Catalytic dehydration of d-xylose to 2-furfuraldehyde in the presence of Zr-(W,Al) mixed oxides. Tracing by-products using two-dimensional gas chromatography-time-of-flight mass spectrometry

Article abstract of DOI:10.1016/j.cattod.2012.03.066

Zirconium-tungsten mixed oxides (ZrW) are effective catalysts in the aqueous-phase dehydration of d-xylose to 2-furfuraldehyde (Fur), at 170 °C. The texture and acid properties of ZrW materials were modified to enhance Fur yields. The catalysts prepared by co-condensation without a templating agent (ZrW(X), X = NO₃, Cl; X is related to the type of zirconium precursor) possess relatively low specific surface area and amounts of accessible acid sites, leading to a modest Fur yield of ca. 35% at 99% conversion. The use of a templating agent in the preparation of mesoporous ZrW (ZrW-MP) increased considerably the specific surface area and the amount of accessible acid sites, which resulted in enhanced Fur yields (42%) reached at comparable conversions. Further improvements in Fur yields at high conversions were accomplished by introducing aluminium in the catalyst preparation procedure to give ZrAlW-MP (51% yield at 98% conversion). Fairly good catalytic results were also obtained in the case of the ZrAlW-MP catalyst, using solely water as solvent (46% yield at 93% conversion). A study of the identification of the reaction by-products by two-dimensional gas chromatography (GC × GC) combined with time-of-flight mass spectrometry (ToFMS) was carried out.

Full text of DOI:10.1016/j.cattod.2012.03.066

Catalysis Today 195 (2012) 127–135
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Catalytic dehydration of d-xylose to 2-furfuraldehyde in the presence of
Zr-(W,Al) mixed oxides. Tracing by-products using two-dimensional gas
chromatography-time-of-flight mass spectrometry
a
a
b
a
a
Margarida M. Antunes , Sérgio Lima , Auguste Fernandes , Joana Candeias , Martyn Pillinger ,
c
b
a,∗
Sílvia M. Rocha , Maria Filipa Ribeiro , Anabela A. Valente
Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
Institute for Biotechnology and Bioengineering, Centre for Biological and Chemical Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
Department of Chemistry, QOPNA, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Zirconium-tungsten mixed oxides (ZrW) are effective catalysts in the aqueous-phase dehydration of
d-xylose to 2-furfuraldehyde (Fur), at 170 C. The texture and acid properties of ZrW materials were
modified to enhance Fur yields. The catalysts prepared by co-condensation without a templating agent
Received 9 January 2012
Received in revised form 22 March 2012
Accepted 28 March 2012
Available online 15 May 2012
◦
(ZrW(X), X = NO3, Cl; X is related to the type of zirconium precursor) possess relatively low specific surface
area and amounts of accessible acid sites, leading to a modest Fur yield of ca. 35% at 99% conversion. The
use of a templating agent in the preparation of mesoporous ZrW (ZrW-MP) increased considerably the
specific surface area and the amount of accessible acid sites, which resulted in enhanced Fur yields
Keywords:
Xylose
(42%) reached at comparable conversions. Further improvements in Fur yields at high conversions were
2
-Furfuraldehyde
accomplished by introducing aluminium in the catalyst preparation procedure to give ZrAlW-MP (51%
yield at 98% conversion). Fairly good catalytic results were also obtained in the case of the ZrAlW-MP
catalyst, using solely water as solvent (46% yield at 93% conversion). A study of the identification of the
reaction by-products by two-dimensional gas chromatography (GC × GC) combined with time-of-flight
mass spectrometry (ToFMS) was carried out.
Acid dehydration
Zirconium
Tungsten
By-products identification
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
monosaccharides into Fur and Hmf, respectively: the two reaction
systems are acid-catalysed.
The production of non-petroleum derived energy, fuels and
Fur is produced worldwide on an industrial scale (e.g. China,
chemicals is nowadays a global concern, principally due to oil
depletion. Plant biomass is a natural renewable resource that
through chemical and/or biological transformations can give a
plethora of bio-derived compounds which can become increas-
ingly competitive alternatives to some petrochemicals [1–12].
Several low-cost kinds of plant biomass may be utilised, such
as agricultural and forest waste and surpluses, plants grown
on arid land, and aquatic plants. The major components of
plant-derived biomass are carbohydrates which can be converted
to furanic aldehyde compounds such as 2-furfuraldehyde (Fur)
and 5-(hydroxymethyl)-2-furaldehyde (Hmf), with wide applica-
tion profiles [13,14]. These reactions involve the hydrolysis of
di/polysaccharides into hexoses (e.g. glucose, fructose) and pen-
toses (xylose is the most abundant), and the dehydration of these
South Africa, Dominican Republic), commonly using H SO4 as the
2
catalyst [15,16]. Liquid mineral acids pose risks to human health,
the environment (due to the difficult catalyst recovery/recycling,
formation of sulphur containing by-products, large amounts of neu-
tralisation waste), and equipment (e.g. corrosion hazards). As an
alternative to toxic liquid acids as catalysts for converting car-
bohydrates into furanic aldehydes, porous solid acids seem quite
promising [17,18]; advantages may include the easier separation of
the catalyst from the reaction products (separation processes typi-
cally constitute more than half of the total investment in equipment
[13,19,20]), extended catalyst lifetimes and recycling, the avoid-
ance of downstream neutralisation steps, and a plethora of families
of porous solids with tunable texture/acid properties can be found.
