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N. Hiyoshi et al. / Journal of Catalysis 252 (2007) 57–68
catalysts in 2-propanol, indicating that hydrochloric acid en-
hanced the direct hydrogenation of 3-tert-butylphenol to cis-3-
tert-butylcyclohexanol in scCO2.
The initial cis ratio in scCO2 was higher than that in
2
-propanol for both HCl–Rh/C and Rh/C. However, in all
the cases, the cis ratio decreased with the progress of the
subsequent hydrogenation of 3-tert-butylcyclohexanone, and
the final cis ratio was in the following order: HCl–Rh/C in
scCO2 > Rh/C in scCO2 > Rh/C in 2-propanol > HCl–Rh/C
in 2-propanol. Interestingly, the final cis ratio was decreased by
the addition of hydrochloric acid in 2-propanol, whereas it was
increased in scCO2.
A characteristic of 3-tert-butylphenol hydrogenation is the
decreased cis ratio with the progress of consecutive hydro-
genation of 3-tert-butylcyclohexanone, which indicates that
the hydrogenation mechanism of 3-tert-butylphenol differs
from those of 2- and 4-tert-butylphenol. Scheme 4 shows the
mechanism of 3-tert-butylphenol hydrogenation based on par-
tial hydrogenation to enol, followed by tautomerization and
then complete hydrogenation. In the hydrogenation of 3-tert-
butylphenol, 3-tert-butylcyclohexanone formed on the active
sites by tautomerization of enol had a large steric hindrance
due to the tert-butyl group and cyclohexane ring (step II
in Scheme 4); thus, desorbed 3-tert-butylcyclohexanone was
readsorbed on the active sites from another side (step V in
Scheme 4). Consequently, in the initial stage of hydrogena-
tion, cis-3-tert-butylcyclohexanol was formed by the direct
hydrogenation of enol or 3-tert-butylcyclohexanone (steps III
and IV in Scheme 4), and trans isomer was formed by the
hydrogenation of enol after the flipping of enol (step III in
Scheme 4); however, in the consecutive hydrogenation of
3-tert-butylcyclohexanone, trans isomer was formed by the hy-
drogenation of enol or 3-tert-butylcyclohexanone (steps VII
and VIII in Scheme 4), and cis isomer was formed by the
hydrogenation of enol after the flipping of enol (step VII in
Scheme 4). In addition, direct hydrogenation without flipping
would be kinetically more favorable than that through the flip-
ping of enol. Thus, the cis ratio would decrease with the pro-
gression of consecutive hydrogenation of 3-tert-butylcyclohex-
anone.
Fig. 11. Selectivity to 3-tert-butylcyclohexanone (a) and cis ratio (b) as a func-
tion of conversion of 3-tert-butylphenol in scCO (10 MPa) over Rh/C ("), in
2
scCO (10 MPa) over HCl–Rh/C (!), in 2-propanol (10 mL) over Rh/C (Q),
2
and in 2-propanol (10 mL) over HCl–Rh/C (P). Catalyst weight, 0.02 g; hy-
drochloric acid, 3.9 µmol; partial hydrogen pressure, 2 MPa; catalyst weight,
0.02 g; temperature, 313 K.
than in scCO2 (0.014 mmol for Rh/C and 0.029 mmol for HCl–
Rh/C). Note that the cis ratio decreased with increasing reaction
time in both cases, in contrast to the hydrogenation of 2- and
4
-tert-butylphenol. The final cis ratio was greater in scCO2 than
in 2-propanol.
Fig. 11 shows the changes in the selectivity to 3-tert-
butylcyclohexanone and cis ratio as a function of conver-
sion of 3-tert-butylphenol. In scCO , the selectivity to 3-tert-
2
butylcyclohexanone remained constant up to 70% conversion
and began to decrease once most of 3-tert-butylphenol was con-
verted, indicating that the consecutive hydrogenation of 3-tert-
butylcyclohexanone to 3-tert-butylcyclohexanol occurred af-
ter most of 3-tert-butylphenol was hydrogenated. In contrast,
the selectivity to 3-tert-butylcyclohexanone in 2-propanol de-
creased linearly from the beginning of the reaction, indicat-
ing that the consecutive hydrogenation of ketone occurred in
the presence of 3-tert-butylphenol. In both the cases, a de-
creased cis ratio accompanied the decreased selectivity to
In the direct hydrogenation of 3-tert-butylphenol to 3-tert-
butylcyclohexanol, the enhanced cis ratio by the addition of
hydrochloric acid to Rh/C in scCO could be explained by the
2
enhanced hydrogenation of the adsorbed 3-tert-butylcyclohexa-
none intermediate to cis-3-tert-butylcyclohexanol. However,
the cis ratio of 3-tert-butylcyclohexanol formed in the con-
secutive hydrogenation of 3-tert-butylcyclohexanone would be
decreased by the addition of hydrochloric acid, because the hy-
drogenation of readsorbed 3-tert-butylcyclohexanone to trans-
3-tert-butylcyclohexanol would be enhanced. With the combi-
3
-tert-butylcyclohexanone. This suggests a lower cis ratio
of 3-tert-butylcyclohexanol in the hydrogenation of 3-tert-
butylcyclohexanone compared with that in the hydrogenation
of 3-tert-butylphenol.
It is interesting to note that at up to 80% conversion of 3-
tert-butylphenol, the selectivity to 3-tert-butylcyclohexanone
nation of scCO and HCl–Rh/C, a greater amount of 3-tert-
2
for HCl–Rh/C was lower than that for Rh/C in scCO , indicat-
butylcyclohexanol was formed directly from 3-tert-butylphenol
2
ing that the extent of direct hydrogenation of 3-tert-butylphenol
to 3-tert-butylcyclohexanol was increased by the addition of
hydrochloric acid in scCO2. Also, at up to 80% conversion
of 3-tert-butylphenol, the cis ratio for HCl–Rh/C was higher
than that for Rh/C in scCO2 and was almost the same for both
than from 3-tert-butylcyclohexanone consecutively, and thus
the final cis ratio was higher. In the combination of 2-propanol
and HCl–Rh/C, the amount of trans-3-tert-butylcyclohexanol
from 3-tert-butylcyclohexanone was greater, leading to a lower
final cis ratio.