Metal-doped carbon aerogels as catalysts for the aromatization of n-hexane

Article abstract of DOI:10.1016/j.apcata.2011.09.021

Metal (Cr, Mo, W) - doped carbon aerogels were synthesized from resorcinol-formaldehyde polymerization, characterized textural and chemically and used as aromatization catalysts. Catalytic performance depended on the thermal treatments applied to the samples and on the nature of the metal phases formed (pure carbon is inactive). Only cracking and aromatization reactions were detected and benzene was produced by dehydrogenation and direct 1-6 ring closure. The partial reduction of the metal oxides led to less acidic surfaces enhancing the aromatization versus cracking. Benzene selectivity of 60% without deactivation was obtained. However, when Ni and Co were reduced to the zero valence, strong interactions with the hexane led to a 100% selectivity to methane.

Full text of DOI:10.1016/j.apcata.2011.09.021

Applied Catalysis A: General 408 (2011) 156–162
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Applied Catalysis A: General
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Metal-doped carbon aerogels as catalysts for the aromatization of n-hexane
F.J. Maldonado-Hódar∗
Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada 18071, Granada, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal (Cr, Mo, W) – doped carbon aerogels were synthesized from resorcinol–formaldehyde polymeriza-
tion, characterized textural and chemically and used as aromatization catalysts. Catalytic performance
depended on the thermal treatments applied to the samples and on the nature of the metal phases formed
(pure carbon is inactive). Only cracking and aromatization reactions were detected and benzene was pro-
duced by dehydrogenation and direct 1–6 ring closure. The partial reduction of the metal oxides led to less
acidic surfaces enhancing the aromatization versus cracking. Benzene selectivity of 60% without deac-
tivation was obtained. However, when Ni and Co were reduced to the zero valence, strong interactions
with the hexane led to a 100% selectivity to methane.
Received 16 June 2011
Received in revised form
16 September 2011
Accepted 18 September 2011
Available online 22 September 2011
Key words:
n-Hexane aromatization
Carbon aerogels
Active phase
© 2011 Elsevier B.V. All rights reserved.
Acidity
Pre-treatments
1. Introduction
recently that the catalysts control is due to the entry of hexane into
the lobes of the L-zeolite containing the active sites [7].
The preparation of aromatic compounds from alkanes by cat-
alytic reforming is an interesting reaction from an economic point
of view because they are used as feedstock in many industrial syn-
thetic processes. Platinum is the typical active metal in naphtha
reforming catalysts used in the production of high-octane gasoline.
Industrial catalysts are normally based on Pt, mainly supported
on chlorinated alumina [1] and great efforts have been made to
improve their performance [2–4]. Alternative catalysts are also
being sought using mainly different types of zeolites as Pt-supports
[5–11].
Many papers have tried to explain the exceptional behaviour
of PtKL catalysts first reported by Bernard [5]. The debate on the
performance of this catalyst traditionally centres on its porous and
chemical characteristics. The main hypotheses proposed to explain
the properties of the Pt clusters on basic supports are (i) the role of
the basicity of the support and of the metal–support interactions
(ii) the influence of the geometry of the channels leading to the
concepts of the molecular die and of the structural recognition and
preorganisation of the alkane molecule, and (iii) the platinum clus-
ter size. However, as Meriaudeau pointed out in a classical review
of aromatization [6] none of these hypotheses by itself can fully
explain this effect and the exceptional Pt/KL catalytic performance
for aromatization remains a matter of debate. Thus, it was proposed
In previous papers [8–11], we showed how the ionic exchange of
beta zeolite with alkaline metal simultaneously modifies the acid-
ity, the porosity, the ability to obtain very small Pt-clusters, and the
interaction of these clusters with the neighbouring alkaline atoms
of the zeolite. Pt/KL was used as a reference. The Pt/Csbeta and
Pt/KL samples show similar catalytic performance, which strongly
differs from that of the Pt/Nabeta. The steric constraints induced by
the increase in the size of the alkali result in a decrease in both the
Pt content and the adsorption capacity of the support. The acidity
also decreases as the cation size increases. The acidity favours unde-
sired reactions such as cracking and isomerisation, also inducing
coke deposition. This not only leads to the loss of valuable prod-
ucts, but can also cause catalyst deactivation [8,9]. Furthermore,
the alkali metal strongly affects the size of the Pt particles and their
electronic behaviour as shown following the adsorption of CO by
FTIR. Smaller Pt particles are obtained in the presence of the heav-
ier alkali metals, suggesting that the more basic supports stabilize
the smaller Pt species. The similar behaviour of the Pt/Csbeta and
Pt/KL catalysts emphasizes the combined roles of the alkali and
their porous environment in determining the acid–base properties
of the zeolitic support, the stabilization of the Pt species, and the
catalytic behaviour.
