Promoting effect of zirconium oxide on Cu-Al2O3 catalyst for the hydrogenolysis of glycerol to 1,2-propanediol

Article abstract of DOI:10.1039/c6cy00085a

In this work, ZrO₂-promoted Cu-Al₂O₃ catalysts prepared by the co-precipitation method were used for the hydrogenolysis of glycerol to 1,2-propanediol in a fixed-bed reactor. These catalysts were fully characterized by BET, ICP, N₂O chemisorption, XRD, H₂-TPR, NH₃-TPD, XPS, TEM and TGA. The relationship between the catalytic activity and the metal-support interaction was studied in detail. The experimental results showed that the addition of ZrO₂ to Cu-Al₂O₃ could greatly enhance glycerol conversion and 1,2-propanediol selectivity. This improvement was related to the increase in the acidity and Cu dispersion on the catalytic surface. The optimal 20ZrCu-Al₂O₃ catalyst attained 97.1% glycerol conversion and 95.3% 1,2-propanediol selectivity. Furthermore, the effects of process parameters like solvent, reaction temperature, operating pressure, glycerol concentration and liquid flow rate on glycerol hydrogenolysis together with the catalyst stability were deeply investigated. Compared with the Cu-Al₂O₃ catalyst, the ZrO₂-promoted Cu-Al₂O₃ catalyst had better stability and potential for practical application, which was likely due to the high Cu dispersion and strong interaction between copper and zirconium species.

Full text of DOI:10.1039/c6cy00085a

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DOI: 10.1039/C6CY00085A
Journal Name
ARTICLE
Promoting effect of zirconium oxide on Cu-Al O catalyst for the
2
3
hydrogenolysis of glycerol to 1,2-propanediol
Received 00th January 20xx,
Accepted 00th January 20xx
Fufeng Cai, Wei Zhu and Guomin Xiao*
DOI: 10.1039/x0xx00000x
In this work, the ZrO
of glycerol to 1,2-propanediol in a fixed-bed reactor. These catalysts were fully characterized by BET, ICP, N
chemisorption, XRD, H -TPR, NH -TPD, XPS, TEM and TGA. The relationship between the catalytic activity and metal-
support interaction was studied in detail. The experiment results showed that addition of ZrO to Cu-Al could greatly
enhance glycerol conversion and 1,2-propanediol selectivity. This improvement was related to the increases in the acidity
and Cu dispersion on the catalytic surface. The optimal 20ZrCu-Al catalyst attained 97.1% glycerol conversion and
5.3% 1,2-propanediol selectivity. Furthermore, the effects of process parameters like solvent, reaction temperature,
operating pressure, glycerol concentration and liquid flow rate on glycerol hydrogenolysis together with the catalyst
stability were deeply investigated. Compared with Cu-Al catalyst, the ZrO -promoted Cu-Al catalyst had better
2 2 3
-promoted Cu-Al O catalysts prepared by co-precipitation method were used for the hydrogenolysis
2
O
www.rsc.org/
2
3
2
2 3
O
2
O
3
9
2
O
3
2
2 3
O
stability and prospective for practical application, which was likely due to the high Cu dispersion and strong interaction
between copper and zirconium species.
dehydrogenation, various Cu-based catalysts such as Cu/Al
Cu/ZrO , Cu/ZnO, Cu/SiO , and Cu/MgO have been selected to
glycerol hydrogenolysis.
drawbacks such as poor catalytic activity and stability for the Cu-
2 3
O ,
1
. Introduction
In the last decades, increasing environmental issues and declining
2
2
1
3–16
Nevertheless, there are still some
petroleum resources have encouraged the development of
biofuels. In this context, biodiesel produced by transesterification
1
2,13
1
based catalysts that need to be overcome.
It is well documented that the acidity of the catalyst has greatly
reaction has received significant interest because of its renewable
and environmental-friendly property. However, large amounts of
glycerol are simultaneously generated as a by-product in the
production of biodiesel. Converting the oversupplied glycerol to
valuable chemicals can effectively enhance the added value of the
biodiesel. For this reason, several routes have been proposed for
the transformation of glycerol to different chemicals by catalytic
4
,17
impact on the glycerol conversion.
On the other hand, suitable
acid components are necessary for glycerol hydrogenolysis on the
1
8,19
basis of the dehydration-hydrogenation reaction mechanism.
Thus, modification of Cu-based catalysts by the promoter with an
appropriate acidity will help to improve the catalytic activity for
glycerol hydrogenolysis. Recently, the strong acidic nature of ZrO
2
2
0,21
22
2
has been fully discussed in the literature.
the ZrO -based catalysts were responsible for the acetol produced
from glycerol which could further undergo hydrogenation reaction
Raju et al. published
processes. In particular, much efforts have been focused on the
hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO) which is
widely applied in the manufacture of the important chemicals such
as cosmetics, pharmaceuticals, functional fluids, and polyesters
2
23
and produce 1,2-PDO. Sharma et al. performed that the ZrO
2
-
3
promoted Cu/ZnO/Cr catalysts had high Cu dispersion with
2
O
3
resins. Nevertheless, 1,2-PDO at present is mainly produced from
suitable acidity and showed the superior catalytic activity for
glycerol hydrogenolysis. However, no literature has been reported
petroleum derivates. Thus, the production of 1,2-PDO from glycerol
has attracted considerable attention in academia and industry
concerning ZrO
hydrogenolysis.
2
-promoted Cu-Al catalysts for glycerol
2
O
3
because this process provides
production of 1,2-PDO.
a sustainable route for the
In this work, the ZrO
2
-promoted Cu-Al O catalysts prepared by
2 3
A variety of heterogeneous catalysts based on noble metals and
non-noble metals have been used for the hydrogenolysis of glycerol
co-precipitation method were used for the hydrogenolysis of
glycerol to 1,2-PDO in a fixed-bed reactor. The catalysts developed
in this study were well characterized by different techniques. The
physicochemical properties of catalysts were correlated with the
obtained catalytic performance. Further, the effects of different
process parameters such as solvent, reaction temperature,
operating pressure, glycerol concentration and liquid flow rate on
glycerol hydrogenolysis together with the catalyst stability were
investigated in detail.
4
–12
to 1,2-PDO in recent years.
Particularly, because of their lower
ability to cleave C-C bond and high efficiency for C-O bond hydro-
School of Chemistry and Chemical Engineering, Southeast University, Nanjing
211189, China. E-mail address: xiaogm@seu.edu.cn. Tel./Fax: +86 25 52090612.
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Journal Name
2
. Experimental
The reduction behaviors of the calcined catalysts were evaluated
View Article Online
DOI: 10.1039/C6CY00085A
by
H
2
temperature-programmed reduction (TPR) technique.
