Synergistic effect between copper and different metal oxides in the selective hydrogenolysis of glucose

Article abstract of DOI:10.1039/c8nj05815f

Copper catalysts have been extensively applied in saccharide hydrogenolysis for their high selectivity to C-O bond cleavage. The hydrogenolysis of glucose contains many reaction procedures, which need the synergistic effect of different active sites. A series of Cu catalysts supported on metal oxides with different surface physicochemical properties were prepared. The metal oxide supports not only influence the properties of Cu, such as dispersion and the electronic state, but also affect the activity of C-C and C-O bond cleavage in glucose. Furthermore, the coordination of a large amount of Lewis acid sites and hydrogenation sites on a Cu/γ-Al₂O₃ catalyst can promote C-C and C-O bond cleavage, leading to the selective conversion of glucose to glycol (selectivity of 66.6%). A Cu/MgO catalyst with a large amount of basic sites and metal sites could accelerate the retro-aldol condensation and isomerization reactions simultaneously, resulting in the main products of C₂, C₃, and C₄ polyols. A study of the synergistic effect between other transition metals and γ-Al₂O₃ showed that Pd had high activity for central C-C bond cleavage in glucose. Ru provided extremely strong activity for C-C bond cleavage at the terminal of the carbon chain in glucose, with the main product being methane (selectivity of 66.4%).

Full text of DOI:10.1039/c8nj05815f

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January 2016 Pages 1–846
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Synergistic effect between copper and different metal oxides in the selective
hydrogenolysis of glucose
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2,3
1,4
Chengwei Liu , Zhaonan Zhang , Xuefeng Zhai , Xianzhou Wang , Jianzhou Gui *,
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Chenghua Zhang *, Yulei Zhu , and Yongwang Li
1
State Key Laboratory of Separation Membranes & Membrane Processes, College of
Environment and Chemical Engineering, Tianjin Polytechnic University, Tianjin
300387, China
2
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, People’s Republic of China
3
1
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
11
China
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School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin
300387, China
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National Energy Center for Coal to Liquids, Synfuels China Technology Co., Ltd.,
15
Huairou District, Beijing 101407, People’s Republic of China
*Corresponding authors
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E-mail address: jzgui@hotmail.com. Tel.: +86-22-83955668 (J. Gui).
E-mail address: zhangchh@sxicc.ac.cn. Tel.: +86-10-69667802 (C. Zhang).
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Abstract
Copper catalysts had been extensively applied in saccharide hydrogenolysis for their
high selectivity to C-O bond cleavage. The hydrogenolysis of glucose contains many
reaction procedures, which needs the synergistic effect of different active sites. A series
of Cu catalysts supported on metal oxides with different surface physicochemical
properties were prepared. The metal oxide supports not only influence the properties of
Cu such as dispersion and electronic state, but also affect the activity of C-C and C-O
bond cleavage in glucose. Furthermore, the coordination of a large amount of Lewis
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acid sites and hydrogenation sites on Cu/γ-Al
2
O catalyst can promote the C-C and C-
3
1
O bond cleavage, leading to the selective conversion of glucose to glycol (selectivity of
66.6%). The Cu/MgO catalyst with a large amount of basic sites and metal sites could
accelerate the retro-aldol condensation and isomerization reactions simultaneously,
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1
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3
resulting the main products of C
2
, C
3
, and C
4
polyols. The study of the synergistic effect
showed that Pd had high activity for the
14
between other transition metals and γ-Al
2
O
3
15
central C-C bond cleavage in glucose. Ru provided an extremely strong activity for the
C-C bond cleavage at the terminal of the carbon chain in glucose, with the main product
of methane (selectivity of 66.4%).
16
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7
8
Keywords: glucose, hydrogenolysis, glycol, copper, acid/basic sites
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1. Introduction
The depletion of the non-renewable fossil fuel resources, combined with the ongoing
climate change, indicates the necessity of the shift of the feedstock base towards
sustainable biorefinery approach.1-5 Since cellulose is the most abundant renewable
resource in nature and may not compete with the production of edible crops, the
efficient conversion of cellulose and its derivatives has attracted much attention. D-
glucose, as the only monomer of cellulose, is a particularly simple and promising
platform molecule of biomass for better understanding the complex process occurring
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in the conversion of polysaccharides. Therefore, the catalytic conversion of glucose
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plays an important role in the production of a variety of valuable chemicals in the
utilization of biomass resources. Furthermore, significant efforts have been devoted to
the transformation of glucose by various catalytic processes involving hydrogenation,
hydrogenolysis, dehydration, isomerization, oxidation and so on.7-11 In this respect, the
hydrogenation/hydrogenolysis of saccharides and polyols, by selectively breaking C-C
and C-O bonds, provides an efficient and environmental benign process for the catalytic
conversion of glucose to value-added chemicals and liquid hydrocarbons such as
sorbitol, glycerol, propanediol (PDO), ethylene glycol (EG), hexane, etc.12-14
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Copper catalysts were often used for sugar hydrogenolysis at 235-250 C.
They
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were found to be excellent alternatives for sugar and sugar alcohol hydrogenolysis due
to their relatively low activity for the hydrogenolysis of C-C bonds and high efficiency
for C-O bond cleavage.17,18 For example, the copper chromium oxide catalyst has been
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21
22
used by Zartman and Alkins for the hydrogenolysis of glucose in supercritical ethanol,19
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with 60% yield of 1,2-PDO. Furthermore, copper catalysts were often selected for the
hydrogenolysis of glycerol and sorbitol to PDO (1,2-PDO and 1,3-PDO).18,20,21
Typically, Chaminand obtained nearly 100% selectivity of 1,2-PDO at a low glycerol
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conversion on a Cu-ZnO catalyst. Guo tested a series of Cu/γ-Al
different amounts of copper, with 96.8% selectivity and 49.6% conversion achieved at
493 K and 1.5 MPa H pressure. However, the coordination of copper and acid/basic
2
O catalysts with
3
2
sites has not been studied systematically.
