Direct methylation of N-methylaniline with CO2/H2 catalyzed by gold nanoparticles supported on alumina

Article abstract of DOI:10.1039/c5ra20991a

Small gold nanoparticles (～3 nm) loaded onto various supports have been prepared by a deposition-precipitation method and studied for direct methylation of N-methylaniline with CO₂/H₂. Among the catalysts examined, an acid-base bifunctional support γ-alumina supported gold catalyst (Au/γ-Al₂O₃) exhibits the best catalytic performance. Au/γ-Al₂O₃ catalysts with controlled mean Au particle sizes (1.8-8.3 nm) have also been successfully prepared by regulating the concentration of HAuCl₄ in solution, aging time, aging temperature and mole ratio of urea to gold in the process of deposition-precipitation with urea. The turnover frequency (TOF) values for direct methylation of N-methylaniline with CO₂/H₂ increase on decreasing the mean size of Au nanoparticles (from 8.3 to 1.8 nm), showing that methylation of N-methylaniline with CO₂/H₂ is a structure-sensitive reaction. A fast increase in TOF occurs when the mean Au particle size becomes smaller than 3 nm. Through TEM (transmission electron microscope), gold L₃-edge XAFS (X-ray absorption fine structure) and CO₂- and NH₃-TPD (temperature programmed desorption) analysis, we can conclude the Au particle sizes, oxidation state of the gold species and acid-base properties of the supports are responsible for the high catalytic activity of direct methylation of N-methylaniline with CO₂/H₂.

Full text of DOI:10.1039/c5ra20991a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Direct methylation of N-methylaniline with CO2/H2
catalyzed by gold nanoparticles supported on
alumina†
Cite this: RSC Adv., 2015, 5, 99678
a
b
b
a
b
*
*
Gao Tang, Hong-Liang Bao, Chan Jin, Xin-Hua Zhong and Xian-Long Du
Small gold nanoparticles (ꢀ3 nm) loaded onto various supports have been prepared by a deposition–
precipitation method and studied for direct methylation of N-methylaniline with CO2/H2. Among the
catalysts examined, an acid–base bifunctional support g-alumina supported gold catalyst (Au/g-Al2O3)
exhibits the best catalytic performance. Au/g-Al2O3 catalysts with controlled mean Au particle sizes (1.8–
8.3 nm) have also been successfully prepared by regulating the concentration of HAuCl4 in solution,
aging time, aging temperature and mole ratio of urea to gold in the process of deposition–precipitation
with urea. The turnover frequency (TOF) values for direct methylation of N-methylaniline with CO2/H2
increase on decreasing the mean size of Au nanoparticles (from 8.3 to 1.8 nm), showing that methylation
of N-methylaniline with CO2/H2 is a structure-sensitive reaction. A fast increase in TOF occurs when the
mean Au particle size becomes smaller than 3 nm. Through TEM (transmission electron microscope),
gold L3-edge XAFS (X-ray absorption fine structure) and CO2- and NH3-TPD (temperature programmed
desorption) analysis, we can conclude the Au particle sizes, oxidation state of the gold species and acid–
base properties of the supports are responsible for the high catalytic activity of direct methylation of N-
methylaniline with CO2/H2.
Received 10th October 2015
Accepted 10th November 2015
DOI: 10.1039/c5ra20991a
www.rsc.org/advances
(DMC),9 formamides10 and carboxylic acids,11 activated by rele-
vant reactive substrates such as epoxides, alcohols, amines and
Introduction
olens respectively.
Carbon dioxide (CO2) is a greenhouse gas and the accumulation
of a huge amount of CO2 has caused signicant and negative
effects on the global environment.1 It is crucial, therefore, to
take effective measures to reduce CO2 emission and utilize CO2
resources. In this regard, carbon dioxide has been used as an
inexpensive and nontoxic C1 feedstock for producing
commodity chemicals, fuels and various materials in the last
few decades.2–4 Owing to its thermodynamic stability, highly
efficient activation of CO2 and a strong thermodynamic driving
force are required for efficient transformation of CO2. In this
respect, only a handful of industrial processes have been real-
ized for CO2 utilization, such as the production of urea, salicylic
acid, and carbonates.5 Additionally, the catalytic hydrogenation
of CO2 to methanol, formic acid or its derivatives have been
reported by using various catalysts.6,7 Carbon dioxide can also
be converted into cyclic carbonates,8 dimethyl carbonate
Methylamines, including dimethylamine, trimethylamine,
methylamine and dimethylformamide (DMF), have been
usually used as platform chemicals for preparing fertilizers,
fungicides, synthetic leathers or polymers.12 Hazardous alky-
lating agents, such as methyl iodide, dimethyl sulfate, and
dimethyl carbonate, are traditionally preferred for the prepa-
ration of methylamines.13 Meanwhile, dimethylsulfoxide and
formic acid have also been found to be the available carbon
sources for methylation of amines by catalysts.14 Great efforts on
the other hand have been made to develop new catalytic process
for preparation of N-methylated compounds using CO2 as a C1
building block in the absence of the toxic alkylating agents. In
early 2013, two catalytic systems were independently reported
by two research groups for the methylation of amines with CO2
and silanes by homogeneous catalysts, producing siloxane as
a by-product.15 Shortly aerwards, a green method for methyl-
ation of amines with CO2/H2 has been reported using homo-
geneous Ru-based catalysts by the groups of Klankermayer and
Beller, generating water as the only by-product.16 Lately, two
new metal-free homogeneous catalysts for methylation of
amines using CO2 as a C1 source, were developed by the groups
of Dyson and Cantat.17 Despite the signicant progress that has
been made in the area of amine methylation using CO2/H2 with
homogeneous systems, there are still few available relevant
aKey Laboratory for Advanced Materials, Institute of Applied Chemistry, East China
University of Science and Technology, Shanghai 200237, P.R. China. E-mail:
zhongxh@ecust.edu.cn
bKey Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied
Physics, Chinese Academy of Sciences, Jialuo Road 2019, Shanghai 201800, P.R.
China. E-mail: duxianlong@sinap.ac.cn
† Electronic supplementary information (ESI) available: Details containing gures
of TEM, catalysis activity, XRD. See DOI: 10.1039/c5ra20991a
99678 | RSC Adv., 2015, 5, 99678–99687
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
reports on the heterogeneously catalyzed methylation of amines deionized water at room temperature, the pH was adjusted to
with CO2/H2. Recently, N-methylation of amines with CO2/H2 9.0 by dropwise addition of NH4OH (2.5 M). The resultant hydro
was realized by heterogeneous catalysts such as CuAlOx,18 Pt– gel was washed with deionized water until free of NO3ꢁ ions.
MoOx/TiO2 (ref. 19) and Pd/CuZrOx (ref. 20) respectively. These The precipitate was then dried at 110 ꢂC overnight and calcined
ꢂ
heterogeneous systems however are limited by their low turn- at 500 C for 4 h in air. Then the particles were passed a 200-
over frequencies (TOF, <3.3 hꢁ1) and prolonged reaction times mesh sieve to get the qualities of particles with diameters of less
(24–48 h) for achieving the high product yields (>75%).