2
.1. Chemicals and materials
Glycerol (99.0 wt.%), ethylene glycol (99.0 wt.%), methanol (99.5 He (30 mL/min) for 1 h at 400 ℃, followed by cooling at room
wt.%), ethanol (99.7 wt.%), 1-propanol (99.0 wt.%), 2-propanol temperature. After pretreatment, the reduction agent of a 5%
99.7 wt.%), Cu(NO ·3H O (99.0 wt.%), Al(NO ·9H O (99.0 wt.%) /N flow (30 mL/min) was introduced. Then, the sample was
and K CO (99.0 wt.%) were obtained from Sinopharm Chemical heated to 700 ℃ at a rate of 10 ℃/min. The outlet gas was
Typically, the sample (50 mg) was loaded and pretreated in flowing
(
3
)
2
2
3
)
3
2
H
2
2
2
3
Reagent Co., Ltd., Shanghai, China. 1,2-Propanediol (99.5 wt.%), 1,3- collected by passing it through a silica gel trap to remove water. The
propanediol (99.5 wt.%), acetol (90.0 wt.%), 1,4-butanediol (99.5 consumption of hydrogen was detected by a thermal conductivity
wt.%) and ZrO(NO
Aladdin Industrial Corporation, Shanghai, China. Hydrogen and
nitrogen of high purity (≥99.99%) were obtained from Nanjing were measured by the dissociative N
special gas factory Co., Ltd., Nanjing, China and were used directly the sample (50 mg) was pretreated in flowing He (30 mL/min) for 1
3
)
2
·xH
2
O (99.5 wt.% metal) were purchased from detector.
The Cu dispersion and specific surface area (SCu) in the catalysts
O chemisorption. Typically,
2
in this work from the cylinders.
.2. Catalyst preparation
The ZrO -promoted Cu-Al
precipitation method. In a typical preparation, copper nitrate 0.5 h at 50 ℃ to oxidize the surface Cu to Cu
Cu(NO ·3H O, 3.80 g), zirconyl nitrate (ZrO(NO ·xH O, 2.54 g) carried out to reduce the Cu O to Cu by raising the temperature to
and aluminium nitrate (Al(NO ·9H O, 36.80 g) were dissolved in 400 ℃with the reduction agent of a 5% H /N flow (30 mL/min).
00 mL of deionized water. The mixture was vigorous stirred at The acidities of the catalysts were determined by ammonia
room temperature for 2 h. A 0.2 g/mL aqueous solution of temperature-programmed desorption (TPD) technique. Prior to the
potassium carbonate was added at a speed of 0.5 mL/min to the adsorption of NH , the sample (100 mg) was pretreated in flowing
h at 400 ℃, followed by cooling at room temperature. The sample
was reduced by increasing the temperature to 400 ℃ at a rate of
2
1
0 ℃/min with the reduction agent of a 5% H
catalysts were prepared by co- Subsequently, the sample was exposed to N O flow (30 mL/min) for
O. Next, TPR was
2 2
/N flow (30 mL/min).
2
O
2 3
2
0
2
(
3
)
2
2
3
)
2
2
2
3
)
3
2
2
2
5
3
solution until the pH was 9.0. The precipitate was aged at room He (30 mL/min) for 1 h at 400 ℃ to remove moisture and other
temperature for 48 h, filtered off, and washed thoroughly with adsorbed gases. After cooling to 100 ℃, the sample was saturated
deionized water until the effluent was neutral. The precipitate was with pure NH
dried at 100 ℃for 24 h and calcined at 400 ℃in stationary air for 4 mL/min) for 1 h to remove the physically adsorbed NH
3
for 0.5 h and subsequently purged with He (30
. Next, the
3
h. Then, the catalyst was ground and pressed at 10 MPa to form sample was heated to 750 ℃ at a rate of 10 ℃/min and the NH
3
pellets. The pellets were sieved to retain particles with sizes desorption was monitored by a thermal conductivity detector.
comprised 20 to 40 mesh. Prior to the reaction, the catalyst was
reduced in flowing H (100 mL/min) at 400 ℃for 2 h. The calcined on the Thermo Scientific ESCALAB 250Xi X-ray Photoelectron
and reduced catalysts were labelled in the form xZrCu-Al , where Spectrometer with Mg Kα radiation source (1253.6 eV) at a
x indicated the nominal weight percentage of zirconium on the pressure of 3.0×10 mbar. The collected binding energies were
Al support. In all catalysts, the nominal weight loading of copper corrected by the C1s peak at 284.6 eV as the reference. The binding
on the Al support was fixed at 20 wt.%. For example, 10ZrCu- energies were determined within a precision of ±0.1 eV.
Al refers to the catalyst with 20% mass fraction of copper and The transmission electron microscope (TEM) images of the
0% mass fraction of zirconium on the Al support. The catalysts catalysts were obtained with FEI Tecnai G2 electron microscope
with different zirconium mass fraction were also prepared as per operating at a 200 kV voltage. The sample was suspended in
X-ray photoelectron spectroscopy (XPS) analysis was performed
TM
TM
2
2 3
O
-7
2 3
O
2
O
3
2 3
O
1
2 3
O
the above-mentioned procedure.
ethanol with an ultrasonic dispersion for 0.5 h. Drops of the
suspension were deposited on copper grid coated with amorphous
carbon film.
2.3. Catalyst characterization
The BET surface area (SBET) and pore volume (V
p
) of the catalysts
The SDT-Q600 thermogravimetric analysis (TGA) instrument was
2
were measured by N adsorption-desorption method at liquid used for the characterization of the spent catalysts. A small quantity
nitrogen using Beishide instrument (3H-2000PS1). Prior to the of the sample (200 mg) was placed in an aluminium sample cup and
measurement, the sample was degassed under vacuum at 200 ℃ the temperature was increased from 50 ℃ to 900 ℃ at a rate of
for 12 h. The BET surface area was obtained as per the desorption 10 ℃/min in air atmosphere.
branch of the isotherms.
2
.4. Catalytic activity test
Inductive coupled plasma optical emission spectroscopy (Optima
300 DV, PerkinElmer) was used to measure the Cu content of the
7
The glycerol hydrogenolysis was carried out in a vertical fixed-bed
prepared catalysts and the chemical composition of the liquid reactor with a back pressure regulator to control the system
product.
pressure. The reactor was stainless steel tube with an internal
The X-ray diffraction (XRD) patterns were determined with a diameter of 11 mm and a length of 950 mm. A schematic diagram
D2/max-RA X-ray diffractometer (Bruker, Germany) operating with
Cu Kα radiation at 40 kV and 30 mA with a scanning angle (2θ)
ranging from 10° to 90° at the scanning rate of 6°/min. The particle
size was calculated using the Debye-Scherrer equation.
