Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its derivatives

Article abstract of DOI:10.1016/j.cclet.2016.03.036

Pd/TiN nanocomposite catalysts were fabricated for one-step selective hydrogenation of phenol to cyclohexanone successfully. High conversion of phenol (99%) and selectivity of cyclohexanone (98%) were obtained at 30?°C and 0.2?MPa H₂for 12?h in the mixed solvents of H₂O and CH₂Cl₂. The Pd nanoparticles were stable in the reaction, and no aggregation was detected after four successive runs. The catalytic activity and selectivity depended on slightly the Pd particle sizes. The generality of the catalysts for this reaction was demonstrated by the selective hydrogenation of phenol derivatives, which showed that the catalyst was selective for the formation of cyclohexanone.

Full text of DOI:10.1016/j.cclet.2016.03.036

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CCLET-3656; No. of Pages 5
Chinese Chemical Letters xxx (2016) xxx–xxx
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Chinese Chemical Letters
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Original article
Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol
and its derivatives
a
a
a,b
a,b
c,
a,
Hai-Feng Li , Qin-Sheng Zhang , Zeng-Bo Pang , Mi Tian , Ping Gao *, Lai-Lai Wang *
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b
Graduate University of Chinese Academy of Sciences, Bejing 100039, China
c
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Pd/TiN nanocomposite catalysts were fabricated for one-step selective hydrogenation of phenol to
cyclohexanone successfully. High conversion of phenol (99%) and selectivity of cyclohexanone (98%)
Received 3 February 2016
Received in revised form 9 March 2016
Accepted 15 March 2016
Available online xxx
2 2 2 2
were obtained at 30 8C and 0.2 MPa H for 12 h in the mixed solvents of H O and CH Cl . The Pd
nanoparticles were stable in the reaction, and no aggregation was detected after four successive runs.
The catalytic activity and selectivity depended on slightly the Pd particle sizes. The generality of the
catalysts for this reaction was demonstrated by the selective hydrogenation of phenol derivatives, which
showed that the catalyst was selective for the formation of cyclohexanone.
Keywords:
Phenol
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Hydrogenation
Cyclohexanone
TiN support
Pd/TiN catalysts
1
. Introduction
reported that dual-supported Pd Lewis acid catalyst was a selective
3
and efficient catalyst, the AlCl assisted activation of the phenyl
Cyclohexanone, which was produced by oxidation of cyclohex-
ring, and the prevention of further hydrogenation to cyclohexanol,
which imposes severe limitations on the applications of hydro-
genation in general and adds a chemical sensitivity that restricts
substrates, purity, and reaction conditions. Recently, Wang et al.
[20] described that Pd nanoparticles (PdNPs) supported on a mpg-
ane or the hydrogenation of phenol, is a key intermediate for the
manufacture of both caprolactam for Nylon 6 and adipic acid for
Nylon 66 [1–5]. Oxidation of cyclohexane requires high tempera-
tures (140–180 8C) and high pressures (8–20 bar), and generates
some byproducts such as cyclohexanol and cyclohexyl hydroper-
oxide [6]. The phenol hydrogenation route can be generated
cyclohexanone much more convenient and efficient in a ‘‘one-
step’’ or a ‘‘two-step’’ process, in which the ‘‘two-step’’ process
involves hydrogenated phenol to cyclohexanol [7], and the
production dehydrogenates to cyclohexanone at high tempera-
tures [8,9]. In contrast, the direct ‘‘one-step’’ hydrogenation of
phenol to cyclohexanone process is more advantageous and
identified as an atom economic method in which the endothermic
step of cyclohexanol dehydrogenation can be avoided [2,3,10].
Some catalysts have been evaluated in the one-step hydro-
genation of phenol, and the results showed that the selectivity of
cyclohexanone is largely dependent on the catalyst properties
C
3
N
4
, is an efficient and selective catalyst in hydrogenation of
phenol in an aqueous media. However, the preparation of the mpg-
support involved the use of aqueous ammonium bifluoride
HF ) and/or hydrogen fluoride (HF) which are hazardous and
3 4
C N
(NH
4
2
not environmentally friendly. Therefore, it is desirable to search for
an alternative heterogeneous catalyst for selective hydrogenation
of phenol under ambient conditions with high activity and
selectivity.
