NbWx (x representing W/Nb ratios of 10 and 5)
samples were prepared in a similar fashion using physical
mixtures of appropriate amounts of WCl6 and NbCl5. TiWx
(W/Ti ratios of 10 and 5) were prepared by adding an appro-
priate amount of WCl6 to 160 mL water first and injecting
the correct amount of TiCl4 within 10 s using a Finn pipette
with the tip submerged in the water. After drying, the materi-
als were used without further treatment.
Powder X-ray diffraction (XRD) patterns were recorded
on a Bruker Endeavour D2 Phaser diffractometer using Cu
Kα radiation with a scanning speed of 0.6° min−1 in the
range of 5°≤2θ≤60°. Crystal phases were identified using
the DIFFRAC.EVA software package and the PDF-2 crystal-
lographic database (version 2008).
Fourier-transformed infrared (FT-IR) was used to evalu-
ate acidic properties of the materials. Spectra were recorded
in the range of 4000–1200 cm−1 at a resolution of 2 cm−1 on
a Bruker Vertex V70v equipped with a DTGS detector and
CaF2 windows. A total of 64 scans were averaged for each
spectrum. Typically, finely powdered material was pressed
into self-supporting wafers with density ρ ≃ 25 mg cm−2
using a pressing force of 3000 kg, and placed inside a vari-
able temperature IR transmission cell coupled to a closed
gas circulation system. The samples were then outgassed
at 70 °C in vacuo until a pressure of 2×10−5 mbar or lower
was reached.
2.3 Characterization
Elemental analysis was performed on a Spectroblue EOP
ICP optical emission spectrometer with axial plasma view-
ing, equipped with a free-running 27.12 MHz generator
operating at 1400 W. Prior to the measurement, the samples
were digested using a 4 M KOH solution. For Ti containing
materials an equal volume of 8 vol% HF was added to dis-
solve the titanium under gentle heating (~40 °C).
X-ray photoelectron spectroscopy (XPS) was performed
on a Thermo Scientific K-alpha equipped with a monochro-
matic small-spot X-ray source and a 180° double focusing
hemispherical analyzer with a 128-channel detector. Ini-
tial pressure was 8×10−8 mbar or less which increased to
2×10−7 mbar due to the active argon charge compensation
dual beam source during measurement.
Prior to pyridine adsorption, the sample was kept at 70 °C
and a sample background was recorded. Pyridine was then
introduced into the cell until saturation was reached, and
physisorbed pyridine was removed in vacuo for 1 h. A sec-
ond spectrum were recorded in situ at this point.
Difference spectra were obtained by subtraction of the
sample background from the recorded spectra. Processing
and deconvolution of the signals was performed with Fityk
curve fitting program.
For a typical sample preparation, fresh catalyst was
pressed down on carbon tape supported by an aluminium
sample plate. Spectra were recorded using an AlKα X-ray
source (1486.6 eV, 72 W) and a spot size of 400 µm. Survey
scans were taken at a constant pass energy of 200, 0.5 eV
step size, region scans at 50 eV constant pass energy with a
step size of 0.1 eV.
2.4 Catalytic Activity Tests
XPS spectra were calibrated to the C–C carbon signal
(284.8 eV) obtained from adventitious carbon and decon-
voluted with CasaXPS. The peak areas thus obtained were
used to estimate surface chemical composition.
Batch reactions were performed at 120 °C under autogenous
pressure in Pyrex tubes (inner volume 12 mL) equipped with
a magnetic stirring bar. For a typical experiment, 5 tubes
were each charged with 40 mg glucose dissolved in 4 mL
of a biphasic H2O/THF mixture, volume ratio 1/9, in which
40 mg of catalyst was suspended and sealed with a PTFE
stopper. After a corresponding reaction time, the reaction
was quenched by immersion of the tube in an ice/water bath.
Reaction times of 30, 60, 120, 180 and 240 min were used.
Nitrogen sorption data was recorded on a Micrometrics
Tristar 3000 in static measurement mode at −196 °C. The
samples (typically 150 mg) were pretreated at 120 °C under
a gentle N2 stream overnight prior to the sorption measure-
ments. The Brunauer–Emmett–Teller (BET) equation was
1 3

J. J. Wiesfeld et al.
Aliquots were filtered through a 0.45 µm PTFE filter
and analyte concentrations were determined by a Shimadzu
HPLC system equipped with autosampler and column oven.
