Transfer hydrogenation of 4-propylphenol using ethanol and water over charcoal-supported palladium catalyst

Article abstract of DOI:10.1246/cl.160204

The aromatic hydrogenation of 4-propylphenol to 4-propylcyclohexanone, cis- and trans-4-propylcyclohexanols proceeded over a charcoal-supported palladium catalyst (Pd/C) in water-ethanol cosolvent at 573 K without using any external hydrogen gas. The ring hydrogenation activities in water-ethanol cosolvent over Pd/C were higher than those with a conventional method using externally supplied hydrogen gas. Both water and ethanol were indispensable for the ring hydrogenation in the water-ethanol cosolvent at 573 K.

Full text of DOI:10.1246/cl.160204

Received: February 29, 2016 | Accepted: March 21, 2016 | Web Released: March 30, 2016
CL-160204
Transfer Hydrogenation of 4-Propylphenol Using Ethanol and Water
over Charcoal-supported Palladium Catalyst
Yoshiyuki Nagasawa,1 Hidetaka Nanao,1 Osamu Sato,2 Aritomo Yamaguchi,2,3 and Masayuki Shirai*1,2
1Department of Chemistry and Bioengineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551
2Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST),
4-2-1 Nigatake, Miyagino, Sendai, Miyagi 983-8551
3JST, PRESTO, 4-2-1 Nigatake, Miyagino, Sendai, Miyagi 983-8551
(E-mail: mshirai@iwate-u.ac.jp)
The aromatic hydrogenation of 4-propylphenol to 4-
propylcyclohexanone, cis- and trans-4-propylcyclohexanols
proceeded over a charcoal-supported palladium catalyst (Pd/C)
in water-ethanol cosolvent at 573 K without using any external
hydrogen gas. The ring hydrogenation activities in water-
ethanol cosolvent over Pd/C were higher than those with a
conventional method using externally supplied hydrogen gas.
Both water and ethanol were indispensable for the ring
hydrogenation in the water-ethanol cosolvent at 573 K.
phenol, and a mixture of water and ethanol as solvent, were
loaded into the reactor. Composition of water-ethanol mixture
was varied as 3.0 mL of water (0 ethanol molar fraction), 2.9 mL
of water + 0.1 mL of ethanol (0.011), 2.5 + 0.5 (0.057), 2.0 +
1.0 (0.13), 1.5 + 1.5 (0.23), 1.0 + 2.0 (0.38), and 0.5 + 2.5
(0.60), and 3.0 mL of ethanol (1.0). The reactor was thoroughly
purged by argon gas ten times for the removal of any residual
air. Then the reactor was submerged in a sand bath (ACRAFT,
model AT-1B) and was maintained at 573 K. After a given
reaction time, the reactor was taken out from the sand bath and
submerged in a water bath for rapid cooling to room temper-
ature. Gaseous products were collected by a syringe through
sampling loops attached to a gas chromatograph with a thermal
conductivity detector (GC-TCD) (SHIMADZU, model GC-8A).
After sampling the gaseous products, the slurry was filtered to
remove the catalyst and the clear liquid fraction was recovered
with tetrahydrofuran (THF). The products dissolved in THF
were analyzed by a gas chromatograph with a flame ionization
detector (GC-FID) (SHIMADZU, model GC-14B) and by a gas
chromatograph with a mass spectrometer (GC-MS) (Agilent
Technologies, model HP-7890) with a DB-WAX capillary
column.
Hydrogenation over Pd/C with gaseous hydrogen was
carried out as follows. After the removal of residual air in the
reactor having 0.10 g of 4-propylphenol, 0.15 g of Pd/C, and
3.0 mL of water, 1.0 and 3.0 MPa of hydrogen gas was
introduced into the reactor. Then the reactor was heated in the
sand bath at 573 K. After 1 h, the reactor was cooled in a water
bath and the gaseous and liquid products were analyzed by GC-
TCD and GC-FID, respectively.
