Air-promoted selective hydrogenation of phenol to cyclohexanone at low temperature over Pd-based nanocatalysts

Article abstract of DOI:10.1007/s11426-017-9095-4

Attaining high activity with high selectivity at low temperature is challenging in the selective hydrogenation of phenol to cyclohexanone due to its high activation energy (E_a, 55–70 kJ/mol). Here we report a simple and efficient strategy for p

Full text of DOI:10.1007/s11426-017-9095-4

SCIENCE CHINA
Chemistry
November 2017 Vol.60 No.11:1444–1449
doi: 10.1007/s11426-017-9095-4
• ARTICLES •
Air-promoted selective hydrogenation of phenol to cyclohexanone
at low temperature over Pd-based nanocatalysts
Qing Guo, Shiguang Mo, Pengxin Liu, Weidong Zheng, Ruixuan Qin, Chaofa Xu,
Youyunqi Wu, Binghui Wu* & Nanfeng Zheng*
Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Chemical Productions of
Alcohols-Ethers-Esters, State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry
for Energy Materials, and Engineering Research Center for Nano-Preparation Technology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, China
Received March 31, 2017; accepted June 12, 2017; published online August 11, 2017
Attaining high activity with high selectivity at low temperature is challenging in the selective hydrogenation of phenol to
cyclohexanone due to its high activation energy (Ea, 55–70ꢀkJ/mol). Here we report a simple and efficient strategy for phenol
hydrogenation catalyzed by Pd in aqueous phase at 30ꢀ°C by introducing air to promote the catalysis. With the assistance of
air, >99% conversion and >99% selectivity were achieved over Pd(111)/Al2O3 with an overall turnover frequency (TOF) of
621ꢀh−1, ~80 times greater than that of the state-of-art Pd catalyst at 30ꢀ°C. Mechanism studies revealed that phenol was activated
to generate phenoxyl radicals. The radicals were yielded from the reaction between phenol and hydroxyl radicals in the presence
of hydrogen, oxygen and protic solvent on Pd. The phenoxyl pathway resulted in a low apparent Ea (8.2ꢀkJ/mol) and thus high
activity. More importantly, this strategy of activating substrate by air can be adapted to other Pd based catalysts, offering a new
thinking for the rational design of cyclohexanone production in industry.
selective hydrogenation, phenol, palladium, nanocatalysis, radical
Citation:
Guo Q, Mo S, Liu P, Zheng W, Qin R, Xu C, Wu Y, Wu B, Zheng N. Air-promoted selective hydrogenation of phenol to cyclohexanone at low temperature
over Pd-based nanocatalysts. Sci China Chem, 2017, 60: 1444–1449, doi: 10.1007/s11426-017-9095-4
1 Introduction
or liquid-phase reactions. The gas-phase reactions usually
have high selectivity (>95%) and turnover frequency (TOF)
but low conversion (<80%), and only occur at elevated
temperature (150 to 250ꢀ°C) [1,2,18–21]. On the other hand,
the liquid-phase reactions can occur under mild temperature
(<100ꢀ°C) with satisfying selectivity (>95%) and gratifying
conversion (>90%), but usually with low TOF because of
the lower reaction temperature in comparison with gas-phase
hydrogenation [3,7,12–14,16,17,22]. Hence, achieving high
TOF with high conversion and selectivity for phenol hydro-
genation is desirable.
Selective hydrogenation of phenol has attracted wide at-
tention because it was considered as a promising route to
produce cyclohexanone [1–7], an important industry inter-
mediate in the synthesis of nylons and polyamide resins [8].
Compared with oxidation of cyclohexane [9,10] which is
another industrial route to prepare cyclohexanone, selec-
tive hydrogenation of phenol can occur at relatively low
temperature without excessive energy consumption and
side reactions [1,2,11–17]. The hydrogenation of phenol to
cyclohexanone can be conducted through either gas-phase
Generally, the reaction can be accelerated either through
increasing reaction temperature or decreasing apparent acti-
vation energy (Ea) according to Arrhenius equation [15,23].
