Carbonaceous deposits on alumina as catalysts and supports

Article abstract of DOI:10.1016/j.jpcs.2003.08.042

Studies of carbonaceous deposits, intercalated within alumina pores, were carried out from the point of view of their catalytic activity for a number of reactions of alkyl-substituted benzenes. Alumina was not active for ammoxidation of ethylbenzene and m-xylene as well as oxidative dehydrogenation of ethylbenzene until certain amount of the deposit was generated on acid sites of alumina surface. Carbonaceous deposits formed during reactions of decomposition of 2-propanol and hexafluoro-2-propanol were used as supports for platinum catalysts for hydrosilylation of allyl chloride and 1-octene to result in excellent selectivity. Precursor of carbonaceous deposit formed from hexafluoro-2-propanol has been identified and scheme of the deposit formation was proposed.

Full text of DOI:10.1016/j.jpcs.2003.08.042

Journal of Physics and Chemistry of Solids 65 (2004) 627–632
www.elsevier.com/locate/jpcs
Carbonaceous deposits on alumina as catalysts and supports
*
Ryszard Fiedorow , Rafał Franski, Alina Krawczyk, Sławomir Beszterda
´
Faculty of Chemistry, Adam Mickiewicz University, Pozna n´ , Poland
Accepted 19 August 2003
Abstract
Studies of carbonaceous deposits, intercalated within alumina pores, were carried out from the point of view of their catalytic activity for a
number of reactions of alkyl-substituted benzenes. Alumina was not active for ammoxidation of ethylbenzene and m-xylene as well as
oxidative dehydrogenation of ethylbenzene until certain amount of the deposit was generated on acid sites of alumina surface. Carbonaceous
deposits formed during reactions of decomposition of 2-propanol and hexafluoro-2-propanol were used as supports for platinum catalysts for
hydrosilylation of allyl chloride and 1-octene to result in excellent selectivity. Precursor of carbonaceous deposit formed from hexafluoro-
2-propanol has been identified and scheme of the deposit formation was proposed.
q 2003 Elsevier Ltd. All rights reserved.
Keywords: B. Chemical synthesis
1
. Introduction
carbon-mineral adsorbents [6,7] and catalysts [8]. Results of
our earlier study on catalytic activity and selectivity of
fluorinated carbon-supported platinum catalysts for
hydrosilylation of alkenes and their derivatives [9] has
encouraged us to investigate the performance of supports of
mixed nature, such as fluorine-containing coke on alumina,
in hydrosilylation reactions.
In reactions of organic compounds proceeding on oxide
catalysts, secondary products in the form of carbonaceous
deposit are often accumulated in the catalysts pores.
Properties of the carbonaceous deposit depend on the nature
of catalyst and reactants as well as reaction temperature.
Carbonaceous deposits formed on oxide surfaces contain
carbon, hydrogen and, in some cases, other elements as
well. Hydrogen content decreases with increase in
temperature and time on stream. At temperatures above
2. Experimental
5
00 8C polyaromatic products accumulate on catalysts,
2.1. Preparation of catalysts and their characterisation
whereas at lower temperatures condensation products of
considerably lower C/H ratios are formed. Both forms of
carbonaceous deposits are called coke by some
authors, whereas the others use the term coke only for
high-temperature products [1]. The deposition of coke is
usually an undesirable phenomenon, because it leads to
catalyst deactivation as a result of blocking the access to
active centres. However, it was reported in the literature that
a few cokes are catalytically active, e.g. carbonaceous
deposits capable of catalysing reactions of oxidative
dehydrogenation of alkylbenzenes [2–4] and ammoxidation
of toluene [5]. The formation of coke can be also useful as a
way of preparation of mixed-type catalyst supports and
Alumina (alu) was obtained by hydrolysis of aluminium
isopropoxide followed by drying at 100 8C and calcining at
00 8C. Carbonaceous deposits (i-coke and f-coke) were
formed in alumina pores by performing reactions of
4
decomposition of 2-propanol (i) or hexafluoro-2-propanol
(
f) at 300 8C in a flow reactor. Catalysts containing 1%Pt on
coke-covered alumina (Pt/i-coke and Pt/f-coke) were
prepared by incipient wetness method using
platinum acetylacetonate solution in chloroform, followed
by three-hour reduction in hydrogen flow at 160 8C.
The reduction temperature was established in preliminary
TPR experiments performed on a Chemisorb 2705
apparatus (Micromeritics). Fluorine presence in f-coke
was tested by XPS measurements carried out on a VG
*
Corresponding author. Fax: þ48-618658008.
E-mail address: fiedorow@amu.edu.pl (R. Fiedorow).
0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2003.08.042

