Stereoselective Aromatic Ring Hydrogenation over Supported Rhodium Catalysts in Supercritical Carbon Dioxide Solvent

Article abstract of DOI:10.1002/tcr.201800128

The combination of supported rhodium metal catalysts and supercritical carbon dioxide solvent was effective for the stereoselective ring hydrogenations of aromatic compounds at low temperature. Higher solubility of hydrogen in supercritical carbon dioxide provides higher concentration of hydrogen on the rhodium surface, but lower that of the intermediate on rhodium surface, which suppresses the flipping of surface intermediate, leading to higher catalyst activities and cis selectivities to the corresponding ring-hydrogenated products as compared with those in organic solvents.

Full text of DOI:10.1002/tcr.201800128

Personal Account
DOI: 10.1002/tcr.201800128
Stereoselective Aromatic Ring
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Hydrogenation over Supported Rhodium
Catalysts in Supercritical Carbon Dioxide
Solvent
T H E
C H E M I C A L
R E C O R D
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[
a]
[b]
[c]
M. Shirai,* N. Hiyoshi, and C. V. Rode
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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Abstract: The combination of supported rhodium metal catalysts and supercritical carbon
dioxide solvent was effective for the stereoselective ring hydrogenations of aromatic compounds
at low temperature. Higher solubility of hydrogen in supercritical carbon dioxide provides higher
concentration of hydrogen on the rhodium surface, but lower that of the intermediate on
rhodium surface, which suppresses the flipping of surface intermediate, leading to higher catalyst
activities and cis selectivities to the corresponding ring-hydrogenated products as compared with
those in organic solvents.
Keywords: hydrogenation, supercritical carbon dioxide, stereoselectivity, rhodium catalyst
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[
2,3]
without distillation are possible using scCO2 solvent.
1
. Introduction
Organic transformations using hydrogen gas with solid
Catalytic hydrogenation is an important core unit process in
catalysts in scCO solvent (multi-phase hydrogenation with
2
[
1]
chemical industry. The stereoselective hydrogenation of
aromatic compounds to the corresponding saturated com-
pounds is also important for chemical industry. For example,
cis-alkylcyclohexanols are important intermediates for the
production of fragrance and perfume industry, which could
be obtained by the hydrogenation of corresponding alkylphe-
nols. cis-Decalin, obtained by the hydrogenation of tetralin
and/or naphthalene, is a starting chemical for production of
sebacic acid that is used in the manufacture of 6, 10-nylone
and plasticizer and proposed as hydrogen storage media.
Vapor and liquid phase hydrogenation of aromatic
compounds having high boiling points, over supported metal
catalysts were commercially operated. Higher activities could
be obtained for vapor phase hydrogenation; however, the
yields and selectivities are low because coking of metal
catalysts occurred on acidic supports and the formation of
byproducts. Higher selectivities could be obtained for liquid
phase hydrogenation with organic solvents; however, reaction
rates are low and the separation of products from solvents is
critical. Moreover, eliminating the use of organic solvents is
highly desirable for environmentally benign processing.
supported metal catalysts and scCO ) has several other
2
advantages: (1) higher solubility of hydrogen in scCO2,
thereby controlling the product selectivity and activity, (2)
easy separation of products and catalysts and (3) maintaining
clean active sites of solid surfaces by their rinsing with scCO
solvent. Conceptual representation of the hydrogenation
2
[
4]
with supported metal catalysts in scCO is shown in Figure 1.
2
Several groups have succeeded in demonstrating the applica-
tion of the multiphase hydrogenation over supported metal
catalysts in scCO solvent for enhancing the activities and
2
[
2–8]
chemoselectivities for unsaturated compounds.
In this
account, we focus on the stereoselective ring hydrogenation of
aromatic compounds with the multiphase hydrogenation over
supported metal catalysts in supercritical carbon dioxide
solvent.
