One-pot synthesis of Cu/ZnO/ZnAl2O4 catalysts and their catalytic performance in glycerol hydrogenolysis

Article abstract of DOI:10.1039/c3cy00661a

In this work, a series of Cu/ZnO/ZnAl₂O₄ catalysts with different metal molar fractions (Cu:Zn:Al) were successfully prepared using a one-pot method via the evaporation-induced self-assembly (EISA) of Pluronic P123 and the corresponding metal precursors. The catalysts were characterized using N₂ adsorption, H₂ temperature-programmed reduction (H₂-TPR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS). The catalytic properties of the resulting Cu/ZnO/ZnAl₂O₄ with different molar fractions of metals were investigated for the selective hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO). It was observed that the ZnAl₂O ₄ support exerts a strong positive effect on the catalytic activity of the copper-based catalysts, and the presence of ZnO further improves the catalytic activity of the Cu/ZnAl₂O₄ catalysts. The Cu/ZnO/ZnAl₂O₄ catalyst (Cu₁₀Zn ₃₀Al₆₀, Cu/Zn/Al molar ratio is 10:30:60), which was the best catalyst, exhibited the highest yield (79%) of 1,2-PDO with 85.8% glycerol conversion and 92.1% 1,2-PDO selectivity at 180 °C reaction temperature in 80 wt% glycerol aqueous solution over 10 h reaction time. The high catalytic activity was attributed to the presence of the ZnAl₂O₄ support, the strong interaction between ZnO and Cu nanoparticles and the small particle size of ZnO and Cu. Moreover, the Cu/ZnO/ZnAl₂O₄ catalysts exhibited higher stability than Cu/ZnO and Cu/ZnO/Al₂O ₃ catalysts prepared by a co-precipitation method during consecutive cycling experiments, which is due to the high chemical and thermal stability of crystalline ZnAl₂O₄ under harsh reaction conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00661a

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Page 1 ofJ11ournal Name
DOI: 10.1039/C3CY00661A
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
One-pot synthesis of Cu/ZnO/ZnAl2O4 catalysts and their catalytic
performance in glycerol hydrogenolysis
Hua Tan,*a Mohamed Nejib Hedhill,b Yilin Wang,b Jizhe Zhang,c Kun Li,b Salim Sioud,a Zeyad A. Al-
Talla,a Maan H Amad,a Tong Zhan,d Omar E Tall, a and Yu Han c
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In this work, a series of Cu/ZnO/ZnAl2O4 catalysts with different metal molar fractions (Cu:Zn:Al)
were successfully prepared using a one-pot method via the evaporation-induced self-assembly (EISA) of
Pluronic P123 and the corresponding metal precursors. The catalysts were characterized using N2
10 adsorption, H2 temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), transmission
electron microscopy (TEM) and X-ray photoelectron spectra (XPS). The catalytic properties of the
resulting Cu/ZnO/ZnAl2O4 with different molar fractions of metals were investigated for the selective
hydrogenolysis of glycerol to 1, 2-propanediol (1, 2-PDO). It was observed that the ZnAl2O4 support
exerts a strong positive effect on the catalytic activity of the copper-based catalysts, and the presence of
15 ZnO further improves the catalytic activity of the Cu/ZnAl2O4 catalysts. The Cu/ZnO/ZnAl2O4 catalyst
(Cu10Zn30Al60, Cu/Zn/Al molar ratio is 10:30:60), which was the best catalyst, exhibited the highest yield
(79%) of 1, 2-PDO with 85.8% glycerol conversion and 92.1% 1, 2-PDO selectivity at 180 oC reaction
temperature in 80 wt% glycerol aqueous solution over 10 h reaction time. The high catalytic activity was
attributed to the presence of the ZnAl2O4 support, the strong interaction between ZnO and Cu
20 nanoparticles and the small particle size of ZnO and Cu. Moreover, the Cu/ZnO/ZnAl2O4 catalysts
exhibited higher stability than Cu/ZnO and Cu/ZnO/Al2O3 catalysts prepared by co-precipitation method
during consecutive cycling experiments, which is due to the high chemical and thermal stability of
crystalline ZnAl2O4 under harsh reaction conditions.
9
45 pharmaceuticals.8, Different heterogeneous catalysts, such as
25 Introduction
supported Ru, Pt, Pd and Rh catalysts have been widely
employed for the hydrogenolysis of glycerol to 1, 2-PDO.10, 11
Supported Ru catalyst has been considered as an effective
catalyst for glycerol hydrogenolysis.12 However, its selectivity for
50 glycerol hydrogenolysis to 1, 2-PDO is not sufficient because it
can cleave C-C bonds, which leads to degradation products, such
as ethylene glycol and methane.13
Supported Cu catalyst are well known for their poor
hydrogenolysis activity toward C-C bonds and high selectivity for
55 C-O bond hydro-dehydrogenation.13, 14 Huang et al. reported 73-
80% glycerol conversion with 98% selectivity over Cu/SiO2
catalysts at 200 °C and 6.4 or 9 MPa H2.15, 16 A Cu/Al2O3 catalyst
prepared using an incipient wetness impregnation method
exhibited 49.6% glycerol conversion with approximately 96.8%
60 selectivity to 1, 2-PDO at 220 °C and 1.5 MPa H2.17
Recently, biodiesel has received considerable attention as a
renewable, clean-burning diesel replacement because sustainable
energy sources are essential for the continued development of our
society.1 Biodiesel is produced via transesterification of vegetable
30 oil with ethanol or methanol in the presence of basic or acidic
catalysts.2 As the major byproduct of this process, glycerol
accounts for 10 wt% of the biodiesel produced,3 with a fast-
growing biodiesel market, glycerol is now available in large
amounts and is expected to exceed demand in the near future.4, 5
35 Therefore, the effective utilization of glycerol by conversion into
value-added chemicals is important to ensure that biodiesel
production is an economically feasible process. Many processes
have been developed to transform glycerol into different
intermediates for chemical production, such as reforming,
40 oxidation, dehydration and etherification.6 Among these
processes, the catalytic hydrogenolysis of glycerol to produce 1,
2-propanediol (1, 2-PDO) is one of the promising processes.7
This process can be used to manufacture such compounds as
unsaturated polyester resins, functional fluids, cosmetics and
Many other supports were also selected for Cu-based catalysts
to promote their catalytic performance.18, 19 The Cu/ZnO catalyst
was chosen for glycerol hydrogenolysis because of its superior
selectivity to 1, 2-PDO and wide range of industrial applications
65 in methanol synthesis, water gas shift reaction, and methanol
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steam reforming.20 Wang et al. obtained a selectivity of 83.6% to
1, 2-PDO at a 22.5% glycerol conversion rate with a Cu-ZnO
catalyst at 200 °C and 4.2 MPa initial H2 pressure over 12 h.21
mesoporous Cu/ZnO/ZnAl2O4 catalysts were synthesized based
