Mesoporous tantalum phosphates: Preparation, acidity and catalytic performance for xylose dehydration to produce furfural

Article abstract of DOI:10.1039/c6ra07830c

Mesoporous tantalum phosphates (TaOPO₄-m) with varying P/Ta molar ratios (m = 0.41-0.89) were prepared, comprehensively characterized by ICP-AES, N₂ physisorption, small-angle XRD, TEM, Raman, FT-IR, NH₃-TPD and IR of pyridine adsorption and employed to catalyze the dehydration of xylose to produce furfural in a biphasic batch reactor. The physicochemical properties of these TaOPO₄-m samples were affected significantly by variation of m. More ordered mesopores were formed in the sample with a higher m. On the other hand, the density of acidity decreased but the ratio of Br?nsted acidity to Lewis acidity (B/L) increased with the increase in m. TaOPO₄-0.84, which showed adequate mesoporosity and a high B/L ratio, was identified as the best performing catalyst among these TaOPO₄-m catalysts in terms of high furfural selectivity (ca. 72 mol%). Correlating the catalyst performance with its acid property showed that the xylose consumption rate decreased with the increasing B/L ratio, while furfural selectivity showed a volcano-type dependence on the B/L ratio. Besides, the huge decrease in the furfural selectivity after poisoning the Br?nsted acid sites by adding 2,6-dimethyl pyridine revealed a kind of Br?nsted acid catalysis for selective furfural production.

Full text of DOI:10.1039/c6ra07830c

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PAPER
Mesoporous tantalum phosphates: preparation,
acidity and catalytic performance for xylose
dehydration to produce furfural†
Cite this: RSC Adv., 2016, 6, 59081
*
Yanran Xing, Bo Yan, Zifei Yuan and Keqiang Sun
Mesoporous tantalum phosphates (TaOPO4-m) with varying P/Ta molar ratios (m ¼ 0.41–0.89) were
prepared, comprehensively characterized by ICP-AES, N2 physisorption, small-angle XRD, TEM, Raman,
FT-IR, NH3-TPD and IR of pyridine adsorption and employed to catalyze the dehydration of xylose to
produce furfural in a biphasic batch reactor. The physicochemical properties of these TaOPO4-m
samples were affected significantly by variation of m. More ordered mesopores were formed in the
sample with a higher m. On the other hand, the density of acidity decreased but the ratio of Brønsted
acidity to Lewis acidity (B/L) increased with the increase in m. TaOPO4-0.84, which showed adequate
mesoporosity and a high B/L ratio, was identified as the best performing catalyst among these TaOPO4-
m catalysts in terms of high furfural selectivity (ca. 72 mol%). Correlating the catalyst performance with
its acid property showed that the xylose consumption rate decreased with the increasing B/L ratio, while
furfural selectivity showed a volcano-type dependence on the B/L ratio. Besides, the huge decrease in
the furfural selectivity after poisoning the Brønsted acid sites by adding 2,6-dimethyl pyridine revealed
a kind of Brønsted acid catalysis for selective furfural production.
Received 25th March 2016
Accepted 8th June 2016
DOI: 10.1039/c6ra07830c
www.rsc.org/advances
because they are more environment-friendly and easier to
separate. Various solid acid catalysts have been used to catalyze
the dehydration of xylose to furfural, including zeolites,8–11 ion-
1. Introduction
The selective transformation of biomass-derivatives to value-
added chemicals has attracted more and more attention
during the last few decades, because biomass is one of the most
promising alternatives to fossil fuels for the production of
chemicals in terms of its huge reserves and being carbon
neutral.1,2 Carbohydrates, which account for 75% of the annual
production of biomass, contain two types of sugars: hexoses
and pentose.3 Xylose is the most common pentose derived from
xylan hemicelluloses and the second most abundant sugar.4
Catalytic dehydration of xylose to furfural could be an impor-
tant route in the utilization of xylose, as furfural is an important
raw material in production of various value-added chemicals,
such as solvents, pharmaceuticals and synthetic resins.5
The commercial production of furfural involves dehydration
of xylose in the presence of mineral acids such as sulfuric acid,
acetic acid.6,7 These mineral acids are awed by their corro-
siveness and toxicity. On the other hand, the separation of
homogeneous catalysts from products is energetically intensive
and economically unfeasible. Alternatively, solid acid catalyst
has emerged as an ideal catalyst for the production of furfural,
exchange resins,12,13 bulk heteropolyacids,14 metal oxides,15,16
exfoliated titanate, niobate and titanoniobate nanosheets.17
Furfural formation is always accompanied by the formation of
humins, as depicted in Scheme 1. Choudhary et al. studied the
conversion of xylose using HCl as a Brønsted acid catalyst and
CrCl3 as Lewis acid catalyst.18 It is shown that xylose could
undergo a triple dehydration reaction to furfural directly cata-
lyzed by Brønsted acid sites (I), and that it could also be iso-
merized to xylulose catalyzed by Lewis acid sites (II), and then
dehydrate to furfural rapidly catalyzed by Brønsted acid sites
(III). Using the combination of Lewis and Brønsted acids,
a much higher furfural yield can be obtained than using HCl
alone. Furfural can further polymerize to humins catalyzed by
Lewis and Brønsted acid sites (IV) and react with xylose or
xylulose to form humins (V and VI) catalyzed by Lewis acids.19 It
seems generally understood that Brønsted acid catalysts are
more selective for furfural production than Lewis acid
catalysts.19
Besides the acidic property of catalyst, the solution of the
reaction also plays an important role in furfural selectivity. A
much higher furfural selectivity can always be obtained in
a biphasic system (water phase + organic phase), because the
formed furfural can be extracted from the aqueous phase by an
organic phase to minimize the condensation between xylose
and furfural.20,21 Various organic solvents were applied, such as
Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular
Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, China.
