Hydrogenation of Phenol to Cyclohexanone over Bifunctional Pd/C-Heteropoly Acid Catalyst in the Liquid Phase

Article abstract of DOI:10.1007/s10562-019-02852-1

Abstract: Cyclohexanone is an important intermediate in the manufacture of polyamides in chemical industry, but direct selective hydrogenation of phenol to cyclohexanone under mild conditions is a challenge. Hydrogenation of phenol to cyclohexanone has been investigated in the presence of the composite catalytic system of Pd/C-heteropoly acid. 100% conversion of phenol and 93.6% selectivity of cyclohexanone were achieved within 3?h under 80?°C and 1.0?MPa hydrogen pressure. It has been found that a synergetic effect of Pd/C and heteropoly acid enhanced the catalytic performance of the composite catalytic system which suppressed the hydrogenation of cyclohexanone to cyclohexanol. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02852-1

Catalysis Letters
https://doi.org/10.1007/s10562-019-02852-1
Hydrogenation of Phenol to Cyclohexanone over Bifunctional Pd/
C‑Heteropoly Acid Catalyst in the Liquid Phase
Shiwei Liu1 · Jing Han1 · Qiong Wu1 · Bing Bian2 · Lu Li1 · Shitao Yu1 · Jie Song3 · Cong Zhang1 · Arthur J. Ragauskas4
Received: 26 March 2019 / Accepted: 21 May 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Cyclohexanone is an important intermediate in the manufacture of polyamides in chemical industry, but direct selective
hydrogenation of phenol to cyclohexanone under mild conditions is a challenge. Hydrogenation of phenol to cyclohexanone
has been investigated in the presence of the composite catalytic system of Pd/C-heteropoly acid. 100% conversion of phenol
and 93.6% selectivity of cyclohexanone were achieved within 3 h under 80 °C and 1.0 MPa hydrogen pressure. It has been
found that a synergetic efect of Pd/C and heteropoly acid enhanced the catalytic performance of the composite catalytic
system which suppressed the hydrogenation of cyclohexanone to cyclohexanol.
Graphic Abstract
P
d
HO
/
C
+
O
H
P
A
Cl
C
H
Cl
H
Stainless steel reactor
Keywords Cyclohexanone · Catalysis · Hydrogenation · Pd/C-heteropoly acid · Phenol
1 Introduction
Cyclohexanone is well known as one of the key intermedi-
ates in the synthesis of caprolactam and adipic acid, which
are the main ingredients for the preparation of Nylon 6,
Nylon 66, and polyamide resin [1–3]. The process for the
production of cyclohexanone in industry contains the oxida-
tion of cyclohexane [4] and the hydrogenation of phenol [5].
Compared with the hydrogenation of phenol, the oxidation
of cyclohexane has almost disadvantage because it requires
high temperature and high pressure, and generates a large
number of undesirable byproducts. In the route of the hydro-
genation of phenol, cyclohexanone is generally produced
through one-step or two-step process [6]. The two-step
process involves hydrogenation of phenol to cyclohexanol
* Shiwei Liu
liushiweiqust@126.com
1
College of Chemical Engineering, Qingdao University
of Science and Technology, Qingdao 266042, China
2
College of Chemical and Environmental Engineering,
Shandong University of Science and Technology,
Qingdao 266510, China
3
4
Department of Chemistry and Biochemistry, University
of Michigan, Flint 48503, MI, USA
Department of Chemical & Biomolecular Engineering,
University of Tennessee Knoxville, 419 Dougherty
Engineering Bldg, Knoxville 37996, TN, USA
Vol.:(0123456789)
1 3