of the experimental apparatus is displayed in Figure S1. In a typical
run, 4.0 g catalyst (20-40 mesh, ca. 5 mL) was charged in the
constant temperature section of the reactor tube, with quartz sand
packed in both ends. Before the catalytic activity test, the catalyst
2
| J. Name., 2012, 00, 1-3
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Cat al el ysis Science & Techn oi nl ogy
Journal Name
ARTICLE
was in situ reduced in pure H
2
with a flow rate of 100 mL/min at
molesof glycerol(in)-molesof glycerol(out)
View Article Online
100
Conversion(%)
DOI: 10.1039/C6CY00085A
400 ℃for 2 h. After reduction, the temperature of the reactor was
molesof glycerol(in)
decreased to 230 ℃and pressured to 3.5 MPa. A 20 wt.% glycerol
ethanol (or water, methanol, 1-propanol) solution was continuously
fed into the reactor with a flow rate of 27.8 mL/h, which
corresponded to WHSV ((weight flow rate of the feed
solution)×(weight fraction of glycerol in the feed)/(weight of the
moles of carbon in a specific product
moles of carbon in glycerol consumed
Selectivity (%) 
100
Conversion (%)Selectivity (%)
Yield (%) 
1
00
-1
catalyst)) of 1.21 h , in pure H
2 2
(or N ) flow at 100 mL/min (mole
3
. Results and discussion
ratio of H :glycerol = 95:1). The reaction products were cooled in a
2
condenser and collected in a gas-liquid separator. The liquid
samples were withdrawn at regular intervals of time (0.5 h). The
collected liquid samples were analyzed by a gas chromatography
3
.1. Catalyst Characterization
3.1.1. Physicochemical properties of the catalysts
(
Shimadzu GC-2014), equipped with a flame ionization detector and
a capillary column (Rtx-WAX, 30 m × 0.25 mm). 1,4-Butanediol was in Table 1. Compared with Cu-Al
added to the liquid sample as an internal standard substance for Al catalysts had high BET surface area and pore volume.
the GC analysis. The steady state performance of the reactor was However, the incorporation of ZrO into Cu-Al slightly reduced
the weight loading of Cu and the pore size of the catalysts (Figure
S2). On the other hand, addition of ZrO to Cu-Al notably
decreased Cu particle size and enhanced the Cu dispersion and
specific surface area. These behaviours showed that the ZrO
The physicochemical properties of the catalysts are summarized
catalyst, the ZrO -promoted Cu-
O
2 3
2
O
3
2
2
2 3
O
observed by GC analysis of the liquid samples after reaction for 2 h
and data acquisition was carried out after steady operation for 3 h.
The obtained results were averages of 3-4 data points at the same
conditions. Each data point was analyzed three times by GC. The
mass balances were greater than 95% for each set of data
acquisition. The liquid products were also identified by GC-MS
2
2 3
O
2
contributed to restraining the aggregation of Cu particles during the
thermal treatment, which might be due to the strong interaction
2
4,25
between zirconium and copper species.
Similar behavior was
-promoted Cu/ZnO/Cr
(
Shimadzu GCMS-QP2010). The conversion of glycerol and
also reported elsewhere for the ZrO
2
2 3
O
selectivity of products were defined as follows. The standard
uncertainties of the conversion and selectivity in the samples were
23
catalysts.
1%.
Table 1 Physicochemical property of the catalysts
SBET
Vp
Cu content
dCu
Cu dispersion
SCu
dCu
Acidity
Acidity 420-750 ℃
Catalyst
2
-1
3
-1
a
b
c
2
-1 c
c
-1 d
-1 d
(m g )
(cm g )
0.236
0.279
0.302
0.317
0.295
(wt.%)
(nm)
(%)
(m g )
(nm)
(μmol NH3 g )
(μmol NH3 g )
Cu-Al
2
O
3
64.2
89.5
103.4
117.9
17.6
17.2
16.9
16.3
19.4
15.7
9.8
6.5
5.9
7.8
52.8
77.1
101.5
126.5
117.0
12.8
8.8
6.7
5.3
5.8
627
1436
1686
2175
2261
3
-TPD.
444
5
1
2
3
ZrCu-Al
2
O
3
11.4
15.0
18.7
17.3
1242
1487
1972
2121
0ZrCu-Al
0ZrCu-Al
0ZrCu-Al
2
2
2
O
3
O
3
O
3
100.1
15.1
a
b
c
d
Measured from ICP. Estimated from XRD. Measured from N
2
O chemisorption. Measured from NH
3.1.2. Structural properties of the catalysts
2
of ZrO could suppress the increase of Cu particles, which resulted
in the increased Cu dispersion.
The XRD patterns of the catalysts after calcination at 400 ℃for 4
h are presented in Figure 1 A. The presence of the peaks at 2θ =
2
3
(A)
3
2.5°, 35.5° and 38.7° corresponded to CuO (PDF#48-1548), peaks
＃
◇ Al O
2 3
△
CuO
26
at 2θ = 46.3° and 67.0° to Al
2
O
3
(PDF#13-0373), and peaks at 2θ =
＃
＃
＃
m-ZrO
2
2
5
2
3.9°, 29.5° and 33.9° to monoclinic ZrO
zirconium content, the intensity of diffraction peaks for monoclinic
ZrO increased remarkably. Interestingly, almost no peaks of CuO
were observed on the ZrO -promoted Cu-Al catalysts, which was
2
.
With the increase in
30ZrCu-Al O3
2
2
2
2
O
3
20ZrCu-Al O
2
3
likely related to the decreased Cu loading and homogeneous
distribution of CuO particles on the support surface. As shown in
Figure 1 B, after reduction at 400 ℃for 2 h, the diffraction peaks of
1
0ZrCu-Al O
2 3
5
ZrCu-Al O
2 3
△
△
△
◇
◇
monoclinic ZrO
2
and CuO disappeared, while the peaks of Cu
Cu-Al O
2 3
23
(
PDF#04-0836) were detected in the catalysts. The diffraction
10
20
30
40
50

60
70
80
90
peaks of Cu enlarged and their intensities notably decreased with
an increase in zirconium content. The Cu particle size (given in Table
2
1) was remarkably decreased from 19.4 nm for Cu-Al
2 3
O to 5.9 nm
for 30ZrCu-Al . The results above indicated that the introduction
2
O
3
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DOI: 10.1039/C6CY00085A
(
B)
△
△
Cu
(A)
△
△
Cu-Al O3
2
Cu-Al O
2
3
5
1
ZrCu-Al O
2 3
5
ZrCu-Al O
2
3
0ZrCu-Al O
2
3
a
1
0ZrCu-Al O
2 3
ß
2
3
0ZrCu-Al O
20ZrCu-Al O
2
2
3
3
0ZrCu-Al O
2
3
3
0ZrCu-Al O
2
3
10
20
30
40
50

60
70
80
90
100
200
300
400
500
600
700
2
Temperature (℃)
Figure 1. XRD patterns of the catalysts upon calcination (A);
reduction (B).
(
B)
3
.1.3. Reducibility and surface acidic properties of the catalysts
The H -TPR profiles of the catalysts are displayed in Figure 2 A. In
order to get a deeper understand about the results of H -TPR, the
2
2
3
0ZrCu-Al O
2
3
reduction peak can be classified into two sections, one at low
temperature (peak α) and another at relatively high temperature
2
0ZrCu-Al O
2
3
1
0ZrCu-Al O
2
(
peak β). The low temperature peak α can be associated with the
3
5
ZrCu-Al O
2 3
reduction of highly dispersed CuO with smaller particle size, while
Cu-Al O
2
3
high temperature peak β can be ascribed to the reduction of bulk
1
7
Al O
2
3
CuO with a larger size. As can be seen, the Cu species in Cu-Al
catalyst was mainly present in the form of bulk CuO. For the ZrO
promoted Cu-Al catalysts, the intensity of low temperature peak
2 3
O
2
-
100
200
300
400
500
600
700
Temperature (℃)
2 3
O
α increased significantly while that of high temperature peak β
declined notably with the increase in zirconium content, suggesting
2 3
Figure 2. H -TPR (A) and NH -TPD (B) profiles of the catalysts.
addition of ZrO
dispersed Cu species.
