Aerobic dehydrogenation of cyclic ketones into corresponding phenols catalyzed by heterogeneous Pd nanocatalysts

Article abstract of DOI:10.1016/j.catcom.2016.10.019

The Pd, Ni, Ni-Sn, Cu and Ag@Pd nanocatalyst having different shape and size were used for direct dehydrogenation of cyclic ketones to the corresponding phenols in the presence of O₂ as the oxidant. Among all catalyst tested, metallic Pd-1 nanoparticles exhibited excellent catalytic performance both in presence and absence of p-toluenesulfonic acid. The dehydrogenation of cyclohexanone catalyzed by Pd-1 nanocatalyst led to a 93% yield of the desired cyclohexenone and phenol. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that there was no change in morphology, crystal phases and particles size of Pd-1 nanocatalyst after the reaction.

Full text of DOI:10.1016/j.catcom.2016.10.019

Aerobic dehydrogenation of cyclic ketones into corresponding phenols
catalyzed by heterogeneous Pd nanocatalysts
Mazloom Shah, Qing-Xiang Guo, Yao Fu
PII:
DOI:
Reference:
S1566-7367(16)30385-5
doi: 10.1016/j.catcom.2016.10.019
CATCOM 4824
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
3 August 2016
16 September 2016
23 October 2016
Please cite this article as: Mazloom Shah, Qing-Xiang Guo, Yao Fu, Aerobic dehydro-
genation of cyclic ketones into corresponding phenols catalyzed by heterogeneous Pd
nanocatalysts, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.10.019
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

ACCEPTED MANUSCRIPT
Aerobic dehydrogenation of cyclic ketones into corresponding phenols catalyzed
by heterogeneous Pd nanocatalysts
*1,2,3
1
1
Mazloom Shah
, Qing-Xiang Guo* , Yao Fu
1
Department of Chemistry, University of Science and Technology of China, Hefei,
30026, P.R.China
2
2
Xishuangbanna Tropical Botanical Garden Chinese Academy of Science, Kunming,
6
50223, P.R.China
3
Department of Chemistry, Women University of Azad Jammu and Kashmir, Bagh,
2500, Pakistan
1
For submission to Catalysis Communications
(August. 2016)
*
Corresponding author: Tel.: +86-186-8711-5949.
E-mail address: mazloom@mail.ustc.edu.cn, qxguo@ustc.edu.cn.
1

ACCEPTED MANUSCRIPT
Abstract
The Pd, Ni, Ni-Sn, Cu and Ag@Pd nanocatalyst having different shape and size were
used for direct dehydrogenation of cyclic ketones to the corresponding phenols in the
presence of O as the oxidant. Among all catalyst tested, metallic Pd-1 nanoparticles
2
exhibited excellent catalytic performance both in presence and absence of
p-toluenesulfonic acid. The dehydrogenation of cyclohexanone catalyzed by Pd-1
nanocatalyst led to a 93% yield of the desired cyclohexenone and phenol. X-ray
diffraction(XRD) and transmission electronmicroscopy(TEM) confirmed that there
was no change in morphology, crystal phases and particles size of Pd-1 nanocatalyst
after the reaction.
Keywords：Palladium Nanoparticles, Heterogeneous Catalysts, Dehydrogenation,
Cyclic ketones, Phenols
1
. Introduction
The transformation of cyclohexanones into corresponding phenols is a fundamental
reaction in organic chemistry. Phenols and its derivatives are commonly used in
pharmaceuticals, bulk chemicals and polymers [1]. The preparation of phenols from
ketones has been investigated previously [2-10]. Condensation of acyclic ketones with
β-ketoaldehydes or β-diketones enable direct access to phenols but low product yields,
limited access to starting materials, and formation of isomeric products have restricted
the utility of these procedures [2]. The dehydrogenation of cyclic ketones affords
cyclic enones or aromatic alcohol via removal of 1 or 2 equivalent of H , respectively.
2
Methods for formation of phenols via dehydrogenation of cyclohexenones typically
2