Tungsten carbides have been reported as efficient supports for
the preparation of platinum-based electrocatalysts [21]. The role of
semiconducting supports in heterogeneous catalysts and their
ability to modify the electron density of the supported metal
thereby modifying their activities has been highlighted [22]. TiN is
a biocompatible material with excellent physical and chemical
properties including thermal stability and acid resistance
[
2
10–17]. Rode et al. [18] used supercritical CO as a solvent, which
requires high H and CO pressures (>7.0 MPa). Han et al. [19]
2
2
[23,24]. Thus, it could be applied as support for preparing
nanoscale catalysts. Ni/TiN catalysts for the hydrogenolysis of
aryl ethers as models for lignin [25] have been reported. The close
contact between TiN and Ni was observed by electron transfer from
*
Corresponding authors.
E-mail addresses: gaoping@licp.cas.cn (P. Gao), wll@licp.cas.cn (L.-L. Wang).
http://dx.doi.org/10.1016/j.cclet.2016.03.036
1
001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-F. Li, et al., Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its
derivatives, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.036

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CCLET-3656; No. of Pages 5
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H.-F. Li et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Ni to TiN, and the electron transfer promoted the catalytic activity
of Ni. In this work, a new method using Pd/TiN catalysts for the
synthesis of cyclohexanone by the selective hydrogenation of
phenol were described. It was found that TiN as support enhanced
the catalytic activity of PdNPs.
2
. Experimental
2.1. Materials and instruments
TiN was purchased from Hefei Kaier Nanometer Energy &
Technology Co., Ltd. 5.0 wt% Pd/C was purchased from Shanxi
Kaida Chemical Engineering Co., Ltd. All other chemical reagents
were purchased from Kailaiying Medical Chemical (Tianjin) Co.,
Ltd. The morphology and the particle size distribution of the TiN
supported PdNPs were determined by a JEM-2010 TEM with an
accelerating voltage of 200 kV. Powder X-ray diffraction (XRD)
measurements were performed on an X’Pert Pro multipurpose
diffractometer (PANalytical, Inc.) with Nifiltered Cu K
0.15046 nm) at room temperature from 10.08 to 80.08 (wide
angle) and 0.68 to 58 (small angle). X-ray photoelectron spectra
XPS) analyses of the catalysts were performed on a Thermo Fisher
Scientific KAlpha spectrometer.
a radiation
1
.3
(
Fig. 1. XRD patterns of Pd/TiN with different Pd loadings. (1) TiN; (2) Pd/TiN ; (3)
2
.5
5
5
8.5
Pd/TiN ; (4) Pd/TiN ; (5) Pd/TiN (after four reaction cycles); (6) Pd/TiN
.
(
5
for Pd/TiN is as high as 5.0 wt%, the PdNPs are still not detected by
XRD, which possibly indicates that the supported PdNPs have small
particle sizes and are homogeneously dispersed on the surface of
the TiN. The diffraction peaks showed no significant changes for
2.2. Fabrication of Pd/TiN catalysts
5
An aqueous solution of H
2
PdCl
4
was initially prepared by
the used four reaction cycles of Pd/TiN (Fig. 1, curve 5). The results
mixing of PdCl into 10% (v/v) HCl aqueous solution by stirring at
room temperature until the salt was homogeneously dissolved. A
certain amount of the TiN powder was impregnated with the
2
indicated that TiN can efficiently stabilize the PdNPs catalysts in
the selective hydrogenation of phenol. However, with the Pd
loadings increasing, the characteristic diffraction peaks of the Pd
were obviously found (Fig. 1, curve 6).
H
2
PdCl
4
solution and stirred at room temperature for 8 h. Then
aqueous solution was slowly added to such a
excess NaBH
4
suspension mixture under ice bath conditions. The corresponding
supported PdNPs were separated by centrifugation, washed
sequentially using with distilled water and absolute ethanol
several times, and dried at 60 8C overnight in a vacuum oven to give
expected Pd/TiN catalysts.