As shown in Figure 2 B, the acidity of the catalyst was measured
by NH -TPD. Based on the temperature of NH desorption peak, the
2
to Cu-Al
2
O
3
favored the formation of highly 3.1.4. Surface element compositions of the catalysts
The surface element compositions of catalysts were analyzed
using XPS technique. Figure 3 displays the Cu 2p and Zr 3d XPS
spectra of the catalysts and the binding energy (B.E.) values of Cu,
Zr, Al and O are given in Table S1. As shown in Figure 3 A, the
calcined catalysts showed the B.E. values of about 934.5 and 954.5
eV respectively corresponding to Cu 2p3/2 and Cu 2p1/2. Meanwhile,
the main peaks were followed by two strong satellite peaks at ca.
3
3
acid sites of the catalyst can be divided into three sections, weak
acid (150-250 ℃), medium acid (250-420 ℃), and strong acid (420-
27,28
7
50 ℃) strength.
desorption peak was observed on Al
catalyst showed a broad peak of NH
As can be seen from Figure 2 B, almost no NH
3
2
O
3
support. The Cu-Al O
2
3
3
desorption at 150-350 ℃,
2
+
9
43 and 963 eV, implying the presence of Cu species in the
implying the presence of weak and medium acid sites, while small
amounts of strong acid sites were evident from another broad peak
1
6,31
catalysts.
After reduction at 400 ℃ for 2 h, the two peaks
(
Figure 3 B) at ca. 932.5 and 952.5 eV on XPS spectra for the
2 2 3
at 420-750 ℃. In the case of ZrO -promoted Cu-Al O catalysts, a
catalysts were respectively ascribed to Cu 2p3/2 and Cu 2p1/2 of
strong NH desorption peak in the temperature range of 450-750 ℃
3
0
17
Cu . The low B.E. values and absence of satellite peaks showed
was observed, indicating the existence of large amounts of strong
acid sites. To illustrate, the acidities of catalysts are listed in Table 1.
2
+
0 8
that the Cu species in the catalysts were fully reduced to Cu .
Furthermore, the two peaks (Figure 3 C) at ca. 182.0 and 184.5 eV
for the catalysts were attributed to Zr 3d5/2 and Zr 3d3/2 peaks of
It can be observed from Table 1 that incorporation of ZrO
Al catalyst not only enhanced the acid quantity, but also
increased the strong acid sites, which might be related to the strong
2
into Cu-
2 3
O
4
+
24
Zr , respectively. As can be seen from Table S1, the catalysts
showed the B.E. values of about 73 and 74 eV respectively ascribed
2
9,30
acid nature of ZrO
NH desorption peak centred at ca. 630 ℃ for the ZrO
Cu-Al catalysts slightly moved to higher temperature with the
2
.
Moreover, it should be pointed out that the
3
+
to Al 2p3/2 and Al 2p1/2, confirming the existence of Al species. The
O 1s spectra for the catalysts exhibited two peaks at ca. 531 and
3
2
-promoted
2
O
3
533 eV. The peak at ca. 531 eV was ascribed to the lattice oxygen,
increase in zirconium content, confirming the higher the zirconium
content the higher the acid strength.
while the peak at ca. 533 eV could be related to the presence of
1
0
hydroxyl species. Interestingly, the Cu 2p peaks (Figure 3 B) for
the catalysts slightly shifted to higher B.E. when adding of ZrO to
Cu-Al , while the Zr 3d spectra (Figure 3 C) moved to lower B.E.,
2
2
O
3
showing the existence of charge transfer from the copper metal
4
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ions to the ZrO
due to the strong interaction between the CuO and ZrO
Al
elsewhere.
almost the same Cu loading, the introduction of ZrO
notably increased the surface Cu/Al atomic ratio (Table S1), which alumina. In the case of Cu-Al
2
and/or Al
2
O
3
species. This behaviour was probably
The catalytic activity of the catalyst was studied in a fixed-bed
View Article Online
DOI: 10.1039/C6CY00085A
2
and/or reactor at 230 ℃ and 3.5 MPa using ethanol as a solvent. For the
2
O
3
species. Similar behaviours were also presented in purpose of comparison, catalytic activity of alumina prepared by
1
7,24,31
In addition, it’s also worth noting that in spite of precipitation method is also listed in Table 2. As can be seen, very
into Cu-Al low glycerol conversion and 1,2-PDO selectivity were obtained on
catalyst, the glycerol conversion
2
2 3
O
2 3
O
showed the addition of ZrO
dispersion.
2
was beneficial to improve the Cu and 1,2-PDO selectivity respectively were 64.9% and 82.7%. The
glycerol conversion and 1,2-PDO selectivity notably increased when
ZrO
2
was added to Cu-Al
2 3
O . Among the catalysts tested, the
2
0ZrCu-Al O
2 3
catalyst obtained the excellent catalytic performance
(A)
Cu 2p
with 97.1% glycerol conversion and 95.3% 1,2-PDO selectivity. This
result was much better than that attained by other Cu-based
catalysts.
led to slight decrease of 1,2-PDO selectivity, which might be due to
the reduction of Cu dispersion in the catalyst, as confirmed by N
chemisorption. On the other side, addition of ZrO to Cu-Al O
2 3
Cu-Al O3
2
2p1/2
2p3/2
3
2–34
5
ZrCu-Al O3
Nevertheless, further increase in zirconium content
2
1
0ZrCu-Al O3
2
2
O
2
3
0ZrCu-Al O3
2
2
decreased the selectivities to ethylene glycol and acetol, suggesting
that the C-C bond breaking reaction for glycerol was restrained to
0ZrCu-Al O3
2
some extent and the introduction of ZrO
hydrogenation of acetol. The results above obviously showed the
high catalytic performance of ZrO -promoted Cu-Al catalysts.
2
contributed to the
9
60
950
940
930
2
2 3
O
Binding energy (eV)
Table 2 Catalytic results of glycerol hydrogenolysis on different
a
catalysts
b
(B)
Conversion Selectivity (%)
Cu 2p
Catalyst
(
%)
1,2-PDO EG
Acetol Others
Al
Cu-Al
2
O
3
1.2
0.2
Trace 98.3 1.5
2
p1/2
2p3/2
Cu-Al O3
2
O
3
64.9
80.2
86.4
97.1
98.5
82.7
85.9
92.6
95.3
92.0
4.8
3.7
2.4
1.2
1.9
4.2
2.6
0.9
0.3
1.4
8.3
7.8
4.1
3.2
4.7
2
5ZrCu-Al
2 3
O
5
ZrCu-Al O3
2
10ZrCu-Al
2
2
2
O
3
O
3
O
3
1
2
0ZrCu-Al O3
20ZrCu-Al
0ZrCu-Al
3
2
3
0ZrCu-Al O3
2
a
Reaction conditions: 20 wt.% glycerol ethanol solution; liquid flow
rate, 27.8 mL/h; hydrogen flow rate, 100 mL/min; H :glycerol = 95:1
mole ratio); catalyst weight, 4.0 g (ca. 5 mL); WHSV = 1.21 h ;
0ZrCu-Al O3
2
2
-1
(
960
950
940
930
Binding energy (eV)
reaction temperature, 230 ℃; operating pressure, 3.5 MPa; data
acquisition after steady operation for 3 h. 1,2-PDO = 1,2-
b
propanediol; EG = ethylene glycol; others include methanol, 1-
propanol, 2-propanol, 1,3-propanediol, etc.