S. Liu et al.
followed by dehydrogenation to cyclohexanone [7]. In
contrast, the one-step selective hydrogenation of phenol
to cyclohexanone is more advantageous. For instance, it
avoids the endothermic dehydrogenation step. The one-step
hydrogenation of phenol can be conducted in either the gas
phase or liquid phase, and the selectivity of cyclohexanone
is largely dependent on the catalyst properties. Currently, the
supported Pd catalysts is frequently used in the hydrogena-
tion of phenol [8, 9]. Pd/Al2O3 catalysts obtained approxi-
mately 50~60% selectivity of cyclohexanone with 20% con-
version of phenol in the gas-phase hydrogenation (160 °C)
at 0.2 MPa hydrogen pressure [10]. At 160 °C and 0.45 MPa
hydrogen pressure, 95% selectivity with 40% conversion was
obtained when Pd supported on calcined Mg/Al hydrotalcite
was used [11]. When Ba-modifed Pd/Al2O3 was used, the
conversion of phenol reached 100% and the selectivity of
cyclohexanone was up to 80% in situ hydrogenation (220 °C)
at 3.5 MPa hydrogen pressure [9]. Palladium nanoparticles
supported on polyaniline-functionalized carbon nanotubes
(Pd-PANI/CNT) has a high activity towards the direct hydro-
genation of phenol to cyclohexanone, and the phenol conver-
sion and the cyclohexanone selectivity were more than 99%
under atmospheric pressure of hydrogen in aqueous media
at 80 °C for 9 h [12]. Zhong and coworkers reported that Pd
nanoparticle supported on a mesoporous-graphitic carbon
nitride (Pd@mpg-C3N4) showed high activity and promoted
the formation of cyclohexanone under atmospheric pressure
of hydrogen in aqueous media without additives. Phenol
conversion reaching 99% and cyclohexanone selectivity
was more than 99% at 65 °C. Even at room temperature,
99% conversion and 96% selectivity could still be obtained
[13, 14]. Gas phase phenol hydrogenation requires normally
high temperature (150–300 °C), and generates carbonaceous
deposits (coking) in the course of the reaction leads to deac-
tivation of the supported catalyst. In contrast, liquid-phase
process reduces cost and energy savings, because hydro-
genation reaction is performed at relatively low tempera-
ture. However, another important and inherent drawback of
current liquid-phase hydrogenation relates to the high cost
and the complex preparation process of the catalyst. In this
regard, a highly selective catalyst for the liquid-phase hydro-
genation is still desirable.
nitroaromatics with excellent catalytic performance in
homogeneous phase [22]. Co-based catalysts modifed with
Keggin type HPA salts showed the improving of selectivity
on hydrogenation of crotonaldehyde [23]. The one-pot trans-
formation of (+)-citronellal to menthol over Pd-H3PW12O40/
SiO2 catalyst obtained 92% yield of menthol with 100% cit-
ronellal conversion and 85% stereo selectivity of the desired
(-)-menthol [24]. The results indicated that the catalyst
modifed with HPAs had better catalytic performance for
the hydrogenation than traditional catalysts. Therefore, the
hydrogenation of phenol using the bifunctional Pd/C-HPA
catalyst in the liquid phase was studied in this paper.
2 Experimental Section
2.1 Materials
Pd/C (10 wt%) was purchased from Sigma Aldrich. Phos-
photungstic acid (PTA, H3O40PW12·xH2O, Mw=2880) and
tungstosilicic acid (TTA, H8Si(W2O7)6·xH2O, Mw=2915)
were purchased from Sinopharm Chemical Reagent. H2
(99.99 wt%) was produced by Qingdao Analytical Instru-
ment Company. Other chemicals (analytic purity) were com-
mercially available and used without further purifcation.
2.2 Hydrogenation Reaction
The hydrogenation reaction was carried out in the stainless
steel reactor equipped with a magnetic stirrer. Amounts of
phenol, Pd/C-HPA and solvent were added into the reac-
tor. The reactor was fushed with 0.2 MPa H2 for fve times
to remove air and 1.0 MPa H2 was introduced. Then the
mixture was heated to a desired temperature and reacted
for a certain time. After the reaction was fnished, the reac-
tor was cooled to room temperature and depressurized.
The reaction mixture was centrifuged to separate the cata-
lysts and got the product mixture. The separated catalysts
were directly used in the recycle experiments. The sample
of the upper layer were characterized qualitatively with
HP6890/5973 GC/MS equipped with an HP-5MS column,
30 m × 0.25 mm × 0.25 μm, and its quantitative analysis
were determined by GC using HP6890 GC equipped with
an HP-5 column, 30 m×0.32 mm×0.25 μm. The contents
of the reactants and products were directly showed by the
system of GC chemstation according to the area of each
chromatograph peak. The conversion of phenol was defned
as Conv. %, which was the wt% of phenol consumed in the
reaction. The selectivity of one product was calculated by
Sel. %=Wp/Wall ×100, where Wp was the mass of one prod-
uct, and Wall was the total mass of all products, including
cyclohexanone and cyclohexanol. All the experiments were
repeated for fve times to confrm the results efectiveness.
Catalysis by heteropoly acids (HPAs) and related poly-
oxometalate compounds is a feld of increasing importance.
