Effects of steam on toluene hydrogenation over a Ni catalyst

Article abstract of DOI:10.1016/j.apcata.2019.117374

The catalytic toluene hydrogenation over Ni/SiO₂ was carried out using H₂ or a H₂/H₂O mixture. The toluene conversion and MCH selectivity were evaluated under partial steam pressures 0?10 kPa, at H₂/t

Full text of DOI:10.1016/j.apcata.2019.117374

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Effects of Steam on Toluene Hydrogenation over a Ni Catalyst
Ryosuke Atsumi, Keisuke Kobayashi, Cui Xieli, Tetsuya Nanba,
Hideyuki Matsumoto, Keigo Matsuda, Taku Tsujimura
PII:
S0926-860X(19)30529-0
DOI:
https://doi.org/10.1016/j.apcata.2019.117374
APCATA 117374
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Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
22 July 2019
29 November 2019
6 December 2019
Please cite this article as: Atsumi R, Kobayashi K, Xieli C, Nanba T, Matsumoto H, Matsuda K,
Tsujimura T, Effects of Steam on Toluene Hydrogenation over a Ni Catalyst, Applied Catalysis
A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117374
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Effects of Steam on Toluene Hydrogenation over a Ni Catalyst
Ryosuke Atsumi1*, Keisuke Kobayashi2, Cui Xieli1, Tetsuya Nanba1, Hideyuki Matsumoto1, Keigo Matsuda2, Taku
Tsujimura1
1National Institute of Advanced Industrial Science and Technology, 2-2-9 Machiikedai, Koriyama, Fukushima 963-
0298, Japan
2Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata
University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, JAPAN
*Corresponding author
Graphical abstract

Highlights

Toluene hydrogenation with steam-containing hydrogen (wet-H2) was conducted using an industrial
Ni catalyst for H2 storage.


Methylcyclohexane selectivity increased with increasing steam partial pressure.
Wet-H2 produced in a water electrolysis system can be used directly in toluene hydrogenation.
Abstract
The catalytic toluene hydrogenation over Ni/SiO2 was carried out using H2 or a H2/H2O mixture. The
toluene conversion and MCH selectivity were evaluated under partial steam pressures 0-10 kPa, at H2/toluene
molar ratios 1-5, and at temperatures 393-453 K. In the absence of steam, toluene conversion increased with
increasing H2 partial pressure, and the MCH selectivity decreased with increasing temperature. In the presence of
steam, the toluene conversion showed near-constant activities over the entire range of steam partial pressures, and
the MCH selectivity increased with increasing steam partial pressure. This result indicated that the addition of
steam inhibited byproduct formation. To clarify the effects of steam on the side-reaction, the byproducts
composition was analyzed using a gas chromatograph. The results showed that the addition of steam inhibited
demethylation, the ring-opening reaction of MCH, the disproportionation of toluene, and isomerization to convert
six-membered ring compounds into five-membered ring compounds.
Keywords

Toluene hydrogenation: Nickel catalyst: Low-grade hydrogen: Methylcyclohexane: Hydrogen carrier
1. Introduction
Hydrogen (H2) storage using liquid organic hydride is important to the utilization of renewable electricity
generated from wind, solar, and hydropower [1-4]. Methylcyclohexane (C7H14, MCH) is one of the candidates for
chemical H2 storage materials [1-4]. MCH can store H2 via toluene (C7H8) hydrogenation:
C7H8 + 3H2  C7H14 – 205 kJ mol-1
(1).
This reaction has been studied extensively in the petrochemical industry, and the infrastructure for MCH synthesis
and transportation has been established [2]. MCH also has relatively high volumetric and gravimetric H2 densities
(47 kg-H2 m-3 and 6.1 wt% respectively), good transportability, and relatively low toxicity [2,4]. The Fukushima
Renewable Energy Institute, AIST (FREA) has demonstrated a pilot-scale toluene hydrogenation system in which a
150-kW-class alkaline water electrolysis (AWE) system has been installed to generate renewable H2 [5]. The
demonstration system can utilize a power input that fluctuates with time to generate H2, which simulates the surplus
energy of renewable energy sources [5]. Kojima et al. [5] reported that the H2 production rate was highly
synchronized with power input fluctuations and relatively short time delay. Although it has been reported that AWE
efficiency decreases with increased input current frequency [8,10,11], AWE shows second-order response
fluctuation in input power [8,9]. To achieve steady operation of the hydrogenation reactor, the toluene
hydrogenation system needs a buffer tank (and a H2 compressor) or batteries to absorb the fluctuation of H2 or
surplus energy, yet this can increase the capital cost of the system. Therefore, we believe that directly providing
fluctuating H2 from AWE to the reactor is important to utilize renewable H2 effectively.

