Glucose dehydration to 5-hydroxymethylfurfural over phosphate catalysts

Article abstract of DOI:10.1016/j.jcat.2012.12.028

Glucose dehydration has been studied over aluminum (AlPO), titanium (TiPO), zirconium (ZrPO), and niobium phosphates (NbPO). The experimental results suggest that the activity of glucose transformation decreases in the order: NbPO > ZrPO > TiPO > AlPO, which was determined to correspond to the amount of strong acid sites on these catalysts. The selectivity to 5-hydroxymethylfurfural (HMF) varies in the range of 30-60% and depends on the ratio of Broensted to Lewis acid sites. Excess of Lewis acidity on the catalyst leads to unselective glucose transformation into humins. Modification of the catalysts by a silylation procedure or by deeper treatment with phosphoric acid leads to drastic increase in the selectivity to HMF due to the deactivation of unselective Lewis acid sites. It is proposed that a synergism of a protonated phosphate group and a nearby metal Lewis acid site in the two-stage glucose transformation into HMF leads to a highly selective glucose dehydration.

Full text of DOI:10.1016/j.jcat.2012.12.028

Journal of Catalysis 300 (2013) 37–46
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Glucose dehydration to 5-hydroxymethylfurfural over phosphate catalysts
a
b
a
a
a,
⇑
V.V. Ordomsky , V.L. Sushkevich , J.C. Schouten , J. van der Schaaf , T.A. Nijhuis
a
Laboratory of Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Department of Chemistry, Moscow State University, Leninskye Gory 1/3, 119991 Moscow, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Glucose dehydration has been studied over aluminum (AlPO), titanium (TiPO), zirconium (ZrPO), and nio-
bium phosphates (NbPO). The experimental results suggest that the activity of glucose transformation
decreases in the order: NbPO > ZrPO > TiPO > AlPO, which was determined to correspond to the amount
of strong acid sites on these catalysts. The selectivity to 5-hydroxymethylfurfural (HMF) varies in the
range of 30–60% and depends on the ratio of Brönsted to Lewis acid sites. Excess of Lewis acidity on
the catalyst leads to unselective glucose transformation into humins. Modification of the catalysts by a
silylation procedure or by deeper treatment with phosphoric acid leads to drastic increase in the selec-
tivity to HMF due to the deactivation of unselective Lewis acid sites. It is proposed that a synergism of
a protonated phosphate group and a nearby metal Lewis acid site in the two-stage glucose transformation
into HMF leads to a highly selective glucose dehydration.
Received 3 October 2012
Revised 26 November 2012
Accepted 23 December 2012
Available online 1 February 2013
Keywords:
Glucose
Dehydration
HMF
Phosphates
Ó 2013 Elsevier Inc. All rights reserved.
1
. Introduction
catalysts for HMF production demonstrate a low HMF yield and re-
quire a high reaction temperature.
Over the last years, the dehydration of carbohydrates to
-hydroxymethylfurfural (HMF) has attracted increasing attention
Glucose dehydration over zirconia and titania oxides by Watan-
abe et al. [10] resulted in a yield of HMF lower than 30% at 200 °C.
Yan [11] used sulfated zirconia for glucose dehydration to HMF
with a 36% yield at 200 °C. The most promising systems for glucose
dehydration to HMF were shown to be niobic acid and tantalum
hydroxides treated with phosphoric [12,13]. The yield of HMF
was close to 50% in a biphasic system of water-2-butanol at 160 °C.
The main reason for the unselective process of glucose transfor-
mation over heterogeneous catalysts in our point of view lies in
unbalanced functions of isomerization of glucose into fructose
and dehydration of fructose into HMF. This leads to a requirement
for more severe conditions for the reaction and unselective con-
densation of sugars into humins.
In this work, we studied the role of the type and amount of
Lewis and Brönsted acid sites in glucose transformation over differ-
ent phosphates. The strength of the acid sites was varied by appli-
cation of different metals in the row Al, Ti, Zr, and Nb. The Lewis
acidity of phosphates was modified by silylation with tetrathoxysi-
lane (TEOS).
5
due to its possible application as a substitute for petroleum based
building blocks [1]. HMF and its derivatives can be applied as plat-
form chemicals, precursor for polymers, fuels, or solvents [2].
Fructose dehydration to HMF is easy to realize over homoge-
neous and heterogeneous acidic catalysts in aqueous or organic
phase [3,4], multiphase systems [5], and ionic liquids [6]. The
low availability and high price of fructose in comparison with glu-
cose, which is the main building block of biomass, increase interest
in the methods for effective glucose dehydration to HMF. Unfortu-
nately, glucose as an aldohexose leads to low HMF selectivity in the
direct dehydration due to side reactions like cross-condensation
with formation of humins.
The most popular method of glucose dehydration to HMF at the
moment is based on the application of metal halides like CrCl in dif-
2
ferent ionic liquids or organic solvents [7,8]. The catalyst promotes
the isomerization of glucose to fructose followed by a dehydration
to HMF. A combination of acidic and basic heterogeneous catalysts
like Amberlyst-15 and hydrotalcite in the presence of aprotic organ-
ic solvents also gives high yields of HMF [9]. However, the applica-
tion of ionic liquids and high boiling solvents results in a significant
increase in the price of the process due to difficult separation and
purification stages. It would be preferable to use water in the
dehydration process. At the moment, in literature heterogeneous
2
. Experimental
2.1. Catalysts
Aluminum, titanium, zirconium, and niobium phosphates were
prepared using different procedures described in literature
[14–17]. Aluminum phosphate (AlPO) was prepared by the
addition of aqueous ammonia (25 wt.%) to an aqueous solution
⇑
Corresponding author.
E-mail address: t.a.nijhuis@tue.nl (T.A. Nijhuis).
0
021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.12.028

3
8
V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
containing equimolar quantities of aluminum chloride and ortho-
phosphoric acid (1 M) until a pH 7.0 was reached [14]. Titanium
phosphate (TiPO) was prepared by a fast addition of an aqueous
the catalysts were pressed in self-supporting disks and activated in
the IR cell attached to a vacuum line at 300 °C for 4 h. Adsorption of
pyridine (Py) was performed at 150 °C for 30 min. The excess of Py
was further evacuated at 150 °C for 1 h. The adsorption–evacuation
was repeated several times until no changes in the spectra were
observed.
solution of TiCl
4
(7.5 g) in 1 M HCl (20 mL) to orthophosphoric acid
(
5.92 g in 20 mL water) while intensively stirring [15]. Phosphoric
acid (23.3 g) in water (470 mL) was added quickly to a rapidly stir-
red solution of zirconyl chloride (22.5 g) in water (140 mL) at
ambient temperature (ZrPO) [16]. Niobium phosphate (NbPO)
was synthesized by a treatment of hydrated niobium oxide pro-
vided by CBMM in Brazil (5 g) with diluted orthophosphoric acid
31P solid-state NMR was performed using a Bruker Avance-400
spectrometer.
