Selective hydrogenation of phenol to cyclohexanone by SiO2-supported rhodium nanoparticles under mild conditions

Article abstract of DOI:10.1016/j.jcat.2018.06.002

A silica-supported rhodium catalyst for the selective hydrogenation of phenol to cyclohexanone under mild conditions has been developed. As the Rh concentration on the catalyst increased from 0.5 to 15 wt%, the conversion (at phenol/Rh mole ratio 100/1) dropped whereas the initial selectivity to cyclohexanone increased. The direct hydrogenation to cyclohexanol occurred in parallel with partial hydrogenation to cyclohexanone. The negative correlation between selectivity and Rh dispersion suggests that direct hydrogenation occurs at low coordination sites whereas dissociation of phenol to phenoxy followed by hydrogenation to cyclohexanone takes place at higher coordinated terrace sites. DFT calculations revealed that the activation barrier for O–H bond cleavage is lower for phenol adsorbed on a Rh(1 1 1) flat surface than on small particles. By blocking the low coordination edge and step sites through grafting with (3-mercaptopropyl)trimethoxysilane, the cyclohexanone selectivity was improved from 82 to 93% at 100% conversion. The catalyst is active at room temperature and 1 atm H₂ pressure and can be easily activated by in-situ reduction.

Full text of DOI:10.1016/j.jcat.2018.06.002

Journal of Catalysis 364 (2018) 354–365
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective hydrogenation of phenol to cyclohexanone by SiO -supported
2
rhodium nanoparticles under mild conditions
a
a
b
c
c
a
Hongwei Zhang , Aijuan Han , Kazu Okumura , Lixiang Zhong , Shuzhou Li , Stephan Jaenicke ,
a,
⇑
Gaik-Khuan Chuah
a
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-machi Hachioji-city, 192-0015 Tokyo, Japan
Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A silica-supported rhodium catalyst for the selective hydrogenation of phenol to cyclohexanone under
mild conditions has been developed. As the Rh concentration on the catalyst increased from 0.5 to
Received 7 November 2017
Revised 19 April 2018
Accepted 3 June 2018
15 wt%, the conversion (at phenol/Rh mole ratio 100/1) dropped whereas the initial selectivity to
cyclohexanone increased. The direct hydrogenation to cyclohexanol occurred in parallel with partial
hydrogenation to cyclohexanone. The negative correlation between selectivity and Rh dispersion
suggests that direct hydrogenation occurs at low coordination sites whereas dissociation of phenol to
phenoxy followed by hydrogenation to cyclohexanone takes place at higher coordinated terrace sites.
DFT calculations revealed that the activation barrier for O–H bond cleavage is lower for phenol adsorbed
on a Rh(1 1 1) flat surface than on small particles. By blocking the low coordination edge and step sites
through grafting with (3-mercaptopropyl)trimethoxysilane, the cyclohexanone selectivity was improved
Keywords:
Rhodium
Phenol hydrogenation
Cyclohexanone
Particle size
Site specificity
Thiol-modification
2
from 82 to 93% at 100% conversion. The catalyst is active at room temperature and 1 atm H pressure and
can be easily activated by in-situ reduction.
Ó 2018 Elsevier Inc. All rights reserved.
1
. Introduction
selectivity for cyclohexanone can be influenced by a number of
parameters including the type of metal [5,9–11], its particle size
[12–14], and the nature of the support [7,12,14–18]. Various met-
als such as palladium [6,7,12,14–18], Raney nickel [19], platinum
[10,20], rhodium [21–23], as well as bimetallic Pd-Mg, Pd-Ce and
Au-Pd [9,24,25] are active for this reaction. In general, palladium
catalysts show good selectivity to cyclohexanone although the
activity is not high. Therefore, high hydrogen pressure (>5 bar),
temperature (>50 °C) and phenol/Pd molar ratios of 5–20 are typ-
ically used. Liu et al reported that the activity and cyclohexanone
selectivity could be enhanced to >99.9% when Pd/C was used in
Cyclohexanone is an important intermediate in the synthesis of
caprolactam for nylon-6 and adipic acid for nylon-6,6 with about
7% of the annual production being devoted for this purpose
1,2]. The balance is used as building block in the synthesis of phar-
9
[
maceuticals, herbicides, insecticides and as a specialty solvent for
resins and lacquers. Commercially, cyclohexanone is prepared by
the catalytic oxidation of cyclohexane or via the hydrogenation
of phenol in either a one- or two-step process [3]. In the two-
step process, phenol is fully hydrogenated to cyclohexanol fol-
lowed by an endothermic dehydrogenation step to cyclohexanone
combination with AlCl
gested to activate the benzene ring for hydrogenation while
inhibiting the formed cyclohexanone. However, AlCl is hygro-
3
[7]. The addition of the Lewis acid was sug-
(Scheme 1). A one-step process where phenol is selectively hydro-
genated to cyclohexanone is certainly preferred due to savings in
costs and energy.
Phenol hydrogenation can be carried out in the gas or liquid
phase [4–10]. Attaining high selectivity at elevated conversions
under mild reaction conditions is a challenging catalytic problem
as it is difficult to stop the reaction at cyclohexanone. The
3
scopic and reacts with moisture to form corrosive HCl, posing dif-
ficulties in handling and reusability. Hence, recent attention has
focused on finding suitable supports that can enhance the activity
and selectivity of Pd under moderate temperatures and hydrogen
pressure even when water is used as the solvent. Several materials
have been reported including high surface area Al
26], hydroxyapatite [27], TiN [28], hydrophilic carbon [29], metal
organic frameworks MIL-101 [14] and ZIF-67 [29], alkali
2 3
O [15] and ceria
[
⇑
Corresponding author.
E-mail address: chmcgk@nus.edu.sg (G.-K. Chuah).
https://doi.org/10.1016/j.jcat.2018.06.002
021-9517/Ó 2018 Elsevier Inc. All rights reserved.
0