However, very high aromatization selectivity was also described
for other catalytic systems, offering attractive alternatives to the
Pt/zeolites catalysts. Ciuparu [12] obtained values of almost 90%
for aromatization selectivity using Pd supported on MgO–ZrO2
composites. This is favoured by the basicity of the MgO, while
on a ZrO2 rich support isomerization increases at the expense of
∗
Corresponding author. Tel.: +34 958240444; fax: +34 958248526.
E-mail address: fjmaldon@ugr.es
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.021

F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
157
aromatization. Aromatization catalysts based on transition metal
2.2. Procedure of characterization
oxides were also developed. Al-Kandari [13] studied the corre-
lation between the catalytic activity and the surface structure of
molybdenum-based catalysts. Different active Mo-phases were
obtained by reduction as a function of temperature and the
n-hexane aromatization to benzene was shown to be related to
the formation of metallic Mo0. Molybdenum carbide (Mo2C) is
also a traditional alternative to the Pt-catalysts as an aromatiza-
tion catalyst and research in this field is continuously appearing
[14,15]. The n-octane aromatization selectivity reaches in this case
values of 50–60% [15]. Another interesting alternative is the use of
carbon/inorganic oxide composites [16]. The carbon phase itself is
inactive, but it cooperates with the metal oxides (titania, zirconium
or hafnium), so creating a catalyst (80% selectivity) that is highly
selective for the aromatization of n-octane into ethylbenzene and
o-xylene.
In our laboratory we have been studying the preparation of
metal-doped carbon aerogels and their application as catalysts in
certain reactions of energy-related or environmental interest for
many years [17–21]. However, as far as the author knows, car-
bon aerogel-based catalysts have never been used as aromatization
catalysts. In view of the results previously exposed, it seemed inter-
esting to explore the possibilities of these catalysts in this reaction,
because they can be prepared with a large variety of chemical and
structural characteristics. As a first step in this new direction, a
series of carbon aerogels doped with group-6 metals were prepared
using doping or impregnation techniques, and extensively charac-
terized. Their textural and chemical properties were then compared
with their catalytic behaviour.
The textural characteristics of all the samples were obtained
by adsorption of N2 and CO2 at 77 and 273 K, respectively, and
mercury porosimetry up to 4200 kg cm−2 (Quantachrome Autoscan
60 Pororosimeter). 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 mean micropore width,
L0, were obtained [22,23]. The following parameters were obtained
from mercury porosimetry experiments: pore size distribution,
PSD, of pores with a diameter greater than 3.7 nm; pore volume
for pores with a diameter of between 3.7 and 50 nm, V2, referred
to as mesopore volume, although in fact the mesopore volume is
between 2 and 50 nm [24]; pore volume of pores with a diameter
larger than 50 nm, or macropore volume, V3, and particle density,
ꢀp.
The morphology of the sample, the chemical nature and dis-
persion of the metallic phases were studied by XRD, SEM and TEM.
X-ray diffraction experiments, XRD, were carried out with a Phillips
PW1710 diffractometer (40 kV and 40 mA) using Cu K␣ radiation.
SEM experiments were carried out with a ZEISS DSM 950 (30 kV)
microscope. TEM was carried out with a Phillips CM-20 electron
microscope.
In order to evaluate the surface acidity of the catalyst, the
decomposition of isopropanol was studied. Catalytic experiments
were carried out in a quartz microreactor at atmospheric pressure
with 0.15 g catalyst. The reaction was performed in a He flow satu-
rated with the alcohol at 0 ◦C. The total flow rate was 60 cm3 min−1
and the partial pressure of the alcohol 8.44 Torr. Analysis of reaction
products was done by online gas chromatography using a Variant
gas chromatograph, with a flame ionization detector and a column
Carbopack B80/120.
