Kinetic analysis of 4-isopropylphenol hydrogenation over activated carbon-supported rhodium catalysts in supercritical carbon dioxide solvent

Article abstract of DOI:10.1039/c1gc16038a

Hydrogenation behavior of 4-isopropylphenol to 4-isopropylcyclohexanol over activated carbon-supported rhodium catalysts in supercritical carbon dioxide (scCO₂) at 313 K was studied in a batch reactor and the results were compared with those in 2-propanol. Higher yields of cis-4-isopropylcyclohexanol were obtained in scCO₂ than in 2-propanol, and the formation of a byproduct, isopropylcyclohexane, was suppressed in scCO₂. The catalyst modification with hydrochloric or phosphoric acid enhanced the yield of cis-4-isopropylcyclohexanol in both scCO₂ and 2-propanol solvents. Kinetic analyses of the reaction profiles revealed higher reaction rates in scCO₂ than those in 2-propanol for the 4-isopropylcyclohexanol formation both by the direct hydrogenation of 4-isopropylphenol and by the consecutive hydrogenation of 4-isopropylcyclohexanone, and also revealed that the addition of hydrochloric acid increased the consecutive hydrogenation rate of 4-isopropylcyclohexanone to cis-4-isopropylcyclohexanol, which reduced the total reaction time needed for the complete hydrogenation of 4-isopropylphenol to 4-isopropylcyclohexanol. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c1gc16038a

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PAPER
Kinetic analysis of 4-isopropylphenol hydrogenation over activated
carbon-supported rhodium catalysts in supercritical carbon dioxide
solvent†‡
Norihito Hiyoshi,a Osamu Sato,a Aritomo Yamaguchi,a Chandrashekhar V. Rodeb and Masayuki Shirai*a
Received 23rd August 2011, Accepted 11th November 2011
DOI: 10.1039/c1gc16038a
Hydrogenation behavior of 4-isopropylphenol to 4-isopropylcyclohexanol over activated
carbon-supported rhodium catalysts in supercritical carbon dioxide (scCO2) at 313 K was studied
in a batch reactor and the results were compared with those in 2-propanol. Higher yields of cis-
4-isopropylcyclohexanol were obtained in scCO2 than in 2-propanol, and the formation of a byproduct,
isopropylcyclohexane, was suppressed in scCO2. The catalyst modification with hydrochloric or
phosphoric acid enhanced the yield of cis-4-isopropylcyclohexanol in both scCO2 and 2-propanol
solvents. Kinetic analyses of the reaction profiles revealed higher reaction rates in scCO2 than those
in 2-propanol for the 4-isopropylcyclohexanol formation both by the direct hydrogenation of
4-isopropylphenol and by the consecutive hydrogenation of 4-isopropylcyclohexanone, and also revealed
that the addition of hydrochloric acid increased the consecutive hydrogenation rate of
4-isopropylcyclohexanone to cis-4-isopropylcyclohexanol, which reduced the total reaction time needed
for the complete hydrogenation of 4-isopropylphenol to 4-isopropylcyclohexanol.
We have reported that hydrogenations of aromatic compounds
proceeded more rapidly over carbon-supported rhodium catalysts
Introduction
The stereoselective hydrogenation of 4-alkylphenols to 4-alkyl-
cyclohexanols is of great commercial interest because the cis
forms of 4-alkylcyclohexanols are useful fragrance intermedi-
ates. cis-4-Alkylcyclohexanols are usually synthesized by the
liquid-phase hydrogenation of 4-alkylphenols over supported
metal catalysts in organic solvents,1–5 in which the thermodyna-
mically more stable trans isomer is also formed simultaneously.
The addition of hydrochloric acid to the catalytic system is also
known to be effective in controlling the stereoselectivity of
4-tert-butylphenol hydrogenation in organic solvents.1 However,
the use of toxic organic solvents and inorganic acid is a critical
issue, and separation and neutralization of products is energy
consuming. Therefore, the development of a highly efficient
stereoselective catalytic system that can be operated under mild
reaction conditions without toxic materials is desirable.
in scCO2 medium than in organic solvents, because of higher
solubility of hydrogen in scCO2.9–16 We have also reported that
the scCO2 solvent is very effective for the stereoselective hydro-
genation of naphthalene and tetralin to cis-decalin10–13 and that
of alkylphenols to cis-alylcyclohexanols compared with the tra-
ditional liquid-phase hydrogenation using organic solvents.14–16
Moreover, leaching of inorganic acid from acid-modified cata-
lysts is prevented by using scCO2.15 In this paper, we report
higher hydrogenation rates of 4-isopropylphenol and higher
selectivity to cis-4-isopropylcyclohexanol over activated carbon-
supported rhodium catalysts modified with inorganic acids in
scCO2, and kinetic analyses revealing the effect of scCO2
solvent and inorganic acids as additives on the catalyst surface.