The mechanism for the dehydration of xylose to Fur is complex,
involving a series of elementary steps, and it may be accompanied
by several types of undesirable side reactions such as fragmentation
of xylose and reactions of Fur with other products (e.g. conden-
sation) or with itself (so-called “resinification”) [15,21–32]. The
understanding of the influence of the acid/texture properties of
∗ Corresponding author. Tel.: +351 234 370603; fax: +351 234 401470.
E-mail address: atav@ua.pt (A.A. Valente).
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.03.066

1
28
M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
porous solids on the catalytic reaction is not straightforward partly
due to the complex reaction system (many types of side reactions
may occur, leading to complex mixtures of by-products), and each
step may depend differently on the catalytic properties. Recently,
Weingarten et al., in a comparative study using different types of
solid acids as catalysts in the aqueous-phase dehydration of xylose,
reported that a commercial zirconium–tungsten mixed oxide cat-
alyst (XZO 1251, MEL Chemicals, WO3 content of 15 wt.%), with
relatively high concentration of Lewis acid sites, was more active
using an agate pestle and mortar, and sieved to give particles of
less than 150 ␮m width. A zirconia material (denoted ZrO ) was
2
prepared in a similar fashion but without adding the tungsten pre-
cursor.
Two mesoporous ZrW-MP and ZrWAl-MP materials were
prepared via the incipient wetness impregnation method for
introducing tungsten (and aluminium) on a pre-prepared tem-
plated zirconium hydroxide support (denoted Zr(template)). The
Zr(template) solid was prepared by following the procedure
(
and less selective to Fur) than an amorphous (mainly Brønsted
described by Ciesla et al. [51]. A solution of 70 wt.% Zr(O-nPr) in 1-
4
type) zirconium phosphate catalyst, and less active than (Brönsted)
microporous HY zeolite (Si/Al = 30) in Fur loss reactions (strong
adsorption and subsequent decomposition of Fur on the catalyst’s
microporous surface) [27]. In a different approach, one may focus
on a specific family of catalysts to help establish structure-activity
relationships by modifying the physicochemical properties.
Zirconium–tungsten mixed oxides (ZrW) are fairly stable,
versatile solid acid catalysts (and commercially available) used
in different types of catalytic reactions, including dehydrations
such as those of 2-butanol [33–35], tert-butanol [36], methanol
propanol (11.34 mL, 25.64 mmol) was added slowly with stirring
to a solution of hexadecyltrimethylammonium bromide (5.00 g,
13.72 mmol) in a mixture of water (230 g) and 37 wt.% HCl (37.6 ml,
457.9 mmol). After stirring for 30 min a solid was obtained, to which
was added a solution of (NH ) SO (3.40 g, 25.73 mmol) in water
4
2
4
(46 g). The resultant mixture was stirred for 1 h, and then trans-
◦
ferred to a polypropylene bottle and heated at 100 C for 72 h.
Finally, the suspension was filtered, and the solid washed consecu-
tively with deionised water (200 mL), ethanol (200 mL), deionised
◦
water (200 mL), followed by drying at 100 C overnight to give
[
34,37,38], 1-propanol and 2-propanol [36], 2-propenol [39], and
Zr(template).
of glycerol to acrolein [40–47]. The ZrW-type materials have also
been used to study structure-activity relationships in different
types of acid-catalysed reactions (the texture and acid properties
of ZrW materials may be finely-tuned through different synthetic
approaches/conditions). In the case of the dehydration of xylose
into Fur, it may be advantageous to prepare mesoporous versions
The alumina-doped tungstated mesoporous zirconia (denoted
ZrAlW-MP) was prepared as follows: a solution of ammonium
metatungstate hydrate (0.213 g, 0.0668 mmol) and aluminium
nitrate nonahydrate (0.210 g, 0.560 mmol) in a mixture of water
and ethanol (10 mL, 1:1 v/v) was added drop-wise with stirring
◦
at 100 C to Zr(template) (3.215 g) [49]. The drop-wise addition of
(
denoted MP) of ZrW catalysts with enhanced amounts of effec-
small aliquots of the solution was alternated with partial drying of
◦
tive acid sites. Catalyst preparation approaches in this direction
can involve using a templating agent (for generating mesoporosity)
and doping the catalyst with aluminium (attempting to enhance
acidity) [48,49].
