Selective epoxidation of cyclohexene with molecular oxygen on catalyst of nanoporous Au integrated with MoO3 nanoparticles

Article abstract of DOI:10.1016/j.apcata.2016.10.010

Selective oxidation of olefins to desired products such as epoxides is highly demanded in chemical industry. It remains challenging to achieve high selectivity for production of epoxides over heterogeneous catalysts through using molecular oxygen as the oxidant. Nanoporous reverse catalysts (MoO₃@np-Au) consisting of pure nanoporous gold (np-Au) and MoO₃ nanoparticles anchored on Au ligaments were synthesized for selective oxidation of cyclohexene with molecular oxygen. By controlling the loading of molybdenum and thermal treatment condition, MoO₃ nanoparticles with size of ～5?nm were uniformly anchored on the surface of gold ligaments (30–50?nm) of pure nanoporous gold (np-Au). These synthesized MoO₃@np-Au catalysts exhibited high selectivity of 58%–73% for production of cyclohexene oxide at conversion of 4%–11% of cyclohexene by using molecular oxygen as the oxidant. Compared to MoO₃@np-Au, the selectivity for the production of cyclohexene oxide on pure np-Au catalyst is only 6% under the same catalytic condition as that on MoO₃@np-Au. The observed high selectivity for production of cyclohexene oxide on MoO₃@np-Au can be rationalized with a bi-functional mechanism of a reverse metal/oxide catalyst. The in-situ formed surface molybdenum oxo-peroxo species are suggested to be responsible for selective oxidation of cyclohexene to cyclohexene oxide, while the MoO₃/Au interface activates molecular oxygen to regenerate the molybdenum oxo-peroxo active centers.

Full text of DOI:10.1016/j.apcata.2016.10.010

Accepted Manuscript
Title: Selective Epoxidation of Cyclohexene with Molecular
Oxygen on Catalyst of Nanoporous Au Integrated with MoO3
Nanoparticles
Author: Jian Dou Franklin Feng Tao
PII:
S0926-860X(16)30502-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.10.010
APCATA 16026
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
18-7-2016
28-8-2016
11-10-2016
Please cite this article as: Jian Dou, Franklin Feng Tao, Selective Epoxidation
of Cyclohexene with Molecular Oxygen on Catalyst of Nanoporous
Au Integrated with MoO3 Nanoparticles, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.10.010
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Selective Epoxidation of Cyclohexene with Molecular Oxygen on Catalyst
of Nanoporous Au Integrated with MoO3 Nanoparticles
Jian Doua,b, Franklin (Feng) Taoa,b,*
a Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, KS, 66045, USA
b Department of Chemistry, University of Kansas, Lawrence, KS 66045, USA
* To whom all correspondences should be sent to. Email: franklin.feng.tao@ku.edu.
Corresponding Author
*Email: franklin.feng.tao@ku.edu
1

Graphical abstract
Selective oxidation of cyclohexene on nanoporous Au
integrated with MoO3 nanoparticles
Research highlights
 MoO3@np-Au catalyst is active for selective oxidation of cyclohexene with molecular oxygen.
 MoO3@np-Au catalysts exhibited high selectivity of 58%-73% for cyclohexene oxide at
conversion of 4%-11% of cyclohexene.

MoO3@np-Au catalyst behaves as a bi-functional catalyst.
2

Abstract
Selective oxidation of olefins to desired products such as epoxides is highly demanded in chemical
industry. It remains challenging to achieve high selectivity for production of epoxides over heterogeneous
catalysts through using molecular oxygen as the oxidant. Nanoporous reverse catalysts (MoO3@np-Au)
consisting of pure nanoporous gold (np-Au) and MoO3 nanoparticles anchored on Au ligaments were
synthesized for selective oxidation of cyclohexene with molecular oxygen. By controlling the loading of
molybdenum and thermal treatment condition, MoO3 nanoparticles with size of ~5 nm were uniformly
anchored on the surface of gold ligaments (30-50 nm) of pure nanoporous gold (np-Au). These
synthesized MoO3@np-Au catalysts exhibited high selectivity of 58%-73% for production of cyclohexene
oxide at conversion of 4%-11% of cyclohexene by using molecular oxygen as the oxidant. Compared to
MoO3@np-Au, the selectivity for the production of cyclohexene oxide on pure np-Au catalyst is only 6%
under the same catalytic condition as that on MoO3@np-Au. The observed high selectivity for production
of cyclohexene oxide on MoO3@np-Au can be rationalized with a bi-functional mechanism of a reverse
metal/oxide catalyst. The in-situ formed surface molybdenum oxo-peroxo species are suggested to be
responsible for selective oxidation of cyclohexene to cyclohexene oxide, while the MoO3/Au interface
activates molecular oxygen to regenerate the molybdenum oxo-peroxo active centers.
Keywords: Cyclohexene oxidation, catalyst, epoxidation
3

