group 6 metal oxide-carbon aerogels. their synthesis, characterization and catalytic activity in the...
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Group 6 metal oxide-carbon aerogels. Their synthesis, characterizationand catalytic activity in the skeletal isomerization of 1-butene
Carlos Moreno-Castillaa,*, Francisco Jose Maldonado-HoÂdara,Jose Rivera-Utrillaa, Enrique RodrõÂguez-CastelloÂnb
aGrupo de InvestigacioÂn en Carbones, Departamento de QuõÂmica InorgaÂnica, Facultad de Ciencias,
Universidad de Granada, 18071 Granada, SpainbDepartamento de QuõÂmica InorgaÂnica, Universidad de MaÂlaga, MaÂlaga, Spain
Received 11 December 1998; received in revised form 1 March 1999; accepted 5 March 1999
Abstract
Group 6 metal±organic aerogels were prepared by polymerization of a resorcinol±formaldehyde mixture which contained
metallic salts of the elements of this group. The aerogels prepared were carbonized both at 773 and 1273 K to obtain the
corresponding metal oxide±carbon aerogels. All these materials were characterized by different techniques including gas
adsorption, mercury porosimetry, XRD, XPS and decomposition of isopropanol to determine their textural characteristics,
state of the metallic phase and surface acidity, respectively.
The metal oxide±carbon aerogels were tested in the isomerization reaction of 1-butene. The most active catalyst was the
tungsten oxide±carbon aerogel pretreated at 773 K, which presented an isobutene yield of 40% at 448 K. This was due to its
high surface acidity and more adequate porosity. Isobutene and trans-2-butene were the main reaction products obtained and
no C3±C5 and C2±C6 by-products were observed. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Organic aerogels; Metal oxide±carbon aerogels; 1-Butene isomerization
1. Introduction
Organic aerogels have been synthesized recently
following Pekala's method to prepare aerogels by
polycondensation of certain organic monomers [1±
11]. Carbon aerogels and activated carbon aerogels
can be obtained from the corresponding organic aero-
gels by carbonization and activation [8±11]. These
materials present very large meso- and microporosity.
Very recently [11,12], we demonstrated that transi-
tion-metal-containing carbon aerogels can also be
prepared by adding the adequate metallic salt to the
original recipe of the aerogel. The textural character-
istics of these materials depended on the nature of the
metal. Thus, whereas the sample containing a very
small amount of Pt had the largest meso- and macro-
pore volume (0.822 and 2.982 cm3 gÿ1), those con-
taining either Pd or Ag were essentially microporous.
Hence, metal±carbon aerogels are very new materials
which, due to their method of synthesis, can provide
the metal dispersed inside a carbon matrix with a very
well developed porosity. This could, therefore, be a
Applied Catalysis A: General 183 (1999) 345±356
*Corresponding author. Tel.: +34-958-243323; fax: +34-958-
248526; e-mail: [email protected]
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 0 6 8 - X
suitable method to prepare carbon supported metal
catalysts.
It is known [13,15] that in conversion reactions of
hydrocarbons, such as the skeletal isomerization of n-
butene, microporous materials with proper channel
dimensions and pore structure as well as acidity are
required. Thus, the high selectivity to isobutene and
catalytic activity of 10-membered ring zeolites in the
above reaction can be explained by shape selectivity of
these materials [14±18]. These catalysts are selective
since intermediates leading to by-products cannot be
formed freely inside the pores and channels as well as
deactivation is slow because polymerization is also
restricted [13,15].
Bulk tungsten oxide is one of the catalysts that have
been found to be active in the skeletal isomerization of
1-butene due to its strong acid sites [19±25]. The use
of tungsten oxide-carbon aerogels as catalysts in this
reaction could have the advantage, against other tung-
sten oxide catalysts, that the porosity of the carbon
matrix within which is trapped the metal could control
the formation of dimers and polymers leading to by-
products, due to the shape and size of the pores. Thus,
they would act as materials with high shape selectivity,
similar to the above zeolite catalysts, which would
allow to control the activity, selectivity and deactiva-
tion of the tungsten oxide-carbon aerogel.
For this reason, we have prepared a tungsten oxide±
carbon aerogel to be used as catalyst in the skeletal
isomerization of 1-butene. In addition, other group 6
metal oxide±carbon aerogels were prepared because
chromium and molybdenum oxides have also been
found to be active in conversion reactions of hydro-
carbons [25±27]. The skeletal isomerization of 1-
butene is of great practical interest because isobutene
is used to produce methyl-tert-butyl-ether (MTBE), an
octane booster additive in gasoline. To our knowledge,
this is the ®rst time that these metal±oxide-containing
carbon aerogels have been synthesized, characterized
and used in the above reaction.