With respect to the former researches on glucose hydrogenolysis, many
disadvantages also exist which prevent the selective conversion of the promising
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feedstocks. Alkali additives such as NaOH and Ca(OH)
2
had often been used to promote
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the C-C cleavage activity.23,24 However, the alkali existing in the reaction solution could
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cause the excessive degradation and product separation problem. Obviously, many
other drawbacks, such as the low selectivity, harsh conditions and environmental
pollution also inhibit the effective conversion of glucose. In order to develop an
economical and environmental benign process for glucose conversion, heterogeneous
catalytic system was selected in this text.
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As widely accepted that the bifunctional catalysts containing metal sites and
acid/basic sites are most efficient for the hydrogenolysis of glucose, with the synergistic
effect of different active sites. Furthermore, the elemental reactions of saccharide
hydrogenolysis, occurring at the hydrogenation conditions with metal catalysts, mainly
contain hydrogenation/dehydrogenation, isomerization, retro-aldol/aldol condensation,
19
20
21
22
and dehydration reactions.1,2 The hydrogenation and dehydrogenation reactions are
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conducted on the transition metal sites. The retro-aldol condensation reaction that leads
to the cleavage of C-C bonds occurs on metal sites. The C-O bond cleavage of polyols
can be catalyzed by the acid sites through dehydration.1,26 However, the transition metal
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sites could also catalyze the cleavage of C-C/C-O bonds through the direct
hydrogenolysis reaction, which makes the reaction more complicated.23,24 For glucose
transformation, the basic sites and the Lewis acid sites are also responsible for the
isomerization between glucose and fructose. Based on the reaction pathway, the
synergistic effect between metal and metal oxides with different acid/basic sites plays
an important role in the product selectivity. For example, Wang studied the
hydrogenolysis of sorbitol over MgO-supported Ni, Cu, and Co catalysts.25,27
1
C and
2
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C polyols such as glycerol, 1,2-PDO, and EG were the main products. Huang et al.
3
1
1
2
3
have compared the activity and selectivity of Cu supported on SiO
2
, Al
2
O
3
MgO, ZrO ,
2
28
CeO
2
, and ZnO for xylitol hydrogenolysis. Cu-SiO
2
showed the highest selectivity for
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EG and PDO among the prepared catalysts. However, the coordination effects between
copper and different metal oxides in the bifunctional catalysts for the selective
conversion of glucose have not been studied yet.
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In this article, a series of metal oxides with different surface acid/basic properties
were selected for copper catalysts. The relationships between the physicochemical
properties and catalytic performance of copper catalysts were studied in detail.
19
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Furthermore, the synergistic effects between γ-Al
2
O and different transition metals
3
21
were also studied.
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2. Material and Methods
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2.1. Catalysts preparation
ZrO and TiO were purchased from Nanjing Haitai nanometer material Co., Ltd. γ-
was from sinopharm chemical reagent Co., Ltd. MgO was prepared by a co-
2
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Al
2
O
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precipitation method using the 0.5 M magnesium nitrate aqueous solution and 0.5 M
ammonia at pH 9. The precipitate was aged for 1 h, then separated with filtration and
washed with deionized water until the filtrate was neutral. Then the precursor was dried
o
o
at 120 C overnight and calcined at 500 C for 5 h. SiO
2
was prepared by colloidal silica
aqueous solution (SiO
2
, 30 wt%, Qingdao, Ocean Chemical Co., Ltd., China)
o
o
o
dehydration at 60 C followed by dried at 120 C and calcined at 500 C for 5 h. All the
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metal oxide supports were granted into size 20-40 mesh before used.
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All the catalysts were prepared by an incipient wetness impregnation method. The
contents of Cu and Ni were designed as 5 wt%, and Pd and Ru were 2 wt%. Take Cu/γ-
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2
3
Al
2
O
3
for example, 10 g γ-Al
2
O was impregnated with 13 mL copper nitrate aqueous
3
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solution containing 1.9 g Cu(NO
3
)
2
·3H
2
O. After impregnation, the sample was dried at
o
o
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120 C overnight and then calcined at 500 C for 5 h.
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2.2 Catalysts characterization
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ICP optical emission spectroscopy (Optima2100DV, PerkinEler) was performed to
measure the copper loading of the calcined copper catalysts.
The BET surface area of the calcined catalysts was measured on a micromeritics
19
o
o
20
ASAP 2420 instrument. All the samples were degassed at 110 C for 1 h and 350 C for
21
8 h before the measurement.
22
Powder X ray diffraction (XRD) patterns were obtained on a D/max-RA X-ray
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diffractometer (Bruker, Germany), with Cu-Kα radiation operating at 30 KV and 100
o
o
mA. The XRD patterns were recorded in 2θ values ranging from 10 to 80 .
DRIFTS spectra of CO adsorption were collected using an infrared spectrometer
(VERTEX70, Bruker, Germany). The infrared cell with ZnSe windows was connected
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to a gas-feed system, which allowed the in situ measurement of the adsorption of CO.
o
Prior to measurement, the catalysts were reduced in situ at 250 C for 10 h in H
2
flow
o
(20 mL/min). The sample was flushed with helium for 1 h at 250 C. Then the system
o
was cooled to 30 C in helium flow for the CO-FTIR record.
NH
3
temperature-programmed desorption (NH
3
-TPD) measurements and CO
2
1
temperature-programmed desorption (CO
2
-TPD) measurements were conducted on an
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AutoChem II 2920 equipment (Mircromeritics, USA). In a typical run, 200 mg catalyst
was put into a u-shaped quartz tube. The sample was first pretreated in a helium flow
1
1
2
3
o
o
at 500 C for 1 h. After cooling to 100 C in the helium flow, the sample was adsorbed
with NH or CO until saturation, then flushed with helium for 1 h to remove the
physical adsorption. The NH
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3
2
15
3
or CO
2
desorption pattern was recorded using MS
o
o
o
16
detectors from 100 C to 500 C at a heating rate of 10 C/min.
1
1
7
8
The Py-FTIR spectra were obtained using a VERTEX70 Bruker FT-IR spectrometer.