(C)
Zr 3d
It has been well established that the acid-catalyzed
hydrogenolysis of glycerol proceeds by the C-O bond scission step
that involves the dehydration of glycerol molecules to produce
acetol on acid sites of support, and followed by hydrogenation of
3d3/2
3d5/2
3
0ZrCu-Al O
2
3
17–19,28,35
2
0ZrCu-Al O
2
3
acetol to produce 1,2-PDO on metal active sites of catalyst.
Thereby, the amounts of acid and metal active sites in the catalyst
have great effects on the hydrogenolysis of glycerol to 1,2-PDO. As
shown in Figure 4 A, very low glycerol conversion and 1,2-PDO
selectivity were achieved on alumina, most probably due to the lack
of acid and metal active sites on alumina surface. Conversely, the
1
0ZrCu-Al O3
2
5
ZrCu-Al O3
2
195
192
189
186
183
180
177
Binding energy (eV)
catalytic activity increased significantly for the Cu-Al O3 catalyst
2
(
Figure 4 B) due to the presence of acid and Cu active sites .
However, because the Cu-Al catalyst was very low in acid, as
proved by NH -TPD, only 64.9% glycerol conversion was obtained.
As ZrO was introduced into Cu-Al catalyst, the acidity and Cu
Figure 3. Cu 2p XPS profiles of calcined (A) and reduced (B) catalysts,
Zr 3d XPS profiles of reduced catalysts (C).
2
O
3
3
2
2 3
O
3
.2. Glycerol hydrogenolysis studies
.2.1. Catalytic activity
dispersion were enhanced notably, which resulted in the improved
catalytic performance for glycerol hydrogenolysis (Figure 4 C).
3
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ARTICLE
2 3 2 3 2 3
Figure 4. Proposed reaction pathway for glycerol hydrogenolysis in Al O (A), Cu-Al O (B) and 20ZrCu-Al O (C).
hydrogenolysis. To get a deeper understand about the nature of
active species in the hydrogenolysis of glycerol to 1,2-PDO on the
ZrO -promoted Cu-Al O catalysts, we attempted to explore the
2 2 3
(A)
100
relationship between structural properties of catalysts and catalytic
performance for glycerol hydrogenolysis. As shown in Figure 5 A, a
clear correlation between glycerol conversion and total acidity was
observed. Glycerol conversion increased with increasing total
acidity of the catalyst. Numerous studies have suggested that the
acid-catalyzed hydrogenolysis of glycerol was first dehydrated to
acetol at Lewis acid sites or 3-hydroxypropanal at Bronsted acid
sites, which were subsequently hydrogenated to 1,2- and 1,3-PDO,
90
80
70
60
50
1
9,37
respectively.
2 2 3
Hence, the acidity of the ZrO -promoted Cu-Al O
500
1000
1500
2000
2500
catalysts might be mainly composed of Lewis acid because a very
low level of 1,3-PDO and high concentrations of 1,2-PDO were
detected in this work. Nevertheless, it should be noted that the
Total acidity (umol NH /g)
3
presence of the Cu active sites was necessary to the hydrogenolysis
100
17,18
(B)
of glycerol to 1,2-PDO.
A closely linear correlation between 1,2-
PDO yield and Cu dispersion was presented in Figure 5 B, showing
that the Cu active sites on the support surface were mainly
responsible for the hydrogenation of acetol to 1,2-PDO. Therefore,
90
80
70
60
50
introduction of ZrO2 into Cu-Al O3 not only improved 1,2-PDO
2
selectivity but also decreased the selectivity to acetol. The excessive
acetol on Cu-Al
the presence of more available Cu active sites on ZrO
Cu-Al catalysts. In short, the results in this work demonstrated
O
2 3
catalyst can be transformed into 1,2-PDO due to
2
-promoted
2
O
3
that both acid sites and Cu active sites in the catalysts were the key
parameters for the efficient hydrogenolysis of glycerol to 1,2-PDO.
10
15
20
Cu dispersion (%)
3.2.2. Effect of solvent
It's well known that the glycerol can be generated as a by-
Figure 5. Variation of glycerol conversion with total acidity in the
catalysts (A) and 1,2-PDO yield as a function of Cu dispersion for the product in the manufacture of biodiesel and is presented in water
catalysts (B). Reaction conditions: 20 wt.% glycerol ethanol solution; phase. And water is a by-product of the glycerol hydrogenolysis.
liquid flow rate, 27.8 mL/h; hydrogen flow rate, 100 mL/min; Therefore, water seems to be the most suitable solvent for the
2
H :glycerol = 95:1 (mole ratio); catalyst weight, 4.0 g (ca. 5 mL);
WHSV = 1.21 h ; reaction temperature, 230 ℃; operating pressure,
hydrogenolysis of glycerol to 1,2-PDO in view of environment
protection and economic benefits. On the other side, different
alcohols have been widely used as the solvents and/or hydrogen
-
1
3.5 MPa; data acquisition after steady operation for 3 h.
3
2–34,40–44
donors for the hydrogenolysis of glycerol.
Typically,
As stated above, it has been reported that the hydrogenolysis of
3
1,36–
methanol and 1-propanol were used for studying the effect of
glycerol was strongly influenced by the acidity of the catalysts.
3
8
solvent on glycerol hydrogenolysis over 20ZrCu-Al O3 catalyst. As
2
Also, the role of acid and metal active sites on glycerol
1
7–19,39
displayed in Figure 6, the catalytic activity of the catalyst was
strongly affected by the selection of solvent. The maximum (97.1%)
and minimum (56.4%) glycerol conversion respectively were
hydrogenolysis has been investigated by different researchers.
Their results indicated that more acid and metal active sites in the
catalysts favored the increased catalytic performance for glycerol
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achieved when ethanol and water were selected as solvents. In Table 3 Different hydrogen donors for glycerol hydrogenolysis on
View Article Online
DOI: 10.1039/C6CY00085A
a
2
0ZrCu-Al
2
O
3
catalyst
addition, in the case of 1-propanol as a solvent, glycerol conversion
and 1,2-PDO selectivity were 94.6% and 92.4%, respectively, slightly
greater than those in the presence of methanol as a solvent.