Experimental
Hydrogenation using solid catalysts and supercritical carbon
dioxide (scCO2) solvent is an attractive process option because
of higher reaction rates under mild temperature conditions.6–8
An activated carbon-supported rhodium catalyst (Rh/C) with
metal loading of 5 wt% (Wako Pure Chemical Ind. Ltd., Japan)
was used without further treatment. The average size of rhodium
particles was 3.4 nm, which was determined by transition elec-
tron microscopy (Figure S1 of Supplementary Information‡).
Activated carbon-supported rhodium catalysts containing hydro-
chloric acid (HCl-Rh/C) or phosphoric acid (H3PO4-Rh/C) were
prepared by mixing hydrochloric acid (36 wt%) or phosphoric
acid (10 wt%) with Rh/C in a sealed vial. The weighed amounts
of catalyst and 4-isopropylphenol (2.0 mmol) were placed in a
stainless steel high-pressure reactor (50 cm3 capacity), and the
reactor was flushed three times with carbon dioxide. Once the
aResearch Center for Compact Chemical System, National Institute of
Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake,
Miyagino, Sendai 983-8551, Japan. E-mail: m.shirai@aist.go.jp;
Fax: (+81) 22-237-5224
bChemical Engineering and Process Development Division, National
Chemical Laboratory, Pune, 411008, India
†This article was submitted following the 11th International Conference
on Carbon Dioxide Utilization, June 27-30, 2011 (Dijon-France).
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c1gc16038a
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Green Chem., 2012, 14, 633–638 | 633

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desired temperature was attained in an oil bath, hydrogen
(38 mmol) and then carbon dioxide were introduced into the
reactor to the desired pressure (12 MPa) and the contents were
magnetically stirred. The stirring speed was maintained at 1000
rpm at which the hydrogenation rate of 4-isopropylphenol was
independent of the stirring speed, confirming that the catalyst
was in complete suspension and external mass transfer resist-
ances were unimportant. After the reaction period, the reactor
was rapidly cooled in an ice bath, the pressure was slowly
released, and the contents were discharged to separate the cata-
lyst by simple filtration. The unreacted substrate and the products
were recovered with acetone, which showed a material balance
of >95%. 2-Propanol (Wako Pure Chemical Ind. Ltd., Japan)
was used as a solvent for studying liquid-phase hydrogenation
under 2 MPa hydrogen and 0.1 MPa carbon dioxide. Quantitat-
ive analysis was conducted by GC-FID (HP-6890) and GC-MS
(HP-6890 and 5973) equipped with a DB-WAX capillary
column. Conversion, selectivity, and cis ratio were defined as
follows:
Results and discussion
Product selectivity
Table 1 shows the results of the hydrogenation of 4-isopropyl-
phenol over Rh/C in scCO2 and 2-propanol. Higher cis ratio was
obtained in scCO2 than that in 2-propanol, and the formation of
isopropylcyclohexane, a dehydroxylation compound, was con-
siderably suppressed in scCO2 as reported for the hydrogenation
of 4-tert-butylphenol.15,16
Table 1 also shows the results for 4-isopropylphenol hydro-
genation over HCl-Rh/C and H3PO4-Rh/C. The cis ratio
increased from 0.78 to 0.88 by adding hydrochloric acid to Rh/C
in scCO2. Higher cis ratio was obtained in scCO2 than that in
2-propanol with HCl-Rh/C. In addition, the amount of 4-isopro-
pylcyclohexane formed was smaller in scCO2 than that in 2-
propanol, though it increased by the addition of hydrochloric
acid in both scCO2 and 2-propanol. Although the effect of phos-
phoric acid was lower than that of hydrochloric acid, the cis ratio
was also increased to 0.83 by adding phosphoric acid to Rh/C in
scCO2, indicating that the cis ratio was increased by protons but
not by chloride anions.