-2
o
All the samples were pretreated by degassing to a pressure of 10 MPa at 350 C for 1
o
o
o
19
h. The Py-FTIR spectra were measured at 30 C after evacuations at 30 C, 200 C, and
o
20
350 C for 30 min.
21
Adsorptive decomposition of N O was conducted using the method reported
2
22
previously.29-31 The exposed copper surface area was calculated based on the assuming
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spherical shape of the copper metal particles and 1.4×10 copper atoms/m . For one
o
measurement, the catalyst was reduced at 350 C in a 10 vol% H
2
/Ar flow for 15 min
-TPR was
o
firstly. Then it was exposed to N
recorded with the 10 vol% H
2
O at 40 C for 30 min. Finally, the H
2
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0
2
/Ar flow after flushing with Ar (50 mL/min) for 30 min.
2.3. Catalytic reaction process
The catalytic tests were carried out in a tubular fixed-bed reactor (i.d. 12 mm, length
600 mm). For a typical run, 6 mL catalysts were packed at the flat-temperature zone
o
and in situ reduced at 250 C for 10 h with a stream of pure hydrogen. The reactant was
introduced into the reactor with an HPLC pump along with a co-feed H of gas flowing.
2
1
The liquid products collected in the cold trap were analyzed by a high performance
liquid chromatograph (HPLC) equipped with an RID detector. The gas products were
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1
1
2
3
analyzed in situ by gas chromatograph (GC) (models 6890N and 4890D; Agilent).
o
Typical reaction conditions are as follows: 180 C, 4 MPa H
2
, 5 wt% glucose (sorbitol,
14
fructose, glycerol) aqueous solution, ratio of H to glucose = 161:1 (molar ratio), LHSV
2
= 0.2 h-1.
15
16
The total carbon (TC) balance was also measured. TC in feedstock and liquid products
after reaction was detected by German elementar (Vario EL cube). Sulfanilamide was
used as a standard sample which contains 41.81% of C. The carbon content in gas was
calculated from concentration values obtained by GC chromatographic analysis. Then
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19
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TC was calculated by the following equation:
Carbon in liquid product + Carbon in gas product
2
2
1
2
TC(%) =
× 100
Carbon in feedstock
The conversion of glucose and the selectivity of products were quantified by the
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following equation:
carbon mol of reactant after reaction
carbon mol of reactant in feedstock
2
3
Conversion (%) = 100 −
× 100
carbon mol of each product
Selectivity (%) =
× 100
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5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
sum of carbon mol for all products
4
3. Results and discussion
5
6
3.1. Physicochemical properties of the prepared catalysts
Table 1. Physicochemical properties of the prepared catalysts
Cu
S
BET
S
Cu
Acidity
(μmol/g)d
21.7
Basicity
Catalyst
(mmol/gCat)a
0.71
(m /g)
171.4
171.1
53.0
2
b
(m /g)
0.5
2
c
(μmol/g)e
23.9
Cu/SiO
Cu/γ-Al
Cu/MgO
Cu/TiO
2
2
O
3
0.73
44.8
11.2
3.0
88.8
40.0
0.73
16.7
45.0
2
0.72
23.9
29.6
23.5
Cu/ZrO
2
0.73
20.4
8.0
23.1
26.1
a
ICP method. b BET method. c Copper metal surface area, determined by the N
d
2
O-decomposition method.
e
Measured by NH
3
-TPD. Measured by CO
2
-TPD.
7
8
9
0
1
2
3
The supports such as γ-Al
properties were tested. γ-Al
2
3
O
3
, MgO, ZrO
2
, TiO
2
, and SiO with special surface
2
2
O
, with the large amount of acid sites, is extensively used
and TiO are favorable for
3
2
in industrial processes. The amphoteric properties of ZrO
2
2
1
1
1
1
saccharide conversion which needs the synergetic catalysis of different active sites.33
MgO also plays a good performance on polyols conversion to glycerol and glycol.25
SiO has been widely researched for glycerol conversion, with a high selectivity of PDO
2
obtained.34
14
The physicochemical properties of the copper-based catalysts are listed in Table 1.
The small discrepancy of Cu loadings could be due to the loss in preparation procedure.
1
5
16
2
The surface areas of the calcined copper catalysts decreased with the order: Cu/SiO ～
9

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2
2
2
2
2
2
2
2
2
2
3
3
3
3
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Cu/γ-Al
2
O
3
> Cu/MgO > Cu/TiO
2
> Cu/ZrO .
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
3
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
Figure 1. XRD patterns of the calcined catalysts.
The X ray diffraction patterns were obtained to detect the dispersion of copper
4
5
6
7
8
9
0
1
2
o
o
(Figure 1). Most of the catalysts showed the CuO peaks at 2θ = 35.5 and 38.7 (JCPDS
48-1548) except Cu/γ-Al . It indicates that CuO particles were dispersed very well
on the large surface of γ-Al . The N O adsorptive decomposition method is reliable
2
O
3
2
O
3
2
to character the copper surface area. The results showed that Cu/γ-Al
2
O had the highest
3
2
copper surface area (44.8 m /g), which was consistent with the XRD results. However,
copper surface areas showed a great discrepancy with the catalyst surface areas, which
suggested that the copper surface area not only depended on the physical dispersion of
the supports, but also had correlation with the metal-support interaction. The lowest
1
1
1
13
dispersion of Cu on SiO depends strongly on the preparation method of the catalysts,
2
3
5
1
1
1
1
4
5
6
7
which can cause the weak metal-support interaction. For example, the high calcination
of SiO and the catalyst precursor could decrease the surface silanol concentration,
which is disadvantage to the dispersion of copper nanoparticles.36,37 The dispersion of
copper on different supports decreased with the following order: Cu/γ-Al O >
2
2
3
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Cu/MgO > Cu/ZrO
2
> Cu/TiO
2
> Cu/SiO .
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
Figure 2. CO-DRIFT patterns of the reduced catalysts.