Catalysts based on supported gold nanoparticles (NPs) have phase (based on XRD analysis, see Fig. S1†).
been found to be versatile catalysts for many chemical trans- Mn2O3 powders were prepared by calcination of MnCO3
than 0.074 millimeter. The crystal structure of Al2O3 was gamma
ꢂ
formations in various mild chemical processes, since Haruta and sample (AR) at 500 C in owing air (1 atm) for 4 h and then
his associates discovered gold particles (<5 nm) supported on cooled.29
transition metal oxides were very active for CO oxidation at low
Co3O4 powders were prepared by a conventional precipita-
temperature.21 It is generally accepted that the mean size of gold tion method. 10.0 g Co(NO3)2$6H2O was dissolved in 200 mL
NPs, supports and preparation methods markedly inuence the deionized water at room temperature, the pH was adjusted to
catalytic performances of supported gold catalysts.22 Bus et al. 9.0 by dropwise addition of NH4OH (2.5 M). The resultant hydro
ꢂ
reported that small gold particles (size < 2 nm) loaded on g-Al2O3 gel was washed with deionized water and dried at 110 C over-
were active for selective hydrogenation of cinnamaldehyde to night. The product was calcined at 400 ꢂC for 4 h in air and then
cinnamyl alcohol.23 It was previously reported that both size and cooled.
support of supported gold catalyst affected the reaction rates for
the water–gas shi reaction.24 Sun et al. showed that basic resin
Catalyst synthesis
R201 supported Au catalyst displayed good performance in the
The theoretical Au loading for each catalyst used in this work
one-pot synthesis of styrene carbonate from styrene and CO2.25
was 1 wt%, unless otherwise stated. Au/SiO2 was prepared
Recently, Liu et al. has reported the effective synthesis of benz-
according to sonication-aided impregnation method reported
imidazoles from 2-nitroanilines and CO2/H2 using supported gold
by Wang et al.30 The calculated amount of SiO2 (Degussa, Aerosil
catalyst.26 Besides, as reported by the group of Manzoli, gold sites
380, specic surface area: 380 m2 gꢁ1) was suspended in an
were essential for hydrogen dissociation on metal oxide supports,
aqueous solution of HAuCl4, and the suspension was placed
which could activate hydrogen.27 These established knowledge
into the sonication bath with irradiation at 40 kHz and 200 W
provide inspiration for our current study of methylation of N-
output power. Aer sonication, the water was removed by
methylaniline using CO2/H2 with supported gold catalysts.
evaporation at 80 ꢂC. The solid powder was nally reduced in H2
We previously reported that Au/Al2O3-VS (very small gold NPs
atmosphere at 350 ꢂC for 2 h. The deposition–precipitation with
loaded on g-Al2O3) catalyzed direct methylation of aniline using
urea method was used for the preparation of Au/Al2O3, Au/MgO,
CO2/H2 with total conversion of aniline and excellent selectivity
Au/Co3O4, Au/Mn2O3 and Au/CeO2 catalysts. In a typical proce-
towards N,N-dimethylaniline (92%) at 140 ꢂC for 7 h.28 To the best
dure, urea was dissolved in HAuCl4 solution at room tempera-
of our knowledge, it is the highest TOF value ever reported to date
ture. The support was then added to this clear solution, and the
for methylation of aniline with CO2/H2 in this catalysis system
temperature of the resulting slurry was increased gradually to
based total gold atoms (26 hꢁ1). Small Au NPs (ꢀ2 nm) supported
a xed temperature. The temperature was maintained for
on alumina is critical for high activity for methylation of amines
a certain time. Aer cooling to room temperature, the solid was
using CO2/H2, in comparison with Au NPs supported on other
recovered by ltering and washing several times wꢂith distilled
supports.28 We herein extend the previous work, and systemati-
water. The sample was dried under vacuum at 50 C for 12 h,
cally study the effects of the mean size of Au NPs, gold species and
acid–base properties of supports on catalytic activity of Au/Al2O3
for 2 h. The standard conditions were as follows: [HAuCl4] ¼
catalyst for direct methylation of N-methylaniline with CO2/H2. We
have also examined the mechanism of methylation of N-methyl-
mole ratio of urea to Au ¼ 200. The concentration of gold by
aniline in the presence of CO2/H2 catalyzed by Au/Al2O3 catalyst.
weight was conrmed by ICP-AES.
ꢂ
followed by reduction with a stream of 5 vol% H2/Ar at 350 C
0.48 mmol Lꢁ1; aging time ¼ 6 h; aging temperature ¼ 80 C;
ꢂ
Experimental
Characterization
Chemicals
Au loadings in all of the catalysts were measured by ICP-AES
with an ICAP 6300 instrument when the sample was dissolved
in aqua regia and diluted with distilled water to the proper
concentration. The crystal structures of the prepared catalysts
were characterized with powder X-ray diffraction (XRD) on
a Bruker D8 Advance X-ray diffractometer using the Ni-ltered
Cu Ka radiation source at 40 kV and 40 mA. TEM images for
supported gold catalysts were taken with a Tecnai G2 electron
Commercially available organic and inorganic compounds were
used without further purication. Gold catalysts including 1
wt% Au/TiO2 (catalogue number 79–0165), 1 wt% Au/ZnO
(catalogue number 79–0170) were supplied by Mintek.
Synthesis of Al2O3, Mn2O3 and Co3O4 supports
Al2O3 powders were prepared by a conventional precipitation microscope operating at 200 kV. Before being transferred into
method. 45.0 g Al(NO3)3$9H2O was dissolved in 500 mL the TEM chamber, the samples dispersed with ethanol were
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 99678–99687 | 99679

View Article Online
RSC Advances
Paper
deposited onto a carbon-coated copper grid and then quickly methylation of 1a with CO2/H2 gave better conversion than the
moved into the vacuum evaporator. The size distribution of the use of polar solvents, which shows that the polarity of solvents
metal nanoclusters was determined by measuring about 200 has a remarkable inuence on the conversion of 1a with CO2/H2
random particles on the images. NH3-TPD and CO2-TPD were in the presence of Au/Al2O3 catalyst.
performed on an AutoChem 2950 HP instrument. Typically, the
Subsequently, the effects of some typical supports on cata-
sample loaded in a quartz reactor was pretreated with high lytic activity were studied for methylation of 1a with CO2/H2.