E-mail: kqsun@mail.tsinghua.edu.cn; Fax: +86-10-62771149; Tel: +86-10-62772592
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07830c
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Scheme 1 Liquid-phase dehydration of xylose and its competing reactions.18,19
DMSO, 1-butanol, toluene, and methyl isobutyl ketone,8,11,13–15,17 added to the solution under continuous stirring until a homo-
among which toluene is found to be the best solvent for furfural geneous solution was obtained. The solution was further mixed
production.11,13–15,17
with 14 mL (NH4)2HPO4 (98+%, Beijing Modern Eastern Fine
As water is always used as solvent in this reaction, it is crucial Chemical Co., Ltd) solution containing the desired amount of P/
to develop water-tolerant heterogeneous catalysts. The group VB Ta ratios (m). 15 mL of 0.256 M hexadecyltrimethylammonium
(i.e. V, Nb and Ta) pentoxides and phosphates are very attractive bromide (CTAB, 99+%, J&K Scientic Co., Ltd) solution in
due to their strongly acidic and highly water-resistant prop- deionized water was added dropwise (0.5 mL minꢁ1) to the
erty.15,22–24 However, the catalytic activity of these group VB mixture under vigorous stirring at 35 ꢀC and stirred for another
catalysts is limited by their relatively low specic surface area. 60 min. The nal mixture was transferred to a 25 mL Teon
ꢀ
As is well-known, synthesis of mesoporous is one of the most lined autoclave and annealed at 130 C for 24 h. The obtained
important way to increase the surface area and improve the precipitates were washed with deionized water for 3 times and
diffusion for reactants and products. Thus, various mesoporous then anhydrous ethanol for 3 times. The resulted precipitates
niobium phosphate and mesoporous tantalum phosphate were were dried at 60 ꢀC for 12 h and then calcined at 550 ꢀC in
prepared and used as catalysts for many reactions.25–29 Meso- owing air for 6 h to obtain TaOPO4-m samples (the molar P/Ta
porous tantalum phosphate is known as a strongly acidic solid ratios in the calcined samples were m ¼ 0.41, 0.61, 0.75, 0.84,
catalyst with high surface area and large pore size, but its and 0.89, as determined by inductively coupled plasma-atomic
potential for the liquid-phase dehydration of xylose has been emission spectroscopy (ICP-AES)).
rarely explored.
Hydrated tantalum oxide samples were prepared by precip-
In this paper, a series of mesoporous tantalum phosphate itation of tantalum penta-ethoxide (Ta(OC2H5)5, 99.9+%, Ning-
with varying molar P/Ta ratios (m) were prepared to regulate xia Orient Tantalum Industry Co., Ltd) in ethanol solution with
surface Lewis and Brønsted acidity and used to catalyze the ammonium hydroxide solution (NH3$H2O, 7 M, Beijing Modern
dehydration of xylose in a biphasic batch reactor. Discussion of Eastern Fine Chemical Co., Ltd) at room temperature. 15 g
the correlation between the ratio of Brønsted acidity to Lewis tantalum penta-ethoxide was rst dissolved in 120 mL anhy-
acidity (B/L) and xylose consumption as well as furfural selec- drous ethanol, then 90 mL ammonium hydroxide solution (7 M
tivity is also presented.
NH3$H2O) was added dropwise (1 mL minꢁ1) to the solution
under vigorous stirring. The resulted suspension was reuxed at
80 ꢀC under vigorous stirring for 2 h and then aged for 3 h. The
precipitate was then ltered, and washed with deionized water
until conductivity of the ltrate was less than 10 mS cmꢁ1
(detected by DDS-307 W conductivity meter). The washed
precipitate was dried at 110 ꢀC overnight and calcined at
different temperature in owing air for 5 h. The resulted
samples were denoted as Ta2O5-T, where T is the calcination
temperature (T ¼ 300, 500 ꢀC).
2. Experimental
2.1 Sample preparation
Mesoporous tantalum phosphate samples were prepared by
hydrothermal method described in references.25,26 In a typical
synthesis procedure, 0.012 mol tantalum penta-ethoxide
(Ta(OC2H5)5, 99.9+%, Ningxia Orient Tantalum Industry Co.,
Ltd) and 0.036 mol tartaric acid (C4H6O6, 99.5%, J&K Scientic
Co., Ltd) were dissolved in 50 mL anhydrous ethanol to produce
tantalum tartrate complex solution as the tantalum precursor.
