Au/CuMgAl-hydrotalcite catalysts promoted by Cu+ and basic sites for selective oxidation of glycerol to dihydroxyacetone

Article abstract of DOI:10.1007/s13404-017-0222-z

CuMgAl-hydrotalcite-supported Au catalysts were prepared and tested in the selective conversion of glycerol to dihydroxyacetone. The electron density of Au was decreased by Cu embedded in the supports, arising from the electron transfer from Au to Cu sites. The valence state (+?1) of Cu ions was detected. Both Cu⁺ and basic sites (Mg–O) affected the catalytic activity of Au catalysts. The Cu⁺ sites promoted the selective oxidation of glycerol to dihydroxyacetone, while basic sites boosted the selectivity oxidation of glycerol to glyceric acid. The synergy of Cu⁺ sites and basic sites could effectively promote the activity and selectivity of Au catalysts in the selectively conversion of glycerol to dihydroxyacetone. A 53% conversion of glycerol and 72% of dihydroxyacetone selectivity were obtained under optimum reaction conditions.

Full text of DOI:10.1007/s13404-017-0222-z

Gold Bull (2017) 50:319–326
https://doi.org/10.1007/s13404-017-0222-z
ORIGINAL PAPER
+
Au/CuMgAl-hydrotalcite catalysts promoted by Cu and basic
sites for selective oxidation of glycerol to dihydroxyacetone
1
,2
1,2
1,2
Yanrui Yin & Tian Tang & Chunli Xu
Received: 28 June 2017 /Accepted: 2 November 2017 /Published online: 22 November 2017
Springer International Publishing AG, part of Springer Nature 2017
#
Abstract CuMgAl-hydrotalcite-supported Au catalysts were
prepared and tested in the selective conversion of glycerol to
dihydroxyacetone. The electron density of Au was decreased
by Cu embedded in the supports, arising from the electron
transfer from Au to Cu sites. The valence state (+ 1) of Cu
through the routes of oxidation. GL is a highly functionalized
molecule and contains two primary OH groups and one sec-
ondary OH group [3]. Oxidation of GL is a very complex
process, including multiple parallel reactions, and generating
a variety of products, such as glyceraldehyde (GLD), dihy-
droxyacetone (DHA), glyceric acid (GLA), tartronic acid
(TAA), oxalic acid (OXA), glycolic acid (GLCA), glyoxylic
acid (GA), and formic acid (FA) [4, 5]. Therefore, controlling
the reaction selectivity is necessary to obtain the desired prod-
ucts. DHA, formed by oxidation of the secondary OH groups
of GL, is one of the desirable products. It is an economically
interesting product, applying in cosmetics industry as a self-
tanning agent and the fine chemicals industry as a chemical
intermediate in organic synthesis [6, 7]. Controlling the selec-
tivity to DHA depends on the design of catalysts [8].
+
ions was detected. Both Cu and basic sites (Mg–O) affected
+
the catalytic activity of Au catalysts. The Cu sites promoted
the selective oxidation of glycerol to dihydroxyacetone, while
basic sites boosted the selectivity oxidation of glycerol to gly-
+
ceric acid. The synergy of Cu sites and basic sites could
effectively promote the activity and selectivity of Au catalysts
in the selectively conversion of glycerol to dihydroxyacetone.
A 53% conversion of glycerol and 72% of dihydroxyacetone
selectivity were obtained under optimum reaction conditions.
+
.
.
.
Au catalysts had attracted considerable attention as prom-
ising aerobic alcohol oxidation catalysts owing to the high
selectivity and resistance against over-oxidation [9]. Liu
et al. [10] reported the performance of Au NPs on different
oxides in catalyzing GL oxidation for DHA formation at ele-
vated temperature. Especially, Au/CuO catalyst exhibited a
high DHA selectivity. However, the catalytic activity of Au/
CuO catalyst was low for the transformation of GL to DHA.
The activity of Au could be promoted by base, such as
NaOH or basic supports [10–12]. However, the main product
was GLA, not DHA, when using Au as catalysts in the pres-
ence of base promoters [13]. Since the activity of Au could be
promoted by basic sites and the selectivity to DHA could be
adjusted by Cu sites [10], we prepared Au/CuMgAl-HTs cat-
alysts, which contained metal sites (Au), basic sites (Mg–O),
Keywords Glyceroloxidation Aucatalyst Cu sites Basic
.
sites Dihydroxyacetone
Introduction
Much attention has been given to transformation of glycerol
(
GL) to value-added fine chemicals and fuel [1, 2], especially
Electronic supplementary material The online version of this article
https://doi.org/10.1007/s13404-017-0222-z) contains supplementary
material, which is available to authorized users.