Supercritical carbon dioxide (scCO2 having, critical
temperature of 304.2 K and critical pressure of 7.38 MPa) is
a prominent candidate as an alternative solvent because of its
non-toxic and non-flammable properties. Also, higher reac-
tion rates and simple separation of products from the system
Figure 1. Multiphase hydrogenation with supported metal catalysts and
supercritical carbon dioxide solvent.
[a] Prof. M. Shirai
Faculty of Science and Technology
Iwate University
4
-3-5, Ueda, Morioka, Iwate 020-8551, Japan
E-mail: mshirai@iwate-u.ac.jp
b] Dr. N. Hiyoshi
2. Experimental
[
Research Institute for Chemical Process Technology
Advanced Institute of Science and Technology (AIST)
-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan
The catalytic hydrogenation over supported metal catalysts in
scCO was studied in a batch reaction system (Figure 2). The
2
4
system was composed of hydrogen and carbon dioxide
cylinders, carbon dioxide feed pump, stainless steel reactor
(50 ml), oil bath with jacket, and back-pressure valve.
[c] Dr. C. V. Rode
Chemical Engineering and Process Division
National Chemical Laboratory
Dr. Homi Bhabha Road, Pune 411008, India
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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reactor was rapidly cooled in an ice bath, the pressure was
slowly released, and the contents were discharged to separate
the catalysts by simple filtration with organic solvent.
For the catalytic hydrogenation in organic solvents, the
same stainless reactor was used. After the weighed amounts of
catalyst, substrate, and organic solvent were placed in the
reactor, the reactor was purged with nitrogen to remove air
and heated in an oil bath to the desired temperature.
Hydrogen was introduced into the reactor to the desired
pressure levels and the contents were stirred magnetically.
After the desired time, the reactor was rapidly cooled in an
ice bath, the pressure was slowly released, and the contents
were discharged to separate the catalysts by simple filtration
with the organic solvent.
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Commercially available 5 wt% charcoal-supported metal
[
9]
catalysts (rhodium (Rh/C), metal dispersion 12% ), ruthe-
Figure 2. Apparatus for multiphase hydrogenation with supported metal
catalyst and supercritical carbon dioxide.
[
9]
[9]
nium (Ru/C, 25% ), palladium (Pd/C, 3% ), and plati-
[
9]
num (Pt/C, 8% )) from Wako Chemicals, were used in this
study.
The hydrogenation procedure with scCO solvent was as
2
follows. After the weighed amounts of catalyst, substrate and
a magnetic bar were placed in the reactor, the reactor was
purged with carbon dioxide to remove air and heated in an
oil bath to the desired temperature. Hydrogen and then
carbon dioxide were introduced into the reactor to the desired
pressure levels and contents were stirred magnetically. We
designated the reaction start time as the completion of the
hydrogen introduction. The carbon dioxide pressure was
determined from the difference of the total pressure and
hydrogen pressure introduced. After the desired time, the
3. Hydrogenation of Aromatic Ring
3.1. Phenol Hydrogenation
Adipic acid, which is an intermediate for nylon 6, 6, is
manufactured by the oxidation of cyclohexanone (None),
cyclohexanol (Nol) or both. None and Nol are industrially
produced by vapor phase ring hydrogenation of phenol over
Pd catalysts for which high temperature is employed causing
catalyst deactivation due to coke formation. Liquid phase
Masayuki SHIRAI received Ph.D. at Uni-
versity of Tokyo in 1993. After working as
an associate professor at Tohoku Univer-
sity and as a team leader at National
Institute of Advanced Industrial Science
and Technology (AIST), he has been a full
professor of Iwate University since 2013.
His research interest is green chemistry
using supported metal catalysts in carbon
dioxide and/or water solvents under high-
pressure including supercritical conditions.
the development of catalytic materials,
their application, and characterization us-
ing aberration-corrected (scanning) trans-
mission electron microscopy.
Chandrashekhar RODE obtained his
Ph.D. from University of Pune in 1993.