2. Experimental
2.1. Synthesis of the catalysts
2.3. Catalytic performances in the n-hexane conversion
Four organic aerogels were synthesized by polymerization
of resorcinol and formaldehyde. Resorcinol and formaldehyde
(37 wt.%) were mixed in a molar ratio R/F = 1/2 with the corre-
sponding amount of metal precursor dissolved in distilled water.
Chromium nitrate, ammonium heptamolybdate and ammonium
tungstate were used as catalyst precursors. The amount of these
compounds added to the solutions was calculated to obtain 1% by
weight of the metal in the initial solution. Another aerogel, to be
used as a 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 into 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, according to the metal
that contain. Pyrolisis of the aerogels to obtain the correspond-
ing carbon aerogels was carried out in N2 flow, 100 cm3 min−1, by
heating up to 500 or 1000 ◦C with a heating rate of 1.5 K min−1
and a soaking time of 5 h. The carbon aerogels will be referred to
in the text by adding the carbonization temperature to the aero-
gel name. The exact metal content of the supported catalyst was
obtained by burning a fraction of it in a thermobalance at 1123 K
under air flow up to constant weight. The metal-doped carbon aero-
gels were used directly as catalysts, because the precursor salts
decompose during the carbonization process. For comparison, a
supported catalyst, with a tungsten loading of 5% w/w was pre-
pared by impregnation of the blank sample A1000 with ammonium
tungstate, this catalyst will be referred to as A1000-5W and was
pretreated 2 h at 500 ◦C in He or H2 flow (60 cm3 min−1) before
reaction.
The aromatization of n-hexane was performed with the same
experimental procedure described for isopropanol decomposi-
tion. In this case, the reactor was fed with a continuous flow,
60 cm3 min−1, of a mixture 50%He/50%H2 bubbled in a hexane sat-
urator cooled to fix the hexane concentration at 2.5% v/v. Products
were analyzed by gas chromatography, using a flame ionization
detector and a column Chromosorb 102. Conversion was defined
on the basis of the n-hexane disappearance and selectivity as the
fraction of hexane molecules transformed into each detected prod-
uct. Prior to any catalytic test, the metal-doped carbon aerogels
were heated in a He flow at 200 ◦C for 2 h to clean their surfaces
while the supported A1000-5W catalyst was pretreated as previ-
ously commented.
3. Results
3.1. Morphology and textural characteristics
The textural characteristics of the samples are set out in Table 1
and the corresponding pore size distributions (PSD) are shown in
Fig. 1. These doped-metal carbon aerogels were mainly macrop-
orous materials, as was the pure carbon aerogel used as a reference
material. Mesoporosity was detected only in the case of W-doped
catalysts, and is quite insignificant. The macropore volume var-
ied according to the sequence ACr500 < AMo500 < AW500 and
decreased with increasing carbonization temperature to obtain
AW1000. For the PSD the opposite occurred (Fig. 1). The larger
macropores correspond to ACr500 with pores of over 2000 nm in
diameter, AMo-500 showed a monomodal PSD centred on pores of

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F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
Table 1
Textural characteristics of metal-doped carbon aerogels.
Sample
Metal (%)
ꢀp (g cm−3
)
SN (m2 g−1
)
V3
(cm3 g−1
V2
W0
L0 (nm)
2
)
ACr500
AMo500
AW500
AW1000
3.6
1.9
1.4
2.1
–
0.56
0.50
0.43
0.54
0.72
0.72
397
481
528
610
470
410
0.99
1.40
1.53
1.32
0.63
0.61
0.00
0.00
0.05
0.01
0.00
0.00
0.17
0.18
0.20
0.26
0.27
–
1.03
1.12
1.06
1.06
1.08
–
A1000
A1000-5W (He/500 ◦C/2 h)
5
carbon gels, many papers have analyzed the control of carbon aero-
gel porosity by adjusting these experimental conditions [21,25–28].
SEM images showing the surface morphology of samples are
depicted in Fig. 2a and b, as an example. They are composed of fused
microbead particles, and the particle size was strongly influenced
by the nature of the metal. The textural properties may be related to
the size of the aerogel microparticles. Thus, the smaller micropar-
ticles give rise to a pore network containing smaller inter-particle
voids which range from macro to mesopores. Results indicate that
the morphology and pore texture of the aerogel depended on the
nature of the metal salt [17–21]. Intra-particle microporosity is
generated during carbonization, thus, W0 and L0 increased with
carbonization temperature due to the large weight loss during car-
bonization.