on the evaporation-induced self-assembly (EISA) procedure
using Pluronic P123 as a structure directing agent according to
Bienholz et al. reported 46% glycerol conversion with 90% 60 the previous reports.27, 37 In a typical synthesis, 1.0 g of Pluronic
5
selectivity to 1, 2-PDO over a Cu–ZnO catalyst after a 24 h
reaction.22 The ZnO present in the catalyst drives the dehydration
step to yield acetol from glycerol, which is further hydrogenated
on Cu sites to yield 1, 2-PDO. Glycerol conversion and 1, 2-PDO
P123 (Aldrich) was dissolved in 20 mL of anhydrous ethanol
(Aldrich), and the mixture was stirred at room temperature for 4
h. Then, 1.6 mL of 65 wt% nitric acid (Aldrich) and the
appropriate quantity of copper acetate (Aldrich), zinc nitrate
selectivity varied with the ZnO and Cu particle size, and small 65 hexahydrate (Fisher Scientific) and aluminium isopropoxide
10 ZnO and Cu particle size gave positive effects on glycerol
conversion and 1, 2-PDO selectivity.21, 23
Although these catalysts are highly selective toward 1, 2-PDO,
they still face several challenges to achieve high catalytic
(98+%, Alfa Aesar) were added into the above solution with
vigorous stirring. The total amount of metal species was kept at
10 mmol, and the molar ratio of copper oxide, zinc oxide and
alumina was adjusted accordingly. The beaker containing the
performance: first, their catalytic activity is relatively low. 70 mixture was covered with PE film and stirred at room
15 Normally a long reaction time is needed to obtain decent glycerol
conversion. This long reaction time is necessary because Cu-ZnO
catalysts prepared using the co-precipitation method have low
surface area.23 As a result, Cu in this catalyst is not well-
temperature for approximately 6 h. Then, the final solution was
put into a drying oven and underwent the solvent evaporation at
60 °C for two days. Calcination was carried out by slowly
increasing temperature from room temperature to 600 °C (1
dispersed and cannot catalyse glycerol hydrogenolysis efficiently. 75 °C/min ramping rate) and holding at 600 °C for 4 h under air.
20 Second, a tremendous loss of Cu surface area was found in the
Cu-ZnO catalyst during the course of the reaction, which could
be a possible cause for catalyst deactivition.22 In previous studies,
Bienholz et al. modified the Cu/ZnO catalyst with Ga2O3 to
Before the catalytic reaction, the calcined samples were reduced
at 400 °C under H2 stream for 3 h. The calcined and reduced
samples were labelled in the form CuxZnyAlz, where x, y, and z
indicate the molar percentage of copper, zinc, and aluminium. For
enhance the stability of the catalyst; no deactivation was observed 80 example, Cu10Zn30Al60 refers to the mesoporous aluminium oxide
25 even after four consecutive experiments over Cu/ZnO/Ga2O3
with 10% molar fraction of copper, 30% molar fraction of zinc,
and 30% molar fraction of aluminium oxide. Cu/ZnO (Cu/Zn
molar ratio is 1:1) catalyst and Cu/ZnO/Al2O3 (Cu/Zn/Al molar
ratio is 1:1:4) catalyst were prepared as reference catalysts using
85 the co-precipitation method following the procedure reported in
the previous publications.4, 12
catalysts.24 Cu/ZnO/Al2O3 catalysts were also prepared to
23, 25, 26
catalyze the glycerol hydrogenolysis.4,
It was found that
the presence of Al2O3 in Cu/ZnO catalysts can increase the
stability of the catalysts.
30 In this work, the convenient one-pot sol-gel method is used to
prepare the ZnAl2O4 supported Cu-ZnO catalysts for glycerol
hydrogenolysis. Zinc aluminate (ZnAl2O4), which has been
widely studied for use as catalysts and catalyst supports in
cracking, dehydration, hydrogenation and dehydrogenation,30-
35 32was used to replace the traditional Al2O3 and ZnO supports for
copper catalysts because it has a good metal dispersion capacity,
hydrophobic characteristics, and high thermal and chemical
stability.33-35For example, Mierczynski et al. prepared Cu/support
Catalyst characterization
90
The transmission electron microscope (TEM) images of the
samples were examined with an FEI Titan 80 (300 kV). The
samples were prepared by dispersing the catalysts in ethanol
using an ultrasonic bath and evaporating a drop of suspension
onto the carbon coated copper grid. X-ray diffraction (XRD)
catalysts (ZnO, Al2O3, and ZnAl2O4) and tested their activities on 95 patterns of the samples were recorded on a Bruker D8 Advance
40 methanol synthesis. The results showed that the most active
catalyst was Cu supported on ZnAl2O4.36 In addition, compared
to the conventional catalyst preparation method, such as co-
precipitation method, this one-pot method was reported to yield
diffractometer with Cu Kα radiation (λ = 1.5418 Å) with a
graphite monochromator (40 kV, 40 mA). Particle size of Cu and
ZnO were calculated by using the Debye-Scherrer equation. The
X-ray photoelectron spectroscopy (XPS) was recorded with an
high surface area supports and the strong metal-support 100 Axis Ultra DLD system (Kratos, U.K.) by using Al Kα radiation
45 interaction.27-29 Thus, there is a possibility to increase the catalytic
activity and stability of Cu based catalysts on glycerol
hydrogenolysis by using ZnAl2O4 as a catalyst support. The as-
prepared Cu/ZnO/ZnAl2O4 samples were characterized by
source (hυ = 1486.8 eV). The binding energy was calibrated by
taking the C 1s peak (284.8 eV) as a reference. The XPS peaks
are analysed using a Shirley-type background and a nonlinear
least-squares fitting of the experimental data based on a mixed
various techniques and studied for glycerol hydrogenolysis to 105 Gauss/Lorentz peak shape. XPS quantification was performed by
50 examine their catalytic activity and stability.
applying the appropriate relative sensitivity factors (RSFs) to the
integrated peak areas. Nitrogen adsorption–desorption isotherms
were obtained on a Micromeritics ASAP 2420 instrument at −196
°C. Prior to the measurements, all samples were degassed under
110 vacuum for approximately 12 h at 200 °C. The BET surface area
was calculated from the data in a relative pressure range from
0.05 to 0.2. By using the Barrett–Joyner–Halenda (BJH) model,
the pore volumes and pore size distributions were derived from
the adsorption branches of isotherms. H2 temperature-
Experimental
Catalyst preparation
55 All materials were reagent grade and used without further
purification. Anhydrous ethanol was used as a solvent. The
2
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programmed reduction (TPR) was performed in a Micromeritics
Autochem II 2950 Chemisorption Analyzer. The catalysts
containing 5mg of Cu were placed in a U-type quartz cell, and the
temperature was raised linearly from 30 °C to 600°C with a
heating rate of 10 °C /min in a flow of 10% H2/Ar (50 ml/min).