*Corresponding authors (emails: binghuiwu@xmu.edu.cn; nfzheng@xmu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

Guo et al. Sci China Chem November (2017) Vol.60 No.11
1445
While increasing reaction temperature makes the reaction
60ꢀ°C for another 2ꢀh.
more energy consuming, the increased reaction temperature
also accelerates the over-hydrogenation (herein cyclohex-
anone to cyclohexanol) and thus decreases the selectivity
[1–3,7,14,24]. For this reason, it is highly desirable to
develop a room-temperature catalytic process that offers
both high catalytic activity and selectivity [25–27]. Never-
theless, due to high apparent Ea (55–70ꢀkJ/mol) of selective
hydrogenation from phenol to cyclohexanone [18,28,29], it
is challenging to achieve high TOF at low temperature.
Herein, we report a facile and efficient method to reduce
apparent Ea in phenol hydrogenation to cyclohexanone at
30ꢀ°C by simply introducing air. With the assistance of air
and using Pd(111)/Al2O3 as model catalyst, the apparent Ea
was reduced from 68.3 to 8.2ꢀkJ/mol. In an aqueous-phase
reaction, 99% conversion of phenol was achieved with >99%
selectivity to cyclohexanone within 7ꢀh in a mixture of H2
and air (1:1, in volume) at 30ꢀ°C. The overall TOF was
greater than the highest value by a factor of 80 reported in
liquid-phase hydrogenation at 30ꢀ°C [17,25,26] and also a
little higher than that of gas-phase hydrogenation operated at
180ꢀ°C [2,20,21]. It is demonstrated that the reaction under-
goes an alternative mechanism involving hydroxyl radicals
(·OH) and phenoxyl radicals (·OPh). This study provides
new insights into the catalytic hydrogenation of phenol to
cyclohexanone for industrial applications.
2.2 Characterization
Transmission electron microscopy (TEM), including
high-resolution transmission electron microscopy (HRTEM),
and high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) studies were performed
on a TECNAI F-30 high-resolution transmission electron
microscopy (Philips, Netherlands) operating at 300ꢀkV. The
samples were prepared by dropping the ethanolic dispersion
of samples onto 300-mesh carbon-coated copper grids and
immediately evaporating the solvent.
The UV-Vis absorption spectra were taken on UV-2600
(Shimadzu, Japan). DMF was used as solvent during mea-
suring H2PdCl4 and [Pd2(μ-CO)2Cl4]2− cluster. γ-Al2O3 and
freshly prepared Pd(111)/Al2O3 were directly measured in the
form of solid.
CO stripping was carried out on CHI660E (CH Instru-
ments, USA). Pd(111)/Al2O3-modified working electrode
was fabricated by depositing the ethanolic dispersion of
freshly prepared catalyst onto a glassy carbon electrode
followed by drying under an infrared radiation (IR) lamp. A
saturated calomel electrode (SCE) and a platinum foil were
used as the reference and counter electrode, respectively. The
CO stripping voltammogram was recorded in 0.1ꢀM H2SO4
at a sweep rate of 2ꢀmV/s without introducing any additional
CO.
2 Experimental
CO-titration was carried out on a Micromeritics Auto
Chem II 2920 (USA) chemical adsorption instrument with
TCD detector to determine the metal dispersion for TOF cal-
culations. Before analysis, Pd(111)/Al2O3 was exposed in air
for 3ꢀd to release CO completely, then 100ꢀmg Pd(111)/Al2O3
was treated under argon flow (30ꢀmL/min) at 200ꢀ°C for
2ꢀh. Then the catalyst was cooled down to 40ꢀ°C and pulse
mode CO titration was performed. The metal dispersion of
Pd(111)/Al2O3 was calculated to be 0.46 by assuming that
the surface stoichiometry of Pd/CO was 2.