(
*
Chunli Xu
xuchunli@snnu.edu.cn
+
+
1
and Cu sites. The synergy of basic sites and Cu sites on the
activity of Au catalysts and the selectivity to DHA for GL
oxidation was investigated for the aim to increase both the
activity and selectivity of Au catalysts in the transformation
of GL to DHA.
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry
of Education, Shaanxi Normal University, Xi’an 710119, People’s
Republic of China
2
School of Chemistry and Chemical Engineering, Shaanxi Normal
University, Xi’an, People’s Republic of China

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Gold Bull (2017) 50:319–326
Experimental
instrument. The position and area of the desorption peak are
directly correlated with the basic strength and basic amount,
respectively. Typically, the sample was pretreated with high-
purity He at 350 °C for 1 h. After cooling to 50 °C, CO2
adsorption was performed by switching the He flow to 10%
Preparation of supports
A series of supports were prepared by a co-precipitation meth-
od as described in our report [14]. Typically, a total concen-
tration of 1.5 M mixed aqueous solution of Cu(NO ) ∙3H O,
CO /He gas mixture at 100 °C for 1 h. The gas−phase (and/or
2
weakly adsorbed) CO , was purged by high-purity He at the
3
2
2
2
Mg(NO ) ∙6H O, and (Al(NO ) ∙9H O) were dissolved in
same temperature. CO -TPD was then performed in the He
flow by raising to 800 °C at a rate of 10 °C min . The
3
2
2
3 3
2
2
−
1
1
60 mL of deionized water by continuous stirring with an
alkaline solution of Na CO −NaOH. The pH of the mixture
desorbed CO were detected by MKS Cirrus 2 mass
spectrometer.
2
3
2
was kept at values between 9.5 and 10.5. The molar ratio of
2−
3+
CO3 to Al was 2:1. After being aged at 60 °C for 18 h with
stirring, the slurry was filtered and washed. The precipitate
was dried at 110 °C for 12 h. The obtained materials were
denoted as CuMgAl-HTs (Cu/Mg/Al, 1:1:1, 1:4:1, 1:5:1,
X-ray photoelectron spectra (XPS) were conducted on an
AXIS ULTRA spectrometer (Kratos Analytical Ltd., Japan)
with Al Kα x-ray source (1486.6 eV, 15 kV, 5 mA) at energy
of 150 W. In this study, XPS was used to measure the valence
of Au and Cu. Chemical analyses of metals were performed
by using inductively coupled plasma mass spectrometry (ICP-
MS, M90, Bruker).
1
:7:1, 1:9:1, 1:13:1, 1:15:1, 1:20:1, 1:30:1).
The MgAl-HTs (9:1) supports were prepared by the similar
procedure as CuMgAl-HTs, with the pH of slurry keeping at
0 without the Cu(NO ) ∙3H O aqueous solution. CuAl-HTs
1
3
2
2
sample was prepared by using a similar procedure without the
Reaction procedure
Mg(NO ) ∙6H O aqueous solution.
3
2
2
G L o x i d a t i o n w a s c a r r i e d o u t i n 1 0 0 - m l
polytetrafluoroethylene-lined stainless steel autoclaves (Parr
autoclaves). The catalyst of 0.088 g and GL solution (0.3 M)
were admitted into the reactor, purged with oxygen, and ad-
justed to the desired pressure of 3 bar, heated to 60 °C for 4 h,
cooled to room temperature. Using high-pressure liquid chro-
matography (HPLC) equipped with refractive index and ultra-
violet detectors to analyze the reaction mixture. Authentic
samples were used to identify products. An external calibra-
tion method was used to quantify the products and reactants
remained.
Synthesis of the Ausupported catalyst
The Au/CuMgAl-HTs catalysts were prepared by an impreg-
nation−reduction approach [15]. Typically, the CuMgAl-HTs
supports were added to HAuCl ∙4H O (1 mg/mL) aqueous
4
2
solution under vigorous stirring. After 1 h of vigorous stirring
at room temperature, the slurry was stirred at 85 °C for 1.5 h,
followed by drying at 100 °C for 12 h, and reduction in H at
2
3
00 °C for 2 h. If there is no other special statement, the
amount of theoretical Au loading is 1 wt%.