Adsorption of glucose or fructose on the catalysts was studied
by addition of 1 g of catalyst to a solution of 1.85 mmol of glucose
or fructose in 10 mL of water. The mixture was stirred for 0.5 h at
298 K. The amount of adsorbed glucose or fructose was determined
using HPLC (Shimadzu) equipped with refractive index detector by
determining the remaining concentration in the water.
(
2
9 g in 160 mL of H O) at 70 °C for 7 h [17]. NbPO-f was synthe-
sized by the same treatment of the fresh niobium oxide. The fresh
niobium oxide was prepared by preliminary fusion of niobium
oxide with KOH by gradually heating up to 450 °C, dissolution in
hot water, and precipitation by addition of nitric acid. The resulting
phosphates were aged at ambient temperature and then filtered
and washed with a large amount of water. The solid was dried
2.3. Sugars dehydration
(
100 °C) and calcined at 400 °C for 3 h in a flow of air.
Na/NbPO-f and Na/ZrPO were prepared by threefold ion ex-
Experiments were carried out in a 100-mL stirred autoclave
working in a batch mode. The procedure for testing catalysts was
as follows: catalyst (2 g) and water (40 mL) were poured into the
autoclave. Methylisobutylketone (MIBK) or 2-methyltetrahydrofu-
ran (MTHF) was added to the autoclave in the experiments with
solvent. The autoclave was purged with nitrogen. Fructose or glu-
cose (4 g) dissolved in 20 mL of water was forced into the autoclave
change of NbPO-f and ZrPO at 50 °C with 1 M solution of sodium
carbonate.
Aluminum, titanium, and zirconium oxides were prepared by
similar methods. Aluminum (Al
nium (ZrO ) oxides were prepared by the addition of aqueous
ammonia (25 wt.%) to an aqueous solution of chlorides. Niobium
oxide (Nb ) was used from the described procedure for the prep-
aration of NbPO-f. The resulting oxides were aged at ambient tem-
perature and then filtered and washed with a large amount of
water. The solid was dried (100 °C) and calcined at 400 °C for 3 h
in a flow of air.
The surface modified samples ZrPO/Si, NbPO/Si, and Nb
were prepared by the chemical liquid deposition of silica over
ZrPO, NbPO, and Nb , respectively, using TEOS as the silylation
agent. 4 g of dried catalysts were added to a solution of 1 g of TEOS
in 100 mL of n-hexane. The mixture was stirred at 50 °C for 24 h.
The samples were subsequently washed with water and dried.
2 3 2
O ), titanium (TiO ), and zirco-
2
2
from a storage vessel by high-pressure N after the temperature
2 5
O
had been increased to 135 °C, after which the catalytic experiment
was started. The agitation speed was 500 rpm.
Periodically, liquid samples were taken from the autoclave,
which were analyzed using HPLC (Shimadzu) equipped with
refractive index and UV–Vis detectors with a BIO-RAD Aminex
HPX-87H column.
2 5
O /Si
Reactant conversion and product selectivity was defined as
follows:
2 5
O
Conversion ðmol:%Þ ¼ ðmoles of fructose or glucose reactedÞ=
ðmoles of initial fructose or glucoseÞꢂ100%
2
.2. Characterization
Selectivity ðmol:%Þ ¼ ðmoles of produced HMF; fructose;
glucose or levulinic acidÞ=ðmoles of
The chemical composition of the samples was determined by
inductively coupled plasma (ICP) spectroscopy. All measurements
were performed on an ELAN ICP-MS machine (Perkin Elmer). Prior
to the measurements, the exact amount of catalyst (ꢀ100 mg) was
dissolved in 0.4 g of concentrated hydrofluoric acid (40%). This
solution was diluted by distilled water to obtain a concentration
of measured ions of about 1 mg/L. Solutions for calibration were
prepared from the standards with addition of the corresponding
amount of HF to level the matrix effect. Sorption–desorption iso-
therms of nitrogen were measured using an automated porosime-
ter (Micrometrics ASAP 2000). Prior to the measurements, the
samples were evacuated at 300 °C. The XRD patterns were
recorded with a DRON-3M diffractometer, applying Cu Ka radia-
tion. The acidic properties were studied by temperature-pro-
fructose or glucose reactedÞ ꢂ 100%
3. Results and discussion
3.1. Physico-chemical properties of catalysts
The properties of the catalysts are given in Table 1. All studied
catalysts show XRD patterns typical for an amorphous material
(not shown). Nitrogen adsorption data indicate that the amor-
phous phosphates have a high surface area with a very broad dis-
tribution of pore sizes (Fig. 1). Sizes of the pores for phosphates are
higher than 100 Å, except for TiPO that has narrow pore size distri-
bution in the range 50–120 Å. The amorphous phosphates have
completely different M/P molar ratios, which is due to the different
ways of their preparation. They are most probably composed of a
mixture of phosphates, polyphosphates, and hydrophosphates
with different M/P ratios. The ZrPO catalyst has the highest content
of phosphorus, which can be explained by the formation of a lay-
ered structure with bridged phosphate groups between layers
[18]. The lowest content of phosphorus has the NbPO sample due
to the incomplete transformation of niobium oxide into phosphate
[16,17]. Indeed, in this case, only a niobium oxide surface area is
exposed to interaction with phosphoric acid in comparison with
the direct mixing of chlorides with phosphoric acid. All the
attempts to increase the phosphorus content in the sample NbPO
3
grammed desorption of ammonia (NH TPD). Prior to adsorption,
the samples were calcined in situ in a flow of dry air at 400 °C
for 1 h and, subsequently, in a flow of dry helium for 1 h and cooled
down to ambient temperature. For NH
subjected to a flow of diluted NH for 30 min at ambient tempera-
ture. The physisorbed NH was removed in a flow of dry He at
00 °C for 1 h. Typical TPD experiments were carried out in the
temperature range of 100–800 °C in a flow of dry He. The rate of
heating was 7 °C/min. The desorbed NH was analyzed by a ther-
3
adsorption, a sample was
3
3
1
3
mal conductivity detector (TCD) detector. The water formed during
desorption at high temperatures was trapped using a cold trap.
IR spectra were recorded with a Nicolet Protégé 460 FT-IR spec-
ꢁ
1
trometer with 4 cm optical resolution. Prior to the measurements,

V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
39
Table 1
Characterization of catalysts.