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
355
(
1
2
synthesized). For
a
typical preparation of 5 wt% Rh/SiO
27.9 mg (0.049 mmol) RhCl O, 0.95 g (15.8 mmol) SiO and
ꢁ3H
0 ml deionized water were added into a 100 ml beaker. After
2
,
3
2
2
stirring at room temperature for 3 h, the orange colored slurry
was heated to almost dryness and placed overnight in an oven at
9
0 °C. The sample was calcined at 400 °C for 4 h in air. Samples
with 0.5 to 15 wt Rh% were similarly prepared.
2
.1.2. Grafted Rh catalysts
Scheme 1. Hydrogenation of phenol.
The 5 wt% Rh/SiO sample was grafted with molecules of differ-
2
metal-promoted TiO
polyaniline-functionalized carbon nanotubes [31]. Notably, aque-
ous phase systems using palladium supported on polymeric meso-
2
[30], poly(N-vinyl-2-pyrrolidone) [30] and
ent chain length and chemical groups as shown in Table 1. The
hydroxyl groups of the support react with the methoxysilane moi-
ety to chemically bind the molecules to the surface. In a typical
porous graphitic carbon nitride (mpg-C
N
3 4
) [6,32] and TiO
2
-C
2
synthesis, 0.1 g of 5 wt% Rh/SiO catalyst, (3-mercaptopropyl)trime
composites [18] gave >99% selectivity to cyclohexanone at 100%
conversion. Palladium nanoparticles supported on a specially
designed mesostructured silica (MMT-1) was found to exhibit high
phenol conversion with 98% selectivity at room temperature and
thoxysilane (molar ratio to Rh = 2:1) and 20 ml toluene were
placed in a two-necked 50 ml round bottom flask equipped with
a septum port and a reflux condenser. After stirring at 110 °C for
24 h, the slurry was filtered, washed with acetone three times
and dried at room temperature overnight. The samples are named
2
atmospheric H pressure [33]. Besides gaseous hydrogen, potas-
sium formate, sodium formate and formic acid have also been used
for phenol hydrogenation [34–36]. However, these alternative
hydrogen sources adsorb competitively at the catalyst surface
and an optimized ratio must be worked out to avoid inhibition of
the reaction.
Rhodium is known for its high activity for hydrogenation of the
aromatic ring under very mild conditions [22,37,38]. However,
there are only a few studies on its use for phenol hydrogenation
due to poor selectivity to cyclohexanone. For example, the use of
2
as molecule-5 wt% Rh/SiO where molecule represents Amine-n,
Glycidyl, Aniline, Thiol and Chloro.
2.2. Catalyst characterization
The surface area and porosity of the catalysts were determined
from N adsorption/desorption isotherms (Micromeritics Tristar
2
3000). Prior to the measurement, the sample was pretreated under
a nitrogen flow at 300 °C for 5 h. Powder X-ray diffraction was per-
formed with a Bruker D8 Advance diffractometer equipped with Cu
anode, variable slits and a LynxEye XE detector. The 2h range from
20 to 80° was measured using a step size of 0.02° and a dwell time
of 1 s. Transmission electron micrographs (TEM) were obtained
using a JEOL 3010 operated at 200 kV. The sample was finely
ground and suspended in 2-propanol. A drop of the suspension
was placed onto a carbon-coated copper grid and dried at room
temperature.
2
carbon nanofiber-supported rhodium in supercritical CO resulted
in 100% phenol conversion within 0.5 h (at phenol/Rh molar ratio
of 436) but the selectivity to cyclohexanone was only 43% [21].
Kempe’s group reported that small rhodium nanoclusters of
ꢀ
1.6 – 2.8 nm stabilized in a polymerderived silicon carbonitride
SiCN) matrix formed highly active catalysts for the selective
hydrogenation of phenolic compounds [39]. At 25 °C and 6 bar
, 99% phenol conversion was obtained in comparison to 49%
and 36% for Al - and C-supported Rh, respectively. The selectiv-
(
H
2
O
2 3
X-ray photoelectron spectroscopy (XPS) was performed using a
VG-Scientific ESCALAB Mark 2 spectrometer equipped with a
ity to cyclohexanone for the three catalysts was only 73–78%. In
comparison, a high selectivity of 92% at >95% conversion was found
hemispherical electron analyzer and a Mg K
a anode (1253.6 eV)
for Rh@S-MIL-101 catalyst operating at 50 °C and 5 bar H
2
[40].
operating at 300 W (15 kV ꢂ 20 mA). Wide and detailed spectra
were collected in constant analyzer energy mode with a step of 1
and 0.05 eV, respectively. The analyzed area was 3.0 mm in diam-
eter with medium magnification for samples. The binding energy
of the elements was referenced to the C 1 s signal of ubiquitous
carbon at 285 eV. The spectra were evaluated using a nonlinear
(Shirley) background subtraction.
The good performance was attributed to host-guest cooperation
between the rhodium nanoparticles and sulfonated MIL-101
framework as well as the presence of Cr(III) Lewis acidic sites in
the support. Kuklin et al reported 100% yield of cyclohexanone
using polyacrylic acid-stabilized rhodium nanoparticles modified
2
with 20-fold excess cyclodextrin at 80 °C and 10 to 40 atm H using
n-hexyltriethylammonium bromide as solvent [41].
The elemental composition of the samples was measured by
inductively coupled plasma optical emission spectroscopy (ICP-
OES) using an Optima 5300 DV ICP-OES system. To dissolve the
sample, about 5 mg sample was placed in a Teflon liner with
1 ml of hydrofluoric acid (40%) and heated at 98 °C for an hour, fol-
lowed by the addition of a mixture of concentrated HCl (37%) and
HNO (69%) (volume ratio: 20:1). The sample was then placed in a
3
microwave oven and heated at 200 °C for 2 h. The obtained
solution was diluted to 10 ml before analysis.
Although these results showed that selective hydrogenation to
cyclohexanone could be obtained over Rh catalysts at higher
temperatures and pressures, we were interested in whether the
same could be achieved under ambient conditions. This work
investigates if metal loading, particle size, support, and selective
inhibition of certain active sites can improve the selectivity to
cyclohexanone without compromising on the mild reaction condi-
tions. Selective inhibition of the metal sites was carried out by
grafting of organic functional groups with amine and thiol moieties
onto the catalyst.
Infrared spectroscopy for CO adsorption was performed using a
Perkin Elmer Spectrum Two spectrometer. The sample was pressed
into a self-supporting wafer and mounted in an evacuable quartz
IR cell with CaF windows. After evacuation, the sample was
2
reduced under H flow at room temperature for 1 h. The cell was
2
2
. Experimental
evacuated to 10- mbar and CO was introduced. The sample was
equilibrated in 1 atm CO for 1 h. After pumping off the gas, FTIR
measurements were performed at different time intervals using a
3
2
2
.1. Catalyst preparation
ꢃ1
.1.1. Synthesis of metal oxide-supported Rh catalysts
resolution of 2 cm and 32 scans. In order to determine the metal
Rhodium was supported on the following metal oxides – SiO
2
2
dispersion, pulsed CO chemisorption was carried out using a
homemade temperature programmed desorption apparatus
(
Merck), La
2
O
3
,
TiO
2
(Degussa), MgO (Merck) and ZrO