2.1 Preparation of catalysts
The synthesis of Pd(111)/Al2O3 catalysts (1ꢀwt% Pd loading;
Pd nanosheets on γ-Al2O3) was conducted via a modified
method, using carbon monoxide as a reducing and shape-con-
trolling agent [30]. In a typical experiment, 20ꢀμL of
1ꢀM H2PdCl4 aqueous solution was added to 15ꢀmL an-
hydrous dimethyformate (DMF). [Pd2(μ-CO)2Cl4]2− was
formed after the solution was treated under 1-atm CO for
10ꢀmin. γ-Al2O3 (200ꢀmg) was dispersed in 5ꢀmL anhydrous
DMF and slowly added into the above mixture while stir-
ring. The final mixture was kept under 1-atm CO for more
5ꢀmin and the reaction was monitored by UV-Vis absorption
spectroscopy (Figure S1, Supporting Information online).
The solid was collected by centrifugation and washed with
ethanol twice. Purified product was dried under vacuum at
room temperature.
2.3 Catalytic experiments
All reactions were conducted in a 48-mL glass pressure
vessel charged with: (1) 0.2ꢀMPa H2 (first bubbled with
N2 to remove any air, then purged with pure H2 to re-
move N2 and finally charged with totally 0.2ꢀMPa H2); (2)
0.1ꢀMPa H2+0.1ꢀMPa air (not bubbled to remove air; directly
charged with 0.1ꢀMPa H2).
The Pd NP/Al2O3 (1ꢀwt% Pd loading; NP, nanoparticle)
nanocatalyst was prepared by incipient wetness impreg-
nation of γ-Al2O3 with an aqueous solution of H2PdCl4
(20ꢀμL, 1ꢀM). The solvent was then gradually evaporated
by heating the solution at 120ꢀ°C until dried. The sample
was then calcined under air flow (30ꢀmL/min) at 300ꢀ°C for
2ꢀh and further reduced under H2 flow (30ꢀmL/min) at
For phenol hydrogenation catalyzed by Pd(111)/Al2O3, the
freshly-prepared catalyst of Pd(111)/Al2O3 (0.1ꢀμmol Pd) and
phenol (200ꢀμmol dispersed in 10ꢀmL water) were added to
the glass pressure vessel and kept stirring (1500ꢀr/min) at
30ꢀ°C in a water bath. Then 0.2ꢀMPa pure H2 or 1:1 H2&air
was applied into the vessel. The reaction was carried out for
various time durations and diluted with ethanol for further

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Guo et al. Sci China Chem November (2017) Vol.60 No.11
use in gas chromatography. The mixture was well dispersed
using ultrasonic. The products were identified by gas chro-
matograph-mass spectrometer (GC-MS, QP2100 plus, Shi-
madzu, Japan). Gas chromatography analysis was carried
out on FULI 9790 II (China) with a KB-5 capillary column
(30ꢀm×0.32ꢀmm×0.33ꢀμm) using acetophenone as an internal
standard. The GC was operated at 90ꢀ°C for 3ꢀmin and the
temperature was raised to 220ꢀ°C at a speed of 10ꢀ°C/min. Ni-
trogen was used as carrier gas. Injection port and flame ion
detector (FID) were kept at 250ꢀ°C.
For phenol hydrogenation catalyzed by commercial Pd
NP/C and home-made Pd NP/Al2O3, the amount of Pd was
0.2ꢀμmol and the reactions were kept under the same condi-
tion as for Pd(111)/Al2O3.
For the hydrogenation of cyclohexanone catalyzed by
Pd(111)/Al2O3, the fresh prepared catalyst of Pd(111)/Al2O3
(2ꢀμmol Pd) and cyclohexanone (200ꢀμmol, dispersed in
10ꢀmL water) were added to the glass pressure vessel.
0.2ꢀMPa pure H2 was applied into the vessel. Then the vessel
was kept stirring (1500ꢀr/min) in a water bath at different
temperatures for 3ꢀh. The analyzed condition was the same
with mentioned above.