2. Experimental
Four organic aerogels were synthesized by poly-
merization of resorcinol and formaldehyde following
Pekala's method [1,11,12]. Brie¯y, 24.7 g of resorci-
nol and 36.2 g of formaldehyde (37 wt%) were mixed
with the corresponding amount of metal precursor
dissolved in 33.4 g of distilled water. Chromium
nitrate, ammonium heptamolybdate and ammonium
tungstate were used as catalyst precursors. The
amount of these compounds added to the solutions
were calculated to obtain 1% by weight of the metal in
the initial solution. Another aerogel, to be used as
blank, was prepared with the same recipe but without
adding any metal salt.
The mixtures were stirred to obtain homogeneous
solutions which were cast into glass molds (25 cm
length�0.5 cm internal diameter). After the curing
period, the gel rods were cut in 5 mm pellets and
supercritically dried with carbon dioxide to form the
corresponding aerogels. The aerogels will be referred
to as ACr, AMo, AW and A (the blank), respectively.
Pyrolisis of the aerogels to obtain the corresponding
carbon aerogels was carried out in N2 ¯ow,
100 cm3 minÿ1, by heating up to 773 or 1273 K with
a heating rate of 1.5 K minÿ1 and a soaking time of
5 h. The carbon aerogels will be referred to in the text
by adding the carbonization temperature to the aerogel
name. The aerogel Awas only carbonized at 1273 K to
obtain sample A1273. The exact metal content of the
supported catalyst was obtained by burning a fraction
of it in a thermobalance at 1123 K under air ¯ow up to
constant weight.
Textural characteristics of all samples were
obtained by adsorption of N2 and CO2 at 77 and
273 K, respectively, and mercury porosimetry up to
4200 kg cmÿ2 (Quantachrome Autoscan 60 Pororosi-
meter). The BET equation was applied to the N2
adsorption data to obtain the nitrogen surface area,
SN2, and the Dubinin±Radushkevich equation to the
CO2 adsorption data, from which the micropore
volume, W0, and the structural constant, B, were
obtained [28,29].
The characteristic adsorption energy, E0, is related
with the B constant via the following equation:
E0�kJ molÿ1� � 8:3244� 10ÿ3=B1=2: (1)
E0 is a function of the micropore size distribution of
the adsorbent. It has been shown [30] that for slit-
shaped and cylindrical model micropores, the adsorp-
tion potential is related [31] to the width of these
pores, L0, according to following equation:
L0 � 4:699 exp�ÿ0:666 E0�: (2)
346 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356
From mercury porosimetry experiments the follow-
ing parameters were obtained: pore size distribution,
PSD, of pores with a diameter greater than 3.7 nm;
surface area of these pores, which will be referred to as
the external surface area, Sext; pore volume corre-
sponding to pores with a diameter between 3.7 and
50 nm, V2, referred to as mesopore volume, although
in fact the mesopore volume is within 2 and 50 nm
[32]; pore volume of pores with a diameter larger than
50 nm, or macropore volume, V3, and particle density,
�p.
X-ray diffraction experiments, XRD, were carried
out with a Phillips PW1710 diffractometer (40 kV and
40 mA) using Cu K� radiation.
X-ray photoelectron spectroscopy, XPS, measure-
ments were carried out with a Physical Electronic
5700 equipment with Mg K� X-ray excitation source
(h��1253.6 eV) and hemispherical electron analyzer.
Accurate (�0.1 eV) binding energies were determined
with respect to the position of the C1s peak at 284.8 eV.
Prior to the analysis, the samples were heated in situ at
673 K under high vacuum and introduced in the
analysis chamber without any contact with the atmo-
sphere. The residual pressure in the analysis chamber
was maintained below 10ÿ9 Torr during data acquisi-
tion. Survey and multiregion spectra were recorded at
W4f, Mo3d, Cr2p, O1s and C1s photoelectron peaks.
Each spectral region of photoelectron interest was
scanned several times to obtain good signal-to-noise
ratios. The atomic concentrations were calculated
from the photoelectron peak areas, using Shirley back-
ground subtraction [33] and sensitivity factors pro-
vided by the spectrometer manufacturer PHI [34].
In order to evaluate the surface acidity of the
catalyst, the decomposition of isopropanol was stu-
died [35±40]. Catalytic test were carried out in a glass
microreactor at atmospheric pressure with 0.2 g cat-
alyst. The reaction was performed in a He ¯ow
saturated with the alcohol at 273 K. The total ¯ow
rate was 62 cm3 minÿ1 and the partial pressure of the
alcohol 8.44 Torr. Analysis of reaction products was
done by on-line gas chromatography using a Perkin-
Elmer gas chromatograph, model 8500, with ¯ame
ionization detector and a column Carbopack B80/120.