The Au/MgAl-HTs (9:1), Au/CuO, and Au/CuAl-HTs cat-
alysts were prepared according to the above steps. The Au
loadings was 1 wt%.
Results and discussion
Catalyst characterization
Catalyst characterization
The Micromeritics ASAP 2020 instrument was used to mea-
sure the surface area, pore characteristics, and average pore
diameter of the materials. The samples were degassed at
BET surface area and pore size analysis
The nitrogen physisorption data of supports and Au catalysts
are shown in Table 1. Increasing the Mg content resulted in
continued lowering of the surface area of CuMgAl-HTs sup-
1
40 °C for 4 h. Nitrogen adsorption/desorption isotherms
were calculated at 77 K, the relative pressure range from
.06 to 0.30. Total pore volumes were estimated at a relative
2
−1
0
ports from 93 to 3 m g . The pore volume of the supports
3
−1
pressure of 0.995.
also decreased from 0.66 to 0.018 cm g (entries 1–5). The
specific surface areas of supports increased after Au doping.
For example, the surface area of Au/CuMgAl-HTs (1:30:1)
X-ray diffraction (XRD) patterns were estimated over a
D/Max-2550VB+PC x-ray powder diffractometer (Rigaku
Co., Japan), using a Cu–Ka source fitted with an Inel CPS
2
−1
was up to 174 m g , which was attributed to the high tem-
perature reduction process resulting in the formation of mixed
oxides [15]. The catalysts’ pore volume also decreased from
1
20 hemispherical detector. The scanning transmission elec-
tron microscopy (STEM), and energy-dispersive x-ray were
conducted on a Tecnai G2F20 (USA).
3
−1
0.85 to 0.32 cm g with the increase of Mg content. The
nitrogen adsorption isotherms of the samples exhibited the
characteristic type IV shape [16], together with the pore size
CO -temperature-programmed desorption (CO -TPD)
2
2
were performed on a Micromeritics AutoChem 2920 II

Gold Bull (2017) 50:319–326
321
Table 1 Nitrogen physisorption
data of CuMgAl-HTs supports
and Au catalysts with different
Cu/Mg/Al molar ratio
Entry
Cu/Mg/Al
Supports/catalysts
BET surface area
Pore diameter
(Å)
Pore volume
(cm g )
2
−1 a
b
3
−1 b
(m g )
1
2
3
4
5
1:1:1
93/111
39/140
35/166
27/142
3/174
285/306
259/133
202/99
228/95
330/74
0.66/0.85
0.25/0.47
0.18/0.41
0.16/0.34
0.018/0.32
1:4:1
1:9:1
1:20:1
1:30:1
a
Calculated by the BET method
b
Calculated by the BJH method from the desorption isotherm
distribution results, suggested that the samples were mesopo-
rous materials (Figs. S1 and S2).
The size and composition of catalysts
The morphology and structural of Au/CuMgAl-HTs samples
were characterized by STEM. As shown in Fig. 2, the size of
Au NPs gradually decreased with increasing Mg content. The
average size was approximately 5.0 nm with Cu/Mg/Al molar
ratio of 1:1:1. The size of Au NPs was 3.2 nm for molar ratio
of 1:4:1. Lower mean size of 2.0–2.5 nm was observed for Au
NPs with molar ratio of 1:9:1 (or 1:20:1, 1:30:1). The differ-
ence in size of Au NPs can be attributed to the content and the
dispersion of Cu in the supports, which played an important
role in the dispersion of the Au NPs. In our former reports, it
was found that the dispersion of Cu ions, as configurational
ion of HTsupports, could control the dispersion of the Au NPs
through their interaction [15].
The crystalline phases of samples
The XRD patterns of Au/CuMgAl-HTs catalysts with differ-
ent Cu/Mg/Al molar ratio were shown in Fig. 1. These results
showed the diffraction peaks of MgO, which were observed
irrespective of the Cu/Mg/Al molar ratio and distinct from the
corresponding peaks of their supports (Fig. S3). Moreover, the
Cu O diffraction peaks were observed. Increasing the content
2
of Mg, that is, lowering the content of Cu, the diffraction peak
intensity of Cu O was weaker. The results showed that the
2
+
basic sites (Mg–O) coexisted with metal sites Cu in our cat-
alysts. No diffraction peaks corresponding to Au were ob-
served for catalysts, which may be attributed to its low loading
or low size [17].
