Hydrogenolysis of glycerol over a highly active CuO/ZnO catalyst prepared by an oxalate gel method: Influence of solvent and reaction temperature on catalyst deactivation

Article abstract of DOI:10.1039/b914523k

The hydrogenolysis of glycerol was performed in an autoclave at temperatures between 190 and 225 °C and at a H₂ pressure of 5 MPa over a CuO/ZnO catalyst prepared by an oxalate gel (OG) method. Compared to a CuO/ZnO catalyst prepared by coprecipitation, much higher conversions of glycerol and space-time yields up to 9.8 g_{propylene glycol} g _Cu^-1 h^-1 are achieved with CuO/ZnO-OG, whereas both catalysts produced propylene glycol with selectivities of about 90%. Additionally, the influence of the temperature and the solvent was examined. Compared to a conversion of glycerol of only 5% in an aqueous glycerol solution, the use of 1,2-butanediol as a solvent leads to a high conversion of 55%. Moreover, experiments were carried out in pure glycerol and from transmission electron microscopy images of fresh and spent catalysts, it was obvious that the morphology of the catalyst changed during the reaction. By X-ray diffraction and N₂O chemisorption, it was proved that a tremendous loss of copper surface area occurred during the hydrogenolysis of glycerol. Taking together the influence of the solvent on the conversion of glycerol and the results of the catalyst characterization, it can be concluded that water, as an unavoidable by-product of the reaction, is responsible for a strong deactivation of the catalyst.

Full text of DOI:10.1039/b914523k

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PAPER
www.rsc.org/greenchem | Green Chemistry
Hydrogenolysis of glycerol over a highly active CuO/ZnO catalyst
prepared by an oxalate gel method: influence of solvent and reaction
temperature on catalyst deactivation
Arne Bienholz, Frederick Schwab and Peter Claus*
Received 20th July 2009, Accepted 20th October 2009
First published as an Advance Article on the web 26th November 2009
DOI: 10.1039/b914523k
The hydrogenolysis of glycerol was performed in an autoclave at temperatures between 190 and
◦
2
25 C and at a H
2
pressure of 5 MPa over a CuO/ZnO catalyst prepared by an oxalate gel (OG)
method. Compared to a CuO/ZnO catalyst prepared by coprecipitation, much higher conversions
-
1
-1
of glycerol and space–time yields up to 9.8 gpropylene glycol
g
Cu
h are achieved with CuO/ZnO-OG,
whereas both catalysts produced propylene glycol with selectivities of about 90%. Additionally,
the influence of the temperature and the solvent was examined. Compared to a conversion of
glycerol of only 5% in an aqueous glycerol solution, the use of 1,2-butanediol as a solvent leads to
a high conversion of 55%. Moreover, experiments were carried out in pure glycerol and from
transmission electron microscopy images of fresh and spent catalysts, it was obvious that the
morphology of the catalyst changed during the reaction. By X-ray diffraction and N O
2
chemisorption, it was proved that a tremendous loss of copper surface area occurred during the
hydrogenolysis of glycerol. Taking together the influence of the solvent on the conversion of
glycerol and the results of the catalyst characterization, it can be concluded that water, as an
unavoidable by-product of the reaction, is responsible for a strong deactivation of the catalyst.
1
. Introduction
Among the different noble metal catalysts, Cu catalysts turned
out to combine high selectivities for propylene glycol with
satisfying conversion levels of glycerol.
As a byproduct of biodiesel production, glycerol is a highly-
functionalized platform chemical available in large amounts
and, thus, at low costs. Therefore, the development of new
catalytic processes to convert glycerol to value-added products,
5,6,14–17
However, the reaction times needed to achieve high glycerol
conversions are rather long. Additionally, Cu catalysts suffer
from deactiviation in the hydrogenolysis of glycerol. It is
observed in the literature that a strong decrease in glycerol
conversion occurs if a Cu/Zn/Al catalyst is used in a second
run, regardless of whether the spent catalyst was dried or
1
e. g. dihydroxyacetone or hydrogen, is the subject of an
increasing number of investigations. One of the promising
approaches is the hydrogenolysis of glycerol, leading to 1,2-
propanediol (propylene glycol). Propylene glycol has a wide
range of practical applications, like its use as a heat transfer fluid
or its polymerisation to polyesters and polyurethanes. Supported
Pt, Rh, Co, Ru and Cu catalysts are used for the hydrogenolysis
17
calcinated before it was reused. Moreover, Montassier et al.
reported the deactivation of Cu/C because of the sintering
of copper particles in the hydrogenolysis of glycerol in a tube
15
reactor.
