Catalytic Partial Oxidation of Cyclohexane by Bimetallic Ag/Pd Nanoparticles on Magnesium Oxide

Article abstract of DOI:10.1002/chem.201605941

The liquid-phase oxidation of cyclohexane to cyclohexanol and cyclohexanone was investigated by synthesizing and testing an array of heterogeneous catalysts comprising: monometallic Ag/MgO, monometallic Pd/MgO and a set of bimetallic AgPd/MgO catalysts. Interestingly, Ag/MgO was capable of a conversion comparable to current industrial routes of approximately 5 %, and with a high selectivity (up to 60 %) to cyclohexanol, thus making Ag/MgO an attractive system for the synthesis of intermediates for the manufacture of nylon fibres. Furthermore, following the doping of Ag nanoparticles with Pd, the conversion increased up to 10 % whilst simultaneously preserving a high selectivity to the alcohol. Scanning transmission electron microscopy and energy dispersive spectroscopy of the catalysts showed a systematic particle-size-composition variation with the smaller Ag–Pd nanoparticles being statistically richer in Pd. Analysis of the reaction mixture by electron paramagnetic resonance (EPR) spectroscopy coupled with the spin-trapping technique showed the presence of large amounts of alkoxy radicals, thus providing insights for a possible reaction mechanism.

Full text of DOI:10.1002/chem.201605941

A Journal of
Accepted Article
Title: Catalytic partial oxidation of cyclohexane by bimetallic Ag/Pd
nanoparticles on magnesium oxide
Authors: Xi Liu, Marco Conte, Qian He, David Knight, Damien Murphy,
Stuart Taylor, Keith Whiston, Christopher Kiely, and Graham
John Hutchings
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201605941
Link to VoR: http://dx.doi.org/10.1002/chem.201605941
Supported by

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
Catalytic partial oxidation of cyclohexane by bimetallic Ag/Pd
nanoparticles on magnesium oxide
[
a,b,]
[a,c,]
[a,d]
[a]
[a]
[a]
Xi Liu,
Marco Conte,
Qian He, David W. Knight, Damien M. Murphy, Stuart H. Taylor,
[
e]
[d]
[a]
Keith Whiston, Christopher J. Kiely, Graham J. Hutchings,*
Abstract: The liquid phase oxidation of cyclohexane to cyclohexanol
and cyclohexanone was investigated by synthesizing and testing an
array of heterogeneous catalysts comprising: monometallic Ag/MgO,
monometallic Pd/MgO and a set of bimetallic AgPd/MgO catalysts.
Interestingly, Ag/MgO was capable of a conversion comparable to
current industrial routes of ca. 5%, and with a high selectivity (up to
conversion increased up to 10% whilst simultaneously preserving a
high selectivity to the alcohol. Scanning transmission electron
microscopy and energy dispersive spectroscopy of the catalysts
showed a systematic particle size composition variation with the
smaller Ag-Pd nanoparticles being statistically richer in Pd. Analysis
of the reaction mixture by Electron Paramagnetic Resonance (EPR)
spectroscopy coupled with the spin trapping technique showed the
presence of large amounts of alkoxy radicals, thus providing insights
for a possible reaction mechanism.
60%) to cyclohexanol, thus making Ag/MgO an attractive system for
the synthesis of intermediates for the manufacture of nylon fibres.
Furthermore, following the doping of Ag nanoparticles with Pd, the
Introduction
capable of an enhanced conversion for cyclohexane oxidation of
>
10% in the absence of organic initiators, while still preserving a
high selectivity to cyclohexanol as shown by Au/MgO
The catalytic oxidation of hydrocarbons is a research area or
[11]
[
1,2]
catalysts.
It was noted however that monometallic Au
great importance both in industry and academia,
by virtue of
nanoparticles supported on MgO required the presence of a
radical initiator such as AIBN or TBHP to show an appreciable
catalytic activity. These studies and their context prompted us to
consider the use of Ag and Ag-Pd nanoalloys for cyclohexane
oxidation. In this paper, we therefore explore the effect of adding
Pd to Ag nanoparticles, and although Ag has proved extremely
the variety of products that can be obtained from this
[3,4]
feedstock.
2
The use of O or air as oxidants for the partial
oxidation of hydrocarbons to alcohols or ketones, gained
[
5]
considerable attention in recent years. This is because aerobic
oxidation processes are greener and cheaper from a reagent
cost perspective compared to the use of oxygen transfer
reagents, which are more expensive and usually lead to
[
12]
useful for epoxidation reactions of alkenes, to date it has been
largely neglected for the direct oxidation of saturated
hydrocarbons.
[
6]
undesired
by-products.
However,
the
oxidation
of
hydrocarbons by means of molecular oxygen is a complex
process comprising both catalytic oxidation and autoxidation
[
7]
routes, with the latter precluding selective oxidation processes Results and Discussion
to a specific or desired product. In this context, the aerobic
oxidation of cyclohexane to cyclohexanol and cyclohexanone as
precursors for the production of nylon fibres is one of the most
important and challenging processes in the petrochemical
industry. In fact, this reaction is industrially carried out by
Catalytic activities of Ag Pd and Ag-Pd particles supported
on MgO
Monometallic Ag and Pd catalysts prepared using a sol-
[13]
immobilization (SI) protocol
benchmark for investigating the activity of Ag-Pd nanoalloys
Table 1). Pd/MgO was virtually inactive with a conversion of
.5%, which is similar to that of a blank oxidation test (i.e. one in
were initially tested as a
exploiting
autoxidation
pathways
promoted
by
Co
[
8,9
complexes. There is a strong commercial incentive to run such
processes at higher conversion in order to reduce the extent of
cyclohexane recycle and therefore save steam. However, the
conversion of cyclohexane has in practice to be kept low, in the
range of 3-8%, to reduce the presence of uncatalysed and
unselective free radical reaction pathways, which occur
increasingly at higher conversion and give rise to undesired by-
products. Low cyclohexane conversion is therefore required to
control the process at acceptable selectivities > 80% towards the
two major products commercial products cyclohexanol and
cyclohexanone. In a previous study, by using Au-Pd nanoalloys
(
0
which no catalyst is present). In contrast, Ag/MgO gave a
significant conversion of ca. 5%, with an excess of alcohol (56%
selectivity), together with a low selectivity to adipic acid (ca. 7%).
These conversion levels and selectivity indicate that Ag/MgO
[14]
could be a promising catalyst for this reaction. However, the
catalytic activity of Ag nanoparticles showed marked
a
dependence on the catalyst preparation method used. In fact, if
a Ag/MgO catalyst was synthesised via impregnation (Table 1),
a lower conversion of 2.6% was observed. It should be
emphasised that both of these Ag/MgO catalysts display a high
selectivity to the alcohol (> 55% in each case). However, the
product distribution between the two differently synthesised
catalysts is quite different, with Ag/MgO prepared by sol-
immobilization produced some adipic acid (ca. 7%), whereas
Ag/MgO prepared by impregnation led to some cyclohexyl
hydroperoxide (CHHP) (ca. 7%).