Glucose and fructose were separated on a Shodex Asahipak
NH2P-50 2D kept at 35 °C and detected by ELSD (ELSD-
LT II operating at 40 °C, gas pressure 350 kPa) using 70:30
MeCN:H2O modified with 0.001 M TEA as mobile phase
(0.2 mL min−1). HMF was measured by UV–Vis (SPD-
M20A operating at 40 °C, λmax 284 nm) using a Phenom-
enex Kinetex 5u EVO C18 100A reversed phase column at
40 °C for separation and 5:95 MeCN:H2O as mobile phase
(0.4 mL min−1).
content. Deconvolution of the tungsten 4f region yields
two separate W 4f7/2/W 4f5/2 doublets, presented in Fig. 1.
For the W 4f7/2 peaks, the binding energies were 34.8 and
35.9 eV, which could be assigned to WV and WVI oxida-
tion states, respectively [35]. Deconvolution of the peaks
related to the dopants (Nb 3d and Ti 2p) yielded a single
doublet for either material. Binding energies for the Nb 3d5/2
and Ti 2p3/2 components were 207.6 and 459.2 eV and were
assigned to the fully oxidated states (NbV and TiIV) [36, 37].
Of note is that the amount of WV species differs from
material to material. The undoped oxide contains~5.6% as
WV. Increasing levels of Nb raise the level of WV to 6.9%
for NbW10 (W/Nb ratio of 10) and to 11% for NbW5 (W/Nb
ratio of 5). Reduction of W due to the inclusion of niobium
has been described before [38, 39]. Doping WO3 with Ti
also increased the WV content, but to a lesser extent. Care
was taken to include the Nb 4p and Ti 3p regions, since both
overlap with the W 4f region and would present incorrect
WVI/WV ratios if excluded [40]. Finally, no surface chlorine
was present on the samples, indicated by the absence of the
Cl 2p signal.
Conversion, yield and selectivities of the respective com-
pounds were calculated using the following formulas and
converted to percentages when appropriate:
(
)
(
)
np
nt
X = 1 −
; Yp =
n0
n0
where X is the conversion of glucose, YP the product yield,
n0 and nt the glucose concentrations at t=0 and t=reaction
time, and nP the product concentration.
Textural properties of the bulk materials were evaluated
using N2 physisorption. Surface areas are summarized in
Table 2. NbW5 has the highest surface area (35 m2 g−1) and
TiW5 the lowest (5 m2 g−1). Isotherms of all materials, dis-
played in Fig. 2, were of the type IV shape with a H3 hys-
teresis loop, indicative of aggregates of plate-like materials
forming slit-like macropores.
3 Results and Discussion
Hydrolysis of WCl6 in water initially yielded a gray suspen-
sion, which turned yellow overnight. Collection and drying
of the formed material provided a vividly yellow, easy to
disperse solid. Hydrolysis of the NbCl5/WCl6 physical mix-
tures behaved similarly and yielded similarly yellow colored,
more voluminous oxides NbW10 and NbW5 (W/Nb= 10
or 5). On the other hand, the synthesis of TiW10 and TiW5
(W/Ti=10 or 5) yielded pale-yellowish, very dense, grainy
materials.
TEM micrographs present a sheet-like morphology for
WO3, NbW5 and TiW5 (Fig. 3), which is in keeping with the
physisorption measurements. The sheets of WO3 and NbW5
have well-defined edges in contrast to the quite corrugated
plane edges of TiW5.