Sum of products ðmolÞ
ꢀ
ꢁ
Conversion ð%Þ ¼
ꢁ 100
Sum of products ðmolÞ þ
unreacted 4 ꢀ isopropyl phenol ðmolÞ
Reaction profile and kinetic analysis
Fig. 1, 2 and, 3 show the reaction profiles for the hydrogenation
of 4-isopropylphenol over Rh/C in scCO2 and in 2-propanol, and
over HCl-Rh/C in scCO2, respectively. Lines shown in these
figures are obtained by curve fitting with rate equations as dis-
cussed later. In each reaction profile (Fig. 1–3), the amounts
of cis-4-isopropylcyclohexanol, trans-4-isopropylcyclohexanol,
and 4-isopropylcyclohexanone increased from the beginning
of the reaction, indicating that both 4-isopropylcyclohexanol
and 4-isopropylcyclohexanone were formed directly from 4-
isopropylphenol. As the reaction proceeded, the amount of
4-isopropylcyclohexanone decreased, and those of 4-cis-isopro-
pylcyclohexanol and 4-trans-isopropylcyclohexanol increased,
indicating the consecutive hydrogenation of 4-isopropylcyclo-
hexanone to 4-cis-isopropylcyclohexanol and 4-trans-isopropyl-
cyclohexanol in both solvents. In scCO2 (Fig. 1 and 3), the
ratio of 4-isopropylcyclohexanol to 4-isopropylcyclohexanone
was almost constant in the presence of 4-isopropylphenol,
indicating that the consecutive hydrogenation was inhibited by
ð1Þ
Each product ðmolÞ
ꢀ
ꢁ
Yield ð% Þ ¼
Sum of products ðmolÞþ
unreacted 4 ꢀ isopropyl phenol ðmolÞ
ꢁ 100
ð2Þ
ð3Þ
Each product ðmolÞ
Selectivity ð% Þ ¼
ꢁ 100
Sum of products ðmolÞ
cisꢀ4 ꢀ Isopropylcyclohexanol ðmolÞ
ꢀ
ꢁ
cis ratio ¼
ð4Þ
cisꢀ4 ꢀ Isopropylcyclohexanol ðmolÞþ
transꢀ4 ꢀ isopropylcyclohexanol ðmolÞ
Table 1 Hydrogenation of 4-isopropylphenola
Selectivity (%)
Conv.
(%)
cis-Isopropyl-
cyclohexanol
trans-Isopropyl-
cyclohexanol
Isoporopyl-
cyclohexanone
Isopropyl-
cyclohexane
Ring-opened cis
Catalyst
Solvent
alcohol
ratio
b
Rh/C
scCO2
100
76.6
66.6
84.4
73.1
78.5
21.2
24.5
11.7
15.3
18.2
0.5
0.1
0.2
0.1
1.1
1.0
8.7
3.0
11.4
1.9
0.5
0.1
0.7
0.1
0.4
0.78
0.73
0.88
0.83
0.81
Rh/C
2-Propanolc 99.9
b
HCl-Rh/Cd
HCl-Rh/Cd
scCO2
99.9
2-Propanolc 99.9
b
H3PO4-Rh/Ce scCO2
99.9
a Reaction conditions: initial 4-isopropylphenol, 2.0 mmol; initial hydrogen pressure, 2 MPa; catalyst weight, 0.06 g; temperature, 313 K; reaction
time, 60 min. b Total pressure (hydrogen + carbon dioxide), 12 MPa. c 10 cm3. d Hydrochloric acid content, 0.2 mmol g-cat.−1 e Phosphoric acid
content, 0.2 mmol g-cat.−1
.
.
634 | Green Chem., 2012, 14, 633–638
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Fig. 1 Hydrogenation of 4-isopropylphenol over Rh/C in scCO2: (●)
4-isopropylphenol, (○) 4-isopropylcyclohexanone, (■) cis-4-isopropyl-
cyclohexanol, (□) trans-4-isopropylcyclohexanol, (▲) isopropylcyclo-
hexane, (4) ring-opened alcohol. Solid line shows calculated value
using eqn (12)–(18). Reaction conditions: catalyst weight, 0.02 g; initial
hydrogen pressure, 2 MPa; total pressure (hydrogen + carbon dioxide),
12 MPa; reaction temperature, 313 K.
Fig. 3 Hydrogenation of 4-isopropylphenol over HCl-Rh/C in scCO2:
(●) 4-isopropylphenol, (○) 4-isopropylcyclohexanone, (■) cis-4-iso-
propylcyclohexanol, (□) trans-4-isopropylcyclohexanol, (▲) isopropyl-
cyclohexane, (4) ring-opened alcohol. Solid line shows calculated value
using eqn (12)–(18). Reaction conditions: catalyst weight, 0.02 g; hydro-
chloric acid content, 0.2 mmol g-cat.−1; initial hydrogen pressure, 2
MPa; total pressure (hydrogen + carbon dioxide), 12 MPa; reaction
temperature, 313 K.