He has been working as a Scientist at
CSIR-National Chemical Laboratory,
Pune India and currently, he is an Emer-
itus Scientist at NCL Pune. He is a Fellow
of Indian National Academy of Engineer-
ing (FNAE). His research interests include
Catalysis for Sustainable Development
involving bio-mass utilization to value
added products and Catalysis in super-
critical carbon dioxide medium.
Norihito HIYOSHI received his Ph.D.
form Hokkaido University in 2002. He
worked as a researcher at Research Insti-
tute of Innovative Technology for the
Earth (RITE), from 2002 to 2004. He
then moved to National Institute of
Advanced Industrial Science and Technol-
ogy (AIST). His research interests include
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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hydrogenation under mild conditions is therefore, more
desirable. Hence, the ring hydrogenation of phenol was
studied over commercially available supported noble metal
indicating that the dehydroxylation could be successfully
suppressed in scCO solvent.
2
The hydrogenation of 4-t-BNone, which is a partial
hydrogenation product from 4-t-BP, was also studied over
Rh/C under the same reaction conditions (Table 2). The cis
catalysts in scCO . The catalyst screening results for phenol
2
hydrogenation with commercially available charcoal-sup-
ported metal catalysts at 328 K under 10 MPa of scCO are
ratio obtained for t-BNone hydrogenation in scCO and in 2-
2
2
[
10,11]
shown in Table 1.
Rh/C, Ru/C and Pd/C catalysts were
propanol was almost the same as those for 4-t-BP hydro-
genation. It was confirmed that the hydrogenation of 4-t-BP
and 4-t-None in 2-propanol without hydrogen did not
proceed, indicating that hydrogen transfer from 2-propanol
did not occur and hydrogen atoms from hydrogen gas reacted
with 4-t-BP molecules under the reaction conditions.
active for the ring hydrogenation of phenol to None and Nol.
On the other hand, Pt/C, which is an active catalyst for
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hydrogenation, was inactive in scCO . It is probable that
2
carbon monoxide, which was formed by the water gas shift
reaction during hydrogenation in scCO , deactivate the
2
platinum sites. Table 1 shows that rhodium is the most active
Figure 3(a) shows the reaction profile for the hydro-
metal for the benzene ring hydrogenation in scCO while, Pd
genation of 4-t-BP in 15 MPa of scCO solvent. The ring
2
2
was found to be the most active catalyst under non-
hydrogenation reaction, c-4-t-BNol, t-4-t-BNol, 4-t-BNone,
and small amounts of other side products (4-t-BH and ring-
opened alcohol) were formed. The selectivity pattern and cis
ratio were almost constant up to 60 min, indicating that 4-t-
BNols and 4-t-BNone were formed directly from 4-t-BP.
Once most of 4-t-BP was hydrogenated (within 60 min), the
amount of 4-t-BNone decreased and the amounts of c-4-t-
BNol, t-4-t-BNol increased, indicating that consecutive
hydrogenation of 4-t-BNon to 4-t-BNols proceeded. Small
amounts of ring-opened alcohol also were formed after
60 min. It was confirmed separately that the isomerization of
c-4-t-BNol to trans isomer t-4-t-BNol did not occur under
the reaction conditions.
supercritical conditions. Under the scCO conditions, higher
2
hydrogen solubility in scCO does contribute to the higher
2
catalyst activity but additionally, the adsorption characteristics
of phenol on active metal sites and the electronic structures of
metal particles also influence the activity and selectivity
[10]
behavior in supercritical CO solvent.
2
[
a]
Table 1. Catalyst screening for phenol hydrogenation.
Selectivity (%)
Catalyst
Conversion (%)
None
Nol
TON
Rh/C
Ru/C
Pd/C
53
30
3
17
5
54
97
83
95
46
3
8244
759
304
125
[
b]
Pt/C
1
[
a]
Reaction temperature 328 K, reaction time 2 h, H
pressure
2
[
b]
1
0 MPa, CO pressure 10 MPa, Reaction temperature 353 K.
2
3.2. Alkylphenol Hydrogenation
Stereoselective hydrogenation of 4-tert-butylphenol (4-t-BP)
produces cis-4-tert-butylcyclohexanol (c-4-t-BNol), which is
an important raw material for the production of perfume and
[12]
fragrance intermediates. The multiphase hydrogenation of
4-t-BP was studied using Rh/C catalyst in scCO . Table 2
2
gives the results of the hydrogenation of 4-t-BP under
10 MPa of scCO and in 2-propanol solvents over 0.02 g of
2
[
13,14]
Rh/C at 313 K.