◦
of glycerol at temperatures between 170 and 220 C under
elevated hydrogen pressures and in dilute aqueous solutions
Besides the preparation of copper catalysts by
5,14,17
precipitation,
different preparation methods like
2–8
of glycerol. Several attempts were made to enhance glycerol
conversion by the addition of solid acids like ion-exchange
resins or zeolites and sulfated zirconia to supported noble
metal catalysts, suggesting that the hydrogenolysis of glycerol
occurs in two steps. Initially, glycerol is dehydrated to acetol
over acid sides followed by hydrogenation over metal catalysts.
Indeed, acetol was detected by several groups, particularly when
the hydrogenolysis of glycerol is carried out in the vapour phase
and, thus, under rather low hydrogen pressures.
18
the template preparation of Cu–Cr catalysts and the
precipitation–gel method for
19
a
Cu/SiO
2
catalyst have
9,10
11
been applied. However, the Cu/ZnO catalysts used in the
hydrogenolysis of glycerol have usually been prepared by
precipitation and the effect of the Cu/ZnO catalyst synthesis
procedure was not investigated, although the properties
of Cu/ZnO catalysts, and, thus, their performance in the
6
methanol synthesis from CO and H
strongly dependent on the preparation method.
2
, have been shown to be
12,13
20–22
Here, we
report the preparation of a copper catalyst by an oxalate gel
method, its characterization and its use in the hydrogenolysis
of glycerol. Additionally, the deactivation of the catalyst was
examined with regard to the reaction temperature and the
generation of water as a by-product of the hydrogenolysis of
glycerol.
Department of Chemistry, Ernst-Berl-Institute/Chemical Technology II,
Technical University Darmstadt, Petersenstr, 20, 64287, Darmstadt,
Germany. E-mail: claus@ct.chemie.tu-darmstadt.de; Fax: +49 6151 16
4
788; Tel: +49 6151 16 5369
2
90 | Green Chem., 2010, 12, 290–295
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2
. Experimental
2.3 Hydrogenolysis of glycerol
In a standard experiment, 140 mL pure glycerol and 3 g catalyst
were loaded into a stainless steel reactor (Parr). The reactor was
pressurized with hydrogen to a pressure of 5 MPa and heated
2
.1 Preparation of the catalysts
The preparation of the CuO/ZnO catalyst was carried out
23
by an oxalate gel coprecipitation method. Cu(NO
4.87 g, 20.9 mmol) and Zn(NO O (12.2 g, 41 mmol)
·6 H
were dissolved in 65 ml ethanol. Then, oxalic acid (6.68 g,
4.3 mmol) in 40 ml ethanol was added to form a light blue
3
)
2
·2.5H
2
O
◦
to a temperature of 200 C. The samples were taken at desired
(
3
)
2
2
time intervals and analyzed with a Hewlett-Packard 6890 gas
chromatograph equipped with a DB-WAX GC column and a
flame ionization detector. The selectivity of the products was
calculated on a carbon basis.
In order to investigate the role of water (section 3.4) in the
deactivation of the catalyst, 1,2-butanediol was used as a solvent.
Glycerol and the reaction products, including several alcohols,
have to be dissolved easily in the solvent making the use of an
7
precipitate. After one hour, the precipitate was separated by
filtration, washed with 500 ml deionised water and finally dried
overnight at 70 C. Afterwards, the precipitate was calcined at
◦
◦
150, 200 and 300 C for one hour at each temperature, followed
◦
by a final step at 360 C for four hours. Hereafter, this catalyst
will be referred to as the CuO/ZnO-OG catalyst.
alcohol desirable. Furthermore, no C
1
to C components were
3
As a reference, aCuO/ZnOcatalyst was prepared by coprecip-
chosen as solvent due to their possible formation during the
itation. Copper acetate (3.8 g, 20.9 mmol) and Zn(NO
3
)
2
·6H
2
O
hydrogenolysis of glycerol. Therefore, this C
the solvent.