0.1ꢀmg Pd(111)/Al2O3 and a 10-mL aqueous solution contain-
ing 0.5ꢀmM TA and 2ꢀmM NaOH were added into a 48-mL
glass pressure vessel. The solution was bubbled with nitrogen
to remove air if needed to deoxygenate. Then the vessel was
charged with pure H2 or 1:1 H2&air to 0.2ꢀMPa and kept at
30ꢀ°C in a water bath for assorted time duration with stirring.
The fluorescence emission spectra excited at 315ꢀnm from
the 2-hydroxyterephthalic acid were captured on a Hitachi
F-7000 fluorescence spectrophotometer (Japan).
Detection of phenyl radicals (·Ph): 1ꢀmg Pd(111)/Al2O3
was dispersed in 1ꢀmL water in a 48-mL glass pressure ves-
sel. The vessel was purged with nitrogen to remove air then
charged with pure H2 to 0.2ꢀMPa and kept in water bath at
30ꢀ°C. 20ꢀμL phenol and 100ꢀmmol DMPO were dispersed
in 1ꢀmL water and then injected into the above vessel. After
stirring for 5ꢀmin, H2 was removed and the solution was bub-
bled with air. The reaction mixture was filtered off and the
filtrate was detected by ESR.
ESR spectra were recorded on a Bruker EMX-10/12
microspectrometer (Germany) at 30ꢀ°C. The operation
frequency and power were 9.43ꢀGHz and 19.83ꢀmW, respec-
tively. Liquid samples were injected into capillary tubes
which were placed into NMR tubes for measurements. The
data fitting process was conducted on an EPR simulator
(www.eprsimulator.org) whose code was written by Vic-
tor Chechik at the University of York. Different species
(DMPO-OH, DMPO-Ph and DMPO-degrade) were involved
in the fitting process.
TOF was defined as mole of consumed reactant of
1ꢀmol surface Pd atom in 1ꢀh in a defined time period. The
initial TOF was calculated as the initial reaction rates when
the conversion was below 10%.
The apparent Ea of phenol hydrogenation with or without
deoxygenation was calculated. The calculations were based
on data collected from experiments under assorted tempera-
ture ranged from 30 to 70ꢀ°C catalyzed by Pd(111)/Al2O3:
(1) For the determination of Ea in the absence of air, the
fresh prepared Pd(111)/Al2O3 catalyst (2ꢀμmol Pd) and phenol
(200ꢀμmol) were dispersed in 10ꢀmL water and then added
into the vessel. The solution was bubbled with nitrogen to
remove air. Then 0.2ꢀMPa H2 was added and kept at 30ꢀ°C in
a water bath.
(2) For the determination of Ea in the presence of air, the
freshly-prepared Pd(111)/Al2O3 catalyst (0.1ꢀμmol Pd) and
phenol (200ꢀμmol) were dispersed in 10ꢀmL water and then
added into the vessel. Then 0.1ꢀMPa H2 was added and kept
at 30ꢀ°C in a water bath.
3 Results and discussion
Our experiments showed that by simply introducing air into
the reaction system, the conversion of phenol to cyclohex-
anone at 30ꢀ°C was dramatically increased using commer-
cial Pd NP/C catalyst (Figure 1(a) and Figure S2). Further-
more, the enhancement on the initial reaction rate by air was
confirmed when home-made Pd NP/Al2O3 (Figure S3) was
employed as catalyst under same condition, also shown in
Figure 1(a). The initial reaction rates using Pd NP/C and Pd
NP/Al2O3 as catalyst were amplified by 6 and 50 times, re-
spectively. This effect unveils a progressive and robust ap-
proach to high conversion of phenol hydrogenation at low
temperature.
2.4 Mechanism analysis
For further utilization and industrial application of this
method, it is demanding to understand the role of air and
the reaction mechanisms over Pd-based catalysts. However,
Al2O3 supported Pd NPs have a complex structure containing
different ration of edge, corner, terrace and defect sites [31].