Hydrogenation of aromatic rings of 4-t-
BP proceeded and c-4-t-BNol, trans-4-tert-butylcyclohexanol
(t-4-t-BNol), 4-tert-butylcyclohexanone (4-t-BNone) were
formed for both the solvent systems. It is noteworthy that
higher selectivity to c-4-t-BNol and higher cis ratio (c-4-t-
BNol/(c-4-t-BNol+t-4-t-BNol)*100) were obtained in
scCO compared with those in 2-propanol. The amounts of
2
undesirable byproduct due to dehydroxylation (tert-butylcy-
clohexane (4-t-BH)) and ring-opening reactions in 10 MPa of
carbon dioxide was six times lower than those in 2-propanol,
Figure 3. Hydrogenation of 4-t-BP over Rh/C in carbon dioxide (15 MPa)
(a) and 10 mL of 2-propanol (10 mL) (b) [14]. Catalyst weight, 0.005 g,
hydrogen pressure 2 MPa, reaction temperature 313 K.
Chem. Rec. 2019, 19, 1–10
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Table 2. Hydrogenation of 4-tert-butylphenol (4-t-BP) and 4-tert-butylcyclohexanone (4-t-BPNone) over supproted rhodium catalyst.
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[
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substrate 0.02 g. hydrigen pressure 2 MPa, carbon dioxide pressure 10 MPa, 2-propanol 10 mL, catalyst weight 0.02 g. temperature 313 K,
reaction time 60 min.
The hydrogenation profile in 10 mL of 2-propanol
solvent is shown in Figure 3(b). The initial hydrogenation
rate of 4-t-BP hydrogenation was half of that in 15 MPa of
CO . c-4-t-BNol, t-4-t-BNol, 4-t-BNon, 4-t-BH, and ring-
2
opened alcohol also were formed from the beginning of the
reaction in 2-propanol. For the parallel hydrogenation of 4-t-
BP proceeded in 2-propanol; however, the initial selectivity to
4-t-BNone (70%) in 2-propanol was higher than that in the
scCO (55%) at the beginning of the reaction. After 40 min,
2
the selectivity to 4-t-BNone decreased and that to c-4-t-BNol
and t-4-t-BNol increased, indicating that the consecutive
hydrogenation of 4-t-BNone to c-4-t-BNol and t-4-t-BNol
proceeded simultaneously in the presence of 4-t-BP. The cis
ratio in 2-propanol was as low as 0.70. It was also confirmed
that the isomerization of c-4-t-BNol to trans isomer did not
occur in 2-propanol over Rh/C.
It is reported that partially hydrogenated enol species are
formed and they are interconverted rapidly to the correspond-
Scheme 1. Hydrogenation of aromatic compounds over metal catalyst.
[15–17]
ing cyclohexanone species
and cis form of cyclohexanol
was formed from the enol species by the cis addition of
hydrogen atoms and trans form of cyclohexanol was formed
by the flipping of the enol species and the following cis
addition of hydrogen atoms for ring hydrogenation of
phenols (Scheme 1).