4
5
6
7
8
9
0
CO is used as a probe molecule to study the metal-support interaction, as shown in
Figure 2. The bands of CO adsorption on the reduced catalysts changed along with the
-
1
chemical environment of the metal. Generally, the bands at 2110 cm and below are
-
1
ascribed to CO on zero-valent copper metal particles. The bands between 2110 cm and
-1
38
2135 cm are assigned to CO irreversibly adsorbed on cuprous ions. The pattern of
-1
Cu/TiO
2
shows a broad band with the peak at 2123 cm after purging helium for half a
-1
1
minute, which shifted to a lower band at 2115 cm after purging helium for 15 min.
11
This indicates that there are at least two different sites on the Cu/TiO
2
surface. Therefore,
-1
-1
1
1
1
1
2
3
4
5
it’s reasonable to spite the band into two peaks at 2123 cm and 2115 cm . The band
-1
at 2115 cm is ascribed to CO adsorbed on imperfect sites, such as step and edge sites.
-1
The band at 2123 cm could be assigned to CO adsorbed on isolated Cu atoms and/or
small 2D Cu particles on the TiO surface. Small 2D Cu particles in Cu/TiO are
partially electropositive as a result of interactions with oxygen atoms on the surface of
the support, as reported by Boccuzzi et al.39,40 The Cu/SiO
catalyst showed the band at
2
2
16
1
7
2
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1
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2
2
2
2
2
2
2
2
2
3
3
3
3
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3
3
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2119 cm , which was easily removed after purging helium for a short time. Ferullo and
Castellani have assigned this band to CO adsorbed on electropositive copper linked to
41
a Si-O* site when the peak is reversible. The band of CO adsorbed on Cu/ZrO
2
was
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
+
42
stable at 2115 cm , which could be ascribed to CO adsorbed on Cu sites. The bands
-1
-1
of CO adsorbed on Cu/MgO and Cu/γ-Al
2
O were at 2110 cm and 2113 cm , which
3
almost disappeared after purging helium for 15 min. It is suggested that the bands could
be assigned to CO-Cu sites.38,43 Therefore, as a result of the interaction between metal
oxides and copper, copper species with different valences existed on the catalyst surface.
0
In summary, CuO was easily reduced on MgO and γ-Al
2
O
3
, resulting in the main species
and ZrO , which
0
1
of Cu on the catalyst surface. CuO was difficult to be reduced on TiO
2
2
11
could be due to the strong interaction between Cu and the metal oxides. CuO was also
1
1
2
3
not easy to be reduced on the SiO
CuO.
2
support, which is due to the large particle size of
14
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2

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1
2
2
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2
2
2
2
3
3
3
3
3
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3
3
3
3
4
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6
0
1
2
3
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5
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9
0
1
2
3
4
5
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9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Figure 3. NH
3
-TPD (A) and CO
2
-TPD (B) patterns of the calcined copper
3
4
catalysts.
NH
3
-TPD and CO
2
-TPD were performed to identify the acidity and basicity of the
5
catalysts respectively. The amounts of acid and basic sites, as listed in Table 1, were
calculated by the integrating curves and calibrated by standard NH and CO gases. The
amounts of acid sites on the catalyst surface decreased in the following order: Cu/γ-
6
3
2
7
8
Al
2
O
3
> Cu/TiO
2
> Cu/ZrO
2
> Cu/SiO
2
> Cu/MgO (Table 1). As shown in Figure 3A,
o
o
9
Cu/γ-Al
2
O
3
had a main peak at 172 C with a shoulder peak at around 258 C, indicating
10
that there were weak and medium acid sites on the surface of Cu/γ-Al
of Cu/TiO , Cu/ZrO , Cu/SiO
2
O
3
. In the case
o
1
1
1
2
2
2
2
, and Cu/MgO catalysts, only one peak at about 150 C
was observed, which can be ascribed to the weak acid sites. The amounts of the basic
sites in the copper catalysts decreased with the following order: Cu/MgO > Cu/γ-Al
Cu/ZrO > Cu/SiO ~ Cu/TiO . Figure 3B shows the CO -TPD profiles of the copper
, Cu/ZrO , Cu/SiO , and Cu/TiO catalysts, only a broad
13
2
3
O >
1
4
2
2
2
2
15
catalysts. For the Cu/γ-Al
2
O
3
2
2
2
o
1
6
peak at 150 C was observed, which is corresponding to the weak basic sites on the
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3

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catalysts surface. However, a new shoulder peak at about 212 C was observed on
Cu/MgO, which could be attributed to the medium basic sites on the catalyst surface.
The results indicate that there were large amounts of weak and medium acid sites and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
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0
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0
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2
3
4
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9
0
weak basic sites on the surface of the Cu/γ-Al
2
O
3
catalysts, with the acid/basic sites mol
are changeable with the processing
ratio of 2.2. The surface properties of γ-Al
2
O
3
method. For example, the calcination process could cause the remove of the hydroxyl
group on γ-Al surface, which could bring new Lewis acid sites by the electron
deficient aluminium ions. Many researchers have also detected the surface acid and
base properties using CO -TPD and NH
-TPD.44-47 For example, Kumar et al.
performed the CO -TPD and NH -TPD of two kinds of commercial Al
turned out that one Al
2
O
3
2
3
.47 The results
1
2
3
2
O
3
11
2
O
3
sample showed nearly the same desorption amounts of CO
2
-1
-1
1
1
2
3
and NH
Cu/ZrO
surface. Relatively low amounts of acid and basic sites are present on the Cu/SiO
3
, with the values of 56.6 μmol g and 55.6 μmol g , respectively. Both the
2
and Cu/TiO catalysts had small amounts of acid and basic sites on the catalyst
2
14
2
15
catalysts surface. The Cu/MgO catalyst displayed the largest amounts of weak and
medium basic sites and the lowest amount of acid sites.