ꢂ
ꢂ
purity He at 300 C for 1 h. Aer cooling the sample to 50 C, The mean size of Au NPs loaded on various supports at around 3
NH3 adsorption was performed by switching the He ow to nm was successfully obtained by using the DPU method and
a NH3–He (10 vol% NH3) gas mixture and then maintaining the sonication-aided impregnation techniques. The typical TEM
sample at 50 ꢂC for 1 h. The gas-phase (and/or weakly adsorbed) micrographs and corresponding Au particle size distributions
NH3, was purged by high-purity He at the same temperature. for these catalysts are shown in Fig. S2.† Of the different
NH3-TPD was then performed in the He ow by raising the supports tested, Al2O3 (Table 1, entry 1) provided the best result
temperature to 700 C at a rate of 10 C minꢁ1. The desorbed in terms of high conversion (76%) and excellence selectivity
NH3 molecules were detected by using a MKS Cirrus 2 mass (>99%). The use of SiO2, a-Mn2O3, CeO2 and Co3O4 resulted in
spectrometer at the signal of m/z 17. CO2-TPD was performed by low yields of 1b (<4%) under similar reaction conditions (Table
using a similar procedure. The X-ray absorption data at the Au 1, entries 8–11). Moderate yield of 1b (15%) could be obtained
L-edge of the samples were recorded at room temperature in when Au/TiO2 catalyst was used (Table 1, entry 12). However, 1b
transmission mode using ion chambers at beam line BL14W1 of was not detected when MgO, ZnO and C were used as supports
the Shanghai Synchrotron Radiation Facility (SSRF), China. The (Table 1, entries 13–15). In addition, the use of the Au0 powder,
synchrotron radiation was monochromatized with a Si (111) did not promote the reaction at all (Table 1, entry 16). It was
double crystal monochromator, and mirrors were used to reported that MgO has strong basic character and SiO2 is an
eliminate higher harmonics. The incident and transmitted acidic to neutral oxide.30 Al2O3 with moderate electronegativity
beam intensities were monitored using ionization chambers can be considered to have both acidic and basic surface sites.31
lled with pure N2. During the measurement, the synchrotron These results may suggest that the acid–base properties of the
was operated at energy of 3.5 GeV and a current of 250 mA. Data support can affect the catalytic behavior of the supported Au
processing was performed using the program ATHENA and catalysts for methylation of 1a with CO2/H2.
ꢂ
ꢂ
ARTEMIS. The X-ray absorption near-edge structure (XANES)
Next, the acid–base properties of the typical supported Au
was normalized with edge height and the rst-order derivatives catalysts were examined by temperature-programmed desorp-
was taken to compare variation of absorption edge energies.
tion (TPD) of NH3 and CO2. As shown in Fig. 1, desorption peaks
of NH3 are observed from Au/Al2O3, Au/Mn2O3, Au/TiO2, Au/
ZnO, Au/CeO2 and Au/SiO2, which indicates that acidic sites
exist on these catalysts. From the temperature and intensity of
the NH3 desorption peak, we can see that Au/Al2O3 possesses
relatively strong acidity. No signicant acidity, on the other
hand, exists on Au/MgO, Au/C and Au/Co3O4, as no desorption
peak of NH3 can be detected from these catalysts. CO2-TPD
proles in Fig. 2 suggest that Au/Al2O3 and Au/MgO both have
relatively strong basic sites, whereas no basic sites can be
observed on Au/TiO2, Au/Co3O4, Au/C, Au/ZnO and Au/SiO2.
Weak desorption peaks of CO2 are detected in Au/Mn2O3 and
Au/CeO2, which suggests weak basic sites exist on these cata-
lysts. According to the NH3-TPD and CO2-TPD results, Au/Al2O3
possesses both relatively strong basicity and acidity, while Au/
Co3O4 and Au/C have neither acidic nor basic sites. Therefore,
we have observed signicant support effects in the Au-catalyzed
Catalytic testing
A mixture of solvent (10 mL), N-methylaniline (1.0 mmol), and
supported Au catalysts (Au 2.7–5.0 mmol) were charged into a 50
mL Hastelloy-C high pressure Parr reactor. The reactor was then
exchanged with H2, followed by introducing 1.0 MPa CO2 and
3.0 MPa H2. The reaction was reacted at desired temperature for
given reaction time under magnetic stirring. Subsequently, the
autoclave was cooled to room temperature, and n-octane (1
mmol, internal standard) was added for quantitative analysis by
GC-FID (Agilent 7820A).
Results and discussion
Solvent and support effects
In the course of identifying and optimizing key reaction methylation of N-methylaniline with CO2/H2, and our studies
parameters for N-methylaniline (1a) with CO2/H2, several suggest that the acid–base properties of the supports play
solvents were investigated. The Au/g-Al2O3 catalyst with very important roles in determining both its activity and the selec-
small Au NPs (2.0 nm) was used as the standard catalyst. The tivity. The strong basicity of supports may promote the
highest yield of N,N-dimethylaniline (1b) was obtained when adsorption of CO2 and accelerate departure of product from the
cyclohexane was used as the reaction medium (Table 1, entry 1). surface of catalysts. The strong acidity of supports may be
In contrast, the reaction in hexane, toluene, 1,4-dioxane, CH2Cl2 important to anchor the amines (reactive molecules). So the
and THF (Table 1, entries 2–6) gave moderate to good yield (24– strong acidity and basicity will benet the methylation reaction.
61%) with excellent selectivity towards 1b (>90%), and in
In summary, acidity and basicity of supports in some typical
ethanol (Table 1, entry 7) obtained the lowest yield (2%) with supported gold catalysts were successfully detected in our
poor selectivity towards 1b (18%). Based on above results, we experiment and had signicant effects on conversion of 1a and
empirically concluded that the use of apolar solvents for direct selectivity towards 1b during direct methylation of 1a with CO2/
99680 | RSC Adv., 2015, 5, 99678–99687
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
a
Table 1 Effects of different solvent and support on the synthesis of N,N-dimethylaniline with CO2/H2
Entry
Solvent
Catalyst
Au Sizeb (nm)
Au contentc (wt%)
Conv.d (%)
Sel.e (%)
1
2
3
4
5
6
7
8
Cyclohexane
Hexane
Toluene
1,4-Dioxane
CH2Cl2
THF
Au/Al2O3
Au/Al2O3
Au/Al2O3
Au/Al2O3
Au/Al2O3
Au/Al2O3
Au/Al2O3
Au/SiO2
Au/a-Mn2O3
Au/CeO2
Au/Co3O4
Au/TiO2
Au/MgO
Au/ZnO
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.7
2.4
3.7
4.4
2.8
2.6
2.8
4.0
150
0.73
0.73
0.73
0.73
0.73
0.73
0.73
0.97
0.95
0.74
0.96
1.00
0.91
1.00
1.00
100
76
61
52
25
34
29
12
1
1
3
7
15
n.r.
n.r.
n.r.
n.r.
>99
>99
>99
96
>99
91
Ethanol
18
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
>99
>99
>99
42
>99
—
—
—
—
9
10
11
12
13
14
15
16
Au/C
Au powder
a
Reaction conditions: 0.5 mol% Au, N-methylaniline (1 mmol), CO2 (1 MPa), H2 (3 MPa), solvent (10 mL), T ¼ 140 ꢂC, t ¼ 5 h, n. r. ¼ no reaction.
b
Obtained from TEM. c Measured by ICP-AES. d Conversation and selectivity were determined by GC with n-octane as the internal standard. e N-
methylformanilide is the only other product.