ACCEPTED MANUSCRIPT
use
undesirable
stoichiometric
reagents,
such
as
DDQ
(2,3-dichloro-5,6-dicyano-1,4-benzoquinone); use stepwise procedures, such as
bromination/ dehydrobromination; or require harsh reaction conditions (≥200 °C) that
limit functional group compatibility [2-8, 11]. Recently, the group of Stahl presented a
Pd-based homogeneous system using O as the terminal oxidant for the aromatization
2
of substituted cyclohexanones. Later a thorough investigation of the Pd
(
TFA) /2-Me Npy-catalyzed dehydrogenation of cyclohexanones and cyclohexenones,
2 2
showing that an initial, highly active molecular Pd (II) species generate soluble Pd
nanoparticles that serve as the active catalyst during steady-state dehydrogenation of
the substrate [12-14]. In contrast, no effective heterogeneous catalytic method for
dehydrogenation of substituted and unsustituted cyclohexanones exist. Its great
opportunity to create nanoparticle-based heterogeneous catalytic system for this
important transformation. The heterogeneous catalyst can be easily separated from the
reaction mixture by simple filtration and recycled several times. Here, we report a
different Pd, Ni, Ni-Sn, Cu and Ag-Pd heterogeneous nano-catalyst systems, for direct
dehydrogenation of cyclohexanones (1a) and 3-methyl-cyclohexanone (2a) to the
corresponding phenols, in the presence of O as the oxidant.
2
2
. Experimental
2
.1 Materials
Sodium
polyvinylpyrrolidone with an average molecular weight of 10,000 (PVP), acetone
CH COCH ), ethanol (C H OH), L-ascorbic acid ( C H O ), palladium acetate (Pd
borohydride
(NaBH4),
ethylene
glycol
(HOCH CH OH),
2 2
(
3
3
2
5
6
8
6
3

ACCEPTED MANUSCRIPT
(
CH COO) ), potassium bromide (KBr), p-toluenesulfonic acid (CH C H SO H),
3
2
3
6
4
3
cyclohexanone (C H O), cyclohexenone (C H O), toluene (C H CH ), mesitylene
6
10
6
8
6
5
3
(
C H (CH ) ), ethyl acetate (CH COOC H ), acetic acid (CH COOH),
6 3 3 3 3 2 5 3
dimethylformamide ((CH ) NC(O)H), dimethylacetamide (CH C(O)N(CH ) ),
3
2
3
3 2
1
-methyl-2-pyrrolidinone (C H NO), phenol (C H O), 3-methyl phenol (C H O) were
5 9 6 6 7 8
purchased from Sinopherm chemical reagent Co. Ltd., palladium nitrate hydrate
Pd(NO ) •2H O) from Aladdin, Sodium tetrachloropalladate (Na PdCl •nH O) from
(
3
2
2
2
4
2
Shanxi kaida chemical engineering Co. Ltd., copper acetate monohydrate (Cu
CH COO) •H O) from Shanghai chemical reagent company, Sodium sulfate (Na SO )
(
3
2
2
2
4
from Shanghai rich joint chemical reagent Co. Ltd., 3-methyl cyclohexanone
(CH C H O) from Tokyo chemical industry Co. Ltd., 3-methyl cyclohexenone
3 6 9
(
CH C H O) from Energy chemical, p-anisaldehyde from Aladdin chemistry Co. Ltd.,
3
6
7
and nickel (II) formate dihydrate (Ni (HCOO) •2H O) and silver nitrate (AgNO )
2
2
3
from Alfa Aesar.
2
2
.2 Methods
.2.1 Preparation and catalytic application of nanoparticles
All commercially available compounds and solvents were purchased and used as
received. Pd-3 [19-20], Cu, Ni and Ni-Sn [21], Pd-1 and Ag@Pd [22], and Pd-2 [23]
nanocatalysts were prepared according to condition suggested in literature.
In a typical dehydrogenation reaction, catalyst (2.8 to 5.1 mole %), p-toluenesulfonic
acid (15 mole %), the cyclic ketone (0.5 mmol) and DMSO (0.5 ml) were added to a
1
0 ml schlenk tube loaded with stir bar. The tube was sealed with a rubber septum and
4