The amount of hydrogen gas consumed by the reduction was
measured by a thermal conductivity detector (TCD). Dissociative
N2O adsorption–H2 TPR reverse titration experiments were
5
carried out as described in the literature.38 In
a typical
10 experiment, TPR was carried out to reduce the CuO to Cu (0)
with a 10% H2–Ar mixture and a heating rate of 10 °C /min. Then,
the sample, which is in the form of Cu (0), is exposed to N2O to
oxidize Cu to Cu2O by adsorptive decomposition of N2O at 80 °C
by a continuous N2O flow for 0.5 h. Finally, TPR was carried out
15 again on the re-oxidized Cu2O surface to reduce Cu2O to Cu. The
Cu loadings in all of the samples were measured using
inductively coupled plasma-optical emission spectrometer (ICP-
OES).
20 Catalytic activity measurement
The glycerol hydrogenolysis reaction was carried out in a
50mL stainless steel Parr reactor equipped with a temperature
controller. Twenty millilitres of aqueous glycerol solution (20-
25 100 wt% glycerol) and CuxZnyAlz catalysts (reduced CuxZnyAlz
samples, catalyst weight: glycerol/Cu ratio is 200) were added
into the reactor, then the reactor was sealed and flushed with
hydrogen three times to eliminate air. The glycerol
hydrogenolysis reaction was conducted at a temperature of 180
30 °C , a hydrogen pressure of 4.0 MPa, and a stirring speed of 600
rpm. After 10 h, the reactor was cooled to room temperature. The
liquid products were separated using a gas chromatograph with a
capillary column (DB-Wax, 30 m × 0.25 µm) and detected using
a flame ionization detector (FID). Conversion of glycerol is
35 defined as the ratio of the number of moles of glycerol consumed
in the reaction to the total moles of glycerol initially added.
Selectivity is defined as the ratio of the number of moles of the
product formation to that of the glycerol consumed in the
reaction, taking into account the stoichiometric coefficient. Yield
40 is defined as the number of moles of 1, 2-PDO in the sample
multiplied by the ratio of glycerol to 1, 2-PDO.
60 Figure 1 TEM images of pure alumina and reduced CuxZnyAlz samples, (a)
pure alumina, (b) reduced Cu10Zn0Al90, (c) reduced Cu10Zn15Al75, (d) High
resolution TEM image of reduced Cu10Zn15Al75, (e) reduced Cu10Zn30Al60, (f)
High resolution TEM image of reduced Cu10Zn30Al60, (g) Cu10Zn45Al45, (h)
High resolution TEM image of Cu10Zn45Al45.
Results and Discussion
45 Catalysts Characterization
65 When 15% or 30% molar fraction of Zn was introduced into this
one-pot synthesis, the resulting materials showed decreased
structural ordering in the mesoscale. In the TEM images of the
reduced Cu10Zn15Al75 and Cu10Zn30Al60 samples (Figure 1(c) and
(e)), ordered mesoporous channels were not visible. Accordingly,
70 only a weak and broad peak could be observed in their low-angle
XRD patterns. However, they are also highly mesoporous with
rather uniform mesopore sizes, as demonstrated later by the N2
physisorption experiments. These results suggest an “ordered” to
“wormlike” transformation of the mesoporous structure taking
75 place with the involvement of a secondary component, which is a
common phenomenon in the synthesis of mesoporous alumina. 28,
37The devolution of structural ordering in the mesoscale caused
The pure alumina sample prepared using the one-pot sol-gel
method had an ordered mesostructure with p6mm symmetry, as
determined by TEM (Figure 1(a)) and low-angle XRD (Figure 2),
50 which was consistent with the results reported in the previous
article.39 With the addition of copper acetate, the Cu10Zn0Al90
sample, synthesized from the same system, also exhibited an
ordered mesoporous structure with small Cu nanoparticles in the
size of approximately 3 nm highly dispersed on the amorphous
55 alumina wall after H2 reduction at 400 °C. The results indicate
that the introduction of Cu up to 10% molar fraction and high
temperature calcination and reduction steps do not affect the
formation of ordered mesopores.
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by the incorporation of Zn can be clearly seen in the low-angle 45 for the mesoporous materials. The steepness of the capillary
XRD patterns of these samples (Figure 2). Despite their less
ordered mesostructures, highly dispersed Cu nanoparticles can
still be formed on the surface of mesopores in the reduced
Cu10Zn15Al75 and Cu10Zn30Al60 samples from Figure 1(c) and (e).
condensation step indicated uniformity of mesopores. The H1
shaped hysteresis loops also suggested that the mesopores in
these two samples were cylindrically shaped channels, which
were also confirmed by the TEM images (Figure 1(a) and (b)).
5
Only the size of Cu nanoparticles was increased to approximately 50 Compared to the pure mesoporous alumina, the calcined
5nm and 7nm in the reduced Cu10Zn15Al75 and Cu10Zn30Al60
sample, respectively. In HRTEM image of the reduced
Cu10Zn15Al75 sample shown in Figure 1(d), the support exhibited
Cu10Zn0Al90 sample also exhibits high specific surface areas of
271.9 m2/g and large pore volumes of 0.50 cm3/g, as shown in
Table 1. In addition, the BJH pore size distribution of the
calcined Cu10Zn0Al90 sample shown in Figure 3 (b) revealed a
10 the characteristic ZnAl2O4 (111) plane spacing (0.47 nm),
indicating the amorphous support was transformed into 55 narrow pore size distribution centred at approximately 7 nm,
crystalline ZnAl2O4. The ZnAl2O4 crystalline phase was possibly
formed by the solid state reaction between alumina and zinc
oxide during the calcination step. For the reduced Cu10Zn30Al60
15 sample, surprisingly, it was found in the HRTEM image of the
which was consistent with the TEM observation (Figure 1(b)).