4
polyol was used as
(
12,2 g, 41 mmol) were dissolved in 300 ml deionised water.
Precipitation was achieved by the dropwise addition of 140 ml
of a 1 M NaOH solution at 80 C. The precipitate was aged for
2
◦
3
. Results and discussion
h at a constant pH value of 9 at the same temperature. Finally,
the precipitate was separated by filtration, washed with one litre
of deionised water and dried over night at 70 C. The calcination
3.1 Preparation method of the catalyst
◦
Initially, the CuO/ZnO catalysts prepared by the two different
preparation methods were tested in the hydrogenolysis of
glycerol. According to Table 1, no differences between the
two catalysts in terms of the selectivities are observed. Both
CuO/ZnO-OG and CuO/ZnO-CP catalysts exhibited high
selectivities towards propylene glycol of 90 and 87%, respectively.
Moreover, the selectivities towards undesired side products like
ethylene glycol and acetol were low. Concerning the activity
of the two catalysts, a much higher conversion of glycerol was
achieved in the case of the CuO/ZnO-OG catalyst compared to
CuO/ZnO-CP. Using the CuO/ZnO-OG catalyst, a conversion
of 46% is achieved, whereas the conversion at the CuO/ZnO-
CP catalyst is only 17%. With the CuO/ZnO-OG catalyst, the
of this catalyst was the same as that for the CuO/ZnO-OG
catalyst. This catalyst will be referred to as the CuO/ZnO-
CP catalyst. The copper oxide content of both catalysts was
3
3 wt%.
2
.2 Catalyst characterisation
Copper surface areas were determined by N
followed by temperature-programmed reduction (TPR) of the
formed Cu O species in a stream of H (Cu O-TPR). All these
treatments of the catalyst were carried out by means of a TPDRO
2
O chemisorption
2
2
2
1
100 device (Porotec). Before the analysis, in situ pre-reduction
-
1
-1
-1
of the catalysts in a stream of 4.95% H
2
in Ar (20 ml min ), at
space–time yield amounts to 9.8 gpropylene glycol
g
Cu h . Compared
◦
-1
◦
19
a heating rate of 10 C min , to a temperature of 330 C was
performed. This temperature was held for 60 min. The catalyst
was cooled down in a stream of He in order to oxidize Cu to
to other copper-based catalysts like Cu/SiO
2
and CuO/ZnO,
14
prepared by coprecipitation, which produce propylene glycol
-
1
-1
with space–time yields of 3.2 and 0.8 gpropylene glycol
g
Cu
h ,
-
1
◦
Cu
2
O with 1% N
2
O in He (10 ml min ) at 20 C. Finally, Cu
2
O-
respectively, the CuO/ZnO-OG catalyst is extremely active.
Therefore, the CuO/ZnO-OG catalyst was chosen for further
investigations.
-
1
TPR was carried out in 4.95% H
rate of 10 C min , to a temperature of 330 C. From the amount
2
in Ar (20 ml min ) at a heating
◦
-1
◦
of H
2
needed to completely reduce Cu
2
O, the specific copper
In order to determine the reason for the different conversions
surface area, the dispersion and the mean particle diameter were
calculated.
Additionally, the morphology of the catalysts was examined
by transmission electron microscopy (HRTEM JEOL, JEM
of glycerol, N
2
O chemisorption was carried out with both
◦
catalysts whichwerereduced in 4.95%H
2
in Ar at330 C(heating
◦
-1
rate = 10 C min ) for 60 min prior to N
2
O chemisorption. The
resulting copper surface areas, dispersions and mean copper
particle diameters are listed in Table 2. The copper surface
3
010, 300 kV). Powder X-ray diffraction (XRD) patterns of the
˚
2
-1
catalysts were recorded using Co Ka1 radiation (l = 1.78896 A).
area of the CuO/ZnO-CP catalyst amounts to 16.8 m g , and
Table 1 Hydrogenolysis of glycerol over the CuO/ZnO-OG and CuO/ZnO-CP catalysts, both with a copper oxide content of 33 wt%
Selectivity(%)
b
Catalyst
Conversion (%)
Propylene glycol
Ethylene glycol
Acetol
Others
a
CuO/ZnO-OGa
CuO/ZnO-CP
46
17
90
87
1
0
1
1
8
12
a
◦
b
Reaction conditions: 140 mL pure glycerol, 3 g catalyst, 5 MPa H
2
, 200 C, 7 h. Unidentified side products.
