Electrocatalytic hydrogenation of toluene using a proton exchange membrane reactor: Influence of catalyst materials on product selectivity

Full text of DOI:10.1246/bcsj.20180021

As a candidate organic chemical hydride, methylcyclohexane
(MCH) which is obtained by hydrogenation of toluene has received
much attention due to its high availability, relative low melting point,
and low toxicity.2 Therefore, to realize an organic chemical hydride
system, development of an efficient synthetic process for MCH pro-
duction by hydrogenation of toluene is highly required.
Short Article
Electrocatalytic Hydrogenation of
Toluene Using a Proton Exchange
Membrane Reactor: Influence of
Catalyst Materials on Product
Selectivity
It is well known that the chemical hydrogenation of aromatic com-
pounds has been carried out using transition metal catalysts such as
platinum, palladium, ruthenium, etc.5,6 However, this reaction involves
theoretical loss of heat because it is an exothermal reaction.5-7 To
overcome this problem, we have investigated one-step electrocatalytic
hydrogenation of toluene in a PEM reactor with various transition
metal catalysts.8-10 This reaction system can reduce the theoretical
loss of heat because the reaction in this system involves electrolysis
of water.5-7 In addition, Matsuoka et al. also reported electrocatalytic
hydrogenation of toluene using a PEM reactor with Pt and PtRu cata-
lysts.11 In these previous reports, by-products such as methylcyclo-
hexenes and methylcyclohexadienes were not detected at all. However,
to establish further a strong and reliable hydrogenation system, a more
detailed reaction mechanism involving by-product formation should be
clarified. This knowledge is very important to apply for industrial
processes in which electrochemical hydrogenation is conducted for an
extremely long time. In this case, by-products may accumulate in the
reactor and would probably have a negative effect on the processes.
Therefore, in this work, we have tried to detect by-products in the
hydrogenation of toluene using a PEM reactor with various transition
metal catalysts. In previous work, the hydrogenation of toluene in a
PEM reactor was carried out in a single-flow operation,9,10 and hence
the concentration of by-products after the hydrogenation might be
too low to detect by GC analysis. To overcome this problem, in the
present work, we carried out the hydrogenation in a PEM reactor by a
circulation-flow operation in order to accumulate by-products.
Atsushi Fukazawa,1 Ken Takano,1
Yoshimasa Matsumura,1
Kensaku Nagasawa,2
Shigenori Mitsushima,2,3
and Mahito Atobe*1,2
M. Atobe
1Department of Environment and System Sciences,
Yokohama National University, 79-7 Tokiwadai,
Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
2Institute of Advanced Sciences, Yokohama National
University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama,
Kanagawa 240-8501, Japan
3Green Hydrogen Research Center, Yokohama National
University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama,
Kanagawa 240-8501, Japan
E-mail: atobe@ynu.ac.jp
Received: January 17, 2018; Accepted: February 22, 2018;
Web Released: March 10, 2018
2. Experimental
2.1 General Experimental. Gas chromatography (GC) analyses
were performed by using a Shimadzu gas chromatograph (GC2014)
with a capillary column (0.50 ¯m, 100.0 m, 0.25 mmID (HP-1, Agilent
Technologies, Inc.)). The temperature program used was 15 min at
35 °C, ramp to 45 °C at 5 °C min¹1, hold for 13 min, ramp to 60 °C
at 1 °C min¹1, hold for 10 min, ramp to 120 °C at 9 °C min¹1 and hold
for 3 min at 120 °C. Gas chromatography-mass spectrometry (GC-MS)
analyses were performed by using a Shimadzu gas chromatograph
mass spectrometer (GCMS-QP2014) with a capillary column (0.50
¯m, 100.0 m, 0.25 mmID (HP-1, Agilent Technologies, Inc.)).
Constant-current electrolyses were carried out with a DC power
supply (PK-80M, Matsusada Precision Inc.). In this case, the anode
worked as a counter electrode (C.E.) and a reference electrode (R.E.,
reversible hydrogen electrode (RHE)). Therefore, cathode potential
was monitored during the constant-current electrolysis as needed.