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R. Fiedorow et al. / Journal of Physics and Chemistry of Solids 65 (2004) 627–632
Scientific ESCA-3 photoelectron spectrometer and the
identification of coke precursor formed as a result of
hexafluoro-2-propanol decomposition on alumina was
performed by GC/EI-MS technique on an AMD-402
two-sector mass spectrometer (AMD Intectra) of B/E
geometry. High-resolution data were obtained on the same
instrument by using V/E high-resolution scan. The
compounds were introduced into the mass spectrometer
using a gas chromatograph Hewlett-Packard Model 5890II.
The instrument was equipped with a DB-1 fused-silica
capillary column (J and W Scientific, 25 m £ 0.2 mm i.d.).
Flow rate of carrier gas (helium) was 1 ml/min and column
temperature 40 8C. The GC/CI-MS analysis was performed
on a Saturn 2160 GC/MS instrument (Varian). The GC
parameters were the same as in the case of GC/EI-MS
analysis, except for the column type which was DB-5.
Surface area and pore size distribution were determined
on the ground of low-temperature nitrogen adsorption
measurements performed on an ASAP 2010 soprtometer
Fig. 1. Catalytic activity of carbonaceous deposit on alumina for
ammoxidation of alkyl-substituted benzenes.
with increasing time on stream is the source of the catalytic
activity, thus belonging to the family of carbonaceous
deposits reported in the literature as catalytically active for
a number of other reactions of ammoxidation and oxydehy-
drogenation [2–5]. Catalytic activity for both reactions, after
a period of rise, reaches a quasi-stationary level, which
represents equilibrium between the formation of coke and its
deep oxidation to carbon oxides.
(
Micromeritics).
2
.2. Catalytic reactions
Ammoxidation of ethylbenzene and m-xylene was
conducted in a flow reactor at 400 8C. Mole ratio of
hydrocarbon:oxygen:ammonia was 1:5:2.5.
In our earlier paper [4] we have described the formation of
carbonaceous deposit that was active for catalysing oxidative
dehydrogenation of ethylbenzene with nitrobenzene to yield
styrene and aniline as the reaction products. In the present
study, we have generated carbonaceous deposit in alumina
pores by passing separately parent compounds and products
of the reactions through alumina bed in a reactor at 450 8C for
1.5 h (nitrobenzene, styrene, aniline) or 6 h (ethylbenzene).
The reason for a longer time of alumina exposition to the
latter compound was low formation of coke from ethylben-
zene (0.7 wt% during 6 h, compared to 2.9, 3.5 and 10.2 wt%
in the case of styrene, aniline and nitrobenzene, respectively,
during 1.5 h). The study, aimed at determining which
components of the reaction system discussed are the main
contributors to the formation of catalytically active carbon-
aceous deposit, was performed by pulse technique which
consisted in introducing pulses of the reactant mixture
Oxidative dehydrogenation of ethylbenzene using
nitrobenzene as a hydrogen acceptor was studied on coke
catalysts obtained by passing different compounds through
alumina at 450 8C. The coke precursors included parent
compounds and products of the latter reaction, which
differed considerably in their capability of forming
carbonaceous deposits on alumina. Pulses of ethylbenze-
ne/nitrobenzene mixture (mole ratio of 1:1) were injected
into helium carrier gas flowing through microreactor loaded
with coke-containing alumina. The reaction temperature
was 450 8C and reaction products were transported by
carrier gas to an on-line gas chromatograph.
Hydrosilylation of allyl chloride and 1-octene on Pt/coke
catalysts was carried out in a batch reactor at 60 8C for 3 h
followed by analysis of post-reaction mixture on a
Perkin-Elmer Auto System XL gas chromatograph.
3
. Results and discussion
Ammoxidation of ethylbenzene to benzonitrile and that of
m-xylene to isophthalic acid dinitrile proceed on transition
metal oxides such as vanadia and molybdena but not on
alumina. The lack of activity for the above reactions in the
case of pure alumina clearly results from Fig. 1 in which it is
seen that at the initial period on stream, conversion to the
above products is zero. However, alumina is well-known for
its surface acidity and acid centres are the sites on which so-
called coke, intercalated within alumina pores, is formed.
This coke, as evidenced by the appearance and rise of activity
Fig. 2. Catalytic activity for oxidative dehydrogenation of ethylbenzene
with nitrobenzene shown by carbonaceous deposits formed from ethylben-
zene, nitrobenzene, styrene and aniline.