The XRD pattern of WO3 (Fig. 4) shows that it consists
primarily of tungstite [PDF 043-0679] with hydrotungstite
[PDF 018-1420] as a minor fraction. Both phases consist
of stacked planes built up of equatorial corner-sharing
WO5·H2O octahedral and each plane is terminated with
alternating double bonded oxygen or coordinated H2O. The
planes are held together by hydrogen bonds. The phases pri-
marily differ in an extra layer of water intercalated between
every two hydrotungstite planes. Both NbW10 and NbW5
materials comprise a pure hydrotungstite phase, decreasing
in crystallinity with increasing Nb content. TiW10 and TiW5
are mostly amorphous, although a hydrotungstite phase can
still be identified in both samples. Probably, this is caused
by the tetrahedral coordination that TiIV can adopt, disrupt-
ing the otherwise planar tungstite layers [34]. Substituting
WVI with NbV on the other hand will maintain the planar
ordering as NbV can take on a square pyramidal coordination
with four equatorially placed oxygens. The planar morphol-
ogy of all materials affirm the findings found with sorption
and TEM measurements. No Nb2O5 or TiO2 phases were
XPS and ICP results are summarized in Table 1. All
doped materials contain the desired amount of dopant as
evidenced by ICP. The bulk and surface W/Nb ratios of
NbW10 and NbW5 match quite well, indicating no surface
segregation. However, the content of titanium in the bulk of
the TiWx samples is significantly higher than the surface
Table 1 Atomic surface concentrations of WVI/WV and surface and
bulk W/dopant ratios
Sample
WVI 4f7/2
WV 4f5/2
eV/%
W/dopant
– (XPS)
W/dopant
– (ICP)
eV/%
WO3
35.9/94.4
35.9/93.6
35.9/88.8
35.9/94.2
35.9/93.5
34.8/5.6
34.8/6.9
34.8/11.2
34.8/5.8
34.8/6.5
–
–
NbW10
NbW5
TiW10
TiW5
10.4
5.78
15.3
11.9
10.1
4.98
11.9
4.44
1 3

Early Transition Metal Doped Tungstite as an Effective Catalyst for Glucose Upgrading to…
Fig. 1 XPS spectra of the as-
prepared materials. Experimen-
tal data are represented by open
circles and the fit as a black
curve. W 4f fits are composed
of WVI (blue) and WV (green).
Nb 4p and Ti 3p fits overlapping
with the W 4f region are indi-
cated in red. Dopant Ti 2p and
Nb 3d fits are displayed in blue
Table 2 Physico-chemical properties of undoped and doped tungsten
oxides
Material SBET
(m2 g−1
NLAS,pyr
NBAS,pyr
)
(mmol g−1/µmol m−2
)
(mmol g−1/µmol m−2
)
WO3
NbW10 24
NbW5
TiW10
TiW5
17
0.058/3.44
0.076/3.15
0.099/2.84
0.008/1.42
0.005/1.37
0.029/1.70
0.034/1.44
0.079/2.27
0.022/4.13
0.039/9.91
35
5.4
3.9
found in the diffractograms, showing full incorporation of
the dopants into the WO3 framework. This was additionally
confirmed by aid of STEM–EDX. Although minor inhomo-
geneities were observed, no segregation between dopant
oxides and tungstite could be detected (Fig. 5).
Fig. 2 N2 sorption isotherms of as-prepared materials
Previous work shows that doping WO3 with niobium
increased the Brønsted acidity of the material [33]. The
exchange of Lewis acidic WVI cations by NbV species
induces a charge defect which is compensated by acidic
protons, thus increasing the amount of Brønsted acid sites
(BAS). Concurrently, this exchange decreased the number
of Lewis acid sites (LAS) as the relative amount of WVI is
lowered.
integrated molecular absorption coefficients as established
by Datka et al. [41] Results are presented in Table 2. Expect-
edly, undoped WO3 has the highest density of LAS, but also
possesses a reasonable density of BAS. The latter are likely
derived from the partial reduction of WVI to WV. Generally,
increasing amounts of either dopant lowers the amount of
LAS and increases the amount of BAS. Additionally, the
effects of Ti doping are more pronounced than inclusion of
The as-produced materials were therefore analyzed on
acidic properties using pyridine as probe molecule using
1 3

J. J. Wiesfeld et al.
Fig. 3 Representative TEM images of WO3, NbW5 and TiW5
55%, indicating that the Brønsted acidity of the undoped
oxide is too low to efficiently perform the dehydration steps.
In contrast, the doped materials were all able to complete this
dehydration unaided and with a much higher activity, leading
to selectivities of roughly 55–60%. The remainder of the con-
verted glucose is inadvertently lost due to side-product forma-
tion such as humins, which can form from glucose, 5-HMF,
fructose and other intermediates. Interestingly, the TiWx and
NbWx materials perform equally well, although TiWx has an
order of magnitude lower surface area than NbWx. This can be
ascribed to the substantially higher density of BAS in TiWx.
As control experiments, the catalytic performance of com-
mercial Nb2O5 and TiO2 powders was tested using the same
conditions as for the undoped and doped WO3 oxides (Fig. 7).