2.2 Sample characterizations
Excess ethanol was carefully removed by heating the solution at The actual P/Ta ratios (m) for TaOPO4-m samples were deter-
80 ꢀC under vigorous stirring. Then 38 mL deionized water was mined by ICP-AES analysis on a Perkin Elmer Optima 3300 RL
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instrument. The samples were dissolved by hydrouoric acid the reactor in an ice-water bath. The selective poisoning
(HF, 48–51%, J&K Scientic Co., Ltd) for the ICP-AES tests. experiments were performed by adding different amount of 2,6-
Transmission electron microscopy (TEM) images of TaOPO4-m dimethyl pyridine (99%, J&K Scientic Co., Ltd) when loading
samples were recorded on a JEOL JEM-2010 system operated at the reactor.
120 kV. The small-angle XRD patterns for TaOPO4-m samples
The aqueous phase and organic phase products were
were recorded on a D-Max-2200PC diffractometer at a scanning collected by ltration and phase separation. The separated
rate of 0.5ꢀ minꢁ1 in the range of 0.6–10ꢀ using Cu Ka radiation liquid products were analyzed by a LC-10 HPLC (Jiangshen,
(l ¼ 0.15418 nm) as the light source at 40 kV and 30 mA. The Dalian, China) equipped with an ultraviolet (280 nm) detector
surface area, pore volume, and average pore diameter data were (UVD) as well as a refractive index detector (RID). Furfural in
obtained from nitrogen adsorption–desorption isotherms at 77 both organic phase and aqueous phase were quantied by using
K on a Micromeritics TriStar II 3020 instrument. Before the a SB-C18 column (Agilent Zorbax) at 40 ꢀC with a 70 vol%
isotherm measurements, the samples were degassed under methanol/water (ow rate 0.7 mL minꢁ1) eluent. For analysis of
vacuum at 90 ꢀC for 2 h and then 200 ꢀC for another 2 h. Raman xylose in the aqueous phase, a P-OA 2000-0 column (Benson
ꢀ
spectra were recorded at room temperature on a Jobin-Yvon Polymeric Inc., USA) operated at 40 C was used with 5 mM
T64000 triple-stage spectrometer at a resolution of 2 cmꢁ1 H2SO4 solution (ow rate 0.7 mL minꢁ1) as eluent.
with a 325 nm He–Cd laser at output power of 25 mW. Fourier
The xylose conversion, product selectivity and yield were
transform infrared (FT-IR) spectra were recorded at room dened as follows:
temperature on a Perkin-Elmer FT-IR System 2000 spectropho-
moles of xylose consumed
moles of xylose in the feed
tometer at a resolution of 4 cmꢁ1 in transmission mode using
self-supported wafers of the catalyst samples.
Xylose conversion ð%Þ ¼
ꢂ 100;
Temperature programmed desorption of NH3 and IR of
pyridine adsorption were measured to evaluate the acidic
properties of the catalyst surfaces. NH3-TPD proles were
recorded on Cat-Lab (BEL JAPAN, INC.) apparatus equipped
with an on-line QIC-200 quadrupole mass spectrometer
(Inprocess Instruments, GAM 200) as detector, as detailed
previously.30 In order to prevent a possible intervention of H2O
desorption, the signal of m/z ¼ 15 was recorded to measure the
NH3 desorption proles, rather than m/z ¼ 16 or 17. Relative
populations of Lewis and Brønsted acid sites on the catalyst
surfaces were measured by IR of adsorbed pyridine carried out
on the Perkin-Elmer FT-IR System 2000 spectrophotometer in
transmission mode. A quartz-lined stainless steel IR cell with
optical path length less than 5 mm was used for the measure-
ments. The self-supported sample wafers (15 mg cmꢁ2) were
pretreated in a owing of 20 vol% O2/Ar (40 mL minꢁ1) at 300 ꢀC
for 2 h. Adsorption of pyridine was done at 100 ꢀC, by switching
the sample to a pyridine-containing Ar (10 000 ppm pyridine, 40
mL minꢁ1) for 30 min. The samples were purged in owing Ar
(40 mL minꢁ1) at 100 ꢀC for 1 h before recording the IR spectra
to remove the reversibly adsorbed pyridine. Each spectrum was
Product selectivity ðmol%Þ ¼
moles of carbon atoms in the product defined
ꢂ 100;
moles of carbon atoms in xylose consumed
moles of furfural produced
Furfural yield ðmol%Þ ¼
ꢂ 100
moles of xylose in the feed
The catalytic activity was expressed as area-specic xylose rxn
rate according to the consumption of xylose, which were ob-
tained using the following equation:
Area-specific xylose rxn rate ¼
moles of xylose consumed per h ðmmol hꢁ1
surface area of the catalyst ðm2Þ
Þ
3. Results and discussion
3.1 Physicochemical properties
recorded for 20 scans at a resolution of 4 cmꢁ1
.
The P/Ta molar ratios (m) in TaOPO4-m samples were deter-
mined by ICP-AES and the results are summarized in Table 1.