the solid at 120 C for 1 h; after all the solution had been added, the
◦
mixture was dried in an oven at 100 C for 24 h, and the solid was
◦
then calcined at 630 C for 5 h. The tungstated mesoporous zirco-
nia (denoted ZrW-MP) was prepared using the procedure described
above for ZrAlW-MP: a solution of ammonium metatungstate
hydrate (0.150 g, 0.047 mmol) in a mixture of water and ethanol
In the present work, mixed zirconium tungsten oxides pre-
pared by co-condensation (ZrW(X), X = NO , Cl), or impregnation
3
◦
(
with or without aluminium) on mesophases of zirconia (ZrW-
(10 mL, 1:1 v/v) was added drop-wise with stirring at 100 C to
MP, ZrAlW-MP), were investigated as solid acid catalysts in the
aqueous phase dehydration of d-xylose into Fur, at 170 C. A study
Zr(template) (1.332 g).
◦
of the identification of reaction by-products by two-dimensional
gas chromatography (GC × GC) combined with time-of-flight mass
spectrometry (ToFMS) was carried out.
2.2. Catalyst characterisation
Powder XRD data were collected at room temperature
on
a Philips X’Pert MPD diffractometer, equipped with an
X’Celerator detector,
a
graphite monochromator (Cu-K␣ X-
2
. Experimental
radiation, ꢀ = 1.54060 A˚ ) and a flat-plate sample holder, in a
Bragg–Brentano para-focusing optics configuration (40 kV, 50 mA).
Scanning electron microscopy (SEM), EDX analyses, and elemental
mapping were performed using a HR-SEM-SE/EDS Hitachi SU-70
UHR Schottky instrument and Bruker Quantax 400. ICP-AES mea-
2
.1. Preparation of the catalysts
Two samples of zirconium tungsten mixed oxide ZrW(X)
−
1
(
X = Cl, NO ) with W/Zr atomic ratio of ca. 0.1 were prepared
surements for Zr, W and Al (error < 10%; detection limit, 20 ␮g L
)
3
by the co-precipitation method, in a similar fashion to that
reported in the literature [50]. Ammonium metatungstate hydrate
were carried out using a Horiba Jobin Yvon Activa-M spectrometer.
The BET specific surface areas (SBET, calculated for relative pres-
sures in the range 0.01–0.10) were estimated from N2 adsorption
(
(NH ) W12O40·13H O; Sigma–Aldrich, 99%) was used as the
4
6
2
◦
source of tungsten and ZrOCl ·8H O (Sigma–Aldrich, 99%) or
isotherms measured at −196 C using a Micromeritics instrument
2
2
◦
ZrO(NO ) ·6H O (Sigma–Aldrich, 99%) were used as the source of
Corp Germini model 2380 (sample pre-treatment: 250 C, vacuum):
3
2
2
zirconium. For each ZrW(X) material, three solutions were pre-
the BJH pore size distribution curve was calculated from the adsorp-
tion branch of the isotherm. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were carried out under air,
pared: 50 mL of 0.5 M ZrOCl ·8H O or ZrO(NO ) ·6H O in deionised
2
2
3
2
2
water (solution A); 1 L of distilled water with pH adjusted to
◦
−1
1
0 using aqueous NH OH (solution B); 0.2 mmol ammonium
with a heating rate of 10 C min , using Shimadzu TGA-50 and
DSC-50 systems, respectively. Raman spectra were recorded on
4
metatungstate dissolved in 20 mL of solution B (solution C). Solu-
tion A was added drop-wise to solution C, and the pH was adjusted
to ca. 10 using NH OH (Fluka, 25% NH , ca. 13.3 M). The resultant
–1
a Bruker RFS FT-Raman spectrophotometer (100 s , ꢀ = 1064 nm,
−
1
200–5000 scans with a resolution of 4 cm ).
4
3
slurries were hydrothermally treated in teflon-lined stainless steel
autoclaves heated at 195 C for 24 h. Subsequently, the solids were
The acid properties were measured using a Nexus-Thermo Nico-
let apparatus (64 scans and resolution of 4 cm ) equipped with a
◦
−1
filtered; in the case of ZrW(Cl) the solid was washed with deionised
specially designed cell, using self-supported discs and pyridine as
the basic probe. Gas-phase characterisation of acid sites has been
quite useful for investigating the influence of the acid properties of
solid acid catalysts in the aqueous phase reaction of xylose [16,25].