It is too complicated to figure out why air can boost the
hydrogenation conversion when Pd NPs were used. Thus,
an uncomplicated and surface-clean Pd catalyst (i.e. Pd(111)
nanosheets supported on γ-Al2O3) was chosen for further
studies. The synthesis process was monitored by UV-Vis
Spin trapping of OH radicals (·OH) by 5,5-dimethyl-1-pyrro-
line-1-oxide (DMPO): 2ꢀmL (0.05ꢀM) DMPO aqueous solu-
tion and 1ꢀmg Pd(111)/Al2O3 were added into a 48-mL glass
pressure vessel. The vessel was then charged with H2 to
0.1ꢀMPa in the presence of air and kept at 30ꢀ°C in a water
bath for 5ꢀmin with stirring. The reaction mixture was fil-
tered off and the filtrate was detected by electron spin reso-
nance (ESR).
Spin trapping of OH radicals by terephthalic acid (TA):

Guo et al. Sci China Chem November (2017) Vol.60 No.11
1447
drogenated to an enol and then rapidly isomerized to form cy-
clohexanone. Cyclohexanone would be further hydrogenated
into cyclohexanol [4,17]. However, our experimental data
and literature result indicated that direct hydrogenation of
phenol was hindered by high apparent Ea at low temperature.
After introducing air into reaction system, the Ea was cur-
tailed from 68.3 to 8.2ꢀkJ/mol (Figure 1(d)), indicating that
phenol was efficiently activated after introducing air. The low
Ea in our case suggested a different reaction mechanism in the
presence of air.
To better understand the role of air in the catalyzed reaction,
firstly we figured out whether oxygen accelerated the catalytic
activity. Various amount of pure oxygen was injected to the
reaction system for further investigation, as nitrogen is known
as an inert gas. From Figure 2(a), we can certainly conclude
that the conversion of the reaction is highly dependent on the
amount of oxygen.
It has been reported that active oxygen species, such as hy-
droxyl radicals, can be formed on Pd-based catalyst in the
presence of both H2 and O2 [32,33]. The existence of ·OH
was testified by fluorescence spectroscopy using TA as cap-
ture agent [34]. The presence of ·OH was only observed when
air was introduced to the system, suggesting that OH radi-
cals were generated from oxygen reacting with hydrogen cat-
alyzed by Pd (Figure 2(b)).
Figure 1 (a) Comparison of initial reaction rates of phenol hydrogena-
tion catalyzed by commercial Pd catalyst (Pd NP/C) and home-made Pd
NP/Al2O3 in the presence of 0.2ꢀMPa H2 or 0.1ꢀMPa H2+0.1ꢀMPa air; (b)
HR-TEM image of as-prepared Pd(111)/Al2O3; (c) time profiles of the
conversion and selectivity of phenol hydrogenation to cyclohexanone cat-
alyzed by Pd(111)/Al2O3 at 30ꢀ°C; (d) apparent Ea of phenol hydrogenation
catalyzed by Pd(111)/Al2O3 (color online).
The existence of OH radicals indicated that the reaction
underwent a new pathway involving free radicals. To fur-
ther understand the new mechanism, ESR was carried out
using DMPO as spin trapping agent to detect free radicals
in the reaction system. The ESR spectra (Figure 3(a)) from
the reaction without phenol showed hyperfine coupling con-
stants where aN=15ꢀG and aHβ=15ꢀG, which were identified
as DMPO-OH radicals [35]. This observation further con-
firmed the existence of hydroxyl radicals, which is consistent
with the result in Figure 2(b).
spectra (Figure S1), and the structure and exposed facets
of as-prepared Pd(111)/Al2O3 was determined by TEM, CO
stripping and IR (Figure 1(b), Figures S4–S6).
As we expected, the air promoting effect was also observed
in phenol hydrogenation using the as-prepared Pd(111)/Al2O3
catalysts. Conversion exceeding 99% was achieved with
over 99% selectivity within 7ꢀh in the presence of H2 and
air at 30ꢀ°C in aqueous solution (Figure 1(c)). Furthermore,
the conversion was maintained above 90% with selectivity
over 99% after 5 cycling tests (Figure S7). On the contrast,
the reaction barely happened in the absence of air, with
conversion lower than 5% under similar condition. Results
have also shown that the overall TOF in the presence of
air is 621ꢀh−1 which is greater than the best reported value,
7.5ꢀh−1 [17,25,26], by a factor of 80 at the same temperature.
