cyclohexene conversion over pure alumina exceeds 40% as
shown in Fig. 1, the conversion decreases steeply on deposi-
tion of a small amount of carbon. Fig. 9 shows variations in
the selectivities to benzene (a), 1,3-cyclohexadienes (b), isomer-
ization products such as 1- and 3-methylcyclopentene (c) and
cyclohexane (d), with the carbon content. Table 2 summarizes
the conversion and the yields of the products over the
C/Al O samples. Comparing among the samples with di†er-
ent carbon sources at the same carbon content, they show dif-
ferent selectivities. However, the variations in the selectivity
with carbon content for di†erent carbon sources resemble
each other. The selectivity to isomerization products decreases
steeply with increasing carbon content [Fig. 9(c)]. The selec-
tivity to dehydrogenated products passes through a maximum
at a speciÐc carbon content [Figs. 9(a), (b)]. The selectivity to
cyclohexane, which is produced by hydrogenation or by dis-
proportionation, increases monotonically with increasing
carbon content [Fig. 9(d)]. In addition, over the commercial
active carbon at 773 K, the conversion was 35%, and the
selectivities to benzene, 1,3-cyclohexadiene and cyclohexane
were 35.7, 1.8 and 59.8 mol%, respectively.
present carbon in the CHOÈ823 is considered to have few
oxygen in the polycondensed aromatic units.
The di†erence in deposition behavior results in the Ðlling up
of the pores of alumina support being observed at di†erent
carbon contents: pores of alumina are Ðlled up at lower
carbon content in the CHO samples (Fig. 3). The amount of
deposited carbon is controlled by choosing the deposition
temperature and period (Fig. 2), while the structure of carbon
is modiÐed by using di†erent carbon sources. A similar obser-
vation was seen in the case of silica deposition on alumina
support using tetraethoxysilane.16,17
2
3
Although a resonance peak identiÐed as aromatic com-
pounds is observed in the 13C NMR spectra of samples pre-
pared at temperatures below 923 K (Fig. 4), no peak is
detected in the CHAÈ973 series. This elucidates that, at high
temperature such as 973 K, aromatic compounds are not pro-
duced or decomposed during the carbon deposition process.
In addition, fewer unpaired electrons are detected by EPR
measurement in CHAÈ973 series (Fig. 7). This is explained by
the reports that aromatic compounds formed from
decacyclene19 and polyparaphenylene20 decompose losing
aromaticity above a critical temperature. Thus, we can obtain
C/Al O samples with di†erent structures and carbon content.
2
3
4
Discussion
4.2 Catalytic activity of carbon deposited on alumina
The isomerization of cyclohexene proceeds on the acid sites of
alumina without carbon, and 1- and 3-methylcyclopentenes
are produced [Fig. 1(a)]. Although a trace amount of depos-
ited carbon decreases the cyclohexene conversion, the selec-
tivity to isomerized products is almost unchanged at 773 K.
At 823 K, CHE as carbon is deposited on the acid sites that
are active for isomerization, and the selectivity to isomerized
products decreases with process time [Fig. 1(b)]. At the same
time, the cyclohexene conversion decreases with process time,
whereas the yield of benzene increases. Decrease in the
number of acid sites has been conÐrmed in samples as
Al O /SiO and TiO /Al O , on which cyclohexene depos-
ited at 973 K.21 Furthermore, it is reported that the basicity of
carbon moderates the acidity in an aluminaÈcarbon compos-
ite derived from the solÈgel process of aluminium isopro-
poxide with resorcinol.22 The basic aluminaÈcarbon
composite catalyzes the dehydrogenation of propan-2-ol into
acetone at 473 K. The results in Fig. 1 suggest that dehydro-
genation proceeds over the deposited carbon even under non-
oxidative conditions.
4.1 Carbon deposition
Among the three carbon sources, the reactivity of the sources
to carbon deposition di†ers (Fig. 2). Both CHO with a car-
bonyl group and CHE with p electrons are reactive for carbon
deposition at 823 K. They are probably deposited on the acid
sites of alumina, because the deposition decreases the isomer-
ization activity. In contrast, CHA shows the lowest deposition
reactivity, because CHA has no functional group; the carbon
deposition from CHA requires temperatures º873 K. The dif-
ference in the reactivity for the deposition a†ects the structure
of carbon deposited on alumina.
The commercial active carbon used as reference shows dif-
fraction peaks at 2h \ 25¡ and 43¡ (Fig. 5), which correspond
to the (100) and (200) planes of graphite. The broadness of the
peaks suggests that the graphite-like structure in the active
carbon is disordered by the remaining hydrogen, and that its
size is substantially small. It is reported that carbon deposited
on acidic solids under oxidative conditions consists of oxygen-
containing polycondensed aromatic compounds such as poly-
naphthoquinone.4,6,7,11,12
The present carbon prepared from CHA and CHE also
consists of polycondensed aromatic units without oxygen
because the deposition is carried out under non-oxidative con-
ditions. The results of 13C NMR and XRD measurements
clearly show the formation of condensed aromatic compounds
and its layered structure (Figs. 4 and 5). However, the struc-
ture of the carbon deposited on alumina would di†er depend-
ing on the carbon source as well as on the deposition
temperature.
Carbon species in the CHOÈ823 series show a di†raction
peak at 2h \ 25¡ clearer than that in the CHAÈ973 at the
same carbon content [Figs. 5(c), (h)]. Since the broadness of
the di†raction peak is caused either by disordering of period-
icity or by small crystallite size, the carbon in the CHOÈ823 is
considered to have a layered structure thicker than that in the
CHA series. For the CHOÈ823 series, carbon deposition pro-
ceeds preferentially on the acid sites of alumina. This is prob-
ably the reason for the growth of layered structure in carbon.
In contrast, the deposition of carbon from CHA proceeds
more evenly, resulting in thin and/or disordered carbon layer.
The EPR signals are observed at the same magnetic Ðeld
irrespective of carbon source and carbon content (Fig. 6).
Since it has been reported that the oxidized carbon species has
a resonance shifted with increasing degree of oxidation,18 the
2
3
2
2
2 3
In the reaction of cyclohexene over the C/Al O samples, a
2
3
small carbon content steeply decreases the cyclohexene con-
version (Fig. 8). Since the selectivity to isomerized compounds
is also steeply decreased by small amounts of deposited
carbon, this decrease in conversion is probably due to the
decrease in acid sites. However, the decrease in isomerization
activity over the CHA series with carbon content is larger
than that of CHO and CHE series [Fig. 9(c)]. Therefore, more
acid sites remain on the latter series, resulting in higher activ-
ities on isomerization. This indicates that CHA covers the
whole acid sites and that CHO and CHE cover the strong
acid sites at the initial deposition.
The yields of dehydrogenated products such as benzene and
1,3-cyclohexadiene increase with increasing carbon content
(Table 2), and the yield of hydrogenated product such as
cyclohexane increases. In oxidative dehydrogenation, H on
2
carbon abstracted from reactant is oxidatively removed as
H O.7,15 The H produced on the carbon must be removed
2
2
via other mechanisms to complete the catalytic cycle under
non-oxidative conditions. The following three reactions are
considered to be alternatives: release of H molecule, hydro-
2
genation of cyclohexene to cyclohexane, and formation of
smaller hydrocarbons by hydrogenolysis of cyclohexene.
When the second reaction is dominant, the overall reaction is
described as disproportionation.
Phys. Chem. Chem. Phys., 2001, 3, 873È879
877
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