The P/Ta molar ratios (m) of the samples increased with
2.3 Catalytic reaction and product analysis
Biphasic batch reactions of xylose dehydration were conducted increasing the starting ((NH4)2HPO4/Ta(OC2H5)5) ratio in the
in a 25 mL stainless steel autoclave at 140 ꢀC with magnetic preparation solution. However, the P/Ta ratio determined by
stirring. In a typical reaction, 300 mg xylose, 100 mg catalyst, 6 ICP-AES was lower than the theoretical P/Ta ratio in the starting
mL deionized water and 14 mL toluene were loaded into the solution. Similar deviation was also observed in the literature.29
reactor at room temperature. Aer purging with nitrogen for 6 The loss of P should be attributed to equilibrium during ligand
times, the reactor was pressurized to 0.8 MPa with nitrogen. The exchange between tantalum tartrate complex and (NH4)2HPO4.
magnetic stirrer (900 rpm) was switched on when the reaction The nearly invariant m for samples with theoretical P/Ta ratio in
temperature (140 ꢀC) was reached. The reaction was performed starting solution higher than 2.5 suggested a saturation in the
within kinetic controlling regime as no diffusion limitation was ligand exchange reaction.
observed in our preliminary experiments conducted at different
N2 adsorption–desorption isotherms of the TaOPO4-m
stirring speeds (600–1100 rpm). Termination of the reaction was samples are shown in Fig. 1. For TaOPO4-0.41 and TaOPO4-0.61
done by switching off the stirrer and immediately quenching samples, their isotherms resembled with type II isotherms
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Table 1 Composition, texture properties, and acidity of the catalyst samples
Area-specic acidity
m
PDd (nm)
(mmol mꢁ2
)
Sample
Theoreticala
ICP
SAb (m2 gꢁ1
)
PVc (cm3 gꢁ1
)
Adsorption
Desorption
Totale
BASf
LASg
B/Lh
TaOPO4-0.41
TaOPO4-0.61
TaOPO4-0.75
TaOPO4-0.84
TaOPO4-0.89
Ta2O5-500
0.5
1.0
1.5
2.5
3.5
—
0.41
0.61
0.75
0.84
0.89
—
110
156
218
275
298
98
0.10
0.13
0.17
0.22
0.18
0.12
0.19
4.52
3.96
3.14
2.96
2.56
4.12
4.06
4.83
4.37
3.65
3.25
2.60
4.36
4.27
3.036
2.769
2.197
2.138
1.846
1.710
2.231
0.509
0.756
0.748
1.055
0.970
—
2.527
2.013
1.450
1.084
0.876
1.710
2.231
0.20
0.38
0.51
0.98
1.11
—
Ta2O5-300
—
—
157
—
—
a
The starting (NH4)2HPO4/Ta(OC2H5)5 ratio in the preparation solution. b BET surface area. c Pore volume measured at P/P0 ¼ 0.991. d Average pore
diameter measured from the adsorption and desorption branch of isotherms according to BJH method. e Measured by NH3-TPD. f Calculated from
the bands at 1540 cmꢁ1
. . Measured by pyridine-IR and calculated using equations proposed by Emeis,
Calculated from the bands at 1445 cmꢁ1
g
h
assuming the integrated molar extinction coefficients are 2.22 and 1.67 cm mmolꢁ1 for Lewis and Brønsted acid sites, respectively.31
Textural properties derived from the N2 physisorption
isotherms are also summarized in Table 1. Increasing m resul-
ted in continuous increase in the specic surface area from 110
m2 gꢁ1 for TaOPO4-0.41 to 298 m2 gꢁ1 for TaOPO4-0.89. The
average pore diameter decreased from 4.83 nm to 2.60 nm at the
same time, which could be due to the increasing number of
pore walls with increasing phosphate content. As more ordered
mesopores formed, the pore volume increased from 0.10 cm3
gꢁ1 to 0.20 ꢃ 0.02 cm3 gꢁ1 with m increasing from 0.41 to 0.89.
The effect of P/Ta molar ratios (m) on the mesoporosity of the
TaOPO4-m samples was investigated using small-angle XRD, as
shown in Fig. 2. TaOPO4-0.41 and TaOPO4-0.61 samples
appeared to be disordered materials, with no distinct X-ray
diffraction peak in the small-angle region detected (0.6–10ꢀ).
For the TaOPO4-m samples of m $ 0.75, a broad peak centered
Fig.
1 N2 adsorption–desorption isotherms of the TaOPO4-m
samples with different molar P/Ta ratio m.
dened by IUPAC with no distinct hysteresis loops. For samples
with m ¼ 0.75, 0.84 and 0.89, the isotherms were typical type IV
sorption isotherms with hysteresis loops in the range of 0.4 < P/
P0 < 1.0, which were characteristic for mesoporous materials.