The base properties of catalysts
The basicity of the Au/CuMgAl-HTs catalysts was determined
by the CO -TPD. As shown in Fig. 3, the desorption peaks
2
located at similar position for all the samples, which contained
two desorption peaks at 100 and 425 °C, respectively. It sug-
gested that the Au/CuMgAl-HTs catalysts contained weak
basic sites and medium basic sites. The desorption peaks dif-
fered in height and width. Increasing the content of Mg result-
ed in the increase in the height and width of the desorption
peaks, such as the desorption peaks height of Au/CuMgAl-
HTs with Cu/Mg/Al molar ratio of 1:30:1 was twice of that for
Cu/Mg/Al molar ratio of 1:1:1 at 100 °C. This indicated that
the Cu/Mg/Al molar ratio markedly affects the base properties
of Au catalysts.
XPS analysis
As shown in Fig. 4a, the XPS spectra of Au/CuMgAl-HTs
showed peaks of the Au 4f5/2 and Au 4f7/2 states. Increasing
the content of Cu resulted in the positive shifts to higher BE of
2
Theta (Degree)
Au 4f , moving from 83.8 to 84.5 eV, which may be attrib-
Fig. 1 XRD patterns of Au/CuMgAl-HTs samples with different Cu/Mg/
7/2
Al molar ratio
uted to the transfer of electrons from Au to Cu.

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Gold Bull (2017) 50:319–326
Fig. 2 STEM images and
particle size distributions for Au/
CuMgAl-HTs with different Cu/
Mg/Al molar ratio
2
2
1
1
5
0
5
0
Mean=5.0 nm
1:1:1
5
0
3
4
5
6
7
Particle diameter (nm)
2
0
6
2
Mean=3.2 nm
:4:1
1
1
1
8
4
0
1
2
3
4
5
6
Particle diameter (nm)
3
3
2
2
1
1
5
Mean=2.5 nm
1:9:1
0
5
0
5
0
5
0
0
.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Particle diameter (nm)
3
5
Mean=2.4 nm
3
2
2
1
1
0
5
0
5
0
1:20:1
5
0
1
.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Particle diameter (nm)
3
2
2
1
1
0
5
0
5
0
Mean=2.0 nm
1:30:1
5
0
0
.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Particle diameter (nm)

Gold Bull (2017) 50:319–326
323
1
-30-1
0
0
0
.78
.52
.26
Figure 4b showed the XPS spectra of the Cu 2p region. The
BE of Cu 2p3/2 was 932.5 eV and no satellite peaks was ob-
2
+
served, suggesting that the catalysts may not contain Cu
2
+
0
0
.00
.78
because the BE of Cu was 933.6 eV and accompanied by
1
-20-1
2+
the characteristic Cu satellite peaks (938–945 eV).
0
0
.52
.26
+
0
Therefore, it was presumed the presence of Cu or Cu species
in catalyst. It is quite difficult to distinguish the chemical states
0
0
.00
.78
+
0
of Cu or Cu species, because of the similar electron binding
energy and peak shape [18]. Auger Cu LMM spectra [19] can
1
-9-1
0
0
.52
.26
+
0
be used to distinguish the Cu and Cu species. The kinetic
energy (KE) of Cu was at ~ 916.3 eV (Fig. 4c), which corre-
0
0
.00
.78
+
1
-4-1
-1-1
sponds to the Cu . In combination with the spectrum of Cu
2
+
0
0
.52
.26
2p3 and Cu LMM, it can deduce that Cu was reduced to
/2
+
Cu in the process of preparing the catalyst of Au/MgCu-HTs.
0
.00
.78
The measured results were in agreement with XRD data.
However, they were different from the results by Xu et al.
1
0
0
0
0
.52
.26
.00
[10], in which the valence state of Cu was 2+.
+
The effect of basic sites and Cu sites on GL oxidation
2
00
400
600
800
°
Temperature ( C)
-TPD profiles of Au/CuMgAl-HTs catalysts with different
Cu/Mg/Al molar ratio
Fig. 3 CO
2
As shown in Table 2, the catalytic performance of the supports
and Au catalysts for GL oxidation was investigated. The GL
Fig. 4 XPS spectra of Au/
CuMgAl-HTs catalysts with
different Cu/Mg/Al molar ratios.