Vasiliadou et al suggested that there was a linear relationship
between the glycerol conversion and hydrogen concentration in the
b
Conversion Selectivity (%)
Solvent
(
%)
1,2-PDO EG
Acetol Others
3
3
Methanol
Ethanol
1-Propanol 17.2
13.1
19.7
24.9
30.4
28.6
0.6
1.1
0.8
65.3
59.6
62.1
9.2
8.9
8.5
solution when 1-butanol was used as
a
solvent for the
a
hydrogenolysis of glycerol. Thereby, the concentration of hydrogen
Reaction conditions: 20 wt.% glycerol concentration; liquid flow
was supposed to be a key factor in glycerol conversion. In this work, rate, 27.8 mL/h; nitrogen flow rate, 100 mL/min; catalyst weight,
the glycerol conversion in different solvents decreased in the order 4.0 g (ca. 5 mL); reaction temperature, 230 ℃; operating pressure,
b
of ethanol > 1-propanol > methanol > water, which was in 3.5 MPa; data acquisition after steady operation for 3 h. 1,2-PDO =
1
,2-propanediol; EG = ethylene glycol; others include methanol,
agreement with the results of the solubility of hydrogen in the
4
5,46
ethanol, formaldehyde, acetaldehyde, propionaldehyde, 1-propanol,
corresponding solvents.
On the other hand, because the
2-propanol, etc.
alcohols such as methanol, ethanol, 1-propanol, 2-propanol, and 1-
butanol were considered to be the appropriate hydrogen donors for
glycerol hydrogenolysis, the catalytic hydrogen transformation from
alcohols to glycerol also favored the improved glycerol conversion
It should be emphasized, however, that 98.9% 1,2-PDO selectivity
was achieved in the presence of water as a solvent, as shown in
Figure 6. In comparison to the alcohols like methanol, ethanol, and
1
1,40–44
compared to water as a solvent.
effect of solvent on glycerol hydrogenolysis, different alcohols
methanol, ethanol and 1-propanol) were used as hydrogen donors
To further understand the
1-propanol, the water had the higher polarity and proton transfer
ability so that the target product 1,2-PDO could more easily migrate
from the catalytic surface in an aqueous solution, which resulted in
the high selectivity to 1,2-PDO.
alcohols were weaker with lower proton transfer ability, which led
to the formation of more by-products. But any way, the best
catalytic performance was achieved in the case of ethanol as a
solvent. Therefore, further experiments were performed on
(
4
3,47
for the hydrogenolysis of glycerol. Table 3 summarizes the catalytic
performance for glycerol hydrogenolysis in different hydrogen
donors. As can be seen, the best catalytic performance (19.7%
glycerol conversion and 30.4% 1,2-PDO selectivity) was achieved
when ethanol was adopted as a hydrogen donor, but the selectivity
to acetol was very high (59.6%), suggesting the lack of available
hydrogen for the hydrogenation of acetol to 1,2-PDO. The glycerol
conversion and 1,2-PDO selectivity respectively were 17.2% and
Conversely, the polarities of
2 3
20ZrCu-Al O catalyst using ethanol as a solvent.
3.2.3. Effect of reaction temperature
28.6% when using 1-propanol as a hydrogen donor, greater than
those in the case of methanol as a hydrogen donor. The glycerol
conversion for different hydrogen donors followed this order:
ethanol > 1-propanol > methanol, consistent well with the literature
reports. In sum, the results above clearly demonstrated that
ethanol was the most effective hydrogen donor, which could be
another reason for the high catalytic activity when glycerol
hydrogenolysis was performed in ethanol solution.
5
4
3
2
1
0
100
90
41
Conversion
,2-PDO
EG
1
80
Acetol
70
60
50
40
Glycerol conversion
100
1
,2-PDO selectivity
80
60
40
20
0
1
80
200
220
240
260
Reaction temperature (℃)
Figure 7. Effect of reaction temperature on glycerol hydrogenolysis
over 20ZrCu-Al catalyst. Reaction conditions: 20 wt.% glycerol
ethanol solution; liquid flow rate, 27.8 mL/h; hydrogen flow rate,
2 3
O
-1
1
00 mL/min; catalyst weight, 4.0 g (ca. 5 mL); WHSV = 1.21 h ;
operating pressure, 3.5 MPa; data acquisition after steady
operation for 3 h.
Ethanol
Water Methanol 1-Propanol
The effect of reaction temperature on glycerol hydrogenolysis
2 3
was investigated in the range of 180 to 250 ℃ on 20ZrCu-Al O
catalyst. As displayed in Figure 7, the glycerol conversion rapidly
enhanced from 42.4 to 100% when the reaction temperature was
increased from 180 to 250 ℃. Nevertheless, the selectivity to 1,2-
PDO was found to be slightly declined with the increase of reaction
temperature. It should be noted that the selectivity to acetol was
very low in the full temperature range (180-250 ℃), indicating the
Figure 6. Effect of solvent on glycerol hydrogenolysis over 20ZrCu-
Al catalyst. Reaction conditions: 20 wt.% glycerol concentration;
liquid flow rate, 27.8 mL/h; hydrogen flow rate, 100 mL/min;
:glycerol = 95:1 (mole ratio); catalyst weight, 4.0 g (ca. 5 mL);
reaction temperature, 230 ℃; operating pressure, 3.5 MPa; data
acquisition after steady operation for 3 h.
2 3
O
H
2
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ARTICLE
Journal Name
2 3
high efficiency of 20ZrCu-Al O catalyst for glycerol hydrogenolysis. 3.2.5. Effect of glycerol concentration
View Article Online
DOI: 10.1039/C6CY00085A
Conversely, the selectivity to ethylene glycol increased marginally
from 0.2 to 2.7%. The improvement in selectivity of ethylene glycol
was likely related to the fact that the hydrogenolysis of 1,2-PDO to
ethylene glycol might occur on Cu active sites of the catalyst at high
5
4
3
2
1
0
100
95
3
2
reaction temperature. In addition, the high reaction temperature
Conversion
1,2-PDO
EG
9
also favored direct hydrogenolysis of glycerol to ethylene glycol.
90
Thus, an appropriate reaction temperature was necessary for the
hydrogenolysis of glycerol to 1,2-PDO. The temperature of 230 ℃
was selected to further study.
85
80
75
70
Acetol
3.2.4. Effect of operating pressure
The glycerol hydrogenolysis was performed at different operating
pressures to assess the catalytic activity of the 20ZrCu-Al
As shown in Figure 8, the glycerol conversion was remarkable
increased from 89.4 (2.5 MPa) to 100% (4 MPa) and then remained
2
O
3
catalyst.