TaOPO4-0.75 and TaOPO4-0.84 samples displayed type H4
hysteresis loops at relative nitrogen pressure (P/P0) below 0.80
and type H3 hysteresis loops at higher pressures. The Type H4
loop involves in narrow slit-like pores while the type H3 loop
characterizes slit-shaped pores formed from aggregates of plate-
like particles.32 TaOPO4-0.89 sample had a type H2 hysteresis
loop, which was associated with complex interconnected
networks of pores with narrow necks and wide bodies (i.e., ink
bottle pores).32 The change of hysteresis loop type could be due
to the fast precipitation of tantalum phosphate. The pH of the
starting solution was too high during the preparation of
TaOPO4-0.89 as (NH4)2HPO4 was an alkaline compound. Higher
pH resulted in the fast precipitation of tantalum phosphate
without sufficient interaction with surfactant.29
Fig. 2 Small-angle XRD patterns for the TaOPO4-m samples of (a) m
¼ 0.41, (b) m ¼ 0.61, (c) m ¼ 0.75, (d) m ¼ 0.84, (e) m ¼ 0.89.
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Fig. 3 TEM images of the TaOPO4-m samples of (a) m ¼ 0.41, (b) m ¼ 0.61, (c) m ¼ 0.75, (d) m ¼ 0.84, (e) m ¼ 0.89.
at 2q ¼ 2.4ꢀ appeared, indicating a worm-like pore structure.33 phosphate contained in the framework of TaOPO4 samples
The average inter-pore distance in TaOPO4-0.75 sample was made mesoporous structure more stable.
calculated to be 3.68 nm according to the position of the peak.
Surface acidity of solid acid catalyst is the key to the catalytic
The shi of diffraction peak to lower angle with increasing the performance for dehydration reactions. The Brønsted acid site
P/Ta molar ratio (m) suggested a larger inter-pore distance at of TaOPO4-m is due to the presence of P–OH, which was
higher P/Ta ratio. No peak appeared at higher 2q angles, sug- investigated by Raman and FT-IR. In order to compare the
gesting that all of these samples showed no long-range order. relative intensity of P–O bond among these TaOPO4-m samples,
These observations were further supported by TEM results as the Raman and FT-IR spectra was normalized by the height of
shown in Fig. 3. It can be seen that the worm-like pores were not Ta–O vibration peak at 660 and 655 cmꢁ1, respectively.
obvious for TaOPO4-0.41 and TaOPO4-0.61 samples, while they
Raman spectra of TaOPO4-m samples are shown in Fig. 4A.
were much more apparent for the TaOPO4-m samples of m $ As a comparison, the spectrum of Ta2O5-300 is also presented.
0.75, which was consistent with N2 adsorption–desorption For the Ta2O5-300 sample, a strong Raman band appeared at
isotherms and small-angle XRD patterns. Thus, more 660 cmꢁ1, which was characteristic for Ta–O stretching
Fig. 4 (A) Raman spectra and (B) FT-IR patterns of (a) Ta2O5-300, the TaOPO4-m samples of (b) m ¼ 0.41, (c) m ¼ 0.61, (d) m ¼ 0.75, (e) m ¼ 0.84,
(f) m ¼ 0.89.
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vibration mode.26 Similar features were also observed in the
In order to understand the effect of m on the type and
TaOPO4-m samples. Besides, a distinct band centered at 1013 number of acidic sites on the TaOPO4-m samples, NH3-TPD and
cmꢁ1, which was attributed to the P–O asymmetric stretching IR spectra of pyridine adsorption were measured, respectively.
vibrations of phosphate species, can be found for all TaOPO4-m NH3-TPD proles of TaOPO4-m and Ta2O5-T samples are shown
samples, suggesting that Ta–O–P bond was formed in meso- in Fig. 5 and the corresponding quantitative results are given in
porous TaOPO4 samples.26 The band observed between 200 and Table 1. NH3-TPD proles for TaOPO4-m samples showed
400 cmꢁ1 can be assigned to O–Ta–O bending modes for the a broad peak (130–500 C) with maximum at 180–350 C. The
Ta2O5-300 sample and to various O–P–O and O–Ta–O bending proles could be divided into two desorption peaks (peak at 220
modes for TaOPO4-m samples.34 Furthermore, with increasing and 320 ꢀC), which was attributed to the weak acid sites and
m, the area ratio of the peak assigned to P–O to the peak strong acid sites.37 The high-temperature peak became more
ꢀ
ꢀ
assigned to Ta–O increased.
distinct and lager amount of desorbed ammonia was detected
The FT-IR spectra for TaOPO4-m and Ta2O5-300 samples are with increasing m, which could be explained by more P–OH
presented in Fig. 4B. The broad and intense band at 655 cmꢁ1 in groups introduced with more phosphate content. However, for
both Ta2O5-300 and TaOPO4-m samples could be assigned to the sample TaOPO4-0.89, the desorption peak remained almost
Ta–O stretching vibration mode.35 The 1050 cmꢁ1 band only unchanged compared with sample TaOPO4-0.84, which was
present in TaOPO4-m samples corresponded to the P–O asym- consistent with the almost unchanged phosphate content. The
metric vibration stretching mode of the phosphate groups.36 proles for Ta2O5-T (T ¼ 300 ꢀC, 500 ꢀC) samples were different
The shoulder at about 1200 cmꢁ1 was also assigned to dP–OH of from that for mesoporous TaOPO4-m samples. The samples of T
uncoordinated phosphate groups.26 The band at 1620 cmꢁ1 ¼ 300 ꢀC produced two desorption peaks, featuring medium
assigned to H–O–H bending mode was associated to molecu- strong acid sites and strong acid sites, while high-temperature
larly adsorbed water. As similar to the Raman data, intensity of peak became almost invisible and the low-temperature peak
peak (1050 cmꢁ1) associated with P–O bond increased with reduced to a lower one on the proles for Ta2O5-500.37 The area
increasing m.