water and the AgNO test was used to confirm the efficient removal
3
−
◦
of Cl ions. The prepared materials were dried overnight at 110 C
and then calcined under air at 800 C for 3 h, and finally ground
◦

M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
129
Pyridine was chosen since its critical dimension of ca. 6.5 A˚ [52] is
comparable with the molecular diameters of xylose (6.8 A˚ along
chromatograph, with a dual stage jet cryogenic modulator (licensed
from Zoex) and a secondary oven. The detector was a high-speed
ToF mass spectrometer. A non-polar/polar set of columns was used:
a HP-5 column (30 m × 0.32 mm I.D., 0.25 ␮m film thickness, J&W
Scientific Inc., Folsom, CA, USA) was used as first-dimension col-
umn and a DB-FFAP (0.79 m × 0.25 mm I.D., 0.25 ␮m film thickness,
J&W Scientific Inc., Folsom, CA, USA) was used as a second-
dimension column. The carrier gas was helium at a constant flow
◦
the longest axis, [53]). After in situ outgassing at 400 C for 2 h
−
5
(
1
10 mbar), pyridine (99.99%) was contacted with the sample at
◦ ◦ ◦
50 C for 10 min and then evacuated at 150 C and 350 C (30 min)
under vacuum (10 mbar). The IR bands at ca. 1540 and 1450 cm
−5
−1
are related to pyridine adsorbed on Brønsted (B) and Lewis (L) acid
sites, respectively, and were used for quantification [54].
−
1
rate of 3.0 mL min . The primary oven temperature was pro-
◦
◦
grammed from 50 C (3 min) to 230 C (10 min) at a heating rate
2.3. Catalytic tests
◦
−1
of 10 C min ; the secondary oven temperature was programmed
from 70 C (3 min) to 250 C (10 min) at 10 C min . The MS trans-
fer line temperature and the MS source temperature was 250 C.
◦
◦
◦
−1
Batch catalytic experiments were performed under nitrogen
◦
atmosphere in tubular glass micro-reactors with pear-shaped bot-
toms and equipped with an appropriate PTFE-coated magnetic
stirring bar and a valve for gas purging. In a typical procedure, d-
xylose (30 mg), powdered catalyst (20 mg) and either water (1 mL,
The modulation time was 5 s, and the modulator temperature was
◦
kept at 20 C offset (above primary oven). The ToFMS was operated
−
1
at a spectrum storage rate of 100 spectra s . The mass spectrome-
ter was operated in the EI mode at 70 eV using a range of m/z 33–500
and the voltage was −1626 V. Total ion chromatograms (TIC) were
processed using the automated data processing software Chro-
maTOF (LECO) at signal-to-noise threshold of 100. Contour plots
were used to evaluate the separation general quality and for manual
peak identification; a signal-to-noise threshold of 50 was used. Two
commercial databases (Wiley 275 and US National Institute of Sci-
ence and Technology (NIST) V. 2.0 - Mainlib and Replib) were used.
The majority (86%) of the identified compounds showed mass spec-
tral similarity matches >850. Furthermore, a manual inspection of
the mass spectra was done, combined with the use of additional
data, such as the retention index (RI) value, which was determined
according to the Van den Dool and Kratz RI equation [55]. For the
determination of the RI, a C8 C20 n-alkanes series was used, and
denoted W) or a solvent mixture (denoted W–T) comprising H O
2
(
0.3 mL) and toluene (0.7 mL) were added to the reactor. The reac-
tion mixtures were heated with a thermostatically controlled oil
bath under magnetic stirring at 700 rpm, which was optimised
to avoid external mass transfer limitations; e.g. for ZrW(Cl), the
initial reaction rates (based on conversion at 30 min reaction)
were similar for stirring rates at or above 700 rpm (9.4, 12.4 and
−1
−1
1
2.1 mmol gcat
h at 500, 700 and 900 rpm, respectively). Zero
time (considered as the instant the reaction began) was taken to be
the instant the micro-reactor was immersed in the oil bath. After
a batch run, the catalyst was separated from the reaction mixture
by centrifugation, thoroughly washed with deionised water and
◦
finally dried at 55 C overnight (giving the washed/dried solid) and
◦
◦
−1
calcined at 450 C (1 C min ) for 5 h (giving the recovered solid).
The products present in the aqueous phase were analysed using
a Knauer K-1001 HPLC pump and a PL Hi-Plex H 300 mm × 7.7 mm
as some volatile compounds were eluted before C , the solvent n-
8
hexane was used as C6 standard. The experimentally calculated RI
values were compared, when available, with values reported in the
literature for similar chromatographic columns employed as the
first dimension. The results were analysed qualitatively and thus
the relative amounts of the products are not considered in this
work.
(
i.d.) ion exchange column (Polymer Laboratories Ltd., UK), coupled
to a Knauer 2300 differential refractive index detector (for xylose)
and a Knauer 2600 UV detector (280 nm, for Fur). The mobile phase
was 0.001 M H SO . The analytical conditions were the following:
2
4
−1
◦
flow rate 0.6 mL min , column temperature 65 C. The Fur present
in the organic phase was quantified using a Gilson 306 HPLC pump
and a Spherisorb ODS S10 C18 column, coupled to a Gilson 118
UV–vis detector (280 nm). The mobile phase consisted of 37% v/v
3. Results and discussion
−
1
methanol and 63% v/v H O (flow rate of 0.5 mL min ). Authentic
2
3.1. Characterisation of the catalysts
samples of d-xylose and Fur were used as standards and calibration
curves were used for quantification. The Fur yield (%) was calcu-
lated using the formula: [(moles of Fur formed)/(initial moles of
xylose) × 100]. For each reaction time, at least two replicates of
an individual experiment were made; the reported results are the
average values.