6
ACr500
AMo500
AW500
AW1000
A-1000
4
2
0
1
10
100
Radius (nm)
1000
10000
Fig. 1. Pore size distribution (PSD) obtained by mercury porosimetry of metal-doped
carbon aerogels.
3.2. Chemical nature and dispersion of the metals
The metal nature and dispersion were studied by XRD and TEM.
XRD did not show any diffraction peaks for the metal phase. These
results are similar to those previously obtained [17] and confirm
the good dispersion of the metals. Previous XPS analysis showed a
partial reduction of Cr(VI) to Cr (III) and from Mo (VI) to Mo (V),
while tungsten remained as W(VI) after carbonization at 500 ◦C.
Fig. 3 shows some examples of the good dispersion obtained in all
doped metal catalysts. It is noteworthy that metals are attached to
or trapped by the organic gels during gelification and metal parti-
cles are formed during the carbonization of the organic aerogels.
Thus, there are good contacts or links between both organic and
inorganic phases. As can be observed, a high dispersion of metal
nanoparticles of different size and morphology is formed on the
carbon surface depending of the metal nature and thermal treat-
ment [17,29], indicating that metal-doped catalysts are resistant
to sintering given the drastic thermal treatments they have under-
gone. Nevertheless, the metal particle size observed is large enough
to produce XRD diffraction, indicating their amorphous character.
around 1500 nm in diameter, while AW-500 presented a bimodal
PSD with two maxima at around 180 and 1500 nm. The increasing
carbonization temperature led to the disappearance of the nar-
rower macropores, while increasing the volume of the larger ones.
The blank sample presents a monomodal macroporosity around
pores of 130 nm in diameter, and is significantly less porous and
therefore denser. The supported A1000-5W catalyst after pretreat-
ment in He flow presents a certain decrease of SN regarding the
2
A1000 support, as consequence of the blockage of some porosity by
the metal phase deposited, but the macroporosity is not apparently
influenced.
The microporosity (W0), and as a result the surface area values
(SN2 ), varied in the same order indicated for the macroporosity but
were enhanced by the increasing carbonization temperature. The
largest micropores (L0) were detected for the AMo-500 catalyst. The
porosity of carbon aerogels was defined by the experimental con-
ditions of four critical steps during the synthesis: (a) gelation (b)
drying (c) carbonization and (d) activation. Since the discovery of
Fig. 2. SEM microphotograph of (a) ACr500 and (b) AW500.

F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
159
Fig. 3. TEM microphotographs of samples (a) ACr500 (b) AMo500 and (c) AW500.
3.3. Determination of the acidity of catalysts by isopropanol
decomposition
fell, which is indicative of a lower acidity strength of the acidic
sites due to the progressive reduction of the oxide by the carbon
matrix with increasing temperature.
Metal oxides behave as acid catalysts in the composite, and
their performance depends on the nature of the metal oxide and
the thermal treatment. Reactions between both metallic and car-
bon phases are favoured by increasing carbonization temperature
(Section 3.2), while the organic carbon phase can be more or less
graphitized [17–20,30,31]. On this basis, the acidic characteristics
of the samples were studied by isopropanol decomposition as pro-
posed by Ai [32,33] and used previously to characterize carbon
aerogels–inorganic oxide composites [17,18,34]. Ai assumed that
the dehydration of isopropanol is catalyzed by acid sites, whereas
dehydrogenation is catalyzed by both acid and base sites through
a concerted mechanism. Thus, the acidity is related to the reac-
tion rate of dehydration to obtain propene, and the basicity to
the ratio of the reaction rates for dehydrogenation (to acetone)
and dehydration. The results obtained are compiled in Fig. 4. As
observed, catalytic isopropanol conversion increased in the order
AW500 > AMo500 > >ACr500 and AW500 was strongly more acidic
than AMo500 and ACr500, producing a higher propene/acetone
ratio (Fig. 4b). When analyze the performance of tungsten-doped
carbon aerogels carbonized at 500 and 1000 ◦C respectively a sim-
ilar activity was detected for both catalysts. However the product
distribution changed significantly and the propene/acetone ratio
3.4. Aromatization of n-hexane
The catalytic behaviour of metal-doped carbon aerogels in the
n-hexane conversion reaction is summarized in Fig. 5. The pure
carbon aerogel used as a reference (A1000) is inactive in n-hexane
conversion in the temperature range used. The amounts of the
various products expected to be produced by the isomerisation
(branched alkanes, methylcyclopentane, and cyclohexane) or inter-
mediate dehydrogenation (olefins) processes are always negligible.