specic acid density was obtained via NH3-TPD proles of
TaOPO4-m and Ta2O5-T. The density of acid sites decreased
from 2.231 mmol mꢁ2 to 1.710 mmol mꢁ2, when the T of Ta2O5-T
increased from 300 ꢀC to 500 ꢀC. The area specic acidity of
TaOPO4-m, normalized to the surface area, decreased from
3.036 mmol mꢁ2 to 2.769, 2.197, 2.138, and 1.846 mmol mꢁ2
,
when the m increased from 0.41 to 0.61, 0.75, 0.84, and 0.89,
respectively.
Pyridine-IR spectra are shown in Fig. 6. All the spectra
showed the bands centered at 1445 and 1610 cmꢁ1, which
featured adsorbed pyridine on Lewis acid sites. These Lewis
acid sites could be due to the existence of octahedral TaO6 and
tetrahedral TaO4.38 For TaOPO4-m samples, the new bands at
1540 and 1638 cmꢁ1 are the characteristic bands of Brønsted
acid sites due to the presence of OH groups associated with P on
the surface.39 While Ta2O5-T samples showed no detectable
band at ca. 1540 cmꢁ1, indicating that Ta2O5-T samples
possessed almost no Brønsted acidity. This result was consis-
tent with our previous report on the acidity of hydrated
Fig. 5 NH3-TPD profiles for the Ta2O5-T samples of (a) Ta2O5-500, (b)
Ta2O5-300 and the TaOPO4-m samples of (c) m ¼ 0.41, (d) m ¼ 0.61,
(e) m ¼ 0.75, (f) m ¼ 0.84, (g) m ¼ 0.89.
Fig. 6 Pyridine-IR spectra (A) for the TaOPO4-m samples of (a) m ¼ 0.41, (b) m ¼ 0.61, (c) m ¼ 0.75, (d) m ¼ 0.84, (e) m ¼ 0.89; pyridine-IR
spectra (B) for the tantalum compound samples of (a) TaOPO4-0.84, (b) Ta2O5-500, (c) Ta2O5-300.
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tantalum oxide though TaCl5 was used as the source of Ta.30 0.89. Thus, TaOPO4-0.84 was identied as the best performing
Meanwhile the peak area ratio of 1540 cmꢁ1 to 1450 cmꢁ1 catalyst for furfural production, which gave the highest furfural
increased with increasing m, illustrating that the higher phos- selectivity (ca.72 mol%) and yield (ca. 35 mol%). The reaction
phate content, the more Brønsted acid sites were formed. rate is competitive compared to other catalyst systems in the
Quantication of Lewis and Brønsted acid sites was obtained reference (Table S1†) and the selectivity of furfural over the
according to the integrated areas of the two peaks at 1540 and TaOPO4-0.84 catalyst is higher compared with those obtained
1450 cmꢁ1, respectively, calculated through the basis of equa- over the tantalum-based and niobium-based catalyst under the
tions proposed by Emeis, given in Table 1.31
similar reaction conditions.15,19,24
The catalytic performance of TaOPO4-0.84 catalysts was also
compared with that of Ta2O5-300 and Ta2O5-500, as shown in
Fig. 8. Ta2O5-300 and Ta2O5-500 exhibited a higher xylose
conversion than TaOPO4-0.84 in the initial 2 h. However, it
should be noted that when the duration lasted to 240 min,
TaOPO4-0.84 showed the highest xylose conversion, indicating
higher durability of TaOPO4-0.84 compared to Ta2O5-T. Ta2O5-
300 and Ta2O5-500 produced much lower furfural selectivity
(<55 mol%) at the whole reaction (4 h). Thus, hydrated tantalum
oxides are poorer than mesoporous tantalum phosphates for
the dehydration of xylose.