Fig. 2 CO2-TPD profiles for some typical supported Au catalysts: (a)
Au/C, (b) Au/ZnO, (c) Au/Co3O4, (d) Au/CeO2, (e) Au/Mn2O3, (f) Au/
SiO2, (g) Au/MgO, (h) Au/TiO2, (i) Au/Al2O3.
Fig. 1 NH3-TPD profiles for some typical supported Au catalysts: (a)
Au/CeO2, (b) Au/C, (c) Au/ZnO, (d) Au/Co3O4, (e) Au/Mn2O3, (f) Au/
SiO2, (g) Au/MgO, (h) Au/TiO2, (i) Au/Al2O3.
methylation of 1a with CO2 and H2 can be carried out smoothly
under relatively mild conditions although the reaction time
becomes longer (18 h). It is noteworthy that the selectivity is
constant (>99%) during the process, which suggests the reac-
tion has high selectivity towards 1b. Besides, increasing the
reaction temperature and pressure respectively can boost the
conversion of 1a to a certain extent (see Fig. S3 and Fig. S4 in the
ESI†). However, the conversion of 1a and selectivity towards 1b
kept nearly the same when the total initial pressure and
H2. The Au/Al2O3 catalyst with relatively strong acidity and
basicity contributes to high conversion of 1a and excellent
selectivity towards 1b, whereas other catalysts with only strong
basic (Au/MgO) or acidic to neutral sites (Au/SiO2 and Au/C) are
almost inactive. Catalysts with weak acid–base properties (Au/
Mn2O3 and Au/ZnO) also have no signicant activity. There are
moderate activity and excellent selectivity when TiO2 and CeO2
with a few acidic sites are used as a support in Au/TiO2 and Au/
CeO2 catalysts. Au/Co3O4 with neutral sites has very low activity
and poor selectivity (<45%) towards 1b.
ꢂ
temperature were higher than 4 MPa and 140 C respectively.
Obviously, 1a could not be fully converted by increasing the
total initial pressure or boosting reaction temperature under
A further study to investigate the effect of reaction parame-
ters was carried out in our experiment. As shown in Fig. 3, the
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 99678–99687 | 99681

View Article Online
RSC Advances
Paper
DPU process could be an important factor in determining the
mean size of Au NPs supported on alumina. As is well known,
the rates of nucleation and growth determine the size of
a particle formed in the solution. Increasing the aging
temperature from 20 to 80 ꢂC can increase the nucleation rates
and the pH values in solution because of urea decomposition.
At 80 ꢂC, when the mole ratio of urea to Au decreased to 50, the
pH value could not reach at 8 as too little urea was added in
solution. So a large Au particle size (5.7 nm) was formed (Table
2, entry 3). When aging time was 1 h instead of 6 h, large mean
Au NPs size (3.6 nm) was obtained (Table 2, entry 4), indicating
that less aging time caused the formation of large Au particle.
Alternatively, prolonged aging time favored the formation of
small Au particles in the DPU process.32 However, pH value was
over 8 when urea added in solution was enough, which
produced the smallest mean Au NPs size (1.8 nm) (Table 2, entry
6). Fig. 4 shows the TEM micrographs and the Au particle size
distributions for these Au/Al2O3 samples. Grisel et al. reported
the Au loading depended strongly on the nal pH of the
precipitation solution and a sharp decrease in Au loading was
observed with pH value over 8.5.33 Low Au loadings (yields:
<50%) due to incomplete decomposition of urea in solution
were obtained when aging temperatures were below 80 ꢂC
(Table 2, entries 1–2). Most Au was loaded on alumina (yield:
>70%) when ꢂthe mole ratio of urea to gold was less than or equal
to 200 at 80 C (Table 2, entries 3–5). Too much urea added in
solution on the other hand caused low Au loading (yield: 40%)
because of high pH value over 8.5 (Table 2, entry 6). Thus,
regulating pH of the solution during DPU process contributed
to the formation of small Au particle and could cause the
alteration of Au loadings.
Fig. 3 Effects of reaction time on catalytic activity of Au/Al2O3 catalyst
for direct methylation of N-methylaniline with CO2/H2. Reaction
condition: 0.5 mol% Au, 1.0 mmol N-methylaniline, cyclohexane 10
mL, CO2 (1 MPa), H2 (3 MPa), T ¼ 140 ꢂC, t ¼ 5 h.
the present reaction conditions. Nevertheless, 1a could be fully
converted with excellent selectivity towards 1b when reaction
time was up to 18 h under the standard conditions. Therefore,
total initial pressure and temperature both affected the yield of
1b when the reaction time was xed at 5 h with the yield of 1b
lower than 76%. In order to get 1a totally converted, prolonged
reaction time is required under proper total initial pressure and
at appropriate temperature.
Preparation and characterization of Au/g-Al2O3 with
controlled mean Au particle sizes
There are still challenges to prepare supported Au catalysts with
controlled Au sizes. It was found that aging temperature, aging
time and the mole ratio of urea to Au used in DPU process
played key roles to obtain controlled Au particle sizes. Louis
et al. reported that the pH increased with time due to urea
decomposition and most gold was deposited within the rst
Catalytic properties of Au/g-Al2O3 catalysts with various Au
NPs sizes for direct methylation of N-methylaniline with CO2/
H2
hour in the preparation of Au/TiO2 by DPU method.32 Urea The dependence of catalytic activity on the mean size of Au NPs
begins to decompose when aging temperature is over 60 ꢂC. in Au/Al2O3 catalysts prepared by DPU with various preparation
Compared with Au/TiO2 prepared by DPU, we found that by parameters was shown in Table 3. The highest yield (83%) of 1b
regulating aging temperature from 20 to 80 ꢂC in the DPU (Table 3, entry 1) was acquired with the smallest mean Au NPs
process could markedly decrease the mean size of Au NPs from size (1.8 nm). Small Au NPs (2.0–5.7 nm) could lead to moderate
8.3 to 2.0 nm of Au/Al2O3 catalysts (Table 2, entries 1–2 and 5). yields (61–76%) and high selectivity (>99%) towards 1b (Table 3,
These results claried that aging temperature used during the entries 2–7). However, on the other hand, low yields (<8%) of 1b
Table 2 Preparations by deposition–precipitation with urea: influence of aging temperature, aging time and the mole ratio of urea to Au on the
mean Au NPs sizea
Mole ratio of urea : Au
(mol : mol)
Entry
Aging temperature (ꢂC)
Aging time (h)
Au loadingb (wt%)
Mean diameterc (nm)
1
2
3
4
5
6
20
50
80
80
80
80
6
6
6
1
6
6
200
200
50
200
200
400
0.47
0.46
0.71
0.70
0.73
0.40
8.3
7.2
5.7
3.6
2.0
1.8
a
If not mentioned, preparation conditions were: [HAuCl4] ¼ 0.48 mmoL Lꢁ1, aging time ¼ 6 h, aging temperature ¼ 80 ꢂC, mole ratio of urea to Au
¼ 200, theoretical Au loading ¼ 1 wt%. b Measured by ICP-AES. c Obtained from TEM.