a Au 4f XP spectra. b Cu 2p XP
spectra. c XAES spectra of Cu
LMM for Au/CuMgAl-HTs
Mg 2s
a
b
1:30:1
1:20:1
0
:9:1
Cu 2p1/2
Cu 2p3/2
Au4f7/2
1
:30:1
:20:1
:9:1
(1:1:1) catalysts
1
1
1:9:1
1:4:1
1:1:1
1
1
:4:1
:1:1
94
92
90
88
86
84
82
965 960 955 950 945 940 935 930
Binding energy (eV)
c Cu LMM
Binding energy (eV)
Cu +
924
921
918
915
912
909
906
Kinetic energy (eV)

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Gold Bull (2017) 50:319–326
Table 2 Activity and selectivity
for GL oxidation over different
supports and catalysts
Entry Catalysts
TOF
GL conv. Product selectivity (%)
(%)
−
1 b
(h )
GLA GLCA TAA GA OXA DHA FA
1
2
3
4
5
6
CuAl-HTs
–
< 1
1.7
–
–
–
–
–
–
–
MgAl-HTs
–
42.9
50.0
–
11.4
13.0
–
–
–
3.9
5.1
5.2
6.8
10.0
–
33
28
–
CuMgAl-HTs
Au/CuAl-HTs
Au/MgAl-HTs
–
3.8
–
–
–
5
4.5
–
–
92.0
28.0
64.0
19
18.0
42.0
45.0
11.0
6.0
8.6
8.0
2.2
2.6
–
–
Au/CuMgAl-HTs 292
4
a
Reaction conditions: 15 mL, 0.3 M GL, 1 wt% Au, GL/Au = 1000 mol/mol, 60 °C, PO2 = 3 bar, stirring
speed = 400 rpm, time = 4 h, Mg/Al = 9:1, Cu/Mg/Al = 1:9:1
b
Turn over frequency = (mole of glycerol converted after 0.5 h)/(reaction time (0.5 h) × mole of total loading of
gold); the amount of catalyst was 0.088 g
conversion of the pure supports (CuAl-HTs, MgAl-HTs,
CuMgAl-HTs) as catalysts were low. The maximum GL con-
version was 3.8% for CuMgAl-HTs (entry 3). MgAl-HTs ex-
hibited the next activity, which was 1.7% (entry 2). The CuAl-
HTs showed the lowest activity (GL conversion, < 1%). The
basic sites (Mg–O) and metal ions (Cu) sites were coexisted in
CuMgAl-HTs. The MgAl-HTs contained only basic sites
6). The lowest selectivity to DHA of 28% was observed for Au/
MgAl-HTs (entry 5). The results demonstrated that Cu sites
+
promoted the selectivity to DHA for GL oxidation by Au cata-
lysts. For the selectivity to GLA, Au/MgAl-HTs catalyst exhib-
ited the highest selectivity of 45% (entry 5). Next was Au/
CuMgAl-HTs with 11% of the selectivity to GLA. However,
no GLAwas found for Au/CuAl-HTs catalyst. The oxidation of
the primary OH groups of GL sequentially generates GLA. It
indicated that the basic sites could promote the oxidation of pri-
mary OH groups of GL.
(
(
Mg–O). While for CuAl-HTs supports, it had only metal ions
Cu) sites. It indicated that the basic sites facilitated the activ-
ity of supports. The results also showed that the basic sites of
Mg–O and metal ions (Cu) sites together promoted the activity
of CuMgAl-HTs for GL oxidation.
XPS analysis showed that there was electron transfer from
+
Au sites to Cu sites. It was also reported that Cu sites were
The catalytic activity increased after Au doping. The max-
imum of GL conversion was 42% (Entry 6) for Au/CuMgAl-
HTs. It indicated that the Au sites acted as the active sites for
GL oxidation. Moreover, Au/CuMgAl-HTs was the most ac-
tive catalyst, next was Au/MgAl-HTs, and Au/CuAl-HTs
showed the lowest activity. The results were further demon-
active in dehydrogenation of secondary alcohol [20, 21],
where the valence of Cu was 0 or + 2. However, no reports
showed the activity of Cu sites in dehydrogenation of sec-
+
ondary alcohol. Therefore, it was not sure whether the effect
+
of Cu sites on activity and selectivity of Au sites was from the
decrease in electron density of Au sites or the direct activation
−1
+
strated by TOF. The TOF value was 292 h for Au/CuMgAl-
of Cu sites on secondary alcohol. Further study will be con-
HTs (entry 6), 19 h for Au/MgAl-HTs (entry 5), and 5 h−1
for Au/CuAl-HTs (entry 4). The difference in the activity of
three Au catalysts indicated that catalytic activity of Au was
−1
ducted in our future work.