ACCEPTED MANUSCRIPT
O was bubbled through the solution for 10 min at room temperature. The reaction
2
was heated in an oil bath to 80 °C with vigorous stirring under a balloon of O for
2
given time. After the completion of the reaction, the reaction mixture was filtered.
Then, the residue was extracted with ethyl acetate. The organic layer was washed with
water while using p-toluenesulfonic acid, dried over Na SO and the solvent was
2
4
removed under vacuum. The products were identified with GC-MS analysis.
In case of using high pressure reactor, catalyst (2.8 to 5.1 mole %), p-toluenesulfonic
acid (15 mole %), the cyclic ketone (0.5 mmol) and DMSO (1 ml) were placed in a
reactor. After purging the pressure reactor with O , the reactor was loaded with given
2
o
O pressure (25 C). The reaction was heated to 80 °C with vigorous stirring for given
2
time. After the completion of the reaction, the reaction mixture was filtered. Then, the
residue was extracted with ethyl acetate.
3
. Results and discussion
The metallic Pd-PVP (Pd-1), Pd-oleylamine (Pd-2), Pd cube (Pd-3), Cu-PVP and
Ni-PVP nanoparticles having different particle sizes, and bimetallic Ag@Pd coreshell,
and Ni-Sn nanoparticle were successfully synthesized by various methods using
different reaction conditions such as stabilizing agent, reducing agents, solvents,
reaction temperature etc. The synthesis conditions used for the production of
nanoparticles are summarized in Table 1. Nanoparticles ware characterized by powder
X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron
microscopy (TEM), high resolution transmission electron microscopy (HR-TEM) and
energy dispersive X-ray analysis (EDX). The average particle sizes of the Pd-1, Pd-2,
5

ACCEPTED MANUSCRIPT
Pd-3, Ni, Ni-Sn, and Cu were obtained around 3.1 nm, 4.2 nm, 7 nm, 13 nm, 8 nm
and 98 nm respectively. XRD patterns of Pd-1 and Pd-2 nanoparticles are shown in
Fig. 1. Four main characteristic diffraction peaks at 2θ=39.21, 45.87, 68.11 and 82.07°
correspond to the (111), (200), (220) and (311) crystal planes of metallic Pd,
confirming the formation of Pd nanoparticles with no visible impurity phases such as
PdO. The broadening of peaks in the XRD diffractogram is inversely correlated with
short crystallite size of the nanoparticles [15-17]. The detailed experimental
procedures and experiment data for nanoparticles preparation are included in
supplementary information.
The dehydrogenation of cyclohexanone to the corresponding phenols catalyzed by
different nanocatalysts in the presence of O as the oxidant was investigated. Our
2
initial studies targeted the identification of non-acidic reaction conditions for aerobic
dehydrogenation of cyclohexenone. Aiming to ultimately assess the solvent effects on
the aerobic dehydrogenation of cyclohexanone, we first explored the influence of
several solvents in the reaction with cyclohexanone. Table 2 summarizes the results
from the aerobic dehydrogenation of cyclohexanone with Pd-1 nanoparticles
o
conducted in several solvents under 1 atm O (RT) at 80 C for 24 h. The results
2
reveal that the reaction depends heavily upon the solvent. Among different solvents,
Pd-1 showed the best conversion in DMSO (Table 1, entry 6). Then, various catalysts
were tested for the dehydrogenation of cyclohexanone to the corresponding phenols in
dimethylsulfoxide (DMSO) as the solvent under 1 atm of O at 80°C (Table 3). The
2
tested catalysts include Pd-1, Pd-2, Pd-3, Ni, Ni-Sn, Cu, and Ag@Pd nanoparticles.
6