When Zn molar fraction was increased to 15% and 30% in
calcined Cu10Zn15Al75 and Cu10Zn30Al60 samples, respectively, the
two isotherms were still characteristic of the type IV isotherms
reduced Cu10Zn30Al60 sample in Figure 1(f) that Cu nanoparticles 60 with H1 shaped hysteresis loop. For the calcined Cu10Zn15Al75
are in close contact with the surrounding similarly sized ZnO
nanoparticles on the surface of crystalline ZnAl2O4 mesoporous
structures. The ZnO could have originated from the thermal
20 decomposition of zinc nitrate, which did not completely react
with alumina.
sample, a decrease in the steepness of capillary condensation step
is observed with the condensation step shifting slightly to a
greater relative pressure. It is indicated that the mesopores of the
calcined Cu10Zn15Al75 sample become more disordered and
65 larger, which is demonstrated in its BJH pore size distribution
curve (Figure 3 (b)), that shows that the peak become broader and
the peak position shifted to 9nm. As a result, the specific surface
area and pore volume decreased to 207.4 m2/g and 0.40 cm3/g,
respectively.
70
Figure 2 low-angle XRD patterns of pure alumina and reduced CuxZnyAly
25
samples
When zinc molar fraction reached 45% in the reduced
Cu10Zn45Al45 sample, the mesoporous structures were destroyed
and the aggregates of nanoparticles were formed as shown in
Figure 1(g). However, it can be seen from Figure 1(h) that the Cu
30 and ZnO nanoparticles were still in close contact with each other
on the surface of ZnAl2O4 support.
The calcined CuxZnyAlz samples were also measured by TEM,
their morphologies were similar to the corresponding reduced
CuxZnyAlz samples, but copper oxide nanoparticles in calcined
35 CuxZnyAlz samples were not detected even under high
magnification inspection, which indicated that copper oxide was
highly dispersed and incorporated into the mesoporous matrix in
calcined samples.
The nitrogen adsorption-desorption isotherms and BJH pore
40 size distribution curves of pure mesoporous alumina, calcined
and reduced CuxZnyAlz samples are shown in Figure 3. The
isotherms of pure mesoporous alumina and calcined Cu10Zn0Al90
sample in Figure 3 (a) were attributed to the type IV isotherms
with H1 shaped hysteresis loop, which are the significant features
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Catalysis Science & Technology
30 almost the same as the calcined CuxZnyAlz samples.
From the TEM, low angle XRD and N2 physisorption
measurements, it can be seen that mesoporous channels
hexagonally arranged in p6mm symmetry were obtained in the
cases of pure alumina and Cu10Zn0Al90 samples, where the
35 template P123 functions as a ‘structural directing agent” for the
formation of ordered mesostructures. Upon the introduction of
ZnO with molar fraction up to 30%, the resulting materials show
mesoporous structures without long range order in pore
arrangement, which is similar to that observed for mesoporous
40 alumina prepared by self-assembly method in aqueous solution.29
In these cases, P123 simply acts as a “porogen”. However, the
degree of structure ordering would not be crucial for the catalytic
performance.28, 40For the Cu10Zn45Al45 sample, an excess of ZnO
particles in this sample results in the substantial collapse of
45 mesoporosity.
Figure 3 Nitrogen adsorption–desorption isotherms and BJH pore size
distribution curves of calcined CuxZnyAlz samples (pure mesoporous
alumina) (a, b) and reduced CuxZnyAlz samples (c, d).
5
However, for the calcined Cu10Zn30Al60 sample, it is interesting
to note that the specific surface area and pore volume did not
decrease further, but surprisingly increased to 222.3 m2/g and
0.43 cm3/g, respectively. Moreover, the capillary condensation
10 step is steeper and shifted back to lower relative pressure,
indicating the mesopores become narrower and more ordered,
which was also shown by its BJH pore size distribution curve in
Figure 3(b). When the Zn molar fraction was further increased to
45%, the specific surface area and pore volume of calcined
15 Cu10Zn45Al45 sample greatly decreased to 76.7 m2/g and 0.19
cm3/g, respectively. The small adsorption quantity and broad
capillary condensation step indicated a substantial collapse of
mesoporous structure.
After the calcined samples were reduced under H2, the reduced
20 CuxZnyAlz samples showed rather increased specific surface area,
except for the reduced Cu10Zn45Al45 sample, compared to their
corresponding calcined samples in Table 1. As discussed above,
copper oxide species were uniformly dispersed in the mesoporous
skeleton after calcination, then they were reduced to form the
25 copper nanoparticles under H2 reduction. The formation of
copper nanoparticles on the surface of mesoporous ZnAl2O4 may
be attributed to the increased surface area. The types of
isotherms, hysteresis loops and BJH pore size distribution curves
of the reduced CuxZnyAlz samples in Figure 3(c) and (d) remained
Figure 4 wide angle X-ray diffraction patterns of calcined CuxZnyAly
50
samples (a) and reduced CuxZnyAly samples (b).
Figure 4 shows wide angle XRD patterns for calcined and
reduced CuxZnyAlz samples. In Figure 4(a), the diffraction peaks
corresponding to CuO or Cu2O phases were not detected at all in
the calcined CuxZnyAlz samples, indicating a homogenous
55 dispersion of copper oxide species in the entire mesopore
skeleton on the atomic level. Therefore, the calcined Cu10Zn0Al90
sample only showed the amorphous nature of alumina support.
When zinc nitrate was added to Cu10Zn15Al75 sample, ZnAl2O4
was formed due to a solid-state reaction, and as a result, the
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Table
1 Physicochemical properties of pure mesoporous alumina,
calcined CuxZnyAlz samples, and reduced CuxZnyAlz sample.
50
BET surface area
(m2/g)
Crystallite size
after reduction
BJH pore size (nm)
Pore volume (cm3/g)
Cu surface areab (m2/g,
Cu)
Samples
ZnO
Cu sizea
sizea
Before
Before
After
After reduction Before reduction After reduction
reduction
reduction reduction
(nm)
(nm)
Alumina
319.6
271.9
207.4
222.3
76.7
ND
6.6
7.8
7.9
7.2
5.8
ND
7.7
7.9
7.1
5.6
0.64
0.50
0.40
0.43
0.19
ND
0.56
0.44
0.45
0.20
ND
3.2
4.8
6.8
12.1
NA
NA
ND
9.7
NA
Cu10Zn0Al90
Cu10Zn15Al75
Cu10Zn30Al60
Cu10Zn45Al45
297.3
225.6
235.4
79.1
205.8
177.3
126.6
16.6
57.5
a: Average crystallite size determined from XRD data using Scherrer equation.
b: Specific copper surface area is based on the mass of copper and determined by N2O chemisorption method.