Effect of Cu/Mg/Al molar ratio on GL oxidation over Au
catalysts
affected by their supports. Either basic sites or metal ions
+
(
Cu ) in supports could promote the catalytic activity of Au
+
catalysts for GL oxidation. However, the coexistence of basic
The basic sites and Cu sites of supports not only affect the GL
+
sites and metal ions (Cu ) in supports was better than single
conversion, but also the products selectivity over Au catalysts.
In order to increase both the activity and selectivity of Au
sites in promoting the activity of Au.
The products distribution over pure supports and Au catalysts
for GL oxidation was shown in Table 2. When the pure supports
+
catalysts, the effect of the amounts of the basic sites and Cu
sites on GL oxidation was investigated by changing Cu/Mg/
Al molar ratio.
(
MgAl-HTs, CuMgAl-HTs) were used as catalysts, the main
products were GLA and FA. The selectivity toward GLA was
in the range of 40–50%. The selectivity to FAwas around 30%.
This indicated that the pure supports promoted the formation of
over-oxidation products. While the products mainly included
DHA and GLA after Au doping. The Au/CuAl-HTs catalyst
exhibited the highest DHA selectivity of 92% (entry 4). The next
highest selectivity to DHAwas 64% for Au/CuMgAl-HTs (entry
The activity of Au/CuMgAl-HTs versus Cu/Mg/Al molar ra-
tio for GLoxidationwere showninTable 3. When the Cu/Mg/Al
molarratiowas1:1:1,theGLconversionwaslow,only8%(entry
1). Increasing the Mg content (or decreasing the Cu content)
resulted in increasing the GL conversion. The maximum GL
conversion was 46% (entry 7), as the Cu/Mg/Al molar ratio
was 1:15:1. Then, the GL conversion decreased to 16% with

Gold Bull (2017) 50:319–326
325
FA
Table 3 Effect of Cu/Mg/Al
molar ratio on activity and
selectivity of Au/CuMgAl-HTs
for GL oxidation
Entry
Cu/Mg/Al
GL conv. (%)
Product selectivity (%)
GLA
GLCA
TAA
OXA
DHA
1
2
3
4
5
6
7
8
9
1:1:1
8
23
26
33
42
43
46
21
16
9.2
12.0
12.0
9.9
7.3
7.5
–
9.6
12.0
11.0
10.4
10.0
11.2
10.2
13.2
12.9
47.0
62.5
66.0
66.0
64.0
52.4
50.0
19.1
16.5
6.8
0.6
0.6
3.2
4.0
8.0
10.0
7.0
5.0
1:4:1
2.1
2.1
2.3
2.2
3.2
1.9
3.9
4.6
1:5:1
8.5
1:7:1
8.1
1:9:1
11.0
12.0
10.1
33.0
33.0
8.6
1:13:1
1:15:1
1:20:1
1:30:1
12.3
12.9
16.1
18.6
a
Reaction conditions: 1 wt% Au/CuMgAl-HTs, others see the footnote of Table 2
theCu/Mg/Almolarratioof1:30:1(entry9). Itdemonstratedthat
the Cu/Mg/Al molar ratio affected the activity of the Au/
CuMgAl-HTs catalysts. The specific surface area of Au/
CuMgAl-HTs with different Cu/Mg/Al molar ratios (Table 1)
did not have direct relation with their activity. Au/CuMgAl-
HTs (1:30:1) performed low GL conversion but the maximum
specific surface area. As mentioned before, Au/CuMgAl-HTs
Au catalysts was resulted from the effect of Cu sites and basic
sites, but not the size of Au nanoparticles.
The effect of reaction conditions and the stability
of catalysts
+
catalysts contained Au, basic, and Cu sites. Modulating of the
The effect of reaction conditions, such as reaction time, tem-
perature, and catalyst dosage was investigated (Table S1).
Under optimum reaction conditions, 0.088 g of 1 wt% Au/
CuMgAl-HTs catalyst at a temperature of 60 °C, and Cu/Mg/
Al molar ratio of 1:9:1, the conversion of GL reached 53%,
and the selectivity to DHA was up to 72%.
The post-reaction solutions were tested for metal (Au, Cu,
Mg, and Al) by inductively coupled plasma mass spectrome-
try (ICP-MS) and the leaching rates of metals were showed in
Table S2. Leaching rate of all the metals was less than 15%.