The physicochemical properties of the prepared catalysts are
given in Table 1. The semi-quantitative analyses of the crystalline
phases were performed using the Reference Intensity Ratio (RIR)
method applied to the XRPD data (Fig. 1) of the prepared materials.
The ZrW(X) materials are composed predominantly of tetragonal
zirconia (t-ZrO ) and contain ca. 20 wt% of (thermodynamically sta-
2
2.4. Identification of the water-soluble reaction by-products by
ble) monoclinic zirconia (m-ZrO ); the ZrO2 material prepared for
2
SPME/GC × GC–ToFMS analyses
comparison is predominantly monoclinic (contains 8 wt% t-ZrO ).
2
When tungsten is incorporated, the monoclinic phase tends to dis-
appear due to stabilisation of the tetragonal phase [36,37,56–59].
Only t-ZrO2 was detected for the ZrW-MP and ZrAlW-MP materi-
als. For all prepared materials no crystalline phases of WOx were
detected [56].
The SEM images showed that the ZrW(X) materials are com-
posed of particles (ca. 1–50 ␮m size) with irregular morphologies;
the mesoporous materials ZrW-MP and ZrAlW-MP consist of spher-
ical particles with ca. 3 ␮m diameter (Fig. 2). SEM-EDX and chemical
mapping show fairly homogeneous dispersions of Zr, W and Al (in
the case of ZrAlW-MP), and the atomic ratio W/Zr is ca. 0.1 for all
the prepared materials (Fig. 2, Table 1); in the case of ZrAlW-MP
the atomic ratio (W + Al)/Zr is 0.15 (Al/W = 0.07). The bulk atomic
ratios measured by ICP-AES are comparable to those obtained by
EDX (Table 1), further supporting fairly homogeneous dispersions
The liquid phase of the reaction mixture (after separating the
solid phase by centrifugation) was analysed (qualitatively) on the
same day that the respective catalytic test was performed. The
vial containing the reaction solution was immersed in a ther-
◦
mostated bath adjusted to 40.0 ± 0.1 C for 5 min. A solid-phase
microextraction (SPME) device with a fused silica fibre coat-
ing (50/30 ␮m divinylbenzene-carboxen-poly(dimethylsiloxane))
was immersed in the reaction solution for 20 min. After the
extraction/concentration step, the SPME coating fibre was manu-
◦
ally introduced into the GC × GC–ToFMS injection port at 250 C
and kept for 30 s for desorption. The injection port was lined
with a 0.75 mm I.D. splitless glass liner; splitless injections were
used (30 s). The LECO Pegasus 4D (LECO, St. Joseph, MI, USA)
GC × GC–ToFMS system consisted of an Agilent GC 7890A gas

1
30
M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
Table 1
Physicochemical properties of the prepared materials.
Crystalline phases (wt%)a
W/Zrb
SBET (m2
g
−1
)
B + L (␮mol g
−1
)
L/B
L350/L150
B350/B150
Sample
m-ZrO2
t-ZrO2
81
77
100
100
8
ZrW(Cl)
ZrW(NO3)
ZrW-MP
ZrAlW-MP
ZrO2
19
23
0
0
92
0.11 (0.10)
0.09
0.10 (0.13)
0.09 (0.07)c
–
48
51
102
133
12
38
1.4
1.2
1.2
1.4
–
0.2
0.2
0.2
0.2
–
<0.1
39
111
127
–
<0.1
<0.1
0.2
–
a
Semi-quantitative analyses of the constituent crystalline phases (monoclinic zirconia, m-ZrO2; tetragonal zirconia, t-ZrO2) determined by RIR method applied to the
XRPD data.
b
Surface atomic ratio of W/Zr determined by EDX. Values in brackets are for the bulk elemental analyses using ICP-AES.
W + Al)/Zr = 0.15 (EDX) or 0.14 (ICP-AES).
c
(
amount of acid sites (B + L) and acid strengths (based on the molar
◦
◦
ratios of L and B acid sites measured at 350 C and 150 C, denoted
L350/L150 and B350/B150, respectively). The total amount of B + L acid
sites is considerably greater for ZrW-MP than for the ZrW(X) mate-
rials which may be partly related to the higher SBET in the former
case, enhancing the accessibility of the acid sites to the base probe
(the W/Zr molar ratios are similar for the three materials). The ZrW-
MP possesses similar acid strengths to the ZrW(X) materials. In
comparison to ZrW-MP, the ZrAlW-MP material possesses some-
what higher total amount of B + L acid sites (and SBET), and slightly
stronger B acidity.