Although the purity of H2 produced by AWE is generally lower than that produced using polymer-
electrode electrolysis cells (PEECs) and solid-oxide electrolysis cells (SOECs) due to its diaphragm, AWE is cost-
effective and well-established compared to PEFCs and SOECs [6,7]. Hence it is important to evaluate the effects of
trace impurities from AWE on toluene hydrogenation. Hydrogen generated on the cathode contains trace oxygen
and is saturated with steam at the AWE operating temperature (generally 323 to 353 K) [6,7,34,35]. Hydrogen from
AWE contains alkaline mist, e.g., KOH and NaOH, and a water scrubber removes the alkaline mist. If the water
scrubber works at ambient temperature (298 K), the alkaline-free H2 contains steam with a partial pressure of 3
kPa. The trace oxygen can be removed by a metal-loaded catalyst packed in a toluene hydrogenation reactor. Steam
has been reported to cause the oxidative demethylation of toluene over a Ni catalyst [36,37], as described by the
following equations:
C6H5-CH3 + H2O  C6H6 + 2H2 +CO
C6H5-CH3 + 2H2O  C6H6 + 3H2 +CO2
(3),
(4).
Moreover, steam can also cause oxidation of the Ni surface, which leads to catalytic deactivation. However, there
are only a few reports of toluene hydrogenation in the presence of trace steam and the effects of the partial pressure
of steam on catalytic activity. Practical AWE systems include a pressure swing adsorption (PSA) or temperature
swing adsorption (TSA) process to remove the trace steam from H2 [6,7]. However, when the energy input of the
AWE fluctuates, the dew-point of the purified hydrogen can fluctuate due to the steam flow rate exceeding the PSA
or TSA capacity. To generate pure H2 using a fluctuating power input, it is necessary to load a great deal of
adsorbent, or extend the AWE cycle time (the period from adsorption to desorption). This overloading seems to

lead to higher capital expenditure for the AWE system. Therefore, it is important to estimate the effects of steam on
the toluene hydrogenation process and determine an acceptable partial pressure of steam.
The toluene hydrogenation reaction can be catalyzed by using Pt [2,12-20,30], Ru [21], Ir [22], and Ni
[23-31]. Pt-loaded catalysts exhibit the highest activity [2], while Ni catalysts are the most cost-effective. As
reported by Lindfors et al. [23], Ni catalysts exhibit high activity for excess H2 conditions with H2/toluene ratios
greater than 3. In contrast, the activity drastically declines at H2/toluene ratios below 3. The result suggests that a
H2/toluene ratio below 3 is not favorable for toluene hydrogenation. On the other hand, when the supply of surplus
renewable energy fluctuates on a time scale of seconds, the toluene evaporator is not expected to exhibit an
equivalent time-response. In other words, for the usage of renewable hydrogen, we should consider the situation in
which the H2/toluene ratio will drop below 3. Therefore, it is important to estimate the catalytic activity of Ni
catalysts against a wide range of H2 partial pressures, including H2/toluene ratios below 3.
Nickel catalysts are active for C-C bond cleavage. Grenoble [32] reported the catalytic activities of group
VIIB and VIII metals for toluene hydrodealkylation via the reaction shown in the following equation:
C6H5-CH3 + H2  C6H6 + CH4
(2).
They reported that the catalytic activity of the evaluated metals follows the order Ni > Rh > Ir > Os > Ru > Pt > Pd
> Re [32]. The side reactions in toluene hydrogenation using Ni-loaded catalysts have also been studied [25,30,33].
Cui et al. [33] investigated the iterative utilization of MCH (up to 10 cycles) with bench-scale toluene
hydrogenation and MCH dehydrogenation reactors to demonstrate their practical use. The resulting data showed the
byproduct formation and accumulation behavior for each processing cycle. To our knowledge, there is no

