Aqueous-phase dehydration of xylose to furfural in the presence of MCM-22 and ITQ-2 solid acid catalysts

Article abstract of DOI:10.1016/j.apcata.2011.12.046

The H-MCM-22 zeolite possessing an MWW (medium-pore) framework and its delaminated counterpart, ITQ-2, with enhanced external surface area, are effective and recyclable solid acid catalysts in the batch-wise, aqueous-phase dehydration of xylose, at 170 °C. Up to 71% furfural (Fur) yield is reached at 98% conversion using a biphasic aqueous-organic solvent system (for the simultaneous extraction of Fur); using solely water as the solvent gives up to 54% Fur yield at 97% conversion. Sulfuric acid, used in an approximately equivalent amount to the total acid site concentration of the solid acids, gave 55% Fur yield at 93% conversion. Decreasing the Si/Al ratio of H-MCM-22 zeolite improves the acid properties and consequently the catalytic activity, without affecting significantly the Fur selectivity. While the delamination process considerably enhanced the external surface area of ITQ-2 in comparison to H-MCM-22, it caused modifications in the acid properties, leaving the two prepared materials with the same Si/Al atomic ratio of 24, on a comparable footing in terms of catalytic performance in the studied catalytic reaction. Nevertheless, these solid acid catalysts are fairly stable (similar Fur yields are reached in recycling runs; no structural modifications and no leaching phenomena were detected).

Full text of DOI:10.1016/j.apcata.2011.12.046

Applied Catalysis A: General 417–418 (2012) 243–252
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Aqueous-phase dehydration of xylose to furfural in the presence of MCM-22 and
ITQ-2 solid acid catalysts
Margarida M. Antunesa, Sérgio Limaa, Auguste Fernandesb, Martyn Pillingera, Maria F. Ribeirob,
Anabela A. Valentea,∗
a Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
b Institute for Biotechnology and Bioengineering, Centre for Biological and Chemical Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The H-MCM-22 zeolite possessing an MWW (medium-pore) framework and its delaminated counter-
part, ITQ-2, with enhanced external surface area, are effective and recyclable solid acid catalysts in the
batch-wise, aqueous-phase dehydration of xylose, at 170 ◦C. Up to 71% furfural (Fur) yield is reached
at 98% conversion using a biphasic aqueous–organic solvent system (for the simultaneous extraction of
Fur); using solely water as the solvent gives up to 54% Fur yield at 97% conversion. Sulfuric acid, used in
an approximately equivalent amount to the total acid site concentration of the solid acids, gave 55% Fur
yield at 93% conversion. Decreasing the Si/Al ratio of H-MCM-22 zeolite improves the acid properties and
consequently the catalytic activity, without affecting significantly the Fur selectivity. While the delam-
ination process considerably enhanced the external surface area of ITQ-2 in comparison to H-MCM-22,
it caused modifications in the acid properties, leaving the two prepared materials with the same Si/Al
atomic ratio of 24, on a comparable footing in terms of catalytic performance in the studied catalytic reac-
tion. Nevertheless, these solid acid catalysts are fairly stable (similar Fur yields are reached in recycling
runs; no structural modifications and no leaching phenomena were detected).
Received 26 July 2011
Received in revised form
28 December 2011
Accepted 29 December 2011
Available online 10 January 2012
Keywords:
Xylose
Furfural
Acid dehydration
MCM-22 zeolite
ITQ-2
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
There has been growing interest in the search for heterogeneous
catalytic routes for the production of Fur. Different types of porous
Furfural (Fur; C5H4O2) is a platform chemical derived from
lignocellulosic biomass and has been commercialised since the
beginning of the last century (from cereal waste stockpiles) [1].
The Fur market extends to different sectors of the chemical
industry (e.g., oil refining, resins, agrochemicals and pharma-
ceuticals) and is attracting increasing attention in the biofuels
sector [1–4]. The synthesis of Fur involves the primary hydrolysis
of pentosans (polysaccharides, which may be found in agricul-
tural/forestry wastes/surpluses) to give the respective pentoses
(monosaccharides), followed by the dehydration of the latter to
Fur (Scheme 1); theoretically, water is the sole co-product. The
hydrolysis/dehydration reactions are promoted by acid catalysts
and sulfuric acid is most commonly used in the industrial produc-
tion of Fur. While being quite effective, H2SO4 poses risks to human
health, the environment (difficult catalyst recovery/recycling, pos-
sible formation of sulfur-containing by-products, neutralisation of
effluents) and the process equipment (e.g., corrosion hazards).
solid acids have been investigated as catalysts in the reaction of
xylose in the aqueous phase, such as: (organic) acid resins [5] and
modified carbonaceous spheres [6], (hybrid) silicas functionalised
with sulfonic acid groups [7–9], (inorganic) bulk/supported het-
eropolyacids (HPAs) [10,11], micro/mesoporous transition metal
oxides (Zr, Zn, Ti [12], ZrW [5], Ti/Nb [13], NbSi [14]), sulfated
metal oxides (ZrAl [15,16]), silicoaluminophosphates [17] and alu-
minosilicates [5,18–21]. Important requirements to be put on solid
acid catalysts for this reaction system include water-tolerance
(minimal levelling-off of the acid strength in water), hydrother-
mal stability (crystalline structure integrity and stability towards
metal leaching) and good thermal stability if the catalyst regenera-
tion requires thermal decomposition of accumulated carbonaceous
deposits (typically in the range 350–550 ◦C), or chemical stability
if carbonaceous deposits are to be removed by harsh chemi-
cal treatments (e.g., lower temperature, liquid-phase oxidising
conditions).