NA: not available; ND: not detected.
5
diffraction peaks, which correspond to ZnAl2O4 spinel phase
(JCPDS 05-0669), appeared in the XRD pattern for the calcined
Cu10Zn15Al75 sample. As the Zn molar fraction was increased, the
XRD diffraction peaks for ZnAl2O4 exhibited increasing intensity
in both Cu10Zn30Al60 and Cu10Zn45Al45 samples. In addition, ZnO
10 diffraction peaks (JCPDS 5-0664) appeared in both two samples.
These observations proved that some zinc nitrate did not react
with alumina, but directly decomposed into ZnO nanoparticles
under the experimental conditions.
Figure 4(b) shows the XRD patterns for the reduced CuxZnyAlz
15 samples. Compared to Figure 4(a), the pronounced difference was
that Cu diffraction peaks (JCPDS 1-1241) at 2θ of 43.3º, 50.4º,
and 74.1º appeared in all the samples, but the ZnO and ZnAl2O4
diffraction peaks were not affected by the H2 reduction at 400 ºC.
In addition, the Cu and ZnO diffraction peaks became sharper
20 with increased of ZnO molar fraction in the reduced CuxZnyAlz
samples, indicating the larger size of Cu and ZnO nanoparticles.
Table 1 shows the average crystallite size of Cu and ZnO in the
reduced CuxZnyAlz samples, which were calculated using the
Scherrer equation. The values in Table 1 demonstrate that an
25 increase in Zn molar fraction causes an increase in the size of
copper and zinc oxide nanoparticles.
The XPS spectra of the calcined and reduced CuxZnyAlz
samples are reported in Figure 5. In Figure 5 (a), the intense and
broad photoelectron peak between 930 eV and 937 eV along with
30 the presence of the characteristic shakeup satellite peaks
suggested that all the calcined samples contain Cu2+.15 Because
the reduced copper species, such as metallic copper and Cu+
species, usually appear below 933 eV and Cu2+ typically appears
with binding energies higher than 933 eV and considering the
35 asymmetry of Cu 2p3/2 envelop, the Cu 2p3/2 peaks in all the
calcined samples can be deconvoluted into two contributions,
which are listed in Table 2.41 The peak with binding energy
higher than 933 eV can be assigned to Cu2+ , the other peak with
binding energy lower than 933 eV can be assigned to the Cu+/Cu0
40 species. It also can be observed from Table 2 that the peak for
Cu2+ was shifted to lower binding energy with an increased zinc
molar fraction. It clearly indicated that the electrons were
transferred from zinc oxide to copper, which favors their higher
catalytic activities.
55
Figure 5 Cu 2p XPS spectra of calcined (a) and reduced (b) CuxZnyAly
samples.
sample. From the XPS spectra of the calcined Cu10Zn0Al90
sample, it was confirmed that Cu2+ and Cu+/Cu0 species were
60 present in this sample. According to previous work, CuO reduces
directly to metallic Cu without the formation of an intermediate
or suboxide (such as Cu4O3 or Cu2O).42 Therefore, the peak at
325 ºC could be attributed to the reduction of Cu2+ to Cu0. The
high reduction temperature revealed that Cu2+ species could be
65 incorporated into alumina matrix to form Cu-Al-O network in the
calcined Cu10Zn0Al90 sample because the H2 reduction
temperature of copper oxide in this network (320-390 ºC) was
45 The reducibility of the calcined CuxZnyAlz samples was studied
using H2 temperature programmed reduction (TPR). The TPR
profiles are shown in Figure 6. A sharp reduction peak centred at
approximately 325 ºC is observed in the calcined Cu10Zn0Al90
6
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Catalysis Science & Technology
Table 2 Curve-fitting results of Cu 2p3/2 of CuxZnyAly catalysts.
B.E. (eV) of Cu 2p3/2 for
Cu2+
B.E. (eV) of Cu 2p3/2 for
Reduced CuxZnyAlz
samples
B.E. (eV) of Cu 2p3/2 of
Calcined CuxZnyAlz samples
Cu+/Cu0
932.4
932.6
931.9
932.2
Cu+/Cu0
932.5
932.2
932.0
932.1
Cu10Zn0Al90
Cu10Zn15Al75
Cu10Zn30Al60
Cu10Zn45Al45
934.4(0.91)a
934.0(0.80)a
933.7(0.59)a
933.8(0.49)a
Cu10Zn0Al90
Cu10Zn15Al75
Cu10Zn30Al60
Cu10Zn45Al45
a: Number in the parenthesis represents the relative intensity ratio of the Cu 2p3/2 for Cu2+ versus the Cu 2p3/2 for Cu+/Cu0
40
much higher than that of the supported CuO (approximately 250
ºC).28,43,44 The reduction of Cu2O was more difficult than the
reduction of CuO and required a temperature higher than 600 ºC,
thus the Cu2O reduction peak did not appear in the TPR
profiles.
absence of Cu2+ species. In this work, two overlapping Cu LMM
5
Auger kinetic energy peaks centred at approximately 918.6 and
916.5 eV for Cu0 and Cu+ were observed in the reduced
Cu10Zn0Al90 sample. However, for the rest of reduced samples
45 with high Zn/Cu ratios, it was very difficult to extract the shape
and position of Cu LMM peaks because the Cu LMM peaks
overlapped with the Zn LMM peaks.46, 47 Therefore, the Cu+/Cu0
ratio could not be obtained for the Cu10Zn15Al75, Cu10Zn30Al60,
and Cu10Zn45Al45 samples.
42, 45
When zinc oxide was added in the synthesis of the calcined
10 Cu10Zn15Al75 sample, the linking bridge between Cu and Al was
separated and replaced by ZnAl2O4 as proved by XRD
measurements, as a result, the reduction peak of CuO was shifted
to the low temperature of approximately 285 ºC because Cu
nanoparticles were supported on ZnAl2O4 in the calcined
15 Cu10Zn15Al75 sample rather than alumina. For the calcined
Cu10Zn30Al60 and Cu10Zn45Al45 samples, ZnO appeared in close
contact with CuO, and the reduction peak of CuO was further
lowered to approximately 260 ºC , implying a strong interaction
between zinc oxide and copper exists in these samples, which
20 could promote electron transfer between ZnO and Cu. In
addition, the reduction peaks became broader, indicating that the
size of Cu nanoparticles became larger with increased molar
fraction of Zn.