Niobia and titania convert glucose relatively fast, as no trace of
glucose is found after 30 min of reaction. Fructose was also not
detected at any stage. HMF yields for niobia and titania remain
steady after 30 min at respectively 22 and 0.6%, much lower
than any of the tungsten-based oxides. This shows that doped
tungsten oxides are a promising material for the promotion of
glucose dehydration to 5-HMF.
Fig. 4 X-ray diffractograms of the different materials: tungstite is rep-
resented by diamond, hydrotungstite by circle
From the activity data and the literature insights on glu-
cose dehydration mechanism, we can deduce that Lewis
acid sites catalyze the initial isomerization step of glucose
to fructose [18, 22, 32, 34, 42]. This leads to fructose as an
intermediate. In most experiments, the maximum fructose
yield is observed after a reaction time of 30 min. The yield is
relatively low in all cases, indicating that fructose is rapidly
converted to other products. The remaining steps to obtain
5-HMF from fructose entail dehydration steps, which are
catalyzed by Brønsted acid sites.
Nb. Where NbW10 presents only a minor decrease in LAS
and actually a lower density of BAS, TiW10 loses roughly
half of its LAS and possesses more than double the amount
of BAS compared to undoped WO3. This trend continues in
TiW5 where the density of LAS is reduced slightly and the
BAS density is double that of TiW10. We expect that the
inclusion of Ti4+ induces a larger charge defect as compared
to that Nb5+, which in the extreme case needs compensation
by two acidic protons instead of one.
Activity tests of the five materials are displayed in Fig. 6.
All materials were able to reach full glucose conversion within
4 h. The conversion rate was substantially higher for the doped
materials. Undoped WO3 exhibits a poor selectivity to HMF,
evidenced by the low yield of 15% after 4 h of reaction (Fig. 6,
left graph). Addition of HCl after 3 h increased the yield to
4 Conclusions
Undoped tungsten oxide and niobium- and titanium-doped
tungsten oxides were successfully prepared by aque-
ous precipitation of WCl6, NbCl5 and TiCl4 with final
1 3

Early Transition Metal Doped Tungstite as an Effective Catalyst for Glucose Upgrading to…
Fig. 5 Elemental EDX maps for NbW5 and TiW5
Fig. 6 Activity plots of undoped WO3 (left), NbWx (center) and
TiWx (right). Squares represent glucose conversion, and circles and
triangles represent HMF and fructose yield respectively. Closed circle
for leftmost graph represents HMF yield when no HCl is added after
3 h of reaction time. Gray and black lines in center and right graph
represent activity data for W/dopant=5 and 10 respectively
tungsten/dopant ratios close to desired ratios (5 or 10).
The dopants were shown to substantially impact the phys-
icochemical parameters, most notably on surface area and
crystal morphology. Niobium improved the surface area,
i.e. replacing 20% of tungsten by niobium led to a two
times higher surface area. In contrast, inclusion of 10% of
titanium significantly lowered the surface area. Undoped
WO3 comprises a tungstite crystal phase, while doped
materials additionally contained a hydrotungstite phase.
Separate Nb2O5 and TiO2 phases were not detected and the
dopants were quite homogeneously distributed throughout
the materials. Both niobium and titanium led to increased
Brønsted acid site densities of the tungstite phase, which
had a positive effect on glucose dehydration, although the
final 5-HMF selectivities were mostly unaffected. Opti-
mized 5-HMF selectivities were in the 55–60% range, the
remainder being humins. Interestingly, while both TiW5
and NbW5 have comparable HMF formation rates, their
surface areas differ by an order of magnitude. This indi-
cates that TiW5 is significantly more active on a surface
area base, which is explained by the relatively high density
of Brønsted acid sites. Compared to titania and niobia ref-
erence materials that display high glucose conversion rates
but low 5-HMF selectivity, doped WO3 stand out as cheap
materials with a high density of Lewis and Bronsted acid
sites for efficient conversion of glucose to 5-HMF.
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J. J. Wiesfeld et al.
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Acknowledgements The authors would like to acknowledge A.M.
Elemans-Mehring for assistance with the ICP measurements and
M.W.G.M Verhoeven for support with XPS processing. Further thanks
are extended to F.D. Tichelaar of NCHREM at Delft University of
Techology for performing the STEM–EDX experiments. This work was
supported by the Netherlands Center for Multiscale Catalytic Energy
Conversion (MCEC), an NWO Gravitation programme funded by the
Ministry of Education, Culture and Science of the government of the
Netherlands.
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Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conflict of
interest.
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Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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