From the literature data, certain families of porous inorganic
solids can be pinpointed as fairly robust materials for this catalytic
application, and in particular aluminosilicas (bulk catalysts) are
relatively cheap and versatile materials with respect to the type
of crystalline and porous structures (micro/meso/macropores; 1,
∗
Corresponding author. Tel.: +351 234 370603; fax: +351 234 401470.
E-mail addresses: atav@ua.pt, avalente@dq.ua.pt (A.A. Valente).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.12.046

244
M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
2 or 3-D pore systems), the acid properties and surface polarity
(varying Si/Al ratio), and they may be prepared with parti-
cle/crystallite sizes down to the nano-scale. In pioneering work by
Moreau et al. [18], zeolites revealed to be effective solid acids for
the conversion of saccharides into Fur; in the case of H-Mordenite
and H-Y faujasite with Si/Al atomic ratio in the range 2–15, 90–96%
selectivity was reached at 27–37% conversion, 170 ◦C, albeit selec-
tivity dropped considerably as conversion increased [18]. Aiouache
and co-workers found a similar trend in Fur selectivity with con-
version of xylose for zeolite ZSM-5 as catalyst, and explained these
results on the basis of enhanced Fur loss reactions inside the
pore system, which may eventually cause pore blockage and deac-
tivation of the catalyst [19]. Various synthetic approaches have
improved the catalytic performances of zeolites, such as decreas-
ing the zeolite crystallite sizes to the nano-scale to increase the
surface/pore volume ratio [21], and delaminating lamellar precur-
sors of zeolites into aggregates of sheets with zeolitic nature and
enhanced specific surface area [22].
2. Experimental
2.1. Preparation of the catalysts
The layered zeolite precursors, denoted Pre-MCM-22(X), where
X is the Si/Al molar ratio of 30 or 50 used in the synthesis gel, were
prepared as described in the literature [27,28]. The Si/Al ratios were
chosen to achieve a compromise between enhanced acid site den-
sity and successful delamination: decreasing the Al content of the
zeolite precursor leads to a lower charge density and therefore the
interactions between the zeolitic layers are weaker, facilitating the
delamination process [31–34]. For Pre-MCM-22(30) sodium alu-
minate (Riedel-De Haën, 53% Al2O3, 47% Na2O, 0.587 g, 6.10 mmol)
and sodium hydroxide (Prolab, 98%, 1.03 g, 25.7 mmol) were dis-
solved in water (milli-Q, 156 g, 8.67 mol). Hexamethyleneimine
(Aldrich, 99%, 8.36 g, 84.3 mmol) was then added and the mixture
stirred for 45 min, followed by the addition, under agitation, of sil-
ica (Aerosil 200, Degussa, 11.0 g, 183.1 mmol). The mixture was
stirred for a further 2 h to give a gel which was transferred to a
250 mL PTFE lined stainless-steel autoclave, rotated at 50 rpm, and
heated at 135 ◦C for 11 days; after this period of time the auto-
clave was quenched in cold water. The solid phase was separated by
centrifugation and washed thoroughly with deionised water until
pH < 9.0, and subsequently dried at 25 ◦C overnight. Finally, part
of the solid was calcined under static air at 540 ◦C for 6 h, giving
Na-MCM-22(24) (in parenthesis, the bulk Si/Al atomic ratio mea-
sured by ICP-AES). A similar procedure was followed to prepare
Pre-MCM-22(50), which upon calcination gave Na-MCM-22(38).
The delaminated aluminosilicate, ITQ-2, was prepared from
Pre-MCM-22(30) as follows: Pre-MCM-22(30) (10 g) was dis-
persed in milli-Q H2O (20 g), followed by the addition of aqueous
cetyltrimethylammonium hydroxide/bromide (100 g, 20 wt%; 40%
exchanged Br/OH, prepared by ion-exchange of cetyltrimethylam-
monium bromide (Fluka, ≥96%) using Amberlite IRA-400(OH)) and
aqueous tetrapropylammonium hydroxide/bromide (30 g, 40 wt%;
50% exchanged Br/OH, prepared from a mixture of tetrapropy-
lammonium bromide (Fluka, ≥99%) and tetrapropylammonium
hydroxide (40 wt%, Alfa Aesar)). The resultant mixture possessed
pH ≈ 13.5 and was heated at 80 ◦C with vigorous stirring for 16 h
in order to facilitate the swelling of the layers of the precursor
material; the final pH was 11.9. The suspension was sonicated
using an ultrasound bath (50 W, 50 Hz) during 1 h; after decanta-
tion, the supernatant colloid was separated. The pH of the colloid
was lowered to ca. 2.0 by adding 6 M HCl in order to facilitate
The zeolite MCM-22, which possesses an MWW type framework
and was discovered by Mobil in 1990 [23], may be prepared by
calcination of a lamellar precursor herein denoted as Pre-MCM-
22. The MWW structure consists of two independent pore systems
and specific surface area is commonly in the range 300–500 m2 g−1
:
˚
˚
one pore system is formed by large cages (∼7.1 A Ø, 18.2 A height)
which are accessed through 10-membered ring (10-MR) apertures,
and the other one is defined by a circular 10-MR sinusoidal channel
system (medium-pore). The maximum diameter of the pore open-
˚
˚
ings of MCM-22 is 5.5 A or 6.2 A based on atomic or Norman radii,
respectively [24]. Since xylose molecules possess an approximate
˚
molecular diameter of 6.8 A [25], access to the internal catalyst
surface of MCM-22 through the 10-MR apertures will likely be
impeded. In order to enhance the external surface area available for
the catalytic reactions, Corma and co-workers [26–29] developed a
delamination procedure to transform Pre-MCM-22 into a delam-
inated material known as ITQ-2, possessing very high external
specific surface area (∼600–800 m2 g−1). ITQ-2 consists of 2.5 nm
thick sheets possessing a hexagonal array of cup-shaped open pores
˚
˚
facing out of each side of the sheets (∼7.1 A Ø, 7.0 A height, with
bottoms connected by a double 6-MR unit), and a circular 10-MR
sinusoidal channel system running between the cups, inside the
sheet [26–30]. In the present work, the dehydration of xylose to
Fur is investigated in the presence of MCM-22 and ITQ-2 catalysts,
using a biphasic water–toluene solvent system or solely water as
solvent, at 170 ◦C.