2
Catalyst
Elemental analysis
Surface area, (m /g)
TPD NH
3
(l
mol/g)
Py adsorption (
Brönsted (B)
l
mol/g)
Adsorption (
Fructose
l
mol/g)
M/P
Na/P
Lewis (L)
B/L
Glucose
AlPO
TiPO
ZrPO
NaAZrPO
ZrPO/Si
NbPO
NbPO-f
Na/NbPO-f
1.1
0.9
0.6
0.6
0.6
4.1
2.4
2.4
0
0
0
0.6
0
0
0
0.8
140
80
210
210
210
67
1097
762
1725
–
90
300
280
20
272
75
950
470
1440
1340
620
280
370
90
0.09
0.64
0.19
0.01
0.44
0.27
1.00
0.00
1.2
7.6
39.2
–
20.7
17.2
10.5
–
0
0.6
5.5
–
1.1
2.4
0.2
–
–
345
736
–
70
70
370
0
1
1
.4
.2
1
1121
AlPO
1
077
0
0
0
0
.8
.6
.4
.2
0
TiPO
AlPO
ZrPO
NbPO
1
063
TiPO
ZrPO
0
200
400
600
800
1000
Pore size, Å
1
028
Fig. 1. Pore size distribution in the phosphate catalysts.
NbPO-f
NbPO
by using a higher concentration of orthophosphoric acid or higher
650
temperature lead to drastic loss of the surface area of the catalyst
due to crystallization of the material. Low amount of surface hy-
droxyl groups over niobium oxide leads to a low amount of formed
phosphate groups (Table 1). In order to increase the amount of hy-
droxyl groups over niobic acid, it was transformed into potassium
niobate with subsequent dissolution in water and precipitation by
addition of nitric acid into fresh niobic acid. This activation proce-
dure led to significant increase in the amount of phosphate groups
in NbPO-f after treatment of fresh niobic acid with phosphoric acid
1
028
1
500
1000
500
Wavenumber, cm-1
(
Table 1). The ratio Nb/P decreased from 4.1 to 2.4.
Fig. 2. FTIR spectra in the region of structural stretching vibrations of phosphates.
The IR spectra of the phosphates are shown in Fig. 2. A broad
ꢁ1
band in the range of 1000–1200 cm can be assigned to O@P@O
asymmetric stretching vibrations of phosphate or polyphosphate
ꢁ1
ꢁ20.6 ppm. It was shown earlier that the chemical shift 31P depends
on the amount of Zr atoms attached to P [22,23]. The higher amount
of bonds of P with the metal the higher will be the shift towards
higher magnetic field. Thus, the main peak in phosphates might
be attributed to the formation of three coordinated phosphate
species [19]. This band shifts from 1121 cm
for AlPO to
ꢁ
1
1
028 cm for NbPO. This effect is in line with the decreasing elec-
tronegativity of the metals and the simultaneous increase in the io-
nic character of the bond between metal and phosphate species in
ꢁ
1
the order: Al < Ti < Zr < Nb. The band at 650 cm
for niobium
phosphates is attributed to a skeletal vibration previously reported
for amorphous niobic acid [19,20]. The high M/P ratio for niobium
phosphate indicates a large quantity of metal oxide phase. For the
sample NbPO-f, the modification of the fresh niobic acid leads to a
significant increase in the intensity of the phosphate band and a
decrease in the contribution of niobium oxide (Fig. 2).
(ZrAO)
3
P(OH)
2
. Lines at ꢁ20.6 and ꢁ12 ppm in ZrPO correspond
to formation of (ZrAO)
2
PO(OH) and (ZrAO)PO(OH) . Appearance
2
3
1
of P corresponding to low interaction with Zr indicates the pres-
ence of interlayer phosphate species. Similar lines were observed
over a-zirconium phosphate with the P/Zr ratio 2 [23]. This explains
the low Zr/P ratio in ZrPO. Formation of polyphosphates should be
observed by the appearance of lines in the range ꢁ35 to ꢁ45 ppm
[23], but these lines were not observed in the spectrum (Fig. 3).
The ion exchange of NbPO-f or ZrPO samples does not lead to
any changes of the Nb/P or Zr/P molar ratios (Table 1). Therefore,
we can assume that the ion exchanged materials preserve the
structure of parent phosphates. The content of sodium introduced
is given in Table 1. The results show that ion exchange with sodium
carbonate leads to a deep substitution of the acidic protons with
sodium cations.
The state of phosphate groups in phosphates has been studied by
3
1
the solid-state P magic-angle-spinning NMR. The spectra of ZrPO,
TiPO, and AlPO are presented in Fig. 3. Niobium phosphate was not
included because of the very broad peak, which might be explained
by a wide variation in coordination of the phosphate groups. All
spectra exhibit the main peak in the range of ꢁ24 to ꢁ26 ppm.
The increase in the chemical shift for AlPO to ꢁ24.5 ppm in compar-
ison with ZrPO and TiPO is the result of an increase in the electro-
negativity of the metal and electron density over P [21]. Apart
from the main peak, ZrPO has additional shoulders at ꢁ12 and

4
0
V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
-
26.6
1449
490
1
1
592
1
544
-
-
20.6
NbPO
NbPO-f
Na/NbPO-f
-
25.0
12
*
*
ZrPO
*
ZrPO
*
Na/ZrPO
*
*
ZrPO/Si
TiPO
TiPO
AlPO
*
*
AlPO
50
0
-50
-100
1700
1600
1500
1
400
Chemical shift, ppm
Wavenumber, cm-1
Fig. 3. 31P-NMR spectra of AlPO, ZrPO, and NbPO. ꢄ Spinning side bands.
Fig. 5. FTIR spectra of pyridine adsorbed over phosphates.
The nature of the acid sites was studied by IR spectroscopy of
adsorbed pyridine, and the IR spectra are shown in Fig. 5. The main
bands observed for all the samples can be assigned according to the
1
1
.4
.2
1
AlPO
ꢁ
1
literature data [25,26]: the bands at 1592 and 1490 cm are due
ꢁ1
to H-bonded pyridine, the band at 1544 cm is attributed to pyr-
ZrPO
TiPO
ꢁ1
0
0
0
0
.8
.6
.4
.2
0
idine protonated on Brönsted sites, and the band at 1449 cm is
NbPO-f
assigned to pyridine adsorbed on Lewis acid sites.
Table 1 gives the concentration of the Brönsted and Lewis acid
sites calculated on the basis of the extinction coefficients for bands
ꢁ1
at 1545 and 1454 cm [27]. The intensity of the band assigned to
pyridine adsorbed on Brönsted acid sites increases in the following
order: AlPO ꢃ NbPO < TiPO ꢃ ZrPO (Fig. 5) and correlates mainly
with the ionic character of the bond. Other factors like the amount
of phosphate groups and the surface area also influence the con-
centration of Brönsted acid sites. Indeed, the low adsorption of pyr-
idine over niobium phosphate is the result of low amount of
phosphates groups in NbPO (Table 1). Treatment of fresh niobic
acid with phosphoric acid (NbPO-f) leads to a significant increase
in the amount of Brönsted acid sites and decrease in the contribu-
tion of the Lewis acidity in accordance with the higher amount of
phosphate groups in this sample (Fig. 5, Table 1). The higher sur-
face area of ZrPO in comparison with other phosphates does not re-
sult in an increase in the concentration of Brönsted acid sites, even
though the Lewis acidity is significantly higher. This might be ex-
plained by the presence of phosphate groups in ZrPO with a lower
coordination with Zr atoms (Fig. 3), which might result in a lower
acidity of phosphate groups and a high Lewis acidity. The results
show that the amount of Brönsted acid sites is lower than the
amount of Lewis acid sites for all phosphates. Lewis acidity deter-
mined by Py does not correlate with the electronegativity of metals
but correlates with the amount of desorbed ammonia and reflects
also the high binding energies of Py with Lewis acid sites (Table 1).