The isomerization of 1-butene was performed in the
same experimental conditions as before. In this case,
the reactor was fed with a continuous ¯ow,
60 cm3 minÿ1, of 1-butene/He mixture (1% by volume
of 1-butene), and the reaction temperatures ranged
from 323 to 698 K depending on the catalyst. Products
were analyzed by gas chromatography, using a ¯ame
ionization detector and a GS-alumina capillary col-
umn supplied by J and W Scienti®c. Conversion was
de®ned as:
C �%� � �1-butene�feed ÿ �1-butene�effluent
�1-butene�feed
� 100
(3)
and selectivity to each product as:
S �%� � Nz
�1-butene�feed ÿ �1-butene�effluent
� 100:
(4)
In both the equations, reactant and products are
expressed in moles. Nz in Eq. (4) is the amount of
hydrocarbon z produced in the reaction.
Prior to any catalytic test, the samples were heated
in a He ¯ow at 673 K for 4 h.
3. Results and discussion
3.1. Characterization of catalysts
All the aerogels were opaque. ACr showed an
intense red color and AMo and AW were brown.
The aerogel ACr was prepared with a chromium salt
with the metal in oxidation state (III) whereas with the
other aerogels the metal initially was at oxidation state
(VI). It is noteworthy that when aerogel ACr was
prepared with (NH4)2 CrO4 as precursor salt, it was
not as rigid or hard as the other aerogels and was
converted into powder very easily. After carbonizing
this aerogel in an inert gas ¯ow and when it was
exposed to the atmosphere at room temperature, it
burnt spontaneously. For this reason chromium nitrate
was used as precursor salt.
The textural characteristics of the samples are
compiled in Table 1. The organic aerogel and the
metal oxide±carbon aerogels were mainly macropor-
ous materials with no mesoporosity at all. The macro-
pore volume varied according to the sequence
A�ACr�AMo�AW. After carbonization, the V3
volume increased signi®cantly in Cr-containing±
metal carbon aerogels, and this increase was greater
when carbonization was carried out at 773 K instead
C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356 347
of at 1273 K. The A1273, AMo1273 and AW1273
showed lower V3 values than the corresponding aero-
gels or carbon aerogels heated at 773 K.
PSD of the metal±organic aerogels and metal
oxide±carbon aerogels are shown in Fig. 1(a)±(c).
Samples ACr, ACr773 and ACr1273 show a similar
PSD with pores wider than 2000 nm in diameter.
Samples AMo, AMo773 and AMo1273 also show a
similar PSD with their maximum centered at around
1500 nm in diameter. Therefore, the PSD of Cr and
Mo-containing samples, did not signi®cantly change
from the metal±organic aerogel to the metal oxide±
carbon aerogel. However, this is not the case for W-
containing samples. Thus, the increase in V3 (Table 1)
from AW to AW773 is due to the development of
porosity in the 160 nm diameter range, in which a
bimodal PSD for sample AW773 appears. The
decrease in V3 for sample AW1273 in comparison
to sample AW773 is due to the disappearance of this
peak at 160 nm.
The microporosity, W0, depended on the metal
present, and in the case of the aerogels decreased in
the order ACr>AMo>AW, whereas their mean micro-
pore size, L0, showed the opposite trend (Table 1).
After carbonizing, W0 of the metal oxide±carbon
aerogels increased signi®cantly, and this increase
was greater when the carbonization was carried out
at 1273 K, probably due to the greater weight loss at
this temperature (column 2 of Table 1). There was also
a simultaneous decrease in L0. However, the micro-
porosity of the metal oxide±carbon aerogels obtained
at 773 or 1273 K increased in the reverse order as that
of the aerogels. The reduction in macropore volume
and the increase in microporosity has been previously
described [12,14] in other carbon aerogels.
Table 1
Textural characteristics of aerogels and metal-carbon aerogels
Sample Weight
loss (%)
Metal
(%)
�p
(g cmÿ3)
SN2
(m2 gÿ1)
V3
(cm3 gÿ1)
V2
(cm3 gÿ1)
W0
(cm3 gÿ1)
E0
(kJ molÿ1)
L0
(nm)
A ± ± 0.69 n.d. 0.695 0.000 n.d. n.d. n.d.
ACr ± n.d.a 0.69 n.d. 0.707 0.000 0.108 22.21 1.13
AMo ± n.d. 0.52 n.d. 1.392 0.000 0.096 21.45 1.19
AW ± n.d. 0.47 n.d. 1.429 0.000 0.088 21.11 1.22
ACr773 43.6 3.6 0.56 397 0.994 0.000 0.174 23.61 1.03
AMo773 38.8 1.9 0.50 481 1.399 0.000 0.179 22.35 1.12
AW773 41.0 1.4 0.43 528 1.525 0.047 0.204 23.13 1.06
A1273 54.0 ± 0.72 ± 0.628 0.000 0.266 22.85 1.08
ACr1273 50.3 4.2 0.64 402 0.901 0.000 0.184 22.92 1.08
AMo1273 55.5 2.7 0.53 493 1.297 0.000 0.260 22.80 1.08
AW1273 51.0 2.1 0.54 610 1.320 0.011 0.262 23.14 1.06
a Non-determined.