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Green Chem., 2010, 12, 290–295 | 291

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Table 2 Comparison of the structural properties of the CuO/ZnO-OG
and CuO/ZnO-CP catalysts determined by N
2
O chemisorption
a
2
-1
b
c
Catalyst
S
Cu /m g
D /%
dCu /nm
CuO/ZnO-OG
CuO/ZnO-CP
30.1
16.7
16.8
9.1
6.3
12.1
a
b
c
Copper surface area. Dispersion. Mean copper particle size.
24
the mean copper particle diameter is 12.1 nm. Agrell et al.
prepared a CuO/ZnO catalyst by coprecipitation of an aqueous
Zn(NO and Cu(NO solution and obtained a catalyst with
3
)
2
)
3 2
2
-1
a copper surface area of 21.5 m g . Taking into account the
higher copper oxide loading of 39 wt% in the study of Agrell
compared to 33 wt% of the CuO/ZnO-CP catalyst, the results
are in good agreement. In comparison, the copper surface area
of the CuO/ZnO-OG catalyst is twice as high, which indicates
the reason for the improved activity of this catalyst compared to
the CuO/ZnO-CP catalyst.
3.2 Recycling of the catalyst
The CuO/ZnO-OG catalyst was recovered and used in a second
experiment under the same reaction conditions. However, a
glycerol conversion of just 10% was achieved, although the high
selectivity for propylene glycol was preserved (97%). In order
to examine the reason for the loss of activity, the catalyst was
recovered after the first experiment and analysed by XRD and
N O chemisorption. The XRD pattern of the spent catalyst,
2
along with the pattern of the fresh catalyst, is shown in Fig. 1.
The fresh CuO/ZnO-OG catalyst contained merely CuO
and ZnO crystallites. After the hydrogenolysis of glycerol at
◦
2
00 C and at a hydrogen pressure of 5 MPa, the CuO phase
was completely reduced to metallic copper. The crystallite
sizes of the CuO, Cu and ZnO were calculated using the
Scherrer equation. The resulting crystallite sizes, together with
the copper surface areas and copper particle sizes obtained from
N O chemisorption, are listed in Table 3. Before the reaction,
2
the mean diameters of the CuO and ZnO crystallites were
determined to be 15 and 10 nm, respectively. After the reaction,
the mean crystallite diameters obtained from XRD of both the
Cu and ZnO crystallites were calculated to be 40 nm. Therefore, a
strong increase in the copper crystallite size during the reaction
was observed, leading to a decrease in the active surface area
and, thus, to a loss of activity. These results were confirmed
by N O chemisorption as the copper surface area decreases
2
2
-1
2
-1
during the reaction from 30.1 m g to 5.3 m g . Moreover,
a strong increase in the ZnO crystallite size is also observed. A
loss of activity may occur as well because of the agglomeration
of the ZnO crystallites, as glycerol may be dehydrated to acetol
5
and glycidol at the ZnO surface. The loss of active copper
25
surface area is also known for methanol synthesis. Muhler et al.
reported the growth of copper particles in a Cu/ZnO methanol
catalyst with increasing time on stream.
Fig. 1 XRD patterns of the CuO/ZnO-OG catalyst before (a) and
after (b) use in the hydrogenolysis of pure glycerol or of glycerol with
To further investigate the deactivation of the CuO/ZnO-OG
catalyst, TEM images of the catalyst were recorded before and
after the reaction (Fig. 2). From the TEM images, a change in
morphology is clearly observed. Before the catalyst was used in
the hydrogenolysis of glycerol, almost spherical particles with
1
,2-butanediol (c) or water (d) as a solvent. Reaction conditions: 140 mL
pure glycerol (b) or 140 mL 50 wt% glycerol in 1,2-butanediol (c) or water
◦
(
d), 3 g catalyst, 5 MPa H
2
, 200 C, 7 h. * ZnO, ᭺ CuO, # Cu.