3.2 Catalytic performance
The xylose dehydration reaction was conducted in batch oper-
ation, using a water–toluene solvent mixture at 140 ꢀC. Xylose is
dissolved and the catalyst is suspended in aqueous phase, and
the formed furfural can be extracted from water phase to
toluene phase. Fig. 7 shows xylose conversion and furfural
selectivity as a function of reaction time for these TaOPO4-m
catalysts. Xylose conversion increased almost lineally at the
initial 60 min, on the other hand, the furfural selectivity
increased at the initial 120 min, then decreased with further
reaction time. As shown in Scheme 1, the formed furfural could
be further converted to humins, thus the furfural selectivity
3.3 The stability and regenerability of catalysts
decreased in the last two hour due to the secondary reactions.19 In order to evaluate the stability of the spent TaOPO4-0.84
The catalytic reaction data obtained at 2 h are given in Table catalyst, N2 physisorption, small-angle XRD, TEM, and NH3-
2 to show the catalytic activity and product selectivity over these TPD of the spent TaOPO4-0.84 sample were performed. N2
mesoporous TaOPO4-m catalysts. The main product was adsorption–desorption isotherms of the spent TaOPO4-0.84
furfural (60–72 mol%), which could be accurately quantied by sample was shown in Fig. S1.† For the spent TaOPO4-0.84
HPLC. However, lots of solid products called humins were also sample, the hysteresis loop was not as obvious as the fresh one.
produced, which was difficult to quantify and denoted as others The small-angle XRD pattern of the spent catalyst was shown in
in the Table 2. Humins were highly polymerized insoluble Fig. S2† and no distinct X-ray diffraction peak in the small-angle
carbonaceous species formed by the reaction of furfural with region was detected. The TEM result was shown in Fig. S3.† It
itself or between furfural and xylose.19 The consumption rate of can be seen that the worm-like pores were not obvious for the
xylose at the reaction time of 20 min decreased from 0.164 spent catalyst and the carbon deposition was observed in the
mmol hꢁ1 mꢁ2 to 0.083 mmol hꢁ1 mꢁ2 with the increase in m image. NH3-TPD prole was shown in Fig. S4.† Less desorbed
from 0.41 to 0.89, while the furfural selectivity increased from ammonia was detected in spent catalyst compared with the
ca. 60 mol% to 72 mol% when m increased from 0.49 to 0.84, fresh one, which indicated the decreasing of surface acidity.
but decreased to ca. 60 mol% when m was further increased to These observations can indicate that solid humins deposited on
Fig. 7 Performance of the TaOPO4-m catalysts with different molar P/Ta ratio m by the time courses of (A) xylose conversion, (B) furfural
selectivity: m ¼ 0.41 (-), m ¼ 0.61 (B), m ¼ 0.75 (:), m ¼ 0.84 (A), and m ¼ 0.89 (P).
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Table 2 Effect of molar P/Ta ratio m on xylose conversion and product distribution of catalytic xylose dehydration over theTaOPO4-m catalystsa
Product selectivity (mol%)
Catalyst
Xylose conv. (%)
Rxn rateb (mmol hꢁ1 mꢁ2
)
Furfural
Others
Furfural yield (%)
TaOPO4-0.41
TaOPO4-0.61
TaOPO4-0.75
TaOPO4-0.84
TaOPO4-0.89
33.3
40.9
50.4
49.0
49.8
0.164
0.154
0.150
0.122
0.083
59.6
63.3
65.7
72.1
60.4
40.4
36.7
34.3
28.0
39.6
19.9
25.9
33.1
35.1
30.1
a
Rxn conditions: 100 mg catalyst; 300 mg xylose; 140 ꢀC; rxn solvent, 6 mL of water and 14 mL of toluene; rxn time, 2 h. b Rxn rate aer reaction for
20 min.
Fig. 8 Performance of the Ta2O5-T and TaOPO4-m catalysts by the time courses of xylose conversion (A), furfural selectivity (B): Ta2O5-300 (,),
Ta2O5-500 (C), TaOPO4-0.84 (:).
the catalyst surface and blocked the mesoporosity of the cata-
lyst. Leaching of tantalum phosphate from the solid catalyst to
the aqueous phase was measured by ICP-AES. No tantalum was
detected and the concentration of phosphate in solution was
2.507 mg mLꢁ1, which represented only 0.17 wt% of the phos-
phate initially present in the solid catalyst. This demonstrated
that no leaching process existed in the whole reaction. The
reusability of the TaOPO4-0.84 catalyst was tested with the
catalyst separated by ltration from the aqueous phase. The
spent catalyst was thermally treated in air at 550 ꢀC for 4 h
before reuse. The results in Fig. 9 indicate that the thermal
treatment regenerated the catalyst performance as the xylose
conversion and furfural yield did not decreased a lot. The
mesoporous tantalum phosphate exhibits fairly good stability
Fig. 9 Reuse study for the TaOPO4-0.84 catalyst at 140 ꢀC after 2 h
reaction. Rxn conditions: 100 mg catalyst; 300 mg xylose; rxn solvent,
6 mL of water and 14 mL of toluene; rxn time, 2 h. For the second and
third run, the catalyst is regenerated at 550 ꢀC for 4 h in air.
when used in the xylose dehydration reaction.
