group 6 metal oxide-carbon aerogels. their synthesis, characterization and catalytic activity in the...

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: cmoreno@goliat.ugr.es

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.

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