3
56
H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
Table 1
2
Nomenclature of molecules grafted on 5 wt% Rh/SiO .
Molecule
Name
Molecule
Name
Aniline
Glycidyl
Amine-1
Amine-2
Chloro
Thiol
Amine-3
equipped with a quadrupole mass spectrometer. The sample was
periodic topping up) and 30 °C. The reaction mixture consisting
of 3 mmol (0.282 g) in 50 ml of cyclohexane as solvent was placed
in a Teflon liner and an amount of catalyst corresponding to Rh:
phenol mole ratio 1:100 was added. The system was purged with
helium and heated. Once the temperature was stable, hydrogen
gas was introduced to 1 bar gauge and the reaction was started.
Aliquots were removed at regular time intervals and analyzed
using an Agilent HP 6890 gas chromatograph (GC) equipped with
placed in a quartz reactor and pretreated in flowing H
temperature for 1 h. After flushing in 50 ml min He for 2 h, 50
2
at room
ꢃ1
ll pulses of CO was introduced to the sample until the CO signal
reached saturation. From the total uptake of CO, the metal disper-
sion was calculated based on the FTIR peak areas using integrated
ꢃ1
absorption coefficients of 13, 42 and 130 cm
l
mol for linear-,
bridge- and germinal-bonded CO on Rh [42]. Thermogravimetric
analysis (TGA) was carried out with a TA Instruments Discovery
instrument. About 10 mg of the grafted sample was kept at 100
a HP-5 column (30 m ꢂ 0.32 mm ꢂ 0.25
lm film) and a flame ion-
ization detector. The GC program was as follows: initial tempera-
ture 60 °C, constant pressure, dwell time 8 min, ramp at 20 °C/
min to 200 °C and hold for 3 min. The conversion and cyclohex-
anone selectivity were calculated based on the GC peak areas using
experimentally determined calibration factors:
°
C for 1 h under a flow of purified air to remove physically
adsorbed water before raising the temperature at 5 °C/min to
50 °C. From the weight loss, the mass of the organic component
4
in the sample could be calculated.
Rhodium K edge (23.2 keV) XAFS data were collected at the
BL01B1 station of the Japan Synchrotron Radiation Research Insti-
tute (JASRI). A Si(1 1 1) single crystal was used to obtain a
monochromatic X-ray beam. The measurement was carried out
C
cyclohexanone þ Ccyclohexanonol
Con
version ð%Þ ¼
C
cyclohexanone þ Ccyclohexanol þ Cphenol
C
cyclohexanone
in the quick mode. Ion chambers filled with Ar (100%) and N
75%)/Ar (25%) were used to determine I and I, respectively. The
2
Cyclohexanone selecti
v
ity ð%Þ ¼ C
cyclohexanone þ Ccyclohexanol
(
0
samples were pressed into self-supporting wafers. The data analy-
sis was performed using the REX2000 Ver. 2.0.4 program (Rigaku).
3
3. Results and discussion
.1. Supported Rh catalysts
Fourier transformations of k
v (k) data were performed in the k
-
1
range of 30–160 nm .
3
2
.3. Catalytic tests
3.1.1. Textural properties
The x-ray diffractograms of the Rh/SiO
tion at 400 °C show only the broad peaks of amorphous SiO
2
catalysts after calcina-
The reaction was typically carried out in a two-necked round
2
bottom flask containing 1.5 mmol phenol (0.141 g) in 25 ml of
cyclohexane at ambient temperature (25 °C). The catalyst at a
Rh:phenol mole ratio of 1:100 was added. To check for mass bal-
ance, 1.5 mmol dodecane was added as internal standard. Unless
(Fig. S1). No peaks of rhodium or rhodium oxide could be seen,
even for the highest loading of 15 wt% Rh, suggesting that the Rh
particle size is <5 nm. This is supported by TEM results where
the average Rh particle size was between 1.59 and 3.66 nm for
samples with 0.5–15 wt% Rh ( Figs. 1 and S2). For each loading,
the particles are narrowly distributed about the mean size with a
standard deviation of 0.17–0.51 nm.
otherwise stated, the catalyst was pretreated in a H
2
:He gas flow
ꢃ1
(
2:18 ml min ) at room temperature for an hour prior to use.
The reaction flask was purged with helium followed by hydrogen
before placing a hydrogen filled balloon over one of the necks.
The other neck was closed off with a rubber septum. Under these
conditions, the hydrogen pressure is very close to atmospheric
throughout the reaction. Reactions were also carried out in a Ber-
The nitrogen-sorption isotherms of all samples exhibit type IV
o
isotherms with hysteresis at P/P ꢀ 0.7–0.9, indicating the pres-
2
ence of mesopores (Fig. 2). The SiO support has a wide spread of
pores from 5 to 23 nm with a mean pore size 17.8 nm. After
impregnating with Rh, there was a shift to smaller pore sizes
2
ghof HR100 autoclave at 1 bar H gauge (pressure maintained by

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
357
(
a)
5
nm
(
b)
1
0 nm
(
c)
2
0 nm
2
Fig. 1. TEM images and particle size distribution for (a) 0.5 (b) 5 and (c) 15 wt% Rh/SiO .
2
ꢃ1
<
18 nm. Irrespective of the Rh loading, the pore size distribution
was rather similar with the mean at ꢀ11.5 nm. Interestingly,
SiO -H O, a sample prepared by the same wet impregnation proce-
support has a high surface area in excess of 400 m g even after
calcination at 400 °C for 4 h (Table 2). With increase of Rh loading
2
2
2
from 0 to 15 wt%, the surface area decreased from 406 to 324 m /g
3
ꢃ1
dure but using deionized water without any Rh, also suffered a
similar decrease in pore size as well as pore volume. This shows
that SiO
and the pore volume dropped from 2.3 to 1.0 cm g . The similar-
ity of the pore size distribution curves suggests that the metal is
uniformly distributed within the pore walls and no pore blockage
occurred.
2
underwent textural changes with collapse of the larger
pores as a result of the aqueous treatment. Despite this, the SiO
2

3
58
H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
1
1
600
200
0.40
(b)
(a)
0.30
0.20
0.10
0.00
8
00
00
0
4
0
0.2
0.4 0.6
P/Po
0.8
1
0
10
20
30
Pore diameter (nm)
Fig. 2. (a) Nitrogen sorption isotherms and (b) pore size distribution of (d) SiO
2
(
) SiO
2
2 2
-H O and Rh/SiO catalysts with ( ) 0.5 ( ) 2 ( ) 5 ( ) 10 and ( ) 15 wt% Rh.
Table 2
Textural properties of Rh/SiO
after 9 h under these mild conditions (Table 3, entry 1). Pretreating
the catalyst to a 10% oxygen in helium flow mixture at 300 °C for 1
h did not affect the activity and selectivity, which were essentially
similar to that of the untreated catalyst (Table 3, entry 2). In con-
trast, after pre-reduction at 300 °C for 1 h under a gas flow of
2
catalysts.
Rh loading
wt%)
Surface area
Pore volume
(cm g )
Mean Rh
size (nm)
2
ꢃ1
3
ꢃ1
(
(m
g
)
SiO
SiO
2
2
406
404
405
397
376
367
324
2.3
1.4
1.5
1.4
1.4
1.2
1.0
–
–
-H
2
O
2
10% H in helium, full conversion was reached in a much shorter
0
2
5
1
1
.5
1.59
1.83
2.39
3.25
3.66
time of 3.5 h (Table 3, entry 3). However, the selectivity to cyclo-
hexanone was reduced to 68% as further hydrogenation to cyclo-
hexanol occurred. With a gentler reduction at room temperature
instead of 300 °C, 100% phenol conversion was obtained in 5 h
and the cyclohexanone selectivity was 80% (Table 3, entry 4). Con-
ducting the reactions in an autoclave gave very similar results. Due
0
5
3
3
.1.2. Catalytic activity
.1.2.1. Effect of support. The hydrogenation of phenol was carried
pressure in cyclohexane. A preliminary
2
to the higher pressure of 1 bar H pressure and 30 °C, full phenol
conversion could be achieved in a shorter time of 3–4 h (Table 3,
entries 5 and 6).
out at 25 °C and 1 atm H
2
study carried out using different solvents showed that the highest
yield of cyclohexanone was obtained with cyclohexane (Table S1).
Moreover, it has a low boiling point 80.74 °C, which facilitates sep-
aration from the higher boiling products.
TiO
Over an unreduced 5 wt% Rh/TiO
verted to cyclohexanone after 10 h (Table 3, entry 7). The cyclohex-
2
-supported Rh samples also do not need any pre-reduction.
2
, 70% of the phenol was con-
anone selectivity at 80% was similar to that for the SiO
sample. In contrast, the as-formed La -, MgO-, ZrO
2
-supported
-supported
Samples containing 5 wt% Rh supported on various supports
2 3
O
2
were tested. Good activity was obtained for the SiO
2
-supported
Rh catalysts were inactive and had to be reduced (Table 3, entries
sample. Even without any pre-reduction of the catalyst, 100%
conversion and 82% selectivity to cyclohexanone was obtained
9–14). After reduction at 300 °C for 1 h, phenol was fully converted
after 3.5 h over Rh/ZrO but required a longer time of 24 h for
2
Table 3
Selective hydrogenation of phenol over 5 wt% Rh and Pd catalysts.
Entry
Catalyst
Pretreatment
Reaction
Time
(h)
Conv.
(%)
Sel. C@O
(%)
Conditions[
a]
1
2
3
4
5
6
7
8
9
Rh/SiO
Rh/SiO
Rh/SiO
Rh/SiO
Rh/SiO
Rh/SiO
2
2
2
2
2
2
None
Oxidized
Balloon
Balloon
Balloon
Balloon
Autoclave
Autoclave
Balloon
Balloon
Balloon
Balloon
Balloon
Balloon
Balloon
Balloon
Balloon
Balloon
9
9
3.5
5
4
3
10
3
24
24
24
7
24
3.5
48
48
100
100
100
100
100
100
70
100
0
100
0
100
0
100
11.1
84.2
82.6
82.8
68.1
80.5
80.6
53.1
80.5
72.1
0
74.3
0
73.5
0
[
b]
Reduced[
Reduced[
Oxidized
Reduced[
None
c]
d]
b]
c]
[
Rh/TiO
Rh/TiO
2
Reduced[
None
c]
c]
c]
c]
c]
2
Rh/La
Rh/La
2
O
O
3
3
10
11
12
13
14
15
16
2
Reduced[
None
Rh/MgO
Rh/MgO
Rh/ZrO
Rh/ZrO
Pd/C
Reduced[
None
2
Reduced[
None
78.2
100
82.4
2
Pd/C
Reduced[
[a]
2
Reaction conditions: balloon – phenol (1.5 mmol), cyclohexane (25 ml), catalyst (0.03 g), H in balloon, 25 °C; autoclave – phenol (3 mmol), cyclohexane (50 ml), catalyst
(
0.06 g), 1 bar gauge H
2
pressure in autoclave, 30 °C.
[
b]
ꢃ1
Catalyst pre-oxidized at 300 °C for 1 h in O
Catalyst pre-reduced at 300 °C.
2
/He gas flow (2:18 ml min )
[
c]
[
d]
ꢃ1
Room temperature for 1 h in H
2
/He gas flow (2:18 ml min ).