65
10
20
30
40
50
60
Glycerol concentration (wt.%)
at 100% with the increase in the operating pressure. As the Figure 9. Effect of glycerol concentration on glycerol hydrogenolysis
solubility of hydrogen in ethanol solution was raised with the over 20ZrCu-Al catalyst. Reaction conditions: solvent, ethanol;
improvement of operating pressure, more hydrogen molecules liquid flow rate, 27.8 mL/h; hydrogen flow rate, 100 mL/min;
2 3
O
4
5
catalyst weight, 4.0 g (ca. 5 mL); reaction temperature, 230 ℃;
operating pressure, 3.5 MPa; data acquisition after steady
operation for 3 h.
were available to be absorbed on catalytic surface, which led to the
increase of glycerol conversion. Meanwhile, the improvement of
operating pressure also favored the sequential hydrogenolysis of
glycerol to 1,2-PDO. Nevertheless, the selectivity to acetol notably
declined with the increase of operating pressure. The lack of
available hydrogen molecules at low operating pressure might lead
to significant accumulation of acetol. Conversely, hydrogen
molecules could more easily reach the Cu active sites of the catalyst
at high operating pressure, which resulted in the rate of acetol
hydrogenation to 1,2-PDO exceed by far its formation rate via
glycerol dehydration. For this reason, it was understood that the
selectivity to acetol decreased with the increase in operating
pressure. In general, glycerol hydrogenolysis at low operating
pressure was the key to reduce the equipment cost and ensure an
economic operation. Thus, the hydrogen pressure of 3.5 MPa was
supposed to be the optimum value in consideration of the catalytic
activity and economic interest.
To obtain maximum productivity of 1,2-PDO, different glycerol
concentrations were selected for studying the catalytic
performance for glycerol hydrogenolysis. It can be observed from
Figure 9 that the glycerol conversion was rapidly decreased from
1
1
00 to 68.2% when the glycerol concentration was increased from
0 to 60 wt.%. The reasons for this behaviour might be associated
with the two factors: (1) when the glycerol concentration was
raised, the increase in the viscosity of the solution would lead to
33
33
more external mass transfer limitations, which was not beneficial
to the glycerol hydrogenolysis; (2) as the quantity of catalyst was
maintained constant, the growing number of glycerol molecules
resulted in the reduction in catalyst to glycerol ratio, and
consequently less number of Cu active sites was available to the
hydrogenolysis of glycerol. Similarly, the selectivity to 1,2-PDO
slightly decreased from 96.7 to 90.2% with the increase of glycerol
concentration from 10 to 60 wt.%. As the glycerol concentration
was increased, the large amounts of glycerol molecules were
presented in the feedstock, which would lead to the reduction of
8
100
6
4
2
0
33
95
90
85
80
the solubility of hydrogen in solution. Thus, the selectivity to 1,2-
PDO was diminished slightly while the selectivity to acetol
enhanced slowly with the increase in glycerol concentration. In
addition, very low ethylene glycol (< 2%) was detected within the
whole range of glycerol concentration (10-60wt.%), suggesting the
Conversion
,2-PDO
EG
1
Acetol
high selectivity of the 20ZrCu-Al
.2.6. Effect of liquid flow rate
In order to enhance the efficiency of glycerol hydrogenolysis, the
effect of liquid flow rate on glycerol hydrogenolysis was tested in
the range of 13.2 to 88.8 mL/h on 20ZrCu-Al catalyst. As shown
2 3
O catalyst.
3
2
3
4
5
Operating pressure (MPa)
2 3
O
Figure 8. Effect of operating pressure on glycerol hydrogenolysis
over 20ZrCu-Al catalyst. Reaction conditions: 20 wt.% glycerol
ethanol solution; liquid flow rate, 27.8 mL/h; hydrogen flow rate,
in Figure 10, the glycerol conversion was significantly dropped with
the increase in liquid flow rate. The reason for the reduction in
00 mL/min; catalyst weight, 4.0 g (ca. 5 mL); WHSV = 1.21 h ; glycerol conversion might be attributed to the decrease in
reaction temperature, 230 ℃; data acquisition after steady residence time of glycerol on the catalytic surface when the liquid
2 3
O
-1
1
operation for 3 h.
flow rate was increased. On the other side, the catalyst particles
were partially wetted at a low liquid flow rate. The hydrogen
8
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molecules could more easily to get intimate contact with glycerol glycerol conversion and 1,2-PDO selectivity were slowVileywdAertcicrleeaOsnelinde,
molecules on catalytic surface because of the less external mass but their values were still higher than 90D%OIa:f1t0e.r101309/0C6hCYti0m0e08o5An
transfer limitations. Subsequently at a high liquid flow rate, the stream on 20ZrCu-Al
2 3
O catalyst in the presence of ethanol as a
increase in wetted fractions of catalytic surface would lead to the solvent. Conversely, serious deactivation was observed on Cu-Al
2 3
O
3
2
reduction of the reaction rate. Similarly, the selectivity to 1,2-PDO catalyst after 56 h continuous operation when ethanol was selected
was found to be decreased from 96.2 to 75.1%, while the selectivity as a solvent. Moreover, the Cu-Al catalyst exhibited the loss of
to acetol enhanced dramatically from 0.2 to 16.4% when the liquid catalytic activity after only 16 h in the case of water as a solvent. By
flow rate was increased from 13.2 to 88.8 mL/h. The less residence contrast, the catalytic activity of 20ZrCu-Al was relatively stable
2 3
O
2 3
O
time at high liquid flow resulted in the lack of the intimate contact for 30 h under the same reaction conditions. But the further
between the hydrogen molecules and glycerol molecules on the extension of reaction time, the deactivation of catalyst was also
catalytic surface. Therefore, the selectivity to acetol increased at observed. But anyway, the results of the stability test indicated that
the expense of 1,2-PDO with the increase in liquid flow rate.
2 2 3
the ZrO -promoted Cu-Al O catalyst had a better stability and
2 3
prospective for practical application compared to Cu-Al O catalyst.
1
00
1
1
8
4
0
6
△
△
Cu
Conversion
1,2-PDO
EG
△
2
8
6
4
0
0
0
△
(
f)
Acetol
(e)
(d)
(c)
(
b)
a)
(
10
20
30
40
50
60
70
80
90
Liquid flow rate (mLh)
10
20
30
40
50

60
70
80
90
2
Figure 10. Effect of liquid flow rate on glycerol hydrogenolysis over
0ZrCu-Al catalyst. Reaction conditions: 20 wt.% glycerol ethanol Figure 12. XRD patterns of the catalysts: (a) fresh 20ZrCu-Al O ; (b)
2
2 3
O
2
3
solution; hydrogen flow rate, 100 mL/min; catalyst weight, 4.0 g (ca. spent 20ZrCu-Al O3 using ethanol as a solvent; (c) spent 20ZrCu-
2
5
mL); reaction temperature, 230 ℃; operating pressure, 3.5 MPa; Al O using water as a solvent; (d) fresh Cu-Al O ; (e) spent Cu-Al O
2 3 2 3 2 3
data acquisition after steady operation for 3 h.
using ethanol as a solvent; (f) spent Cu-Al O3 using water as a
2
solvent.