Only small quantities of ethylene and butanes (<1%) were some-
times observed, and were evaluated as cracking products. Thus
cracking (C1–C5) and benzene were the main reaction products.
For metal-doped aerogels, the n-hexane conversion increased lin-
early with temperature but in this sense cracking was progressively
favoured versus aromatization, mainly in the case of AW500. All the
metal-doped carbon aerogels demonstrated a good stability of the
catalytic performance.
As the carbon is inactive, the catalytic performance of the metal-
doped carbon aerogels must be related to the properties of the
oxide phase. If we compare the results for isopropanol and hex-
ane conversion, we can see that the aromatization ability of the
150
B
50
A
100
50
0
25
0
320
370
420
470
520
320
370
420
470
520
T (K)
T (K)
Fig. 4. Variation of (A) isopropanol conversion and (B) propene/acetone ratio against the reaction temperature; ꢀ ACr500, ♦ AMo500, ꢀ AW500 ꢁ AW1000 (0.15 g of catalyst,
flow rate 60 cm3 min−1, isopropanol partial pressure 8.44 Torr).

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F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
a
50
100
75
50
25
0
b
40
30
20
10
0
520
540
560
580
600
520
540
560
580
600
Temperature (ºC)
Temperature (ºC)
Fig. 5. Catalytic performance of metal-doped carbon aerogels (a) n-hexane conversion (b) selectivity to benzene; ꢀ ACr500 ꢁ AMo500 ꢀ AW500 (0.15 g of catalyst,
50%He/50%H2 flow rate 60 cm3 min−1, 2.5%, v/v hexane).
50
40
30
20
10
0
100
80
60
40
20
0
a
b
520
540
560
580
600
620
520
540
560
580
600
620
Temperature (ºC)
Temperature (ºC)
Fig. 6. Influence of the carbonization temperature of tungsten-doped carbon aerogels on their catalytic performance (a) n-hexane conversion (b) selectivity to benzene
(ꢂAW500, ♦AW1000; experimental conditions as in Fig. 5).
catalysts is related at low temperature to their acidic character,
varying at 525 ◦C in the order AW500 > AMo500 > ACr500. However,
with increasing temperature, cracking processes are enhanced on
acid catalysts in such a manner that AW500 shows from 550 ◦C
an aromatization performance similar to ACr500. AMo500 shows
high activity in both processes, indicating a large number of active
centres, and these acid centres are weak enough (Fig. 4b) to main-
tain high aromatization activity. The importance of the strength of
the active sites was also pointed out by comparing the catalytic
performance of AW500 and AW1000. Thus it can be observed in
Fig. 6 that AW1000 is not only more active, but also more selec-
tive than AW500 practically over the whole temperature range.
Together with the transformations of the metallic phase, the sur-
face area development with increasing carbonization temperature
(Table 1) can also favour catalytic activity. Previous authors also
indicated that the acidity of supported or bulk oxides catalysts is a
possible cause of their aromatization ability [35].
In an attempt to improve the activity and selectivity of tungsten
catalysts, a 5%W catalyst (A1000-5W) was prepared by impreg-
nation on the pure carbon aerogel. Two different pre-treatment
atmospheres were used (He or H2) so as to produce a different
distribution of active sites. The catalytic results are shown in Fig. 7.
Again, the activity observed is similar, but the decline in selectivity
is slower when pretreated in H2 flow. It is also noteworthy that
in these experimental conditions aromatization of n-hexane was
favoured against cracking over the whole temperature range
studied (S Ar > 50%). Fig. 8 shows the catalytic performance of this
supported catalyst pretreated in He flow at 525 ◦C. It is noteworthy
80
60
b
a
60
40
20
0
40
20
0
500
525
550
575
600
520
540
560
580
600
Temperature (ºC)
Temperature (ºC)
Fig. 7. Influence of the pretreatment atmosphere (ꢀ He/500 ◦C/2 h, ꢃ H2/500 ◦C/2 h) on the catalytic performance of A1000-5W catalysts (a) Conversion (b) Selectivity to
benzene (experimental conditions as in Fig. 5).