No gaseous product was formed and the hydrogenation of
4-propylphenol did not proceed in the absence of any supported
metal catalysts in a water-ethanol cosolvent system (2.0 mL of
water and 1.0 mL of ethanol (ethanol molar fraction 0.13)) at
573 K. However, various gaseous products were formed over
the supported metal catalysts in the water-ethanol cosolvent,
indicating that ethanol would be gasified over the supported
metal catalysts in water. Among the several catalysts screened in
this study, Pt/C was the most active for the hydrogen production
in the water-ethanol cosolvent system; however, the hydro-
genation of 4-propylphenol did not proceed over Pt/C. On the
other hand, a Pd/C catalyst was active for the hydrogenation
of 4-propylphenol to give 4-propylcyclohexanone, and cis- and
trans-4-propylcyclohexanols under the present reaction condi-
tions. A control experiment without 4-propylphenol showed the
formation of hydrogen along with carbon monoxide, carbon
dioxide, and hydrocarbons (methane and ethane), indicating that
ethanol could be gasified over Pd/C also and hydrogen formed
Keywords: Aromatic hydrogenation
| Palladium | Biomass
Many important chemicals are produced from petroleum,
which is a finite resource. For the sustainable production of
chemicals, it is inevitable to utilize renewable resources like
biomass (mainly lignocellulose). Valuable chemicals can be
obtained by partial decomposition, dehydration and/or hydro-
genation of lignocellulose, which is a polymer containing
functional group with several oxygen atoms. The research on
catalytic conversion of lignocellulose to chemicals is being
pursued extensively over last two decades.1-10 Here, we report
the development of synthetic strategies to obtain high-value-
added chemicals from lignocellulose by reacting two or more
biomass-derived compounds with each other. As an example,
hydrogenation of lignin-derived alkylphenols with cellulose-
derived bioethanol to produce alkylcyclohexanone and alkyl-
cyclohexanols will be demonstrated in this communication.
Liquid-phase hydrogenation of alkylphenols to the correspond-
ing alkylcyclohexanone and alkylcyclohexanols, which are
useful intermediates for fragrance and perfume, is well reported
over supported rhodium catalysts under high pressure of external
hydrogen gas.11-14 However hydrogen gas is highly explosive
and inflammable; thus, use of external hydrogen gas poses a
serious problem of its safe handling and it becomes a major cost
center in the whole process economics. This can be overcome
using ethanol as a hydrogen donor.15,16 So far, there is no report
on the ring hydrogenation of aromatic compounds with ethanol
and supported metal catalysts, which will be reported in this
paper for the first time. This approach will open a new route
to convert lignin-derived phenols into valuable chemicals using
cellulose-derived ethanol as a hydrogen donor.
Commercially available charcoal-supported 5 wt % palladi-
um (Pd/C), rhodium (Rh/C), platinum (Pt/C), and ruthenium
(Ru/C) catalysts, and 4-propylphenol (97%) procured from
Wako Pure Chemical Ind. Ltd., Japan, were used in this study
without any further pretreatment.17 Catalytic hydrogenation was
conducted in a 316 stainless steel tube reactor having an internal
volume of 6.0 cm3.18 Desired amounts of catalyst, 4-propyl-
Chem. Lett. 2016, 45, 643–645 | doi:10.1246/cl.160204
© 2016 The Chemical Society of Japan | 643

would be responsible for the hydrogenation of 4-propylphenol,
which will be discussed later. Compared with the Pt/C and Pd/C
catalysts, a large amount of methane was formed among the
gaseous products over Ru/C and Rh/C catalysts in the water-
ethanol cosolvent system; however, the aromatic hydrogenation
did not proceed. Pd/C was the sole active catalyst for the
hydrogenation of the aromatic ring of 4-propylphenol in water-
ethanol cosolvent without using gaseous hydrogen. The gas-
ification of ethanol proceeded at the lower reaction temperatures;
however, the hydrogenation did not proceed over Pd/C in
water-ethanol cosolvent at 523 K. Hence, we focused on the
hydrogenation behavior of the Pd/C catalyst in water-ethanol
cosolvent at 573 K.