2
92 | Green Chem., 2010, 12, 290–295
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Table 3 Comparison of the structural properties of fresh and spent CuO/ZnO-OG catalysts
d
d
d
e
2
-1
e
Catalyst
d
CuO /nm
d
ZnO /nm
d
Cu /nm
S
Cu /m g
dCu /nm
Fresh CuO/ZnO-OG
15
—
—
—
10
40
40
40
—
40
40
80
30.1
5.3
4.2
2.7
6.3
a
Spent Cu/ZnO OGb
38.2
47.5
75.6
Spent Cu/ZnO OG
Spent Cu/ZnO OG
c
a
◦
b
Reaction conditions: 140 ml pure glycerol, 3 g catalyst, 5 MPa H , 200 C, 7 h. Reaction conditions: 140 ml 50 wt% glycerol in 1,2-butanediol, 3 g
2
◦
c
◦
d
catalyst, 5 MPa H
2
, 200 C, 7 h. Reaction conditions: 140 ml 50 wt% glycerol in water, 3 g catalyst, 5 MPa H
ZnO crystallite sizes determined by XRD. Copper surface area and mean copper particle size determined by N
2
, 200 C, 7 h. Mean CuO, Cu and
O chemisorption.
e
2
In order to determine the reason for the poor conversion of
an aqueous solution of glycerol compared to 1,2-butanediol,
the catalysts were recovered after the reaction and characterised
by XRD and N O chemisorption. The XRD patterns of the
2
two recovered catalysts are shown in Fig. 1. From the XRD
patterns, the CuO, Cu and ZnO crystallite sizes were calculated
and are shown in Table 3, together with the copper surface areas
determined after the reaction. Before the catalyst CuO/ZnO-
OG was used in the hydrogenolysis of glycerol, the mean
crystallite size of the CuO and ZnO crystallites was determined
to be 15 and 10 nm, respectively (Table 3). Regarding the ZnO
crystallite size, a strong increase (d = 40 nm) is observed, which
is not affected by the choice of solvent (Table 3). However, the
calculated size of the Cu crystallites after the reaction strongly
depends on the solvent. In comparison to 1,2-butanediol as a
solvent, the size of the Cu crystallites is, in the case of water,
twice the size. This increase in the copper particle size can be
Fig. 2 TEM images of the CuO/ZnO-OG catalyst before (left) and
after (right) use in the hydrogenolysis of glycerol. Reaction conditions:
◦
1
40 ml pure glycerol, 3 g catalyst, 5 MPa H
2
, 200 C, 7 h.
a mean diameter of about 10 nm are visible, while after
the reaction, the catalyst consists of rod-shaped particles with a
considerably larger size.
confirmed by N O chemisorportion as the copper surface is also
2
influenced by the choice of solvent. The copper surface area
strongly decreased in the case of water as a solvent, from 30.1
3
.4 The effect of the solvent
Montassier et al. proved that the size of copper particles in
a Cu/C catalyst increased if the catalyst is stirred in water
2
-1
2
-1
m g before to 2.7 m g after the reaction, while in the case of
2
-1
1
,2-butanediol the copper surface area diminished to 4.2 m g .
15
at elevated temperatures. As no water is loaded into the
reactor at the beginning of the reaction, the deactivation of the
CuO/ZnO-OG catalyst might be due to formation of water as an
unavoidable by-product during the hydrogenolysis of glycerol.
Because of this, the deactivation of the catalyst should occur
much faster if the hydrogenolysis of glycerol is carried out with
an aqueous glycerol solution.
3
.5 Reaction temperature
As well as the solvent, the reaction temperature might also play
an important role in the deactivation process of the Cu/ZnO-
OG catalyst. Thus, the hydrogenolysis of glycerol was performed
◦
in the temperature range between 190 and 225 C. The effect of
Therefore, a 50 wt% aqueous solution of glycerol was loaded
into the reactor. As a reference, the same experiment was
repeated but now with a solution of glycerol in 1,2-butanediol.
