Catalytic hydrogenation of aromatic rings catalyzed by Pd/NiO

Article abstract of DOI:10.1039/c3ra45600e

A simple and efficient heterogeneous palladium catalyst was prepared for aromatic ring hydrogenation. The catalyst was prepared by a reduction-deposition method and exhibited high activity and selectivity for the hydrogenation of a variety of substituted aromatic compounds to the corresponding cyclohexane and cyclohexanol derivatives with up to 99% yields. The catalyst was characterized by BET, TEM, XRD, XPS and ICP. Meanwhile the reusability of the catalyst was investigated, and it can be reused for several runs without significant deactivation.

Full text of DOI:10.1039/c3ra45600e

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Catalytic hydrogenation of aromatic rings
catalyzed by Pd/NiO†
Cite this: RSC Adv., 2014, 4, 2729
a
a
a
Yanan Wang,ab Xinjiang Cui, Youquan Deng and Feng Shi
*
Received 5th October 2013
Accepted 14th November 2013
DOI: 10.1039/c3ra45600e
www.rsc.org/advances
A simple and efficient heterogeneous palladium catalyst was prepared hydrogenated to the corresponding cyclohexane and cyclohexanol
for aromatic ring hydrogenation. The catalyst was prepared by a derivatives in good yields, but the reusability of the catalyst was
reduction-deposition method and exhibited high activity and selec- poor. Rhodium on graphite (Rh/Gr, C24Rh) was described as an
tivity for the hydrogenation of a variety of substituted aromatic active catalyst for the hydrogenation of carbocyclic and heterocy-
compounds to the corresponding cyclohexane and cyclohexanol clic aromatic compounds, too.10 Although the experimental
derivatives with up to 99% yields. The catalyst was characterized by conditions are mild enough, the selectivity needs to be improved.
BET, TEM, XRD, XPS and ICP. Meanwhile the reusability of the catalyst According to the former reports, the catalytic activity for hydro-
was investigated, and it can be reused for several runs without genation of aromatic rings decreases in the order Rh > Ru > Pt > Ni
significant deactivation.
> Pd > Co.18 Moreover, among these catalysts, Ru, Ni or Pd cata-
lysts are commonly chosen owing to their high thermal stability.
Nagashima et al.11 investigated the complete hydrogenation of
aromatic rings over Ru/CNFs. Among those Ru/CNFs, Ru/CNF-p
showed excellent catalytic activity towards the hydrogenation of
benzene derivatives and pyridine at 100 ꢀC under 30 atm
hydrogen pressure. A new method was developed for the hydro-
genation of phenol and its derivatives under microwave irradia-
tion.17 Zhao et al. found that phenol was transformed efficiently to
cyclohexanone with >98% yield under microwave irradiation.
However, the species of the phenols are limited. Therefore, the
development of clean and economic catalyst system for the
hydrogenation of aromatic rings is still desirable. Here an active
Pd/NiO catalyst was prepared for the hydrogenation of aromatic
rings. It exhibited good generality to various aromatic compounds
and the yields of the hydrogenated products were high enough.
The Pd/NiO catalyst was prepared by reduction-deposition
method using nickel oxide as support, ethanol as reducing
agent and polyvinyl pyrrolidone (PVP) as surfactant.19 The TEM
images of Pd/NiO are presented in Fig. 1 to illustrate the Pd
particle dispersion and catalyst morphology. The TEM results
revealed that the Pd nanoparticles have an average diameter of
5.2 nm and dispersed on NiO homogeneously, and the Pd (111)
crystal lattice can be observed from Fig. 1(d).