Figure 1 shows the effect of ethanol-water molar fraction
on the hydrogenation of 4-propylphenol over the Pd/C catalyst
at 573 K. Gaseous products were not produced and hydro-
genation of 4-propylphenol did not occur over Pd/C in pure
water i.e. when ethanol molar fraction was zero; gaseous
products were formed by the addition of ethanol. The amount of
each gas increased with an increase in ethanol molar fraction,
and a greater amount of hydrogen gas could be obtained than the
other gases at all the molar fractions of ethanol in water. The
maximum amount of hydrogen gas was obtained at 0.60 molar
fraction of ethanol (Figure 1a). The yield of ring hydrogenation
products increased with an increase in ethanol molar fraction and
reached maximum at 0.13 (Figure 1b). Formation of propyl-
benzene was observed at 0.011 of ethanol molar fraction by the
hydro-deoxygenation of 4-propylphenol. Ethylpropylphenol was
formed in water-ethanol cosolvent and its yield increased with
an increase in ethanol molar fraction, as a result of alkylation of
aromatic compounds with ethanol. In spite of the formation of
hydrogen along with other gases in ethanol solvent (1.0 molar
fraction of ethanol), hydrogenation of 4-propylphenol did not
proceed over Pd/C; however, the formation of ethylpropyl-
phenol (determined by GC-MS but unable to ascertain the
position of ethyl group) was observed, indicating that alkylation
proceeded under supercritical conditions in ethanol over Pd/C.
Figure 1 shows that hydrogen was formed from ethanol over
Pd/C in water at 573 K and the amount of hydrogen increased
with an increase in ethanol molar fraction; however, the
hydrogenation product yields were maximized at an intermittent
ethanol molar fraction of 0.13. One probable explanation for
this observation is that the transferred hydrogen from ethanol
was active for the hydrogenation of 4-propylphenol molecules
dissolved in the given composition of water-ethanol system.
Phase of water-ethanol mixture at 573 K was estimated with a
process simulation software (VMG, VMG-Sim) and it showed
that two phases (liquid and gas) exist in water-ethanol mixture
from 0 to 0.13 of ethanol molar fraction while, a single phase of
steam dissolved in supercritical ethanol was formed from 0.23
to 1.0 ethanol fraction at 573 K. We could not determine the
solubility of 4-propylphenol in the system; however, the amount
of 4-propylphenol molecules dissolved in the liquid phases
would be larger and the adsorbed 4-propylphenol molecules
react with activated hydrogen atoms on palladium sites. The
hydrogenation product yields would increase in the liquid phase
up to 0.13 ethanol molar fraction with an increase in the number
of activated hydrogen atoms on the palladium sites. On the other
hand, the yield decreased beyond 0.23 ethanol molar fraction in
a single phase in spite of the fact that larger amount of hydrogen
molecules was present. The amount of 4-propylphenol mole-
cules dissolved in the supercritical phase would be lower than
those in the liquid phase; hence, lower ring hydrogenation yields
were obtained for more than under 0.23 ethanol molar fraction.
Further study is necessary on this aspect; however, the water
molecules would be involved in the hydrogenation steps
(formation of transfer hydrogen atoms and/or adsorption of 4-
propylphenol molecules) on palladium sites because the product
yield decreased with an increase in ethanol molar fraction and it
became zero in ethanol solvent.
Table 1 shows the effect of external gaseous hydrogen on
the hydrogenation of 4-propylphenol over the Pd/C catalyst in
water for 1 h at 573 K under 1.0 and 3.0 MPa pressure. Under
1.0 MPa of hydrogen pressure at 573 K, 4-propylcyclohexanone
and 4-propylcyclohexanols were not formed; however, propyl-
benzene was formed indicating that the hydrogenation of 4-
propylphenol did not proceed over Pd/C in water solvent under
external gaseous hydrogen pressure of 1.0 MPa. Nevertheless, as
hydrogen pressure was increased to 3.0 MPa, aromatic hydro-
genation proceeded and both 4-propylcyclohexanone and 4-
propylcyclohexanols were formed in water, although yields of
4-propylcyclohexanone and 4-propylcyclohexanols were one-
sixth and one-third, respectively, of those in water-ethanol
cosolvent system (0.13 ethanol molar fraction), indicating that
ethanol addition is more effective for the hydrogenation of
the aromatic ring of 4-propylphenol in water than the external
hydrogen gas under high-pressure conditions. The hydrogen
atoms generated from ethanol and activated on palladium sites,
would be more active than the adsorbed hydrogen atoms from
gas phase under high-pressure conditions.
