M.J. Hidajat et al.
Molecular Catalysis 512 (2021) 111770
activities, of which the activities and selectivities can be controlled, are
required [23, 26, 27]. Noble metal-based catalysts are known for their
high hydrogenation activities in carboxylic acid hydrogenation [28-32].
However, all reported studies on the hydrogenation of octanoic acid are
based on non-noble metal catalysts, such as In2O3/CuP [33], NiMo/ZrO2
[10], Ni/Al2O3 [34], and Ni/ZrO2 [17]. The hydrogenation of octanoic
acid over In2O3/CuP resulted in 86 % conversion, but only 55% of
octanol selectivity was achieved under harsh reaction conditions [33].
The addition of a second metal to Ni resulted in an improvement in the
activity of NiMo/ZrO2 in the hydrogenation of octanoic acid. Although
conversion up to 93 % was achieved at 350◦C, cracking and esterifica-
tion limited the selectivity to octanol [10]. Furthermore, the addition of
In2O3 to Ni/ZrO2 for the hydrogenation of octanoic acid increased the
selectivity to octanol; however, this result was still unsatisfactory, as a
conversion of only 80 % and selectivity to octanol of 70 % were achieved
[34]. These studies indicate that non-noble metal catalysts have a lim-
itation in the selective production of alcohol from carboxylic acid.
The use of noble metal catalysts for the hydrogenation of various
carboxylic acids has been widely reported; specifically the use of Ru,
owing to its higher hydrogenation activity compared to other noble
metals [26, 35]. Ru-based catalysts have been investigated for the hy-
drogenation of propionic acid [28, 36], oleic acid [37], and lactic acid
[38]. For example, Ru catalysts with different supports (TiO2, ZrO2,
SiO2, Al2O3, SiO2-Al2O3) have been examined for aqueous phase hy-
drogenation of propanoic acid [28]. It is reported that in the hydroge-
nation of propanoic acid, the formation of acyl was the key intermediate
and the C=O hydrogenation activity depended on the formation of
metal-acid sites on the catalyst. It is also stated that decarbonylation of
acyl was preferred on support with lower acidity whereas hydrogenation
was more likely to occur on support with higher acidity such as Al2O3.
Therefore, the hydrogenation over Ru/Al2O3 started from the adsorp-
tion of C=O to form acyl and followed by H transfer to the acyl to form
alcohol. While the high hydrogenation activity of Ru ensures high
conversions of acids, it has a limitation that must be overcome. Ru is
active for C-C bond hydrogenolysis, which can lead to
over-hydrogenation of the desired products and a reduction in the
selectivity towards the alcohol [28, 39]. Thus, the addition of a second
metal to Ru-based catalysts, to control the metal activity, has been
extensively studied [40].
the surface of the catalyst. This finding is contradictive to the previous
report by Pouilloux et al. [50] that states Ru0 and SnOx were the main
active sites for hydrogenation. In the report, Luo et al. proposed that
Ru3Sn7 was the main active site while the presence of SnOx catalyzed the
esterification of the alcohol. They stated that in Ru3Sn7 alloy phase
electrons were transferred from Sn to Ru, which increased the electron
density on Ru, resulting in a suppression of reduction ability towards
C=C bond. The C=O bond of acid attached to the Sn site and H-transfer
from Ru occurred to form alcohol [51]. Thus, based on this report, the
presence of RuSn alloy is the key for the high activity of carboxylic acid
hydrogenation.
In our previous study, the vapor phase hydrogenation of butyric acid
was studied over RuSn/ZnO [52]. Almost complete conversion of
butyric acid and more than 98 % selectivity to butanol were observed
when performing the reaction at 265◦C, 25 atm, and a weight hourly
space velocity (WHSV) of 0.9 hꢀ 1. The reported catalyst activity was
maintained over 3500 h without any significant deactivation.
In this study, the vapor phase hydrogenation of octanoic acid was
conducted in a fixed-bed continuous-flow reactor over a bimetallic
RuSn/ZnO catalyst. To the best of our knowledge, the hydrogenation of
octanoic acid over noble metal-based bimetallic catalysts has not yet
been reported. The effects of the operating conditions (temperature,
total pressure, H2 partial pressure, and H2 flow rate) and the addition of
Sn to Ru/ZnO were investigated in the hydrogenation of octanoic acid.
The catalyst stability was also tested over 1000 h.