F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
161
stepwise dehydrogenation of the alkane into mono-alkene, diene,
triene followed by catalytic or thermal cyclization, the so-called
‘triene mechanism’ [16]. The bifunctional considerations for the
dehydrocyclization of alkanes suggest that aromatic formation
develops by cyclization to a five carbon ring followed by ring
expansion to a six carbon ring and subsequent dehydrogenation to
aromatics. In our case, the fact that there is no isomerisation prod-
uct under any of the experimental conditions or catalysts tested
leads us to the conclusion that the aromatization of hexane in this
kind of catalyst takes place according to the triene mechanism.
The gaseous n-hexane should be adsorbed on the catalyst sur-
face, with carbons having bonds with the metallic surface leading
to dehydrogenation and weakening the C–C bonds in such a way
that rupture occur forming different fragments depending on the
adsorption position. In hexane conversion, three different positions
C1–C2, C2–C3 and C3–C4 are possible. The C1–C5 hydrocarbons can
therefore be formed by hydrogenolysis occurring on metallic sites
20
10
0
70
60
50
40
30
20
Conversion
S benzene
S Cracking
0
200
400
600
800
1000
1200
Time (min)
Fig. 8. Stability of the catalytic performance of A1000-5W supported catalyst pre-
treated in He flow/2 h/500 ◦C (reaction in continuous flow at 525 ◦C, 0.15 g of catalyst,
50%He/50%H2 flow rate 60 cm3 min−1, 2.5%, v/v hexane).
or by bifunctional hydrocracking reactions involving acid and metal
ꢀ
that the conversion increases at the beginning of the reaction,
which suggests certain activation of the surface sites. This is
accompanied first by an increase in cracking at the expense of
aromatization, however, after that, the opposite was observed,
and the stationary state was reached after around 5 h on stream.
However, the small changes in the slope of the selectivity curves
showing certain oscillations from this point onwards seem to indi-
cate that there is a continuous exchange between aromatization
and cracking sites, probably through the deposition/gasification of
adsorbed molecules or light coke precursors. The catalyst is stable
at 525 ◦C during at least 20 h on stream with an aromatization
selectivity of around 60% at n-hexane conversion values of 12%.
This activation effect disappears at high reaction temperatures.
In fact, conversion and aromatization decrease slowly at the
beginning of the reaction and are quickly stabilized. Similarly,
after pretreatment in H2, aromatization increased at the beginning
of the reaction when reaction temperatures were low and slowly
decreased at high reaction temperatures. No XRD peaks were
detected after pretreatment or even after reaction, suggesting a
good dispersion of the active phase and high sintering resistance,
but limiting the discussion about the nature of the active sites.
In view of these results the performance of Ni and Co-doped car-
bon aerogels was also studied. These metal-doped carbon aerogels
were selected on the basis of a good dispersion and reducibility
of metal nanoparticles, their high porosity and their proven abil-
ity (especially of Ni based catalysts) in aromatization processes
[36,37]. In this case, both materials have a high mesopore volume
and surface area. As occurred previously, both metals are well dis-
persed inside the carbon matrix and do not present DRX peaks.
However, these metals are easily reduced by carbon below 500 ◦C,
according to the Ellhingam diagram, and in fact, both appeared
completely reduced after carbonization at 1000 ◦C [26]. The XRD
patterns for the used catalyst present small peaks corresponding
to Ni0, indicating the sintering and reduction of the metallic Ni-
particles during reaction and a catalytic system like Pt0 supported
on non-acidic supports was expected. However, the two catalysts
only produce the cracking of the n-hexane molecule, with propane
as the main reaction product at very low temperature (at around
150 ◦C) and increasing selectivity to methane with reaction tem-
perature. Only methane is produced from 300 ◦C onwards in both
cases.
sites [38,39]. The coefficient Mf, defined as Mf =
(C2–C5)/C1 pro-
vides information about the metal or acidic character of cracking
reactions. The Mf values described for acid catalysts are greater than
45, while all of our catalysts present Mf values of less than 5. This
also suggests that cracking is produced by hydrogenolysis on metal
sites, and that our catalysts behave as monofunctional catalysts.