16
1
1
7
8
Py-FTIR spectra were obtained to identify the nature of acid sites on the different
supported catalysts, as illustrated in Figure 4. For the spectra of pyridine adsorbed on
o
-1
-1
19
Cu/γ-Al
2
O when outgassing at 30 C, the bands at 1577 cm and 1593 cm represent
3
4
8
20
physically adsorbed pyridine and hydrogen-bonded pyridine respectively, considering
-1
21
their disappearance after evacuation at higher temperature. The band at 1448 cm , 1489
-1
-1
22
cm , and 1610 cm were ascribed to the coordination of Py molecules to Lewis acid
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4

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sites and consequent formation of LPy species. Moreover, the decrease of the intensity
and the increase in the frequency of the peak after evacuation at subsequent higher
temperatures obviously indicated that there were both weak and medium Lewis acid
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
sites on the surface of Cu/γ-Al
2
O
3
.1,49,50 The bands around 1445, 1489, 1574, and 1604
and Cu/ZrO at 30 °C showed the strong features of
-1
cm in the spectrum of Cu/TiO
2
2
4
9
the formation of LPy species. However, the peak of the LPy species was very weak
o
and almost disappeared after evacuation at 200 C, suggesting that there were only small
amounts of weak Lewis acid sites on the surface of Cu/TiO
2
and Cu/ZrO
2
. The peak of
o
o
pyridine adsorbed on Cu/SiO
2
at 30 C dramatically decreased at 200 C, which
1
indicated there was only weak acid sites on the catalyst surface. The bands at 1439,
-1
o
11
1485, and 1597 cm in the spectrum of the Cu/MgO catalyst after evacuation at 30 C
1
1
2
3
represent hydrogen-bonded pyridine. The peaks totally disappeared as expected.
-1
In this context, only a weak shoulder band at around 1540 cm was found for the
14
Cu/γ-Al
2
O
3
catalyst. The results were consistent with Chen’s report, which also showed
51
15
that only a negligible amount of Brønsted acid existed on the Al
2
O
3
surface. However,
-1
16
no band at 1540 cm was found for the other catalysts, indicating the absence of
1
1
7
8
Brønsted acid sites. For the Cu/γ-Al
acid sites. There were only weak Lewis acid sites on the surface of Cu/SiO
and Cu/ZrO catalysts. The Cu/MgO catalyst was lack of acid sites. These were
2
O
3
catalyst, there were weak and medium Lewis
2
, Cu/TiO ,
2
19
2
20
consistent with the NH -TPD results.
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5

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0
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9
0
1
2
3
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Figure 4. Py-FTIR spectra of the calcined catalysts after adsorption of pyridine and
subsequent evacuation at different temperatures for 30 min: (a) Cu/ZrO , (b) Cu/MgO,
(c) Cu/TiO , (d) Cu/SiO , (e) Cu/γ-Al
3
4
5
6
7
8
9
0
1
2
2
2
2
O .
3
3.2. The catalytic performance of copper-based catalysts in glucose hydrogenolysis
The results of glucose hydrogenolysis over different supported Cu catalysts were
shown in Table 2. Significant efforts had been devoted to the qualitative/quantitative
measurements of the product distributions using the HPLC and GC/GC-MS methods.
Glucose was almost totally converted over the prepared catalysts. CO
2
, CH or other
4
1
1
carbon containing gas products were not detected in the tail gas. The products in liquid
phase mainly included sorbitol, glycerol, EG, and PDO. Small amounts of other
products such as methanol, ethanol, propanol, butanol, 1,2-butanediol, 1,4-butanediol,
erythritol, and xylitol were also detected in the liquid phase. Soluble polymeric
byproducts were detected in the liquid after reaction. The carbon balance was generally
12
1
3
14
1
5
better than 88% over most of the catalysts except Cu/TiO , with only 28.8%. Most of
2
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6

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the glucose feedstock was transformed into insoluble humins.
Cu/γ-Al
2
O showed high selectivity for EG and PDO, with a combined selectivity of
3
66.6% (8.6% for EG and 58.0% for PDO) at 99.4% conversion of glucose. Including
glycerol, the selectivity to the sum of these polyols (EG, PDO, and glycerol) reached
77.7%, which was much higher than those on the other supported catalysts. Cu/MgO
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
and Cu/ZrO also showed relatively high glycol selectivity (55.0% and 40.0%
2
respectively). However, it is interesting to note that, the selectivity of EG and glycerol
on Cu/MgO, with 14.0% and 20.0% respectively, was higher than those on other
catalysts. The total selectivity of these polyols (EG, PDO, and glycerol) on Cu/TiO
2
1
catalysts was only 4.2%. Furthermore, the aqueous soluble polymers were the major
11
products in the liquid. The results of Cu/SiO showed that the hydrogenation reaction
2
1
1
2
3
was dominating, leading to the highest sorbitol selectivity of 82.4%. In contrast, only a
small amount of the lower hydrogenolysis products were present in the liquid. The
selectivity of glycol was only 5.0%.