3.2. The TEM images of Pd/TiN catalysts
The TEM images shows the morphology and the particle size
distribution of the TiN supported PdNPs prepared at four kinds of
5
x
catalysts with different Pd loadings Pd/TiN (Fig. 2a) and Pd/TiN
x = 1.3, 2.5, 8.5, x represented wt% Pd loadings) (Fig. S1 in
(
2
.3. Hydrogenation of phenol
Supporting information). These images reveal well-dispersed Pd
particles and no agglomeration. The average particle sizes of Pd/
1
.3
2.5
5
A typical procedure for the hydrogenation of phenol was as
TiN
(Fig. S1a), Pd/TiN
(Fig. S1b) and Pd/TiN (Fig. 2a) are
follows: phenol (47 mg, 0.5 mmol), Pd/TiN (2.5–5 mol%) and H
2 mL), CH Cl (1 mL), were placed into a 50 mL stainless steel
autoclave. The reactor system was purged with N three times
followed by H three times. The autoclave was pressurized with
.2 MPa of H . The reaction mixture was stirred vigorously at the
2
O
7.5 Æ 1.1 nm, 7.7 Æ 0.9 nm and 10.5 Æ 1 nm, respectively. It is most
likely that the TiN supports play a key role in preventing the
uncontrollable growth of the PdNPs and stabilizing the formed PdNPs
effectively. However, as shown in Fig. 2b, the PdNPs particle size
(
2
2
2
2
5
0
2
increased slightly after four reaction cycles of Pd/TiN and the
desired reaction temperature. After a prescribed reaction time, the
autoclave was cooled to room temperature and the residue gas was
released. The catalyst was removed from the liquid by filtration,
and then the organic phase was extracted. The conversion and
selectivity were determined by a GC 112A equipped with a FID
catalytic activity of these PdNPs correspondingly decreased in the
selective hydrogenation of phenol. The Pd particle sizes obviously
increased to 16.5 nm with the increasing Pd loading to 8.5 wt% (Fig.
S1c). The corresponding mean particle diameters were measured and
calculated by counting 200 particles from the enlarged photographs.
detector and an SE-54 column (30 m  0.25 mm  0.25
mm film
thickness).
3.3. The XPS characterization of Pd/TiN5
5
3
. Results and discussion
.1. The XRD patterns of the Pd/TiN catalysts
Fig. 1 shows the XRD patterns of the Pd/TiN catalysts. As can be
The Pd/TiN was characterized by XPS. The results are shown in
Fig. 3. The spectra present a doublet corresponding to Pd 3d5/2 and
3d3/2. The Pd 3d5/2 peak at 335.6 eV and 3d3/2 peak at 340.9 eV is
attributed to Pd (metallic palladium), which shows that nearly all
3
0
2
+
0
of the Pd ions were reduced, and Pd was the only form in the
seen, the diffraction peak at 36.18 in the XRD patterns can be
assigned to the TiN (1 1 1) [23] (Fig. 1, curve 1). The diffraction
peaks at 40.18, 46.78, and 68.18 can be assigned to the (1 1 1),
4
PdNPs prepared by NaBH [26].
3.4. Hydrogenation
(
2 0 0), (2 2 0) planes of the face centered cubic Pd particles [26],
1.3
2.5
5
respectively. The patterns of Pd/TiN , Pd/TiN , and Pd/TiN only
show (1 1 1) diffractions, other diffraction peaks of the Pd are not
observed at all (Fig. 1, curves 2–4). Although the Pd loading content
Table 1 presents the results of phenol hydrogenation under
different conditions over the Pd/TiN catalysts. The four kinds of
catalysts synthesized by varying the Pd loadings from 1.3 wt% to
Please cite this article in press as: H.-F. Li, et al., Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its
derivatives, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.036

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3
5
5
Fig. 2. TEM images and the particle size distribution of (a) Pd/TiN and (b) Pd/TiN (after four reaction cycles).
Table 1
Hydrogenation of phenol with various Pd/TiN catalysts.
OH
O
Cat. Pd/TiN
+
H (0.2 MPa)
2
H O-CH Cl
2
2
2
phenol
cyclohexanone
Entry
Catalysts
T (8C)
t (h)
Con. (%)
Sel. (%)
C–OH
C5O
a
1.3
1
Pd/TiN
60
60
2
2
44
45
49
28
10
26
24
99
99
95
92
99
0.0
8
8
92
92
97
96
>99
82
97
98
98
60
96
98
0
a
2.5
2
Pd/TiN
a
5
3
Pd/TiN
60
2
3
a
8.5
4
Pd/TiN
60
2
4
b
5
5
Pd/TiN
60
2
<1
19
3
c
5
6
Pd/TiN
60
2
d
5
7
Pd/TiN
60
2
5
8
9
Pd/TiN
60
2
2
5
Pd/TiN
100
100
30
1
2
5
Fig. 3. XPS spectra of Pd/TiN .