Fig. 1. Pore size distribution obtained by mercury porosimetry of
aerogels and their carbonized derivatives. (a) Aerogels; (b) AX773
series; (c) AX1273 series. (X�Cr (Ð), Mo (....), W (ÐÐÐ)).
348 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356
XRD patterns of some samples are shown, as
examples, in Fig. 2. The organic aerogels presented
no diffraction peaks, indicating that these are amor-
phous materials. After carbonization, at both 773 and
1273 K, only two bands around 2� equal to 268 and
438 were observed, which correspond to the graphite
microcrystals of the carbon matrix. No diffraction
peaks corresponding to the metal phase were observed
in any case except in sample AW1273 in which
diffraction peaks corresponding to tungsten carbide
and rhombic WO3 were detected. Therefore, in this
case, the carbon matrix was able to reduce partially, at
1273 K, the tungsten oxide to metallic tungsten and
react with the carbon matrix to yield the corresponding
carbide. This phenomenon has been described else-
where [41] with other tungsten catalysts supported on
activated carbons and heat treated at 1273 K in an inert
atmosphere.
The XRD patterns of all metal oxide±carbon aero-
gels that did not present any diffraction peak corre-
sponding to the metallic phase (ACr773, ACr1273,
AMo773, AMo1273 and AW773) indicate that, at this
measuring scale, the metallic phase was well dispersed
and intimately linked or trapped inside the organic
matrix, forming either amorphous or microcrystallites
less than 4.0 nm in size.
XPS was used to provide information about the
metal oxidation states, their proportion and the metal
dispersion. The spectra of the Cr2p, Mo3d and W4f
levels for the different catalysts studied are depicted in
Figs. 3±5, respectively. These ®gures also include the
curve-®tted spectra. The BEs of the principal peaks,
the percentage of the different metallic species and the
metal/C surface atomic ratio are compiled in Table 2.
In both chromium-oxide-containing carbon aero-
gels the metal was in two oxidation states, (III) at
576.9 eV and (VI) at 579.2 eV in agreement with
values reported in the literature [26,42,43]. The per-
centage of both species in samples ACr773 and
ACr1273 were similar. The metal/C ratio dropped
sharply, indicating a loss of dispersion, when the
carbonization temperature increased from 773 to
1273 K.
Fig. 2. XRD patterns of some aerogels and metal oxide-carbon
aerogels. ((~) graphite, (*) WC, (*) WO3).
Fig. 3. Curve-fitted XPS spectra for chromium catalysts. (a)
ACr773; (b) ACr1273.
C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356 349
Sample AMo773 showed two doublets with the
BE values of the Mo 3d5/2 peaks at 233.4 and
232.1 eV, which were characteristic of Mo(VI) and
Mo(V), respectively. Thus, Hercules et al. [44]
reported, for Mo supported on Al2O3, BE values
of 233.1 and 231.8 eV for Mo(VI) and Mo(V),
respectively. A large proportion of the total Mo,
82%, was present as Mo(V). Sample AMo1273
showed 55% of Mo(V) and 45% of Mo(III), the
BE of this last species was also similar (228.8 eV)
to that found in the case of Mo supported on Al2O3
[44]. Values of the metal/C surface atomic ratio
re¯ected a decreased dispersion with increased
carbonization temperature.
Fig. 4. Curve-fitted XPS spectra for molybdenum catalysts. (a)
AMo773; (b) AMo1273.Fig. 5. Curve-fitted XPS spectra for tungsten catalysts. (a) AW773;
(b) AW1273.
Table 2
XPS parameters derived from curve fitting envelopes for the different catalysts
Catalyst BE (eV) Chemical state
of metal
Proportion
(%)
Surface atomic
ratio M/C (�103)
ACr773 576.9 Cr (III) 72 21.9
579.2 Cr (VI) 28
ACr1273 576.9 Cr (III) 71 6.6
579.1 Cr (VI) 29
AMo773 231.1 Mo (V) 82 7.6
233.4 Mo (VI) 18
AMo1273 228.7 Mo(III) 45 4.4
232.2 Mo (V) 55
AW773 35.7 W (VI) 100 1.3
AW1273 32.1 WC 7 2.1
35.9 W (VI) 93
350 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356
Sample AW773 only presented one doublet with the
BE of the W 4f7/2 peak at 35.7 eV, corresponding to
W(VI). However, sample AW1273 showed two doub-
lets with the BEs of the W 4f7/2 peaks at 32.1 and
35.9 eV which corresponded to W (IV) of tungsten
carbide and W(VI) of WO3, respectively. The ®rst
value is very near to that found in the literature for
tungsten carbide (32.2 eV) [45]. The tungsten carbide,
present in a proportion of 7%, and the tungsten oxide
were also detected by XRD as mentioned above. In
this case there is a segregation of the metal to the
surface of the catalyst particles, since there is an
increase in the M/C surface atomic ratio.