The decrease in the excess of Lewis acid sites in zirconium phos-
phate was realized by treatment of the catalyst with TEOS. This
procedure is well known for deactivation of acid sites on the exter-
nal surface of zeolites [28] but was not applied yet for modification
of phosphates. The modification does not lead to any changes in
NbPO
0
100
200
300
400
500
600
700
800
Temperature, °C
3
Fig. 4. TPD-NH profiles.
The amount and strength of the acid sites of phosphate catalysts
have been studied by temperature-programmed desorption of
ammonia (Fig. 4). The strength of acid sites and the temperature
of NH
3
desorption should increase according to the increase in
pro-
the ionic character in the row Al < Ti < Zr < Nb. The TPD-NH
3
files show one peak in the temperature range of 100–500 °C for
all the samples studied. For NbPO-f and AlPO, this peak is rather
narrow and is shifted to lower temperature, which points to acid
sites with medium strength. Differently, NbPO, TiPO, and ZrPO des-
orb ammonia over a wide range of temperatures from 100 to
5
00 °C, which corresponds to the medium and the strong acid sites.
The total amount of the acid sites determined by TPD-NH corre-
3
late with the surface area of samples (Table 1). The broad peaks
of ammonia desorption over phosphates which do not correspond
with the row of electronegativity are the result of the higher heat
of adsorption of ammonia over Lewis acid sites compared to that
over Brönsted acid sites [24]. This results in desorption of ammonia
with a broad distribution of energies from different types of Lewis
acid sites.

V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
41
changes in the electronic state of the Lewis acid site after exchange
+
Si(OH)3
Zr
of a nearby proton with Na . Protons compete with metal Lewis
-O
Zr
O
O
O
O
O
O
+
Si(OEt)4
H2O
sites for electrons of oxygen. Ion exchange with Na should lead
HO
Zr
Zr
to a decrease in the positive charge over Nb and strength of the Le-
wis acid site. The effect of ion exchange with alkali metal on the
spectra of Py adsorption over Lewis acid sites provides information
on the proximity of Brönsted and Lewis acid sites on the catalyst
NbPO-f. The influence of the strength that coexisting Brönsted
and Lewis acid sites have on each other was shown earlier in the
literature [31].
At the same time, adsorption of Py over Na/ZrPO shows that the
elimination of protons does not lead to a significant decrease in the
amount of Lewis acid sites (Fig. 5, Table 1). This observation might
be explained by a high amount of unsaturated Lewis acid sites iso-
lated from phosphate groups. Ion exchange with Na+ should not
significantly affect the amount and strength of Lewis acid sites in
this case.
O
Scheme 1.
the texture of the catalyst, which can be observed by similar results
in the nitrogen adsorption (Table 1). Treatment of zirconium phos-
phate with TEOS leads mainly to a deactivation of the Lewis acidity
with a low effect on the Brönsted acid sites (Fig. 5, Table 1). This
fact might be explained by a low stability and easy hydrolysis of
the SiAOAP bonds in comparison with SiAOAAl bonds. Lewis acid
sites in phosphates can be divided into two groups: Lewis acid sites
formed by metal attached to the phosphate group (AZrAOAPA)
[
29] and Lewis acid sites formed by coordinatively unsaturated
To summarize, our results suggest that the amount of strong
Brönsted acid sites increases in accordance with the decrease in
the electronegativity of the metals. Lewis acidity in phosphates
consists of metal sites bound to phosphate groups and isolated
coordinatively unsaturated sites. Treatment of phosphates with
TEOS results in a decrease in the amount of isolated Lewis acid
sites.
metal ions like in oxides [30]. Treatment of the catalyst with TEOS
might lead to a hydrolysis of TEOS on unsaturated metal sites with
the formation of SiAOAZr bonds and a deactivation of this type of
Lewis acid sites (Scheme 1). The first type of the Lewis acidity in
ZrAOAP groups should be less affected by the treatment with TEOS
due to the saturated metal.
The presence of the Lewis acid sites bound with a phosphate
group might be observed by catalyst treatment with Na
ment of NbPO-f with Na CO leads to a disappearance of Brönsted
acid sites (Fig. 5). At the same time, the intensity of the peak at
2
CO
3
. Treat-
3.2. Dehydration of sugars
2
3
3.2.1. Glucose dehydration over phosphates
The conversion of glucose and the selectivity to HMF, fructose,
and levulinic acid during dehydration of glucose over the different
ꢁ1
1
449 cm , attributed to Py adsorption over Lewis acid sites, also
significantly decreased. This observation might be explained by
TiPO
70
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
a
b
NbPO
NbPO/Si
NbPO-f
6
0
0
ZrPO
5
AlPO
NbPO
NbPO-f
40
30
20
ZrPO/Si
ZrPO/Si
TiPO
ZrPO
NbPO/Si
10
AlPO
without catalyst
0
0
100
200
300
400
500
600
0
20
40
60
Glucose conversion, %
Time, min
8
7
6
5
4
3
2
1
0
10
9
8
7
6
5
4
3
2
1
0
c
d
NbPO/Si
AlPO
NbPO-f
TiPO
NbPO
ZrPO
NbPO/Si
NbPO
ZrPO/Si
ZrPO/Si
NbPO-f
AlPOTiPO
ZrPO
60
0
20
40
80
0
20
40
60
80
Glucose conversion, %
Glucose conversion, %
Fig. 6. Glucose conversion versus time (a), selectivity to HMF (b), selectivity to acids (c), and fructose (d) versus glucose conversion over phosphates. Dotted arrows show
results of the experiment after filtering of ZrPO and heating of the solution for an additional 2 h.

4
2
V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
catalysts are shown in Fig. 6. Besides, these main products furfural
was detected in the reaction system by HPLC. The selectivity to fur-
fural did not exceed 1 mol.%, and therefore, we did not investigate
its formation further. The appearance of furfural was explained
previously by a fast reverse-aldol cleavage of carbohydrates [32].
Formic and levulinic acids are the products of HMF rehydration
over acidic catalysts [33]. The main side reaction is the formation
of polymeric humins by condensation of HMF and sugars [34].
Humins could not be observed by HPLC, and their amount was
calculated from the carbon balance.