3
3
.2. Catalytic studies
.2.1. Catalytic performances of the prepared catalysts
The mesoporous ZrW-MP and ZrAlW-MP catalysts lead to higher
Fur yields at comparable reaction times, in comparison to the
ZrW(X) catalysts (Fig. 5a), which correlates with the higher total
amounts of acid sites of the former two catalysts (Table 1). In com-
parison to ZrW-MP, the ZrAlW-MP catalyst leads to higher Fur
yields at high conversions (51% yield at 98% conversion, Fig. 5b),
suggesting that the combined effects of enhanced SBET and surface
acidity favour the dehydration of xylose into Fur.
Fig. 1. XRPD patterns of the unused and recycled catalysts. For ZrW(Cl) and
ZrW(NO3) the main peaks of the ZrO2 crystalline phases are marked as (ꢀ) m-ZrO2
and (᭹) t-ZrO2.
The thermally regenerated ZrW-MP and ZrAlW-MP catalyst
(details given in Section 2.3) were re-used in three consecutive
4
h-batch runs (similar conditions have been reported previously
of the constituent elements (W, Zr, Al), and consistent with the
absence of crystalline WOx phases in the XRPD patterns of all the
prepared materials.
for efficiently regenerating a zirconia catalyst used in the conver-
sion of cellulose at 180 C [64]). The used catalysts were brownish
◦
in colour and the solvent washing procedures failed to restore the
original white colour of the as-prepared catalysts (in contrast to
that observed for the thermal treatment). For each catalyst, the
Fur yields in recycling runs are comparable (Fig. 6). It is worth
mentioning that for the washed/dried ZrW-MP catalyst which
was not subject to the thermal treatment, the Fur yield dropped
considerably from 41% for the first batch run to 13% for the sec-
ond one, possibly due to catalyst surface passivation by adsorbed
organic compounds. The XRPD patterns and Raman spectra for the
used/recovered ZrW-MP and ZrAlW-MP catalysts are similar to
those for the respective unused catalysts (Figs. 1 and 4; the Raman
spectra for the used/recovered catalysts exhibit higher background
noise). The stability of the mesoporous structure was confirmed
by the similar texture properties (type IV isotherms, SBET and pore
◦
The N2 sorption isotherms measured at −196 C for the ZrW(X)
materials exhibit a hysteresis loop at high relative pressures (above
0.6) and very broad pore size distributions (exemplified in Fig. 3 for
ZrW(Cl)); the values of SBET for these materials are similar and in
agreement with those reported in the literature for related mate-
rials [37,50,60–62]. The ZrW-MP and ZrAlW-MP materials exhibit
type IV isotherms, and possess higher SBET (Table 1) and narrower
mesopore size distributions (between ca. 2 and 7 nm width, inset
of Fig. 3) when compared against the ZrW(X) materials.
The Raman spectra of the prepared materials are shown in
Fig. 4; that of (essentially monoclinic) ZrO is given for comparison.
2
The spectra of the tungsten-containing materials show similarities
between each other and are very different from that of monoclinic
2
−1
;
ZrO , partly due to the predominant t-ZrO2 phase in the former
size distribution) of the used/recovered ZrW-MP (109–133 m g
mesopore size distributions between ca. 2 and 7 nm width) and
2
materials [37]. Bands characteristic of crystalline WO species (typ-
3
−1
−1
2
−1
ically at ca. 800–810 cm and 710–720 cm ) are hardly detected
ZrAlW-MP (133–143 m g ; mesopore size distributions between
ca. 2.5 and 6 nm width) catalysts (exemplified in Fig. 3 for ZrW-MP).
ICP-AES analyses of the aqueous phase of the reaction mixtures
indicated no significant leaching of Zr, W or Al. Based on these
results it seems that ZrW-MP and ZrAlW-MP are fairly stable cata-
lysts.
(
9
consistent with the XRPD data) [61]. Broad bands in the region
30–1020 cm may be assigned to the terminal W O bonds which
−1
may be isolated or polymeric mono oxo tungsten species [37,63].
All the prepared materials possess both Brønsted (B) and Lewis
◦
(
L) acid sites interacting with pyridine after outgassing at 150 C;
the molar ratio L/B varies in the range 1.2–1.4 (Table 1). The acid
properties of the ZrW(X) materials are similar in terms of the total
The amount of water-insoluble organic matter (hereafter des-
ignated as InsolOrg) was determined by TGA of the washed/dried

M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
131
Fig. 2. SEM image (top) and chemical mapping (Zr – red; W – dark blue; Al – light blue) for: (a) ZrW(Cl), (b) ZrW-MP and (c) ZrAlW-MP. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of the article.)
solids (after reaching 98–100% xylose conversion), based on the
total volume of reaction mixture were the same for the two sets
of reaction conditions). The Fur yields at similar conversions are
lower for the water system than for the biphasic solvent system
(Fig. 5a). The water and biphasic solvent systems lead to com-
parable Fur yields until ca. 90% conversion (46–48% Fur yield);
afterwards the Fur yield drops to 36% as conversion reaches 100%
in the case of the water system, whereas in the case of the bipha-
sic solvent system the Fur yield increases to 51% at 98% conversion
(Fig. 5b). A greater amount of InsolOrg is formed for the water sys-
◦
mass loss in the temperature range 200–600 C, and represented
−
1
as [(mass of InsolOrg)/(initial mass of xylose)] (mg gxylose ). DSC
analyses (exemplified for ZrAlW-MP in Fig. 7) of the used cata-
lysts exhibited two exothermic bands in the temperature range
◦
2
00–550 C which did not appear for the respective unused cata-
lysts and are therefore attributed to the decomposition of InsolOrg.