1. Introduction
Selective oxidation of olefins is an extremely important reaction in chemical industry.1 For instance,
the oxidized products such as epoxides are widely used for production of resins, paints, surfactants and
intermediates for organic synthesis.2 Epoxidation of the simplest olefins, ethylene to ethylene oxide has
long been established in industry by reacting ethylene with molecular oxygen over silver catalysts.
However, it remains challenging for direct oxidation of higher olefins such as propylene and cyclohexene
using molecular oxygen instead of peroxide as oxidant.3 Generally, oxidation of higher olefins can proceed
with various molecular catalysts or supported metal catalysts through using peroxides such as hydrogen
peroxide or tert-butyl hydroperoxide as an oxidant4-9. However, the high cost of peroxide oxidants and
costly footprint of handling large amount of peroxides have a considerable influence on whether oxidation
of higher olefins such as cyclohexene should be processed with peroxides at industrial scale or not.10-17
Molecular oxygen, a cheap and readily available gas is definitely a green, low-cost oxidant for selective
oxidation reactions. Thus, it is highly desirable to develop a catalyst active for selective epoxidation of
cyclic olefins using molecular oxygen at a mild condition.
Gold-based catalysts have been widely used for selective oxidation of olefins.18-30 For example,
activated carbon supported gold nanoparticles with size of 5-50 nm have been successfully utilized for
oxidation of cyclohexene with hydroperoxide as oxidant by using 1,2,3,4-tetramethylbenzene as solvent.18
In addition, gold nanoparticles with a size less than 10 nm supported on modified silica supports were
effective in oxidation of cyclohexene using only molecular oxygen as the oxidant 19-21; however, the
selectivity for production of cyclohexene oxide (or called epoxide) on these catalysts is only about 9-
14%.19,21
4

Oxidation of cyclohexene has also been investigated under solvent free conditions.31-34 For example,
using silica or manganese oxide supported gold catalysts, selectivity for production of epoxide (i.e.,
cyclohexene oxide) is generally lower than 10% when molecular oxygen was used.31,32 In order to improve
the selectivity for production of cyclohexene oxide, gold nanoclusters (i.e., Au9 and Au101) were anchored
onto WO3 nanoparticles35; the selectivity for production of cyclohexene oxide on Au101/WO3 catalyst was
improved largely to 35% comparing to 7% of Au101/SiO2 catalyst when molecular oxygen or peroxide is
used as the oxidant.35 This difference suggests the promotion effect of the support, WO3 on catalytic
selectivity of cyclohexene oxide. Even though these efforts have been made in using molecular oxygen
as oxidant and a reasonable conversion of cyclohexene in selective oxidation has achieved, high
selectivity for production of epoxides by using molecular oxygen as an oxidant in a solvent free condition
has still remained challenging.
It is well known that MoO3-based catalyst is efficient in selective oxidation of hydrocarbon while
peroxide is used as the oxidant. For example, epoxidation of cyclohexene on MoVIO3 supported on silica
using hydrogen peroxide or tert-butyl hydroperoxide as the oxidant was investigated. Furthermore,
molybdenum based oxides and heteropolyacids have been reported as excellent promoter or support for
noble metals (i.e., Pt and Au) for hydrogenation, water gas shift and electrochemical reactions.36-40 As
reported in literature, np-Au is highly active in activating molecular oxygen;41,42 this activated molecular
oxygen can spillover to surface of MoO3 where oxidization of cyclohexene to products is performed. Thus,
we expect that the integration of MoO3 nanoparticles on internal surface of ligaments of np-Au could tune
catalytic performances of np-Au in selective oxidation of cyclohexene with molecular oxygen.
5