ACCEPTED MANUSCRIPT
The metallic Pd-1 nanoparticles exhibited excellent catalytic performance as
compared with Ni, Ni-Sn, Cu and Ag@Pd nanoparticles. We found that 3.1 nm Pd-1
nanoparticles were a highly active heterogeneous catalyst for the dehydrogenation of
cyclohexanone. In case of Ni, Ni-Sn and Ag@Pd nano-catalysts, addition of organic
acid such as p-Toluenesulfonic acid and increase of O pressure also failed to increase
2
the yield of the phenol product significantly (Table 3, entries 7 to 12). The
dehydrogenation of cyclohexanone in the absence of catalyst was quite slow resulted
in no conversions after 24 hours (Table 3, Entry 1).
In case of Pd-1, addition of organic acid such as p-toluenesulfonic acid and increase
of O pressure also increase the yield of the phenol product. When O pressure
2
2
increased from 1 to 5 atm the phenol yield from 15 to 49 % was increased and further
increase in the O pressure up to 10 atm also increased product yield and phenol yield
2
(
Table 3, entries 3 to 5). The Pd-1 catalyst at low O pressure favours formation of
2
cyclohexenone, whereas at high O pressure promotes highly selective formation of
2
phenol (Table 3, entries 3 to 5).
With the increase in the concentration of Pd-1 nanocatalyst up to 5.1 mole % the
catalytic activity towards the dehydrogenation of cyclohexanone was increased (Table
3
, Entries 2 to 3). The use of p-toluenesulfonic acid led to a substantial increase in the
yield of phenol as compared to that where no additive was used (Table 3, Entry 3 and
Entry 13). The best results were obtained by using 15 mole % p-toluenesulfonic acid
with Pd-1 nanocatalyst in dimethylsulfoxide (DMSO) under 1 atm of O at 80 °C
2
(Table 3, Entry 14). This catalyst system led to a 93 % yield of the desired enone 2
7

ACCEPTED MANUSCRIPT
and phenol 3. After optimization of the reaction conditions for Pd-1 nanocatalyst, we
prepared Pd-2 and Pd-3 nanocatalyst. These catalysts have different method of
preparation, stabilizing agents, particle size and shapes. Pd-1 nanocatalysts reported
here exhibited higher catalytic activity for the dehydrogenation of cyclohexanone to
the corresponding phenols in dimethylsulfoxide as compare to Pd-2 and Pd-3
o
nano-catalysts. The order of activity at 80 C was Pd-1>Pd-2> Pd-3 nano-catalyst in
DMSO (Table 3, Entries 14, 17 and 19). In the view of industrial purposes, reusability
of the catalyst was tested by carrying out repeated runs of the reaction on the same
batch of the catalyst. In order to regenerate the catalyst, after each run, it was
separated by a filtration, washed several times with deionized water, acetone and ethyl
acetate. Then, it was dried under vacuum at 30 °C and reused in the subsequent run.
The catalyst can be reused without any significant loss of activity. XRD and TEM
results confirmed that there is no change in morphology, particle size and XRD
pattern of the Pd-1 nanoparticle after reaction (Fig 1 and Fig 2). According to other
researchers, polyvinylpyrrolidone capped noble metal, when heated, may lead to
o
deactivation [18]. However, we tested the PVP-stabilized Pd- nanocatalysts at 80 C,
and found no evidence for its decomposition.
4
. Conclusions
In summary, we have identified a Pd nanocatalyst heterogeneous system that enables
direct dehydrogenation of cyclic ketones to the corresponding phenols in the presence
of O as the oxidant. The tested catalysts include Pd-1, Pd-2, Pd-3, Ni, Ni-Sn, Cu, and
2
Ag@Pd. The metallic Pd-1 nanoparticles exhibited excellent catalytic performance as
8

ACCEPTED MANUSCRIPT
compared with Pd-2, Pd-3, Ni, Ni-Sn, Cu and Ag@Pd nanoparticles. When O2
pressure increased from 1 to 5 atm the phenol yield from 15 to 49 % was increased
and further increase in the O pressure up to 10 atm also enhanced product yield and
2
phenol yield. The best results were obtained by using (15 mole %) p-toluenesulfonic
acid with Pd-1 nanocatalyst in dimethylsulfoxide (DMSO), under 1 atm of O at
2
8
0 °C. The ability to replace stoichiometric reagents with O as an oxidant has
2
important implications for large scale applications of these methods in pharmaceutical
and fine chemical synthesis.
Acknowledgements: This work is supported by the NSFC (21325208, 21172209,
2
1272050), CAS (KJCX2-EW-J02) and FRFCU (WK2060190025, WK2060190033).
References
[
[
[
1] J.H.P. Tyman, Synthetic and Natural Phenols; Elsevier: Amsterdam 1996; Vol.52.
2] P. Bamfield, P.F. Gordon, Chemical Society Reviews 13 (1984) 441.
3] E.C. Horning, M.G. Horning, Journal of the American Chemical Society 69 (1947)
1
359.
[
[
4] P.P. Fu, R.G. Harvey, Chemical Reviews 78 (1978) 317.
5] T. Moriuchi, K. Kikushima, T. Kajikawa, T. Hirao, Tetrahedron Letters 50 (2009)
7
385.
[
[
[
6] C.Y. Yi, D.W. Lee, Organometallics 28 (2009) 947.
7] P.F. Schuda, W.A. Price, Journal of Organic Chemistry 52 (1987) 1972.
8] J. Muzart, J.P. Pete, Journal of Molecular Catalysis 15 (1982) 373.
9