10
Pd/C
1
40
4
5
1
1
2
Pd/TiN
10
12
2
5
1
Pd/TiN
30
2
x
13
TiN
60
0
8
.5 wt% Pd/TiN (x = 1.3, 2.5, 5.0, 8.5) were evaluated. It can be seen
that as Pd loadings were increased from 1.3 wt% to 5.0 wt%, the
conversion and the selectivity were also gradually increased
correspondingly, and the conversions of phenol were 44%, 45%, and
Reaction condition: phenol (47 mg, 0.5 mmol), Pd/TiN (5 mol% relative to phenol),
0.2 MPa H , H O (2 mL), CH Cl (1 mL).
2
2
2
2
a
Pd (2.5 mol% relative to phenol).
Pd (2.5 mol% relative to phenol), H
b
c
2
O (2 mL), CH
O (3 mL), CH
O (1 mL), CH
2
Cl
Cl
Cl
2
(3 mL).
(1 mL).
(2 mL).
4
9% with the selectivity of cyclohexanone were 92%, 92%, and 97%
Pd (2.5 mol% relative to phenol), H
2
2
2
under 0.2 MPa of H at 60 8C for 2 h (Table 1, entries 1–3),
2
d
Pd (2.5 mol% relative to phenol), H
2
2
2
respectively. These results may be due to the fact that the same
amount of reactive Pd sites, the content of TiN was different, and
the residual TiN partially influences catalytic activity by hindering
access of reactants to active Pd sites. However, when the amount of
Pd loading was increased from 5.0% to 8.5 wt%, the selectivity of
cyclohexanone is still maintained at around 96%, but the
conversion of phenol declined drastically to 28% (Table 1, entry
particle size of Pd was increased to about 16.5 nm as the amount of
Pd loading was increased. These results may depend sensitively
weak on the sizes of the Pd particles [27,28]. When using Pd/TiN
5
as a catalyst, the excellent conversion of 99% and selectivity of 98%
5
was obtained (Table 1, entry 8), and the catalyst Pd/TiN was
4
). The reason for such a phenomenon can be attributed to the
chosen for the investigation of reaction parameters in the
hydrogenation of phenol.
PdNPs was not stable and tend to aggregate slightly when the
Please cite this article in press as: H.-F. Li, et al., Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its
derivatives, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.036

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H.-F. Li et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
2 2 2
The mixed solvents of H O and CH Cl were used in the
selective hydrogenation of phenol and the effect of different
solvent ratio on the conversion and selectivity was investigated
(
Table 1, entries 5–7). Initially, 2 mL of H
selected as mixture solvents and the conversion of phenol was only
0%, while the selectivity of cyclohexanone up to more than 99%
Table 1, entry 5). The conversion of phenol 26% and a declined
selectivity of cyclohexanone 82% were obtained when increased
the H O to 3 mL and decreased CH Cl to 1 mL (Table 1, entry 6).
The volumes of H O and CH Cl were further adjusted to 1 mL and
mL, respectively, the conversion did not changed obviously,
while the selectivity of cyclohexanone was rebounded to 97%
Table 1, entry 7). The above results showed that H O was
beneficial to the conversion of water soluble phenol, while CH Cl
was beneficial to the selectivity of oil soluble cyclohexanone. And
when the mixed solvents of H O (2 mL) and CH Cl (1 mL) were
2 2 2
O and 3 mL CH Cl was
1
(
2
2
2
2
2
2
2
(
2
2
2
2
2
2
selected as mixture solvents (Table 1, entry 8), the conversion and
selectivity were up to 99% and 98%, respectively. It was observed
that the higher temperature will accelerate the hydrogenation of
phenol, for example, with increasing the reaction temperature
from 60 8C to 100 8C, the reaction time was shortened obviously,
while the conversion and selectivity still remained (Table 1, entry
5
Fig. 4. The recyclability of Pd/TiN catalyst. (Reaction condition: phenol (47 mg,
5
0.5 mmol), Pd/TiN (5 mol% relative to phenol), 0.2 MPa H
2
, H
2
O (2 mL), CH
2
Cl
2
(1 mL), 30 8C, 12 h.)
100 8C for 20 h (Table 2, entry 5). When the substituents on the
aromatic ring were changed, the conversion of the substrates and
the selectivity of the products were changed accordingly.
Methylphenol derivatives gave methylcyclohexanones with ex-
cellent conversion and selectivity, while p-tert-butylphenol
showed a lower conversion and the selectivity (95%). This may
be due to the bulky functional group inhibiting an attack of a
palladium hydride species towards the phenol ring [29].