50
Catalytic activity measurements
Table 3 summarizes the activities of glycerol hydrogenolysis
over CuxZnyAlz (reduced CuxZnyAlz samples), Cu/ZnO (1:1), and
Cu/ZnO/Al2O3 (1:1:4) catalysts. Approximately 49.6% of
55 glycerol conversion in 80% glycerol aqueous solution with 80.2%
selectivity to 1, 2-PDO was achieved over the Cu10Zn0Al90
catalyst at 180 ºC with 4.0 MPa of H2 pressure over 10 h. This
result suggested that the alumina supported copper catalyst could
effectively catalyse this reaction. The high catalytic activity can
60 be attributed to the high dispersion of copper nanoparticles on
high surface area alumina in this catalyst.
Table 3 Glycerol hydrogenolysis over CuxZnyAlz, Cu/ZnO (1:1) and
Cu/ZnO/Al2O3 (1:1:4) catalysts.a
Glycerol
conversion
(%)
Selectivity (%)
Catalyst
1,2-PDO
EG
Acetol Othersb
Cu10Zn0Al90
Cu10Zn15Al75
Cu10Zn30Al60
Cu10Zn45Al45
49.6
71.7
85.8
73.4
80.2
91.8
92.1
93.3
8.2
2.8
2.3
1.9
8.4
2.8
1.1
1.4
3.2
2.6
4.5
3.4
Cu/ZnO (1:1)
47.3
84.5
7.6
5.7
2.2
Cu/ZnO/Al2O3
(1:1:4)
45.5
86.7
6.2
2.3
4.8
a:Reaction conditions: 80% aqueous solution of glycerol 20 mL, total H2
pressure 4.0 MPa, 180 oC, 10 h, catalyst weight: glycerol/Cu ratio: 200
b: methanol, ethanol, and 1-propanol were identified as by-products.
25
Figure 6 H2 temperature-programmed reduction profiles for calcined
CuxZnyAly samples
When 15% molar fraction of zinc was added to this one-pot
synthesis, the glycerol conversion rose substantially to 71.7%,
and the selectivity to 1, 2-PDO also increased to 91.8%. As the
zinc molar fraction increased to 30%, the glycerol conversion was
further increased to 85.8% while retaining similar selectivity to 1,
2-PDO. The Cu10Zn45Al45 catalyst, which contains the highest
molar fraction of zinc, showed lower glycerol conversion (73.4%)
than the Cu10Zn30Al60 catalyst. For the purpose of comparison,
Cu/ZnO (1:1) and Cu/ZnO/Al2O3 (1:1:4) catalysts, which are the
best catalysts in the previous work on glycerol hydrogenolysis,4,
12 were also prepared by the co-precipitation method as reference
For reduced CuxZnyAlz samples in Figure 5(b), the binding
energy of Cu 2p3/2 shifted to a lower binding energy between
30 932.0 and 932.5 eV, and the satellite peaks completely
disappeared, indicating that Cu2+ completely reduced to Cu0; Cu+
as a hard-reducible species in calcined CuxZnyAly samples could
not be reduced under the experimental conditions. Therefore, the
reduced CuxZnyAlz samples should contain Cu0 and Cu+. The Cu
35 LMM peaks, in addition to the modified Auger parameter and Cu
2p3/2 peak position and shape could normally be used to
determine Cu0 versus Cu+ qualitatively and quantitatively in the
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Catalysis Science & Technology
Page 8 of 11
catalysts, but their catalytic activity is much lower than that of the
ratio by co-precipitation method and found that Cu/ZnO/Al2O3
Cu10Zn15Al75, Cu10Zn30Al60, and Cu10Zn45Al45 catalysts. It 60 catalyst with a Cu/Zn/Al ratio of 1:1:4 is most active on glycerol
indicated that the catalytic activity of the ZnAl2O4 supported Cu
catalyst (Cu10Zn15Al75 catalyst) was much higher than that of the
Cu/Al2O3, Cu/ZnO, and Cu/ZnO/Al2O3 catalysts. Moreover, the
presence of ZnO in the ZnAl2O4 supported Cu catalyst further
increased its catalytic activity.
It was reported in previous studies that the observed glycerol
hydrogenolysis activity was linearly related to the Cu metal
10 surface area and smaller sizes of Cu and ZnO led to higher
catalytic activity.12, 21In this work, an N2O chemisorption
hydrogenolysis.4 This Cu/Zn ratio of 1:1 is reasonable. If the
Cu/Zn ratio was lower than 1, the concentration of ZnO would
not be enough to catalyse the glycerol dehydration. However, if
the Cu/Zn ratio was higher than 1, the concentration of Cu would
65 not be enough to catalyse the second hydrogenation step.
Unfortunately, XPS measurements cannot give us information
about the Cu/ZnO ratio in CuxZnyAlz samples because both ZnO
and ZnAl2O4 contain Zn and Zn 2p3/2 peaks due to ZnO and
ZnAl2O4 could not be distinguished.49 Nevertheless, considering
5
technique was employed to measure the copper surface area of 70 the initial molar fraction of the Cu, Zn, and Al sources and the
the catalysts. However, the results listed in Table 1 show that the
surface area of Cu decreased from 205.8 m2/g to 57.5 m2/g with
15 increased zinc molar fraction from 0 to 45%. Moreover, the
particle sizes of both of Cu and ZnO increased with increased
fact that part of the zinc nitrate converted to ZnO, it was possible
that the Cu/ZnO ratio in the Cu10Zn30Al60 catalyst was close to 1,
while the Cu/ZnO ratio is larger than 2 in the Cu10Zn15Al75
catalyst. Therefore, the catalytic activity of Cu10Zn45Al45
zinc molar fraction as listed in Table 2. Thus, the results implied 75 decreased.