Scheme 1. Simplified representation of the conversion of carbohydrate biomass into furfural.

M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
245
1455 cm−1 are related to pyridine adsorbed on B and L acid sites,
respectively [37].
the flocculation of the delaminated solid. The solid was separated
by centrifugation and washed with distilled water. After drying at
60 ◦C for 12 h, the solid was calcined at 540 ◦C for 3 h under a flow of
For collidine as basic probe, the FT-IR spectra were recorded
with a resolution of 2 cm−1 on a Nicolet Nexus spectrophotometer
equipped with an MCT detector. The powdered solids were pressed
into self-supported disks (10 mg cm−2) and placed in a quartz IR
cell with KBr windows. After in situ outgassing at 450 ◦C for 4 h,
collidine was contacted with the sample at 200 ◦C for 30 min and
then evacuated at 200 ◦C (30 min) under vacuum. The IR band at ca.
1635 cm−1 is related to collidine adsorbed on B acid sites.
N2 (300 mL min−1), and then during 6 h under air (300 mL min−1
)
[29,35]. The catalysts were manually ground using an agate pestle
and mortar and subsequently sieved to give a powder with particle
sizes of less than 106 ␮m width.
An ion-exchange/calcination procedure was applied to Na-
MCM-22(24) and Na-MCM-22(38) to give H-MCM-22(24) and
H-MCM-22(38), respectively. The ion-exchange procedure con-
sisted of suspending 1 g of the solid material in 15 mL of 1 M
NH4NO3 and stirring the resultant suspension for 24 h at 80 ◦C; the
ion-exchanged materials were washed thoroughly with deionised
water, dried at 50 ◦C overnight, and finally calcined in air at 540 ◦C
(heating rate of 1 ◦C min−1) for 6 h.
2.3. Catalytic tests
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,
denoted W) or a solvent mixture (denoted W-T) comprising H2O
(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. Zero time was taken to be
the instant the micro-reactor was immersed in the oil bath. It may
be assumed that external mass transfer limitations are avoided at
this stirring rate, since tests showed that the initial reaction rates
(based on conversion at 1 h reaction) were similar for stirring rates
at or above 700 rpm: 1.6, 2.4 and 2.3 mmol gcat−1 h−1 at 600, 700
and 800 rpm, for H-MCM-22(24); 2.1, 2.0 and 2.1 mmol gcat−1 h−1
at 600, 700 and 800 rpm, for ITQ-2.
2.2. Catalyst characterisation
ICP-AES measurements for Si and Al (error of 5–10%) were
carried out at the Central Laboratory for Analysis, University of
Aveiro (by L. Carvalho). 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-radiation, ꢀ = 1.54060 A) and a flat-plate sample holder, in a
Bragg–Brentano para-focusing optics configuration (40 kV, 50 mA).
Samples were step-scanned in 0.04◦ 2ꢁ steps with a counting time
of 6 s per step. Scanning electron microscopy (SEM) was carried
out on a Hitachi SU-70 UHR Schottky instrument. Transmission
electron microscopy (TEM) was carried out on Hitachi H9000 and
JEOL 2200FS instruments. Samples were prepared by spotting holey
amorphous carbon film-coated 400-mesh copper grids (Agar Scien-
tific) with a suspension of the solid sample in ethanol. The following
textural parameters were estimated from N2 adsorption isotherms
measured at −196 ◦C using a Micromeritics instrument Corp Ger-
mini model 2380 (sample pre-treatment: 250 ◦C, vacuum): BET
specific surface area (SBET, calculated for relative pressures in the
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
(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 H2SO4. Analysis conditions: flow rate 0.6 mL min−1
,
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) methanol and 63% (v/v)
H2O (flow rate 0.5 mL min−1). Authentic samples of d-xylose and
Fur were used as standards and calibration curves were used for
quantification. The Fur yield (%) was calculated 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 max-
imum average absolute deviation calculated for xylose conversion
and Fur yield was less than 4%.
range 0.01–0.10), microporous external specific surface area (Sext
;
t-plot method), and BJH pore size distribution curve calculated from
the adsorption branch of the isotherm. Thermogravimetric analy-
sis (TGA) and differential scanning calorimetry (DSC) were carried
out under air, with a heating rate of 10 ◦C min−1, using Shimadzu
TGA-50 and DSC-50 systems, respectively.
The 27Al magic-angle spinning (MAS) NMR spectra were mea-
sured at 104.26 MHz with a Bruker Avance 400 (9.4 T) spectrometer,
using a contact time of 0.6 ␮s, a recycle delay of 0.8 s, and a spinning
3+
rate of 15 kHz; chemical shifts are quoted in ppm from Al(H2O)6
.
Infrared spectra were recorded on a Unican Mattson Mod 7000 FT-
IR spectrophotometer using KBr pellets. The liquid state 1H NMR
and 13C NMR spectra were recorded on a Bruker DRX 300 MHz
spectrometer at 20 ◦C.