[
10]
supported on MgO,
we found that these catalysts were
[
a]
Dr. X. Liu, Dr. M. Conte, Prof. D. W. Knight, Prof. D. M. Murphy,
Prof. S. H. Taylor, Prof. G. J. Hutchings*
Cardiff Catalysis Institute, School of Chemistry, Cardiff University,
Cardiff, CF10 3AT (UK)
The difference in catalytic activity between these two catalysts is
E-mail: hutch@cardiff.ac.uk
[
[
[
[
b]
c]
d]
e]
Dr. X. Liu
tentatively attributed[ to the formation of Ag
2
O
for the
15]
Syncat@Beijing, Synfuels China Technology Co.,Ltd,
Beijing, 101407 (China)
Dr. M. Conte
Department of Chemistry, Dainton Building, University of Sheffield,
Sheffield, S3 7HF (UK)
Dr. Q. He, Prof. K. J. Kiely
impregnated catalyst.
On the contrary, the sol immobilized
material presented, rather surprisingly, only metallic Ag (see
section: Characterization of the Ag/MgO catalysts using electron
microscopy). In view of this, Ag
2
O and MgO were individually
tested (Table 1). Ag O showed a conversion of ca. 2% and a
2
similar product distribution to that of the blank test, (i.e. a large
amount of CHHP). In comparison, MgO was highly selective
towards the alcohol, but with a conversion equal to the blank test
Department of Materials Science and Engineering, Lehigh University
5
East Packer Avenue, Bethlehem, PA 18015-3195 (USA)
Dr. K. Whiston
INVISTA Textiles (UK) Limited,
P.O. Box 2002, Wilton, Redcar, TS10 4XX (UK)
These two authors contributed equally to this work.
(
ca. 1%). Furthermore, despite the conversion levels with MgO
and Pd/MgO being statistically identical, no CHHP was detected
when Pd/MgO was used. This demonstrates that the metal can

1
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
promote the decomposition of this CHHP species, which is an
important intermediate in the oxidation of cyclohexane (see
section: Mechanistic insights and CHHP decomposition).
Characterization of the Ag/MgO catalysts using electron
microscopy
In an effort to structurally characterise these Ag/MgO catalysts,
as well as provide possible explanations for the dependence of
the catalytic activity on the preparation method, high angle
annular dark field (HAADF) STEM imaging studies were
employed (Figure 1). The monometallic Ag/MgO sample
prepared via sol-immobilization showed supported Ag metal
nanoparticles in the 5-10 nm size-range having an f.c.c. crystal
structure (Figure 1(d)). Many of these primary colloidal Ag
particles were also found to have congregated into larger
polycrystalline aggregates about 20-30 nm in size (Figure 1(c)).
It is quite remarkable that although this sample had been
exposed to air for many months before STEM examination and
yet still presented as metallic Ag. We speculate that the stability
of the Ag nanoparticles towards low oxidation states might be
Table 1. Conversion and product distribution for a series of Pd and Ag
catalysts supported over MgO for the partial oxidation of cyclohexane.
o
Reaction conditions: T = 140 C, P = 3 bar, reaction time: 17 h.
Catalyst
Conversion
%)
Selectivity (%)
(
[
c]
[d]
[e]
[f]
[g]
K
A
CHHP
AA
Total
[
[
[
a]
a]
b]
Pd/MgO
Ag/MgO
Ag/MgO
MgO
0.5
4.6
2.6
1.0
1.9
1.1
32
31
27
31
31
22
66
56
55
67
38
40
0
0
0
98
97
90
98
94
97
7
0
1
0
0
7
attributable to the PVA surfactant used in the preparation
0
[16]
method
- a stability that also reflects in a high catalyst re-
usability for sequentially repeated catalytic tests - with high
consistency in conversion and selectivity values (Table 2).
Ag
2
O
25
35
Blank
By way of contrast, the monometallic Ag/MgO catalyst
synthesised by the impregnation route presented a distinct
bimodal size distribution of Ag containing particles (Figure 1(a)).
The larger particles had diameters in the 20-40 nm range,
whereas the smaller population were in the 1-5 nm size range
[
a] Catalyst prepared by sol immobilization method. [b] Catalyst prepared by
impregnation method. [c] ketone, cyclohexanone. [d] alcohol,
cyclohexanol. [e] CHHP = cyclohexyl hydroperoxide. [f] AA = adipic acid. [g]
Total observed selectivity.
K
=
A =
(
Figure 1(b)). Interestingly these smaller Ag particles were more
These data prompted us to consider the use of supported Ag-Pd
nanoalloys prepared using a sol-immobilization protocol to
explore the possibility of synergistic effects between Ag and Pd,
analogous to that observed for Au-Pd nanoalloys supported over
raft-like and irregularly shaped in character and exhibited a
definite epitaxial cube-on-cube orientation relationship with the
underlying MgO support. The reason for the poorer activity of
the impregnation Ag/MgO catalyst is not obvious from this
nanostructural analysis.
[
10]
MgO.
A molar ratio of 1:1 between these two metals was
initially prepared and tested (Table 2). The simultaneous
presence of the two metals (retaining the same total metal
loading of 1% wt, as used for the monometallic species, so that
the total metal-to-substrate ratio is not changed), doubled the
conversion up to ca. 10%.
Importantly, this increase in conversion also preserved a product
distribution similar to that observed for the monometallic
Ag/MgO catalyst obtained via sol -immobilization (i.e. with an
alcohol-to-ketone ratio, A/K, of ca. 2 and adipic acid < 10%). In
order to confirm this result, reusability tests were carried out
(
Table 2). Repeated testing up to four consecutive runs did not
show any decrease in catalytic activity, nor any changes in
selectivity compared to the initial test, thus showing that the
AgPd/MgO catalyst is highly stable under the tests conditions.
Therefore, we consider these results highly significant for the
development of catalyst materials active towards cyclohexane
oxidation.
Table 2. Catalytic test and reusability test result using a AgPd/MgO catalyst
prepared via sol immobilization and a Ag:Pd ratio of 1:1 for the partial
o
oxidation of cyclohexane. Reaction conditions: T = 140 C, P = 3 bar, reaction
time: 17 h.