14
15
The catalytic conversion of glucose to the desired product needs the selective
cleavage of C-C and C-O bonds. Therefore, the integral C-C and C-O bond cleavage
ratios were summarized in Table 2. By assuming that the soluble polymer and the
insoluble humins were regarded as the products with no C-C and C-O bond cleavage,
the detail definition is as follows:
16
1
1
7
8
19
ꢁ
ꢂꢃ ꢄꢅꢆꢇ ꢈꢅꢉ ꢅꢊ ꢋꢌꢅꢇꢍꢎꢏ
20
C − O (%) = ꢀ1 −
C − C (%) = (1 − ꢁ
ꢔ × 100
) × 100
(1)
(2)
ꢁꢂꢃ ꢄꢅꢆꢇ ꢈꢅꢉ ꢅꢊ ꢎꢅꢆꢐꢑꢌꢏꢑꢇ ꢒꢉꢍꢎꢅꢓꢑꢓ
ꢁ
ꢂꢁ ꢄꢅꢆꢇ ꢈꢅꢉ ꢅꢊ ꢋꢌꢅꢇꢍꢎꢏ
21
ꢂꢁ ꢄꢅꢆꢇ ꢈꢅꢉ ꢅꢊ ꢎꢅꢆꢐꢑꢌꢏꢑꢇ ꢒꢉꢍꢎꢅꢓꢑꢓ
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Table 2. The activities and selectivities of glucose hydrogenolysis on the prepared catalystsa
C-C/C-O bond cleavage ratio
(%)
Selectivity (on a carbon basis, %)
Catalyst
Conv. (%)
C
3
(mol%)
C
2
+ C
4
(mol%)
TC (%)
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5
6
Sorbitol
3.5
EG
8.6
14.0
5.4
0.5
0.5
3.4
6.4
0.0
Glycerol
11.1
20.0
9.5
PDO
58.0
41.0
34.6
4.5
Glycolb
66.6
55.0
40.0
5.0
Othersc
18.8
21.6
43.5
11.2
C-C
21.2
19.2
14.5
6.3
C-O
37.8
31.5
32.1
18.3
8.8
Cu/γ-Al
Cu/MgO
Cu/ZrO
2
O
3
99.4
99.6
99.2
96.3
97.2
99.0
99.4
95.3
70.6
61.9
47.6
6.2
19.0
24.9
22.7
5.8
100
95.8
80.3
100
28.8
53.8
100
92.6
3.4
2
7.0
Cu/SiO
Cu/TiO
Ni/γ-Al
2
82.4
0.9
1.4
2
0.5
3.2
3.7
94.9
62.9
36.4
99.1d
5.7
5.4
13.8
11.2
24.7
70.2
2
O
3
2.4
2.5
28.8
50.4
0.9
32.2
56.8
0.9
31.8
62.8
9.1
13.9
43.3
10.9
34.3
43.6
84.5
Pd/γ-Al
2
O
3
2.5
4.3
Ru/γ-Al
2
O
3
0.0
0.0
a
o
-glucose = 161:1 (mol ratio), and LHSV = 0.2 h-1. b EG and PDO. Others include xylitol, erythritol, 1,2-butanediol,
c
Reaction condition: 5 wt% glucose aqueous solution, 4 MPa, 180 C, H
2
methanol, ethanol, propanol, butanol, etc. d Include 84.6% selectivity of alkanes from C
to C , with 66.4% methane.
1
6
2
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Figure 5. The correlation between Cu surface area and the C-C/C-O bond cleavage
percent.
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9
0
The results of C-C and C-O bond cleavage ratios of glucose showed that there was
even no cleavage of C-O and C-C bonds occurring on Cu/SiO . The highest selectivity
2
of sorbitol is considered to be related with the nearly neutral surface of the catalyst and
the agglomerated Cu particles, which provide weak dehydration and retro-aldol
condensation reaction activity. In contrast, Cu/TiO
area and hydrogenation activity, resulting in the main products of soluble and insoluble
polymer after reaction. Cu/γ-Al , with the highest copper surface area and acid
2
showed very low copper surface
1
2
O
3
11
amounts, showed the highest C-O and C-C bond cleavage activity. It suggests that the
coordination of acid/basic sites and the copper sites could not only promote the cleavage
of C-C/C-O bonds but also the efficient hydrogenation of the intermediate fragments.
The correlation between Cu surface area and the C-C/C-O bond cleavage percent has
been drawn and provided in Figure 5. The results showed that the C-C bond cleavage
percent of glucose displayed a similar decrease tendency with the decreasing of Cu
1
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New Journal of Chemistry
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surface areas of the supported Cu catalysts. It indicates that the dispersion of Cu played
a significant role in C-C bond cleavage. However, it is known that Cu has lower activity
for C-C bond cleavage through the straightforward hydrogenolysis. Furthermore, many
researchers have also conducted the retro-aldol condensation reaction on metal catalysts,
indicating that the metallic sites are the active site.13,26,52,53 Accordingly, the retro-aldol
condensation reaction can be performed at acid, basic or even neutral conditions. For
example, Torresi has studied the conversion of diols on copper-silica catalysts. They
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pointed out that retro aldol-like reactions occurred on metallic Cu sites. Considering
the carbon balance and the Cu active sites for the retro-aldol condensation reaction, it
suggests that the well dispersed Cu sites could not only inhibit the polymerization of
1
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glucose
and
the
unsaturated
intermediates
by
promoting
their
1
1
2
3
hydrogenation/dehydrogenation reactions, but also promote the retro-aldol
condensation reaction efficiently. The retro-aldol condensation mechanism of the
hydrogenolytic cleavage of polyols was proposed in the 1980s by the group of
Montassier, which has been widely accepted.1,54-56 They postulated that C-C bond
cleavage of polyols was catalyzed by adsorbed hydroxyl species on the metal catalyst
surface. Therefore, we speculated that Cu could promote the retro-aldol condensation
14
15
16
1
1
7
8
0
reaction by the adsorbed hydroxyl groups on the Cu sites. However, the variation trend
19
of the percent of C-O bond cleavage was not strictly followed that of Cu surface area.
The discrepancy could be due to the effect of acidic sites on the catalyst surface. The
20
21
overall C
3
and C
2
+ C
4
product selectivities were also summarized. Cu/γ-Al
2
O showed
3
22
the highest C
3
product selectivity. It is related with the promoting effect of Lewis acid
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Page 21 of 32
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on the isomerization, which were the intermediate reaction procedures of the
hydrogenolysis of glucose to C
product selectivity compared with other catalysts.