The acid±base character of the active sites of the
oxides can be determined by means of indirect deter-
minations, such as the measurements of the catalytic
activity for the decomposition of isopropanol as pro-
posed by Ai [35,36] and used by other authors [37±
40]. Ai assumed that the dehydration of isopropanol is
catalyzed by an acid site, whereas the dehydrogena-
tion is catalyzed by both acid and base sites through a
concerted mechanism. Thus, the acidity is related to
the reaction rate of dehydration to obtain propene, rp,
and the basicity to the ratio of the reaction rates for
dehydrogenation (to acetone) and dehydration, ra/rp.
The results obtained are compiled in Table 3, and
show that when the catalyst were obtained at 773 K
their acidity increased in the order ACr773�AMo773<AW773, which is the same to that found
by other authors [25,37] with the bulk metal oxides.
For the catalysts obtained at 1273 K, the acidity varied
following the same order. However, the acidity of the
catalysts AMo1273 and AW1273 was lower than that
of the respective catalysts obtained at 773 K. This is
due to a decrease in the oxidation state of the metal, as
shown by XPS, and of the dispersion of the metallic
phase with the increase in the treatment temperature of
the catalyst. The decrease in acidity of the Mo-con-
taining carbon aerogel was the highest, because in this
case, the lowering in oxidation state of the metal was
also the highest (see Table 2). Basicity of the catalysts,
given by the ra/rp ratio, increased in the reverse order
to that found for the variation in acidity.
3.2. Catalytic behavior
The catalytic performance of the samples was
studied at temperatures ranging from 323 to 698 K,
using the same catalyst charge, feed composition and
¯ow in all cases. Tables 4 and 5 summarize the results
obtained with both series of catalysts, as a function of
the reaction temperature. The tungsten-containing±
carbon aerogels were much more active than those
of chromium or molybdenum, showing these two
catalysts a similar behavior. The greater catalytic
activity of tungsten oxide with respect to other oxides
has been described previously [15,21,24] and it is due
to the high acidity of this oxide. When comparing both
catalyst series, a loss of activity was observed when
the metal oxide±carbon aerogels were obtained at
1273 K.
Isobutene and trans-2-butene were the main reac-
tion products being the selectivity to cis-2-butene very
low. Chromium and molybdenum catalysts also
favored the dehydrogenation reaction to butadiene,
especially when the catalysts were obtained at 1273 K.
Only a very small amount of butane and isobutane
were found in some experiments. The source of
hydrogen to hydrogenate the 1-butene and isobutene
could be the surface species precursors for coke
[20,46]. The fact that no C3±C5 or C2±C6 by-products
were observed, indicates that the reaction, as in the
case of small pore zeolites [14,17,18], follow a mono-
molecular mechanism, and therefore, that there was no
dimerization.
The formation of butenes against isobutene [21] is
thermodynamically favored as the reaction tempera-
ture increases. According to Ponec [15,47], the iso-
butene yield on the best contemporary catalyst is about
40%, as a result of thermodynamical limitations.
Fig. 6 shows the isobutene yield as a function of
reaction temperature for the series of catalysts
obtained at 773 K. The isobutene yield of catalyst
Table 3
Acidity and basicity of the catalysts evaluated trough the catalytic
decomposition of isopropanol at 403 K
Catalyst rp ra rp/ra
mmol g catÿ1sÿ1
ACr773 0.0006 0.0348 58.00
AMo773 0.0515 0.0285 0.55
AW773 0.2689 0.0055 0.02
ACr1273 0.0007 0.0147 18.37
AMo1273 0.0035 0.0118 3.37
AW1273 0.1962 0.0162 0.08
C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356 351
AW773 rapidly increased with temperature, reaching
a value of 40% at 448 K. This activity for isobutene
formation is higher and obtained at a lower reaction
temperature than in the case of other tungsten oxide
catalysts reported in the literature [20,21]. The iso-
butene yield for AW1273 was lower than AW773,
especially at higher temperatures.
The surface acidity of AW1273 is slightly lower
than that of AW773, being the dispersion of the former
also smaller since the WO3 particles were detected
by XRD. These two facts make AW1273 somewhat
less active than AW773, although the presence of a
small proportion of tungsten carbide could act also
as catalyst for the isomerization of 1-butene due
to this compound has been found active in such
reaction [48].