The reaction was conducted for 2 h and then the catalyst was
filtered out to observe whether the reaction continued after the
catalyst was removed (4 h). The conversion of glycerol over
2 h was 27% and remained unchanged over 4 h after the
catalyst removed. This could prove that there was no apparent
homogeneous catalysis taking place in the glycerol oxidation
for Au/CuMgAl-HTs.
molar ratio of Cu/Mg/Al resulted in the change of the amounts of
+
the basic sites and Cu sites. Thus, the amounts of the basic sites
+
and Cu sites affected the GL conversion.
The selectivity to DHA was 47% when Cu/Mg/Al molar
ratio was 1:1:1. It reached to 62–66% when Cu/Mg/Al molar
ratio was in the range of 1:4:1~1:9:1 (entries 2–5). The selec-
tivitytoDHAbegantodecreasewithincreasingtheMgcontent
(
1
or decreasing the Cu content; entries 6–9). It decreased to
6.5%, as the Cu/Mg/Al molar ratio was 1:30:1 (entry 9). It
+
indicated that the appropriate amounts of basic sites and Cu
sites promoted the selective oxidation of glycerol to DHA over
Au catalysts. The selectivity to GLA changed slightly in the
range of 9.2–12.0% (entries 1–7) when content of Mg was
low. WhentheMgcontent washigh, i.e., Cu/Mg/Almolar ratio
was 1:20:1 or 1:30:1, the selectivity to GLA increased to 33%
(
entry 8). The results showed that the amounts of basic sites
affect the selectivity to GLA. It was found that the similar phe-
nomena were observed for the selectivity to GLCA (or TAA,
OXA). Thus, it indicated that the basic sites promoted the oxi-
dationofprimaryOHgroupandtheover-oxidationofproducts.
The possible reaction pathway was shown in Scheme 1.
Fig. S4a–c shows XRD patterns of Au/MgAl-HTs, Au/
CuAl-HTs, and Au/CuMgAl-HTs catalysts before and after
reaction. The XRD pattern of Au/MgAl-HTs before and after
reaction were similar, displaying typical diffraction peak of
HTs at 11.33°, 22.84°, 34.74°, and 39.13° and MgO at 43.1°
The size of Au nanoparticles normally affected its catalytic
performance [22]. The size for Au/CuMgAl-HTs with 1:9:1 of
Cu/Mg/Al molar ratio was similar with that of 1:20:1 of Cu/
Mg/Al molar ratio (Fig. 2). The similar size eliminated the
effect of size on activity of Au. However, they showed differ-
ent catalytic performance. The former generated 42% of GL
conversion (Table 2, entry 5), while the latter got 21% of GL
conversion. This further demonstrated that the difference in
Scheme 1 Reaction pathway for GL oxidation

3
26
Gold Bull (2017) 50:319–326
4
.
.
Jin X, Zhao M, Zeng C, Yan W, Song Z, Thapa PS, Subramaniam
B, Chaudhari RV (2016) Oxidation of glycerol to dicarboxylic acids
using cobalt catalysts. ACS Catal 6:4576–4583
and 62.3° before reaction. The XRD patterns of Au/CuAl-HTs
before and after reaction were also similar, showing diffrac-
tion patterns characteristic of Cu, CuO, and Cu O with a dif-
2
5
Garcia AC, Kolb MJ, van Nieropy SC, Vos J, Birdja YY, Kwon Y,
Tremiliosi-Filho G, Koper MTM (2016) Strong impact of platinum
surface structure on primary and secondary alcohol oxidation dur-
ing electro-oxidation of glycerol. ACS Catal 6:4491–4500
Hu W, Knight D, Lowry B, Varma A (2010) Selective oxidation of
glycerol to dihydroxyacetone over Pt−Bi/C catalyst: optimization
of catalyst and reaction conditions. Ind Eng Chem Res 49:10876–
10882
ference in the intensity of Cu. Au/CuAl-HTs after reaction
displayed weaker diffraction peak of Cu. Au/CuMgAl-HTs
before reaction showed diffraction peak of MgO and Cu O.
2
6
.
The pattern of Au/CuMgAl-HTs after reaction displayed char-
acteristic peak of HTs, CuO, and MgO. This indicated that
Cu O was oxidized to CuO in the reaction system and the
2
7
8
9
.
.
.