99682 | RSC Adv., 2015, 5, 99678–99687
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
time course of Au/Al2O3 catalysts was done with different mean
Au NPs sizes in the range of 1.8 to 8.3 nm (see Fig. S5 in the
ESI†). No conversion of 1a over Al2O3 (Fig. S5†) indicates that
the Al2O3 species had no active sites for the methylation reac-
tion with CO2/H2, which shows only the reduced gold species
possess active sites for methylation of 1a with CO2/H2. The
conversation of 1a increases almost linearly with reaction time
over the initial 30 min in Fig. S5.† So we can calculate initial
reaction rates by using the slope of the straight line in Fig. S5†
for each catalyst. The results are shown in Table S1.† Obviously,
the initial reaction rates decrease from 5.86 to 0.36 mmol hꢁ1
gꢁ1 (cat.) with increasing mean size of Au NPs from 1.8 to 8.3
nm.
Because the gold atoms contribute to the catalytic reactions,
we have calculated the TOF (the moles of 1a converted per unit
time at the initial stage per mole total gold atoms) using the
initial reaction rate for each Au/Al2O3 catalyst. The results are
shown in Fig. 5, suggesting the intrinsic activity per mole total
gold atoms decreases with increasing the mean Au NPs size and
the methylation of 1a with CO2/H2 is a structure-sensitive
reaction.
Fig. 4 TEM micrographs and corresponding Au NPs size distributions
for the Au/Al2O3 catalysts with various mean sizes of Au NPs: (a) 1.8
nm, (b) 2.0 nm, (c) 3.6 nm, (d) 5.7 nm, (e) 7.2 nm, and (f) 8.3 nm.
(Table 3, entries 8–9) were obtained due to large mean size of Au
NPs (>7.2 nm).
The conversion of 1a at the initial stages over Au/Al2O3
catalysts was recorded to prove the relations between mean Au
NPs size and the intrinsic catalytic reactivity of Au sites. The
Fig. 5 Dependence of TOF on the mean Au NPs size for the Au/Al2O3-
catalyzed methylation of N-methylaniline with CO2/H2. Reaction
conditions: catalyst 0.14 g (Au 2.7–5.0 mmol), 1.0 mmol N-methyl-
aniline, cyclohexane 10 mL, CO2 (1 MPa), H2 (3 MPa), T ¼ 140 ꢂC.
Table 3 Catalytic performance of Au/Al2O3 catalysts with various mean Au NPs size from 1.8 to 8.3 nma
Entry
Preparation conditionsb
Au loading (wt%)
Au sizec (nm)
Conv.d (%)
Sel.e (%)
1
2
3
4
5
6
7
8
9
Urea : Au ¼ 400
0.40
0.73
0.70
0.65
0.60
0.70
0.70
0.46
0.47
1.8
2.0
2.4
2.4
2.6
3.6
5.7
7.2
8.3
83
76
69
70
63
69
61
8
>99
>99
>99
>99
>99
>99
>99
86
Standard condition
[HAuCl4] ¼ 0.12 mmolꢁ1
Aging time ¼ 16 h
[HAuCl4] ¼ 1.92 mmolꢁ1
Aging time ¼ 1 h
L
L
Urea : Au ¼ 50
Aging temperature ¼ 50 ꢂ
C
C
Aging temperature ¼ 20 ꢂ
6
83
a
Reaction conditions: catalyst 0.14 g (Au 2.7–5.0 mmol), N-methylaniline (1 mmol), CO2 (1 MPa), H2 (3 MPa), cyclohexane (10 mL), T ¼ 140 ꢂC, t ¼ 5
h. b If not mentioned, preparation conditions were: [HAuCl4] ¼ 0.48 mmol Lꢁ1, aging time ¼ 6 h, aging temperature ¼ 80 ꢂC, mole ratio of urea to Au
¼ 200. c Obtained from TEM. d Conversation and selectivity were determined by GC with n-octane as the internal standard. e N-Methylformanilide is
the only other product.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 99678–99687 | 99683

View Article Online
RSC Advances
Paper
The TOF values increase from 114 to 287 hꢁ1 with decreasing could be the change of Au particle size and valence of Au
the mean size of Au NPs from 3.6 to 1.8 nm. When the mean species. To investigate the structure of Au species supported on
sizes of Au NPs are larger than 5.7 nm, the TOF values are lower alumina aer the H2 reduction, Au L3-edge XAFS measurement
than 52 hꢁ1. The TOF value (about 300 hꢁ1) with the best of Au/Al2O3 samples and Au reference compounds was carried
catalyst (with 1.8 nm gold size) is much higher than those out. Fig. 6 shows Au L3-edge X-ray absorption near-edge struc-
measured with other heterogeneous catalysts such as CuAlOx tures (XANES) spectra, which are known to be sensitive to the
(0.1 hꢁ1), Pt–MoOx/TiO2 (1.8 hꢁ1) and Pd/CuZrOx (3.3 hꢁ1) for oxidation states of X-ray absorbing atoms. The XANES features
methylation of amines with CO2/H2.18–20
of Au/Al2O3 samples (spectra c–f) aer the H2 reduction with
various temperature from 150 to 450 ꢂC are clearly different
from that of Au2O3 (spectrum a) but rather similar to that of Au
foil (spectrum g), indicating that gold species in these samples
are in a reduced state.
Effects of reduction temperature of Au/Al2O3 catalysts on
direct methylation of N-methylaniline with CO2 and H2
Fig. 7 shows the Fourier transform (FT) of extended X-ray
absorption ne structure (EXAFS) data of Au/Al2O3 samples
reduced at different temperature (150, 250, 350, 450 ꢂC), the Au/
Al2O3 precursor, Au foil and Au2O3. The structural parameters
derived from curve-tting analysis are listed in Table 5. The Au–
O coordination number of 3.8 was observed in pure Au2O3. The
EXAFS of the as-prepared Au/Al2O3 precursor (spectrum b)
showed the Au–O coordination number (CN) and the Au–Au CN
were 1.7 and 3.0 respectively. These results, as reported previ-
The as-prepared supported gold precursors are easily reduced to
the zero state by thermal treatments with reducing gases or
oxidant gases.34 We previously demonstrated that the very small
Au0 NPs supported on alumina treated with a stream of 5 vol%
H2/Ar at 350 ꢂC for 2 h showed high catalytic activity for direct
methylation of amines with CO2/H2.28 In our experiment, the as-
prepared Au/Al2O3 precursor of standard Au/Al2O3 catalyst was
treated with a stream of 5 vol% H2/Ar at 150, 250, 350 and 450 ꢂC
respectively for 2 h, at a heating rate of 5 ꢂC minꢁ1 to obtain Au/
Al2O3 samples. The samples were designated as Au/Al2O3-T,
where T denoted the reduction temperature. With these cata-
lysts, the catalytic activity was investigated for direct methyla-
tion of amines with CO2/H2. Table 4 shows that selective
production of 1b in a 38% yield could be achieved when Au/
Al2O3-150 catalyst was used (Table 4, entry 1). An increase in
ously by Grunert et al. for Au/TiO2-MCM-48 catalyst,36 conrmed
¨
ꢂ
reduction temperature from 150 to 250 C markedly increased
the 1b yield from 38 to 53% (Table 4, entry 2). The highest yield
of 1b was 76% as reduction temperature of the as-prepared Au/
Al2O3 precursor was 350 ꢂC (Table 4, entry 3). However, a further
increase of reduction temperature from 350 to 450 ꢂC led to
a slight decrease in the desired product yield from 76 to 67%
(Table 4, entries 3–4). The high reduction temperature may lead
to decrease of the quantities of active hydroxyl group in alumina
surface and a weak Au-support interaction in Au/Al2O3-450
catalyst, which may cause the slight decrease of the catalytic
activity.35 Notably, the selectivity towards 1b was almost the Fig. 6 Au L3 absorption edge XANES spectra: (a) Au2O3, (b) as
prepared Au/Al2O3 precursor, Au/Al2O3 samples reduced at different
temperatures: (c) 150, (d) 250, (e) 350, (f) 450 ꢂC, and (g) Au foil.