Catalyst
and run)
Conversion
(%)
Selectivity (%)
(
[
a]
[b]
[c]
[d]
[e]
K
A
CHHP
AA
Total
89
st
AgPd/MgO (1 )
9.5
10
26
30
31
28
52
56
57
57
0
0
0
0
10
Figure 1. Representative STEM-HAADF images of Ag/MgO catalysts
prepared by (a, b) the impregnation method and (c, d) the sol immobilization
method. The impregnated Ag catalyst contains (a) relatively large particles
about 30 nm in size, as well as (b) smaller raft-like species in the 1-5 nm size
range. These smaller species show an epitaxial orientation relationship with
the MgO support (inset). The Ag/MgO catalyst prepared via sol immobilization
appears to have a bi-modal Ag size distribution, containing (c) aggregates of
particles about 25 nm in size, and (d) smaller single crystal metallic Ag
particles around 5-10 nm in size.
nd
AgPd/MgO (2
)
8
6
9
94
rd
AgPd/MgO (3 )
10
94
th
AgPd/MgO (4 )
9.8
95
[
a] K = ketone, cyclohexanone. [b] A = alcohol, cyclohexanol. [c] CHHP =
cyclohexyl hydroperoxide. [d] AA = adipic acid. [e] Total observed selectivity.
2
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
Catalytic activities of AgPd/MgO catalysts with different
Ag:Pd molar ratios
In view of these results, and in order to estimate an empirical
optimal composition for the bimetallic Ag-Pd/MgO catalyst with
the aim of enhancing the conversion for this reaction, a series of
catalysts with Pd to Ag molar ratios ranging from 1:10 to 10:1
were prepared via sol-immobilization. A detailed description of
the product distribution for these materials is reported in Table 3.
The highest conversion, in the range of 8-10%, is observed for
catalysts having nominal stoichiometries (molar ratio) of Ag
Ag Pd , Ag Pd and Ag Pd . However, it should be underlined
that Ag and Pd form a continuous solid solution,
1 3
Pd ,
1
1
3
1
5
1
[
17]
and to be
best of our knowledge no superlattice formation is known for
these two elements.
Table 3. Summary of the catalytic activity (conversion and product distribution)
of AgPd/MgO catalysts with different Ag:Pd molar ratios for the oxidation of
cyclohexane. Reaction conditions: 8.5
catalysts, 17 hours, 140°C.
g
cyclohexane,
3
bar
O
2
,
6
mg
Figure 2. Representative STEM-HAADF images and corresponding XEDS
spectra the 1:1Ag-Pd/MgO catalyst prepared by the sol-immobilization
method; (a) shows relatively large particles (>50 nm), and the solid line in (b)
shows the corresponding XEDS spectrum taken from the highlighted particle
in (a). The purple dashed line shows a reference #1 spectrum taken from an
Ag particle in the Ag/MgO sol-immobilized catalyst; (c) shows relatively smaller
particles. The inset shows a higher magnification image of one such typical
metallic Ag-Pd f.c.c. alloy particle viewed along the [110] projection. The solid
line in (d) shows the corresponding XEDS spectra taken from the highlighted
particle in the inset in (c). The red dashed line shows a reference #2 spectrum
[
a]
Catalyst
Conversion
%)
Selectivity (%)
(
[
b]
[c]
[d]
[e]
[f]
K
A
CHHP
AA
Total
Pd/MgO
Ag Pd10/MgO
0.5
2.3
3.2
8
32
38
45
40
26
27
33
66
55
46
45
52
60
58
0
0
0
0
0
0
0
0
6
3
8
98
100
94
1
Ag
1
Ag
1
Ag
1
Ag
3
Ag
5
5
Pd /MgO
3
Pd /MgO
1
Pd /MgO
1
Pd /MgO
1
Pd /MgO
93
taken from a monometallic Pd particle in a suitable reference catalyst,
suggesting that these smaller particles are Pd-rich. Both (b) and (d) show that
mixing of Ag and Pd components in the alloy samples are rather limited.
9.5
10
10
8
89
95
9.3
7
98
The effect of a different Ag:Pd molar ratio impacts only the
conversion; there, however, is no obvious trend with molar
fraction of Ag for the selectivity in terms of ketone to alcohol
molar ratio, K/A ratio, instead. In each case the alcohol is always
in excess in the product, and K/A ratio values are, on average
Ag10Pd
1
/MgO
4.6
4.6
35
31
54
56
0
0
6
7
95
97
Ag/MgO
[
a] Catalysts prepared by sol immobilization method, with different Ag:Pd
molar ratios, as reported by the indexes in subscript next to the metal symbol.
b] K = ketone, cyclohexanone. [c] A = alcohol, cyclohexanol. [d] CHHP =
(
from the amounts of ketone and alcohol in table 3), centred
around 0.7. To explain this experimental observation, we need
to consider the free-radical nature of cyclohexane oxidation. In
[
cyclohexyl hydroperoxide. [e] AA = adipic acid. [f] Total observed selectivity.
[
7,18]
fact, cyclohexane oxidation is a radical-based process
and
our mechanistic study (vide infra) also support this mechanism
when AgPd/MgO catalysts are used. In case of a radical process
From these catalytic data the maximum conversion is observed
the initiation step is the removal of a H· atom from a C
molecule to form a C 11· radical that will further react with
oxygen under a diffusion regime. Because in our case (Tables 1-
) we do not initiate the reaction by means of organic radical
6 12
H
for the compositions: Ag
1
Pd
1
,
Ag
3
Pd
1
,
Ag
5
Pd
1
,
which
6
H
corresponds to a Ag mole fraction xAg in between 0.5 and 0.8. In
an attempt to rationalize these results, we applied STEM-
HAADF imaging together with X-ray energy dispersive
3
initiators, it follows that the initiation step has to be carried out by
the metals present in the system. Our data clearly shows that
AgPd nanoparticles are much more efficient for carrying out this
process than solely Ag or Pd alone. This might be due to a
better surface/substrate interaction or the nanoalloy being more
capable of enabling the C-H abstraction reaction due to a
different electron density of the catalyst active sites when the
two metals are present. Either way, as AgPd nanoparticles are
more active than materials comprising only Ag or Pd, this can
be classed as a synergetic effect between the two metals.
However, whereas the addition of Pd increases the activity of Ag,
the selectivity is essentially unmodified, thus showing that either
the single metal or the nanoalloy decompose CHHP in the same
manner. For the impregnated catalysts we think that changes in
both conversions and selectivity are induced by the probable
spectroscopy (XEDS) analysis to
a Ag-Pd/MgO sample
prepared via the sol-immobilisation route and having a nominal
Ag:Pd molar ratio of 1:1. The results revealed a bimetallic
catalyst possessing a rather complex structure. In fact, the
catalyst was composed of a collection of very large irregularly
shaped agglomerates of primary particles, with dimensions of
around 30-100 nm (Figure 2(a)), together with a multitude of
much smaller isolated particles of 2-5 nm diameter (Figure 2(b)).