information on the effects of fluctuating H2 input on toluene hydrogenation.
In the present study, we aimed to acquire the fundamental knowledge needed for the continued
development of catalysts with applications to fluctuating H2 feed systems. The toluene hydrogenation activity of a
commercial Ni catalyst in a wide range of H2/toluene molar ratios was measured to understand the behavior of not
only main product formation, but also byproduct formation under operation of the fluctuating H2 supply. The
formation of byproducts during hydrogenation was analyzed in detail. Furthermore, toluene hydrogenation was
conducted using H2O-containing H2 (hereafter referred to as wet-H2) to evaluate the effects of steam on the
catalytic activity.
2. Experimental
The toluene hydrogenation activity of a stabilized Ni catalyst (BASF SE, Ni5256) with or without the
addition of H2O vapor was measured in a packed-bed reactor system at ambient pressure, and 3 g of catalyst was
set in the reactor. Fig. 1 shows a schematic diagram of the packed-bed reactor. A 12.7 mm (1/8 inch) diameter
cylinder-shaped industrial catalyst containing 57 wt% Ni was used. The specifications of the catalyst are
summarized in Table 1. The catalyst pellets were packed in a 1/2-inch-diameter stainless steel tube. A K-type
thermocouple was inserted into the bed to monitor the bed temperature. The reactor was heated by using an electric
furnace. The flow rates of the reactants (H2 and toluene) and the inert gas (N2) were controlled using pre-calibrated
mass flow controllers. The flow rate of deionized water was controlled using a syringe pump. Liquid toluene and
water were fed into an evaporator before the reactor to provide the feed of the toluene-steam mixture to the catalyst

bed. The evaporator consisted of a 1/2-inch-diameter stainless steel tube, in which ~1-mm-diameter stainless steel
spheres were packed. The temperature of the stainless sphere bed was monitored using a K-type thermocouple and
was maintained at 423 K using an electric heater. The exhaust gases from the catalyst bed were chilled at 293 K to
recover liquid toluene, MCH, and byproducts. The samples were analyzed using a gas chromatograph (Agilent
Technology Inc., GC system 7890A) equipped with a flame ionization detector (FID) and a DB-1
(dimethylpolysiloxane) column.
The catalyst packed in the reactor was pre-reduced in H2 flow at 473 K for an hour [31]. A 0.15 g min-1
flow of toluene (JUNSEI Chemical co. ltd., Guaranteed Reagent, 99.5%) was supplied to the catalyst bed.
Hydrogen of grade 99.999995 vol% (Taiyo Nippon Sanso Corporation, G1-H2) was supplied at 40.4–201.8 mL
min-1, which corresponded to a H2/toluene molar ratio from 1 to 5 (the stoichiometric ratio was 3, as described in
Eq. 1). In order to evaluate the effects of steam on the catalytic activity, deionized water was supplied at a rate
ranging from 0.4 to 18.6 mg min-1, corresponding to steam partial pressures of 1 to 10 kPa. The toluene conversion
was calculated using the following equation:
(Toluene conversion %) = (1 - XToluene) × 100
(5)
where XToluene is the molar ratio of toluene in the recovered products. Equilibrium toluene conversion was also
calculated by HSC chemistry 7 (Calculation of Equilibrium Composition ver. 7.10). The MCH selectivity was
calculated using the following equation:
(MCH selectivity %) = [XMCH / (1 - XToluene)] × 100
(6).
The byproducts were categorized into five groups: demethylated compounds, ring-opened compounds, six-

membered ring compounds (excluding benzene and cyclohexane), five-membered ring compounds, and dimerized
compounds. The respective selectivity for these byproducts was calculated using the following equation:
(Byproducts selectivity %) = XByproducts / [ξ(1 - XToluene)] × 100
(7)
where ξ is the number of toluene molecules consumed to produce the product molecule. For the dimerized
compounds, ξ = 2, while for the other compounds, ξ = 1. To ensure accuracy, byproduct selectivity was calculated
for compounds with concentration of more than 0.01 mol%. Further, we distinguished these byproduct groups by
the kind of hydrogen carrier: six-membered ring compounds, unusable byproducts; ring-opened compounds and
five-membered ring compounds, toxic or inhibitor byproducts; demethylated and dimerized compounds.
In order to estimate the surface acidic properties of the Ni catalyst with and without steam, temperature-
programmed desorption of NH3 (NH3-TPD) was performed. A total of 0.1 g of the as-prepared catalyst was packed
in a quartz tube with an inner diameter of 6 mm and pre-reduced using 50 mL min-1 of pure H2 at 473 K for an
hour. Then, 200 mL min-1 of dry or humidified argon (Ar, 99.9999% purity, Taiyo Nippon Sanso Corporation, G1-
Ar) gas was flowed into the quartz tube at 298 K, followed by 100 mL min-1 of an Ar/NH3 mixture (argon-balanced
NH3 standard gas). The partial pressures of H2O and NH3 in Ar were 1 vol%, and the Ar was humidified with
deionized water using a bubbler. After the quartz tube was purged with 100 mL min-1 of Ar for 10 min, the sample
with adsorbed NH3 and H2O was heated from 298 to 873 K using an electric heater with an Ar flow rate of 100 mL
min-1 and a heating rate of 3 K min-1. The effluent gas was analyzed by using a Fourier Transform Infrared
Spectrometer (FT-IR) equipped with a multi-reflection gas cell.