same, showing that the selectivity did not depend on reduction
temperature of the as-prepared Au/Al2O3 precursor.
The factor, which caused different catalytic activities of the
Au/Al2O3 catalysts treated by different reduction temperature,
Table 4 Effects of reduction temperature on catalytic performance of
Au/Al2O3 catalyst for direct methylation of N-methylaniline with CO2/
a
H2
Entry Reduction temperature (ꢂC) Au sizeb (nm) Conv.c (%) Sel. (%)
1
2
3
4
150
250
350
450
7.1
4.5
2.0
1.6
38
53
76
67
>99
>99
>99
>99
a
Reaction conditions: 0.5 mol% Au, N-methylaniline (1 mmol), CO2 (1
b
MPa), H2 (3 MPa), cyclohexane 10 mL, T ¼ 140 ꢂC, t ¼ 5 h. Obtained
c
Fig. 7 Au L3 absorption edge EXAFS Fourier transforms: (a) Au2O3, (b)
the Au/Al2O3 precursor, Au/Al2O3 samples reduced at different
temperatures: (c) 150, (d) 250, (e) 350, (f) 450 ꢂC, and (g) Au foil.
from TEM. Conversation and selectivity were determined by GC with
n-octane as the internal standard.
99684 | RSC Adv., 2015, 5, 99678–99687
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 5 Fitting parameters of the curve fitted k3-weighted EXAFS analysis of the Au/Al2O3-T samples, the Au/Al2O3 precursor, Au foil and Au2O3
c
Samplesa
Shell
Nb
R (A)
s
2d (ꢄ10 A )
DE0 (eV)
˚
ꢁ3 ꢀ2
Au-foil
Au/Al2O3-150
Au–Au
Au–O
Au–Au
Au–O
Au–Au
Au–O
Au–Au
Au–O
Au–Au
Au–O
Au–Au
Au–O
11.9 ꢃ 0.5
0.5 ꢃ 0.2
10.7 ꢃ 0.9
0.5 ꢃ 0.2
10.0 ꢃ 1.2
0.8 ꢃ 0.3
7.7 ꢃ 0.8
0.9 ꢃ 0.3
5.6 ꢃ 1.4
1.7 ꢃ 0.4
3.0 ꢃ 1.0
3.8 ꢃ 0.3
2.86 ꢃ 0.01
2.08 ꢃ 0.03
2.85 ꢃ 0.01
2.08 ꢃ 0.03
2.83 ꢃ 0.01
2.09 ꢃ 0.03
2.82 ꢃ 0.01
2.11 ꢃ 0.06
2.77 ꢃ 0.01
2.02 ꢃ 0.01
2.83 ꢃ 0.01
1.99 ꢃ 0.01
8.1 ꢃ 0.3
3.0 ꢃ 1.0
8.8 ꢃ 0.6
3.0 ꢃ 1.0
10.1 ꢃ 1.2
3.0 ꢃ 1.5
10.0 ꢃ 1.3
3.5 ꢃ 1.2
8.9 ꢃ 1.5
5.0 ꢃ 3.0
5.3 ꢃ 2.0
2.0 ꢃ 0.7
3.9
ꢁ7.0
3.8
Au/Al2O3-250
ꢁ6.9
3.5
Au/Al2O3-350
ꢁ3.4
4.3
Au/Al2O3-450
ꢁ2.0
0.5
11.8
1.8
As prepared precursor
Au2O3
11.0
a
b
Reduction temperature of catalysts. Coordination number. c Bond length. d Debye–Waller factor.
that metallic gold species as a minority phase might exist in the
as prepared Au/Al2O3 precursor due to auto reduction. Aer H2
ꢂ
reduction at various temperature in the range of 150 to 450 C
for 2 h (spectra c–f), the Au–Au CN decreased from 10.7 to 5.6
ꢀ
and the Au–Au bond length decreased from 2.85 to 2.77 A. It
should be noted that there is a signicant Au–O path contri-
Scheme 1 Control experiment.
bution for Au/Al2O3-T, such as Au/Al2O3-450. However, this Au–
ꢀ
O path has longer length (2.11 ꢃ 0.06 A) than the Au–O bond in
Au2O3, implying a weak interaction, different from oxidic Au
atoms by oxygen, between Au NPs and Al2O3 support. It implies
that the oxygen of Au–O bond comes from Al2O3 support. This
result can be further suggested from the big uncertainly (ꢃ0.06
ꢀ
A) of the Au–O path length and the absence of Au oxidation
determined by above XANES data. The Au–Au CN in these
samples were much less than that in Au foil while close to that
in Au/Fe–OH or Au/Fe–O catalysts due to the high proportion of
surface atoms, suggesting that the small gold NPs supported on
alumina were obtained.37 TEM measurement in Fig. S6† showed
that the mean gold NPs size remarkably decreased from 7.1 to
Scheme 2 Proposed reaction pathway of Au-catalyzed methylation of
1a with CO2/H2.
1.6 nm with increasing reduction temperature from 150 to 450 Initially, the reduction of the N-methylformanilide (1c) with H2
ꢂC. So the TEM result is in good agreement with the XAFS was investigated in the presence of Au/Al2O3 catalyst. Clearly, 1c
conclusion.
was fully converted and 1b was the major products in 61% yield
Therefore, the EXAFS and XANES results showed that gold (Scheme 1). Aside from methylation of 1c reduced by H2, 1c was
species in the Au/Al2O3 samples were fully reduced aer H2 simultaneously reduced by H2 with Au/Al2O3 catalyst to produce
treatment at different temperature (150, 250, 350, 450 ꢂC) for 2 1a in 39% yield. Under standard reaction conditions, trace
h. The mean Au NPs size decreased with increasing the reduc- amount of 1c could be detected. Moreover, we found that
ꢂ
tion temperature from 150 to 450 C.
increasing the mean size of Au NPs could not only decrease the
conversion of 1a, but also decrease the selectivity towards 1b to
a certain extent as shown in Fig. S7.† These results suggest that
1c could be the key intermediate and signicantly contributed
to methylation of 1a with CO2/H2 catalyzed by Au/Al2O3 catalyst.