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
359
Rh/La
2
O
3
. Despite the different activity of the catalysts, the
cyclohexanone occur. Obviously, the- latter sites can hydrogenate
phenol as well as cyclohexanone. Therefore, the adsorption of
phenol at these sites must be much stronger than that of cyclohex-
anone so that even at low phenol concentration, little cyclohex-
anone was hydrogenated.
selectivity to cyclohexanone was relatively similar, ꢀ73–78%.
A commercial 5 wt% Pd/C catalyst was tested for comparison
(Table 3, entries 15 and 16). Under the experimental conditions,
2
5 °C and 1 atm hydrogen, its activity was low. The reduced cata-
lyst required 48 h to reach 84% conversion with 82% selectivity
to cyclohexanone.
The results show that the metallic state of rhodium is necessary
for reaction. For TiO
effected under very mild conditions either in-situ or ex-situ, at
1 atm pressure and room temperature. In contrast, Rh
supported on La , MgO or ZrO required a high temperature
(300 °C) reduction. Hence, SiO was chosen as the support for fur-
ther studies due to its high activity for the selective hydrogenation
2 2
- and SiO -supported samples, this can be
The kinetics of the reaction were monitored using the 5 wt%
Rh/SiO
was converted within the first 2 h when the reaction was carried
out with H contained in a balloon at atmospheric pressure
Fig. 3a). After this induction time, phenol was converted to
cyclohexanone and cyclohexanol. No other products were formed
2
. Without any pre-reduction of the catalyst, no phenol
H
2
2
O
3
2
2
2
(
2
of phenol at 25 °C and atmospheric H pressure and its ease of
(
Fig. S3). The selectivity to cyclohexanone was ꢀ82% and remained
handling.
constant until all the phenol was converted. Extending the reaction
beyond this time led to the rapid hydrogenation of cyclohexanone
to cyclohexanol. The mass balance closed to within 2% (Fig. S4). In
contrast to the unreduced catalyst, after pretreatment in 10%
3.1.2.2. Effect of metal loading. The reactions were conducted in an
autoclave using 1 bar gauge H2 at 30 °C due to the faster kinetics.
No reaction was observed over the SiO2 support (Table 4). How-
ever, a low Rh loading of 0.5 wt% resulted in a highly active catalyst
with all phenol being converted after only 3.5 h (Fig. 4a). The initial
selectivity to cyclohexanone and cyclohexanol was ꢀ75% and 25%,
respectively. This ratio remained fairly constant until all the phenol
was consumed whereupon rapid hydrogenation of cyclohexanone
to cyclohexanol occurred. Similar kinetic profiles were observed
for 2–10 wt% Rh/SiO2 although full phenol conversion required
longer times of 5 to 8 h (Fig. S5). The initial cyclohexanone selectiv-
ity increased to 82–85% with higher Rh loading. At full conversion,
2
H /He flow for an hour at room temperature, the reaction com-
menced from the start (Fig. 3b). Similarly, no induction period
was observed when the reaction was carried out in an autoclave
(Fig. S5). This can be attributed to a more rapid reduction of the
catalyst under 1 bar gauge H pressure at 30 °C. The constant selec-
2
tivity of about 18% cyclohexanol immediately from the start of the
reaction shows that there are sites that catalyse the complete
hydrogenation of phenol to cyclohexanol and those that catalyse
only partial hydrogenation to cyclohexanone. Only after all the
phenol has been converted does hydrogenation of the formed
5 wt% Rh/SiO has the highest selectivity of 80.5%. In contrast, the
2
1
00
100
80
60
40
20
0
(b)
(a)
80
60
40
20
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
2 2
Fig. 3. (a) Time profile for 5 wt% Rh/SiO (j) conversion and selectivity to ( ) cyclohexanone and ( ) cyclohexanol. (b) Conversion for (j) untreated catalyst and ( ) after H
pretreatment at room temperature for 1 h. Reaction conditions: phenol (1.5 mmol), cyclohexane (25 ml), catalyst (0.03 g), H
catalytic runs: conversion ± 1.6%, selectivity ± 0.5%.
2
in balloon, 25 °C. Standard deviation from 3
Table 4
Selective hydrogenation of phenol over Rh/SiO
2
catalysts.[a]
Conv. (%)
Entry
Rh (wt%)
Time (h)
Sel. (%)
Conv.[b] (%)
Sel.[b] (%)
TOF[b] (hꢃ1
)
N
s
/N
T
D[c]
%)
(
1
2
3
4
5
6
0
0.5
2
5
10
15
24
3.5
5
5
7
0
0
0
–
0
–
–
100
100
100
100
100
69.1
73.2
80.5
65.3
35.1
12.5
10.3
8.5
6.3
3.1
75.6
81.4
83.8
84.8
86.2
41
37
38
36
26
0.62
0.56
0.46
0.36
0.32
68.3
55.7
45.6
34.6
31.5
15
[
a]
2
Reaction conditions: phenol (3 mmol), cyclohexane (50 ml), catalyst (0.06 g), 1 bar H pressure in autoclave at 30 °C.
Conversion and selectivity to cyclohexanone after 0.5 h. Standard deviation from 3 catalytic runs: conversion ± 1.6%, selectivity ± 0.5%.
Rh dispersion from CO chemisorption.
[b]
[
c]