The evolution of the conversion obtained at 498 K
as a function of the time on stream for each catalyst is
depicted in Fig. 7. The ACr773 and AMo773 catalysts
showed initial conversion values of around 8±9%
which reached the steady state after approximately
3 h of reaction, with a decrease in the catalytic activity
more than 50% in this period. For AW773, the con-
version was much higher and decreased from around
Table 4
Conversion and selectivity obtained at different reaction temperatures
Catalyst Temperature (K) Conversion (%) Selectivity (%)
Isobutene T-2-Butene C-2-Butene 1,3 Butadiene
ACr773 323 0.19 95.2 4.8 0.0 0.0
448 1.14 63.4 19.2 15.3 1.9
573 8.35 52.6 41.6 2.8 3.0
673 21.76 47.9 47.9 0.8 3.4
AMo773 323 0.42 89.9 10.1 0.0 0.0
423 2.22 70.6 21.4 7.7 0.2
523 4.83 63.0 28.7 5.4 3.0
623 15.40 59.2 34.9 1.2 4.7
AW773 348 8.30 65.9 31.4 2.7 0.0
373 23.86 60.8 38.4 0.8 0.0
448 83.71 46.8 53.1 0.2 0.0
473 78.80 46.5 53.3 0.2 0.0
WHSV�0.5 hÿ1, P1-but/PHe�0.01.
Table 5
Conversion and selectivity at different reaction temperatures
Catalyst Temperature (K) Conversion (%) Selectivity (%)
Isobutene T-2-Butene C-2-Butene 1,3-Butadiene
ACr1273 423 0.28 90.7 8.0 0.0 1.3
523 0.71 61.6 14.9 0.0 23.5
623 3.50 47.3 28.3 4.6 19.9
673 9.02 42.5 38.6 3.1 15.8
AMo1273 498 1.58 68.6 24.7 0.0 6.7
598 7.19 51.2 33.6 0.0 15.2
648 18.50 49.5 38.6 0.0 11.9
698 31.08 45.8 39.6 0.0 14.6
AW1273 323 1.80 64.1 20.5 15.2 0.3
373 18.93 63.1 36.0 0.8 0.1
423 46.15 63.7 35.9 0.4 0.1
473 50.04 54.6 44.7 0.3 0.4
WHSV�0.5 hÿ1, P1-but/PHe�0.01.
352 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356
Fig. 6. Isobutene yield as a function of the reaction temperature for the different catalysts ((&) ACr773, (~) AMo773, (^) AW773, (})
AW1273).
Fig. 7. (a) Evolution of the conversion with the time on stream for the catalyst series treated at 773 K ((^) fresh catalyst, (&) regenerated).
Reaction temperature: 498 K; (b) evolution of the conversion with the time on stream for the catalyst series treated at 1273 K. (^) Fresh
catalyst, (&) regenerated). Reaction temperature: 498 K.
78% to 55%, corresponding to a much lower loss of
activity (around 30%) than in the other cases.
When the catalysts were regenerated for 1 h in He
¯ow at 673 K (similar to the conditions used in the
catalyst pretreatment) the regenerated ACr773 showed
an initial conversion value of about threefold that
obtained with the fresh catalyst, although this quickly
decreased to the conversion levels previously
observed. The deactivation curve for the regenerated
Mo catalyst was coincident with that corresponding to
the fresh catalyst. In the case of regenerated AW773,
almost the same conversion values were observed with
relation to the fresh one. Therefore, catalyst deactiva-
tion was much greater for the ACr773 and AMo773
catalysts than for the AW773 one.
The greatest activity of AW773 is due to both its
high surface acidity (the highest of all catalysts stu-
died) and porosity, because this sample has the largest
meso and macropore volumes and one of the largest
micropore volumes, which would favour the diffusion
of reactant and products to and from the microporos-
ity. The pore structure of catalyst AW773 will play
also an important role in its deactivation, as shown
above, because it will avoid the formation of inter-
mediates leading to by-products that deactivate the
metal oxide catalyst.
In the catalyst series obtained at 1273 K, Fig. 7(b),
the catalytic activity of Cr and Mo catalysts was so low
that no differences were observed between the fresh
and regenerated catalyst. However, regeneration of
catalyst AW1273 did not increase its activity and
the regenerated catalyst was deactivated faster than
the fresh one.
Variation of product distribution versus reaction
time, with the catalyst series obtained at 773 K, is
shown in Fig. 8. The selectivity to isobutene remained
practically constant in all catalysts. The selectivity to
cis-2-butene increased with reaction time for catalysts
ACr773 and AMo773 and, simultaneously, the selec-
tivity to trans-2-butene decreased. The AW773 did not
produce cis-2-butene or butadiene and the selectivity
to t-2-butene and isobutene were similar in both cases.