The results of the reaction without addition of catalyst (Fig. 6a)
show that the conversion of glucose is lower than 1% indicating a
high stability of glucose at this temperature. Activity of the parent
phosphates in the glucose transformation increases in the order:
AlPO < TiPO < ZrPO ꢃ NbPO. This order corresponds to the decrease
in electronegativity of the metals and the increase in the ionic
character between the metal and the phosphate group. It provided
information on the role of the strength of the acid sites in the glu-
cose dehydration to HMF. The total acidity of the phosphates deter-
90
a
Nb O
2 5
8
7
6
0
0
0
50
ZrO2
4
3
2
0
0
0
TiO
2
10
0
Nb2O5/Si
Al O
2
3
0
100
200
300
400
500
Time, min
b 70
60
Nb O /Si
2
5
5
4
3
0
0
0
3
mined by TPD of NH does not correlate with the activity of the
catalysts (Fig. 4). A similar trend in the activity for this type of cat-
alysts was observed during the Prins condensation of formalde-
hyde with isobutene over phosphates [35].
TiO2
Nb O
2
5
The selectivities to HMF over phosphates have bell-shaped
curves (Fig. 6b). This form might be explained by an initial interac-
tion of glucose with the surface of the catalyst and a decrease in the
selectivity to HMF at high conversions, due to secondary HMF
transformation by rehydration of HMF into formic and levulinic
acids and secondary condensation of HMF into humins [34]. The
selectivity to HMF for TiPO is close to 50% at 20% conversion of glu-
cose. At the same time, the maximum selectivity to HMF for nio-
bium (NbPO) and aluminum phosphate (AlPO) is near 35% and
for zirconium phosphate it is even lower (30%).
The presence of a homogeneous catalyst due to the leaching of
phosphate and metal species could be the reason of the low selec-
tivity of the glucose dehydration over phosphates. In order to
probe this hypothesis in the experiment over ZrPO, the catalyst
was removed by filtration after 6.5 h of the reaction. The filtrate
was subsequently allowed to react at the same temperature for a
further 2 h. The results show (Fig. 6) that the reaction did not con-
tinue once the catalyst was removed and the selectivity to HMF did
not change, indicating that no homogenous catalysis occurred by
leached metal and phosphate ions.
20
Al O
ZrO2
2
3
1
0
0
0
20
40
60
80
100
Glucose conversion, %
c 8
0
70
6
0
Al O
2
3
50
4
3
2
1
0
0
0
0
0
TiO2
ZrO2
Nb O
2
5
Nb O /Si
2
5
The selectivity to formic and levulinic acid is shown in Fig. 6c.
The high selectivity of AlPO and TiPO to acids is the result of the
rehydration of HMF, which is more significant when the catalyst
has a low activity in the dehydration of the more stable glucose
molecule. The selectivity to acids over NbPO and ZrPO slowly in-
creases till 2–3% at 60% conversion of glucose.
0
20
40
60
80
100
Glucose conversion, %
Fig. 7. Glucose conversion versus time (a), selectivity to HMF (b), and selectivity to
fructose (c) versus glucose conversion over oxides.
The dehydration of glucose to HMF, as was shown earlier over
different Lewis acidic systems, contains two stages: isomerization
of glucose to fructose and fructose dehydration to HMF [36,37].
Glucose isomerization to fructose is very significant in the initial
stage of the reaction over ZrPO and NbPO (Fig. 6d). The selectivity
to fructose increases fast to 8% at a low conversion of glucose and
decreases during the reaction to 1%. At the same time, AlPO and
TiPO do not show the formation of fructose. This is the result of
the weak Lewis acid sites of these catalysts, which are not active
in the isomerization of glucose. The formation of a small amount
of fructose in this case should be accompanied by a fast dehydra-
tion to HMF. At the same time, the high amount of formed fructose
in the case of NbPO and ZrPO might be responsible for the low
selectivity to HMF due to the unselective fructose dehydration to
HMF over Lewis acid sites [34].
transformed into humins, which cannot be observed by HPLC. To
verify this and prove the formation of humins during the reaction,
the products of the glucose dehydration over ZrPO after 6.5 h of the
reaction were filtered. The solid part was dried and calcined at
600 °C. Calculation of the burned organic part as the product of
glucose condensations shows that 29% of converted glucose was
transformed into insoluble humins. At the same time, after 6.5 h
of the reaction, 30% of glucose was transformed into HMF (26%),
acids (3%), and fructose (1%). It means that about 40% of glucose
should be in the form of soluble humins in the reaction mixture.
Unselective dehydration of glucose over Lewis acid sites might
be shown by glucose dehydration over oxides (Fig. 7). The activity
of glucose transformation over oxides decreases in the same order
Summation of the selectivity of all detected products like HMF,
acids, and fructose show that approximately 50% of glucose is
like over phosphate catalysts: Nb
This indicates that the activation of glucose molecules occurs
2 5 2 2 2 3
O > ZrO > TiO > Al O (Fig. 7a).

V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
43
preferentially over the strong Lewis acid sites, the amount of which
should decrease from Nb to Al . The selectivities to HMF over
conversion of 40%. HMF transforms over Lewis acid sites mainly
into humins, in comparison with the rehydration reaction to formic
and levulinic acids, which proceeds over Brönsted acid sites [34].
Deactivation of isolated Lewis acid sites leads to an increase in
the contribution of the rehydration reaction with an increase in
the selectivity to acids. As expected, the suppression of the activity
of Lewis acid sites in the catalysts leads to a significant decrease in
the isomerization activity of glucose to fructose (Fig. 6d). The
curves of the selectivity to fructose for NbPO-f, ZrPO/Si, and
NbPO/Si have the same form like those for the parent catalysts,
but they are shifted in the direction of lower conversions of
glucose. The higher isomerization activity of ZrPO and NbPO over
Lewis acid sites is supported by results of fructose and glucose
adsorption (Table 1). A strong adsorption of sugars on NbPO and
ZrPO is the result of the multiple interactions between strong
Lewis acid sites and hydroxyl groups of sugars. These interactions
lead to further isomerization of sugars [39]. It is interesting to note
that the adsorption of fructose is almost 10 times higher over phos-
phates in comparison with glucose. The suppression of the Lewis
acidity of phosphates ZrPO and NbPO after modification might be
observed also from a decrease in sugars adsorption by two times
for ZrPO/Si and NbPO-f (Table 1).
2
O
5
2 3
O
all oxide catalysts were in the range 10–30%, which confirms the
hypothesis about unselective glucose dehydration over Lewis acid
sites (Fig. 7b). It is interesting to note that the shape of the selec-
tivity curves and their positions relative to each other over oxides
resemble the selectivity curves to HMF during glucose dehydration
over phosphates. This is a strong indication that oxide type Lewis
acid sites play a role in the glucose dehydration over phosphates.