3
60
H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
1
00
100
80
60
40
20
0
(a)
(b)
80
60
40
20
0
0
1
2
3
4
5
0
5
10
15
Time (h)
Time (h)
Fig. 4. (a) Time profile of (a) 0.5 and (b) 15 wt% Rh/SiO
cyclohexane (50 ml), phenol/Rh 100:1 (mole ratio), 1 bar H
2
(j) conversion and selectivity to ( ) cyclohexanone and ( ) cyclohexanol. Reaction conditions: phenol (3 mmol),
pressure in autoclave at 30 °C. Standard deviation from 3 catalytic runs: conversion ± 1.6%, selectivity ± 0.5%.
2
ꢃ1
1
(
8
5 wt% Rh/SiO
Fig. 4b). Although the initial cyclohexanone selectivity was high,
6%, it began to decrease with increasing conversion even when
2
sample displayed different reaction kinetics
ꢀ1960 cm
appeared which are assigned to linearly-bonded
Rh-CO and bridged CO-Rh of 3:2 stoichiometry [42]. Furthermore,
for 15 wt% Rh/SiO
of bridged-bonded Rh(CO)
CO are ascribed to crystalline Rh found on larger particles [49].
These results show changes in type of sites with particle growth.
As steps and kink sites are more reactive than terrace sites, it is
not surprising that the initial turnover frequency (TOF) per surface
ꢃ1
2
, the small band at ꢀ1840 cm is characteristic
phenol was still present. Finally, at 100% phenol conversion, the
selectivity to cyclohexanone was reduced to only 35%.
2
[42]. The linear- and bridged-bonded
The results clearly show that the metal loading affects the activ-
ity for phenol hydrogenation and selectivity to cyclohexanone. As
observed by TEM (Fig. 1), the increase in metal loading leads to lar-
ger particles. From CO chemisorption measurements, metal disper-
sion fell from 68.3 to 31.5% for 0.5 to 15 wt% Rh (Table 4). The
ꢃ1
atom was highest at 41 h for 0.5 wt% Rh. For 2–10 wt% Rh, the
ꢃ1
ꢃ1
TOF was rather constant at ꢀ37 h and decreased to 26 h for
number of atoms at the surface, N
cle, N , can also be estimated from the atomic size and crystal
structure of Rh [43–45]. The ratio N /N decreased from 0.62 to
.32 as the Rh loading increased from 0.5 to 15 wt% (Table 4).
S
, and the total atoms in a parti-
15 wt% Rh. The high TOF for 0.5 wt% Rh/SiO is due to a higher rate
2
T
of cyclohexanol formation as its TOF for cyclohexanone formation
was similar to catalysts with higher Rh loading (Fig. S6).
S
T
0
Using density functional theory (DFT), Honkela et al found that
the adsorption and dissociation of phenol on flat Rh(1 1 1) surface
was preferred over that of stepped Rh(2 1 1) due to repulsion of the
hydroxyl group from the step edges [50]. Li et al. also investigated
the role of phenol dissociation in the selective hydrogenation on Pt
(1 1 1) and Pd(1 1 1) using DFT slab calculations [51]. The
calculated energetics and activation barriers indicate that hydro-
genation of phenoxy leads to cyclohexanone while direct hydro-
genation of adsorbed phenol results in cyclohexanol. Therefore,
to gain some insights into the site differentiation on selectivity,
DFT calculations were performed with the Vienna ab initio simula-
tion package (VASP) [52,53]. The projector-augmented wave
method with PBE exchange-correlation functional was used and
the cut-off energy was set as 400 eV [54]. The DFT-D3 method with
Becke-Jonson damping was adopted to include van der Waals
These values are in good agreement with those obtained by CO
chemisorption.
Infrared spectroscopy of adsorbed CO was used to probe the site
distribution of the surface Rh atoms (Fig. 5). The IR spectrum of
ꢃ1
0
.5 wt% Rh/SiO
2
show bands at ꢀ2098 and ꢀ2038 cm , which
are assigned to the symmetric and asymmetric modes of geminal
Rh(CO) , respectively [46–48]. The presence of these bands is
reflective of coordinatively unsaturated Rh centers or dispersed
clusters [22,49]. With higher Rh loading, bands at ꢀ2065 and
2
interactions [55,56]. All structures were relaxed using
a
conjugate-gradient algorithm until the residual force was smaller
than 0.02 eV/Å. The climbing-image nudged elastic band method
was used for transition state search and activation energy
calculation [57]. Rh(1 1 1) was chosen as representative of a big
particle with flat plane. The Rh(1 1 1) surface modelled by a
3
-layer slab model with the bottom layer fixed at their bulk posi-
tions. A 6-atom Rh cluster supported on silica was further studied
to evaluate the size effect on the dissociation of the hydroxyl in
phenol. The cleavage barrier for OAH bond was found to be
0
.557 eV for phenol adsorbed on Rh(1 1 1) surface as compared
to 1.354 for the 6-atom cluster (Fig. S7). Hence, the dissociation
of phenol to phenoxy is easier on flat surfaces than on small
clusters.
Based on our results, we propose the following. Firstly, at least
two different sites are involved for phenol hydrogenation, one
2
Fig. 5. Infrared spectra of CO chemisorbed on Rh/SiO at room temperature.