4. Conclusions
The aerogels and the metal oxide±carbon aerogels
were essentially macroporous materials, and a sig-
ni®cant development of microporosity was observed
after carbonization of the former. The tungsten oxide±
carbon aerogel obtained at 773 K, sample AW773,
showed the largest macro and mesopore volume and
one of the largest micropore volumes of all the cat-
alysts prepared. XRD patterns of the catalysts did not
present any diffraction peak corresponding to the
metallic phase, which indicates that the metal oxide
phase was well dispersed inside the carbon matrix.
The diffraction peaks corresponding to WO3 and WC
only appeared in the case of the tungsten oxide±carbon
aerogel obtained at 1273 K, AW1273, which was also
con®rmed by XPS.
The surface acidity of the metal oxide±carbon
aerogels, prepared at both 773 and 1273 K, was
evaluated by the catalytic decomposition of isopropa-
nol and increased in the order chromium oxide<mo-
lybdenum oxide<tungsten oxide±carbon aerogel.
The tungsten oxide±carbon aerogels were much
more active for the 1-butene isomerization than those
of chromium and molybdenum, and the sample
Fig. 8. Evolution of the product selectivity with the time on stream
for fresh catalysts. ((*) Isobutene, (~) trans-2-butene, (*)
butadiene, (~) cis-2-butene). Reaction temperature: 498 K.
354 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356
obtained at 773 K was more active than that obtained
at 1273 K, which is due to its highest surface acidity
and more adequate porosity. Isobutene and trans-2-
butene were the main reaction products. C3±C5 and
C2±C6 by-products were not observed, likely due to an
effect of the porosity, and indicates that the reaction
must follow a monomolecular mechanism. The cata-
lyst AW773 showed an isobutene yield of 40% at
448 K which was higher and obtained at a lower
reaction temperature than in the case of other
tungsten oxide catalysts described in the literature.
This catalyst also showed a lower deactivation with
reaction time than the other catalysts and the regen-
erated catalyst showed a similar conversion to the
fresh one.
Acknowledgements
Financial support by the DireccioÂn General de
EnsenÄanza Superior e InvestigacioÂn Cientõ®ca, Project
no. PB97-0831, is acknowledged.
References
[1] R.W. Pekala, J. Mater. Sci. 24 (1989) 3221.
[2] R.W. Pekala, Mater. Res. Soc. Proc. 171 (1990) 285.
[3] R.W. Pekala, C.T. Alviso, F.M. Kong, S.S. Hulsey, J. Non-
Cryst. Solids 145 (1992) 90.
[4] R.W. Pekala, C.T. Alviso, J.D. Le May, in: L.L. Hench, J.K.
West (Eds.), Chemical Processing of Advanced Materials,
Wiley, New York, 1992, pp. 671±683.
[5] R.W. Pekala, D.W. Schaefer, Macromolecules 26 (1993)
5487.
[6] R.W. Pekala, in: D.W. Schaefer, J.E. Mark (Eds.), Polymer
Based Molecular Composites, Material Resources Sympo-
sium Proceedings, vol. 171, Pittsburgh, PA, 1990, pp. 285±
291.
[7] R.W. Pekala, S.T. Mayer, J.L. Kaschmitter, F.M. Kong, in:
Y.A. Attia (Ed.), Sol±gel Processing and Applications,
Plenum Press, New York, 1994.
[8] Y. Hanzawa, K. Kaneko, R.W. Pekala, M.S. Dresselhaus,
Langmuir 12 (1996) 6167.
[9] H. Tamon, H. Ishizaka, M. Mikami, M. Okazaki, Carbon 35
(1997) 791.
[10] C. Lin, J.A. Ritter, Carbon 35 (1997) 1271.
[11] F.J. Maldonado-HoÂdar, M.A. Ferro-GarcõÂa, J. Rivera-Utrilla,
C. Moreno-Castilla, Carbon, in press.
[12] F.J. Maldonado-HoÂdar, M.A. Ferro-GarcõÂa, J. Rivera-Utrilla,
C. Moreno-Castilla, Eurocarbon'98 1 (1998) 413 Extended
abstracts, Strasbourg (France).
[13] J. Houzvicka, O. Diefenbach, V. Ponec, J. Catal. 164 (1996)
288.
[14] M.A. Asensi, A. Corma, A. MartõÂnez, J. Catal. 158 (1996)
561.
[15] J. Houzvicka, S. Hansildaar, V. Ponec, J. Catal. 167 (1997)
273.
[16] W.Q. Xu, Y.G. Yin, S.L. Suib, C.L. O'Young, J. Catal. 150
(1994) 34.