9
). Furthermore, the conversion of phenol was increased gradually
from 92% to 99% on prolonging the reaction time from 10 h to 12 h
at 30 8C (Table 1, entries 11 and 12). In order to compare the
selectivity of catalysts, 5.0 wt% Pd/C was used in the hydrogenation
of phenol under otherwise identical conditions and only 60%
selectivity of cyclohexanone was obtained (Table 1, entry 10). The
TiN showed no catalytic activity in the absence of the PdNPs
5
(
Table 1, entry 13). These results revealed that the PdNPs are
3.5. The recyclability of Pd/TiN
efficient and highly selectivity catalysts for the hydrogenation of
phenol to cyclohexanone, which provided a new route for the
fabrication of cyclohexanone via selective hydrogenation of
phenol.
5
The recyclability of catalyst Pd/TiN was investigated in the
selective hydrogenation of phenol in the mixed solvents of H
CH Cl . The recycling data were showed in Fig. 4. It was found that
2
O and
2
2
5
5
To illustrate the general applicability of Pd/TiN , other phenol
the catalyst Pd/TiN exhibited significantly high catalytic activity
and selectivity for the hydrogenation of phenol and the selectivity
of cyclohexanone only dropped from 98% to 94% after four reaction
runs. The results also demonstrated that the PdNPs catalyst have
an excellent stability. From the TEM images (Fig. 2b) of recycled
catalyst, it can be observed that the morphology of recycled
catalyst showed no obvious changes compared to fresh catalyst,
derivatives were investigated. Table 2 shows the results of the
hydrogenation. It can be seen that 99% conversion was achieved in
most cases, and the overall selectivity of cycloketones exceeded
9
3% in all cases. The conversions of p-cresol, o-cresol and m-cresol
were all 99% (Table 2, entries 1–3). Hydrogenation of hydroqui-
none yielded the synthetically very valuable hydroxylcyclohex-
anones with high conversions and selectivities (Table 2, entry 4).
However, the conversion of p-tert-butylphenol was only 90% at
5
except that growth or aggregation and the Pd/TiN particle size
increased from 10.5 Æ 1 nm to 12.4 Æ 0.9 nm slightly after four
reaction cycles. From the above result, it can be concluded that the
catalytic activity of the catalyst decreased with the increase of the
Table 2
5
Hydrogenation of phenol derivatives using Pd/TiN catalyst in mixed solvent of H
2
O
5
and CH
2
Cl
2
.
particle sizes of Pd/TiN in the selective hydrogenation of phenol.
Entry
1
Substract
t (h)
Product
Con. (%)
99
Sel. (%)
OH
8
O
98
4. Conclusion
H
3
C
H3C
In conclusion, Pd/TiN with different size of Pd particles was
fabricated by the chemical reduction with different Pd contents
1.3–8.5 wt%) and successfully applied in the hydrogenation of
2
CH3
8
99
99
CH
3
OH
O
(
phenol and its derivatives. As a result of the uniform distribution of
Pd particles, the catalytic activity and the combined selectivity to
cyclohexanone increased consistently with the increasing Pd
particle size to reach maximum values at around 10 nm in the
range of 7–16.5 nm, which shows the structure sensitivity weak of
3
4
H C
3
OH
OH
8
H C
O
99
99
98
93
3
12
O
O
5
the reaction. Pd/TiN exhibited high activity for the hydrogenation
of phenol, p-cresol, o-cresol and m-cresol to the corresponding
HO
HO
5
OH
20
90
95
2 2 2
cyclohexanone in the mixed solvents of H O and CH Cl under mild
conditions. However, the conversion was lower in the hydrogena-
tion of p-tert-butylphenol. A detailed investigation of the reaction
mechanism including the influence of other solvents and how to
improve the catalytic activity for the hydrogenation of phenol
derivatives will be the subject of future studies.
5
Reaction condition: substrates (0.5 mmol), Pd/TiN (5 mol%), 0.2 MPa H
2 mL), CH Cl (1 mL), 100 8C.
2 2
, H O
(
2
2
Please cite this article in press as: H.-F. Li, et al., Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its
derivatives, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.036

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CCLET-3656; No. of Pages 5
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5
[
14] Y.Z. Chen, C.W. Liaw, L.I. Lee, Selective hydrogenation of phenol to cyclohexanone
Acknowledgment
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(1999) 1–8.
We are grateful to the financial supports granted by the
National Natural Science Foundation of China (No. 21174155).
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Please cite this article in press as: H.-F. Li, et al., Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its
derivatives, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.036