3. Results and discussion
It has been reported in the literature for dehydration reactions
in aqueous solutions with solid acid catalysts that gas-phase char-
acterisation of acid sites can be used to predict catalytic activity in
the aqueous phase [5]. The acid properties of the prepared catalysts
were examined by FT-IR studies of adsorbed basic probe molecules,
namely pyridine and collidine (2,4,6-trimethylpyridine). Pyridine
3.1. Characterisation of the catalysts
The ion-exchange/calcination treatments applied to Na-MCM-
22(24) and Na-MCM-22(38) to give H-MCM-22(24) and H-MCM-
22(38), respectively, did not affect the Si/Al atomic ratio. The
delamination procedure applied to Pre-MCM-22(30) gave the
material ITQ-2 with a slightly lower Si/Al atomic ratio, possibly
due to slight dissolution of silica during the delamination process
[33,34]. The powder XRD patterns of the (Na,H)-MCM-22 sam-
ples are in good agreement with the literature data for a MWW
framework topology (Fig. 1) [26,38]. The XRD pattern of ITQ-2
reveals a significant reduction in the long-range order as a result
of the delamination procedure, in agreement with literature data
[26,28,33,34,39]. The SEM analyses show that the (Na,H)-MCM-22
samples consist of thin plate-like crystallites of a few hundreds
˚
possesses a critical dimension of ca. 6.5 A [36] which is compara-
˚
ble with the molecular diameters of xylose (6.8 A along the longest
axis [25]). For pyridine as basic probe, the acid properties were
measured using a Nexus-Thermo Nicolet apparatus (64 scans and
resolution of 4 cm−1) equipped with a specially designed cell, using
self-supported discs (5–10 mg cm−2). After in situ outgassing at
450 ◦C for 3 h (10−6 mbar), pyridine (99.99%) was contacted with
the sample at 150 ◦C for 10 min and then evacuated at 150 ◦C
(30 min) under vacuum (10−6 mbar). The IR bands at ca. 1540 and

246
M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
Fig. 3. TEM images of Na-MCM-22(24) (a) and an ITQ-2 layer (b), viewed along the
10-ring channels, perpendicular to the c-direction.
crystallite viewed along the 10-MR channels (bright spots), per-
pendicular to the c-direction. The images generally showed the
stacking of MWW sheets with a thickness ranging from 150 to
300 nm, corresponding to the thickness of the platelet crystals. As
expected, the 10-MR channels are separated by 1.2 nm, and the
thickness of the layers is ca. 2.5 nm. HRTEM images of ITQ-2 showed
much more fragmented crystals consisting of fewer sheets than Na-
MCM-22(24), and even single layers of 2.5 nm thickness as shown
in Fig. 3(b).
Fig. 1. Powder XRD patterns of as-prepared and (for H-MCM-22(24) and ITQ-2)
used/calcined (after catalysis) catalysts.
The (Na,H)-MCM-22 materials exhibited type I adsorption
isotherms, typical of microporous materials, with an enhanced
increase in the amount of adsorbed N2 at high relative pressures
(p/p0), which is most likely due to multilayer adsorption on the
external surface of the crystallites (exemplified for Na-MCM-22(24)
in Fig. 4; Figure S2 for H-MCM-22(24), Na-MCM-22(38) and H-
MCM-22(38)). The external surface areas (Sext) of these samples
are significant (48–74 m2 g−1, Table 1), which is consistent with the
rather small crystallite sizes (Fig. 2). The ion-exchange/calcination
treatments did not affect significantly the textural properties of the
Na-MCM-22 samples. The adsorption-desorption isotherm for ITQ-
2 exhibited a hysteresis loop at p/p0 > 0.5, indicating the presence
of mesoporosity; the mesopore size distribution curve exhibited a
of nanometers to ca. 1 ␮m wide, and ITQ-2 consists of parti-
cles of irregular shape (exemplified in Fig. 2 for Na-MCM-22(24)
and ITQ-2; Figure S1 for H-MCM-22(24) and H-MCM-22(38)).
Fig. 3 shows a representative HRTEM image of a Na-MCM-22(24)
Fig. 4. Nitrogen adsorption isotherms measured for Na-MCM-22(24) and ITQ-2, at
−196 ◦C.
Fig. 2. SEM images of (a) Na-MCM-22(24) and (b) ITQ-2.

M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
247
Table 1
Textural and acid properties, and catalytic performances of aluminosilicates tested as catalysts in the reaction of xylose, using a biphasic W-T solvent mixture, at 170 ◦C.a
c
Catalyst (Si/Al)b
SBET (Sext) (m2 g−1
)
[L] + [B] (␮mol g−1
)
[L]/[B]d
t (h)e
Conv. (%)f
YFur (%)g
Ref.
Na-MCM-22(38)
H-MCM-22(38)
Na-MCM-22(24)
H-MCM-22(24)
ITQ-2(24)
H-Nu-6(2)(32)
del-Nu-6(1)(29)
BEA(12)
524 (62)
497 (48)
361 (52)
333 (74)
623 (611)
20 (–)
151 (6)
643 (176)
712 (–)
757 (–)
425
–
168
–
–
0.6
–
–
–
–
–
–
–
–
24/32
24
16
16
6
82/97
98
92
99
59
60/68
47
70
66
28
204
198
ndh
nd
351
209
197
–
0.4
1.0
nd
nd
1.3
1.2
2.3
–
–
[22]
[22]
[21]
[21]
[20]
–
6
87
46
4/6
6/8
6
98/100
94/98
90
54/49
69/74
60
BEA/TUD-1
Al-TUD-1(21)
H-ZSM-5(46)
6/16
88/98
60/61
a
Reaction conditions: 0.67 M xylose, xylose:catalyst mass ratio of 3:2, biphasic W-T solvent mixture, 170 ◦C.
The Si/Al atomic ratio of the catalyst is given in parenthesis.
Sum of the total amount of Brönsted acid sites [B] plus Lewis acid sites [L].
Ratio of the amounts of Lewis to Brönsted acid sites.
Reaction time.
Xylose conversion at the specified reaction times.
Furfural yield at the specified reaction times.