[17] W.Q. Xu, Y.G. Yin, S.L. Suib, J.C. Edwards, C.L. O'Young, J.
Phys. Chem. 99 (1995) 9943.
[18] M. Boronat, P. Viruela, A. Corma, J. Phys. Chem. 102 (1998)
982.
[19] B.G. Baker, N.J. Clark, in: A. Crucq, A. Frennet (Eds.),
Catalysis and Automotive Pollution Control. Studies in
Surface Science and Catalysis, Elsevier, Amsterdam, vol.
30, 1987, p. 483.
[20] P. Patrono, A. La Ginestra, G. Ramis, G. Busca, Appl. Catal.
A 107 (1994) 249.
[21] L.H. Gielgens, M.G.H. van Kampen, M.M. Broek, R. van
Hardelveld, V. Ponec, J. Catal. 154 (1995) 201.
[22] J.C. Yori, C.R. Vera, J.M. Parera, Appl. Catal. A 163 (1997)
165.
[23] Y. Yoshinaga, M. Kudo, S. Hasegawa, T. Okuhara, Appl.
Catal. A 121(122) (1997) 339.
[24] J. Houzvicka, V. Ponec, Catal. Rev.-Sci. Eng. 39 (1997) 319.
[25] J. Bernholc, J.A. Horsley, L.L. Murrell, L.G. Sherman, S.
Soled, J. Phys. Chem. 91 (1987) 1526.
[26] F. Cavani, M. Koutyrev, F. TrifiroÂ, D. Bartolini, D. Ghisletti,
R. Iezzi, A. Santucci, G. Del Piero, J. Catal. 158 (1996) 236.
[27] R.C. Ramani, D.L. Sullivan, J.G. Ekerdt, J. Catal. 173 (1998)
105.
[28] M.M. Dubinin, in: P.L. Walker, Jr. (Ed.), Chemistry and
Physics of Carbon, Marcel Dekker, New York, vol. 2, 1966, p.
51.
[29] R.C. Bansal, J.B. Donnet, H.F. Stoeckli, Active Carbons,
Marcel Dekker, New York, 1988.
[30] D.H. Everett, J. Chem. Soc., Faraday Trans. 1 (1976) 619.
[31] B. McEnaney, Carbon 25 (1987) 69.
[32] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A.
Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem.
57 (1985) 603.
[33] D.A. Shirley, Phys. Rev. B 5 (1972) 4709.
[34] Physical Electronics, 6509 Flying Cloud Drive, Eden Prairie,
Mn 55344, USA.
[35] M. Ai, J. Catal. 40 (1975) 318.
[36] M. Ai, J. Catal. 40 (1975) 327.
[37] A. Gervasini, A. Auroux, J. Catal. 131 (1991) 190.
[38] P. Afanasiev, C. Geatent, M. Breysse, G. Loudurier, J.C.
Vedrine, J. Chem. Soc., Faraday Trans. 90 (1994) 193.
[39] A. Gervasini, G. Bellussi, J. Fenyvesi, A. Auroux, J. Phys.
Chem. 99 (1995) 5117.
[40] M.A. Aramendia, V. Borau, C. JimeÂnez, J.M. Marinas, A.
Porras, F.J. Urbano, J. Chem. Soc., Faraday Trans. 93 (1997)
1431.
[41] C. Moreno-Castilla, M.A. Alvarez-Merino, F. Carrasco-
MarõÂn, Carbon'97 1 (1997) 288 Extended abstracts Penn
State, PA, USA.
C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356 355
[42] G.C. Allen, M.T. Curtis, A.J. Hooper, P.H. Tucker, J. Chem.
Soc., Dalton Trans. 1675 (1973).
[43] W. GruÈnert, E.S. Shpiro, R. Feldhans, K. Anders, G.V.
Antoshin, Kh.M. Minachev, J. Catal. 100 (1986) 138.
[44] M. Yamada, J. Yasumaru, M. Houalla, D.M. Hercules, J.
Phys. Chem. 95 (1991) 7037.
[45] G.E. McGuire, G.K.K. Schweitzer, T.A. Carlson, Inorg.
Chem. 12 (1973) 2451.
[46] J. Abbot, B.W. Wojciechowski, in: M.J. Phillips, M. Teunan
(Eds.), Proceedings of the Ninth International Congress on
Catalysis, Calgary, The Chemical Institute of Canada,
Ottawa, 1988, p. 206.
[47] J. Houzvicka, R. Klik, L. KubelkovaÂ, V. Ponec, Appl. Catal.
A 150 (1997) 101.
[48] V. Kellev, R. Touronde, G. Maire, Catal. Lett. 47 (1997) 63.
356 C. Moreno-Castilla et al. / Applied Catalysis A: General 183 (1999) 345±356