Based on these results, a plausible mechanism is given in
Scheme 2. Under our reaction conditions, the formamide 1c
rst produced due to Au-catalyzed reaction of 1a with CO2/H2.
The formamide 1c could be quickly reduced to give 1b (major)
or be decarbonylated into 1a (minor). Then, 1a continued to
react with CO2/H2 catalyzed by Au/Al2O3 catalyst following
a similar hydrogenation and decarbonylation process as
described above to produce the corresponding products.
Reaction mechanism
The group of Shi reported that using Pd/CuZrOx under milder
conditions or using Pd/ZnZrOx during methylation with CO2/
H2, N-formylation mainly processed instead of N-methylation,
which indicates the N-formamide could be the intermediate.20
Beller et al. also reported that formamide was the intermediate
during methylation of amines with CO2/H2 catalyzed by Ru
catalyst and methanol contributed insignicantly to the reac-
tion.15b In order to better understand the mechanism of the
methylation reaction catalyzed by the Au/Al2O3 catalyst, control
experiments were conducted to conrm the intermediates.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 99678–99687 | 99685

View Article Online
RSC Advances
Paper
4 (a) K. M. Y. Yu, I. Curcic, J. Gabriel and S. C. Edman Tsang,
ChemSusChem, 2008, 1, 893–899; (b) J. Ma, N. N. Sun,
X. L. Zhang, N. Zhao, F. K. Xiao, W. Wei and Y. H. Sun,
Catal. Today, 2009, 148, 221–231; (c) W. Wang, S. P. Wang,
X. B. Ma and J. L. Gong, Chem. Soc. Rev., 2011, 40, 3703–3727.
5 (a) G. Pagani, Hydrocarbon Process., Int. Ed., 1982, 61, 87–91;
(b) S. Walendy, G. Francio, M. Hoelscher and W. Leitner, Exp.
Green Sustainable Chem., 2009, 275–277; (c) S. G. Liang,
H. Z. Liu, T. Jiang, J. L. Song, G. Y. Yang and B. X. Han,
Chem. Commun., 2011, 47, 2131–2133; (d) M. Peters,
Conclusions
Au NPs with uniform mean sizes (ꢀ3 nm) loaded onto various
supports were successfully prepared. These supported Au cata-
lysts were studied in detail to clarify the support and Au particle
size effects in the direct methylation reaction of N-methylani-
line with CO2/H2. Of the supports tested, Al2O3 with relatively
strong acidic and basic surface sites showed the best catalytic
performance. Other supports, such as SiO2 with few acidic sites,
Mn2O3 and ZnO with weak acid–base properties, Co3O4 and C
with neither acidic site nor basic site and MgO with relatively
strong basic sites, have little or low catalytic activity. There is
moderate catalytic activity when Au/TiO2 and Au/CeO2 with
a few acidic sites are used. Supports with relatively strong acid–
base bifunctional sites may be necessary for high efficient
methylation of N-methylaniline with CO2/H2.
By changing preparation parameters during the DPU
process, we obtained the Al2O3 supported Au NPs with
controlled mean sizes in the range of 1.8 to 8.3 nm. We proved
that, during the DPU process, aging time, aging temperature
and mole ratio of gold to urea were the main preparation
parameters affecting the mean size of Au NPs supported on
Al2O3.
¨
B. Kohler, W. Kuckshinrichs, W. Leitner, P. Markewitz and
¨
T. Muller, ChemSusChem, 2011, 4, 1216–1240.
6 (a) S. N. Riduan, Y. Zhang and J. Y. Ying, Angew. Chem., Int.
Ed., 2009, 48, 3322–3325; (b) A. E. Ashley, A. L. Thompson
and D. OHare, Angew. Chem., Int. Ed., 2009, 48, 9839–9843;
(c) S. Wesselbaum, T. vom Stein, J. Klankermayer and
W. Leitner, Angew. Chem., Int. Ed., 2012, 51, 7499–7502.
7 K. Motokura, D. Kashiwame, A. Miyaji and T. Baba, Org. Lett.,
2012, 14, 2642–2645.
8 Q. Su, J. Sun, J. Q. Wang, Z. F. Yang, W. Q. Cheng and
S. J. Zhang, Catal. Sci. Technol., 2014, 4, 1556–1562.
9 Y. Yoshida, Y. Arai, S. Kado, K. Kunimori and K. Tomishige,
Catal. Today, 2006, 115, 95–101.
10 L. Schmid, A. Canonica and A. Baiker, Appl. Catal., A, 2003,
255, 23–33.
11 T. G. Ostapowicz, M. Schmitz, M. Krystof, J. Klankermayer
and W. Leitner, Angew. Chem., Int. Ed., 2013, 52, 12119–
12123.
12 (a) A. Ricci, Modern Amination Methods, Wiley-VCH,
Weinheim, 2000; (b) R. N. Salvatore, C. H. Yoon and
K. W. Jung, Tetrahedron, 2001, 57, 7785; (c) Z. Rapport, The
Chemistry of Anilines, Wiley Interscience, 2007.
The TOF values for direct methylation of N-methylaniline
with CO2/H2 increased with decreasing the mean size of Au NPs
from 8.3 to 1.8 nm and XAFS results demonstrated that only
reduced gold NPs supported on alumina were responsible for
high catalytic activity of Au/Al2O3 catalyst. The results suggested
that the Au-catalyzed direct methylation of N-methylaniline
with CO2/H2 was a structure-sensitive reaction. Then, we
proposed that N-methylformanilide could be a key intermediate
during methylation of N-methylaniline with CO2/H2.
13 (a) M. F. Ali, B. M. ElAli and J. G. Speight, Handbook of
Industrial Chemistry-Organic Chemicals, McGraw-Hill, New
York, 2005; (b) H.-J. Arpe and S. Hawkins, Industrial
Organic Chemistry, Wiley-VCH, Weinheim, 1997; (c)
A. Dhakshinamoorthy, M. Alvaro and H. Garcia, Appl.
Catal., A, 2010, 378, 19–25.
Acknowledgements
This work was nancially supported by the Program of Inter-
national S&T Cooperation (Grant no. 2014DFG60230), the
National Basic Research Program of China (2013CB933104), the
National Natural Science Foundation of China (no. 21303246,
11275258, 91127001).
14 (a) X. Jiang, C. Wang, Y. W. Wei, D. Xue, Z. T. Liu and
J. L. Xiao, Chem. - Eur. J., 2014, 20, 58–63; (b) S. Savourey,
`
G. Lefere, J. C. Berthet and T. Cantat, Chem. Commun.,
2014, 50, 14033–14036; (c) I. Sorribes, K. Junge and
M. Beller, Chem. - Eur. J., 2014, 20, 7878–7883.
Notes and references
1 J. T. Houghton, G. J. Jenkins and J. J. Ephraums, Climate 15 (a) O. Jacquet, X. Frogneux, C. D. N. Gomes and T. Cantat,
Change: The IPCC Scientic Assessment, Cambridge
University Press, Cambridge, UK, 1990.