Not determined.
b
c
d
e
f
g
h
maximum at 3.7 nm (Fig. 4), in agreement with the literature data
[28,40]. The ITQ-2 sample possesses much higher Sext (611 m2 g−1
than the (Na,H)-MCM-22 materials, which is consistent with a suc-
cessful delamination procedure (Table 1).
areas of the respective peaks (AFAl = area of the peak in the range
of 35–70 ppm; AEFAl = area of the peak centered at 0 ppm): the ratio
AFAl/AEFAl equals 8.4 for H-MCM-22(24) and 9.4 for H-MCM-22(38);
these results are in agreement with the literature data in that higher
Si/Al ratios can lead to higher relative amounts of tetrahedral alu-
minium [34]. The 27Al NMR spectrum of ITQ-2 reveals the presence
of FAl (ca. 55 ppm) and EFAl species (0 ppm), in agreement with
the literature data [34,39]. The AFAl/AEFAl ratio for ITQ-2 equals 8.5
which is similar to that observed for its counterpart H-MCM-22(24).
The FT-IR spectrum of ITQ-2 shows a band at ca. 956 cm−1 assigned
to terminal Si-OH groups, which is poorly resolved in the case of
H-MCM-22(24), further suggesting that delamination occurred to
a significant extent in the preparation of ITQ-2 (Fig. 6) [26,28,39].
The acid properties of the prepared catalysts were examined
by FT-IR studies of adsorbed pyridine (Table 1). The H-MCM-22
materials possess mainly Brönsted acid sites ([B]), and compa-
rable ratio of Lewis to Brönsted acid sites ([L]/[B] = 0.4–0.6). The
H-MCM-22(24) sample possesses a higher total amount of Lewis
and Brönsted acid sites ([L] + [B]) in comparison to H-MCM-22(38),
which is consistent with the lower Si/Al ratio for the former.
The ITQ-2 sample possesses a similar total amount of acid sites
)
The 27Al MAS NMR spectra of the Na-MCM-22 samples exhibit
an asymmetric broad peak with maximum at ca. 56 ppm and a
shoulder at ca. 50 ppm (Fig. 5), assigned to tetrahedrally coor-
dinated framework aluminium (denoted FAl), which may be
nonequivalent chemical environments with different T–O–T angles
[30,39–42]. In the case of the H-MCM-22 samples an additional
(narrow, weaker) peak appears at ca. 0 ppm, assigned to extra-
framework aluminium species (denoted EFAl) formed during
the ion-exchange/calcination treatments [39,43,44]. The relative
amounts of the FAl and EFAl species were determined from the
Fig. 5. 27Al MAS NMR spectra of the as-prepared catalysts.
Fig. 6. FT-IR spectra, in the framework region of H-MCM-22(24) and ITQ-2.

248
M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
to H-MCM-22(24), while the [L]/[B] ratio is higher for ITQ-2
(a similar trend has been reported in the literature [27]). The ratios
of the amounts of acid sites measured using pyridine adsorbed at
150 and 250 ◦C ([L]150/[L]250 and [B]150/[B]250) were both equal to
1.1 for H-MCM-22(24), whereas for ITQ-2 [L]150/[L]250 = 1.4 and
[B]150/[B]250 = 2.6. Hence, the surface acidity (particularly the B
acidity) for ITQ-2 is weaker.
For comparison, the acid properties of ITQ-2 and H-MCM-22(24)
were measured using collidine as the basic probe molecule. Due to
˚
its steric bulk (ca. 7.4 A), the collidine molecule is not expected to
enter the pores of the MWW framework structure [45,46], i.e., it
should only interact with B acid sites (denoted [B]col) located on the
external surface or at the pore entrances. On the other hand, col-
∼
∼
=
lidine is a stronger base (pKa 7.4) than pyridine (pK
5.3) [47].
=
a
The [B]col for H-MCM-22(24) and ITQ-2 was 45 and 153 ␮mol g−1
,
respectively (the reproducibility was checked in duplicate experi-
ments). The lower [B]col for H-MCM-22(24) than for ITQ-2 is most
likely due to steric constraints (inaccessibility of the acid sites on
the internal surface to the bulky collidine molecules). For ITQ-2,
[B]col is greater than [B] determined using pyridine, suggesting that
the range of interaction energies with (stronger base) collidine is
wider than that with pyridine. Based on the above results, it seems
that while the delamination procedure led to enhanced external
surface area of ITQ-2, the surface acidity became relatively weak.
3.2. Catalytic studies
3.2.1. Catalytic performance of H-MCM-22 in water-organic
solvent system
The catalysts were tested in the aqueous-phase reaction of
xylose to give furfural (Fur), using a biphasic water–toluene
(0.3:0.7, v/v) solvent system (denoted W-T), at 170 ◦C. Xylose dis-
solves completely in water and is insoluble in toluene, whereas
Fur is distributed in the two liquid phases with a partition ratio
((moles of Fur in T)/(moles of Fur in W)) in the range of 8–10, at
room temperature. Hence, the use of toluene as co-solvent allows
the simultaneous extraction of Fur as it is formed, into the (upper)
organic phase, which may avoid consecutive Fur-loss reactions.
The reaction of xylose in the presence of H-MCM-22(24) gave
70% Fur yield at 92% conversion, reached at 16 h (Fig. 7). The kinetic
curves for H-MCM-22(24) and its parent basic form, Na-MCM-
22(24), are−ro1 ughly coincident (Fig. 7(a)); the initial reaction rate is
2.4 mmol h gcat−1 (based on conversion at 1 h reaction). However,
higher Fur yields are reached for H-MCM-22(24) than for Na-MCM-
22(24) (Fig. 7(b)). The reaction mechanism of the dehydration of
xylose to Fur involves a series of elementary steps leading to the
liberation of three water molecules per Fur molecule formed, and
these reactions take place over acid sites which may be of Brön-
sted or Lewis type [12,20,21,48]. The presence of basic sites in the
catalysts can promote undesirable reactions such as the aldolisa-
tion decomposition of the saccharide into oligomeric acid products
[49,50].