ACCEPTED MANUSCRIPT
[
(
[
[
9] T.T. Wenzel, Journal of the Chemical Society, Chemical Communications 1989
1989) 932.
10] J. Muzart, European Journal of Organic Chemistry 2010 (2010) 3779.
11] D.R. Buckle, InEncyclopedia of Reagents for Organic Synthesis, D Crich, Ed.
Wiley, New York, 2010.
[
[
12] Y. Izawa, D. Pun, S.S. Stahl, Science 333 (2011) 209-213.
13] D. Pun, T. Diao, S.S. Stahl, Journal of the American Chemical Society 135 (2013)
8
213-8221.
14] T. Diao, D. Pun, S.S. Stahl, Journal of the American Chemical Society 135 (2013)
205-8212.
15] G.G. Couto, J.J. Klein, W.H. Schreiner, D.H. Mosca, A.J.A. De Oliveira, A.G
Zarbin, Journal of Colloid and Interface Science 311 (2007) 461-468.
16] M. Niederberger, M.H. Bartl, G.D. Stucky, Chemistry of Materials 14 (2002)
364-70.
[
8
[
[
4
[
[
17] T. Ungar, Scripta Materialia 51 (2004) 777-81.
18] Y. Borodko, S.E. Habas, M. Koebel, P.D. Yang, H. Frei, G.A. Somorjai, Journal
of Physical Chemistry B 110 (2006) 23052.
19] G. Li, H. Kobayashi, J.M. Taylor1, R. Ikeda, Y. Kubota, K. Kato, M. Takata, Y.
Yamamoto, S. Toh, S. Matsumura, H. Kitagawa, Nature Materials 13 (2014) 803-806.
20] B. Lim, M. Jiang, J. Tao, P.H.C. Camargo, Y. Zhu, Y. Xia, Advanced Functional
Materials 19 (2009) 189-200.
21] M. Shah, Q.X. Guo, Y. Fu, Catalysis Communications 65 (2015) 85-90.
[
[
[
1
0

ACCEPTED MANUSCRIPT
[
22] K. Tedsree, T. Li, S. Jones, C.W.A. Chan, K.M.K. Yu, P.A.J. Bagot, E.A. Marquis,
J.D.W. Smith, S.C.E. Tsang, Nature Nanotechnology 6 (2011) 302-7.
23] Z. Yin, W. Zhou, Y. Gao, D. Ma, C.J. Kiely, X. Bao, Chemistry: A European
Journal 18 (2012) 4887-4893.
[
11

ACCEPTED MANUSCRIPT
Figure captions:
Fig 1; Powder XRD patterns of A=Pd-2, B=Pd-1 and C=Pd-1 after reaction.
Fig 2; (a) TEM image of Pd-1, (b) their size distribution (3.1 nm) and (c) TEM image
of Pd-1 after catalysis reaction.
1
2

ACCEPTED MANUSCRIPT
Fig 1;
1
3

ACCEPTED MANUSCRIPT
Fig 2;
1
4

ACCEPTED MANUSCRIPT
Tables:
Table 1; Reaction conditions for the preparation of Pd, Ag@Pd, Ni, Ni-Sn and Cu
nanoparticles
-
1
Nanoparticles
Precursors (mol l )
Protecting Reducing
Reaction
solvent
T (° C)
Reaction
Time (min)
120
agent
agent
Pd
Palladium(II) nitrate hydrate (0.0016)
PVP
-
Ethylene glycol
120
90
Ag@Pd
Palladium(II) nitrate hydrate (0.0016)
Silver(II) nitrate (0.023)
PVP
-
-
Ethylene glycol
120
Pd
Pd
Palladium(II) acetate ( 0.0005)
Oleylamine
PVP
Ethylene glycol
Water
200
80
120
180
Sodium tetrachloropalladate
L-ascorbic
acid
Ni
Nickel (II) formate dehydrate (0.03)
PVP
PVP
NaBH4
Ethylene glycol
Ethylene glycol
195
198
150
150
Ni-Sn
Nickel (II) formate dehydrate (0.03)
Tin (II) chloride dehydrate (0.01)
NaBH4
NaBH4
Cu
Copper (II) acetate monohydrate (0.037)
PVP
Ethylene glycol
190
150
1
5