that the copper surface area or particle size of Cu and ZnO were
not the decisive factors in determining the catalytic activity of the
20 Cu/ZnO-based catalysts on glycerol hydrogenolysis.
Given the above, we can see that choosing the suitable support
for copper base catalysts has great influence on their catalytic
activity for glycerol hydrogenolysis. The presence of ZnO further
increased the intrinsic activity of Cu-based catalysts for glycerol
In the Cu10Zn0Al90 and Cu10Zn15Al75 catalysts, the difference is
that the alumina support was replaced by ZnAl2O4. Thus, the 80 hydrogenolysis, possibly because of the Cu-ZnO synergy. The Cu
enhancement of the catalytic activity of the Cu10Zn0Al90 catalyst
could be attributed to the ZnAl2O4 support, even though the
25 particle size of Cu in the Cu10Zn0Al90 catalyst was larger than that
of Cu in the Cu10Zn15Al75 catalyst. In the Cu10Zn30Al60 catalyst,
ZnO nanoparticles appeared in close contact with Cu on the
surface. Based on the mechanistic studies of Cu/ZnO catalysts on
methanol synthesis and methanol steam reforming, ZnO played a
30 crucial role in creating active Cu sites through the strong
interaction between ZnO and Cu. Such strong interaction can
cause strained Cu lattices, which improved the catalytic activity
of Cu–ZnO catalysts.48 From the TEM observations in Figure 1, it
was found that most of Cu and ZnO nanoparticles were in close
35 contact each other on the surface of the Cu10Zn30Al60 catalyst.
Moreover, from the TPR profiles of the CuxZnyAlz catalysts, with
the increased Zn molar fraction, the reduction peaks for CuO in
the CuxZnyAlz catalysts were shifted to lower temperatures,
indicating that the addition of ZnO to the CuxZnyAlz catalysts
40 facilitated the reduction of the CuO species and improved the
redox nature of the Cu species. The lowest reduction temperature
shown in both of Cu10Zn30Al60, and Cu10Zn45Al45 catalysts
indicated highest redox activity for the Cu species and the
strongest interaction between ZnO and Cu. Furthermore, in the
45 XPS spectra for the CuxZnyAlz catalysts (Figure 5(a)), the same
trend as in the TPR profile was observed; the Cu 2p3/2 peak for
Cu2+ species shifted to lower binding energy, as shown in Table
2. The TEM, TPR, and XPS results confirmed that the strongest
interaction between ZnO and Cu existed in both Cu10Zn30Al60 and
50 Cu10Zn45Al45 catalysts. The strong interaction could be the reason
that the catalytic activities of Cu10Zn30Al60 and Cu10Zn45Al45
catalysts were higher than the Cu10Zn15Al75 catalyst.
loading in each sample was measured by ICP-OES. The Cu
loading decreased from 11.14% to 8.73% with increased zinc
molar fraction, but the results did not influence the above
explanation and conclusion.
85
Figure 7 Effect of glycerol concentration on hydrogenolysis activity over
Cu10Zn30Al60 catalyst. Reaction conditions: aqueous solution of glycerol:
20 mL, H2 pressure: 4.0 MPa, reaction temperature: 180 oC, reaction time:
10 h, catalyst weight: glycerol/Cu ratio: 200.
90
The effect of the glycerol concentration on glycerol
hydrogenolysis over the Cu10Zn30Al60 catalyst was studied and
the results are shown in Figure 7. The glycerol conversion
increased with the increased glycerol concentration from 20% to
95 100%, while the selectivity to 1, 2-PDO is decreased as undesired
degradation of reaction products takes place in the highly
concentrated glycerol aqueous solution. However, the selectivity
to 1, 2-PDO remained more than 90% when the concentration of
glycerol solution is below 80%. According to Figure 7, the
100 highest yield of 1, 2-PDO was obtained from the 80% glycerol
solution.
In previous studies on the Cu/ZnO and Cu/ZnO/Al2O3 catalysts
for glycerol hydrogenolysis, the Cu/ZnO ratio in the catalysts was
55 very important, Wang, et al. demonstrated that the catalytic
activities of Cu/ZnO catalysts changed markedly with varying
Cu/Zn ratios and reached a maximum at a ratio of 1:1.12 Meher, et
al prepared the Cu/ZnO/Al2O3 catalysts with different Cu/Zn/Al
The conversion of glycerol and the selectivity to 1, 2-PDO at
8
| Journal Name, [year], [vol], 00–00
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Page 9 of 11
Catalysis Science & Technology
different reaction time were studied using the Cu10Zn30Al60
catalyst, as shown in Figure 8. The glycerol conversion almost
either used as solvent or formed as an inevitable by-product in the
hydrogenolysis of glycerol.22 Indeed, In Figure 9 (a) and (b), for
linearly increased from 43.8% to 85.8% with the reaction time 45 the Cu10Zn0Al90 (Cu/Al2O3) and Cu/ZnO (1:1) catalysts, due to
before 10 h. It indicates that the Cu10Zn30Al60 catalyst in this
work was very active compared to the Cu/ZnO and
Cu/ZnO/Al2O3 catalysts prepared by co-precipitation method,
because they normally needed 24 h reaction time to achieve high
glycerol conversion. Moreover, the selectivity to 1, 2-PDO
remained almost constant above 90%, irrespective of the reaction
the great loss of copper surface areas in both catalysts after each
reuse, the glycerol conversions decreased from 47.3% to 14.1%
for Cu/ZnO catalysts and from 49.6% to 27.7% for the
Cu10Zn0Al90 catalysts after three reuses. It was reported in the
5
10 time. The results imply that glycerol hydrogenolysis proceeded
preferentially to produce 1, 2-PDO on the Cu10Zn30Al60 catalyst.
50
Figure 8 Effect of reaction time on hydrogenolysis activity over
15 Cu10Zn30Al60 catalyst. Reaction conditions: 80% glycerol aqueous solution:
20 mL, H2 pressure: 4.0 MPa, reaction temperature: 180 oC, catalyst
weight: glycerol/Cu ratio: 200.
A trace amount of acetol as an intermediate was identified from
20 Cu10Zn30Al60 catalysed glycerol hydrogenolysis. Acetol
generation indicated that glycerol hydrogenolysis to 1, 2-PDO
over Cu/ZnO/ZnAl2O4 catalysts proceeds via glycerol
dehydration to form acetol on acidic sites, followed by
hydrogenation to 1, 2-PDO over copper surface, as the reaction
25 mechanism reported by Dasari et al.50 Indeed, the ZnO/ZnAl2O4
catalyst was observed to have no measurable activity.