Fig. 7. Kinetic profiles of the reaction of xylose (a), and Fur yield versus xylose con-
version curves (b), for Na-MCM-22(24) (ꢂ), H-MCM-22(24) (×), H-MCM-22(38) (+)
and ITQ-2 (ꢀ), used as catalysts. Reaction conditions: 0.67 M xylose, xylose:catalyst
mass ratio of 3:2, biphasic W-T solvent mixture, 170 ◦C.
The catalytic activity of (medium-pore) H-MCM-22(24) is inter-
mediate between that previously reported for the small-pore
zeolite H-Nu-6(2) (Si/Al = 32) [22] and large-pore zeolite BEA
(Si/Al = 12) [21], used as catalysts in the same reaction under sim-
ilar conditions (Table 1). Nevertheless, the maximum Fur yields
reached are highest for H-MCM-22(24). For comparison, the reac-
tion of xylose was carried out, under similar reaction conditions,
in the presence of the protonic form of a commercial sample
of ZSM-5 (Alfa Aeser; ammonium form; Si/Al = 46; 425 m2 g−1),
which is a medium-pore zeolite with the MFI framework type.
The as-received zeolite was heated under static air at a ramp rate
of 1 ◦C min−1 to 550 ◦C, and maintained at this temperature for
10 h, giving H-ZSM-5(46). The xylose conversions and Fur yields at
6 h/16 h were 88%/98% and 60%/61%, respectively. Compared with
the prepared H-MCM-22 samples, the H-ZSM-5(46) catalyst is more
active (e.g., for the more active H-MCM-22(24), 73%/92% conver-
sion at 6 h/16 h), but led to lower Fur yields at high conversions (at
ca. 98% conversion: 71%, 69% and 61% Fur yield for H-MCM-22(24),
H-MCM-22(38) and H-ZSM-5(46), respectively). O’Neill et al. inves-
tigated the reaction of xylose in the presence of H-ZSM-5 with
Si/Al = 28 [19]. In that study, a maximum of ca. 33% Fur yield was
reached within 60 min, at 180 ◦C, and afterwards the yield tended to
The influence of the Si/Al ratio of the catalyst on the reaction of
xylose was investigated by comparing the catalytic performances
of H-MCM-22(24) and H-MCM-22(38). For H-MCM-22(38), the ini-
−1
−1
tial reaction rate was 1.0 mmol h
g
cat
and a conversion of 71%
was reached at 16 h reaction (Fig. 7(a)). The higher catalytic activ-
ity observed for H-MCM-22(24) correlates with the higher total
amount of acid sites ([L] + [B]) determined by IR studies of adsorbed
pyridine (Table 1). The Fur yields reached at ca. 98% xylose con-
version were 68% for H-MCM-22(38) and 71% for H-MCM-22(24)
(Fig. 7(b)). Hence, the decrease in the Si/Al ratio (in the range of
24-38) of the H-MCM-22 zeolite leads to an increase in the total
amount of acid sites which, in turn, improves the catalytic activ-
ity in the reaction of xylose, without affecting significantly the Fur
selectivity.

M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
249
drop with time (reaction conditions: 10 wt% xylose, catalyst:xylose
ratio of 0.3 (w/w), under pressurised He atmosphere); the Fur yields
reported for the reaction temperature of 160 ◦C were lower. The
average pore size of the investigated ZSM-5 catalyst was ca. 1.2 nm
which is far greater than the molecular diameters of both xylose and
Fur, and this was considered as a possible cause of the formation of
considerable amounts of by-products (oligomers).
3.2.2. ITQ-2 versus H-MCM-22(24) in water–organic solvent
system
The reaction of xylose was further investigated in the pres-
ence of ITQ-2 (the delaminated counterpart of H-MCM-22(24)),
using the biphasic W-T solvent system at 170 ◦C, which gave a Fur
yield of 66% at 99% conversion (Fig. 7(b)). In terms of the maxi-
mum Fur yield reached, these results are superior to those reported
previously for a delaminated solid acid (denoted del-Nu-6(1)) pre-
pared by exfoliation of the lamellar precursor Nu-6(1) (46% Fur
yield [22]), comparable to a mesoporous Al-TUD-1 material (60%
Fur yield [20]), and lower than that for a BEA/TUD-1 composite
(74% Fur yield [21]), used as catalysts in the same reaction under
similar conditions (ITQ-2 possesses comparatively lower catalytic
activity), Table 1. It is difficult to establish clear structure–activity
relationships between different types of aluminosilicate catalysts;
the catalytic performances may be due to an interplay of different
factors such as morphology, crystalline structure, porosity, sur-
face polarity and acid properties. A common feature, however,
is the apparent fairly high stability of the aluminosilicate cata-
lysts investigated, under the applied reaction conditions (discussed
below for H-MCM-22 and ITQ-2). These comparisons merely sum-
marise literature data obtained under similar reaction conditions,
and do not serve to “rate” the different families of catalysts (the
physical–chemical properties of different types of catalysts may be
optimised).