R. Fiedorow et al. / Journal of Physics and Chemistry of Solids 65 (2004) 627–632
629
addition of chlorosilanes to carbon–carbon multiple
bonds are activated carbon and alumina [10,11] and the
use of fluorinated carbon support resulted in a high
selectivity for hydrosilylation [9]. In this study we have
made an attempt at preparing and characterising a hybrid
support that combined properties of fluorinated carbon
and alumina. The precursor of fluorine-containing coke
was hexafluoro-2-propanol and that of coke used for
comparative purposes—2-propanol. It is well-known that
the latter compound dehydrates on alumina to propene
(which in turn converts to aromatic coke [12]), but it was
necessary to determine the reaction pathway of the
former compound. The identification of products of
hexafluoro-2-propanol decomposition on alumina was
performed by using a gas chromatography-mass spec-
trometry (GCMS) system both with electron ionisation
(EI) and chemical ionisation (CI). In the chromatogram
of the post-reaction mixture (Fig. 3), compound 3 is an
unreacted portion of the parent compound, while
compound 1 is the first identified reaction product
formed by elimination of hydrogen fluoride molecule
from hexafluoro-2-propanol. The elimination results in
the formation of pentafluoropropen-2-ol, which is in
keto-enol tautomeric equilibrium with pentafluoropropan-
2-one. The identification of compound 2 was more
difficult.
Fig. 3. Chromatogram of the mixture collected after hexafluoro-2-propanol
decomposition over alumina.
(
ethylbenzene and nitrobenzene) onto pre-coked alumina.
As concluded from Fig. 2, the highest catalytic activity was
shown by coke of nitrobenzene origin, followed by that
derived from aniline and styrene, whereas the activity of coke
obtained from ethylbenzene was low. It is worth to add that
the carbonaceous deposits prepared from nitrobenzene and
aniline contained nitrogen, in addition to carbon
and hydrogen. The C/N ratios in the above cokes were 9.7
and 9.1, respectively.
Next stage of the present study on the application of
cokes intercalated within alumina pores to catalysis
consisted in the preparation of fluorine-containing coke
that could serve as a support for hydrosilylation catalysts.
Among supports reported for their use in reactions of
In mass spectrum of compound 2 (Fig. 4), molecular ion
was absent because in the case of fluorine-containing non-
aromatic compounds, it easily undergoes fragmentation
Fig. 4. EI-MS spectrum of compound 2.

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R. Fiedorow et al. / Journal of Physics and Chemistry of Solids 65 (2004) 627–632
Table 1
Textural characteristics of coke-containing alumina samples
mass spectrum. The results are given in Table 1 and are in
agreement with the interpretation of compound 2
fragmentation presented above. The GC/CI-MS
analysis has also enabled to confirm molecular weight of
compound 2.
Sample
Surface area
2
m /g)
Average pore
diameter (nm)
Pore volume
3
(cm /g)
(
alu
246
213
216
30
8.4
9.0
0.51
0.48
0.47
0.09
0.13
After the identification of hexafluoro-2-propanol
decomposition products, a question should be answered
how the carbonaceous deposit is formed from them.
Although alumina acidity is of Lewis-type and cokes
formed on alumina surface are generated on Lewis acid
centres, in the case of hexafluoro-2-propanol decompo-
sition, hydrogen fluoride is evolved and a substitution of
some of hydroxyl groups present on alumina surface with
fluorine results in the appearance of Brønsted acid centres
due to fluorine-induced weakening of O–H bond in the
remaining hydroxyl groups. Acidic OH group (Brønsted
acid site) donates proton to 4-(difluoromethyl)-4-hydro-
xyoctafluoropentan-2-one (compound 2) and the resulting
species reacts with enol form of compound 1 (see the
reaction scheme shown in Fig. 5) thus producing carbon-
aceous deposit. Such a structure cannot be long-lasting at
temperature conditions of coke production process, there-
fore alumina covered with carbonaceous deposit should be
removed from the reactor shortly after the deposit was
formed. At prolonged exposure to elevated temperatures,
aromatisation processes will proceed and fluorine and
hydrogen loss will occur. The presence of fluorine in the
carbonaceous deposit was confirmed by ESCA measure-
ments which showed that there is no aluminium fluoride on
the surface of alumina used in hexafluoro-2-propanol
i-coke
Pt/i-coke
f-coke
Pt/f-coke
8.8
12.2
12.6
41
[
13]. Ion of the highest m=z value (m=z 277) is formed by the
elimination of fluorine from molecular ion, while that of m=z
57 is a result of abstraction of fluorine atom and hydrogen
2
fluoride molecule. The elimination of fluorine and carbon
monoxide leads to ion of m=z 249. Fragment ion at m=z 199
has a low relative intensity, however, it is of significant
importance to the confirmation of the structure of
compound 2. It is formed from molecular ion as a result
of the elimination of trifluoroacetyl radical. It is worth to
add that in EI spectrum of compound 2, there are no ions
created by elimination of other fluoroacetyl (di- or mono)
radicals which confirms the proposed structure of compound
2
1
. Ions observed in EI spectrum of compound 2 at m=z 195,
51 and 101 are formed as a result of skeletal rearrange-
ments and a number of mechanisms leading to their
formation can be proposed.
In order to find additional confirmation of the structure of
compound 2, we have carried out also high-resolution
measurements for three ions of the highest m=z values in its
Fig. 5. Scheme of formation of fluorine-containing carbonaceous deposit.