Here we synthesized the catalyst MoO3@np-Au consisting of np-Au and MoO3 nanoparticles
anchored on Au ligaments in the pores of np-Au through different pre-treatments of the catalyst precursor.
They exhibit selectivity up to 73% for epoxidation of cyclohexene to cyclohexene oxide which is much
higher than pure np-Au.
2. Experimental
2.1 Synthesis of nanoporous gold catalysts.
Nanoporous gold sponge (np-Au) was synthesized following literature method.43 In a typical
synthesis of np-Au sponge nanocatalyst, 10 mL of 4.5 mM polyvinylpyrrolidone (PVP, (C6H9NO)n,
Sigma-Aldrich) solution (50 mg PVP in 100 mL ethanol, C2H5OH, Sigma-Aldrich) was mixed with 10
mL of 30 mM gold (III) chloride hydrate (HAuCl4·xH2O, Sigma-Aldrich) solution, followed by stirring
at room temperature for 15 min. After that, 10 mL of freshly prepared 0.1 M sodium borohydride (NaBH4,
Sigma-Aldrich) was poured rapidly into the mixed solution under stirring. The mixture was further stirred
for 15 min before centrifuged and then washed with deionized water. The solid product was dried at room
temperature for overnight, and was denoted as np-Au. The catalyst with integrated np-Au and Mo was
prepared by impregnation with ammonium heptamolybdate ((NH4)6Mo7O24·4H2O, Alfa Aesar) solution.
The impregnated samples were dried at 60oC followed by calcination in statiC-air and/or hydrogen
(5%H2/Ar, Prax Air) at 300oC for 1 h. The final samples were denoted as X%Mo@np-Au-T (X = 1.0 and
2.0, which stands for the percentage of loading of Mo, T stands for the calcination temperature).
6

2.2 Selective oxidation of cyclohexene.
Liquid phase oxidation of cyclohexene was carried out in an autoclave with a capacity of 100 mL, on
which K-type thermocouple and pressure gauge were integrated. The probe of thermocouple was
submerged to the liquid for an accurate measurement of catalytic temperature. In a typical run, 15 mL of
cyclohexene was mixed with 0.5 mL of undecane (C11H24, as an internal standard, Sigma-Aldrich),
followed by adding 10 mg of catalyst and 0.27 mL of tert-butyl hydroperoxide solution (TBHP, C4H10O2,
5.5 M in decane, Sigma-Aldrich). The mole ratio of TBHP to cyclohexene was fixed at 1/100. The reaction
proceeded under vigorous stirring at 80oC for 4 h. Gas chromatography–mass spectrometry (GC-MS,
Agilent, 6890N) was used for qualitatively identification of reaction products. Quantitative determination
of oxidation products (i.e., cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one) was achieved
with gas chromatography equipped with a flame ion detector (FID) and a Restek packed column (30 m ×
0.32 mm × 0.25 μm). Commercial samples of cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-
1-one were used as external standards for calibration of GC peaks. The concentration of cyclohexenyl
hydroperoxide was determined by iodometric titration method.44 Typically, 0.5 mL of analyte was mixed
with 40 mL of isopropyl alcohol and 2 mL of acetic acid, followed by heating to reflux. After that, 2.00 g
of sodium iodide was dissolved in 10 mLof isopropyl alcohol, which was then added to the above solution.
The resulted mixture was further refluxed for 5 min, followed by adding 5 mL of deionized water and
titrated with 0.100 M sodium thiosulfate.
7

2.3 Materials characterization.
The crystallographic structure of the solid samples was investigated using a Bruker X8 Proteum
diffraction system equipped with Helios multilayer optics, and APEX II CCD detector and a Bruker
MicroStar microfocus rotation anode X-ray source (Cu Kα radiation, λ = 1.54178 Å) operating at 45 kV
and 60 mA. Powders were suspended in Paratone N oil and placed into a nylon loop and mounted on a
goniometer head. The sample was scanned at a rotation rate of 2o/second. The dimension, morphology,
and chemical composition of the solid samples were examined using scanning electron microscopy (Versa
3D dual beam, FEI) with a silicon drift energy dispersive X-ray detector (Oxford Instruments), and high
resolution transmission electron microscopy (HRTEM/EDX, FEI Tecnai F20, 200 kV; and Zeiss Libra
200-80, 200 kV). The TEM samples were prepared by dispersion of nanoporous gold catalysts in ethanol
under sonication for 10 min. The suspension was dropped onto a copper mesh grid with lacey carbon film,
followed by drying at room temperature in air. Surface and texture properties of the samples were studied
by nitrogen adsorption-desorption isotherms (Quantachrome NOVA 2200e system) at 77 K. Prior to
measurements, the samples were degassed at 120oC for 6 h. Specific surface areas of the samples were
determined using Brunauer-Emmett-Teller (BET) method. The Mo loading in Mo/np-Au catalysts were
determined by optical emission spectroscopy with inductively coupled plasma as excitation source (ICP-
OES, Horiba Jobin Yvon JY 2000).
8