The reaction pathway of 4-t-BP hydrogenation over active
rhodium sites is shown in Scheme 2. 4-t-BP molecules
adsorbed on rhodium surface (i) are partially hydrogenated to
4-tert-butylcyclohexene-1-ol (4-t-BEnol) by the cis addition
of four hydrogen atoms (ii). 4-t-BEnol adsorbed species are
in equilibrium with 4-t-BNone (tautomerization) (ii). 4-t-
BEnol adsorbed species are hydrogenated to c-4-t-BNnol by
the cis addition of hydrogen atoms (iv), or alternatively, is
flipped over (v) and hydrogenated to t-4-t-Bnol (vi). 4-t-
BNone also is either hydrogenated to c-4-t-BCNol or
desorbed from the surface (viii). The final cis ratio values for
the hydrogenation of 4-t-BP and 4-t-BNone were almost the
same, indicating that reaction steps in the hydrogenation of
4-t-BNone are the same as those of 4-t-BNone formed by the
tautomerization from 4-t-BEnol in the hydrogenation of 4-t-
BP. 4-t-BNone would adsorb on active sites in its chair form
Chem. Rec. 2019, 19, 1–10
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species is critical for the cis selectivity. For the hydrogenation
of 4-t-BP, the cis ratio of 4-t-BNol in scCO was higher than
2
those in 2-propanol. The high selectivity in scCO is because
2
of (1) the higher concentration of hydrogen atoms on
rhodium sites in scCO than in 2-propanol solvent under the
2
same hydrogen pressure, because of the greater solubility of
hydrogen in scCO and (2) the suppression of the flipping of
2
adsorbed 4-t-BEnol in scCO because of the lower solubility
2
of adsorbed 4-t-BEnol species compared with that in 2-
propanol.
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It is a unique feature of scCO solvent that the solubility
2
of reactants can be enhanced by the increase of carbon
dioxide densities. Figure 4 shows the carbon dioxide pressure
on 4-t-BP hydrogenation over Rh/C. The conversion of 4-t-
BP increased; however, cis ratio decreased with an increase in
carbon dioxide pressure. Direct observation using a view cell
showed that 4-t-BP was dissolved in 10 MPa of scCO and
2
2 MPa of hydrogen at 313 K, and most of 4-t-BP was
dissolved in 25 MPa of carbon dioxide and 2 MPa of
hydrogen pressure. The higher solubility of 4-t-BP at higher
carbon dioxide pressures enhanced hydrogenation activities
and decreased the cis ratio.
Figure 4. Carbon dioxide pressure effect on 4-tert-butylphenol hydrogenation
over charcoal-supported rhodium catalyst and phase of 4-tert-butylphenol
under 2 MPa of hydrogen at 313 K in carbon dioxide.
Scheme 2. Proposed mechanism for of 4-tert-butylphenol hydrogenation
over charcoal-supported rhodium catalyst.
The decrease in the cis ratio at higher carbon dioxide
pressures would be caused by the enhanced flipping of 4-t-
BEnol because of the higher solubility of 4-t-BEnol at higher
carbon dioxide pressures. This higher cis selectivity over Rh/
C in scCO2 than in 2-propanol was also observed for
with the tert-butyl group in an equatorial position (ix) and
would not readsorb after flipping over, due to the steric
hindrance of the tert-butyl group (x). This reaction scheme
shows that the adsorption of the intermediate of 4-t-BEnol
Chem. Rec. 2019, 19, 1–10
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[a]
Table 3. Hydrogenation of 4-alkylphenol over Rh/C.