2.2 Materials. All chemicals were used without further purifi-
cation. Toluene and hexane were purchased from Kanto Chemical Co.,
Inc. Methylcyclohexane (MCH), heptane, and 1-propanol were pur-
chased from Tokyo Chemical Industry Co., Ltd. As for manufacturing
the membrane electrode assembly (MEA), Nafionμ perfluorinated
membrane (Nafionμ NRE212), and Nafionμ perfluorinated resin solu-
tion (5 wt.% in mixture of lower aliphatic and water, Nafionμ DE521)
as ionomer solution were purchased from Sigma-Aldrich Co. Fuel cell
catalysts (TEC10E50E; Platinum/Carbon (Pt/C), TECRu(ONLY)E30;
Ruthenium/Carbon (Ru/C), and TEC61E54; Platinum-ruthenium/
Carbon (PtRu/C)) were purchased from Tanaka Kikinzoku Kogyo
K.K. (TKK). Gas diffusion layers (GDL35BC Diffusion Media) were
purchased from Sigracetμ.
Abstract
We have investigated the electrochemical hydrogenation of
toluene using a PEM reactor for development of an organic
chemical hydride system. Especially, the influence of catalyst
materials such as Pt, Ru, and PtRu for a PEM reactor on the by-
product formation and product selectivity in the hydrogenation
of toluene was investigated.
Keywords: Organic chemical hydride
j
Proton exchange membrane (PEM)
j
Hydrogenation
1. Introduction
Hydrogen will play an important role as a new energy carrier in the
next generation. However, hydrogen is the lightest gas at ambient
pressure and temperature. Hence, the most challenging technological
barrier to developing hydrogen systems is how to store hydrogen itself
safely and efficiently. As hydrogen storage methods, hydrogen com-
pression and liquefaction, hydrogen adsorption in metal hydrides,
carbon materials, ammonia, and organic chemical hydrides have been
investigated so far.1-4 Among them, the organic chemical hydride
system is expected to be one of the most promising hydrogen storage
techniques.2,3 In this system, hydrogen is chemically converted to
organic chemical hydride which is in liquid state under ambient
temperature and pressure, then it is stored or conveyed as a hydrogen
carrier. Moreover, the existing infrastructure for inflammable fuels can
be used for its long-term and long-distance storage.
2.3 Preparation of MEA.
MEA was fabricated with 0.01-
¹2
0.5 mg cm of metal loading, and weight ratio of metal catalyst and
Bull. Chem. Soc. Jpn. 2018, 91, 897–899 | doi:10.1246/bcsj.20180021
© 2018 The Chemical Society of Japan | 897

supported ketchen black was 0.85, 0.37, and 1.15 for Pt/C, Ru/C, and
PtRu/C, respectively. Fuel cell catalyst (0.2948 g), deionized water
(1.1893 g), Nafionμ perfluorinated resin solution (0.6472 g), and 1-
propanol (3.9503 g) were stirred in a 100 mL glass beaker cell. In the
current efficiencies in all cases. However, small amounts of partially
hydrogenated product such as 1-methylcyclohexene (1-ene) was also
detected by using the circulation-flow operation.
In the case with Pt catalyst for the PEM reactor, the production
amount of 1-ene was much lower than that with other catalysts
(Table 1, Entry 1). Pt is known as the highest active catalyst for the
hydrogenation.12 Therefore, the hydrogenation of toluene proceeded
completely to afford methylcyclohexane effectively. On the other
hand, the selectivity for 1-ene was highest when Ru catalyst was used
for the hydrogenation (Table 1, Entry 2). In addition, very small
amounts of 3-methylcyclohexene (3-ene) and 4-methylcyclohexene (4-
ene) were also detected by GC-MS analysis as other by-products in
this case (see Scheme 1). It is also known that the Ru-H bond strength
is much weaker compared to the Pt-H bond strength.13 Hence, not so
many adsorbed monoatomic hydrogen species (Had) would be formed
on the Ru surfaces, and consequently partially hydrogenated products
were also formed to some extent. As shown in Entry 3 of Table 1, the
hydrogenation with PtRu alloy catalyst gave excellent high current
efficiency for MCH production (>99% efficiency, 2.3% chemical
yield). Moreover, the use of PtRu alloy catalyst considerably sup-
pressed the formation of by-products. As clarified in our previous
report,9,10 PtRu catalyst exhibits dual functions such as high catalytic
activity by the rich Had on Pt and strong toluene adsorption by Ru. This
synergy would lead to better results in the hydrogenation.
