The monomolecular process is easier for the isomerization
of alkanes with the carbon number greater than four. Butane
is converted into isobutane via the branched primary cation,
which is thermodynamically unstable, after the formation of
a protonated cyclopropane intermediate. On the other hand,
pentane can be isomerized to isopentane through the relatively
stable cation of the secondary ion.
In the isomerization of alkanes over sulfated zirconia, an
induction period is observed in the beginning of reaction; the
monomolecular reaction proceeds in the beginning and induc-
tion period to form isopentane from pentane selectively.24
Afterward, the reaction changes to the bimolecular mechanism
with time to produce isobutane by the reaction of carbenium
ions with alkenes, formed by release of protons from the
carbenium ions, followed by rearrangement and scission.
Sassi and Sommer studied the competitive isomerization and
cracking of branched octanes on sulfated zirconia in relation
to the product distribution expected from bimolecular process
of butane.25 Spectroscopic characterization of the hydrocar-
bon deposits formed during the isomerization of butane on
sulfated zirconia showed the formation of alkenyl and
cycloalkenyl ions, indicating the C8 formation by the bimole-
cular process followed by rearrangement, dehydrogenation,
and cyclization.26
In the superacid-catalyzed isomerization of normal open-
chain alkanes greater than butane, the reaction proceeds by
the monomolecular mechanism giving isoalkanes with the
same carbon number as reactants or by the bimolecular
mechanism forming isobutane together with disproportionated
materials with the carbon number different from the reactants.
The reaction mechanism of cyclic alkanes is different from
that of open-chain alkanes. A known example is the reversible
isomerization between cyclohexane and methylcyclopentane
with preservation of the cyclic structure.27,28 Methylcyclo-
hexane was also converted into dimethylcyclopentanes over
Pt-promoted sulfated zirconia.29 It appears that the skeletal
isomerization of cyclo-alkanes is not based on the bimolecular
process, but the monomolecular reaction. Considering that
the monomolecular mechanism is more simple and plays a key
role in the skeletal isomerization of open-chain alkanes as well
as in the catalytic action, we found it of interest to use more
cycloalkanes as reactants. It seems that in comparison with
open-chain alkanes the isomerization of cyclic ones is more
simple to result in efficiency for clarification of the reaction
mechanism along with the catalytic action.
In continuation of our interest on activation of alkanes cat-
alyzed by solid superacids, we carried out the skeletal isomer-
ization of cycloalkanes with the number of carbons greater
than six, cycloheptane, cyclooctane, cyclodecane, and cyclo-
dodecane, over sulfated zirconia and discussed their reaction
mechanism on the basis of products. In addition, reactions
of methylcyclohexane, ethylcyclopentane, ethylcyclohexane,
dimethylcyclohexane, and [4,4,0]bicyclodecane (decaline) were
performed in order to investigate reaction pathways of the
cyclo-alkanes.
filter for 1 h, followed by filtering, drying in a desiccator at
room temperature, calcining in air at 600 ꢀC for 3 h, and
sealing in an ampoule while hot.
Reaction procedure
The catalytic reactions of n-pentane and cyclohexane were car-
ried out in gas phase with a conventional microflow at atmo-
spheric pressure; 0.15 g of catalyst powder (32–50 mesh) was
placed between glass wool plugs. The reaction was performed
in a single-pass fixed-bed Pyrex reactor equipped with a six-
way stopcock for sampling using helium as carrier gas by
saturating with the reactant vapor at 0 ꢀC, where He gas was
bubbled out of liquid reactants. The catalyst was pretreated
in He at a flow rate of 15 ml minÀ1 at 200 ꢀC for 3 h before
reaction. The effluent products were analyzed with an on-line
gas chromatograph (VZ-7, 3 m).
The liquid–solid reactions of cycloalkanes were performed
in a glass-made apparatus shown in Scheme 1. After 0.05 g
of each catalyst was placed in the minireactor (A) the reactor
stem (E) was connected to a vacuum apparatus. The catalyst
was pretreated in vacuo at the desired temperature, 150–
450 ꢀC, for 3 h; then the reactor tube was sealed in vacuo
on the bottom of the stem E. Cycloalkanes, 0.15 ml (0.12 g
for cyclododecane), were placed in the vertical tube (B), and
the tube stem (F) was connected to the vacuum system; the
cycloalkanes were purified by distilling cycloheptane, methyl-
cyclohexane, and ethylcyclopentane and by drying cyclo-
octane, ethylcyclohexane, 1,2-dimethylcyclohexane, and
cyclodecane over molecular sieves. After cooling the sample
tube to liquid nitrogen, the tube was evacuated and sealed
on the bottom of the stem F. The isomerization reaction was
carried out by breaking the breakable seal D with the glass
bar C followed by cooling the reactor A to liquid nitrogen
before transfering the alkanes B; the whole system was allowed
to stand in an oven at 50 ꢀC. After reaction, the reaction mix-
ture was transferred to B from A by cooling B and analyzed by
a HITACHI 663 GC with an FID detector through a 50 m
CP-Al2O3/KCl capillary column (60-m OV-1701 BONDED
for cyclododecane).
Identification of products from cycloheptane
The reaction of methylcyclohexane was carried out at 50 ꢀC
over the SO4/ZrO2 catalyst after pretreatment with evacuation
at 200 ꢀC, the results being shown in Fig. 1 as a function of
time up to 360 min. The figure shows that the system reaches
Experimental
Catalyst preparation (SO4/ZrO2)
Zirconia gel was obtained by hydrolyzing ZrOCl2 with aqu-
eous ammonia to pH 8 at 70 ꢀC. Aqueous ammonia solution
(25–28%) was added dropwise with stirring into 100 g of
ZrOCl2Á8H2O dissolved in 3 L of distilled hot water; then the
precipitated solution was kept in a water bath warmed at
70 ꢀC for 2 h. Finally the precipitate was washed 2 times with
1 L of hot water, dried at 100 ꢀC for 24 h, and powdered to
32–50 mesh.
The gel was treated with sulfate ion by exposing 2 g of the
dried and powdered material in 30 ml of 1N H2SO4 on a glass
Scheme 1 Reaction apparatus. A: reactor, B: sample reservior, C:
glass bar, D: breakable seal, E, F: joint.
4344
Phys. Chem. Chem. Phys., 2003, 5, 4343–4349