3.2.7. Stability test
The deactivation of Cu-based catalysts has been widely reported
1
2,17,44
in the literature so far. Some researchers
suggested that the
1
00
sintering of Cu particles was mainly responsible for the loss of
catalytic activity in the presence of water as a solvent, while other
3
4,48
8
6
4
2
0
0
0
0
0
studies
showed the reason of deactivation was related to Cu
particles sintering and the presence of adsorbed species on the
catalytic surface. In order to clarify the reason of catalyst
deactivation in this work, the spent catalyst was submitted to
several characterization techniques. First, inductive coupled plasma
optical emission spectroscopy (ICP-OES) was used to measure the
chemical composition of the liquid product. The deactivation of
catalyst cannot be assigned to metal leaching, because the results
of ICP-OES analysis showed negligible copper, zirconium and
aluminium contents in the liquid product. Secondly, the BET surface
area measurement of the spent catalysts was carried out. The BET
Conv. on 20ZrCu-Al O
2
3
Sel. on 20ZrCu-Al O
2
3
Conv. on Cu-Al O
2
3
Sel. on Cu-Al
O
3
2
Conv. on 20ZrCu-Al O -water
2
3
Sel. on 20ZrCu-Al O -water
2
3
Conv. on Cu-Al O -water
2
3
Sel. on Cu-Al O -water
2
3
0
20
40
60
80
100
Time on stream (h)
2 3
Figure 11. The stability test of the catalyst. Reaction conditions: 20 surface area of the spent catalysts slightly decreased (Cu-Al O =
2
-1
2 -1
wt.% glycerol ethanol (or water) solution; liquid flow rate, 27.8 48.7 m g and 20ZrCu-Al
mL/h; hydrogen flow rate, 100 mL/min; H :glycerol = 95:1 (mole selected as a solvent. In comparison to ethanol as a solvent, the use
ratio); catalyst weight, 4.0 g (ca. 5 mL); reaction temperature,
2 3
O = 101.0 m g ) when ethanol was
2
of water as a solvent resulted in a significant reduction in the BET
surface area of the spent catalysts (Cu-Al
0ZrCu-Al O = 86.3 m g ). The decrease in BET surface area might
be related to the collapse of the alumina support and/or partial
pore blockage. Furthermore, the XRD determination was performed
to observe the structure change of the spent catalyst. As shown in
2
-1
2
30 ℃; operating pressure, 3.5 MPa.
2
O
3
= 25.8 m g and
2
-1
2
2 3
As stability is essential to the practical application of catalyst, the
stability of 20ZrCu-Al
addition, the stability of Cu-Al
2
O
3
catalyst has been tested in this work. In
catalyst was also investigated as a
2
O
3
reference in different solvents. As displayed in Figure 11, the
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Figure 12, the Cu particle size of spent 20ZrCu-Al
Journal Name
2
O
3
catalyst slowly displayed in Figure 13, some obvious agglomerates were observed
View Article Online
DOI: 10.1039/C6CY00085A
2 3
increased from 6.5 to 13.9 nm after 100 h continuous operation in on spent Cu-Al O catalyst after 56 h continues operation in the
the case of ethanol as a solvent. In contrast, the Cu particle size of case of ethanol as a solvent. Moreover, the spent Cu-Al
2 3
O catalyst
spent Cu-Al catalyst after only 56 h time on stream dramatically underwent extremely serious aggregation when using water as a
2 3
O
increased from 19.4 to 31.7 nm when ethanol was adopted as a solvent, which was in agreement with the results of XRD analysis. In
solvent. Moreover, the Cu particle size with 44.6 nm was presented contrast, the large quantities of Cu particles were homogenously
on spent Cu-Al
water as a solvent. However, the Cu particle size of spent 20ZrCu- of them also underwent moderate aggregation (Figure 13 c). The
Al catalyst moderately increased from 6.5 to 28.3 nm despite results above demonstrated that addition of ZrO to Cu-Al was
2 3 2 3
O catalyst after 16 h reaction time when using distributed on spent 20ZrCu-Al O catalyst (Figure 13 b), while some
2
O
3
2
2 3
O
with a longer reaction time (44 h) in the presence of water as a beneficial to restrain the aggregation of Cu particles in the reaction
solvent. The results of XRD analysis obviously showed that the process, which was most probably due to the high Cu dispersion
sintering of Cu particles was the key factor for the loss of catalytic and strong interaction between copper and zirconium species on
2 3
activity. The spent catalyst was also characterized by TEM. As 20ZrCu-Al O catalyst.
Figure 13. TEM images of the catalysts: (a) fresh 20ZrCu-Al
using water as a solvent; (d) fresh Cu-Al ; (e) spent Cu-Al
2
O
3
; (b) spent 20ZrCu-Al
2
O
3
using ethanol as a solvent; (c) spent 20ZrCu-Al
2 3
O
2
O
3
2
O
3
using ethanol as a solvent; (f) spent Cu-Al O
2 3
using water as a solvent.
Apart from the above-mentioned characterization techniques,
TGA for the spent catalyst was also carried out. As shown in Figure
100
14, the weight loss between 50 and 200 ℃ for the spent catalyst
was related to the loss of water, while the largest weight loss was
presented between 200 and 400 ℃, most likely due to the presence
of adsorbed reactant and/or products such as glycerol, 1,2-PDO,
9
5
(
d)
(c)
b)
4
8,49
90
acetol and glycerol oligomers on the catalytic surface.
The
(
presence of adsorbed species on catalytic surface was probably one
reason for the loss of catalytic activity. However, it should be noted
that the weight losses between 200 and 400 ℃for spent catalysts
in the case of ethanol as a solvent were greater than that when
using water as a solvent, suggesting the presence of more adsorbed
species on the catalytic surface when ethanol was used as a solvent.
In addition, the weight loss between 400 and 900 ℃for the spent
catalyst was very low, showing the lack of carbonaceous deposits
85
(a)
80
75
1
00 200 300 400 500 600 700 800 900
Temperature (℃)
34
on the catalytic surface. But anyway, the results of TGA clearly
confirmed that the presence of adsorbed species on catalytic
surface might also lead to the deactivation of catalyst.
Figure 14. TGA curves of the catalysts: (a) spent 20ZrCu-Al
ethanol as a solvent; (b) spent Cu-Al using ethanol as a solvent;
c) spent 20ZrCu-Al using water as a solvent; (d) spent Cu-Al
using water as a solvent.
2 3
O using
2
O
3
(
O
2 3
2 3
O
1
0 | J. Name., 2012, 00, 1-3
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Page 11 of 13
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Cat al el ysis Science & Techn oi nl ogy
Journal Name
ARTICLE
1
1
1
1
1
1
1
1
1
1
2
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2
2
2
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3
3
0 V. Rekha, C. Sumana, S. P. Douglas and N. LiVniegwaiAarthic,leAOpnlpinle.
4
. Conclusions
In this work, the ZrO -promoted Cu-Al O catalysts prepared by
2 2 3
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Acknowledgements
This work was financially supported by the Scientific Research
Foundation of Graduate School of Southeast University
(
(
YBJJ1530), the National Natural Science Foundation of China
No. 21276050 and 21076044), the Priority Academic Program
Development of Jiangsu Higher Education Institutions and Key
Laboratory Open Fund of Jiangsu Province (JSBEM201409). The
authors gratefully acknowledge these grants.
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Page 13 of 13
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C6CY00085A
Table of contents
The ZrO
2 2 3
-promoted Cu-Al O catalyst presented excellent catalytic activity for the hydrogenolysis
of glycerol to 1,2-propanediol.