3. Results and Discussion
3.1 Toluene hydrogenation at various H2/toluene ratios
To determine the activity of the catalyst under a fluctuating supply of H2, the toluene conversion and
MCH selectivity at different H2/toluene ratios were evaluated at temperatures from 393 to 453 K. Fig. 2 shows the
observed and equilibrium toluene conversion at H2/toluene ratios of 1–5. The catalyst exhibited higher activity at
lower temperatures and higher H2/toluene ratios. Equilibrium toluene conversion also decrease as temperature
increased. At 393 K and H2/toluene ratios > 3.5, ~100% toluene conversion was achieved. The MCH selectivity
calculated using Eq. 6 is shown as a function of the H2/toluene ratio and temperature in Fig. 3. MCH selectivity at
393 K seems to decrease.
The composition of the byproducts is summarized in Fig. 4, in which the vertical axis represents the
selectivity for each byproduct group, as calculated using Eq. 7. Table 2 lists the compounds detected by GC-FID.
Although the MCH selectivity was relatively stable at different H2/toluene ratios (see Fig. 3), the selectivity for
five-membered ring compounds increased with increasing H2/toluene ratio; in contrast, the selectivity for the six-
membered ring compounds decreased with increasing H2/toluene ratio (see Fig. 4). The ratio of ring-opened and
dimerized products remained nearly constant.
From Fig. 4(a) and (b), it can be seen that the demethylation of toluene and ring-opening selectivities
were almost independent of the H2/toluene ratio; however, these reactions increased probably due to an increase in
the C-C bond cleavage rate over the Ni surface with increasing temperature [32]. This result agrees with the data
reported by Masalska [25]. On the other hand, as shown in Fig. 4(c), the selectivity toward the six-membered ring

compounds decreased with increasing H2/toluene ratio. Possible pathways to six-membered ring compounds, such
as xylenes, are the disproportionation reaction (Eq. 8) and the methylation of toluene, in which the methyl group is
derived from the cracking of aromatic compounds [25].
2C7H8  C6H6 + C8H10
(8).
If disproportionation is dominant, the trend in the graph of six-membered ring compounds will be similar to that of
the demethylated compounds. Since their trend lines were different (Fig 4 (a) and (c)), this means the
disproportionation reaction to form benzene and six membered ring compounds was only a minor pathway. For the
formation of five-membered ring compounds, Upare et al. [38] reported the mechanism of MCH isomerization over
the Ni/SiO2 catalyst, and suggested that MCH was converted to dimethyl cyclopentane and ethyl cyclopentane at
the acid sites on SiO2, followed by the ring-opening of these products to form methylhexane or heptane. In this
study, dimethyl cyclopentane (a C7 compound) was detected; thus, MCH was likely a precursor of dimethyl
cyclopentane. As listed in Table 2, methyl cyclopentane (a C6 compound) and ethyl methylcyclopentane (a C8
compound) were also detected. These byproducts might be formed from a similar reaction mechanism as for
dimethyl cyclopentane formation [25].
To find appropriate operation conditions regarding the temperature and the H2/toluene ratio, the results of
the activity tests were compared again. In the range 393 to 453 K, a favorable temperature was 393 K, because the
highest toluene conversion and MCH selectivity and lowest byproducts formation were obtained. For discussion of
the appropriate H2/toluene ratio, we compared conversion and selectivity at 393 K. When the H2/toluene ratio was
lower than 3, a higher MCH selectivity was obtained. Also, there were fewer unusable and toxic byproducts, and