Fig. 8. DSC curves for the as-prepared H-MCM-22(24) (grey line) and ITQ-2 (black
line) catalysts, and the respective solids recovered from the reaction of xylose after
ca. 98% conversion was reached (used catalysts). Reaction conditions: 0.67 M xylose,
xylose:catalyst mass ratio of 3:2, biphasic W-T solvent mixture, 170 ◦C.
the external surface [45,51]; molecular dynamics simulations indi-
˚
cated that benzene (kinetic diameter of 5.85 A [52]) presents low
diffusivity in either of the two pore systems of the MWW structure
[53]. Assuming that the reaction of xylose takes place essentially
on the external surface of the catalysts, the similar catalytic per-
formances of ITQ-2 and H-MCM-22(24) despite their considerably
different Sext may be due to weaker overall acidity in the former
case (some acid sites may be inactive or possess relatively low
intrinsic catalytic activity).
A comparison of the catalytic results for ITQ-2 and H-MCM-
22(24) shows no major differences in terms of react−io1n ra−te1
(Fig. 7(a)): ITQ-2 gave an initial reaction rate of 2.0 mmol h gcat
After at least 98% xylose conversion was reached, the H-MCM-
22(24) and ITQ-2 catalysts were separated from the reaction
mixtures by centrifugation, washed with methanol and dried at
50 ◦C overnight, giving pale brown powders. For each solid, the
DSC analysis showed an endothermic band below 200 ◦C assigned
to desorption of physisorbed water and strong exothermic bands
above 200 ◦C (which were not observed for the fresh catalysts)
assigned to the combustion of organic matter (Fig. 8). The amount
of organic matter in the used catalysts (measured by the weight
loss in the temperature range of 220–700 ◦C) was similar for H-
MCM-22(24) and ITQ-2 (ca. 12 wt%). However, the DSC profiles
are different, suggesting that the chemical nature of the carbona-
ceous matter is different for the two catalysts. The spectrum of
by-products formed may be influenced by the [L]/[B] acid site ratio
[5] and/or textural properties (by-products may become strongly
adsorbed/entrapped inside the MWW microporous structure).
The stability of the H-MCM-22(24) and ITQ-2 catalysts was
investigated by recycling the solid acids, under biphasic solvent
conditions, at 170 ◦C. The washing of the used catalysts with differ-
ent solvents (methanol, acetone, toluene, water) failed to efficiently
remove the organic matter from the catalysts. Therefore, prior to
reuse, the solids were calcined at either 450 ◦C (ITQ-2) or 550 ◦C (H-
MCM-22(24)) for 5 h (heating rate of 1 ◦C min−1) to leave a residual
amount of organic matter of less than 1 wt%; the higher tempera-
ture required for the complete combustion of the organic matter
in the case of H-MCM-22(24) is consistent with the thermal anal-
ysis data. The Fur yields in four consecutive 6 h-batch runs were
similar for H-MCM-22(24) and ITQ-2 (Fig. 9). When the used H-
MCM-22(24) catalyst was treated at 450 ◦C instead of 550 ◦C, the
Fur yield decreased in recycling runs (Fig. 9), most likely due to
the incomplete removal of organic matter; the calcined solid was
very light brown in color and contained ca. 4 wt% organic matter.
and
2.4 mmol h
a
conversio−n1 at 8 h of 78%, while H-MCM-22(24) gave
−1
g
cat
and a conversion of 83%. These results corre-
late fairly well with the similar total concentration of acid sites
([L] + [B]) measured for the two catalysts using pyridine as probe
molecule (Table 1). The catalytic activity does not correlate with
the [B]col which is higher for ITQ-2 than for H-MCM-22(24); possi-
bly the stronger base collidine interacts with some acid sites which
are too weak for catalysing the reaction of xylose. The Fur yield
versus xylose conversion profiles are also comparable for the two
catalysts, and the Fur yield reached at ca. 99% xylose conversion is
71% for H-MCM-22(24) and 66% for ITQ-2 (Fig. 7(b)). Hence, ITQ-2
stands on a similar footing to H-MCM-22(24) (the Si/Al ratio is the
same for the two materials) in terms of catalytic performance in
the reaction of xylose.
The catalytic performances of ITQ-2 and H-MCM-22(24) are sim-
ilar despite their considerably different Sext. A clear assessment of
the location (external/internal surfaces) of the effective acid sites
is not trivial. As mentioned in Section 1, the intrazeolite void space
of the MWW pore structure is accessible through the 10-MR aper-
˚
˚
tures [26], and the maximum pore diameter is 5.5 A or 6.2 A based
on atomic or Norman radii, respectively [24]. The xylose molecules
(the hemiacetal isomers are predominant in solution) possess an
˚
approximate molecular diameter of 6.8 A [25], and the critical, max-
˚
˚
˚
imum and kinetic diameters of Fur are 4.56 A, 5.99 A and 5.5 A,
respectively [24]. Based on these data, and taking into consider-
ation that in the liquid phase the solute molecules are solvated by
the solvent, the access of xylose (and possibly Fur) molecules to
the internal surface of the MWW-type framework may be severely
hindered. It has been reported for benzene as substrate and zeo-
lite MCM-22 as catalyst that the reaction takes place essentially on

250
M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
Fig. 9. Furfural yields in four consecutive 6 h-batch runs of the reaction of xylose
in the presence of H-MCM-22(24) (regenerated by applying thermal treatment at
either 450 or 550 ◦C) or ITQ-2. Reaction conditions: 0.67 M xylose, xylose:catalyst
mass ratio of 3:2, biphasic W-T solvent mixture, 170 ◦C.
The powder XRD patterns (Fig. 1) and the Si/Al ratios (measured by
ICP-AES) of the used/calcined H-MCM-22(24) and ITQ-2 catalysts
were similar to those of the respective fresh catalysts (Si/Al = 25 and
24, respectively).