−
1
The amount of InsolOrg is similar for ZrAlW-MP (154 mg g
)
xylose
−1
and ZrW-MP (133 mg gxylose ). With respect to the water solu-
ble by-products, only formic acid was detected by HPLC (using a
UV diode array detector), although the yellow colour of the liquid
phases of the reaction mixtures indicated the presence of different
types of by-products. According to the literature, formic acid may
be formed via fragmentation of sugars under acidic hydrothermal
conditions [65,66], or hydrolytic fission of the aldehyde group of
Fur [28–30,67]. A more detailed study on the water-soluble by-
products is discussed ahead (Section 3.2.2).
−
1
tem (197 mg g
) in comparison to the biphasic solvent system
xylose
−
1
(154 mg gxylose ), Fig. 7. The partition ratio of Fur, calculated as
(moles of Fur in toluene)/(moles of Fur in the aqueous phase), mea-
sured at room temperature, varied in the range 7 ± 0.7 throughout
the reaction. Hence, as Fur is formed it is dissolved favourably in the
organic phase, which does not dissolve xylose (or polar intermedi-
ates). The higher Fur yields and lower amount of InsolOrg formed at
high conversions in the case of the biphasic solvent system may be
partly due to (i) reduction in the concentration of Fur in the aqueous
phase and consequently the lower extension of Fur loss reactions
involving xylose and/or polar intermediates [15,28–30,32] and (ii)
For comparison with the biphasic solvent system, the reac-
tion of xylose in the presence of ZrAlW-MP was carried out using
◦
solely water as solvent, at 170 C (the catalyst bulk density and

1
32
M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
Fig. 3. Nitrogen adsorption–desorption isotherms at −196 ◦C for ZrW(Cl) (ꢁ),
ZrW–MP (unused (ꢂ); recycled (×)), and ZrAlW–MP (᭹); the inset shows the respec-
tive pore size distribution curves using the same symbols.
competitive adsorption effects minimising consecutive reactions
of Fur (mainly toluene-solvated) on the catalyst surface (polar,
hydrophilic). The reaction of Fur (as the substrate) in the absence or
◦
presence of ZrAlW-MP, at 170 C (water system) gave 16% and 24%
conversion at 4 h reaction, respectively; in the case of the bipha-
sic solvent system no measurable conversion of Fur was observed,
which is consistent with the favourable effects of the biphasic sol-
vent system in avoiding Fur loss reactions.
Fig. 5. Fur yields versus reaction time (a), and Fur yield versus conversion of xylose
curves (b), for the catalysts ZrW(Cl) (ꢁ), ZrW(NO3) (ꢁ), ZrW–MP (ꢀ), and ZrAlW–MP
(
(+) water–toluene solvent system; (×) water system; the dashed lines are a visual
3
.2.2. Identification of by-products by SPME/GC × GC–ToFMS
analyses
The identification of the by-products may help to identify fac-
◦
guide). Reaction conditions: 30 mg xylose, 20 mg catalyst, 170 C.
tors influencing the side reactions, important for tailoring the
properties of solid acid catalysts for the target reaction. The iden-
tification of the InsolOrg compounds (also known as humins) is
not trivial. Some of the authors have previously attempted the
characterisation of the InsolOrg compounds formed in the reac-
tion of xylose in the presence of micro/mesoporous aluminosilicate
catalysts, under reaction conditions similar to those used in the
13
present work, using FT-IR spectroscopy and C MAS NMR; the
spectra were quite complex and it was only possible to identify
clearly xylose, 2-furfuraldehyde and formic acid (the organic mat-
ter possessed aldehyde/ketone groups, fragments related to xylose
and (un)saturated carbon–carbon bonds) [25,68].
Fig. 6. Fur yield in recycling runs of the reaction of xylose in the presence of
◦
ZrAlW–MP or ZrW–MP, using the biphasic water–toluene solvent system, at 170 C.
Reaction conditions: 10 wt% xylose, 20 mg catalyst, water–toluene (0.3:0.7 v/v),
◦
Fig. 4. Raman spectra of the unused and recycled catalysts.