ACCEPTED MANUSCRIPT
Table 2; Solvents screening for the aerobic dehydrogenation of cyclohexanone
catalyzed by Pd-1 nano-catalyst
Yield (%)
Entry Solvent
Product yield %
2
a
2b
<1
-
1
2
3
4
Toluene
Mesitylene
DMF
1
<2
-
-
1.3
11
<1
2.3
<3
13
DMA
5
6
7
8
Acetic acid
DMSO
1-methyl-2-pyrolidone
Water
40.5
39
3
12.6
15
<1
53
54
<4
<2
1.6
<0.4
Reaction conditions; substrate 0.5 mmol, 3.8 w.t % Pd-1 nanocatalyst, 1 atm O pressure, 80
2
o
C, 24 h.
1
6

ACCEPTED MANUSCRIPT
Table 3; Catalyst optimization for aerobic dehydrogenation of cyclic ketones
Nano-catalyst
(wt. %)
Additives
/TSOH
P
Yield (%)
2a or 3a
Product
Yield %
1
a
o
Entry
T ( C) (atm)
Time h
24
2b or 3b
1
2
No catalyst
Pd-1 (2.5)
Pd-1 (3.8)
Pd-1 (3.8)
Pd-1 (3.8)
Cu (3.8)
-
-
-
-
-
-
80
80
80
80
80
80
1
1
1
5
-
25.2
39
-
0
39.4
54
24
14.2
15
3
4
5
6
24
24
24
24
25
49
74
10
20
55
75
1
10
1
<2
<1
<3
7
8
9
Ni (3.8)
Ni (3.8)
-
15%
-
80
80
80
80
80
24
24
24
24
24
0
23
<0.5
13.3
0.8
23
<1
<1
Ni-Sn (3.8)
Ni-Sn (3.8)
Ag-Pdcore shell
10
1
0.5
9.6
15.4
13.8
10.4
17.7
1
0
15%
-
11
1
2.3
(
3.8)
1
2
Ag-Pdcore shell
15%
80
5
24
0.6
18
18.6
(
3.8)
1
1
1
1
1
1
1
3
4
5
6
7
8
9
Pd-1 (3.8)
Pd-1 (5.1)
Pd-1(5.1)
Pd-1(5.1)
Pd-2 (5.1)
Pd-2 (5.1)
Pd-3 (5.1)
15%
15%
15%
15%
15%
15%
15%
80
80
80
80
80
80
80
1
1
1
1
1
1
1
24
48
24
48
48
48
48
29
15
0
39
78
61
39
53
39
<1
68
93
61
39
63
39
32
1
1
b
b
0
10
0
1
b
31
Reaction conditions; substrate 0.5 mmol [Where cyclohexanone (1a), 3 methyl cyclohexanone
(
1b), cyclohexenone (2a), phenol (2b), 3-methyl cyclohexenone (3a), 3-methyl phenol (3b)],
o
3
.8 w.t % nanocatalyst, 1 atm O pressure, 80 C, 24 h. Yield were determined by using
2
p-anisoldehyde as internal standard.
1
7

ACCEPTED MANUSCRIPT
Graphical Abstract:
1
8

ACCEPTED MANUSCRIPT
Highlights：

Studied the dehydrogenation of cyclohexanone and 3-methycyclohexanone by
nanocatalysts ;




Metallic Palladium nanoparticles was found to be the best nanocatalyst;
Pd-1 showed highest yield of 93% in the presence of p-toluenesulfonic acid ;
Increased in O pressure and addition of p-toluenesulfonic acid enhance phenol yield
2
XRD and TEM results confirmed the heterogeneous nanocatalyst system;
1
9