The composition of these differently sized species were
qualitatively analysed by XEDS (Figures 2(c),(d)) It should be
noted that a strong overlap between the Pd L peak (2.84 keV)
and the Ag L peak (2.98 keV) in this sample precluded any
quantatative compositional analysis. However, by comparison
with XEDS spectra obtained from monometallic supported Ag
and Pd catalyst materials it was possible to deduce the larger
nanoparticles were systematically rich in Ag, whereas the
population of smaller nanoparticles was always rich in Pd. These
observations, combined with the data presented in Table 3,
would suggest that the larger particles, which are richer in Ag
are probably the most active for the oxidation of cyclohexane.
[
15]
2
formation of Ag O.
Mechanistic insights and CHHP decomposition
As all our catalysts presented an excess of alcohol with respect
to the ketone, and cyclohexane oxidation is known to occur as a
free-radical process over Au or Co systems, we were interested
3
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
in exploring the mechanistic features underlying the observed
behaviour for our catalysts. A key intermediate in the oxidation
of cyclohexane is cyclohexyl hydroperoxide (CHHP), which can
Table 4. Conversion and product distribution in the CHHP decomposition by
Pd/MgO, AgPd/MgO and Ag/MgO obtained via sol immobilization. Reaction
conditions: 1 mL solution 2.5 mol% CHHP in cyclohexane, 6 mg catalysts,
[
18,19]
transform to produce cyclohexanone or cyclohexanol.
Given
70°C, 0.5 hour. At this temperature no oxidation of cyclohexane occurs, but
the importance of CHHP in this reaction, it is necessary to
consider the accepted models for the formation of cyclohexanol
and cyclohexanone during an autoxidation process mediated by
the decomposition of CHHP. This will provide a starting point for
considering the catalytic tests using cyclohexyl hydroperoxide,
tert-butyl hydroperoxide, and the spin trapping experiments, as
well as a rationale for the observed selectivity when AgPd/MgO
is used.
CHHP decomposition.
[
a]
Catalyst
Conversion
(%)
Selectivity (%)
[
b]
[c]
[d]
K
A
Total
K/A
0.35
0.43
0.32
Pd/MgO
AgPd/MgO
Ag/MgO
66
100
97
26
30
24
74
70
76
100
100
100
CHHP can decompose by cleavage of the O-O bond of the
hydroperoxide group either via thermal decomposition or
[
20]
assisted by a metal centre. This decomposition leads to the
formation of an alkoxy and a hydroxy radical (eq. 1):
[
a] Catalyst prepared by sol immobilization method, for AgPd/MgO a Ag:Pd
molar ratio of 1:1 was used. [b] K = ketone, cyclohexanone. [c] A = alcohol,
cyclohexanol. [d] Total observed selectivity.
C
6
H
11-OOH → C
6
H11-O· + ·OH
(eqn. 1)
Both of these radicals can react further with cyclohexane to yield
to cyclohexanol and a C 11· radical (eq. 2), as well as forming
water and another C 11· radical respectively (eq. 3):
The catalytic decomposition of CHHP to cyclohexanol and
cyclohexanone is nearly complete when AgPd/MgO and
Ag/MgO are used (ca. 100 %), whereas a value of ca. 66%
conversion was measured in case of Pd/MgO. Therefore the
higher the rate of CHHP decomposition, the higher is the activity
of the catalysts towards cyclohexane oxidation.
6
H
6
H
C
6
H
11-O· + C
6
H
12 → C
6
H
11-OH + C
6
H
11
·
(eqn. 2)
(eqn. 3)
·
OH + C 12 → H O + C H ·
6
H
2 6 11
EPR Spin Trapping for the CHHP Decomposition by Ag- Pd-
and AgPd/MgO catalysts.
Once the C
molecular oxygen, in a diffusion limited reaction step leading
to the formation of cyclohexyl peroxide (C 11-OO·, abbreviated
to CHP) and cyclohexyl hydroperoxide (C
eqns. 4 - 5):
6
H11· radical is formed, this can quickly react with
[21]
6
H
In order to further investigate the correlation between the
catalytic activity and the intermediates involved in this reaction
6
H11-OOH, CHHP)
(
(
such as alkyl peroxide, alkyl hydroperoxide and alkoxy radicals),
we investigated the role of CHHP by using the EPR spin
[
27]
trapping method. The spin-trapping methodology relies on the
trapping of short-lived radicals by a diamagnetic spin trap
C
6
6
H
11· + O
2
→ C
6
H
11-OO·
(eqn. 4)
(eqn. 5)
[
28]
C
H11-OO· + C
6
H
12 → C
6
H
11-OOH + C
6
H
11
·
molecule, forming a stable spin adduct,
i.e. a persistent free
radical with a sufficiently long lifetime to enable detection by
continuous wave (CW) EPR spectrosocpy. In our case 5,5-
dimethyl-1-pyrroline-N-oxide (abbreviated DMPO) was used as
the spin trap (Figure 4, top). As a consequence of the different
hyperfine couplings between the unpaired electron in the spin
adduct and the H in the beta position of DMPO, it is possible to
qualitatively evaluate and indirectly identify the nature of the
The CHP/CHHP molecules can then initiate a series of reactions
for the formation of cyclohexanol and cyclohexanone, involving
[
22]
H- abstraction (eqns. 6-7)
and alkyl peroxide
[23]
disproportionation (the termination reaction given by eqn. 8).
original short-lived radicals present in solution (through the a
and a coupling constants).
H
N
[29,30]
C
C
2
6
H
11-OO· + C
10(·)OOH → C
11-OO· → C
6
H
11OOH → C
10=O + ·OH
11-OH + C
6
H
11OOH + C
6
H
10(·)OOH (eqn. 6)
6
H
6
H
(eqn. 7)
It should also be mentioned that a number of intrinsic pressure
and temperature limitations are associated with this method of
radical detection which collectively prohibit the experiments
C
6
H
6
H
6
H
10=O + O
2
(eqn. 8)
It should be noted that the usual initiation step for the free
radical pathway involves the abstraction of a H atom from
cyclohexane to from a cyclohexyl radical (C
o
[31]
being conducted at 140 C and 3 bar.
As a result, the EPR
spin trapping experiments were carried out at room temperature
and atmospheric pressure. However, owing to the high
sensitivity of the EPR technique, this approach is still sufficient
6
H
12  C H11·), in a
6
[24]
process which can be mediated by the walls of the reactor. In
this scheme no alkoxy radical is needed to initiate the reaction,
and the ketone (cyclohexanone in our case), will always be
formed as a consequence of the autoxidation pathway involving
the CHP/CHHP pair (eqns. 6-8). In other words, if a pure radical
pathway takes place in solution, the ketone will always form in
excess with respect to the alcohol. Current autoxidation models
report a K/A ratio of about 1-1.5 if no selectivity control by a
[32]
to capture and detect the spin adduct species for analysis.
The X-band CW EPR spectra obtained using the spin trap
DMPO during the decomposition of CHHP with Ag-only, Pd-only
and AgPd/MgO catalysts in cyclohexane are shown in Figure 3.