Table 2 shows the results of hydrogenation of 4-propylphe-
nol over the Pd/C catalyst in water-methanol and water-
2-propanol cosolvents. The gaseous yields in water-methanol
cosolvent were higher than those in water-ethanol cosolvent;
however, the gaseous product distribution and hydrogenation
of 4-propylphenol behavior were similar in water-methanol and
water-ethanol cosolvent systems.
Gaseous products were formed by the addition of methanol.
The amount of each gas increased with an increase in methanol
molar fraction, and showed maximum yields of hydrogen,
Figure 1. Hydrogenation products of 4-propylphenol over a char-
coal-supported palladium catalyst (Pd/C) in water-ethanol cosolvent:
(a) Products determined by GC-TCD, ( ) hydrogen; ( ) methane; (
)
carbon dioxide; ( ) carbon monoxide; ( ) ethane. (b) Liquid products
determined by GC-FID, ( ) 4-propylcyclohexanone; ( ) 4-propylcy-
clohexanols (cis + trans); ( ) propylbenzene; ( ) ethylpropylphenol;
(
) 4-propylcyclohexanone + 4-propylcyclohexanols. Reaction con-
ditions: reaction time, 1.0 h; reaction temperature, 573 K; 4-propyl-
phenol, 0.74 mmol; Pd/C, 0.15 g.
644 | Chem. Lett. 2016, 45, 643–645 | doi:10.1246/cl.160204
© 2016 The Chemical Society of Japan

Table 1. Hydrogenation products of 4-propylphenol over Pd/C in water + hydrogen gasa and in water-ethanol cosolvent
Amounts/mmol
Solvents
Hydrogen
4-Propylphenol
4-Propylcyclohexanone
4-Propylcyclohexanols
Propylbenzene
Ethylpropylphenol
Water + 1.0 MPa H2
Water + 3.0 MPa H2
Water-Ethanolb
0.83
3.3
0.85
0.61
0.56
0.34
0.0
0.032
0.18
0.0
0.014
0.045
0.012
0.041
0.0
0.0
0.0
0.015
aReaction conditions: reaction time, 1 h; reaction temperature, 573 K; 4-propylphenol, 0.74mmol; Pd/C, 0.150 g; water 3.0 mL. bWater and ethanol
cosolvent (water (2.0 mL) + ethanol (1.0 mL), ethanol molar fractions 0.13).
Table 2. Hydrogenation products of 4-propylphenol over Pd/C in water-alcohol cosolvent
Gaseous products/mmol
Liquid products/mmol
Alcohol
molar
fraction
Alcohol
Carbon
Hydrogen
Carbon
dioxide
Methane
Ethane 4-Propylcyclohexanone 4-Propylcyclohexanols Alkylpropylphenol
monoxide
Methanol
0.10
0.48
1.0
0.11
0.34
1.0
0.79
2.35
3.6
0.16
0.56
1.2
0.13
0.67
2.1
0.20
0.17
0.55
0.018
0.10
0.28
0.045
0.019
0.086
0.47
0.75
0.67
0.11
0.083
0.080
0
0
0
0
0
0
0.19
0.038
0
0.13
0.25
0.073
0.068
0
0
0.077
0.27
0.32
0
0
0.044b
0
0
0
2-Propanol
b
aReaction conditions: reaction time, 1 h; reaction temperature, 573 K; 4-propylphenol, 0.74mmol; Pd/C, 0.15 g. Methylpropylphenol.
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aromatic ring of 4-propylphenol to 4-propylcyclohexanone and
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molecules enhanced the yield for transfer hydrogenation with
secondary alcohol in high-temperature conditions.
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Chem. Lett. 2016, 45, 643–645 | doi:10.1246/cl.160204
© 2016 The Chemical Society of Japan | 645