Alkyl group
Solvent
Conversion (%)
Selectivity
c-4-RNol
cis ratio
t-4-RNol
t-RNone
4-RXne
Cyclohexyl
tert-Butyl
sec-Butyl
n-Butyl
CO (15 MPa)
99.9
99.8
99.9
99.2
99.9
99.9
99.9
99.9
100
99.9
99.9
99.9
100
74.5
58.7
75.0
66.4
77.5
65.3
75.3
59.2
76.6
66.6
76.0
59.3
73.7
60.1
72.7
63.5
21.5
28.6
20.0
28.5
20.0
23.6
22.3
26.8
21.2
24.4
21.4
26.8
23.1
26.1
22.3
24.5
1.3
0.2
2.7
1.2
0.8
0.2
1.3
0.2
0.5
0.1
0.7
0.2
0.5
0.1
3.0
0.1
2.1
12.5
0.6
3.8
1.3
10.8
2.3
13.5
1.0
8.7
1.4
13.6
1.3
12.6
1.5
11.7
0.77
0.67
0.79
0.70
0.80
0.73
0.77
0.69
0.78
0.73
0.78
0.69
0.76
0.70
0.77
0.72
2
2
-propanol (10 mL)
CO (15 MPa)
2
2
-propanol (10 mL)
CO2 (10 MPa)
2-propanol (10 mL)
CO (10 MPa)
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
2
-propanol (10 mL)
iso-Propyl
n-Propyl
Ethyl
CO (10 MPa)
2
2
-propanol (10 mL)
CO (10 MPa)
2
2
-propanol (10 mL)
CO (10 MPa)
2
2
-propanol (10 mL)
2
99.8
100
Methyl
CO (10 MPa)
2
-propanol (10 mL)
99.9
[
a]
substrate 0.02 mmol; initial H pressure 2 MPa, temperature 313 K, reaction time 60 min.
2
hydrogenation for several types of alkylphenol. Table 3 shows
the hydrogenation of several 4-alkylphenols (4-RP) in scCO2
and in 2-propanol. Higher selectivity to cis-4-alkylcyclohex-
anol (c-4-RNol) was obtained in scCO than in 2-propanol
for the hydrogenation of all 4-RP substrates tested.
promotion of hydrogenation of adsorbed 4-t-BNone to the
cis isomer. These effects are caused by the interaction of
protons with oxygen atoms of 4-t-BNone (Scheme 3).
2
[18,19]
The
selectivity to alkylcyclohexane (4-RX) in scCO was much
2
lower than that in 2-propanol, indicating that dehydroxyla-
tion was suppressed in scCO . The cis ratios for various 4-RP
hydrogenations obtained in scCO were in the range of 0.76–
0.80, while those in 2-propanol were in the range of 0.67–
0.73. Hydrogenation of 4-RP having branched alkyl groups
showed slightly higher cis ratio than those having normal
2
2
alkyl groups in both scCO and 2-propanol.
2
The addition of protonic acids to the catalyst systems is
known to be effective to enhance the cis-selectivity of the 4-
RP hydrogenation. Table 4 also shows the hydrogenation of
Scheme 3. Proposed mechanism for of 4-tert-butylphenol hydrogenation
over HCl-modified charcoal-supported rhodium catalyst.
[
8]
4-RP over hydrochloric acid modified Rh/C (HClÀRh/C)
[18]
catalyst in scCO and 2-propanol. It should also be noted
2
3.3. Naphthalene Hydrogenation
that higher cis ratio values were obtained in scCO than in 2-
2
propanol with HClÀRh/C. The XPS analysis of the
HClÀRh/C catalyst before and after the treatment with
Catalyst screening results showed that Rh/C was the most
active for the naphthalene (Naph) hydrogenation in
scCO2.
solvent, tetrahydronaphthalene (Tetra), octahydronaphthalene
(Octa), cis-decahydronaphthalene (c-Deca) and trans-decahy-
dronaphthalene (t-Deca) were obtained as hydrogenated
products. We could not determine the position of carbon-
carbon double bond in Octa. Among all the catalysts screened
in this work, the Rh/C was also the most active for the ring
[
16]
scCO and 2-propanol showed that most of hydrochloric acid
In the hydrogenating of naphthalene in scCO2
2
remained on the catalyst surface after scCO2 treatment,
whereas a major part of hydrochloric acid was leached in 2-
propanol because of the lower solubility of HCl into
[
13]
scCO2.
The cis ratio with HClÀRh/C did not change
during the direct hydrogenation and the consecutive hydro-
genation, 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-t-BNone, which
is hydrogenated to only the cis isomer and/or (2) the
hydrogenation of Naph in scCO solvent.
Figure 5(a) shows the reaction profile of hydrogenation of
Naph over the Rh/C catalyst under 10 MPa of scCO solvent.
