Chemistry Letters 2001
937
temperature. Although the surface C species is not well under-
1
stood yet at this stage of investigation, the C species is consid-
1
ered to be methylene group (CH =) formed by
2
dechlorination/hydrogenation of adsorbed CHCl , because the
3
chlorine containing C2+ hydrocarbon is not detected in the prod-
ucts.
C2+ hydrocarbons were not produced in the hydrodechlori-
nation of CH Cl and those including alkenes were formed in
2
2
the reaction of CCl over Pd catalysts. The difference of selec-
4
tivity to oligomerization products is explainable by the reactivi-
ty of chloromethanes. The reactivity namely the tendency to
C–Cl bond cleavage increases remarkably with the number of
9
chlorine atoms in it (CH Cl < CHCl < CCl ). It is likely that
2
2
3
4
CH Cl adsorbs dissociatively on Pd at the lower coverage
2
2
because of its lower reactivity, leading to the higher coverage of
adsorbed hydrogen, and the surface C species are attacked by
1
adsorbed hydrogen preferentially to form Cl–H replacement
products, CH and CH Cl. The high reactivity of CCl seems to
4
3
4
cause the higher coverage of C species and chlorine, resulting
1
in the coupling into alkenes and marked deactivation by carbon
deposit because of the shortage of adsorbed hydrogen.
from the consecutive C–C bond formation through the read-
sorption of desorbed molecules.
The selective hydrocarbon formation from CHCl we
3
report here is expected to be a potential process for a methane
conversion into hydrocarbons under mild condition, since CH4
is converted easily to CHCl by the reaction with Cl and HCl
Figure 2 shows the Schultz–Flory plots for compositions of
hydrocarbons formed over Pd/SiO at various temperatures.
2
3
2
The Schultz–Flory distribution is expressed by eq (1); where Cn
formed can be oxidized back to Cl2.
is the mole fraction of the hydrocarbon with n carbon atoms
c
8
and α represents the probability of chain-growth.
Reference
1
a) Y. Takita, H. Yamada, T. Tatsumi, and Y. Mizuhara,
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As seen in Figure 2, the composition of hydrocarbons formed at
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formed through the polymerization of surface C species like a
1
Fischer–Tropsh reaction. The probability of chain-growth (α)
calculated from the slope of each line in Figure 2, was
increased from 0.15 (100 °C) to 0.28 (200 °C) with reaction
2
3
4
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