3.3. The effect of different transition metals in glucose conversion
3
product. Moreover, Cu/MgO showed the highest C +
2
C
4
1
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The above text showed that the hydrogenation activity played an important role on
the product selectivity. Furthermore, the transition metal also has the direct
hydrogenolysis activity towards the C-C/C-O bonds, which will make the reactions and
products of glucose hydrogenolysis more complicated. For example, Ru and Pt
o
57
supported on activated carbon were used for glucose hydrogenolysis at 180 C. The
Pt/AC catalysts displayed high selectivity toward sorbitol. On the other hand, the
1
11
Ru/AC catalysts showed higher selectivity toward C
selectivity of only 55-90%. Therefore, the coordination effect between γ-Al
different transition metals with various hydrogenolysis activities have been studied. The
hydrogenolysis of glucose over Ni/γ-Al , Pd/γ-Al , and Ru/γ-Al were
3
-C polyols, with sorbitol
6
1
1
2
3
2
O and
3
14
2
O
3
2
O
3
2
O
3
15
compared. The product distribution over different metal catalysts varied widely. The
16
glycol product selectivity was very low on Ni/γ-Al
2
O . Aconsiderable amount of xylitol,
3
1
1
7
8
erythritol, and 1,2-butanediol existed in the liquid products. Asmall amount of methane
58
was also detected in the gas phase. This result is similar with the report of Ye. CH
4
19
was also formed in the hydrogenolysis of sorbitol on the Ni/γ-Al
2
O catalyst, with the
3
20
selectivity of 8.3%. It displays that Ni has activity to break C-C bond at the terminal of
the carbon chain of glucose, with the total C-C and C-O bond cleavage ratios of 11.2%
21
22
and 34.3% respectively. Pd/γ-Al
2
O showed very high selectivity for PDO (50.4%). The
3
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1

New Journal of Chemistry
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total C-C and C-O bond cleavage ratios were 24.7% and 43.6% respectively, suggesting
that Pd has high activity for the cleavage of the central C-C bonds of glucose. Ru/γ-
Al
2
O had very low selectivity to polyols, with the total selectivity below 10%. In
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contrast, the alkane selectivity was up to 84.6%, with 66.4% selectivity of methane. It
suggests that Ru has the highest activity for C-C and C-O bond cleavage, with the C-C
and C-O bond cleavage ratios of 70.2% and 84.5% respectively. Furthermore, the C-C
bond cleavage mainly occurs at the terminal of the carbon chain of glucose.59
3.4. The function of supports in glucose conversion
Table 3. The conversion of glucose on different supportsa
Conversion
%)
Selectivity (%)
TC
Catalyst
Blank
(
Fructose Glycerol EG PDO HMF Othersb (%)
33.1
90.8
94.7
37.4
2.8
0.8
0.0
0.0
0.0 14.0 24.1
25.1
89.3
89.9
97.2
33.0
γ-Al
2
O
3
0.8 2.8
0.7 1.2
4.3
3.9
ZrO
2
1.9
29.4
a
o
Reaction condition: 5 wt% glucose aqueous solution, 4 MPa, 180 C, H
2
-glucose =
-
1 b
1
61:1 (mol ratio), LHSV = 0.2 h . Others include small amounts of methanol,
ethanol, propanol, 1,2-butanediol, etc.
10
As mentioned above, the supports can provide not only a surface to disperse the
active component, but also acid/basic sites to promote the isomerization, dehydration,
and retro-aldol condensation reactions. In order to confirm the catalytic effect of the
11
1
1
2
3
supports, two supports such as γ-Al
2
O
3
and ZrO with different acidity and basicity
2
14
were tested. The results are displayed in Table 3. The conversion of glucose without
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any catalysts was only 33.1%. A certain amount of fructose and 5-
hydroxymethylfurfural (HMF) were detected, with the selectivities of 37.4% and 24.1%,
respectively. It suggested that the isomerization and dehydration reactions could be
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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3
3
3
3
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conducted in the hydrothermal condition without any catalyst in the H atmosphere.
2
60
These results are consistent with the report of Sasaki et al. They conducted the
transformation of glucose with none catalyst in supercritical water. Products from the
retro-aldol condensation, isomerization, and dehydration reaction of glucose were
obtained. When the catalytic transformation of glucose was conducted on γ-Al
2
O and
3
ZrO , the conversion of glucose increased significantly. However, the selectivities of
2
1
fructose, PDO, and HMF had decreased. Glucose was mainly converted to both
insoluble humins and soluble polymeric byproducts. This could be attributed to the
oligomerization reaction of glucose and/or the unsaturated intermediates, which can be
carried out in the presence of an acid and basic catalyst through dehydration and aldol
condensation.61,62
11
1
1
2
3
14
1
1
5
6
3.5. The catalytic conversion of Cu/γ-Al
2
O for other polyols
3
a
Table 4. The hydrogenolysis of different materials on Cu/γ-Al
2
O
3
Selectivity (%)
TC
PDO Glycolb Others (%)
Material Conv. (%)
Glucose Sorbitol Mannitol Glycerol EG
Glucose 99.4
Fructose 100
Sorbitol 33.0
-
3.5
0.0
-
0.0
5.3
0
11.1
9.4
36.6
-
8.6
5.3
58.0 66.6
47.2 52.5
18.8
31.8
9.2
100
81.6
100
100
1.0
-
-
14.7 39.5 54.2
11.7 86.9 98.6
Glycerol 46.0
-
-
1.4
a
o
2
Reaction condition: 5 wt% glucose aqueous solution, 4 MPa, 180 C, H -glucose = 161:1 (mol ratio); LHSV =
-1
b
0
.2 h . EG and PDO
17
In order to study the synergistic effect of metal sites and the acid/basic sites on the
18
intermediate products of glucose hydrogenolysis, the catalytic conversion of fructose,
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3

New Journal of Chemistry
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O at 180 C and 4 MPa. The
o
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sorbitol, and glycerol were also performed on Cu/γ-Al
2
3
results are displayed in Table 4. The product species of glucose and fructose
hydrogenolysis were similar. All the products identified in glucose conversion were
detected in fructose conversion. However, the carbon balance and the product
selectivity were lower than that of glucose conversion. It suggested that humins was
easier formed in fructose conversion than in glucose conversion. Glucose was also
detected in the products of fructose conversion. It displayed that fructose could also
isomerized to glucose in this reaction condition. Sorbitol was difficult to be degraded
0
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in the hydrogenation environment with Cu/γ-Al
2
O
3
catalyst, with only 33.0%
1
conversion obtained. Glycerol and glycol were the main products in the liquid phase,
with the yields of 36.6 and 54.2% respectively. It has been identified that sorbitol was
not the intermediate of producing glycerol and glycol in the hydrogenolysis of
11
1
1
2
3
63
glucose. Many reports also proved that the selective hydrogenation of glucose to
14
sorbitol was the main undesirable competitive pathway for glycol producing.64,65 The
conversion of glycerol was similar with that of sorbitol (46.0%). Although the
conversion was not very high, the selectivity of PDO reached 86.9%. Considering the
hydrogenolysis performance of glucose and glycerol, PDO may be generated from the
hydrogenolysis of glycerol or the intermediates of fructose conversion. Furthermore, it
turned out that PDO was easier obtained from glucose than from glycerol in the reaction
condition.