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
361
catalyzing the direct hydrogenation to cyclohexanol (site 1) and
the other (site 2) via cyclohexanone (Fig. 6a). As direct hydrogena-
tion to cyclohexanol was most prominent for the highly dispersed
access of the reactant molecule to the neighbouring low coordina-
tion sites (Fig. 6b). In a previous study, the group of Dyson reported
that the chemoselectivity in the hydrogenation of acetophenone to
cyclohexylacetone could be modified by the addition of phosphine
ligands to PVP-stabilized Rh nanoparticles [58]. It was postulated
that specific sites at the surface of the nanoparticles became
blocked although the nature of these sites were not investigated.
0
2
.5 wt% Rh/SiO , it suggests that site (1) comprises Rh atoms at cor-
ners and edges. Being coordinatively unsaturated, they exhibit
excellent activity for reactions including hydrogen dissociation.
Consequently, there are abundant H atoms to hydrogenate phenol
directly to cyclohexanol. The fraction of such sites decreases with
bigger clusters, and is paralleled by a decrease in the initial cyclo-
hexanol selectivity with Rh loading. Site (2) comprises atoms on
planes and terraces that predominate in larger particles. As shown
by computational studies, these are preferred sites for phenol
adsorption and dissociation [50].
Secondly, the observation that hydrogenation of cyclohexanone
occurs only after all the phenol has been consumed shows that
phenol adsorbs more strongly than cyclohexanone. More impor-
tantly, it also suggests that the sites are independent of each other,
i.e., site (1) does not catalyze the hydrogenation of cyclohexanone
formed at site (2), otherwise increasingly more cyclohexanol
3
.2. Grafting modification on Rh/SiO
.2.1. Catalytic activity
2
3
Trialkoxysilanes containing NH
Table 1) were grafted onto the 5 wt% Rh/SiO
catalysts showed lower hydrogenation activity than the unmodi-
fied 5 wt% Rh/SiO which can be attributed to partial coverage of
the Rh surfaces by the organic molecules (Table 5). Samples grafted
with NH Cl and functional moieties showed lower or
comparable cyclohexanone selectivity to that for 5 wt% Rh/SiO
2
, Cl, S and O functional moieties
(
2
catalyst. The grafted
2
2
,
O
2
.
In contrast, the cyclohexanone selectivity was increased from 82
to 87% for the sample grafted with (3-mercaptopropyl)trimethoxy
silane. However, its conversion was only 47% even after 24 h. To try
2
would be formed with time. Only the 15 wt% Rh/SiO , which had
the largest particle size and hence adequate density of planar sites,
was able to catalyze both phenol and cyclohexanone hydrogena-
tion simultaneously so that at full conversion, the cyclohexanone
selectivity was the lowest. The other lower Rh-containing samples
were able to give higher cyclohexanone yields as the cyclohex-
anone selectivity was maintained until all the phenol was
converted.
The independence of the two hydrogenation sites and striking
feature that cyclohexanone is only reduced in the absence of phe-
nol offer a chance to improve the selectivity to cyclohexanone. We
envisage that if site (1), responsible for direct hydrogenation of
phenol to cyclohexanol, could be deactivated or removed, the for-
mation of cyclohexanol would be minimized. In order to achieve
this, amine, chloro- and thiol-containing molecules were grafted
using the chemical reaction between hydroxyl groups at the cata-
lyst surface and substituted trimethoxysilanes. Those molecules
positioned adjacent to the sides of the metal particle can hinder
Table 5
2
Selective hydrogenation of phenol to cyclohexanone over grafted 5 wt% Rh/SiO .
Entry
Catalyst
Time (h)
Conv. (%)
Sel. (%)
1
2
3
4
5
5 wt%Rh/SiO
2
9
100
100
100
100
47
100
100
42
82
82
80
60
89
93
86
72
84
Glycidyl-5 wt%Rh/SiO2
Aniline-5 wt%Rh/SiO
Chloro-5 wt%Rh/SiO
Thiol-5 wt%Rh/SiO
Thiol-5 wt%Rh/SiO
Amine-1-5 wt%Rh/SiO
Amine-2-5 wt%Rh/SiO
Amine-3-5 wt%Rh/SiO
25
10
24
24
10
8.5
24
24
2
2
2
[a]
6
2
7
8
9
2
2
2
15
Reaction conditions: phenol (1.5 mmol), cyclohexane (25 ml), catalyst (0.03 g), H
2
(1 atm, balloon), 25 °C.
[a]
Pretreated in O
2
/He gas flow (2:18 ml/min) at 200 °C for 1 h.
(
a)
(b)
site 2
site 2
site 1
Fig. 6. (a) Schematic diagram showing the coordinatively unsaturated (cus) site (1) for direct hydrogenation of phenol to cyclohexanol and terrace site (2) for partial
hydrogenation of phenol to cyclohexanone (b) blocking of cus sites at sides of particles by grafted (3-mercaptopropyl)silane groups.

3
62
H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
and improve its activity, the Thiol-grafted catalyst was preheated
in a 10% O /He gas mixture at 200 °C for 1 h so as to pyrolyse some
the conversion of cyclohexanone to cyclohexanol, with 100% con-
version after 6 h at 1 atm H pressure. (Table 6, entry 1). In com-
parison, over the Thiol-grafted catalyst, the conversion was only
28% after 12 h despite the use of 2 bar gauge H pressure, showing
that grafting with the thiol moiety significantly blocked the active
sites for C@O hydrogenation (Table 6, entry 2). Similarly, the
hydrogenation of cyclohex-2-enone to cyclohexanone was also
2
2
of the thiol groups presumably covering the Rh metal. This pre-
treatment proved to be very effective as full conversion could be
obtained after 10 h (Fig. 7a). The results for three series of Thiol-
grafted catalyst are consistent and showed that the initial selectiv-
ity to cyclohexanone was ꢀ98% and decreased slightly to 91–93%
for conversions higher than 30% (Fig. 7b). The selectivity remained
fairly constant even as the conversion increased to 100%.
2
slower for the Thiol-grafted catalyst as compared to Rh/SiO
(Table 6, entries 3 & 4).
2
To gain some insights, both the Rh/SiO
lysts were tested for hydrogenation of cyclohexanone and
cyclohex-2-enone. The Rh/SiO catalyst showed good activity for
2
and Thiol-grafted cata-
Substituted phenols (Table 6, entries 5–18) were also selec-
tively hydrogenated to the corresponding carbonyl products with
a higher selectivity over the grafted catalyst than the ungrafted
2
100
(a)
100
(b)
80
60
40
20
0
8
0
60
40
20
0
0
2
4
6
8
10
12
0
20
40
60
80
100
Time (h)
Conversion (%)
Fig. 7. (a) Time profile of Thiol-5 wt% Rh/SiO
cyclohexanol (b) cyclohexanone selectivity versus conversion for three series of Thiol-5 wt% Rh/SiO
0.03 g), 1 atm H in balloon at 25 °C. Standard deviation from 3 catalytic runs: conversion ± 1.8%, selectivity ± 0.4%.
2
after pretreatment in a 10% O
2
/He gas mixture at 200 °C for 1 h (j) conversion and selectivity to ( ) cyclohexanone and (
)
2
. Reaction conditions: phenol (1.5 mmol), cyclohexane (25 ml), catalyst
(
2
Table 6
Selective hydrogenation with 5 wt% Rh/SiO
2
and Thiol-5 wt% Rh/SiO
Rxn
2
.
Entry
Catalyst
Substrate
Time
h)
Product
Conv.
(%)
Sel.
(%)
(
1
2
Rh
Thiol
Balloon
Autoclave
6
12
>99
28
>99
>99
3
4
Rh
Thiol
Balloon
Autoclave
3.5
4.5
>99
>99
96
>99
5
6
7
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
9.5
28
6
>99
>99
>99
77
>99
93
8
9
1
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
12
30
6
>99
65
>99
75
92
85
0
1
1
1
1
2
3
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
20
24
6
>99
9
57
73
89
81
1
1
1
4
5
6
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
10
24
6
>99
57
83
80
>99
94
1
1
1
7
8
9
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
28
30
6
>99
10
48
76
>99
83
2
2
2
0
1
2
Rh
Thiol
Thiol
Balloon
Balloon
Autoclave
15
30
6
>99
5
39
41
89
73
2 2
Reaction conditions: phenol (1.5 mmol), cyclohexane (25 ml), catalyst (0.03 g), H in balloon at 25 °C or in autoclave at 2 bar gauge H and 30 °C.