3. Results and Discussion
3.1 Preparation and characterization of catalyst of np-Au with integrated MoO3 nanoparticles
(MoO3@np-Au)
Pure nanoporous gold (np-Au) was synthesized through a wet chemistry route using PVP as surfactant
and capping agent in water/ethanol mixed solvent (Figure 1). The crystallinity of as-prepared nanoporous
gold was examined with XRD. As shown in Figure S1, peaks at 38.3, 44.4, 64.6, 77.8, and 82.0o were
attributed to (111), (200), (220), (311), and (222) plane of metallic gold with a face centered cubic phase
(JCPDS 04-0784, space group: Fm-3m, a0 = b0 = c0 = 4.0786 Å). The morphology of as-prepared
nanoporous gold material was characterized by SEM. As presented in Figure S2, the nanoporous gold
consisted of interlinked gold ligaments with diameter of 30-45 nm. The void space between gold ligaments
formed the porous structure with pore diameter in the range of 30-60 nm.
In order to functionalize the nanoporous gold for selective oxidation of cyclohexene reaction,
molybdenum oxide was deposited into the internal surface of np-Au via impregnation. The impregnated
sample was calcined at 200-300oC for 1 h to decompose molybdenum precursor to form molybdenum
oxide nanoparticles on the surface of nanoporous gold. Generally, nanoporous gold materials are
thermally stable up to 300oC; coarsening of gold ligaments occurs when calcination temperature is above
300oC.45-47 On the other hand, the decomposition temperature of ammonium heptamolybdate is 190oC.
Thus, the calcination temperatures of 200 and 300oC were chosen in this work; they are high enough to
decompose the molybdenum precursor to form molybdenum oxide without obvious change of the
nanoporous structure of np-Au.
9

As shown in Figure 2, the porous structure of 1.0wt%Mo@np-Au-200oC catalyst was similar to the
as-synthesized pure nanoporous gold (Figure S2). The size of gold ligament of 1.0wt%Mo@np-Au-200oC
was about 30-50 nm. The void space between the gold ligaments formed the porous structure. The porosity
of pure and molybdenum oxide doped nanoporous gold catalysts were examined by nitrogen adsorption-
desorption method. As shown in Table 1, the specific surface area and pore volume of the as-synthesized
pure nanoporous gold was 2.9 m2/g and 0.078 mL/g, respectively. After doping with 1.0 wt% and 2.0 wt%
molybdenum precursor, the specific surface area decreased to 2.8 and 2.3 m2/g, and total pore volume
decreased to 0.047 and 0.023 mL/g, respectively. The decrease in surface area and total pore volume is
due to deposition of molybdenum oxide nanoparticles into the nanopores of gold sample. In order to
confirm that molybdenum oxide was deposited onto nanoporous gold, the prepared MoO3@np-Au
catalysts were examined by EDX mapping. As shown in Figure 2c-e, molybdenum element was uniformly
dispersed on the surface of nanoporous gold. Quantitative determination of molybdenum loading was
carried out by ICP analysis. Concentration of molybdenum was measured as 0.98wt% and 2.05wt% for
1.0wt%Mo@np-Au and 2.0wt%Mo@np-Au, respectively (Table 1). The morphology of deposited
molybdenum oxide was characterized by HRTEM. As shown in Figures 3a and 3b, MoO3 nanoparticles
with size of ~ 5 nm were dispersed on the surface of nanoporous gold. The lattice spacing of 0.29 and
0.32 nm in Figures 3c and 3d can be assigned to (130) and (021) plane of α-MoO3 (JCPDS 05-0508, space
group: Pbnm, a0 = 3.962 Å, b0 = 13.85 Å, and c0 = 3.697 Å). Thus, both lattice fringe and EDX of TEM
studies confirmed that MoO3 nanoparticles were formed on the surface of np-Au ligaments after thermal
treatment at 200oC in air.
10