Subsequently we examined the effect of catalyst amount on the
current efficiency and selectivity. With Pt catalyst, the current effi-
ciency for MCH production decreased dramatically with a decrease in
Pt catalyst amount loaded on MEA (Table 2, Entries 1-5). In contrast,
the selectivity for 1-ene increased with a decrease in the catalyst
amount even by using Pt catalyst. These results suggest that there are
not enough Had to hydrogenate toluene completely at lower catalyst
amounts. In addition, a considerable amount of hydrogen gas evolution
occurred at lower catalyst amounts. A similar trend can be observed
with Ru catalyst (Table 2, Entries 6-10). Namely, the efficiency for
MCH and the selectivity for 1-ene decreased and increased, respec-
tively, as the catalyst amount was decreased. In addition, 3-ene and 4-
¹2
case of preparing 0.01-0.05 mg cm catalyst loading layer, we added
ketchen black in order to reduce the weight of metal. This mixture and
zirconia balls (diameter: 2.5 mm, 10 pieces) were added to an 80 mL
Teflon vessel. The vessel was placed in a planetary rotation pot mill
(LP-1, Ito Seisakusho Co.) and rotated at 200 rpm for 20 min. After the
rotation, catalyst dispersion was obtained. Gas diffusion layer (2 cm ©
5 cm) was coated with catalyst dispersion on one side by use of a
Teflon spatula with SUS mask for 1 cm © 4 cm. After the coating, it
was dried for 10 min at 60 °C and hot-pressed (0.4 MPa, 120 °C) for
1 min. Two sheets of this catalyst layer were prepared for anode and
cathode. Nafionμ perfluorinated membrane (8 cm © 10 cm) was put
between anode and cathode and their catalyst sides faced toward the
membrane, and finally hot-pressed (0.4 MPa, 120 °C) for 1 min.
2.4 Hydrogenation of toluene in PEM reactor. Electrocatalytic
hydrogenations were carried out by using the PEM reactor (Figure 1).
The MEA (1 © 4 cm2 active area) was integrated into the PEM reactor.
The MEA consists of the proton conducting polymer membrane
sandwiched by a pair of catalyst layers for anode and cathode sides.
They were prepared with a pair of gas diffusion layers which were
coated on one side with carbon-supported noble metal catalysts
(TEC10E50E; Pt/C, TECRu(ONLY)E30; Ru/C, TEC61E54; PtRu/C
(molar ratio of Pt:Ru = 1:1.5), Tanaka Kikinzoku Kogyo K.K.) with
ionomer solution (Nafionμ DE521, Sigma-Aldrich Co.). The Pt load-
¹2
ing amount was maintained to be 0.5 mg cm for an anodic catalyst.
On the other hand, Pt, Ru, and PtRu alloy loaded in the range of 0.01-
¹2
0.5 mg cm were used for cathodic catalysts. For the cathodic cham-
¹1
ber, toluene was provided with flow rates 0.25 mL min by a circu-
lator pump (MSP101-00, Yamazen Corporation) in a closed system
(total volume of toluene: 5 mL) (Figure 1). On the other hand, for the
¹1
anodic chamber, humidified hydrogen gas (flow rate of 30 mL min
was supplied. The hydrogenation was carried out at 12.5 mA cm
)
¹2
current density and at room temperature (r.t.). The obtained reaction
solutions were analyzed by GC.