Cyclohexane derivatives are important moieties in a variety of
organic compounds, such as diesel fuels,1 natural products and
pharmaceuticals.2 Normally cyclohexane derivatives can be
obtained through two ways, i.e. the modication of cyclohexane
and the hydrogenation of aromatic rings. Clearly, the method
through the modication of cyclohexane is very difficult and thus
catalytic hydrogenation of aromatic rings becomes the main route
to synthesize substituted cyclohexane because it is a simple,
convenient and sustainable method and the full hydrogenation of
arenes and phenols represents an important industrial trans-
formation, especially for low aromatic diesel fuels.1,3 Moreover,
the hydrogenation of phenols to alcohols plays an important role
in paper production4,5 and the production of fuels from biomass.6
In most cases, the hydrogenation of aromatic rings is performed
by using heterogeneous catalysts such as supported Rh, Pt, Ru,
Pd, Co and Ni2–4,7–18 due to the ease of separation and reusability of
the catalysts. It is well known that supported Rh catalysts are
generally more active than other catalysts. Recently, Sajiki et al.2
reported an efficient arene hydrogenation system based on the
use of Rh/C catalyst. In his study, a wide variety of arenes were
The diffraction patterns of NiO and Pd/NiO were presented in
Fig. S1.† If compared the NiO support with the Pd/NiO catalyst, it
can be seen that the peaks related to NiO phases were similar and
three weak Pd peaks were observable. The appearance of the Pd
diffraction peaks indicated that the Pd particles possess relatively
good crystallinity, which is in accordance with the TEM results.
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy
of Sciences, No.18, Tianshui Middle Road, Lanzhou, 730000, China. E-mail: fshi@
licp.cas.cn; Fax: +86-931-8277088
bUniversity of the Chinese Academy of Sciences, Beijing, 100049, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45600e
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Table 2 Hydrogenation of phenol derivativesa
Entry
1c
Substrate
T/ꢀC
80
t/h
4
P/MPa
Yield/%b
87
3
1
2c
3c
80
80
10
10
98
89
1
1
Fig. 1 TEM images of Pd/NiO (a and c), particles distribution (b), and
HR-TEM (d).
4c
80
10
8
>99
98
Table 1 Reaction conditions optimization for the hydrogenation of
phenola
5
100
3
6c
7c
100
100
8
3
3
92
98
Entry
Solvents
t/h
Yield/%b
16
1
2
3
4
5
6
7
8
9
MeOH
EtOH
EtOAc
THF
12
12
12
12
12
12
12
4
33
41
Trace
Trace
Trace
92
95
53
87
8
80
80
80
10
10
10
1
2
3
99
98
99
DCM
i-PrOH
n-Hexane
i-PrOH
n-Hexane
9c
4
a
Reaction conditions: 1 mmol phenol, 50 mg Pd/NiO (2 wt%), 2 mL
b
solvent, 3 MPa H2, 80 ꢀC. GC yield determined by GC-FID using
biphenyl as standard material.
10c
XPS study of Pd/NiO was performed to determine the chemical
state of Pd nanoparticles. The Pd3d XPS spectra were shown in Fig
S2.† It can be seen that the peaks of Pd3d are consisted of more
than one contribution. Thus, the Pd3d lines were de-convoluted to
two peaks with the least-squares tting routine using the Gauss
functions. The Pd3d signal of the catalyst showed a maximum at
335.2 eV, which agrees with the binding energy of metallic Pd. The
small peak around 337.5 eV is attributed to PdOx, which was not
strange because the catalyst was reduced by ethanol at r.t., and
calcined at 100 ꢀC in air.20,21
By applying phenol hydrogenation as model reaction, the
inuence of solvents and also the reaction time on the reaction
were investigated (Table 1). The results revealed that the reac-
tion was solvent dependent. Moderate yields were obtained if
the reactions were performed in MeOH and EtOH while almost
no reaction occurred if EtOAc, THF and DCM were used (entries
1–5). To our delight, 92–95% yields were obtained if using n-
11
130
130
10
10
5
5
90
87
12c
13d
14
130
80
12
10
5
3
84
82
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Table 2 (Contd.)
Table 3 Hydrogenation of benzene derivativesa
Yield/%b
Entry
15
Substrate
T/ꢀC
130
t/h
12
P/MPa
3
Yield/%b
90
Entry
Substrate
T/ꢀC
140
140
t/h
24
24
1
2
94
90
16
150
120
24
24
5
3
85
92
3
130
24
92
4c
130
130
130
24
30
28
84
91
93
17
a
5c,d
Reaction conditions: 1 mmol substrate, 2 mL n-hexane, 50 mg catalyst
b
c
(2 wt%). Isolated yield. GC yield determined by GC-FID using
biphenyl as standard material. The product was 2,6-di-tert-butyl-4-
methylcyclohexanone.
d
6c,d
hexane or i-PrOH as solvent (entries 6–7). Moreover, better
result was obtained in n-hexane than in i-PrOH when the
reaction time was reduced to 4 h (Entries 8 and 9). Therefore,
n-hexane was used for the next study.