Fig. 2 Hydrogenation of 4-isopropylphenol over Rh/C in 2-propanol:
(●) 4-isopropylphenol, (○) 4-isopropylcyclohexanone, (■) cis-4-iso-
propylcyclohexanol, (□) trans-4-isopropylcyclohexanol, (▲) isopropyl-
cyclohexane, (4) ring-opened alcohol. Solid line shows calculated value
using eqn (5)–(11). Reaction conditions: catalyst weight, 0.02 g; initial
hydrogen pressure, 2 MPa; 2-propanol, 10 cm3; reaction temperature,
313 K.
Scheme 1 Pathway for hydrogenation of 4-isopropylphenol over Rh/C
and HCl-Rh/C.
pathways in both scCO2 and 2-propanol can be summarized in
Scheme 1 based on the reaction profiles.
4-isopropylphenol in scCO2. In both solvents (Fig. 1–3), a trace
amount of isopropylcyclohexane was also formed from the
beginning of the reaction, and the amount of ring-opened
alcohol increased with a decrease in amount of 4-isopropylcyclo-
hexanone. Separate experiments confirmed that the isomerization
of 4-cis-isopropylcyclohexanol to the trans isomer did not
proceed under 2 MPa of hydrogen at 313 K. Hydrogenation
The rate (r, mol min−1) of each step shown in Scheme 1 was
evaluated by considering first order kinetics using the following
rate equations.
ron ¼ konCph
ð5Þ
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kde’Cph
rcis1 ¼ kcis1Cph
ð6Þ
rde
¼
ð15Þ
1 þ a1Cph
rtr1 ¼ ktr1Cph
rde ¼ kdeCph
rcis2 ¼ kcis2Con
rtr2 ¼ ktr2Con
rro ¼ kroCon
ð7Þ
ð8Þ
k
cis2’Con
rcis2
rtr2
rro
¼
¼
¼
ð16Þ
ð17Þ
ð18Þ
1 þ a2Cph
k
tr2’Con
ð9Þ
1 þ a2Cph
ð10Þ
ð11Þ
kro’Con
1 þ a2Cph
The parameters a1 and a2 were introduced in the above rate
equations for direct hydrogenation of 4-isopropylphenol and for
consecutive hydrogenation of 4-isopropylcyclohexanone,
respectively, to account for the strong adsorption of 4-isopropyl-
phenol on the catalyst surface. These equations predicted reac-
tion profiles which were in excellent agreement with those
obtained experimentally for 4-isopropylphenol hydrogenation in
scCO2 over Rh/C and HCl-Rh/C as shown in Fig. 1 and 3,
respectively. Table 3 shows kinetics parameters for 4-isopropyl-
phenol hydrogenation in scCO2.
The rate inhibition by 4-isopropylphenol observed in scCO2
suggests higher surface coverage by 4-isopropylphenol on active
sites in scCO2 than in 2-propanol, which is indicated by the
larger equilibrium constant of 4-isopropylphenol adsorption (eqn
(19)) in scCO2 than in 2-propanol.
Here, k is a rate constant, C is concentration (amount of sub-
stance/volume of solvent, mol dm−3), and the subscripts “ph”,
“on”, “cis”, “tr” “de” and “ro” denote 4-isopropylphenol, 4-iso-
propylcyclohexanone, cis-4-isopropylcyclohexanol, trans-4-iso-
propylcyclohexanol, isopropylcyclohexane, and ring-opened
alcohol, respectively. In the proposed rate equations, the term for
hydrogen concentration was not included because only 15% of
the total hydrogen introduced was consumed for complete con-
version of 4-isopropylphenol to 4-isopropylcyclohexanol and the
concentration of hydrogen did not significantly decrease during
the reaction. The reaction profiles in 2-propanol solvent fitted
well with the first order equations as shown in Fig. 2, and the
rate constants obtained are summarized in Table 2. On the other
hand, the model predictions of the first order rate equations did
not match well with the experimental reaction profiles in scCO2.