15
16
1
1
7
8
19
20
21
3.6. The synergistic effect between metal and metal oxide on the product
distribution of glucose hydrogenolysis
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As illustrated above, the product distribution of glucose hydrogenolysis depended on
the type and the relative catalytic activity of active sites. Only products from the
dehydration and polymerization of glucose and the unsaturated intermediates were
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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detected with only metal oxide catalysts. The neutral Cu/SiO catalyst could mainly
2
promote the hydrogenation of glucose to sorbitol. However, the synergistic effect
between well dispersed Cu sites and the basic/Lewis acid sites could promote the
hydrogenolysis of glucose to lower polyols. Furthermore, Lewis acid sites on γ-Al
could promote the isomerization of glucose to fructose, which could be transformed to
the C product through retro-aldol condensation reaction on the well dispersed Cu sites.
The unsaturated C products then undergo the dehydration and hydrogenation reactions
2
O
3
3
1
3
11
on Lewis acid sites and well dispersed Cu sites respectively, resulting the highest PDO
1
1
2
3
selectivity on γ-Al
2
O
3
. Cu/MgO showed the highest C selectivity compared with other
2
catalysts. Furthermore, it has been suggested that glucose can be directly converted to
6
0
14
C
2
and C products through retro-aldol condensation. It turned out that the Cu/MgO
4
15
catalyst could accelerate the retro-aldol condensation and the isomerization reaction
rates simultaneously, which is disadvantageous for the enhancement of product
selectivity.
16
1
1
7
8
3.7. The role of support, Cu dispersion and Cu states
19
The NH
3
-TPD and CO -TPD results showed that the oxide supports could provide
2
20
Lewis acid and basic sites. Glucose and the unsaturated intermediates could form
polymers on the acid and basic sites without efficient hydrogenation sites. However, in
the hydrogen atmosphere, the basic sites could facilitate the isomerization and retro-
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aldol condensation reaction. Furthermore, the Lewis acid sites could promote the
isomerization and dehydration reactions of glucose and the intermediates.
Liu et al. has studied the synergistic effect of Ru and WO
x
on the bifunctional Ru-
catalysts for the hydrogenolysis of glucose. They suggested that the retro-
aldol condensation reaction of glucose or fructose was conducted on WO acid sites.
Pang et al. have performed the selective conversion of concentrated glucose to 1,2-
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W/SiO
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propylene glycol and ethylene glycol on RuSn/AC catalysts. They proposed that both
RuSn alloy and the highly dispersed Sn4+ species could promote the retro-aldol
condensation of glucose to form C
2
and C
3
intermediates. Holm and co-workers found
1
that the Lewis acid sites on the zeotype catalysts, such as Ti-, Sn-, and Zr-Beta, could
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catalyze the retro aldol condensation reaction of the glucose and fructose. Therefore,
we proposed that the Lewis acid sites on the catalysts we prepared could also promote
the retro-aldol condensation reaction.
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The hydrogenation and retro-aldol condensation reactions could be conducted on the
Cu sites. Therefore, more Cu active sites could promote the hydrogenation and retro-
aldol condensation reactions.
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The CO-DRIFTS suggested that CuO cannot be completely reduced on the Cu/SiO ,
2
δ+
Cu/ZrO
2
and Cu/TiO
2
catalysts, with some oxidized Cu species existing. As suggested
19
by the Py-FTIR spectra, only Lewis acid sites existed on the catalyst surface. Therefore,
δ+
20
the Cu species may also act as Lewis acid sites on the catalysts and promote the
21
isomerization, retro-aldol condensation, and the dehydration reactions.31,69,70
22
4. Conclusion
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The synergistic effect between transition metal and metal oxides on the
hydrogenolysis of glucose was studied. The product selectivities were strongly
influenced by the coordination of hydrogenation metal sites and acid/basic sites. The
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neutral Cu/SiO catalyst could mainly promote the hydrogenation of glucose. The metal
2
oxides with acid and basic sites could promote the oligomerization reactions of glucose
and the unsaturated intermediates without hydrogenation sites. The coordination of high
dispersed copper sites and the large amounts of weak and middle Lewis acid sites could
tune the isomerization, retro-aldol condensation, and hydrogenation reactions rates,
resulting the main product of C
copper sites with basic sites could accelerate retro-aldol condensation and isomerization
reactions rates simultaneously, resulting the main products of C , C , and C polyols.
3
polyols. The synergy between the high dispersed
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The direct hydrogenolysis of transition metals could also affects the product
selectivity. Pd has high selectivity for the central C-C bond cleavage of glucose.
However, Ru and Ni have excess hydrogenolysis activity for the terminal of the carbon
14
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chain of glucose, with 84.6% alkane selectivity on Ru/γ-Al
2
3
O .
16
Glucose and Fructose, with the unsaturated C=O bond, has higher C-C/C-O cleavage
activity than the saturated polyols (such as sorbitol and glycerol) in hydrogenation
condition. Furthermore, this paper provides a design method of catalyst for the efficient
conversion of the saccharides and polyols.
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Acknowledgement
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This work was financially supported by the Major State Basic Research
Development Program of China (973 Program) (No. 2012CB215305), the Natural
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Science Foundation of Tianjin (No. 18JCQNJC06600) and the Science & Technology
Development Fund of Tianjin Education Commission for Higher Education (No.
2018KJ203). This work was also supported by Synfuels China Co., Ltd.
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