HPAs have several advantages as catalysts, such as a very
strong Brønsted acidity, and fast reversible multi electron
redox transformations, which makes it become a new multi-
functional catalytic material [15] and uses on the hydration
[16], esterifcation [17], condensation [18], polymeriza-
tion [19], alkylation [20], and oxidation reactions [21] with
good catalytic performance. HPAs have been reported in the
hydrogenation reaction. Joshi et al. reported that 12-molyb-
dophosphoric acid-catalyzed transfer hydrogenation of
1 3

Hydrogenation of Phenol to Cyclohexanone over Bifunctional Pd/C-Heteropoly Acid Catalyst…
Table 1 Efects of diferent catalysts on the results of the hydrogena-
3 Results and Discussion
tion
Liquid-phase phenol hydrogenation reduces cost and
energy savings, because the reaction can be performed
at relatively low temperature. Many researchers have
contributed to this area, and multiple catalysts had been
screened, such as Rh/C [25], Pd/hydrophilic C [13], Pd/
Mg and Pd/Fe [26], and Pd/C [26]. Han and co-workers
achieved both excellent conversion and selectivity in the
hydrogenation of phenol by using Pd/C, Pd/Al2O3 or Pd/
NaY assisted by the Lewis acid AlCl3 [27]. However, the
fact that these hosts contain Lewis acids such as AlCl3
imposes severe limitations on their used in hydrogenation
reactions in general and adds another chemical sensitivity
restricting substrates purities and reaction conditions. HPA
is a new multifunctional and organic catalysts. It shows
higher catalytic activity and selectivity when HPA cata-
lyzes organic reactions [17], and it can be used in multi-
phase reactants as phase transference catalysts. Liu and co-
workers achieved both excellent conversion and selectivity
in the hydrogenation of furfural and cinnamic aldehyde by
using modifed heteropoly acid salt catalyst [28]. On this
basis, we have investigated the hydrogenation of phenol to
cyclohexanone in the presence of the composite catalytic
system of Pd/C-Heteropoly acid.
Entry Reagent
Catalyst
Conv. (%) Sel. (%)b
C=O C–OH
1
2
3
4
5
6
7
8
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
Blank
Pd/C
PTA
PMA
0
61.2
0
/
/
89.4
/
10.6
/
0
0
/
/
/
/
TSA
Pd/C-PTA
Pd/C-PMA
Pd/C-TSA
100
100
100
93.6
66.3
90.5
89.5
90.9
83.5
88.0
91.8
/
6.4
33.7
9.5
10.5
9.1
16.5
12.0
8.2
100
100
20.0
9
Pd/C-PTA-Na 100
10
11
12
13a
14
15
16b
17
Pd/C-AcNa
Pd/C-H3PO4
Pd/C-ZnCl2
Pd/C-ZnCl2
67.6
100
1.6
100
61.8
2.8
Cyclohexanone Pd/C-PTA
Cyclohexanone Pd/C
Phenol
Phenol
/
Pd/C-PTA
Pd/C-AlCl3
100
81.39
80.0
93.78 6.22
Phenol 0.1 g, Pd/C 0.005 g, HPA 0.05 g, T=80 °C, t=3 h,
P(H2)=1.0 MPa, dichloromethane 10 ml
PTA Phosphotungstic acid, PMA Phosphomolybdic acid, TSA Tungs-
tosilicic acid, C=O Cyclohexanone, C–OH Cyclohexanol
aPhenol 0.1 g, Pd/C 0.05 g, ZnCl2 0.03 g, T=100 °C, t=6 h
3.1 The Efects of the Diferent Catalysts
bPhenol 10.0 g, Pd/C 0.5 g, HPA 5.0 g
on the Reaction Results
were achieved (Entry 6). The good catalytic performance
may be attributed to the synergistic efect originating
from the Pd/C and PTA, as well the unique acid prop-
erty and the exhibiting fast reversible multi electron redox
transformations of PTA. In order to determine the efects
of the acidic properties on the hydrogenation, sodium
phosphotungstate(PTA-Na), HCOONa and H3PO4 were
selected and used as the co-catalysts (Table 1, Entries
9–11). The results showed that the catalytic performance
of the catalytic system Pd/C-PTA-Na was similar with that
the catalytic system Pd/C-PTA, but it’s catalytic perfor-
mance worse than Pd/C-PTA. These results demonstrated
that HPA and Pd/C have good synergistic efect on the
hydrogenation of phenol to cyclohexanone. And Pd/C-
AlCl3 was used as catalyst to catalyze the hydrogenation of
phenol, under the same reaction conditions, the conversion
of phenol was signifcantly lower than that of Pd/C-HPA
(Table 1, Entries 6 and 17). Otherwise, although Pd/C-
ZnCl2 could reach high conversion and good selectivity, its
catalyst dosage is much more than the Pd/C-HPA (Table 1,
Entries 6 and 13b). This may be due to the larger volume