2. Experimental
2.1. Materials
The chemicals used for the preparation of Ru/ZnO and RuSn/ZnO,
such as RuCl3.xH2O (99.9 %), SnCl4.5H2O (98.0 %), and Zn(NO3)2.6H2O
(98 %), were purchased from Sigma Aldrich Co. Ltd. (USA). NaOH (98
%) was purchased from Samchun Chemicals (Korea) and used to control
the pH of the solution during the catalyst synthesis. Octanoic acid, (>98
%) used as the feed, and n-hexane (>97 %), used as the solvent for GC
analysis, were obtained from Sigma Aldrich Co. Ltd. (USA). The stan-
dards used for GC quantification, such as octanal (99 %) and octanol (99
%), were obtained from Sigma Aldrich Co. Ltd. and Acros Organics
(USA), respectively.
Sn is usually added to Ru to selectively convert carboxylic acids to
alcohols, because of its ability to selectively cleave C-O bonds [41].
Bimetallic RuSn has been used in the hydrogenation of various com-
pounds, such as rosin [42], dicarboxylic acids [43], crotonaldehyde
[44], cinnamaldehyde [45], levulinic acid [46], and succinic acid [47],
resulting in high conversions and selectivities towards the respective
alcohols. Bimetallic RuSn has also been used in the hydrogenation of
ester. Deshpande et al. [48, 49] conducted the hydrogenation of methyl
hexadecanoate, methyl-9-octadecenoate and dimethyl succinate over
Ru-Sn-B catalyst. They found that the addition of Sn tuned the activity of
Ru-B to selectively hydrogenate aldehyde and ester to alcohol. It is re-
ported that the presence of Ru0 and SnOx were important for the high
hydrogenation activity, in which that Ru provided hydride transfer and
SnOx acted as the adsorption sites for C=O due to its Lewis acidity. They
observed the formation of RuSn alloy, but they did not consider the
activity of RuSn alloy because of the small amount as compared to the
SnOx and Ru0. Pouilloux et al. [50] also observed that Ru0 and SnOx
were the major phase and found no formation of RuSn alloy on Ru-Sn-B
catalyst for the hydrogenation of methyl oleate. They also found that
during the hydrogenation of methyl oleate, transesterification of methyl
oleate with oleyl alcohol also occurred depending on the Sn/Ru ratio,
with higher Sn/Ru ratio resulting in a heavier ester compound.
2.2. Synthesis of Ru/ZnO and RuSn/ZnO
Ru/ZnO and RuSn/ZnO were prepared using
a sequential
coprecipitation-deposition method as described in the previous study
[52]. The Ru content for both Ru/ZnO and RuSn/ZnO was set at 1.4 wt
%. The Sn content in RuSn/ZnO was set at a Ru/Sn molar ratio of 1/2.
The coprecipitation method starts from the preparation of aqueous so-
lution containing 0.1 M Zn(NO3)2.6H2O and 1.0 M SnCl4.5H2O. This
solution was then added dropwise to 200 mL of water while adjusting
the pH between 7.2 and 7.5 with continuous addition of NaOH. The
solution was then continuously stirred for 12 h at room temperature. Ru
was then deposited by adding dropwise an aqueous solution of 0.5 M
RuCl3.xH2O while adjusting the pH to 7.2 and stirring the solution
continuously for 5 h at room temperature. The solution was then heated
at 85◦C for 5 h without stirring. After heating, the solution was filtered
and washed with deionized (DI) water to remove the Na+, NO3ꢀ , and Clꢀ
ions. The precipitate was dried overnight in an oven at 120◦C. For
Ru/ZnO, the catalyst was calcined in air at 500◦C for 4 h to remove the
remaining precursor ions, whereas the RuSn/ZnO catalyst was reduced
directly without calcination. The purpose of using direct reduction
without calcination method was to form pure alloy formation, as re-
ported in our previous literature [52]. The catalysts were then pressed
into a disc shape at 400 atm, crushed, and sieved to a specific size
Recently, Luo et al. reported almost complete conversion of coconut
oil and a selectivity towards the alcohol of 96.6 % using RuSn/SiO2 as
the catalyst at 240◦C and 40 atm H2 [51]. According to this report, the
high hydrogenation activity and selectivity to alcohol achieved by the
RuSn bimetallic catalyst were due to the formation of a Ru3Sn7 alloy on
(425–1180
heating rate of 10◦C/min, while the RuSn/ZnO catalyst was reduced
μ
m). The Ru/ZnO catalyst was reduced at 420◦C for 4 h at a
2