In comparison with phosphates, glucose dehydration over oxides
is accompanied by a more intensive glucose isomerization into
fructose over Lewis acid sites (Fig. 7c). Thus, selectivities to fruc-
tose are 60–70% at low conversions even over aluminum and tita-
nium oxides, which were inactive in isomerization in the form of
phosphates. The selectivity decreases fast like over phosphates
with an increase in the conversion of glucose due to subsequent
fructose condensation into humins over Lewis acid sites.
These observations make clear the importance of the right com-
bination of Brönsted and Lewis acidity in a catalyst for an effective
glucose dehydration to HMF. In order to study this effect, we used
modified Brönsted and Lewis acidity of zirconium and niobium
phosphates.
Thus, the modification of zirconium and niobium phosphates
with a decrease in the contribution of Lewis acid sites leads to an
increase in the selectivity of HMF formation. Indeed, the selectivity
to HMF at 20% conversion of glucose depends on the ratio of B/L
acid sites determined by Py adsorption and shows a gradual
increase, indicating the strong influence of the composition of
Brönsted and Lewis acid sites (Fig. 8). The decrease in the selectiv-
ity over catalysts with high Lewis acidity is the result of unselec-
tive glucose and fructose condensation into humins over isolated
Lewis acid sites. The highest selectivity observed over NbPO-f cor-
responds to an equal amount of Lewis and Brönsted acid sites due
to NbAOAP centers.
In order to confirm this hypothesis and to study the role of both
types of acid sites in glucose dehydration over phosphates,
Na-exchanged samples were used. Na-exchanged samples possess
only Lewis acidity and thereby give the opportunity to separate the
process of glucose dehydration in two stages.
3
.2.2. Phosphates modification
The increase in the ratio Brönsted/Lewis (B/L) by modification of
niobium and zirconium phosphates leads to significant changes in
the catalytic properties (Fig. 5, Table 1). NbPO/Si and ZrPO/Si show
an almost two times lower activity in comparison with the parent
catalysts (Fig. 6). At the same time, the selectivity to HMF over
NbPO/Si and ZrPO/Si increases to 60% and 55%, respectively. The
silylation procedure leads mainly to deactivation of isolated Lewis
acid sites from phosphate groups (Fig. 5). This observation indi-
cates a high activity of isolated Lewis acid sites in the unselective
glucose transformation into humins, which was shown by glucose
dehydration over oxides (Fig. 7). A decrease in the amount of active
Lewis acid sites leads to an increase in the selectivity of the pro-
cess. Niobium phosphate on the basis of fresh niobic acid (NbPO-
f) also shows a significantly lower activity in comparison with
NbPO, but its activity is still higher than that of NbPO/Si. The selec-
tivity over NbPO-f is 55% and comparable with the selectivity over
NbPO/Si and ZrPO/Si. This is due to the high amount of phosphate
groups in NbPO-f in comparison with NbPO, with a low amount of
isolated Lewis acid sites leading to an unselective glucose transfor-
mation (Fig. 5). Hydroxyl groups in fresh niobium oxide are more
active in interaction with phosphoric acid; however, these groups
transform into stable Lewis acid sites in time. It is interesting to
note that silylation of NbPO-f does not lead to changes in activity
or selectivity of the catalyst (not shown), which means that already
a low amount of isolated Lewis acid sites is present in this sample.
The poisoning effect of silylation over isolated Lewis acid sites
can be shown over oxide catalysts. Most part of Lewis acid sites
in the case of oxide should be formed by coordinatively unsatu-
rated metal ions subjected to deactivation by a silylation proce-
dure. Indeed, the treatment of niobic acid with TEOS leads to an
almost complete deactivation of the catalyst (Fig. 7a). The perfor-
mance of the silylated catalyst significantly differs from the parent
oxide. It dehydrates glucose to HMF with a much higher selectivity
3.2.3. Dehydration of sugars over Na-exchanged phosphates
Fig. 9 shows a comparison of the fructose and glucose dehydra-
tion over NbPO before and after exchange with Na. The glucose
transformation over NaANbPO has an almost two times higher
60
NbPO-f
50
TiPO
ZrPO/Si
NbPO
4
3
2
1
0
0
0
0
ZrPO
(Fig. 7b). The strong Brönsted acid sites of niobic acid formed by
hydroxyls [38] are apparently not deactivated by TEOS resulting
in a high selectivity to HMF over the silylated catalyst.
The high slope of the line of the HMF selectivity for NbPO-f,
ZrPO/Si, and NbPO/Si with an increase in glucose conversion indi-
cates significant side reactions involving HMF. Indeed, Fig. 6c
shows that the rate of acids formation increased significantly for
modified catalysts. Thus, for NbPO-f, the selectivity to acids
increases to 8%, in comparison with 2% over NbPO at a glucose
0
0
0. 2
0.4
0.6
0.8
1
1.2
Brönsted/Lewis
Fig. 8. Selectivity to HMF versus Brönsted/Lewis ratio determined by Py adsorption
at 20% conversion of glucose.

4
4
V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
was very low (less than 2%) due to the low amount of HMF avail-
a 1
00
NbPO-f
Na/NbPO-f
able for the rehydration reaction in the system and therefore they
could lead only to partial exchange. At the same time, formation of
HMF over an oxide catalyst (Fig. 7) shows that HMF might form
over Lewis acid sites next to the main reactions of glucose isomer-
ization to fructose and the condensation of sugars into humins.
It is interesting to note that the presence of protonated phos-
phate groups significantly suppresses the activity of Lewis acid
sites in glucose transformation. A study of the acidity of NbPO-f
indicates the close arrangement of Brönsted and Lewis acid sites
in this sample (Fig. 5). Isomerization of glucose molecules into
fructose over Lewis acid sites formed by Nb might be accompanied
by subsequent dehydration over protonated phosphate groups
without intermediate desorption of fructose molecules. This pro-
cess can be described by Scheme 2. Protonated phosphate group
might take part in the glucose isomerization by transfer of a proton
between the oxygen of the first and second carbon. This will lead to
a decrease in the accessibility of the Lewis acid site for the transfor-
mation of another molecule of glucose and thereby a lower activ-
ity. At the same time, it should result in a more efficient
transformation of glucose into HMF due to the absence of an inter-
mediate desorption of fructose from the Lewis acid site and a pos-
sible transformation into humins over another Lewis acid site.
The presence of isolated Lewis acid sites in the sodium ex-
changed catalyst should lead to a lower or comparable activity in
glucose transformation in comparison with the parent sample. In-
deed, testing of the catalyst ZrPO with a high isolated Lewis acidity
shows a comparable activity in the glucose transformation before
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Na/NbPO-f
NbPO-f
fructose
glucose
0
100
200
300
400
500
Time, min
fructose
glucose
b
c
60
NbPO-f
5
4
3
2
1
0
0
0
0
0
0
NbPO-f
Na/NbPO-f
Na/NbPO-f
0
20
40
60
80
100
Conversion, %
+
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
and after ion exchange with Na (Fig. 10).