of HPA’s anion, regular tetrahedron and eight surface body
structure that has a good symmetry, which can promote
The activities of diferent catalysts for the hydrogenation
of phenol were investigated and the results were showed
in Table 1. It was seen that cyclohexanone and cyclohex-
anol were the only reaction products observed over the
entire range of conditions studied, and the catalysts had
a remarkable infuence on the selective hydrogenation of
phenol to cyclohexanone. Pd/C was used as the standard
catalyst to compare with other catalysts as it showed good
catalytic performances in many hydrogenations [29]. How-
ever, Pd/C catalyst presented low catalytic activity for the
hydrogenation of phenol. The conversion of phenol and the
selectivity to cyclohexanone were only 61.2% and 89.4%,
respectively. And even the too much cyclohexanone was
hydrogenated to perform cyclohexanol (Entry 2). The
HPAs catalysts, such as phosphotungstic acid (PTA),
phosphomolybdic acid (PMA) and tungstosilicic acid
(TSA) showed no catalytic activity for the hydrogenation
(Entries 3–5). In contrast, the composite catalytic systems
consisting of Pd/C and HPA showed an excellent catalytic
activity for the hydrogenation (Entries 6–8). Especially,
the composite catalytic system Pd/C-PTA had 100% con-
version of phenol and 93.6% selectivity to cyclohexanone
1 3

S. Liu et al.
OH
Table 2 Efects of solvents on the hydrogenation of phenol catalyzed
isomerization
by Pd/C-PTA
O
OH
Inhibit
Entry
Solvent
Conv. (%)
Sel. (%)
C=O
C–OH
[H]
H2
1
2
3
4
5
6
Dichloromethane
Cyclohexane
Tetrahydrofuran
1-Chlorobutane
Trichloromethane
H2O
100
93.6
34.0
76.2
78.9
92.4
15.91
6.4
passivation
HO
99.4
98.3
97.3
8.6
66.0
23.8
21.1
7.6
Pd
electron-loaded
HPA
99.91
84.09
Solvent 10 mL, others the same as Table 1
Scheme 1 Reaction mechanism of phenol over Pd/C-Heteropoly acid
When the other solvents investigated were used, the results
of the hydrogenation were not satisfed. This may be due
to that the solvents had diferent efects on the solubility
of organics or hydrogen, the absorption and desorption of
the catalyst surface, which efects on the reaction rate. The
phenol and the polar solvent have a greater force, so in the
polar solvents, the adsorption of phenol on the surface of
the catalyst is relatively less, which is not conducive to the
hydrogenation of phenol. Therefore, the infuence of other
factors on the reaction was discussed by using dichlorometh-
ane as solvent in the following experiments.
the catalytic reaction. The results can be explained by
the reaction mechanism showed in Scheme 1. One of the
carbons in benzene ring tends to become highly nucleo-
philic, thereby facilitating attack of the benzene ring by
an incipient electrophile. Compared with the palladium
adsorption on phenol, heteropoly acid had stronger adsorp-
tion performance for phenol through the formation of
hydrogen bonding. Therefore, phenol can easily interact
with heteropoly acid through the hydroxyl group to form
the strong O–H–O interactions, and H2 that adsorbed and
activated by Pd formed the active hydrogen. Then when
the O–H–O interactions meets with the active hydrogen,
benzene ring of phenol is partially hydrogenated to the
enol intermediate, which is unstable and isomerize rapidly
to form cyclohexanone. Due to the charge density of het-
eropoly acid is low and the transition metal element in the
heteropoly acid anions is easy to transfer electrons, thereby
accept the lone pair of electrons from the cyclohexanone
carbonyl oxygen atom to form a common election pair,
and to achieve passivation of the hydrogenation activ-
ity of cyclohexanone carbonyl. It’s inhibits the reaction
further hydrogenation. What’s more, there only a weaker
H-bridge donor, so the cyclohexanone leaves the surface of
the catalyst quickly, and being replaced by a more strongly
binding from new phenol molecule. Based on the experi-
mental data presented here and the literature results, it
was concluded that in the hydrogenation of phenol, these
two types of activation worked cooperatively, resulting in
high activity and selectivity for producing cyclohexanone.