The crystal structure of nanoporous gold catalyst (pure np-Au) was characterized with XRD
technique. As shown in Figure 4a, five peaks centered at 38.3, 44.5, 64.6, 77.7, and 82o were assigned to
(111), (200), (220), (311), and (222) plane of cubic phase gold metal (JCPDS 04-0784, space group: Fm-
3m, a0 = b0 = c0 = 4.0786 Å), respectively. After the integration of MoO3 nanoparticles to the pores, the
crystal structures of MoO3@np-Au catalysts remained unchanged. For example, the crystal structures of
MoO3@np-Au catalysts with doping of 1.0wt% (Figure 4b) and 2.0wt% Mo (Figure S3) were nearly
identical to that of pure nanoporous gold catalyst (Figure 4a) although MoO3 nanoparticles were clearly
observed by HRTEM (Figure 3); no diffraction peak of MoO3 was observed in Figure 4a and Figure S3.
The lack of diffraction peak in XRD of MoO3 in the MoO3@np-Au is due to the low loading of
molybdenum. The molybdenum oxide modified nanoporous catalyst was further characterized by Raman
spectroscopy. As shown in Figure S4, the peak at 990 cm-1 could be assigned to terminal Mo=O bond of
MoO3 nanoparticles.48
3.2 Selective oxidation of cyclohexene
The above prepared pure np-Au and MoO3@np-Au were used for selective oxidation of cyclohexene
in liquid phase under solvent free condition. Pure oxygen was used as the sole oxidant and tert-butyl
hydroperoxide was used as the initiator. As shown in Figure 5 and entry 1 of Table 2, around 40% of
cyclohexene was oxidized on the pure nanoporous gold catalyst at 90oC after reaction of 4 h. The high
conversion results from the mechanism of radical chain reaction of oxidation of cyclohexene on pure np-
Au as proposed in literature.31,49. The major product formed through the catalysis on pure nanoporous
11

gold was cyclohexenyl hydroperoxide; the selectivity for the desired cyclohexene oxide was only 7%.
The catalytic performance on np-Au is consistent with literature results on cyclohexene oxidation over
Au nanoparticles supported on SiO2 under solvent-free condition by using molecular oxygen as
oxidant.31,35
Compared to pure np-Au, the conversion of cyclohexene on 1wt%MoO3@np-Au was dropped to
around 4.4%, and the selectivity of cyclohexene oxide was significantly increased from 7% to about 61%
(Figure 5 and Entry 2 in Table 2). Industrial transformation of cyclohexane is in fact processed at a low
conversion in order to avoid formation of excessive undesired byproducts.50,51 Thus, the conversion of
cyclohexene of 4.4% is feasible for industrial production. In the blank experiment without using any
catalyst, the major product formed was cyclohexenyl hydroperoxide. By comparing the blank experiment
with pure nanoporous gold and MoO3@np-Au catalysts (entry 11 vs entry 1 vs entry 2), it can be found
that the selectivity of reaction products varied significantly at similar conversion of 3.2-4.4%. For blank
experiment, no cyclohexene oxide was formed at conversion of 3.2%. With using pure nanoporous gold
catalyst (np-Au-200oC-air), the selectivity of cyclohexene oxide was increased to 5.7%. By modification
nanoporous gold with molybdenum oxide (1.0wt%MoO3@np-Au-200oC-air), the selectivity of
cyclohexene oxide was largely increased to 61.2%, which clearly demonstrate the importance of Mo and
Au in shifting the products selectivity of cyclohexene oxidation. With further increase of the molybdenum
loading to 2wt% (2.0wt%MoO3@np-Au), the conversion of cyclohexene can increase to 11.1%, while the
selectivity for production of cyclohexene oxide still maintained at about 58.6% (entry 3 of Table 2 and
Figure 5). Compared to the high conversion of cyclohexene on pure np-Au, MoO3@np-Au exhibit a low
conversion of cyclohexene but much higher selectivity for production of cyclohexene oxide under the
12