3.4 Correlation between the acid property and catalytic
performance of the TaOPO4-m catalysts
It has been know that the catalytic performance of xylose
Lewis and Brønsted acid sites are important for furfural
dehydration is dependent on the catalyst surface acid property.
production.9,18,19 Therefore, we correlated xylose consumption
Lewis acid site can isomerize xylose to xylulose and Brønsted
acid site can dehydrate xylulose to furfural, indicating that both
rate and furfural selectivity of the TaOPO4-m catalysts with the
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Table 3 Effect of poisoning reagent 2,6-DMP on xylose conversion
and product distribution of catalytic xylose dehydration over the
TaOPO4-0.84 catalysta
ratio of Brønsted acidity to Lewis acidity (B/L), as shown in
Fig. 10. It was clear that the consumption rate of xylose
decreased with increasing the B/L ratio, which agreed well with
earlier reports.18,19 Various solid acids were investigated to
catalyze the aqueous-phase dehydration of xylose, which also
showed that increasing the ratio of B/L decreased the catalytic
activity.19 On the other hand, furfural selectivity showed
a volcano-type dependence on the B/L ratio; the highest furfural
selectivity (ca. 72 mol%) was obtained over TaOPO4-0.84 with
a B/L ratio of 0.98. Although the TaOPO4-0.89 catalyst showed
the highest B/L ratio (ca. 1.1), the furfural selectivity was only ca.
60 mol%. As shown in Scheme 1, Lewis acid sites can reduce
furfural selectivity by catalyzing more side reactions to form
humins. Weingarten et al. also found that high furfural selec-
tivity was usually produced over the catalyst with a high B/L
Product selectivity
(mol%)
Ratiob
Xylose conv. (%)
Furfural
Others
Furfural yield (%)
0
49.0
52.2
53.6
54.1
55.6
58.6
72.1
56.2
54.7
50.3
43.9
26.2
27.9
43.8
45.3
49.7
56.1
73.8
35.3
29.3
29.3
27.2
24.4
15.3
2 : 1
4 : 1
10 : 1
15 : 1
50 : 1
a
Rxn conditions: 100 mg catalyst; 300 mg xylose; 140 ꢀC; rxn solvent, 6
mL of water and 14 mL of toluene; rxn time, 2 h. b Refers to the molar
ratio of 2,6-DMP added to the reaction solution to the total acid sites of
ratio; however, HY zeolite showed a low furfural selectivity the catalyst.
due to strong irreversible adsorption of the furfural in the
micro-pores, causing an increase in the rate of humins forma-
tion, although it has a high B/L ratio.19 The N2 adsorption–
distinguish whether Brønsted acid sites are the active sites for
desorption isotherms showed that TaOPO4-0.89 had complex
interconnected networks of pores with narrow necks, small
average pore diameter and pore volume, which could be
responsible for the low selectivity for furfural. According to the
reference, the selectivity of furfural was restricted by furfural
resinication and condensation reactions.6 On the furfural
resinication reaction, the Brønsted acid shows similar catalytic
activity compared with Lewis acid.19 However, on the furfural
condensation reaction, xylulose isomerized by Lewis acid site is
more reactive and can be converted to polymeric side-products
faster than xylose, which could explain that Lewis acid sites
catalyze more side reactions.40
The results in Fig. 10 reveal that the nature of acid sites or
B/L ratio is the key factor affecting the catalytic activity for the
TaOPO4-m catalysts and furfural selectivity. However, besides
the B/L ratio, the texture properties (surface areas, pore size
and volume) also changed with different P/Ta ratio, which also
played an important role in the catalytic performance. Thus,
we used TaOPO4-0.84 as the only catalyst and regulated its B/L
ratio by using 2,6-dimethyl pyridine to poison the Brønsted
acid sites. The selective and permanent poisoning of protons
with 2,6-dimethyl pyridine has been widely used to
the reaction.41,42 Table 3 presents the catalytic data over the
TaOPO4-0.84 catalyst when various amount of 2,6-dimethyl
pyridine was added to the reaction system. The furfural
selectivity decreased but the xylose conversion and the selec-
tivity for humins increased with increasing the amount of 2,6-
dimethyl pyridine added in the reactor, as more Brønsted acid
sites were poisoned. These results further identify Brønsted
acid sites are more selective for furfural production than Lewis
acid sites.
4. Conclusions
This work provides the rst investigation on the potential of
mesoporous tantalum phosphate (TaOPO4-m) for the liquid-
phase dehydration of xylose to produce furfural. The P/Ta
ratio (m) was found to be the key to the physicochemical
properties and catalytic performance. Higher m resulted in
more mesoporosity and higher ratio of Brønsted acidity to Lewis
acidity (B/L) for the TaOPO4-m samples. TaOPO4-0.84 was
identied as the most selective catalyst for the furfural
production (72 mol%). Correlations between the catalytic
performance and acidic property showed that the consumption
rate of xylose decreased with increasing the B/L ratio, while
higher furfural selectivity was obtained over the catalyst with
a higher B/L ratio. Selective poison of Brønsted acid sites by 2,6-
dimethyl pyridine revealed that the dehydration of xylose to
furfural would be a kind of Brønsted acid-catalyzed reaction. On
the other hand, pores with small pore volume were found to be
adverse to the production of furfural due to a high selectivity for
humins.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant: 21221062) and the National Basic
Research Program of China (Grant: 2013CB933103).
Fig. 10 Effect of molar Brønsted/Lewis ratio on the catalytic perfor-
mance of furfural selectivity.
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22 C. Garcıa-Sancho, M. B. Guemez, P. Maireles-Torres and
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