Na/NbPO-f
The fructose transformation over NbPO and NaANbPO catalysts
occurs with similar activities, which is much higher than the activ-
ity in glucose dehydration over NbPO and NaANbPO catalysts
fructose
glucose
(Fig. 9a). This indicates a higher reactivity of the fructose molecule
Na/NbPO-f
both over Brönsted and over Lewis acid sites in comparison with
glucose due to the furanose ring structure. Fructose transforms
over Lewis acid sites of NaANbPO mainly into humins and glucose
NbPO-f
NbPO-f
(Fig. 9). The selectivity to glucose over NaANbPO is even higher
than the isomerization of glucose into fructose, what might be ex-
plained by a lower rate of consecutive glucose transformation.
The activity of NaANbPO in the fructose dehydration to HMF,
similar to the case of glucose, is also very low and the selectivity
to HMF does not exceed 10%. It is interesting to note that the selec-
tivity of fructose dehydration over NbPO to HMF is lower in com-
parison with the glucose dehydration to HMF. The maximum
selectivity of fructose dehydration to HMF over NbPO-f was only
40%, whereas over NbPO-f glucose dehydrates to HMF with a max-
imum selectivity of 55%. The low selectivity to HMF during fructose
dehydration is the result of a separate unselective fructose trans-
formation over a Lewis site and nearby phosphate group. Glucose
seems to be more stable to condensation into humins over proton-
ated phosphate groups. The adsorption of glucose on Lewis acid
site with subsequent isomerization and further dehydration over
nearby phosphate group results in a higher selectivity of the pro-
cess (Scheme 2). It is interesting to note that glucose and fructose
dehydration proceeds with similar selectivities (30%) over zirco-
nium phosphate (Fig. 10). This is the result of separate stages of
0
20
40
60
80
100
Conversion, %
Fig. 9. Glucose and fructose conversion versus time (a), selectivity to HMF (b), and
fructose or glucose (c) versus glucose or fructose conversion over parent NbPO and
after ion exchange with Na.
activity in comparison with the parent NbPO (Fig. 9a). The main
process over NaANbPO is glucose isomerization to fructose instead
of dehydration to HMF over NbPO (Fig. 9c). Thus, the selectivity to
fructose decreases from 35% with increase in glucose conversion
until almost full disappearance of it due to subsequent transforma-
tion into humins. At the same time, the selectivity to HMF in-
creases with an increase in the glucose conversion to 10%
(
Fig. 9b), which might be due to the unselective fructose dehydra-
tion over Lewis acid sites [34]. Ion exchange of sodium cations with
protons of formic and levulinic acids also might be the reason of
fructose dehydration to HMF. However, the selectivity to acids
O
HO
HO
O
P
O
P
H
C
C
H
C
C
H
H+
2
HO H2C
O
CHO
CH2OH
O
HO
Nb
P
O
P
O
P
HO
Nb
CH OH
O
O
O
OH+
H
C OH
H
O
O
Nb
O
O
Nb
O
O
O
O
Nb
O
O
H
OH
C
O
OH
O
HO
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
(
OH)4H5C4
O
5 4
(OH) H C
(
OH)4H5C4
4
Scheme 2.

V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
45
4
3
0
5
a 1
b
00
Na/ZrPO
ZrPO
Na/ZrPO
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
30
ZrPO
2
2
5
0
ZrPO
ZrPO
15
Na/ZrPO
fructose
glucose
1
0
5
0
Na/ZrPO
0
200
400
600
0
20
40
60
80
100
Conversion, %
Time, min
Fig. 10. Glucose and fructose conversion versus time (a) and selectivity to HMF versus glucose or fructose conversion (b) over parent ZrPO and after ion exchange with Na.
MIBK/water
MTHF/water
70
60
MTHF/water
a
80
b
HMF
acids
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
MIBK/water
water
5
4
3
2
1
0
0
0
0
0
0
water
0
200
400
600
0
20
40
Conversion, %
60
80
Time, min
Fig. 11. Glucose conversion versus time (a) and selectivity to HMF and acids (b) versus glucose conversion over NbPO-f in aqueous phase and with addition of 3 vol. of MIBK
or MTHF.
glucose isomerization over Lewis acid sites with subsequent
desorption and dehydration or condensation over other Lewis acid
sites or phosphate groups.
Thus, the synergism in the two-stage process results in a higher
selectivity of glucose dehydration into HMF. However, at high con-
versions of glucose the selectivity to HMF starts to decrease due to
the secondary processes of HMF rehydration and condensation
with sugars into humins. It was shown earlier that the addition
of organic solvents absorbing HMF leads to a suppression of the
side reactions of HMF transformation [34,40].
of organic solvents leads to a stabilization of the selectivity to HMF
at about 60%. In comparison with the significant beneficial effect of
the addition of organic solvents on the selectivity to HMF during
dehydration of fructose, absorption of HMF during glucose dehy-
dration leads only to a preservation of the maximum selectivity
to HMF [34]. This is a result of the significantly higher rate of sec-
ondary HMF rehydration to acids and HMF condensation with fruc-
tose over Brönsted acidic systems [34]. The addition of solvent in
this case results in a larger effect on the selectivity.
Thus, the presence of organic solvents during the glucose dehy-
dration over phosphates leads to a suppression of the deactivation
of the catalyst and stabilizes the selectivity to HMF at high conver-
sions of glucose.
3.2.4. Glucose dehydration in biphasic system
Fig. 11 shows a comparison of glucose dehydration over NbPO-f
without and in the presence of organic solvents like MIBK and
MTHF. The activity of the catalyst is comparable at low conversion
of glucose, although addition of organic solvents leads to an in-
creased activity at high conversions of glucose. This fact is the re-
sult of the deactivation of the catalyst during the reaction in water
by humins deposition on the surface of the catalyst. Extraction of
HMF and humins by MIBK and MTHF, which is visible by a clearing
of aqueous phase and a darkening of the organic phase, suppresses
the deactivation of the catalyst (Fig. 11).
Extraction ratios of HMF for MIBK and MTHF are 0.9 and 1.5,
respectively. This means that the addition of three volumes of or-
ganic solvent leads to a decrease in the concentration of HMF in
aqueous phase by factor of 3.6 and 6 for MIBK and MTHF, respec-
tively. This decrease in the concentration of HMF in the aqueous
phase leads to a decrease in the amount of formed acids in the
reaction mixture (Fig. 11). Thus, without an addition of organic sol-
vents, the selectivity to acids is 7% at 40% glucose conversion and
the addition of MIBK and MTHF leads to a decrease in the selectiv-
ity to 1.78% and 0.3%, respectively. At the same time, the addition
4. Conclusions
The investigation of the acidic properties of phosphates of Al, Ti,
Nb, and Zr suggests that the amount of strong Brönsted acid sites
increases in accordance with the decrease in electronegativity of
the metals. Lewis acidity in phosphates consists of metal sites
bound to phosphate groups and isolated Lewis acid sites. Treat-
ment of phosphates of Nb and Zr with TEOS allows us to decrease
the amount of isolated Lewis acid sites.