exact same catalytic condition (reaction temperature, molecular oxygen, and solvent free). This distinct
difference in catalytic performances between pure np-Au and Mo@np-Au suggests that MoO3@np-Au
catalyzes the selective oxidation of cyclohexene via a pathway different from the radical reaction route
proposed for pure nanoporous gold catalyst in literature.31,49 Hot filtration has been established as a
standard method to test catalyst leaching.52,53 In present work, the cyclohexene reaction was carried out
in pressurized reactor, and it is difficult to apply the hot filtration method for the evaluation. Thus the
molybdenum loading was examined before and after reaction to verify possible leaching of molybdenum
oxide during reaction. As shown in Figure S4, the molybdenum content was similar before and after
cyclohexene oxidation, which suggests that there is no or no significant leaching of molybdenum oxide
during the reaction. Furthermore, molybdenum oxide is known to have negligible solubility in nonpolar
solvent.54 As nonpolar cyclohexene was used as reactant and solvent in the reaction, it further suggests
that leaching of molybdenum oxide into cyclohexene is unlikely in present work. Furthermore, the carbon
balance for the reaction over nanoporous gold catalysts has been calculated and presented in Table S1. As
can be seen, the total carbon numbers before and after reaction are similar for pure and molybdenum oxide
modified nanoporous gold catalysts, which suggests the major products of this reaction are oxygenated
cyclohexenes.
Homogeneous molybdenum-based molecular catalyst can oxidize cyclohexene to cyclohexene oxide
when peroxide was used as oxidant;55,56 in this homogeneous catalysis, the key step is the activation of
molybdenum atoms of the molecular catalyst by peroxide to form molybdenum oxo-peroxo species which
act as active catalytic site for selective epoxidation of cyclohexene.55,56 Inspired by the proposed pathway
of Mo-based molecular catalyst for selective epoxidation of cyclohexene,55,56 We have proposed a
13

pathway to rationalize the selective oxidation of cyclohexene to cyclohexene oxide over MoO3@np-Au
catalysts. As shown in Figure 6, the initial molybdenum peroxo species (a4 in Figure 6) was formed on
polymeric molybdenum oxide through the reaction of initiator tert-butyl hydroperoxide with surface
polymeric molybdenum oxide (a1 in Figure 6). The formed molybdenum peroxo species act as active
sites. The peroxide oxygen atoms (-O-O-) of the molybdenum peroxo species (b1) attacked the double
bond of cyclohexene directly to produce cyclohexene oxide (b3), by which the molybdenum peroxo
species was reduced to b3 in Figure 6. The reduced form (b3 or c1 in Figure 6) can be oxidized by the
oxygen activated by Au at the Au-MoO3 interface (c2), restoring the catalytic site (c4) for next cycle of
epoxidation of cyclohexane. In this pathway, the formation of cyclohexene oxide occurs at the interface
of Au and MoO3. As shown in Figures 3c and 3d, TEM studies revealed that the surface of nanoporous
gold was partially covered by molybdenum oxide nanoparticles. Au-MO3 interface can be clearly
identified in Figures 3c and 3d. The region consisting of both MoO3 nanoparticles and Au can be
identified readily.
The effect of catalytic temperature on the conversion and selectivity was investigated by varying the
reaction temperature from 80 to 100oC (Figure 7 and Table 2). As expected more cyclohexene was
oxidized with the increase of temperature. For instance, the conversion of cyclohexene was increased to
36% at 100oC. Unfortunately, the selectivity for production of cyclohexene oxide at 100oC dropped
significantly to 12.9%. Meanwhile, the selectivity for production of cyclohexenyl hydroperoxide at 100oC
increased to 30.5% (entry 8 of Table 2). It seems that cyclohexenyl hydroperoxide accumulated very fast
at 100oC (entry 8 of Table 2), and the reaction pathway switched to a radical chain mechanism at this
14