Hence, we proposed rate eqn (12) to (18), taking into account
rate inhibition by the reactant, 4-isopropylphenol.
!
4 ꢀ Isopropylphenol ðdissolvedÞ þ rhodium site
ꢀ isopropylphenol ðadsorbedÞ
4
ð19Þ
kon’Cph
The stronger adsorption of 4-isopropylphenol molecules on
rhodium sites in scCO2 can be explained by the lower solubility
of 4-isopropylphenol in scCO2 than that in 2-propanol.
ron
rcis1
rtr1
¼
ð12Þ
ð13Þ
1 þ a1Cph
kcis1’Cph
¼
1 þ a1Cph
Comparison of scCO2 and 2-propanol
Fig. 4 shows the rate of 4-isopropylphenol consumption (ron
+
ktr1’Cph
¼
ð14Þ
rcis1 + rtr1 + rde) and that of consecutive hydrogenation of 4-iso-
propylcyclohexanone (rcis2 + rtr2 + rro) over Rh/C in scCO2 and
2-propanol as a function of reaction time. The initial rate of
4-isopropylphenol consumption in scCO2 was 1.8 times higher
than that in 2-propanol. Moreover, the maximum rate of con-
secutive hydrogenation of 4-isopropylcyclohexanone in scCO2
was 2.6 times higher than that in 2-propanol. These higher rates
in scCO2 can be attributed to higher miscibility of hydrogen in
scCO2,7,8 which increases the surface concentration of hydrogen
on active sites. We did not include a separate term for the surface
concentration of hydrogen in the proposed rate equations
1 þ a1Cph
Table 2 First order rate constants
k
[10−3 dm3 min−1
]
for 4-
isopropylphenol hydrogenation in 2-propanol over Rh/Ca
kon
kcis1
ktr1
kde
kcis2
ktr2
kor
0.14
0.075
0.036
0.023
0.065
0.030
0.0001
a Reaction conditions: initial 4-isopropylphenol, 2.0 mmol; initial
hydrogen pressure, 2 MPa; catalyst weight, 0.02 g; 2-propanol, 10 cm3;
temperature 313 K.
Table 3 Parameters a [dm3 mol−1] and k′ [10−3 dm3 min−1] of the rate equation for 4-isopropylphenol hydrogenation in scCO2
a
Catalyst
a1
kon′
kcis1
′
ktr1
′
kde′
a2
kcis2
′
ktr2
′
kor′
Rh/C
100
100
5.1
2.8
4.5
2.8
1.3
0.44
0.059
0.085
3000
3000
0.85
1.9
0.25
0.32
0.038
0.080
HCl-Rh/Cb
a Reaction conditions: initial 4-isopropylphenol, 2.0 mmol; initial hydrogen pressure, 2 MPa; total pressure (hydrogen + carbon dioxide), 12 MPa;
catalyst weight, 0.02 g; temperature, 313 K. b Hydrochloric acid content, 0.2 mmol g-cat.−1
.
636 | Green Chem., 2012, 14, 633–638
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Table 4 cis ratio of 4-isopropylphenol hydrogenationa
cis ratio
Solvent
Catalyst
Direct hydrogenation of 4-isopropylphenol
Consecutive hydrogenation of 4-isopropylcyclohexanone
kcis1/(kcis1+ ktr1
0.68
)
kcis2/(kcis2+ ktr2)
0.68
2-Propanolb
Rh/C
k
cis1′/(kcis1′ + ktr1′)
kcis2′/(kcis2′ + ktr2′)
0.78
0.85
c
scCO2c
Rh/C
0.78
0.86
scCO2
HCl-Rh/Cd
a Reaction conditions: initial 4-isopropylphenol, 2.0 mmol; initial hydrogen pressure, 2 MPa; catalyst weight, 0.02 g; temperature, 313 K. b 10 cm3.
c Total pressure (hydrogen + carbon dioxide), 12 MPa. d Hydrochloric acid content, 0.2 mmol g-cat.−1
.