Catalyst stability and reusability are always important in metal-
catalysed liquid phase batch reaction. The enhancement of
stability and reusability on Cu/ZnO/ZnAl2O4 catalyst was also the
30 main aim for this study. To study the stability and reusability of
the Cu10Zn30Al60 catalyst, consecutive cycling experiments for
Cu10Zn0Al90, Cu10Zn30Al60, Cu/Zn (1:1), and Cu/ZnO/Al2O3
(1:1:4) catalysts were performed for glycerol hydrogenolysis. In
the cycling experiment, the catalysts were first separated by
35 centrifugation and washed with deionized water, then dried at 110
ºC under vacuum for 12 h, followed by heating at 500 ºC for 3 h
in a flow of air. Next, the catalyst was reduced by H2 at 400 ºC
for 3 h before it was reused in the next run. Bienholz et al.
reported that a tremendous loss of Cu active surface area during
40 the reaction could be the cause of a strong decrease in catalytic
activity of the Cu/ZnO catalyst after cycling.22 The loss of copper
surface area could be due to the presence of water, which was
Figure 9 Cycling results of Cu10Zn0Al90, Cu10Zn30Al60, Cu/ZnO (1:1), and
Cu/ZnO/Al2O3 (1:1:4) catalysts for glycerol hydrogenolysis (a) and their
copper surface area after each run (b). Reaction conditions: 80%
aqueous solution of glycerol: 20 mL, H2 pressure: 4.0 MPa, reaction
temperature: 180 oC, reaction time: 10 h, catalyst weight: glycerol/Cu
ratio: 200.
55
previous work that the addition of Al2O3 led to an enhancement
in the catalyst stability.23, 51The beneficial effect of these metal
60 oxides in terms of the catalyst stability was assigned to the
presence of a component that prevents the sintering of Cu and
ZnO nanoparticles. It can be seen from Figure 9 that the glycerol
conversion on Cu/ZnO/Al2O3 (1:1:4) catalyst only slightly
decreased from 45.5% to 35.4% due to the presence of Al2O3.
65 However, when Al2O3 was replaced by ZnAl2O4 in the
Cu10Zn30Al60 catalyst, the glycerol conversion was not affected by
consecutive reuses as the copper surface areas of Cu10Zn30Al60
catalysts did not obviously decrease during cycling, indicating its
high reusability and stability for glycerol hydrogenolysis. In
70 addition, Table 4 also listed the physicochemical properties of
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Catalysis Science & Technology
View ArtiPcleaOgnleine10 of 11
DOI: 10.1039/C3CY00661A
fresh and used Cu10Zn0Al90, Cu10Zn30Al60, Cu/ZnO (1:1), and
Cu/ZnO/Al2O3 catalysts prepared by co-precipitation method, the
Cu/ZnO/Al2O3 (1:1:4) catalysts. It can be seen that the 45 Cu10Zn30Al60 catalyst exhibited much higher stability during the
mesoporous structure and particle size of Cu and ZnO remained
almost unchanged in the Cu10Zn30Al60 catalyst. These results
indicate the presence of ZnAl2O4 can also prevent the sintering of
ZnO and Cu nanoparticles, and more importantly, ZnAl2O4 as a
cycling experiments, which could be due to the high thermal and
chemical stability of ZnAl2O4 support under harsh reaction
conditions. The results demonstrated Cu/ZnO/ZnAl2O4 catalysts
could be new and highly active catalysts for glycerol
5
support was more stable than Al2O3 for copper in the high 50 hydrogenolysis.
temperature liquid phase reaction due to its high chemical and
thermal stability.
Acknowledgments
10
Table 4 Physicochemical properties of fresh and used Cu10Zn0Al90
Cu10Zn30Al60, Cu/ZnO (1:1), and Cu/ZnO/Al2O3 (1:1:4) catalysts.
The authors gratefully acknowledge the financial support
55 provided by King Abdullah University of Science and
Technology (KAUST). The contributions from Mr. Khaled
Hallak to this work are also gratefully acknowledged.
BET
surface pore
area size
(m2/g) (nm)
BJH
Pore
ZnO
Cu
Catalysts
volume
size(nm)b size(nm)b
(cm3/g)
Fresh 297.3
7.7
9.2
7.1
0.56
0.39
0.45
0.41
0.17
0.09
0.31
0.22
NA
NA
9.7
10.2
12.6
24.7
9.4
3.2
5.6
6.8
Notes and references
Cu10Zn0Al90
Used a 201.8
Fresh 235.4
Used a 223.2
60 a Analytical core lab, 4700 King Abdullah University of Science and
Technology, Thuwal, 23955-6900, Saudi Arabia. E-mail:
hua.tan@kaust.edu.sa; Tel.: +966-12-8084322
Cu10Zn30Al60
Cu/ZnO (1:1)
7.0
7.4
Fresh
67.5
18.2
35.1
9.3
10.5
35.2
8.6
Used a 33.7
b Nano imaging and fabrication lab, 4700 King Abdullah University of
Science and Technology, Thuwal, 23955-6900, Saudi Arabia
65 cAdvanced Membranes and Porous Materials Center, Chemical and Life
Sciences and Engineering Division, 4700 King Abdullah University of
Science and Technology, Thuwal, 23955-6900, Saudi Arabia
d Water Desalination and Reuse Research Center, 4700 King Abdullah
University of Science and Technology, Thuwal, 23955-6900, Saudi
70 Arabia
Cu/ZnO/Al2O3 Fresh 118.4
(1:1:4)
Used a 87.6
a: after three reuses
b: average crystallite size determined from XRD data using Scherrer
equation
7.6
13.9
12.4
NA: not available
It was confirmed by XPS that Cu+ was present in reduced
15 CuxZnyAlz samples. Although the Cu+/Cu0 ratio was unknown in
this study, Cu+ may be catalytically inactive in glycerol
hydrogenolysis, Cu+ only helped to inhibit the sintering of copper
particles during the reaction rather than exhibit superior activity
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95
319
mesoporous
nanocrystalline
ZnAl2O4
support.
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Cu/ZnO/ZnAl2O4 catalysts showed higher catalytic activity than
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hydrogenolysis to 1, 2-PDO. The Cu10Zn30Al60 catalyst, which
was the best catalyst, exhibited 85.8% glycerol conversion with
92.1% selectivity to 1,2-PDO at 180 °C and 4.0 MPa H2 over 10
h in 80% glycerol aqueous solution. It demonstrated that the
40 support of ZnAl2O4 had a positive effect on the copper based
catalysts and the presence of ZnO further improved the catalytic
activity of the Cu based catalysts due to the strong interaction
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Catalysis Science & Technology
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