relatively high temperature. Thus, 80-90oC is the optimum temperature at this catalytic condition which
offer catalytic selectivity for production of epoxide of 58.6%-73%.
The effect of thermal treatment temperature of MoO3@mp-Au before catalysis on its catalytic
performance was investigated on MoO3@np-Au annealed at 200oC in air and MoO3@np-Au annealed at
300oC in air. As shown in Figure 8 and entry 4 of Table 2, the conversion of cyclohexene on 10 mg of
1.0wt%MoO3@np-Au (annealed at 300oC in air) increased to 43.2% in contrast to the catalyst annealed
at 200oC in air. However, the selectivity of cyclohexene oxide decreased largely to 16.1% and the
selectivity of cyclohexenyl hydroperoxide increased to 43.6%. The decrease and increase of selectivity
for production of epoxide and cyclohexenyl hydroperoxide, respectively on 1.0wt%MoO3@np-Au
(annealed at 300oC in air) suggests that the reaction pathway on the catalyst calcined at 300oC in air
follows a radical chain mechanism similar to that of catalyst MoO3 supported on silica (entry 10, table
2)31,49 In fact, TEM studies of MoO3@np-Au (annealed at 300oC in air) showed that the surface of the
ligaments of this catalyst was nearly fully covered with molybdenum oxide nanoparticles (Figure 9),
which minimized the interfacial sites of Au and MoO3 nanoparticles since most Au surface was covered
with MoO3 nanoparticles. These MoO3 nanoparticles formed an ultrathin film of MoO3. Thus, it is
reasonable to suggest that the MoO3@mp-Au (annealing at 300oC in air) behaves like a MoO3 catalyst.
The distinct different catalytic performance between MoO3@np-Au calcined at 200oC and 300oC
suggested that the molybdenum oxide/gold interface is critical for activation of molecular oxygen (c2 in
Figure 6), by which the oxygen can be activated to generate active oxo-peroxo species on MoO3 surface
for the formation of cyclohexene oxide (c4 in Figure 6).
15

The effect of pretreatment condition was further studied by reducing the air calcined and as-
synthesized Mo/npAu catalysts in hydrogen. As shown in entry 2 and entry 5 of Table 2, the conversion
of cyclohexene was 4.4 and 5.3% for the catalyst treated at 200oC in air and hydrogen at 200oC
respectively. The selectivity of cyclohexene oxide and cyclohexenol was 63.0 and 30.5% for npAu-200oC
H2, which was similar as that of npAu-200 oC in air. It suggested that oxidation or reduction atmosphere
o
at 200 C has negligible effect on the catalytic performance. It was due to that only decomposition of
ammonium heptamolybdate occurred at 200oC, and reduction of molybdenum happened at a higher
o
temperature (i.e., 300 C). For the Mo/npAu-200oC-air-300oC H2, the selectivity of cyclohexene oxide
was further increased to 73.1%, while the conversion of cyclohexene was kept as 3.5% (entry 6 of Table
2). The increased selectivity was possibly due to better dispersion of molybdenum oxide on the
nanoporous gold surface, with help to hydrogen treatment at 300 oC.
X-ray photoelectron spectroscopy was used to examine the oxidation state of Mo and Au of
MoO3@np-Au and pure np-Au. For pure np-Au, one photoemission feature of Au 4f7/2 was observed at
83.9 eV, which is typical for gold at a metallic state (Figure 10a); the O 1s peak appeared at 532.6 and
534.8 eV was attributed to surface hydroxyl group and adsorbed water, respectively (Figure 10b).57 There
was no N 1s peak observed from the wide scan XPS spectrum (Figure S5), which suggests that most of
the surfactant (e.g., polyvinylpyrrolidone) used for preparation of nanoporous gold has been removed
through washing and calcination steps.58 For 1.0wt%MoO3@np-Au annealed at 200oC in air, the
nanoporous gold was also in metallic state as evidenced by the Au 4f7/2 peak at 84.0 eV (Figure 11a). The
fitted Mo 3d5/2 peak was centered at 232.2 eV, which was attributed Mo6+.56 The O 1s photoemission
feature was deconvoluted into three peaks at 530.4, 532.8, and 534.3 eV, which could be assigned to
16

