Ni powders (prepared by calcination of (HCOO)2Ni in air)
were used as catalyst in the hydrogenation of phenol at 280 °C in
octane. The phenol conversion (Entry 6, 83.4%) is lower than
that of the in situ Ni catalyst derived from the decomposition of
(HCOO)2Ni (Table 1, Entry 2, 96.0%), clearly suggesting an
advantage for the in situ catalyst in phenol hydrogenation. The
hydrodeoxygenation of phenol was performed over the combi-
nation of (HCOO)2Ni and different solid acid (Table 1, Entries
7-9). It could be found that the phenol conversion further
increased with the addition of solid acids. Synergy effects
between the hydrogenation and dehydration might be an
important reason. In the reaction process, phenol is converted
by hydrogenation over the in situ formed Ni to form the
intermediates of cyclohexanol, then through dehydration cata-
lyzed by solid acid and subsequent hydrogenation by Ni catalyst.
Over the combination of (HCOO)2Ni and SiZr, the major
products for the phenol HDO were cyclohexane (94.1%),
methylcyclopentane (2.8%), and cyclohexanol (3.1%). Cyclo-
hexanol disappeared from the phenol HDO products when
Nb2O5 and HZSM-5 were added into the reaction system
(Table 1, Entries 7 and 8). The reason is that strong acidity
favors dehydration of intermediate cyclohexanol. In addition,
increased selectivity for the methylcyclopentane was also
observed in the products when HZSM-5 and Nb2O5 were used
as solid acids. This outcome may be also associated with the
acidity of the solid acids because a strongly acidic catalyst
favors the isomerization of intermediates formed during the
process of phenol HDO.12
The effects of combination of nickel precursor and solid acid
on the catalytic performance were investigated. The catalytic
activity of a mechanical mixture of (HCOO)2Ni and solid acid
SiZr is similar to that of the supported catalyst (HCOO)2Ni/SiZr.
Over the two catalysts, phenol can be converted into cyclo-
hexane with a high conversion rate and selectivity (Table 1,
Entries 9 and 12), suggesting an excellent synergy between
in situ Ni and acid sites. However, the conventional supported
catalyst Ni/SiZr exhibits a lower catalytic activity for the phenol
HDO (Table 1, Entry 13). This result also indicates that the
catalyst originating from the in situ method has an advantage
for phenol HDO from another viewpoint.
Apart from phenol, more complex phenolic compounds,
including guaiacol, vanillin, eugenol, anethole, o-cresol, 2,4-
dimethylphenol, and acetylisoeugenol were investigated under
the optimized conditions in octane at 280 °C (Table 2, Entries
1-7). The combination of in situ Ni and SiZr successfully
converted these tested phenolic compounds to their correspond-
ing C6-C9 cyclohexanes as expected. In addition, a small
amount of arenes was also observed, which may result from the
hydrogenolysis of the C-O bond of aryl ethers. The catalytic
performance is comparable to the combined catalysts of Pd/C
and liquid acid H3PO4, Pd/C and HZSM-5, Raneyμ Ni and
Nafion/SiO2.7,13 The octane numbers of the obtained hydro-
carbons mostly range from 80 to 120, indicating that it is
suitable as a high-grade transportation fuel component.5
Furthermore, mixed phenolic compounds were also inves-
tigated over the combined catalyst with in situ Ni and SiZr
(Table 2, Entry 8). Most of the phenolic compounds were
transformed into hydrocarbons. The main products were cy-
clohexane, methylcyclohexane, and propylcyclohexane as ex-
pected, which were similar to the results in the HDO of single
Figure 1. The XRD patterns of Ni obtained from pyrolysis of
(HCOO)2Ni at 280 °C. (a) At solvent of H2O; (b) at solvent of octane.
OH
OH
H
H
H
Ni
H
Scheme 1. Hydrogenation of phenol over in situ Ni catalyst.
However, the catalytic activity for phenol hydrogenation is
deteriorated in the water solvent. Phenol conversions were only
10.3%.
To gain insight into the nature of solvent effects, XRD
analysis of the used catalyst was carried out. Apart from the
characteristic peaks of Ni (2ª = 44.45, 51.81, and 76.32°), the
peaks of NiO (2ª = 19.6, 33.4, and 60.4°) and Ni(OH)2
(2ª = 19.6, 33.4, and 60.4°) were clearly observed for the
catalyst used in water (profile of (a) in Figure 1). In aqueous
solution, H2O can coordinate with the Ni2+, forming a complex
compound [Ni(H2O)6]2+, which can be decomposed into
Ni(OH)2. Thereafter, Ni(OH)2 would be further decomposed
into NiO.8 To make matters worse, part of the Ni catalyst can be
oxidized in the presence of H2O. These lead to a decline of the
catalytic activity for hydrogenation.
In octane, (HCOO)2Ni was directly decomposed into Ni, H2,
CO, etc. The hydrogen derived in situ from the decomposition
of (HCOO)2Ni and the dissolved hydrogen in alkane solution9
can be adsorbed on the surface of cluster Ni, forming an
adsorption of linear H species (Ni-H). This is a weak and
reversible adsorption with high catalytic activity for hydro-
genation reaction. During the hydrogenation of phenol, Ni0
accepts the delocalized aromatic ring electron, resulting in the
activation of the ring,10 which allows the nucleophilic addition
reaction occurring through the adsorbed H attacking the
aromatic ring, finally completing the conversion of phenol to
cyclohexanol (Scheme 1).
However, linear adsorption is unstable. The adsorbed H
atom tends to form the bridge adsorption species (Ni-H-Ni),
which is stable and irreversible adsorption leading to lower ac-
tivity for the hydrogenation of the aromatic ring.11 Fortunately,
bridge absorbed H can prevent the oxidation of Ni during the
reaction process, giving the catalyst longer life with high
catalytic activity for hydrogenation.
© 2015 The Chemical Society of Japan | 649
Zhang, Xinghua