Phosphates of aluminum, zirconium, and niobium are shown to
be effective catalysts for glucose dehydration to HMF. The conver-
sion of glucose over phosphates increases in the following order:
AlPO < TiP < ZrP < NbP in accordance with increase in the strength
of acid sites. The selectivity to HMF increases with the increase in
the ratio of Brönsted to Lewis acid sites with a decrease in the
amount of isolated Lewis acidity. The highest selectivity to HMF
is found to be 55–60% over silylated phosphates.

4
6
V.V. Ordomsky et al. / Journal of Catalysis 300 (2013) 37–46
The selective elimination of Brönsted acidity by Na treatment
[11] H.P. Yan, Y. Yang, D.M. Tong, X. Xiang, C.W. Hu, Catal. Commun. 10 (2009)
558.
12] F. Yang, Q. Liu, X. Bai, Y. Du, Bioresour. Technol. 102 (2011) 3424.
13] F. Yang, Q. Liu, M. Yue, X. Bai, Y. Du, Chem. Commun. 47 (2011) 4469.
1
shows the synergism of nearby Lewis and Brönsted acid sites in
the two-stage process of glucose transformation into HMF. It leads
to a higher selectivity to HMF in comparison with a separate glu-
cose isomerization over Lewis acid sites and fructose dehydration
to HMF over phosphate groups.
[
[
[14] G. Hutchings, I. Hudson, D. Timm, Catal. Lett. 61 (1999) 219.
[
[
[
15] S.M. Patel, U.V. Chudasama, P.A. Ganeshpure, Green Chem. 3 (2001) 143.
16] V.F.D. Alvaro, R.A.W. Johnstone, J. Mol. Catal. A: Chem. 280 (2008) 131.
17] S. Okazaki, A. Kurosaki, Catal. Today 9 (1990) 113.
The addition of organic solvents like MIBK and MTHF leads to a
higher activity of the catalyst and suppression of HMF rehydration
in acids and condensation into humins with stabilization of the
selectivity to HMF on the level of the maximum selectivity during
the aqueous phase dehydration.
[18] M. Xuebing, F. Xiangkai, J. Mol. Catal. A: Chem. 208 (2004) 129.
[
[
19] T. Armaroli, G. Busca, C. Carlini, M. Giuttari, A.R. Galletti, G. Sbrana, J. Mol.
Catal. A: Chem. 151 (2000) 233.
20] D. Prasetyoko, Z. Ramli, S. Endud, H. Nur, Mater. Chem. Phys. 93 (2005) 443.
[21] M.J. Hudson, A.D. Workman, R.J.W. Adams, Solid State Ionics 46 (1991) 159.
[
22] A. Sinhamahapatra, N. Sutradhar, B. Roy, A. Tarafdar, H.C. Bajaj, A.B. Panda,
Appl. Catal. A 385 (2010) 22.
[
[
[
[
23] K. Segawa, Y. Nakajima, J. Catal. 101 (1986) 81.
24] L.C. Jozefowicz, H.G. Karge, E.N. Coker, J. Phys. Chem. 98 (1994) 8053.
25] M.C. Kung, H.H. Kung, Catal. Rev. Sci. Eng. 27 (1985) 425.
26] B.A. Morrow, I.A. Cody, J. Phys. Chem. 80 (1976) 1995.
Acknowledgments
This research is being performed within the framework of the
CatchBio program (053.70.001). The authors gratefully acknowl-
edge the support of the Smart Mix Program of the Netherlands
Ministry of Economic Affairs and the Netherlands Ministry of Edu-
cation, Culture and Science.
[27] H. Miyata, J.B. Moffat, J. Catal. 62 (1980) 357.
[
[
[
[
[
28] S. Zheng, H.R. Heydenrych, A. Jentys, J.A. Lercher, J. Phys. Chem. B 106 (2002)
552.
29] D. Spielbauer, G.A.H. Mekhemer, T. Riemer, M.I. Zaki, H. Knozinger, J. Phys.
Chem. B 101 (1997) 4681.
9
30] G.A.M. Hussein, N. Sheppard, M.I. Zaki, R.B. Fahim, J. Chem. Soc. Faraday Trans.
8
5 (1989) 1723.
31] S. Li, A. Zheng, Y. Su, H. Zhang, L. Chen, J. Yang, C. Ye, F. Deng, JACS 129 (2007)
1161.
1
References
32] D.A. Nelson, P.M. Molton, J.A. Russell, R.T. Hallon, Ind. Eng. Chem. Prod. Res.
Dev. 23 (1984) 471.
[
[
[
[
[
[
[
[
[
1] X. Tong, Y. Ma, Y. Li, Appl. Catal. A 385 (2010) 1.
[
[
33] B.F.M. Kuster, Carbohydr. Res. 54 (1977) 177.
34] V.V. Ordomsky, J. van der Schaaf, J.C. Schouten, T.A. Nijhuis, ChemSusChem 5
2] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411.
3] B.F.M. Kuster, L.M. Tebbens, Carbohydr. Res. 54 (1977) 159.
4] Y. Nakamura, S. Morikawa, Bull. Chem. Soc. Jpn. 53 (1980) 3705.
5] Y. Román-Leshkov, J.N. Chheda, J.A. Dumesic, Science 312 (2006) 1933.
6] C. Lansalot-Matras, C. Moreau, Catal. Commun. 4 (2003) 517.
7] L. Hu, Y. Sun, L. Lin, Ind. Eng. Chem. Res. 51 (2012) 1099.
8] Z. Yuan, C. Xu, S. Cheng, M. Leitch, Carbohydr. Res. 346 (2011) 2019.
9] M. Ohara, A. Takagaki, S. Nishimura, K. Ebitani, Appl. Catal. A 383 (2010) 149.
(
2012) 1812.
[
[
[
[
[
[
35] V.L. Sushkevich, V.V. Ordomsky, I.I. Ivanova, Appl. Catal. A 441–442 (2012) 21.
36] M. Moliner, Y. Roman-Leshkov, M.E. Davis, PNAS 107 (2010) 6164.
37] Y. Zhang, E.A. Pidko, E.J.M. Hensen, Chem. Eur. J. 17 (2011) 5281.
38] K. Tanabe, Mater. Chem. Phys. 17 (1987) 217.
39] M. Moliner, Yu. Román-Leshkov, M.E. Davis, PANS 107 (2010) 6164.
40] V.V. Ordomsky, J. van der Schaaf, J.C. Schouten, T.A. Nijhuis, J. Catal. 287 (2012)
[
10] M. Watanabe, Y. Aizawa, T. Iida, R. Nishimura, H. Inomata, Appl. Catal. A 195
2005) 195.
68.
(