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
363
Rh/SiO
in the hydrogenation of p-cresol, 4-tert-butyphenol, and 4-
methoxyphenol. Hence, grafting of Rh/SiO with (3-mercaptopro
pyl)trimethoxysilane formed a selective catalyst for hydrogenation
of phenol to cyclohexanone under mild conditions of room temper-
2
. For example, >99% selectivity to the ketone was observed
After reaction, the used Thiol-5 wt% Rh/SiO
2
catalyst was recov-
ered by centrifugation, washed with ethanol and dried at room
temperature. When reused for up to five cycles, it was able to
maintain its activity and high selectivity for cyclohexanone
(Fig. 8). No Rh could be detected in the reactant mixture by ICP-
AES, confirming the stability of the catalyst. The surface area and
pore volume of the recycled catalyst were slightly decreased by
2
ature and one atm H
2
pressure.
ꢀ
10% compared to the fresh one (Fig. S8). The Rh dispersion of
recycled catalyst was not significantly affected with reuse, ꢀ41%.
100
3
.2.2. Nature of Thiol-5 wt% Rh/SiO
Thermogravimetric analysis (TGA) was used to determine the
amount of organic component in the Thiol-grafted Rh/SiO sample.
For comparison, the SiO support was also grafted with (3-mercapto
2
8
6
4
2
0
0
0
0
0
2
2
propyl)trimethoxysilane. The weight loss below 450 °C was ꢀ4.77%
for both samples (Fig. S9). Use of TGA-MS showed that the weight
loss can be attributed to desorption of water and pyrolysis of the
(
3-mercaptopropyl)trimethoxysilane moiety (Fig. S10). After some
of the thiol moiety was pyrolyzed during pretreatment in a
flow of 10% O /He for 1 h at 200 °C, smaller weight losses of 1.86%
2
and 1.3% were determined for the catalyst and support, respectively.
From the results, ꢀ0.20 mmol/g of (3-mercaptopropyl)
trimethoxysilane remained on the catalyst. The presence of (3-mer
captopropyl)trimethoxysilane was confirmed by IR measurements
1
2
3
4
5
Cycle
ꢃ1
Fig. 8. Conversion and selectivity at 5 (
recycled Thiol-5 wt% Rh/SiO
,
) and 11 h (
,
) reaction over
(Fig. S11). Absorption bands at ꢀ2930, 2868 and 2549 cm were
2
.
observed. The first two can be assigned to the asymmetric and
Rh(III) 3d5/2
A
B
5% Rh/SiO2
Thiol-5% Rh/SiO2
Rh(0) 3d5/2
R-5% Rh/SiO2
R-Thiol-5% Rh/SiO2
3
17
312
307
302
Binding energy (eV)
C
D
Oscillation
Fourier Transform
1
5
0
5
0
5
16
Rh-Rh
1
1
4
2
1
10
8
6
Rh-O
3
5
7
9
11
13
15
-
4
2
0
-
-
10
15
0
1
2
3
4
5
6
k (Å)
Distance (Å)
Fig. 9. (A) EDS for Rh and S in Thiol-5 wt% Rh/SiO
2
(B) XPS spectra of as-prepared 5 wt% Rh/SiO
2
and Thiol-5 wt% Rh/SiO
2 2
and after reduction in H /He flow at 25 °C; (C)
data with k3 weighting and (D) Fourier transformed k -weighted
3
v
function for Thiol-5 wt% Rh/SiO
experimental
v
2
.