[25]
catalyst surface is occurring. It follows then that the only way
to obtain an excess of alcohol from eqns 1-8 is to have fast
cleavage of the O-O bond in CHHP (eqn. 1).
[
26]
In order to identify any trend between the CHHP decomposition
and the alcohol formation (Tables 1 and 2) during the oxidation
process, the catalysts Pd/MgO, AgPd/MgO and Ag/MgO were
tested using CHHP as a substrate (Table 4).
4
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
(
a)
b)
(
(1)
(2)
(
(
c)
d)
10 G
Figure 3. EPR spectra of DMPO spin adducts obtained during the
decomposition of CHHP in cyclohexane in the presence of: (a) AgPd/MgO, (b)
Pd/MgO, (c) Ag/MgO, and (d) autoxidation with no catalyst. All catalysts were
prepared via sol immobilization.
(3)
(4)
Figure 5. (A) Principle of the spin trapping methodology: fast selective addition
(trapping) of short-lived radicals to a diamagnetic spin trap, usually a nitrone or
a nitroso compound, such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The
product of this addition (spin adduct) is a persistent free radical (nitroxide) with
sufficiently long lifetime to enable detection by conventional EPR spectroscopy.
A representative EPR spin adduct spectrum, including the
combined simulation and deconvoluted single spin adduct
species, is reported for the AgPd/MgO catalyst in Figure 4.
Simulation of the spectrum and comparison with literature values
makes it possible to identify the following species: a di-tert-butyl-
(B) Proposed structures of the DMPO spin-adducts detected in the CHHP
decomposion in cyclohexane by AgPd/MgO catalyst prepared by sol-
immobilization (cross reference with Figure 4): (1) di-tert-butyl-nitroxide
[
33]
nitroxide derivative (with a
spin adduct (a = 13.37, aH() = 5.95, aH() = 1.91 G), a DMPO–
OO–C 11 adduct (a
centred adduct possibly originating from a ring opening species
tentatively assigned as DMPO–C(OH)R (a = 15.93, a = 21.31
N
= 14.30 G),
6 11
a DMPO–O–C H
[
34]
derivative, (2) DMPO–O–C
6
H
11 spin adduct, (3) DMPO–O
2
–C
6
H
11 adduct, and
N
(4) carbon centred adduct, which is possibly a DMPO–C(OH)R
2
species.
[
35]
6
H
N H
= 14.46, a = 10.21 G), and a carbon-
2
N
H
From the illustration of the spin trapping principle (Figure 5A), it
appears that the spin trapping technique only enables one to
obtain a semi-quantitative determination of the spin adducts.
This is due to the fact that the absolute amount of adduct in
solution is the result of several competing factors including the
life-time of the spin adduct itself, the nature of the solvent, the
temperature and most importantly the efficiency of the trapping
[
36]
G) as the catalyst is capable to generate small amounts of
adipic acid.
[
37]
reaction in solution by DMPO.
However, even after taking
(
(
a)
b)
these limitations into account, a semi-quantitative analysis can
still be conducted since all the catalysts were tested under
identical conditions; in other words, any observed differences in
the ratios among different spin adducts will be representative of
[38]
differing catalytic activity.
A summary of the relative
abundances of spin adducts for CHHP decomposition by Ag-
only, Pd-only and AgPd/MgO catalysts is reported in Table 5.
(
(
c)
d)
10 G
(
(
e)
f)
Table 5. Relative abundances (%) of DMPO spin adducts obtained following
CHHP decomposition in cyclohexane by Ag-Pd- and AgPd/MgO catalysts.
[
a]
[b]
Catalyst
Nitroxide
RO
ROO
C
Intensity
Figure 4. Deconvoluted EPR spectra of DMPO spin adducts obtained during
the decomposition of CHHP in cyclohexane in the presence AgPd/MgO with a
Ag:Pd molar ratio of 1:1. (a) experimental spectrum and b simulated spectrum,
Pd/MgO
AgPd/MgO
Ag/MgO
< 0.5%
< 0.5%
< 0.5%
89
90
89
4.6
2.9
6.6
4.6
6.6
4.1
183
369
420
(
b) simulated spectrum, (c) di-tert-butyl-nitroxide derivative, (d) DMPO–O–
11 spin adduct, (e) DMPO–O –C 11 adduct, and (f) carbon centred adduct,
which is possibly a DMPO–C(OH)R species.
C
6
H
2
6
H
2
In order to improve the clarity of our discussion, a schematic
illustration of the spin trapping principle, together with structure
of the spin adducts we have detected, are reported in Figure 5.
[
1
a] Catalyst prepared by sol immobilization method, with a Ag:Pd molar ratio of
-4
:1. [b] Compared to 510 M TEMPO solution in cyclohexane, which was
used as a standard.
5
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
Alkoxy radicals (RO) are intrinsically more reactive than peroxy
AgPd/MgO is used without any organic initiator, the nanoalloy is
as good as organic radicals at initiating the reaction, while still
minimizing by-product formation. Therefore reaction does not
necessarily need to be ‘activated’ by the presence of an organic
radical.
[39]
radicals (ROO),
compared to DMPO-OOR adducts should not be surprising. On
the other hand DMPO-O-C 11 is always present in ca. 90%
and thus an excess of DMPO-OR adducts
6
H
abundance. This means there are no significant variations in the
relative amounts of this species formed using the different
catalysts, and this indicates that large amounts of alkoxy radical
are formed in solution, which in turn implies the formation of an
excess of alcohol with respect to the ketone. However, whilst the
data can explain the observed product distribution for these
Table 6. Catalytic performance of metallic catalysts in presence of TBHP
initiator (TBHP to cyclohexane 0.15 % molar ratio). Reaction conditions:
Pressure = 3 bar, temperature =140°C, reaction time = 17 h.
Catalyst
Conversion
(%)
Selectivity (%)
materials as
a whole, it cannot fully explain the higher
[
c]
[d]
[e]
[f]
[g]
conversions detected in the presence of Ag/MgO and
AgPd/MgO. Since the distribution of the spin adducts is similar
for all three catalysts, the total (integrated) area for all of the spin
adducts was therefore considered as an estimate for the total
amount of intermediates generated during the initial stages of
the reaction. The intensity of the experimental EPR spin adduct
spectra was compared to a standard solution of TEMPO in
cyclohexane (see Table 5, final column).
K
A
CHHP
AA
Total
[
a]
Ag
1
Pd
1
/MgO
12
9
6.3
6.6
32
28
32
33
36
37
52
52
0
0
0
0
32
100
95
95
[
a]
Ag/MgO
Ag/MgO
TBHP
30
11
15
[b]
100
[
a] Catalyst prepared by sol immobilization method. [b] Catalyst prepared by
impregnation method. [c] K = ketone, cyclohexanone. [d] A = alcohol,
cyclohexanol. [e] CHHP = cyclohexyl hydroperoxide. [f] AA = adipic acid. [g]
Total observed selectivity.