Table 3 showed the efects of catalyst amounts and some
reaction conditions on the hydrogenation of phenol. With
increasing of the amount of Pd/C, the conversion of phenol
increased gradually (Table 3, Entries 1–3). When the dos-
age of Pd/C was 0.005 g, the conversion of phenol reached
100%, and the selectivity of cyclohexanone was 93.6%.
After that, when more catalyst Pd/C was used, the selectivity
of cyclohexanone decreased gradually. This may be because
that the excessive amount of catalyst provides the hydro-
genation activity center too much, and the cyclohexanone is
easily hydrogenated to cyclohexanol. Therefore, the amount
of Pd/C as raw material 5 wt% was appropriate. The efect of
the amount of PTA on the hydrogenation was similar with
that amount of Pd/C (Table 3, Entries 2, 4, 5). This may
be because that the active centers of the catalysts increased
with increasing of amount of PTA, so as to improve the
catalytic performance. But when PTA was excessive, the
PTA maybe covers the surface of the hydrogenation centers,
resulting in decrease of the activity of the catalyst. To a cer-
tain extent, the amount of Pd/C and PTA own the synergis-
tic catalytic efect. Therefore, the nice conversion of phenol
and selectivity of cyclohexanone could be obtained at an
appropriate mass ratio of Pd/C and PTA (1:10). The reaction
temperature had a considerable efect on the hydrogenation
(Table 3, Entries 2, 6, 7). 100% conversion of phenol and
93.6% selectivity of cyclohexanone were obtained within 3 h
under the mild conditions of 80 °C and 1.0 MPa hydrogen
pressure. With the temperature continues rise, little efect on
3.2 The Efects of the Reaction Conditions
on the Reaction Results
The efect of solvents on the hydrogenation of phenol to
cyclohexanone were showed in Table 2. Among the investi-
gated solvents, when dichloromethane was used as solvent,
the hydrogenation exhibited the best conversion of phenol
(nearly 100%) and selectivity of cyclohexanone (93.6%).
1 3

Hydrogenation of Phenol to Cyclohexanone over Bifunctional Pd/C-Heteropoly Acid Catalyst…
Table 3 Hydrogenation of
phenol with Pd/C-PTA catalyst
using dichloromethane as
solvent
Entry
Pd/C(g)
PTA(g)
T(oC)
H2 (MPa)
Time(h)
Conv. (%)
Sel. (%)
C=O
C–OH
1
2
3
4
5
6
7
8
0.004
0.005
0.006
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.05
0.05
0.05
0.03
0.07
0.05
0.05
0.05
0.05
0.05
0.05
80
80
80
80
80
60
100
80
80
80
80
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
2.5
1.0
1.0
3
3
3
3
3
3
3
3
3
2
4
90.4
100
100
96.2
100
75.1
100
100
100
100
100
93.9
93.6
89.5
94.0
82.0
94.4
60.7
80.7
55.1
83.6
89.0
6.1
6.4
10.5
6.0
18.0
5.6
39.3
19.3
44.9
16.4
11.0
9
10
11
Conditions: phenol 0.1 g, dichloromethane solvent 10 mL, unless otherwise noted
the reaction conversion rate, but the selectivity of cyclohex-
anone decreased signifcantly (Table 3, Entries 2, 7). This
is probably because that, when the temperature rises, not
only conducive to Pd/C hydrogenation activity generated
more centers to facilitate the hydrogenation reaction of phe-
nol; and also promote phosphotungstic acid generated more
activity center, improve the catalyst the dispersion of the
active ingredient to improve the selectivity of cyclohexanone
[30]. But when the temperature is too high, the heteropoly
acid active branch decomposition and dispersion caused by
uneven reunion, afecting the activity of the catalyst. Thus,
the optimum reaction temperature is 80 °C. Within 3 h of
reaction at 80 °C and 1.0 MPa H2, at an appropriate mass
ratio of Pd/C and PTA (1:10), the conversion of phenol
attained 100% and the selectivity for cyclohexanone was
93.6%.
Conversion
Selectivity
100
80
60
40
20
0
1
2
3
Run
4
5
Fig. 1 Recycling of catalyst in the hydrogenation of phenol
3.3 Reusability of Catalyst
reaction. Therefore, a lot of research work still needs to be
done that about how to avoid catalyst coking which leads to
reduced catalytic activity.