more hydrogen carrier byproducts, than for a H2/toluene ratio above 3. On the other hand, these conditions can lead
to the remainder of unreacted toluene. Therefore, the favorable H2/toluene ratios were stoichiometric or below
stoichiometry.
3.2 Effect of steam on the catalytic activity
Toluene hydrogenation was carried out using wet-hydrogen to determine the effect of steam on the
catalytic activity of Ni. Figs. 5 and 6 show the toluene conversion and MCH selectivity as a function of the steam
partial pressure for H2/toluene ratios of 1–5 at temperatures from 393 to 453 K. The conversion was relatively
constant at the different steam partial pressures at all of the H2/toluene ratios and temperature conditions (Fig. 5).
On the other hand, as shown in Fig. 6, the MCH selectivity increased with increasing steam partial pressure. This
positive effect of steam was observed most clearly at higher temperatures and lower H2/toluene ratios. Even in the
presence of H2O, toluene conversion and MCH selectivity at 393 K were higher than those at other temperatures.
To clarify the details of the side-reaction inhibition effect of steam, the selectivity for each byproduct
group under different steam partial pressures was evaluated using a H2/toluene ratio of 3 (Fig. 7). The decrease in
the selectivity for demethylated compounds was a major cause of the increase in MCH selectivity. Although steam
addition can potentially lead to oxidative demethylation, as described in Eq. 3 and 4 [36, 37], this reaction was not
dominant in this work. It was believed that this was due to the relatively low temperature and steam partial pressure
used in this study. Masalska [25] reported the effect of the metal/Brønsted-acid site ratio on MCH selectivity in
toluene hydrogenation over Ni-loaded ZSM-5 catalysts, and Brønsted acid sites and higher temperature were
suggested to enhance activity for side-reactions such as dehydrogenation, cracking, and disproportionation of

toluene. Upare et al. [38] also reported the reaction pathway of the MCH ring-opening reaction and isomerization
over Ni, as well as on the acid sites of SiO2, which were suggested to be involved in the isomerization of MCH and
ring-opening of the isomers.
To confirm the suppression effect of steam addition on byproducts formation, NH3-TPD measurements
were conducted to assess the contribution of the acid sites. Fig. 8 shows the NH3-TPD profile with or without steam
during pretreatment. Both profiles exhibited a peak at around 350 K. The desorption amounts of NH3 with and
without wet-Ar pretreatment were 4.9 and 5.4 mol gcat-1, respectively. From these results, steam addition appears to
have little influence on the acidic property of the surface of Ni/SiO2. Considering these results, in the presence of
water vapor, the competitive adsorption of H2O and toluene might decrease toluene coverage, resulting in a
decrease in the surface residence time of toluene. If toluene molecules reside on the catalyst surface for a shorter
time, a greater number of molecules desorb before the side-reactions are initiated. According to past research on the
photocatalytic decomposition of volatile organic compounds (VOCs), aromatic compounds and water vapor are
competitively adsorbed on the solid surface, thereby decreasing the VOCs adsorption amount at higher steam
partial pressures.
The present study clarified the robustness of the Ni catalyst against H2O at temperatures below 453 K and
the positive effect of steam addition on MCH selectivity. Although current AWE systems utilize H2 purification,
such as PSA and TSA to remove trace H2O [6,7], our data suggests that wet-H2 can be positive for toluene
hydrogenation, which would decrease the exergy loss during the gas-liquid separation process. When MCH is
synthesized using a H2/H2O mixture, the H2O must be collected in an MCH recovery tank. The water phase is

expected to form under an oil (MCH) phase, which would allow the water phase to be removed from the tank by an
extraction operation.
4. Conclusion
Toluene hydrogenation was carried out using dry- and wet-H2 in a packed reactor with an industrial
Ni/SiO2 catalyst. When MCH was synthesized from toluene using dry-H2, a higher H2/toluene ratio inhibited the
disproportionation of toluene, while higher temperatures accelerated the demethylation reaction, disproportionation
reaction, isomerization reaction, and ring-opening reaction (or cracking) of MCH. The dimerization selectivity
showed only weak correlation with the H2/toluene ratio and temperature under the conditions tested. The toluene
conversion remained near-constant at steam partial pressures ranging from 0 to 10 kPa in the temperature range
from 393 to 453 K, while the MCH selectivity increased with increasing steam partial pressure. The selectivity for
demethylated compounds notably decreased with increasing steam partial pressure. It was hypothesized that
Brønsted acid sites on the surface of the support were occupied by H2O or hydroxyl groups generated from
dissociative H2O adsorption. NH3-TPD measurements were conducted to investigate the acid sites on the Ni/SiO2;
prior to TPD, the catalyst was exposed to wet- or dry-Ar at 298 K. The NH3-TPD profiles indicated that the wet-gas
treatment decreased the number of acid sites, while the acid strength was not affected. These results might suggest
that the occupation of the acid sites on Ni/SiO2 by steam resulted in lesser number of reaction sites available for
catalyzing side-reactions.
Author contribution