Although the aromatization selectivity at low reaction temper-
ature may be related to the acidic character of the composite (only
the stronger and more active sites are able to perform the reac-
tion), with increasing temperature it is clear that these stronger
sites favour cracking at the expense of aromatization. Thus, Mo-500
is the most selective catalyst in the metal-doped series and W1000
is more selective than W500 because of the lower propene/acetone
ratio obtained with these catalysts. The formation of metallic
species with different oxidation states could enhance the catalytic
performance by a synergetic effect. This reduced species can be
formed either by increasing carbonization temperature or by using
reducing atmospheres for the pretreatment of supported catalysts.
However, the reduction of Co or Ni-species until the zero valence
leads to very active catalysts. They are active from 120 to 150 ◦C
while the other doped metal aerogels are only active from 500 ◦C,
but only cracking products were obtained. The high hydrogenolysis
ability of metals such as Ni or Rh was described by previous authors
[37]. Thus, the hydrogenolysis reaction rate in n-heptane conver-
sion using Ni-catalysts was found to be 100 times greater than that
obtained using Pt-catalysts. These results were justified in a strong
adsorption of the alkane to the Ni-particles by terminal C–C bonds,
in such a way that the hydrogenolysis of n-hexane on Ni does not
produce C1 + C5, but the fragment C5 is so strongly linked to Ni that
cleavage continues until n-hexane is converted into six methane
molecules [18]. These results clearly indicate that the reduction
degree of metal-doped carbon aerogels should be optimized.
Regarding the role of the carbon matrix, it was also suggested
[16] that the carbon matrix of the metal–oxide composite enhances
the aromatization ability of the corresponding oxide by favour-
ing dehydrogenation, due to the ability of carbon to interact and
disperse the hydrogen atoms by spill-over. In spite of the low car-
bonization temperature used, the nature of the carbon matrix is
also strongly influenced by the metal present. Thus, some of them,
especially Ni or Cr are highly active inducing the graphitization of
the carbon supports [30]. The good contact between both organic
and inorganic phases needed to promote the hydrogen spill-over is
guaranteed by the synthesis procedure.
4. Discussion
As commented in the introduction, we have previously fit-
ted the characteristics of ␤-zeolite by ionic exchange to optimize
its behaviour as Pt-supports [8–11]. The activity of the catalysts
decreases with increasing the basicity of the exchanged ion (from
Na to Cs) and the selectivity towards aromatic increases in the
Previous authors proposed that the aromatization of alkanes on
metal oxides or metal oxides–carbon composites occurs through
a direct 1–6 ring closure reaction of the C6 chain. The catalysts
are considered as monofunctional and it is assumed that there is a

162
F.J. Maldonado-Hódar / Applied Catalysis A: General 408 (2011) 156–162
same sense. At 450 ◦C the benzene selectivity increased from 10
for Pt supported on Na-exchanged zeolite to 32% for Pt supported
on Cs-exchanged zeolite, tending to the 45% reached by PtKL cat-
alysts used as reference [10]. It is noteworthy that in this study,
a benzene selectivity of 45% was also reached for tungsten doped
carbon aerogel AW500 at 525 ◦C, increasing this value up to 60%
when the catalyst is prepared by impregnation. Similarly, the per-
formance of these metal doped carbon aerogels greatly improves
those presented by Trunschke [16]. In this work, authors used com-
posites with around 80% w/w of inorganic phase and obtained an
aromatic selectivity of 90% in n-octane conversion regardless of the
oxide used, but found that the aromatization centres were largely
deactivated. In this work are developed catalysts with good activ-
ity and selectivity of around 60%, that was stable for a long time
and used catalysts with a metal charge that was always less than
5% w/w. Moreover, it is demonstrated that the catalytic behaviour
is intrinsic of each catalyst and that can be fitted by the ade-
quate synthesis and pretreatments. Therefore, the preparation of
inorganic–organic composites based on carbon aerogels offers an
interesting alternative procedure for developing aromatization cat-
alysts. Many variables such as porosity, the active phase nature and
dispersion, loading, interaction between inorganic–organic phases,
multiple metal doping, etc. can be explored on the basis of the
flexibility of the preparation method.
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Acknowledgment
This research is supported by the project CTM2010-18889 of the
Spanish MCIN.