The reaction of xylose in the presence of −IT1Q-2 at 155 ◦C was
slower (an initial reaction rate of 0.8 mmol h gcat−1) and led to
lower Fur yield (55% yield at 98% conversion, reached at 24 h
reaction) t−h1an that observed at 170 ◦C (initial reaction rate of
2.0 mmol h gcat−1; 66% Fur yield at 99% conversion, reached at
16 h reaction). For the reaction temperature of 170 ◦C, and at con-
stant ITQ-2 catalyst bulk density and catalyst/xylose mass ratio,
changing the W:T solvents ratio affects the Fur yields: for a W:T
ratio of 0.5:0.5, 51% Fur yield was reached at 98% conversion and
16 h reaction, which is lower than that observed for the W:T ratio
of 0.3:0.7.
Fig. 10. Kinetic profiles of the reaction of xylose (a), and furfural yield versus xylose
conversion curves (b), for H-MCM-22(24) (×), ITQ-2 (ꢀ), and H2SO4 (ꢂ), used as
catalysts. Reaction conditions: xylose:solid acid catalyst mass ratio of 3:2 (or 4 mM
H2SO4), solely water as solvent, 170 ◦C.
3.2.3. Catalytic performances of H-MCM-22(24) and ITQ-2 using
solely water as solvent
The reaction of xylose was further investigated in the presence
of ITQ-2 and H-MCM-22(24), using solely water as solvent (denoted
W reaction system), at 170 ◦C; the catalyst bulk density and cata-
lyst/xylose ratio is the same as that for the W:T biphasic system
(the initial concentration of xylose in water is lower in the case
of the W reaction system). The kinetic profiles for ITQ-2 and H-
MCM-22(24) are very similar until 8 h reaction (Fig. 10(a)), which
correlates with the similar total concentration of acid sites ([L] + [B])
for the two catalysts (Table 1). The Fur yield versus xylose con-
version curves are comparable, giving Fur yield of 52–54% at 97%
conversion (Fig. 10(b)). The comparable performances of these two
catalysts using the W reaction system parallels that observed for
the biphasic W-T system. For the two solvent systems, at reaction
times longer than 8 h, ITQ-2 gives slightly higher conversions than
H-MCM-22(24), possibly due to slower catalyst deactivation (cok-
ing) in the former case. The maximum Fur yields reached at high
xylose conversions are somewhat lower when the simultaneous
extraction of Fur is not performed, due to the enhanced forma-
tion of by-products. Accordingly, TGA analyses of the used catalysts
revealed higher amounts of organic deposits using the W reaction
system (18–21 wt% for the solids recovered after 24 h reaction) in
comparison to the W-T one (12 wt%). The catalytic contribution
to the decomposition of Fur was minor: less than 5%, obtained in
separate catalytic tests using Fur as the substrate instead of xylose.
Hence, Fur loss reactions may be essentially due to its reaction with
xylose or intermediates of the reaction of xylose.
To get insight into the type of by-products formed, the reaction
of xylose was carried out in the presence of ITQ-2 using D2O as
solvent at 170 ◦C for 24 h, and the reactions mixture was analysed
by 1H and 13C NMR spectroscopy. The presence of xylose in the
reaction solution was hardly detected in the spectra (not shown),
which is consistent with the catalytic results (97% conversion at
24 h). The main peaks were relative to Fur: 1H NMR, 9.4, 7.8, 7.5 and
6.6 ppm; 13C NMR, 183.6, 155, 153.1, 128.7 and 116.2 ppm. Formic
acid was detected (169 ppm and 8.1 ppm in the 13C and 1H NMR
spectra, respectively), which may be formed via the decomposi-
tion of Fur [19,54–56]. The 1H NMR spectrum exhibited weak to
very weak signals in the range of 3–4.5 ppm which may be due to
carbohydrate H–C–O or H2C–O groups; weak peaks below 2 ppm
may be assigned to methyl/methylene carbon atoms. Fragmenta-
tion reactions of xylose can take place to form oxygenated aliphatic
compounds [56,57]. Scheme 2 shows possible pathways for the
formation of by-products in the xylose-to-Fur reaction systems
[1,19,54–56,58].

M.M. Antunes et al. / Applied Catalysis A: General 417–418 (2012) 243–252
251
Acknowledgements
This work was partly funded by the FCT, POCTI and FEDER
(project POCTI/QUI/56112/2004). The authors thank Dr U. D.
Morales (Instituto de Tecnología Química, CSIC, Universidad
Politécnica de Valencia) for the valuable help with the recipe of ITQ-
2; Marta A. C. Ferro for help with the TEM studies, and acknowledge
the Portuguese network of electron microscopy, the RNME, FCT
Project REDE/1509/RME/2005; Prof. C.P. Neto (University of Aveiro)
for helpful discussions; Dr. F. Domingues (University of Aveiro) for
access to HPLC equipment. S.L. (SRFH/BPD/23765/2005) and M.M.A.
(SFRH/BD/61648/2009) are grateful to the FCT for grants.
Scheme 2. Simplified representation of possible reaction pathways for the forma-
tion of by-products in the reaction of xylose to furfural, with reference to literature
data [1,19,54–56,58].
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.12.046.
For comparison, the reaction of xylose was carried out in the
presence of 4 mM H2SO4 as catalyst instead of the solid acids; the
initial amount of liquid acid was comparable to the total amount
of acid sites in the loaded solid acid catalysts. Although the kinetic
profile until 8 h reaction was similar for H2SO4 and the solid acid
catalysts, H2SO4 gave much lower conversions at longer reaction
times (Fig. 10(a)). The pronounced retardation of the reaction of
xylose in the presence of H2SO4 may be due to the partial decom-
position of the catalyst through its possible participation in the
formation of sulfur-containing by-products [59,60] and/or to the
decrease in the concentration of active Brönsted acid species due
to the protonation of Fur [60]. The Fur yield versus conversion pro-
file for H2SO4 (55% Fur yield at 93% conversion) is comparable to
those for the solid acid catalysts (Fig. 10(b)).
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