Chem. Sci., 2013, 4, 2127–2131; (b) Y. Li, X. Fang, K. Junge
and M. Beller, Angew. Chem., Int. Ed., 2013, 52, 9568–9571.
2 H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, 16 (a) K. Beydoun, T. vom Stein, J. Klankermayer and
E. J. Beckman, A. T. Bell, J. E. Bercaw, C. Creutz, E. Dinjus,
D. A. Dixon, K. Domen, D. L. DuBois, J. Eckert, E. Fujita,
D. H. Gibson, W. A. Goddard, D. W. Goodman, J. Keller,
W. Leitner, Angew. Chem., Int. Ed., 2013, 52, 9554–9557; (b)
Y. Li, I. Sorribes, T. Yan, K. Junge and M. Beller, Angew.
Chem., Int. Ed., 2013, 52, 12156–12160.
G. J. Kubas, H. H. Kung, J. E. Lyons, L. E. Manzer, 17 (a) S. Das, F. D. Bobbink, G. Laurenczy and P. J. Dyson,
T. J. Marks, K. Morokuma, K. M. Nicholas, R. Periana,
L. Que, J. R. Nielson, W. M. H. Sachtler, L. D. Schmidt,
A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stults and
W. Tumas, Chem. Rev., 2001, 101, 953–996.
3 T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007, 107,
2365–2387.
Angew. Chem., Int. Ed., 2014, 53, 12876–12879; (b)
E. Blondiaux, J. Pouessel and T. Cantat, Angew. Chem., Int.
Ed., 2014, 53, 12186–12190.
18 X. J. Cui, X. Dai, Y. Zhang, Y. Deng and F. Shi, Chem. Sci.,
2014, 5, 649–655.
99686 | RSC Adv., 2015, 5, 99678–99687
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
19 K. Kon, S. M. A. H. Siddiki, W. Onodera and K. I. Shimizu, 25 D. Xiang, X. F. Liu, J. S. Sun, F. S. Xiao and J. M. Sun, Catal.
Chem. - Eur. J., 2014, 20, 6264–6267. Today, 2009, 148, 383–388.
20 X. Cui, Y. Zhang, Y. Q. Deng and F. Shi, Chem. Commun., 26 L. D. Hao, Y. F. Zhao, B. Yu, H. Y. Zhang, H. J. Xu and
2014, 50, 13521–13524. Z. M. Liu, Green Chem., 2014, 16, 3039–3044.
21 (a) M. Haruta, Catal. Today, 1997, 36, 153–166; (b) X. Zhang, 27 M. Manzoli, C. Chiorino, F. Vindigni and F. Boccuzzi, Catal.
H. Shi and B. Q. Xu, Angew. Chem., Int. Ed., 2005, 44, 7132– Today, 2012, 181, 62–67.
7135; (c) A. Corma, P. Serna and H. Garcıa, J. Am. Chem. 28 X. L. Du, G. Tang, H. L. Bao, Z. Jiang, X. H. Zhong, D. S. Su
Soc., 2007, 129, 6358–6359; (d) L. He, L. C. Wang, H. Sun, and J. Q. Wang, ChemSusChem, 2015, 8, 3489–3496.
J. Ni, Y. Cao, H. Y. He and K. N. Fan, Angew. Chem., Int. 29 X. Q. Li, J. Xu, F. Wang, J. Gao, L. P. Zhou and G. Y. Yang,
Ed., 2009, 48, 9538–9541; (e) M. D. Hughes, Y.-J. Xu, Catal. Lett., 2006, 108, 137–140.
P. Jenkins, P. McMorn, P. Landon, D. I. Enache, 30 W. H. Fang, J. S. Chen, Q. H. Zhang, W. P. Deng and Y. Wang,
A. F. Carley, G. A. Attard, G. J. Hutchings, F. King, Chem. - Eur. J., 2011, 17, 1247–1256.
E. H. Stitt, P. Johnston, K. Griffin and C. J. Kiely, Nature, 31 K. Shimizu, K. Sugino, K. Sawabe and A. Satsuma, Chem.
2005, 437, 1132. - Eur. J., 2009, 15, 2341–2351.
22 (a) U. Hartfelder, C. Kartusch, M. Makosch, M. Rovezzi, J. Sa 32 R. Zanella, S. Giorgio, C. R. Henry and C. Louis, J. Phys.
and J. A. van Bokhoven, Catal. Sci. Technol., 2013, 3, 454–461; Chem. B, 2002, 106, 7634–7642.
(b) K. R. Souza, A. F. F. de Lima, F. F. de Sousa and 33 R. J. H. Grisel, P. J. Kooyman and B. E. Nieuwenhuys, J.
L. G. Appel, Appl. Catal., A, 2008, 340, 133–139; (c) Catal., 2000, 191, 430–437.
R. Zanella, S. Giorgio, C.-H. Shin, C. R. Henry and 34 G. C. Bond, C. Louis and D. T. Tompson, Catalysis by Gold,
C. Louis, J. Catal., 2004, 222, 357–367; (d) H.-S. Oh, Imperial College Press, 2006.
J. H. Yang, C. K. Costello, Y. M. Wang, S. R. Bare, 35 A. Goguet, R. Burch, Y. Chen, C. Hardacre, P. Hu,
´
´
H. H. Kung and M. C. Kung, J. Catal., 2002, 210, 375–386;
(e) A. Hugon, N. E. Kolli and C. Louis, J. Catal., 2010, 274,
239–250.
R. W. Joyner, F. C. Meunier, B. S. Mun, D. Thompsett and
D. Tibiletti, J. Phys. Chem. C, 2007, 111, 16927–16933.
36 M. W. E. van den Berg, A. de Toni, M. Bandyopadhyay,
¨
23 E. Bus, R. Prins and J. A. van Bokhoven, Catal. Commun.,
2007, 8, 1397–1402.
H. Gies and W. Grunert, Appl. Catal., A, 2011, 391, 268–280.
37 (a) Y. Guo, D. Gu, Z. Jin, P. P. Du, R. Si, J. Tao, W. Q. Xu,
24 M. Shekhar, J. Wang, W. S. Lee, W. D. Williams, S. M. Kim,
E. A. Stach, J. T. Miller, W. N. Delgass and F. H. Ribeiro, J.
Am. Chem. Soc., 2012, 134, 4700–4708.
Y. Y. Huang, S. Senanayake, Q. S. Song, C. J. Jia and
¨
F. Schuth, Nanoscale, 2015, 7, 4920–4928; (b)
D. P. Akolekar, G. Foran and S. K. Bhargava, J. Synchrotron
Radiat., 2004, 11, 284–290.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 99678–99687 | 99687