2
2
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
7

P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
[a]
Table 4. Hydrogenation of 4-alkylphenol over HCIÀRh/C.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
[
a]
substrate 0.02 mmol; initial H
pressure 2 MPa, temperature 313 K, reaction time 60 min.
2
consumed, the amount of Tetra decreased, and those of Octa,
c-Deca and t-Deca increased, indicating that the hydro-
genation of Naph to Deca proceeded consecutively through
the hydrogenation of Tetra and Octa. After most of Tetra and
Octa were hydrogenated, the amounts of c-Deca and t-Deca
did not change and the final cis ratio (c-Deca/c-Deca+t-
[
20]
Deca) was as high as 0.88.
It was confirmed that the
isomerization of c-Deca did not proceed over Rh/C under
[
21]
these reaction condition.
The hydrogenation of Naph over the Rh/C catalyst in n-
heptane is also shown in Figure 5(b). Naph was also
completely converted to Deca over the Rh/C catalyst in n-
heptane and the final cis ratio in n-heptane was 0.86, which
is slightly lower than that obtained in scCO2.
The hydrogenation of Tetra was also studied in scCO2.
Figure 6 shows the Tetra hydrogenation profile over Rh/C
[
22]
under 3 MPa of hydrogen in 10 MPa of scCO2 solvent.
The initial Tetra hydrogenation rate was almost the same as
the Tetra hydrogenation in Naph hydrogenation. The final
cis ratio was 0.88, was the same as that obtained in Naph
hydrogenation. Similar Tetra hydrogenation rates and final cis
ratio in Figures 5 and 6 indicate that hydrogenation of Naph
Figure 5. Naphthalene hydrogenation over Rh/C (0.010 g) under 3 MPa of
hydrogen in 10 MPa of scCO (a) and in 10 mL of n-heptane at 313 K.
2
As the amount of Naph decreasing, Tetra, which is a partially
ring hydrogenated compound, was formed simultaneously
from the beginning of the reaction. After most of Naph was
is a consecutive reaction via Tetra in scCO solvent
2
It has been reported that c-Deca is formed by the
hydrogenation of Tetra adsorbed on metal sites while, t-Deca
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
8

P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
References
[
1] Paul N. Rylander, “Hydrogenation and Dehydrogenation” in
Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH,
Weinheim, 2005. doi:10.1002/14356007.a13_487.
2] P. G. Jessop, W. Leitner in Chemical Synthesis Using Super-
critical Fluids (Eds.: P. G. Jessop, W. Leitner), Wiley, New
York, 1999.
[
[3] A. Baiker, Chem. Rev. 1999, 99, 453–474.
[
[
[
[
4] M. Arai, S. Fujita, M. Shirai, J. Supercrit. Fluids 2009, 47(3),
51–356.
5] B. Subramanian, C. J. Cyon, V. Avunajatesun, Appl. Catal. B
002, 37, 279–292.
6] P. Licence, J. Ke, M. Sokolave, S. K. Ross, M. Poliakoff, Green
Chem. 2003, 5, 99–104.
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
Figure 6. Tetralin hydrogenation over Rh/C (0.010 g) under 3 MPa of
2
hydrogen and 10 MPa of carbon dioxide at 313 K in 10 MPa of scCO (a)
2
and in 10 mL of n-heptane.
7] M. Burgener, D. Ferri, J. D. Grunwaldt, T. Mallat, A. Baiker,
J. Catal. 2005, 109, 16794–16800.
is formed via flipping of the intermediate 2,3,4,5,6,8,10-
[8] J. Kobayashi, Y. Mori, S. Kobayashi, Chem. Commun. 2005,
1,9
octahydronaphthalene (D -Octa), in which it desorbs from
2567–2568.
[
9] T. Sato, C. V. Rode, O. Sato, M. Shirai, Appl. Catal. B 2004,
49(3), 181–185.
the metal surface and immediately readsorbs on its other side,
[
23]
followed by its further hydrogenation to give t-Deca.