3
64
H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
symmetric CH
2
stretching of the propylic chain, respectively while
Appendix A. Supplementary material
ꢃ1
the 2549 cm band is characteristic of the S-H stretching mode
59]. Compared to 5 wt%Rh/SiO , the IR spectrum of chemisorbed
[
2
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.06.002.
CO of the Thiol-grafted sample showed a slightly more pronounced
band of the linearly-bonded CO at 2090 cm while the band due to
ꢃ1
bridge-bonded CO is shifted to lower wavenumbers (Fig. S12). The
differences, although small, point to more planar sites being
accessed by CO. EDS mapping showed that the sulfur is well
dispersed in the proximity of rhodium (Fig. 9A and S13).
References
[1] S. Van de Vyver, Y. Román-Leshkov, Catal. Sci. Technol. 3 (2013) 1465–1479.
[
2] Cyclohexanol and Cyclohexanone, Chemical Economics Handbook IHS Markit,
2017.
XPS measurements revealed that Rh3 was the predominant
+
[
3] M.T. Musser, Cyclohexanol and cyclohexanone, Ullmann’s Encyclopedia Indus.
Chem. (2011), https://doi.org/10.1002/14356007.a08_217.pub2.
surface species in 5 wt% Rh/SiO
Fig. 9B). However, after a mild treatment in H
ture for 1 h, the Rh 3d5/2 binding energy was shifted to 307.2 and
07.0 eV for the Rh/SiO and Thiol-Rh/SiO samples, respectively.
These values fall into the expected binding energy for Rh
60–63]. However, the binding energy was lower in grafted sample
2
and Thiol-5 wt% Rh/SiO
2
(
2
at room tempera-
[4] H. Jiang, Z. Qu, Y. Li, J. Huang, R. Chen, W. Xing, Chem. Eng. J. 284 (2016)
24–732.
7
[
[
5] E. Simón, J.M. Rosas, A. Santos, A. Romero, Catal. Today 187 (2012) 150–158.
6] Y. Wang, J. Yao, H. Li, D. Su, M. Antonietti, J. Am. Chem. Soc. 133 (2011)
2362–2365.
3
2
2
0
[
[
7] H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science 326 (2009) 1250–1252.
8] L.M. Sikhwivhilu, N.J. Coville, D. Naresh, K.V. Chary, V. Vishwanathan, Appl.
Catal., A 324 (2007) 52–61.
[
which suggests a higher electron density at the Rh atom. This could
be due to electron donation from the neighbouring sulfur on the
[9] J. Morales, R. Hutcheson, C. Noradoun, I.F. Cheng, Ind. Eng. Chem. Res. 41
2002) 3071–3074.
(
(
3-mercaptopropyl) group. In order to check if any Rh-S bond could
be discerned, EXAFS measurements were conducted on the
Thiol-5 wt% Rh/SiO sample that had been pretreated in H /He at
[
[
10] S. Srinivas, P.K. Rao, J. Catal. 179 (1998) 1–17.
11] P. Claus, H. Berndt, C. Mohr, J. Radnik, E.-J. Shin, M.A. Keane, J. Catal. 192 (2000)
88–97.
2
2
[12] M. Zhao, J. Shi, Z. Hou, Chin. J. Catal. 37 (2016) 234–239.
[13] L. Cheng, Q. Dai, H. Li, X. Wang, Catal. Commun. 57 (2014) 23–28.
[14] D. Zhang, Y. Guan, E.J.M. Hensen, L. Chen, Y. Wang, Catal. Commun. 41 (2013)
47–51.
room temperature. The EXAFS spectrum showed a rapid oscillation
on the Rh K edge (Fig. 9C). In the Fourier transformed EXAFS func-
tion, two peaks can be observed (Fig. 9D). The peak at 2.4 Å can be
assigned to the Rh-Rh bond length while that at 1.6 Å corresponds
to the Rh-O bond distance [61]. The bulk of rhodium is in the
metallic form, in agreement with the XPS results. Although no
Rh-S bond could be discerned, the adsorption and further hydro-
genation at the C@O end of cyclohexanone is less likely due to
repulsion from the electron-rich Rh. Hence, both steric and elec-
tronic effects could be responsible for the improved cyclohexanone
selectivity in the thiol-grafted catalyst.
[15] Y. Pérez, M. Fajardo, A. Corma, Catal. Commun. 12 (2011) 1071–1074.
[
16] M. Chatterjee, H. Kawanami, M. Sato, A. Chatterjee, T. Yokoyama, T. Suzuki,
Adv. Synth. Catal. 351 (2009) 1912–1924.
[17] P. Makowski, R.D. Cakan, M. Antonietti, F. Goettmann, M.-M. Titirici, Chem.
Commun. (2008) 999–1001.
[
[
[
18] J. Matos, A. Corma, Appl. Catal., A 404 (2011) 103–112.
19] Y. Xiang, L. Ma, C. Lu, Q. Zhang, X. Li, Green Chem. 10 (2008) 939–943.
20] X. Yang, X. Yu, L. Long, T. Wang, L. Ma, L. Wu, Y. Bai, X. Li, S. Liao, Chem.
Commun. 50 (2014) 2794–2796.
[
21] S.-I. Fujita, T. Yamada, Y. Akiyama, H. Cheng, F. Zhao, M. Arai, J. Supercrit. Fluids
54 (2010) 190–201.
[
22] F. Martinez-Espinar, P. Blondeau, P. Nolis, B. Chaudret, C. Claver, S. Castillón, C.
Godard, J. Catal. 354 (2017) 113–127.
4
. Conclusion
[23] C.V. Rode, U.D. Joshi, O. Sato, M. Shirai, Chem. Commun. (2003) 1960–1961.
[24] X. Yang, L. Du, S. Liao, Y. Li, H. Song, Catal. Commun. 17 (2012) 29–33.
[25] H. Li, J. Liu, H. Li, Mater. Lett. 62 (2008) 297–300.
Supported Rh catalysts were evaluated for the selective hydro-
genation of phenol to cyclohexanone at room temperature and
atm H pressure. SiO - and TiO -supported Rh could be easily
reduced under reaction conditions or ex-situ under flowing hydro-
[26] N.C. Nelson, J.S.N. Manzano, A.D. Sadow, S.H. Overbury, I.I. Slowing, ACS Catal.
(2015) 2051–2061.
27] G. Xu, J. Guo, Y. Zhang, Y. Fu, J. Chen, L. Ma, Q. Guo, ChemCatChem 7 (2015)
485–2492.
[28] H.-F. Li, Q.-S. Zhang, Z.-B. Pang, M. Tian, P. Gao, L.-L. Wang, Chin. Chem. Lett. 27
2016) 1500–1504.
5
1
2
2
2
[
2
gen at room temperature, in contrast to the need for high temper-
(
ature activation for La
increase of Rh loading from 0.5 to 15 wt% on SiO
.59–3.66 nm were formed. The catalytic hydrogenation of phenol
2
O
3
, MgO and ZrO
2
-supported samples. With
[
[
29] S. Ding, C. Zhang, Y. Liu, H. Jiang, R. Chen, Appl. Surf. Sci. 425 (2017) 484–491.
30] H. Zhou, B. Han, T. Liu, X. Zhong, G. Zhuang, J. Wang, Green Chem. 19 (2017)
3585–3594.
2
, nanoparticles of
1
[
31] J.-F. Zhu, G.-H. Tao, H.-Y. Liu, L. He, Q.-H. Sun, H.-C. Liu, Green Chem. 16 (2014)
forms both cyclohexanol and cyclohexanone from the start of reac-
tion. At constant phenol:Rh mole ratio of 100:1, the catalytic activ-
ity decreased whereas the initial selectivity to cyclohexanone
increased with Rh particle size. It was proposed that coordinatively
unsaturated sites such as kinks and steps catalyse the direct hydro-
genation of phenol to the final product, cyclohexanol, whereas
higher coordinated terrace sites catalyse the partial hydrogenation
to cyclohexanone. DFT calculations indicated that dissociation of
phenol was more facile on flat surfaces than small particles. Graft-
2664–2669.
[
32] Y. Li, X. Xu, P. Zhang, Y. Gong, H. Li, Y. Wang, RSC Adv. 3 (2013) 10973–10982.
[33] C.-J. Lin, S.-H. Huang, N.-C. Lai, C.-M. Yang, ACS Catal. 5 (2015) 4121–4129.
[34] F. Valentini, N. Santillo, C. Petrucci, D. Lanari, E. Petricci, M. Taddei, L. Vaccaro,
ChemCatChem 10 (2018) 1277–1281.
[
35] R.D. Patil, Y. Sasson, Appl. Catal., A 499 (2015) 227–231.
[
36] D. Zhang, F. Ye, T. Xue, Y. Guan, Y.M. Wang, Catal. Today 234 (2014) 133–138.
[37] V. Mévellec, A. Nowicki, A. Roucoux, C. Dujardin, P. Granger, E. Payen, K.
Philippot, New J. Chem. 30 (2006) 1214–1219.
[
38] S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for
Organic Synthesis, Wiley, New York, 2001.
ing a 5 wt% Rh/SiO
lane improved the cyclohexanone selectivity from 82% to 93% at
00% conversion. This was attributed to a steric effect where access
2
catalyst with (3-mercaptopropyl)trimethoxysi
[39] S. Fehn, M. Zaheer, C.E. Denner, M. Friedrich, R. Kempe, New J. Chem. 40 (2016)
252–9256.
9
[
40] I.E. Ertas, M. Gulcan, A. Bulut, M. Yurderi, M. Zahmakiran, J. Mol. Catal. A:
Chem. 410 (2015) 209–220.
1
to the coordinatively unsaturated sites was blocked. Hence, an
active, selective and recyclable rhodium-based catalyst has been
developed for the selective hydrogenation of phenol to cyclohex-
anone under mild conditions.
[41] S. Kuklin, A. Maximov, A. Zolotukhina, E. Karakhanov, Catal. Commun. 73
2016) 63–68.
(
[
42] P. Rasband, W. Hecker, J. Catal. 139 (1993) 551–560.
[
43] A. Abad, A. Corma, H. García, Chem. Eur. J. 14 (2008) 212–222.
[44] R.E. Benfield, J. Chem. Soc., Faraday Trans. 88 (1992) 1107–1110.
[45] R. Van Hardeveld, F. Hartog, Surf. Sci. 15 (1969) 189–230.
[46] J.A. Anderson, F. Solymosi, J. Chem. Soc., Faraday Trans. 87 (1991) 3435–3442.
[47] R. Cavanagh, J.T. Yates Jr, J. Chem. Phys. 74 (1981) 4150–4155.
[48] J. Yates Jr, T. Duncan, S. Worley, R. Vaughan, J. Chem. Phys. 70 (1979)
1219–1224.
Acknowledgements
This work was supported by the Academic Research Grant,
R-143-000-667-114, National University of Singapore. The award
of an NUS research scholarship for H. W. Zhang is gratefully
acknowledged.
[
[
49] S. Trautmann, M. Baerns, J. Catal. 150 (1994) 335–344.
50] M.L. Honkela, J. Björk, M. Persson, Phys. Chem. Chem. Phys. 14 (2012)
5849–5854.
[51] G. Li, J. Han, H. Wang, X. Zhu, Q. Ge, ACS Catal. 5 (2015) 2009–2016.

H. Zhang et al. / Journal of Catalysis 364 (2018) 354–365
365
[
[
[
[
[
[
52] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169.
53] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15–50.
54] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.
55] S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem. 32 (2011) 1456–1465.
56] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 132 (2010) 154104.
57] G. Henkelman, B.P. Uberuaga, H. Jónsson, J. Chem. Phys. 113 (2000)
[59] E. Finocchio, E. Macis, R. Raiteri, G. Busca, Langmuir 23 (2007) 2505–2509.
[60] R. Duarte, O. Safonova, F. Krumeich, J. van Bokhoven, Phys. Chem. Chem. Phys.
16 (2014) 26553–26560.
[61] J.F. Moulder, J. Chastain, Handbook of X-ray Photoelectron Spectroscopy: A
Reference Book of Standard Spectra for Identification and Interpretation of XPS
Data, Physical Electronics, 1995.
9
901–9904.
58] D.J. Snelders, N. Yan, W. Gan, G. Laurenczy, P.J. Dyson, ACS Catal. 2 (2012)
01–207.
[62] Y. Okamoto, N. Ishida, T. Imanaka, S. Teranishi, J. Catal. 58 (1979) 82–94.
[63] J. Contour, G. Mouvier, M. Hoogewys, C. Leclere, J. Catal. 48 (1977) 217–228.
[
2