Acquisition of data:
Ryosuke Atsumi, Keisuke Kobayashi, Cui Xieli
Analysis and interpretation of data:
Ryosuke Atsumi, Keisuke Kobayashi, Tetsuya Nanba
Drafting of manuscript:
Ryosuke Atsumi, Tetsuya, Nanba
Critical revision:
Tetsuya Nanba, Hideyuki Matsumoto, Keigo Matsuda, Taku Tsujimura

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Table & Figures List
Table 1 Specifications of the commercial Ni/SiO2 catalyst.
Table 2 Major byproducts derived from toluene hydrogenation over Ni/SiO2 catalyst.
Notably, o-xylene and 1,2-dimethyl cyclohexane were not detected herein, and that all the five-membered
ring compounds detected were saturated.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Schematic of the experimental setup.
Toluene conversion over the Ni/SiO2 catalyst vs. the H2/toluene ratio with dry-H2.
MCH selectivity over the Ni/SiO2 catalyst vs. the H2/toluene ratio with dry-H2.
Byproduct selectivity as a function of the H2/toluene ratio over the Ni/SiO2 catalyst.
Toluene conversion over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at a
H2/toluene ratio of 1:5.
Fig. 6
Fig. 7
Fig. 8
MCH selectivity over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at a
H2/toluene ratio of 1:5.
Byproduct selectivity over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at
H2/toluene ratio = 1:5 and temperatures ranging from 393 to 453 K.
Desorbed NH3 in the exhaust gas during the TPD measurement of Ni/SiO2 catalysts with and without
wet-Ar treatment (1 vol% H2O) at 298 K.

Table 1 Specifications of the commercial Ni/SiO2 catalyst.
Shape
Cylinder
Diameter [mm]
12.7
2 – 5
270
57
Height [mm]
BET surface area [m2 g-1]
Ni content [wt%]

Table 2 Major byproducts derived from toluene hydrogenation over Ni/SiO2 catalyst.
Notably, o-xylene and 1,2-dimethyl cyclohexane were not detected herein, and that all the five-membered ring
compounds detected were saturated.
Byproduct group
Carbon number
Compound
Demethylated compounds
6
Benzene
6
Cyclohexane
Ring-opened products
5
Methylbutane
5
Isoprene
8
Methylheptane
8
Dimethylhexane
Trimethylpentane
m-Xylene
8
Six-membered ring compounds
8
8
p-Xylene
8
Ethylbenzene
8
1,3- and 1,4-Dimethylcyclohexane
Ethylcyclohexane
1,3,5-Trimethylbenzene
1.3.5-Trimethylcyclohexane
Methylcyclopentane
dimethylcyclopentane
Ethyl methylcyclopentane
Biphenyl
8
9
9
Five-membered ring
compounds
6
7
8
Dimerized compounds
12
13
Methylbiphenyl

Fig. 1
Schematic of the experimental setup.

Fig. 2
Toluene conversion over the Ni/SiO2 catalyst vs. the H2/toluene ratio with dry-H2.

Fig. 3
MCH selectivity over the Ni/SiO2 catalyst vs. the H2/toluene ratio with dry-H2.

Fig. 4
Byproduct selectivity as a function of the H2/toluene ratio over the Ni/SiO2 catalyst.
(a) Demethylated compounds, (b) ring-opened compounds, (c) six-membered ring compounds, (d) five-membered
ring compounds, and (e) dimerized compounds.

Fig. 5
Toluene conversion over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at
H2/toluene ratio of 1:5.

Fig. 6
MCH selectivity over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at
H2/toluene ratio of 1:5.

Fig. 7
Byproduct selectivity over the Ni/SiO2 catalyst as a function of steam partial pressure (0–10 kPa) at
H2/toluene ratio = 1:5 and temperatures ranging from 393 to 453 K.

Fig. 8
Desorbed NH3 in the exhaust gas during the TPD measurement of Ni/SiO2 catalysts with and
without wet-Ar treatment (1 vol% H2O) at 298 K.
Partial pressures of H2O and NH3 in Ar were 1 vol%, and the Ar was humidified with deionized water using a
bubbler.