oxygen atoms of molybdenum oxide (530.4 eV), surface hydroxyl group (532.8 eV), and surface adsorbed
waters (534.3 eV) species, respectively.54,57,59 Thus, XPS studies confirmed the metallic state of Au in
surface region and the valence state (VI) of MoO3 nanoparticles for MoO3@np-Au. Furthermore, no N 1s
peak was observed in the wide scan (Figure S6), suggesting that most of the molybdenum precursor
(ammonium heptamolybdate) was decomposed during thermal treatment in air.
Structure of the spent 1.0wt%MoO3@np-Au catalysts was characterized by STEM/EDX analysis. As
shown in Figure 12, the porous structure of 1.0wt%MoO3@np-Au almost preserved after cyclohexene
oxidation. The diameters of gold ligaments were about 20 nm, which was similar to that of catalyst before
catalysis. Furthermore, the STEM/EDX showed that MoO3 nanoparticles remained nearly uniformly
dispersed on the nanoporous gold surface.
4. Conclusions
In summary, bi-functional catalyst 1.0wt%MoO3@np-Au consisting of pores of nanoporous gold
catalysts and MoO3 nanoparticles anchored on surface of pores, were successfully synthesized via a wet
chemistry synthetic route. On pure nanoporous gold catalyst, cyclohexene was mainly oxidized to
cyclohexenyl hydroperoxide, which functioned as radical chain propagator to produce other oxidized
products (i.e., cyclohexene oxide, cyclohexenol, and cyclohexenone) via radial chain mechanism. The
integration of MoO3 nanoparticles onto Au ligaments of nanoporous gold significantly increased the
selectivity for production of epoxide in cyclohexene oxidation reaction. Selectivity for production of
cyclohexene oxide up to 73% was achieved over 1.0wt%MoO3@np-Au catalyst. It is expected that the
17

high selectivity of production of cyclohexene oxide on MoO3@np-Au results from the formation of active
sites, molybdenum peroxo species, which oxidize cyclohexene to cyclohexene oxide. The molybdenum
peroxo species can be regenerated by oxygen activated by Au atoms at the interface of Au and MoO3
nanoparticles supported on the surface Au ligaments in np-Au.
Acknowledgement
The authors gratefully acknowledge the financial support provided by the Integrated Mesoscale
Architectures for Sustainable Catalysis (IMASC), Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0012573. The
authors also thank Dr. Zili Wu from Oak Ridge National Laboratory for the Raman characterization.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/
18

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NaBH4
RT
Drying
RT
+
(NH4)6Mo7O24
RT
Drying
60 oC
Air and/or H2
200-300 oC
Figure 1. Scheme of synthesis of catalyst integrating nanoporous Au with MoO3 nanoparticles.
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Figure 2. (a, b) SEM images and (c-e) EDX mapping of 1.0wt%MoO3@np-Au (annealed at 200oC in air).
The red arrow in (b) marks one ligament in a pore of 1.0wt%MoO3@np-Au
.
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0.29 nm
0.29 nm
0.32 nm
Figure 3. (a, b) HRTEM images of 1.0wt%MoO3@np-Au (annealed at 200oC in air) and (c, d) visual
enlargement of the selected sections in (a) and (b).
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Figure 4. XRD patterns of (a) pure nanoporous gold (np-Au) and (b) 1.0wt%MoO3@np-Au (annealed at
200oC in air).
27

70
60
50
40
30
20
10
0
Conversion
Cyclohexene oxide
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
Cyclohexenyl hydroperoxide
0%
1.0%
2.0%
Molybdenum content
Figure 5. Selective oxidation of cyclohexene over catalysts of MoO3@np-Au annealed at 200oC in air
The X-axis is the concentrations of three catalysts including pure np-Au, 1.0wt%MoO3@np-Au (annealed
at 200oC in air), 2.0wt%MoO3@np-Au (annealed at 200oC in air). Reaction condition: 15 ml cyclohexene,
0.5 ml undecane (internal standard), 0.27 ml tert-butyl hydroperoxide, 10mg catalyst, 4 bar O2, catalysis
temperature: 90oC, catalysis time: 4 h.
28

O
H
H
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
HO
O
O
O
O
O
O
O
O
O
Mo
Mo
Mo
Mo
Mo
Mo
O
O
O
O
O
O
O
O
Mo
Mo
Mo
Mo
Mo
Mo
O
H
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(a1)
(a2)
(Oa3)
(a4)
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Mo
Mo
Mo
O
O
O
O
O
O
O
O
Mo
Mo
Mo
Mo
Mo
Mo
+
+
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(b1)
(b2)
(b3)
O
O
O
O
O
O
Au
O
O
O
O
Au
O
O
O
O
O
O
O
O
O
Au
O
O
O
O
O
O
O
O
Mo
Mo
Mo
Mo
Mo
Mo
O
O
O
O
Mo
Mo
Mo
O
O
O
O
Mo
Mo
Mo
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
(c1)
(c2)
(c3)
(c4)
Figure 6. Suggested pathway for epoxidation of cyclohexene over MoO3@np-Au catalyst.
29