7c,d
130
26
87
The catalytic hydrogenation of phenol derivatives with
different substituents was tested over Pd/NiO catalyst to
examine the scope and limitations, and the results were shown
in Table 2. Phenol was converted to cyclohexanol with 87%
yields (entry 1). The mono-substituted phenols were hydroge-
nated smoothly to the corresponding cyclohexanol derivatives
with up to 99% yields (Entries 2–10). The presence of functional
groups such as methyl, tert-butyl, allyl and methoxyl did not
inuence the hydrogenation reactions. The di-substituted and
tri-substituted phenols were also investigated, and they were
hydrogenated to the corresponding substituted cyclohexanol
with excellent yields (entries 11–12). However, relatively higher
8
9
150
120
36
24
93
93
10c
150
24
80
a
Reaction conditions: 1 mmol substrate, 2 mL n-hexane, 5 MPa, 50 mg
b
c
catalyst (2 wt%). Isolated yield. GC yield determined by GC-FID
using biphenyl as standard material. d 5 MPa.
temperature, pressure and longer reaction time were needed in disubstituted benzenes could occur under lower temperature if
order to gain good yields due to the steric hindrance. In the compared to mono-substituted benzenes (entries 5–7). The
extremity, 2,6-di-tert-butyl cyclohexanone was obtained with catalytic hydrogenation of naphthalene was also realized using
84% yield when 2,6-di-tert-butylphenol was employed (entry 13). Pd/NiO as catalyst and 93% yield was obtained (entry 8).
That means the greater steric hindrance inuences the reaction Furthermore, heteroaromatic compounds were reduced to the
pathway. In addition, we explored the Pd/NiO-catalyzed hydro- corresponding heterocyclic compounds with 80–93% yields
genation of catechol and its derivatives (entries 14–16). Fortu- (entries 9 and 10). However, the main product in the hydroge-
nately, catechol and substituted catechols were completely nation of quinoline was tetrahydroquinoline. In one word, the
hydrogenated to the corresponding cyclohexanediol and its hydrogenation of alkylbenzene and biphenyl, as well as naph-
derivatives in good yields. b-naphthol was hydrogenated to thalene, can be realized with good to excellent yields.
decahydro-b-naphthol with 92% yield, too (entry 17).
In order to investigate the stability of the Pd/NiO catalyst, the
The hydrogenation of benzene derivatives was further reusability experiment was performed. As the results shown in
investigated, and the results were shown in Table 3. Clearly, Pd/ Fig. 2, the catalyst was used for ve cycles in the hydrogenation
NiO showed high catalytic activity on the hydrogenation of of phenol and 81% yield was maintained at the 5th run, which
alkyl benzenes and the yields were 92–94% (entries 1 and 2). suggested that the catalyst was very stable during the reaction.
Biphenyl and cumene were successfully converted into the According to ICP-AES analysis, the palladium loading only
corresponding cyclohexane derivatives with excellent yields decreased from 2.1 wt% to 2.0 wt%, which also conrmed the
(entries 3 and 4). Moreover, the catalytic hydrogenation of stability of the catalyst.
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In conclusion, we have introduced a simple and practical Pd/
NiO catalyst for the catalytic hydrogenation of aromatic rings.
Benzene and phenol derivatives with different structures can be
hydrogenated into the corresponding cyclohexane and cyclo-
hexanol derivatives with good to excellent yields. In addition,
this Pd/NiO showed excellent reusability and maintained stable
for at least 5 runs.
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Acknowledgements
Financial support was provided by the National Natural Science
Foundation of China (21303228 and 21073208).
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2732 | RSC Adv., 2014, 4, 2729–2732
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