For Pd/MgO, the least active of the three catalysts, the total
number of spins is also the lowest among these three catalysts
(
Table 5). Therefore, despite the limitation of the semi-
quantitative approach to the analysis of the EPR spin trapping
data, it is still possible to obtain a correlation between the
relative amount of radical intermediates detected and the
catalytic activity. It is clear from this analysis that the more active
the catalyst is, then a larger amount of radical concentrations
are generated in solution (and ultimately trapped by the spin
adducts). It is also clear from the EPR results that the three
primary adducts detected (RO, ROO, C) can most likely be
In this context though, there is no apparent difference between
the blank test in the presence of just TBHP and the test using a
Ag/MgO catalyst prepared by impregnation on MgO. We believe
2
this is due to the presence of Ag O in this particular catalyst. As
TBHP exerts its initiator activity as a consequence of the
cleavage of its O-O bond, this would suggest that sol-
0
immobilized AgPd/mgO catalysts (that are richer in Ag ) are
assigned to the C
6
H
11-O, C
6
6
H11-OO and C H11 radicals, and
more efficient at carrying out this particular reaction.
therefore the generic radical based transformation mechanism
presented in eqns 1-8 above, is likely to be operative under the
current catalytic conditions.
Conclusions
We have shown that silver nanoparticles can be an efficient
catalyst for the oxidation of cyclohexane, with enhanced
selectivity towards cyclohexanol and a limited formation of adipic
acid, and thus are a viable catalytic system for this important
oxidation reaction. The activity of the silver nanoparticles was
found to be highly dependent on the preparation method used to
deposit them onto MgO. The supported nanoparticles were most
active if a sol-immobilization method was used to prepare the
catalyst, whereas a diminished catalytic activity along with a
change in the product distribution was found if the catalysts were
prepared by an impregnation method. This result is tentatively
TBHP decomposition
To further extend our mechanistic model beyond the study of
CHHP decomposition, we also considered the role and
decomposition of tert-butyl hydroperoxide (TBHP). TBHP is a
common radical initiator, as well as an oxygen donor
can undergo homolytic cleavage of the O-O bond in a similar
manner experienced by CHHP (eqs. 9-11).
[
40]
[41]
that
(
(
CH
3
3
)
3
3
C-OO-H → (CH
3
)
3
C-O· + ·OH
(eq. 9)
C-OO·
CH
)
C-O· + (CH C-OO-H → (CH
3
)
3
3
)
3
C-O-H + (CH
3
)
3
2
ascribed to the presence of Ag O species for the impregnated
(
eq. 10)
catalysts, and the presence of reduced Ag metal for the sol-
immobilized nanoparticles. The effect of a second metal such as
Pd, to induce the formation of nanoalloys with Ag, led to a
synergistic effect towards cyclohexane oxidation. In particular,
Ag-Pd/MgO catalysts had higher conversion compared to the Ag
monometallic catalysts, but without losing the high selectivity to
cyclohexanol or without generating large amounts of adipic acid,
and thus showing great potential for the exploitation of silver
nanoalloys in this area. An optimal Ag-Pd composition of these
materials, for enhanced conversion, was identified to be an
equimolar amount of the two metals. Furthermore,
characterization of the catalysts presenting the same nominal
metal loading for Ag and Pd revealed a wide nanoparticle size
range (from ca. 2 nm to about 40 nm) with small particles that
are proportionally richer in Pd, and larger nanoparticles that are
richer in Ag.
Finally, the mechanistic aspects of the catalytic transformation
were investigated using the EPR spin trapping technique, which
showed that AgPd/MgO and Ag/MgO catalysts prepared by sol
immobilization were more active compared to the Pd/MgO
catalyst, as they were capable of forming a higher concentration
of the CHHP species, and in turn alkoxy radicals in solution and
from this high cylohexanol selectivity is possible.
2
3 3 3 3 2
(CH ) C-OO· → (CH ) C-O· + O
(eq. 11)
In previous studies, TBHP was identified as an initiator of the
free radical processes that enhance the conversion by Au and
[
9]
[10]
Pd over Au/MgO and AuPd/MgO
supported nanoparticles
for the oxidation of cyclohexane. This occurs without altering the
K/A ratio of the reaction mixture, but instead by increasing the
formation of undesired adipic acid. Thus we were interested in
the role played by TBHP in the presence of Ag as relevant to the
current study.
Catalytic tests for the cyclohexane oxidation using Ag/MgO and
AgPd/MgO catalysts were carried out by using TBHP in small
amounts only (0.15 mol % with respect to cyclohexane), to
restrict the use of this species to an initiator. TBHP increased
the conversion towards cyclohexane oxidation for all the
catalysts (Table 6), although selectivity to the alcohol is lost with
K/A ratios in the range 0.8-1.0 and results in the formation of
large amounts of adipic acid. This shows similarities with Au and
[
10]
Pd on MgO which give a similar product distribution. On the
other hand, these data (compared to Table 3) also show that if
6
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
h. After reaction, the used catalyst was washed with
cyclohexane and dried at 110oC for 16 h. Afterwards, the
catalytic activity of the used Ag-Pd/MgO catalyst was tested
under same reaction conditions: Ag-Pd/MgO (total metal loading
Experimental Section
Chemicals
1
2
wt%, 6 mg), cyclohexane (8.5 g), at 140 °C and 3 bar O for 17
h). The obtained reaction mixture was analysed by gas
chromatography as described in the catalytic tests and
characterization of the products paragraph. Subsequent re-
usability tests were carried out on the same material following
same procedure.
3 2
AgNO , PdCl , MgO, cyclohexane and other chemicals were
purchased from Aldrich and used without further purification
unless otherwise specified.
Catalyst preparation
CHHP and TBHP decomposition
Ag-Pd/MgO catalysts (1 wt% total metal loading) were prepared
by using a modified sol-immobilisation method as reported in
Cyclohexyl hydroperoxide (CHHP), was synthesized by
a
[
42] and the references therein. The desired amount of AgNO
3
Grignard reagent-oxygen reaction, as reported in [44] and the
references therein. A solution containing 2.5 mol% cyclohexyl
hydroperoxide in cyclohexane was obtained. Catalytic
decomposition of CHHP was carried out in V-Vials with total
volume of 3 mL. The CHHP solution (1 mL) was mixed with the
catalyst (6 mg) under continuous stirring conditions at 70°C for
(
9
1
Sigma Aldrich, assay 99% wt) and PdCl
2
(Sigma Aldrich, assay
9% wt) were added into 800 mL water. After stirring for 15 min,
-1
.3 mL of PVA solution (0.01 g mL ) was added, and the
solution was stirred for a further 15 mins. Subsequently, 3.3 mL
of freshly prepared NaBH solution (0.1 M) was added to
4
generate Ag-Pd nanoalloy particles. After reduction for 45 min,
the MgO support (Sigma Aldrich, 1.98 g) was added to
immobilise the nanoparticles. After filtration and washing, the
solid obtained was dried (110 ºC, 16 h) before use. The relative
amount of Ag and Pd salts used was varied to obtain a
systematic series of supported catalysts with different molar
ratios of Ag to Pd, ranging from 10:1 to 1:10. It is well
established that the use of PVA as a colloid stabiliser prevents
30 min. After reaction, the solution was immediately cooled
using an ice-water bath. The sample was centrifuged and the
liquid analysed by GC.