Moreover, with the assistance of air, the initial TOF can reach
1300ꢀh−1 at 30ꢀ°C in our case, which is a little higher than that
of gas phase hydrogenation of phenol at 180ꢀ°C [2,20,21].
These results revealed that introducing air into the reaction
system readily increased the activity of the Pd catalyst in
the selective hydrogenation of phenol to cyclohexanone at
low temperature. With the enhanced activity at low temper-
ature, a high selectivity toward cyclohexanone (>95%) was
achieved owing to the relatively low activity of over-hydro-
genation of cyclohexanone into cyclohexanol (Figure S8).
It is reported that the proceeds of phenol hydrogenation oc-
curred in a stepwise manner. Phenol was first partially hy-
ESR spectra from the reaction with phenol shown in
Figure 3(b), with hyperfine coupling constants where aN=
Figure 2 (a) Conversion of phenol to cyclohexanone catalyzed by
Pd(111)/Al2O3 as a function of oxygen amount (within 38-mL gas space); (b)
the existence of hydroxyl radicals was testified by fluorescence spectroscopy
using TA as a spin trapping agent. Pd(111)/Al2O3 was used as catalyst
throughout reactions (color online).

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Guo et al. Sci China Chem November (2017) Vol.60 No.11
16ꢀG and aHβ =23ꢀG, revealed the existence of DMPO-Ph
[36,37]. The presence of phenyl radicals (·Ph) can be
explained that phenol reacts with OH radicals to gener-
ate phenoxyl radicals (·OPh) [38] which isomerise into
ketocyclohexadienyl radicals (·Ph) at ambient conditions
(Figure 3(c)) [39–41]. What is more, Li et al. [42] predicted
that phenol dissociation to phenoxyl and further hydrogena-
tion to form cyclohexanone was the major reaction route on
Pd(111) by DFT studying. According to Li’s DFT results and
our experiment data, we can summary that the generation of
phenoxyl species over Pd(111) is promoted through phenol
reacting with OH radicals, leading to high reaction rate
at low temperature (Figure 4). This is consistant with the
previous reported by Nelson et al. [25] that the generation of
phenoxyl species is conducive to promote reaction.
are also elemental for the increase of reaction activity by car-
rying out the reactions at same conditions with protic solvent
(water) or aprotic solvents (n-hexane and ethyl acetate) (Fig-
ure S9). We predicted that the existence of protons is essential
to generate OH radicals. The proton effect is consistent with
the mechanism proposed by Wilson et al. [43], of which pro-
tons boosted oxygen-hydrogen reaction on Pd-based catalysts
at low temperature.
4 Conclusions
In conclusion, we have developed a green, energy-saving
and efficient method for selective hydrogenation of phenol
to cyclohexanone by simply introducing air, over Pd-based
nanocatalysts in aqueous system at low temperature. The
activity was greatly enhanced due to the reduction of Ea as
a result of the formation of phenoxyl radicals, which come
from phenol reacting with hydroxyl radicals in the presence
of oxygen and protic solvent. These findings provide deep
insights into phenol hydrogenation in an alternative pathway
and show great potential for industrial applications.
Based on above results, we can conclude that the formation
of hydroxyl radicals is crucial to boost the hydrogenation of
phenol to cyclohexanone. Thus, it is demanding to identify
any factor that can affect the formation of OH radicals on
Pd-based catalysts from the reaction between H2 and O2. Be-
sides the oxygen amount, we have proved that protic solvents
Acknowledgments This work was supported by the Ministry of Science
and Technology of China (2017YFA0207302, 2015CB93230) and the Na-
tional Natural Science Foundation of China (21420102001, 21333008).
Conflict of interest
The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online
at http://chem.scichina.com and http://link.springer.com/journal/11426.
The supporting materials are published as submitted, without typesetting
or editing. The responsibility for scientific accuracy and content remains
entirely with the authors.
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