R. Fiedorow et al. / Journal of Physics and Chemistry of Solids 65 (2004) 627–632
631
Table 2
Results of high-resolution measurements for ions of the highest m=z values
in the EI-MS spectrum of the compound 2
m=z
Error (ppm)
Composition
Calculated
Obtained
276.9911
256.9849
248.9962
276.9897
256.9847
248.9972
5.1
0.8
C
6
C
6
C
5
2 2 9
H O F
HO
2
F
8
24.0
H OF
2
9
decomposition because binding energy (BE) was 74.3 eV,
which is close to that of g-alumina, boehmite and bayerite
Fig. 7. Addition of trichlorosilane to 1-octene on supported platinum
catalysts.
(
74.2 eV), whereas in the case of AlF it is 76.3 eV. The ratio
3
of F/Al was 1.65 and that of O/Al was 1.47, which means
that most of aluminium was bonded to oxygen and,
therefore, the remaining amount of fluorine is bonded to
carbon.
Figs. 6 and 7 that the use of carbonaceous deposit—mineral
support results in obtaining excellent selectivity since no
products other than desired ones (i.e. 3-chloropropyltri-
chlorosilane and octyltrichlorosilane, respectively) are
formed in the presence of Pt/coke-covered alumina,
contrary to Pt/alumina catalyst. In the case of hydrosilyla-
tion of allyl chloride (Fig. 6), not only selectivity, but also
yield of desired product was higher when coke supports
were used, particularly f-coke which is more hydrophobic.
Surprisingly enough, a reduction in the yield of octyltri-
chlorosilane (Fig. 7) was observed in the case of the latter
support (selectivity was still higher than on coke-free
alumina support).
The decomposition of 2-propanol and hexafluoro-2-
propanol on alumina was carried out until roughly the
same content of i-coke and f-coke (2.14 and 1.98 wt%,
respectively) was reached and this required dosing 30-
times more 2-propanol than hexafluoro-2-propanol at the
same feed flow rate (i.e. i-coke stayed in the reactor 30-
times longer than f-coke), because coke formation was
much slower in the case of the former alcohol. In spite
of similar coke content, surface area of alumina after
becoming covered with f-coke decreased to a consider-
ably greater extent than in the case of covering with
i-coke (Table 2). This suggests that the formation of f-
coke, which occurs much faster, takes place mostly at
pore inlets, while i-coke probably fills alumina pores
with more uniform layer.
4. Conclusions
The two cokes were investigated from the point of view
of their performance as supports of platinum catalysts for
addition of trichlorosilane to 1-octene and allyl chloride,
i.e. reactions which are commercially exploited for the
manufacture of adhesion promoters—3-chloropropyltri-
chlorosilane and octyltrichlorosilane. It can be seen in
Carbonaceous deposits formed on acid sites of alumina
in reactions of ammoxidation of alkyl-substituted benzenes
belong to catalytically active cokes. Alumina itself is
inactive for the above reactions.
Cokes formed in alumina pores by parent compounds and
products of the reaction of oxydehydrogenation of ethyl-
benzene differ considerably in their catalytic activity for the
above reaction, nitrobenzene-derived coke being the most
active and that originated from ethylbenzene showing the
poorest activity.
Decomposition of hexafluoro-2-propanol on alumina
leads to the formation of 4-difluoromethyl-4-hydroxy-
octafluoropentan-2-one which undergoes acid-catalysed
conversion to fluorine-containing carbonaceous deposit.
Carbonaceous deposits on alumina, when used as
supports for platinum, resulted in very selective catalysts
for hydrosilylation of allyl chloride and 1-octene. The yield
of desired products of both reactions carried out on Pt/i-coke
surpassed that obtained on Pt/alumina, while in the case of
Pt/f-coke it was higher only in the reaction of hydrosilyla-
tion of allyl chloride.
Fig. 6. Activity of supported platinum catalysts for hydrosilylation of allyl
chloride.

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R. Fiedorow et al. / Journal of Physics and Chemistry of Solids 65 (2004) 627–632
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