Lari GM, Mondelli C, Pérez-Ramírez J (2015) Gas-phase oxidation
of glycerol to dihydroxyacetone over tailored iron zeolites. ACS
Catal 5:1453–1461
Zhou CH, Beltramini JN, Fana Y, Lu GQ (2008) Chemoselective
catalytic conversion of glycerol as a biorenewable source to valu-
able commodity chemicals. Chem Soc Rev 37:527–549
Dimitratos N, Lopez-Sanchez JA, Anthonykutty JM, Brett G,
Carley AF, Knight DW, Hutchings GJ (2009) Oxidation of glycerol
using gold-palladium alloy-supported nanocrystals. Phys Chem
Chem Phys 11:4952–4961
support partly transformed back to structure of HTs.
The reusability of Au/CuMgAl-HTs was investigated and
showed in Table S3 (supporting information).The conversion
of glycerol on fresh catalyst was 53%. The conversion de-
creased to 47, 43, and 39% in the first, second, and third
run. During the recycle, the descent rate of glycerol conver-
sion was approximately 10%. Thus, the deactivation of the
catalysts was not significant and further proved a better stabil-
ity and reused activity of the used catalyst.
10. Liu S, Sun K, Xu B (2014) Specific selectivity of Au-catalyzed
oxidation of glycerol and other C3-polyols in water without the
presence of a base. ACS Catal 4:2226–2230
1
1. Villa A, Campisi S, Mohammed KMH (2015) Tailoring the selec-
Conclusions
tivity of glycerol oxidation by tuning the acid–base properties of Au
catalysts. Catal Sci Technol 5(2):1126–1132
+
The synergy of Cu sites and basic sites (Mg–O) promoted the
activity of Au catalyst. GL conversion was 42% for Au/
CuMgAl-HTs containing both Cu sites and basic sites, which
was far higher than that of Au/MgAl-HTs containing only
12. Carrettin S, McMorn P, Johnston P, Griffin K, Hutchings GJ (2002)
Selective oxidation of glycerol to glyceric acid using a gold catalyst
in aqueous sodium hydroxide. Chem Commun 7:696–697
+
1
3. Carrettin S, McMorn P, Johnston P, Griffin K, Kiely CJ, Hutchings
GJ (2003) Oxidation of glycerol using supported Pt, Pd and Au
catalysts. Phys Chem Chem Phys 5:1329–1336
+
basic sites (18%) and Au/CuAl-HTs containing only Cu sites
+
(
4.5%). The Cu sites in Au/CuMgAl-HTs and Au/CuAl-HTs
14. Xu C, Sun J, Zhao B, Liu Q (2010) On the study of KF/Zn(Al)O
catalyst for biodiesel production from vegetable oil. Appl Catal B
activated the second OH groups of GL to selectively generate
DHA, and the selectivity of DHA was 64 and 92%, respec-
tively. The basic sites promoted the formation of GLA, and the
maximum selectivity to GLA (45%) was resulted from Au/
MgAl-HTs. Therefore, the coexistence of Cu sites and basic
sites affect both the conversion and selectivity of Au in the GL
9
9: 111–117
1
5. Wang H, Liu D, Xu C (2016) Directed synthesis of well dispersed
and highly active AuCu and AuNi nanoparticle catalysts. Catal Sci
Technol 6:7137–7150
+
16. Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA,
Rouquerol J, Siemieniewska T (1985) Reporting physisorption data
for gas/solid systems with special reference to the determination of
surface area and porosity. Pure Appl Chem 57:603–619
+
oxidation to DHA. The effect depended on the ratio of Cu
sites to basic sites. Under Cu/Mg/Al molar ratio of 1:9:1 and
optimum reaction conditions (60 °C, 6 h, and 0.088 g of
1
7. Xu J, Yue H, Liu S, Wang H, Du Y, Xu C, Dong W, Liu C (2016)
Cu-Ag/hydrotalcite catalysts for dehydrogenative cross-coupling of
primary and secondary benzylic alcohols. RSC Adv 6:24164–
1
wt% Au/CuMgAl-HTs catalyst), the conversion of GL
2
4174
reached 53%, and the selectivity to DHA was up to 72%.
1
1
8. Deutsch KL, Shanks BH (2012) Active species of copper chromite
catalyst in C-O hydrogenolysis of 5-methylfurfuryl alcohol. J Catal
Funding This work was supported by projects funded by the Major
Research Plan of National Natural Science Foundation of China
2
85:235–241
9. Dai W, Sun Q, Deng J, Wu D, Sun Y (2001) XPS studies of Cu/
ZnO/Al O ultra-fine catalysts derived by a novel gel oxalate co-
(Program No. 91545130).
2
3
2 2
precipitation for methanol synthesis by CO +H . Appl Surf Sci
1
77:172–179
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