Scheme 2 Adsorbed intermediates for 4-alkylphenol hydrogenation
over metal catalysts.
hydrogenation of 4-isopropylphenol were close to those in con-
secutive hydrogenation in both scCO2 and 2-propanol over
Rh/C, which suggests that the adsorbed intermediates are the
same in the direct hydrogenation and in the consecutive hydro-
genation. The cis ratio value in scCO2 was higher than that in
2-propanol in the hydrogenation over Rh/C. It has been proposed
that the formation of trans isomer requires flipping via deso-
rption–reversal–re-adsorption of cyclohexene intermediate in the
hydrogenation of aromatic compounds.10–13,17 In addition, it is
known that 4-alkylcyclohexenen-1-ol (enol) is formed on the
catalyst surface in 4-alkylphenol hydrogenation, and this species
is in equilibrium with 4-alkylcyclohexanone (Scheme 2).18
Moreover, the cis isomer is formed from adsorbed 4-alkylcyclo-
hexanone and enol, while trans isomer is formed form enol.15
Based on this reaction mechanism, the cis ratio is determined by
the probability of the flipping of the 4-alkylcyclohexen-1-ol
intermediate. It is probable that the flipping of the 4-alkylcyclo-
hexen-1-ol intermediate is suppressed in scCO2 because of the
low dissolution ability of scCO2.
Fig. 4 (a) Rate of 4-isopropylphenol consumption (ron + rcis1 + rtr1
+
rde) and (b) that of consecutive hydrogenation of 4-isopropylcyclohexa-
none (rcis2 + rtr2 + rro) as a function of reaction time. Reaction conditions
are the same as Fig. 1 and 2 for the reaction in scCO2 and 2-propanol,
respectively.
Comparison of Rh/C and HCl-Rh/C
because partial pressure of hydrogen did not change significantly
during the reactions. Therefore, the rate constants in eqn (5) to
(18) represent a lumped constant which also includes the hydro-
gen concentration.
Table 4 shows cis ratio values of 4-isopropylcyclohexanol in
scCO2 and in 2-propanol. cis ratios were separately obtained in
the direct hydrogenation and in the consecutive hydrogenation as
follows.
The rate constants k′ (Table 3) can be directly compared for each
step in the hydrogenation over Rh/C and HCl-Rh/C because the
values of parameters a1 and a2 for HCl-Rh/C were the same as
those with Rh/C. The values of kon′, kcis1′, and ktr1′ decreased by
adding hydrochloric acid, indicating that the rate of 4-isopropyl-
phenol consumption was adversely affected by addition of
hydrochloric acid. In particular, the decrease in ktr1′ value (0.34
times) was higher than those of kon1′ and kcis1′ (0.55 and 0.62
times, respectively). On the other hand, the rate of consecutive
hydrogenation of 4-isopropylcyclohexanone (kcis2′ + ktr2′ + kro′)
increased. In particular, the increase in kcis2′ value (2.2 times)
was significant, indicating that the hydrogenation of 4-isopropyl-
cyclohexanone to cis-4-isopropylcyclohexanol was promoted by
hydrochloric acid. The promotion of consecutive hydrogenation
cis ratio ¼ rcisi=ðrcisi þ rtriÞ
¼ kcisi=ðkcisi þ ktriÞ ðin 2 ꢀ propanolÞ
or ¼ kcisi’=ðkcisi’ þ ktri’Þ ðin scCO2Þ
ð20Þ
Here, i is 1 or 2 for the direct hydrogenation and the consecu-
tive hydrogenation, respectively. The cis ratios in direct
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in the presence of hydrochloric acid was evident from the fact
that the reaction time needed for complete hydrogenation over
HCl-Rh/C reduced from 270 to 180 min (Fig. 1 and 3).
from The Ministry of Education, Culture, Sports, Science and
Technology, Japan.
The cis ratio with HCl-Rh/C did not change during the direct
hydrogenation and the consecutive hydrogenation (Table 4), and
it was higher than that with Rh/C. The higher cis ratio obtained
by adding hydrochloric acid can be explained by (1) the strong
adsorption of 4-isopropylcyclohexanone, which is hydrogenated
to only the cis isomer and/or (2) the promotion of hydrogenation
of adsorbed 4-isopropylcyclohexanone to the cis isomer. These
effects would be caused by the interaction of protons with
oxygen atoms of 4-isopropylcyclohexanone.19
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Acknowledgements
This work was supported by Special Coordination Funds for
Promoting Science and Technology “Development of Sustain-
able Catalytic Reaction System using Carbon dioxide and
Water”, and Grant-in-Aid for Scientific Research (21750161)
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