170 C, 4 h reaction.

M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
133
Scheme 1. Products formed in the reaction of xylose in the presence of ZrAlW-MP, using solely water as solvent, at 170 ◦C, identified by GC × GC–ToFMS; TIC GC × GC–ToFMS
representation (one- and three-dimensional) of the reaction mixture (on the right).
1
-hydroxy-2-propanone, 2,3-butanedione and 2-butanone
(Table S1). The first four of these are possible sugar fragmen-
tation products [65], and were not detected with Fur as the
substrate. 2,3-Pentadione and hexadione products were formed in
the reaction of xylose and not of Fur. According to the literature
2
,3-pentadione may be formed through a series of reactions
involving aldol reactions of butanedione (detected in this work)
with formaldehyde, or of acetaldehyde (detected in this work) with
1
-hydroxy-2-propanone [76]. The mechanism of formation of the
hexadiones is not clear; it has been reported that 2,5-hexadione
is a possible product of the reaction of 2,5-dimethylfuran (not
detected in this work) with water [77]. It is worth mentioning
that the by-products may react with xylose (e.g. xylose bound to
a non-carbohydrate moiety) and/or Fur, and fragmentation and
recombination of carbohydrate fragments may take place, further
enhancing the complexity of the overall reaction mechanism.
Furanone products were detected, namely 3-methyl-2(5H) fura-
none and 5-methyl-2(5H)-furanone, for xylose and not for Fur
as substrate. A similar compound identified as 5-methyl-3(2H)-
furanone was previously reported as the product of the reaction
of 2-deoxy-d-erythro-pentose in aqueous acidic medium [78].
Several compounds possessing one furan ring were detected
and some (noted with the symbol * in Table S1, e.g. methyl furan-
2-carboxylate) are common to the reactions of xylose and Fur.
Furan derivatives possessing two furan rings were formed (e.g. di-
2-furanyl-ethanedione); related by-products have been reported
previously for the reactions of pentoses [17,79] and hexoses [5].
2,5-Dimethyl-1,4-benzoquinone was detected for the reaction of
xylose and not of Fur, as well as a variety of aromatic by-products
possessing a benzene ring, suggesting that the formation of these
by-products involves intermediates of the reaction of xylose;
monocyclic compounds include hydroxyacetophenones, hydrox-
ybenzaldehydes, and bicyclic compounds are benzopyrone and
benzofuran derivatives. It is worth mentioning that phenolic com-
pounds can be readily transformed into coloured products (as
mentioned above the reaction solutions were yellow). Aromatic
compounds of the type benzopyrone, toluene and acetophenone
possessing hydroxyl functional groups have been identified as by-
products of the reaction of xylose under acidic conditions [80,81].
Fig. 7. TGA (represented as (mass of InsolOrg)/(initial mass of xylose)) and DSC
curves for the ZrAlW–MP catalyst separated (washed/dried) from the reaction mix-
ture after reaching at least 93% xylose conversion, using the biphasic water–toluene
solvent system (black lines) or the water system (grey lines). Reaction conditions:
◦
3
0 mg xylose, 20 mg catalyst, 170 C.
In this work we set out to identify products of the reaction of
xylose in the presence of ZrAlW-MP, using water as solvent, at
◦
1
70 C by GC × GC–ToFMS. This technique allows the separation
and identification of water-soluble reaction products which are suf-
ficiently volatile under the applied analytical conditions (details
given in Section 2.4). For comparison, GC × GC–ToFMS was per-
formed for the reaction of Fur in the presence of ZrAlW-MP, at
◦
1
70 C. A complex mixture of reaction products was obtained for
the reaction of xylose (Scheme 1); the identified products are listed
in Table S1 (many more products were detected, which were not
clearly identified). For Fur as the substrate a much narrower spec-
trum of reaction products was detected (Fig. S1).
The products formed in the reaction of xylose may be grouped
into aliphatic and aromatic compounds (Scheme 1). A major
chromatographic peak was identified as Fur. The aliphatic by-
products consist of linear carbon chains of up to six carbon atoms,
possessing hydroxyl, carbonyl and/or carboxylic acid functional
groups. According to the literature monosaccharides may undergo
fragmentations through complex reaction mechanisms involving
retro-aldolisation, hydrolysis and/or oxidative fission pathways,
which may be accompanied by enolisation and dehydration reac-
tions leading to a variety of compounds [5,69,70–75]. The aliphatic
by-products possessing less than five carbon atoms identified
in the reaction of xylose include acetic acid, acetaldehyde,
4. Conclusions
Zirconium–tungsten mixed oxides are relatively active cata-
◦
lysts in the aqueous phase reaction of d-xylose, at 170 C. The
catalysts prepared by co-condensation led to more than 90% con-
version within 2 h reaction, but 2-furfuraldehyde (Fur) yields were
less than 35%. The Fur yields at similar xylose conversions could

1
34
M.M. Antunes et al. / Catalysis Today 195 (2012) 127–135
be improved by using a templating agent to give ZrW-MP with
enhanced specific surface area and amount of accessible acid sites
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