The influence of the radical initiator, tert-butyl hydroperoxide
(
TBHP) (Sigma Aldrich, 70 wt% in water) was also studied in this
work. Cyclohexane (8.5 g) and TBHP (20 mg) were added into
the glass reactor with or without catalysts, and then catalytic
oxidation was conducted under the same reaction conditions
[
43]
their sintering before their immobilization on a support. As the
surfactant can be easily removed at about 100 C, we consider
o
(
2
140 °C, 3 bar O , 17 hours, 6 mg catalysts). The products were
analysed by the GC following the procedure as described in the
catalytic tests and characterization of the products section.
the activity of these catalysts are not significantly affected by
the presence of residual PVA on the metal or the support.
Mono-metallic Ag and Pd supported catalysts, were also
prepared for comparative purposes using the same total metal
loading (i.e. 1 wt%). An additional Ag/MgO catalyst (1 wt%) was
also prepared using impregnation method for comparative
purposes. For this catalyst, the desired amount of AgNO was
3
added into 20 mL water containing a suspension of MgO. The
resulting slurry was dried at (110 ºC, 16 h), and the catalyst was
EPR experiments
X-band continuous wave (CW) EPR spectra were recorded at
room temperature in deoxygenated cyclohexane, using a Bruker
EMX spectrometer equipped with a high sensitivity Bruker ER
4
3
119 cavity. The typical instrument parameters were: centre field
487 G, sweep width 100 G, sweep time 55 s, time constant 10
2
reduced using H (5% in Ar) at 400 °C for 30 min.
ms, microwave power 5 mW, modulation frequency 100 kHz,
and modulation width 1 G. Spectral analysis was carried out
Catalytic tests and characterization of the products
[
45]
using the WinSim software.
were performed using the following procedure: 5,5-dimethyl-1-
pyrroline N-oxide (DMPO) (0.1 mL of 0.1 solution in
The spin trapping experiments
Oxidation of cyclohexane (Alfa Aesar, 8.5 g, HPLC grade) was
carried out in a glass bench reactor using a fixed mass of
catalysts (6 mg) for all tests. The reaction mixture was
M
cyclohexane) was added to the substrate (0.1 mL of 2.5 mol%
solution of CHHP in cyclohexane) in an EPR sample tube. The
mixture was deoxygenated by bubbling N2 through the solution
for 1 min prior to recording the EPR spectrum in order to
2
magnetically stirred at 140 °C and 3 bar O for 17 h. Samples of
the reaction mixture were analysed by gas chromatography,
using a Varian 3200 GC equipped with a flame ionization
detector. Chromatographic separation and identification of the
products was carried out using a CP-Wax 42 column. Adipic
acid present in the reaction mixture was converted to its
corresponding ester for quantification purposes, and
chlorobenzene was added as an internal standard. The product
distribution as a function of reaction time was monitored by
studying a systematic series of reaction batches subjected to
different reaction times under the same conditions of
temperature and oxygen partial pressure. The experimental
error associated to our methods though does not allow us to
statistically discriminate conversion values less than or equal to
ca. 1%, . Measured conversion levels in this range should
therefore be considered as statistically identical.
[
46]
enhance the signal resolution. For the reactions involving the
Ag/MgO, Pd/MgO and Ag-Pd/MgO catalysts, deoxygenation was
carried out at room temperature 5 min after the mixing of the
catalyst with the reaction mixture.
Electron Microscopy Characterisation
Samples of catalysts were prepared for TEM/STEM analysis by
dry dispersing the catalyst powder onto a holey carbon TEM grid.
Bright field (BF) imaging experiments were carried out on a
JEOL 2000FX TEM operating at 200 kV. High-angle annular
dark field (HAADF) imaging experiments were carried out using
a
200 kV JEOL 2200FS scanning transmission electron
microscope equipped with a CEOS aberration corrector. This
latter microscope was also equipped with a Thermo-Noran X-ray
Re-usability tests
energy dispersive spectroscopy (XEDS)
compositional analysis.
system
for
Re-usability tests were also performed in an identical glass
reactor. Cyclohexane (8.5 g) and an excess of 1 wt% Ag-
Pd/MgO (60 mg) were added into the batch reaction and
2
catalytic oxidation was carried out at 140 °C and 3 bar O for 17
7
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
[
[
21] M. S. Stark, J. Am. Chem. Soc. 2000, 122, 4162–4170.
22] I. Hermans, J. Peeters, P. Jacobs, ChemPhysChem 2006, 7, 1142–
148.
Acknowledgements
1
The authors wish to acknowledge the support of INVISTA
Textiles (UK) Limited. C.J.K gratefully acknowledges funding
from the National Science Foundation Major Research
Instrumentation Programme (GR# MRI/DMR-1040229), X.L.
wish to acknowledge the support of the National Nature Science
Foundation of China grant (Project number 21673273).
[
[
23] W. Partenheimer, Catal. Today 1995, 23, 69–158.
24] M. Conte, H. Miyamura, S. Kobayashi, V. Chechik, Chem. Commun.
2
010, 46, 145–147.
25] I. Hermans, P. A. Jacobs, J. Peeters, Chem. Eur. J. 2006, 12, 4229–
240.
26] J.-R. Chen, H.-H. Yang, C.-H. Wu, Org. Process Res. Dev. 2004, 8,
52–255.
27] C. M. Jones, M. J. Burkitt, J. Am. Chem. Soc. 2003, 125, 6946–6954.
[
[
[
4
2
Keywords: cyclohexane oxidation • silver • palladium • STEM •
EPR
[28] M. Conte, K. Wilson, V. Chechik, Org. Biomol. Chem. 2007, 7, 1361–
367.
1
[
[
1]
2]
M. Eichelbaum, R. Glaum, M. Hvecker, K. Wittich, C. Heine, H.
Schwarz, C.-K. Dobner, C. Welker-Nieuwoudt, A. Trunschke, R.
Schlögl, ChemCatChem 2013, 5, 2318–2329.
[29] E. Finkelstein, G. M. Rosen, E. J. Rauckman, Arch. Biochem. Biophys.
1980, 200, 1–16.