Table 1. Current efficiency for MCH and 1-ene and selectivity
for 1-ene in the electrochemical hydrogenation of toluene
using various cathode catalysts
3. Results and Discussion
As mentioned in the introduction, in our previous work, the
hydrogenation of toluene in PEM reactor was carried out by a single-
Current
efficiency for
MCH
Current
efficiency for
1-ene
¹1
flow operation.9 In this case, the charge passed was 2.2 © 10¹3 F mol
,
Selectivity
for 1-ene
(%)c
Catalyst
materiala
Entry
and hence the concentration of by-products after the hydrogenation
was too low to detect by GC analysis even if they were produced. To
overcome this problem, at first, the electrochemical hydrogenation of
toluene in PEM reactor was carried out by circulating toluene substrate
for 3 hours in order to detect by-products. In this demonstration, the
charge passed (2.0 © 10¹2 F mol¹1) was about 9 times larger than that
of the single-flow operation. In addition, an influence of catalyst
materials such as Pt, Ru and PtRu alloy was investigated (Table 1). As
shown in Table 1, MCH was obtained as a main product with high
(%)b
(%)b
1
2
3
Pt
Ru
PtRu
81
93
>99
0.05
5.6
0.3
0.1
5
0.3
a) Experimental conditions: anode, Pt (loading amount was
0.5 mg cm¹2); cathode, metal loading was 0.5 mg cm¹2; current
density, 12.5 mA cm¹2; cell temperature, r.t.; flow rate of toluene,
0.25 mL min¹1; flow rate of hydrogen, 30 mL min¹1; reaction time,
3 hours. b) Determined by GC. c) Selectivity of 1-ene = (yield of
1-ene/total yield) © 100 (%).
Scheme 1. Methylcyclohexane and by-products formation in the
electrocatalytic hydrogenation.
Figure 1. Schematic representation of PEM reactor.
898 | Bull. Chem. Soc. Jpn. 2018, 91, 897–899 | doi:10.1246/bcsj.20180021
© 2018 The Chemical Society of Japan

Table 2. Catalyst amount effect on current efficiency for MCH
and 1-ene and selectivity for 1-ene in the electrochemical
hydrogenation of toluene
genation of toluene in order to clarify the reaction mechanism involv-
ing by-product formation. By using the circulation-flow operation for
the hydrogenation in a PEM reactor, partially hydrogenated products
such as 1-ene, 3-ene, and 4-ene were also generated other than MCH.
Among of these by-products, 1-ene was found to be the main by-
product. Among all catalyst samples employed, the used of PtRu alloy
catalyst afforded good current efficiencies for the hydrogenation even
if the catalyst loading amount was reduced. This result arose from the
dual functions of PtRu which exhibited high catalytic activity from the
rich Had on Pt and strong toluene adsorption on Ru. Furthermore, the
by-product formation could be suppressed by using PtRu catalyst. We
hope that the electrochemical hydrogenation of toluene will be applied
as an industrial process with the aid of our work.
Catalyst
Current
Current
Selectivity
for 1-ene
(%)c
Catalyst loading efficiency efficiency
Entry
materiala amounta for MCH for 1-ene
¹2
/mg cm
(%)b
(%)b
1
2
3
4
5
Pt
Pt
Pt
Pt
Pt
0.5
0.3
0.1
0.05
0.01
81
21
6.6
2.9
0.5
0.05
0.09
0.10
0.09
0.03
0.1
0.5
2
2
9
6
7
8
9
10
Ru
Ru
Ru
Ru
Ru
0.5
0.3
0.1
0.05
0.01
93
75
46
19
1.0
5.6
3.8
3.5
1.6
n.d.
5
5
7
7
-
This work was supported by the Council for Science, Cross-
ministerial Strategic Innovation Promotion Program (SIP), “energy
carrier” (Funding agency: JST). The Institute of Advanced Sci-
ences (IAS) in YNU is supported by the MEXT Program for
Promoting Reform of National Universities. The authors gratefully
acknowledge JXTG Energy Corporation for the generous gift of
fuel cell catalysts.
11
12
13
14
15
PtRu
PtRu
PtRu
PtRu
PtRu
0.5
0.3
0.1
0.05
0.01
>99
87
76
53
21
0.3
0.4
0.5
0.6
0.5
0.3
0.5
0.7
1
2
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a) Experimental conditions: anode; Pt (loading amount was 0.5
mg cm¹2); current density, 12.5 mA cm¹2; cell temperature, r.t.;
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mL min¹1; reaction time, 3 hours. b) Determined by GC. c)
Selectivity of 1-ene = (yield of 1-ene/total yield) © 100 (%).
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Bull. Chem. Soc. Jpn. 2018, 91, 897–899 | doi:10.1246/bcsj.20180021
© 2018 The Chemical Society of Japan | 899