[
[
[
10] C. V. Rode, U. D. Joshi, O. Sato, M. Shirai, Chem. Commun.
Scheme 4 shows the reaction mechanism of Naph hydro-
genation on Rh/C. Similar to the ring hydrogenation of 4-t-
BP at low temperature described in section 3.2, higher cis
ratio was obtained for hydrogenations of Naph and Tetra in
2003, 1960–1961.
11] C. V. Rode, U. D. Joshi, T. Sato, O. Sato, M. Shirai, Stud.
Surf. Sci. Catal. 2004, 153, 385–388.
12] M. Sekiguchi, S.Tanaka, Jpn. Patent 3528519, 2004.
scCO solvent than organic solvents. Higher hydrogen atom
2
[13] N. Hiyoshi, E. Mine, C. V. Rode, O. Sato, T. Ebina, M. Shirai,
Chem. Lett. 2006, 35(9), 1060–1061.
[14] N. Hiyoshi, C. V. Rode, O. Sato, H. Tetsuka, M. Shirai, J.
Catal. 2007, 252(1), 57–68.
concentration on rhodium sites could enhance the cis
addition of hydrogen atom to the surface intermediate and
lower solubility of surface intermediate would retard the
flipping of the intermediate.
[
[
[
[
15] H. A. Smith, B. L. Stump, J. Am. Chem. Soc. 1961, 83, 2739–
2
743.
16] S. R. Konuspaev, K. N. Zhanbekov, N. V. Kul’kova, D. Y.
Murzin, Chem. Eng. Technol. 1997, 20, 144–148.
17] S. Siegel, G. V. Smith, B. Dmuchovsky, D. Dubbell, W.
Halpern, J. Am. Chem. Soc. 1962, 84, 3136–3139.
18] N. Hiyoshi, K. K. Bando, O. Sato, A. Yamaguchi, C. V. Rode,
M. Shirai, Catal. Commun. 2009, 10(13), 1702–1705.
[19] N. Hiyoshi, O. Sato, A. Yamaguchi, C. V. Rode, M. Shirai,
Green Chem. 2012, 14, 638–633.
[20] N. Hiyoshi, M. Osada, C. V. Rode, O. Sato, M. Shirai, Appl.
Catal. A 2007, 331, 1–7.
[
21] N. Hiyoshi, E. Mine, C. V. Rode, O. Sato, M. Shirai, Chem.
Lett. 2006, 35, 188–189.
1,9
Scheme 4. Formation of c-Deca (a) and t-Deca via
D -Octa in Naph
hydrogenation.
[22] N. Hiyoshi, E. Mine, C. V. Rode, O. Sato, M. Shirai, Appl.
Catal. A 2006, 310, 194–98.
[
23] W. Weitkamp, Adv. Catal. 1968, 18, 1–110.
4. Summary
Multi-phase hydrogenation with supported rhodium catalysts
and supercritical carbon dioxide solvent was effective to give
higher reaction rates and higher cis ratio for ring hydro-
genation of aromatic compounds.
Manuscript received: August 15, 2018
Revised manuscript received: November 20, 2018
Accepted: November 21, 2018
Version of record online: && &&, &&&&
Chem. Rec. 2019, 19, 1–10
© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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PERSONAL ACCOUNT
Prof. M. Shirai*, Dr. N. Hiyoshi,
Dr. C. V. Rode
1 – 10
Stereoselective Aromatic Ring Hy-
drogenation over Supported
Rhodium Catalysts in Supercritical
Carbon Dioxide Solvent
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
In this account, show stereoselective ring
hydrogenations of aromatic compounds
over supported rhodium metal catalysts
in supercritical carbon dioxide. Higher
solubility of hydrogen in supercritical
carbon dioxide provides higher hydrogen
concentration on rhodium surface, but
lower that of the intermediate on
rhodium surface, which suppresses the
flipping of surface intermediate, gave
higher catalyst activities and cis selectiv-
ities to the corresponding ring-hydrogen-
ated products as compared with those in
organic solvents.