[30] D. R. Duling, J. Magn. Reson. B 1994, 104, 105–110.
[31] M. Conte, K. Wilson, V. Chechik, Rev. Sci. Instrum. 2010, 81, 104102.
[32] M. Conte, X Liu, D. M. Murphy, S. H. Taylor, K. Whiston, G. J.
Hutchings, Catal. Lett. 2016, 146, 126–135.
U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R. S. da Cruz, M. C.
Guerreiro, D. Mandelli, E. V. Spinacé, E. L. Pires, Appl. Catal. A: Gen.
2
001, 211, 1–17.
C. Hess, M. H. Looi, S. B. A. Hamid, R. Schlögl, Chem. Commun. 2006,
51–453.
R. Y. Saleh, I. E. Wachs, S. S. Chan, C. C. Chersich, J. Catal. 1986, 98,
02–114
Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 2001, 343, 393–
27.
[
[
[
3]
4]
5]
[33] M. Novak, B. A. Brodeur, J. Org. Chem. 1984, 49, 1142–1144.
[34] S. L. Baum, I. G. M. Anderson, R. R. Baker, D. M. Murphy, C. C.
Rowlands, Anal. Chim. Acta 2003, 481, 1–13.
4
1
[35] M. J. Davies, T. F. Slater, Biochem. J. 1986, 240, 789–795.
[36] E. G. Janzen, C. A. Evans, J. P. Liu, J. Magn. Reson. 1973, 9, 513–516.
[37] P. Ionita, M. Conte, B. C. Gilbert, V. Chechik, Org. Biomol. Chem. 2007,
5, 3504–3509.
4
[
[
6]
7]
J.-M. Brégeault, Dalton Trans. 2003, 3289–3302.
X. Liu, Y. Ryabenkova, M. Conte, Phys. Chem. Chem. Phys. 2015, 17,
[38] M. Conte, H. Miyamura, S. Kobayashi, V. Chechik, J. Am. Chem. Soc.
2009, 131, 7189–7196.
715-731.
[
[
[
8]
9]
W. Yao, Y. Chen, L. Min, H. Fang, Z. Yan, H. Wang, J. Wang, J. Mol.
Catal. A: Chem. 2006, 246, 162–166.
[39] B. W. Griffin, Can. J. Chem. 1982, 60, 1463–1473.
[40] R. Raja, G. Sankar, J. M. Thomas, J. Am. Chem. Soc. 1999, 121,
11926–11927.
C.-C. Guo, M.-F. Chu, Q. Liu, Y. Liu, D.-C. Guo, X.-Q. Liu, Appl. Catal.
A: Gen. 2003, 246, 303–309.
[41] W. Adam, B. Nestler, J. Am. Chem. Soc. 1993, 115, 7226–7231.
[42] J. A. Lopez-Sanchez, N. Dimitratos, P. Miedziak, E. Ntainjua, J. K.
Edwards, D. Morgan, A. F. Carley, R. Tiruvalam, C. J. Kiely, G. J.
Hutchings, Phys. Chem. Chem. Phys. 2008, 10, 1921–1930.
10] X. Liu, M. Conte, M. Sankar, Q. He, D. M. Murphy, D. Morgan, R. L.
Jenkins, D. Knight, K. Whiston, C. J. Kiely, G. J. Hutchings, Appl. Catal.
A: Gen. 2015, 504, 373–380.
[
43] J. A. Lopez-Sanchez, N. Dimitratos, C. Hammond, G. L. Brett, L.
Kesavan, S. White, P. Miedziak, R. Tiruvalam, R. L. Jenkins, A. F.
[
[
11] M. Conte, X. Liu, D. M. Murphy, K. Whiston, G. J. Hutchings, Phys.
Chem. Chem. Phys. 2012, 14, 16279–16285.
Carley, D. Knight, C. J. Kiely, G. J. Hutchings,
Nat. Chem. 2011, 3,
12] Y. Lei, F. Mehmood, S. Lee, J. Greeley, B. Lee, S. Seifert, R. E. Winans,
J. W. Elam, R. J. Meyer, P. C. Redfern, D. Teschner, R. Schlögl, M. J.
Pellin, L. A. Curtiss, S. Vajda, Science 2010, 328, 224–228.
13] N. Dimitratos, L. Prati, Gold Bull. 2005, 38, 73–77.
551-556.
[44] C. Walling, S. A. Buckler, J. Am. Chem. Soc. 1955, 77, 6032–6038.
[45] Simulations were carried out using WinSim software: http://
www.niehs.nih.gov/research/resources/software/tox-
pharm/tools/index.cfm
[
[
14] U. Schuchardt, W. A. Carvalho, R. Pereira, E. V. Spinacé in:
Cyclohexane Oxidation: Can Gif Chemistry Substitute for the Classical
Process? (The Activation of Dioxygen and Homogeneous Catalytic
Oxidation, Eds.: D. H. R. Barton, A. E. Martell, D. T. Sawyer, Springer
US, New York, 1993, pp. 243–255.
[46] M. Conte, Y. Ma, C. Loyns, P. Price, D. Rippon, V. Chechik, Org.
Biomol. Chem. 2009, 7, 2685–2687.
[
[
15] M. Conte, J. A. Lopez-Sanchez, Q. He, D. J. Morgan, Y. Ryabenkova, J.
K. Bartley, A. F. Carley, S. H. Taylor, C. J. Kiely, K. Khalid, G.J.
Hutchings, Catal. Sci. Technol. 2012, 2, 105–112.
16] A. Villa, D. Wang, G. M. Veith, F. Vindigni, L. Prati, Catal. Sci. Technol.
2013, 3, 3036–3041.
[
[
[
[
17] H. N. Vasan, C. N. R. Rao, J. Mater. Chem. 1995, 5, 1755–1757.
18] I. Hermans, P. A. Jacobs, J. Peeters, Chem. Eur. J. 2007, 13, 754–761.
19] B. P. C. Hereijgers, B. M. Weckhuysen, J. Catal. 2010, 270, 16–25.
20] I. A. Salem, M. El Maazawi, A. B. Zaki, Int. J. Chem. Kinet. 2000, 32,
643–666.
8
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201605941
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Au-Pd/MgO nanoparticles were found
to be active for the selective oxidation
of cyclohexane to cyclohexanol and
cyclohexanone.
Xi Liu, Marco Conte, Qian He, David W.
Knight, Damien M. Murphy, Stuart H.
Taylor, Keith Whiston, Christopher J.
Kiely, Graham J. Hutchings,*
Page No. – Page No.
Ag
Catalytic partial oxidation of
cyclohexane by bimetallic Ag/Pd
nanoparticles over magnesium oxide
O2
Layout 2:
FULL PAPER
Author(s), Corresponding Author(s)*
((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))
Page No. – Page No.
Title
Text for Table of Contents
9
This article is protected by copyright. All rights reserved.