Reaction conditions: 0.055 g Pd/C-Heteropoly acid in the
frst run (no further addition), 0.1 g Phenol in each run,
T = 80 °C, t = 3 h, P(H2) = 1.0 MPa dichloromethane sol-
vent 10 mL.
3.4 Generality of Pd/C‑Heteropoly Acid
Performance for the Other Hydrogenations
Figure 1 showed the result of reusability of the composite
catalytic system of Pd/C-Heteropoly acid in the hydrogena-
tion of phenol. It was found that the catalyst still exhibited
high selectivity after using 5 times, but the conversion of
phenol decreased slightly. This stem from the generation of
carbonaceous deposits (coking) in the course of the reaction
leads to deactivation of the catalyst. However, the Lewis
acidity still in heteropoly acid cation which can inhibits the
cyclohexanone further hydrogenate to cyclohexanol. The
results indicated that the structure of the heteropoly acid
was unchanged even after being reused for fve cycles, sup-
porting its good reusability. To further clarifed, the cok-
ing problem of the solid catalyst is also appearance in this
To illustrate the general applicability of Pd/C-Heteropoly
acid, the method was extended to ring hydrogenation of
other hydroxylated aromatic compounds. Table 4 showed
the results of these hydrogenations. As can be seen, 100%
conversion of phenol was achieved in all cases, and the
selectivity toward substituted cyclohexanones were excel-
lent (>96%). As expected, methyl phenol derivatives gave
methyl cyclohexanones with both excellent conversions
and selectivity (Table 4, Entries 1–3). However,it takes
12 h for o-cresol to reach the same conversion rate as other
derivatives. Hydrogenation of hydroquinone, resorcinol and
1 3

S. Liu et al.
Table 4 Results of reduction
of phenol derivatives over the
Pd/C-Heteropoly acid
Entry
1
derivative
T(oC) Time(h) Conv.(%)
product
Sel.(%)
OH
O
80
80
80
80
80
12
3
99
99
99
99
99
90.6
92.4
91.5
94.3
91.7
OH
O
2
3
4
5
OH
O
3
OH
O
3
HO
HO
OH
O
3
OH
OH
OH
OH
O
6
7
80
3
3
99
99
90.1
92.3
OH
O
OH
100
Conditions: Pd/C-Heteropoly acid 0.055 g, substrate 0.5 mmol, P(H2)=1.0 MPa
pyrocatechol (Table 4, Entries 4–6) were produce very valu-
able hydroxyl cyclohexanones, again with high conversion
and selectivity. Furthermore, this Pd/C-Heteropoly acid
catalyst is predicted to be active in the hydrogenation of
other phenol derivatives, such as naphthalene, methoxy-
phenol, aminophenols, etc. Work focused on more detailed
hydrogenation studies of Pd/C-Heteropoly acid material is
going on.
of Pd/C-Heteropoly acid. By varying the reaction conditions
(e.g. the temperature and the amount of Pd/C-Heteropoly
acid), the composite catalytic system of Pd/C-Heteropoly
acid showed 100% conversion and high selectivity for the
hydrogenation of phenol to cyclohexanone under the pre-
ferred reaction conditions, such as the amount of the catalyst
5% of the starting material, the catalyst system is a heter-
opoly acid and Pd/C that mass ratio of 10, the reaction tem-
perature is 80 °C, hydrogen pressure is 1.0 MPa, the reac-
tion time is 3 h. The composite catalyst system was stable
and could be reused fve times, the catalytic activity without
obvious decrease.
4 Conclusions
Within the present study, a new and more sustainable
method for the hydrogenation of phenol to cyclohexanone
was developed by employing the composite catalytic system
Acknowledgements This work was fnancially supported by the Natu-
ral Science Foundation of China (31370570), the Taishan Scholars
1 3

Hydrogenation of Phenol to Cyclohexanone over Bifunctional Pd/C-Heteropoly Acid Catalyst…
Projects of Shandong (ts201511033), the Key R&D Project of Shan-
dong (2017GGX40106), and the People’s Livelihood Science and
Technology Project of Qingdao (173383NSH). The authors are also
grateful for the experimental conditions which the Polyphase Fluid
Reaction and Separation Engineering Key Laboratory of the Shandong
gives.
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Synth Commun 48:223–232
Conflict of interest There are no conficts to declare.
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