The results of these experiments are shown in Table 4. It is
obvious that the conversion of glycerol is strongly affected by
the choice of solvent. Compared to a conversion of glycerol of
only 5% in an aqueous glycerol solution after 7 h, the use of
the reaction temperature on the conversion of glycerol and the
selectivity for propylene glycol are shown in Fig. 3. While there
was no significant influence of the reaction temperature on the
selectivity for propylene glycol (S = 84–92%), the conversion
of glycerol was strongly affected when raising the temperature.
However, there is no consistent dependence of the conversion
on the reaction temperature as would be expected according
to Arrhenius behaviour: the conversion first strongly increases
1
,2-butanediol as a solvent leads to a higher conversion of 55%.
Table 4 Effect of the solvent on the conversion of glycerol and the selectivities towards propylene glycol, ethylene glycol and acetol of the CuO/ZnO-
OG catalyst
Selectivity (%)
a
b
Solvent
,2-Butanediol
Conversion (%)
Propylene glycol
Ethylene glycol
Acetol
Others
1
55
5
86
87
1
9
1
2
12
2
Water
a
◦
b
2
Reaction conditions: 140 ml 50 wt% glycerol dissolved in 1,2-butanediol or water, 3 g catalyst, 5 MPa H , 200 C, 7 h. Unidentified side products.
This journal is © The Royal Society of Chemistry 2010
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glycerol conversion, which proved to be reproducible with a
standard deviation of ± 3%, is only 36% compared to 46%
◦
at 200 C, suggesting that less water is formed at a reaction
◦
temperature of 210 C. From that, an increase of the copper
surface area can be assumed and Fig. 4 shows that this is
2
-1
2
-1
indeed the case (6.0 m g vs. 5.2 m g ). Therefore, it is worth
mentioning that by an increase of reaction temperature from
◦
2
00 to 210 C, the conversion of glycerol decreased (Fig. 3)
although the copper surface area increased (Fig. 4). Thus, the
◦
decrease in glycerol conversion between 200 to 210 C cannot
be attributed to a growth of copper particles. Keeping in mind
that the product distribution remained unchanged during the
◦
course of glycerol hydrogenolysis between 190 and 225 C (1,2-
propanediol is always produced with high selectivity, Fig. 3),
a change of the reaction mechanism should be ruled out, and
the unusual behavior is likely to be due to a difference in the
temperature dependence of the rate constant and the reactants’
adsorption constants.
Fig. 3 Dependence of the conversion (ꢀ) and the selectivity for
propylene glycol (ꢀ) on the reaction temperature. Reaction conditions:
1
40 ml glycerol, 3 g CuO/ZnO-OG catalyst, 5 MPa H
2
, 7 h.
◦
with an increase in temperature from 190 to 200 C, but then,
◦
4
. Conclusions
if the temperature is further increased to 210 C, the conversion
◦
decreases. Raising the temperature further to 215 C and,
A CuO/ZnO catalyst prepared by an oxalate gel method was
proven to be highly active in the hydrogenolysis of glycerol
compared to a CuO/ZnO catalyst prepared by the standard
coprecipitation method. The higher activity can be attributed to
the copper surface area, which was as high as 30.1 m g in the
case of the CuO/ZnO-OG catalyst, whereas the copper surface
◦
finally, to 225 C then gives the expected increase in conversion
again. The reason for this relationship between the reaction
temperature and the conversion during glycerol hydrogenolysis
may be an interference between the acceleration of the reaction
rate due to elevated temperatures and the stronger deactivation
of the catalyst at higher reaction temperatures. Furthermore, an
interplay between the temperature dependence of the adsorption
constants determining the individual adsorption enthalpy of
one or more reactants and the temperature dependence of
the rate constant dictating the activation energy may cause
the observed decrease in conversion with rising temperature.
2
-1
2
-1
area of the CuO/ZnO-CP catalyst amounted to only 16.7 m g .
With both catalysts, a selectivity for propylene glycol of about
90% is achieved.
In the presence of water, the size of the copper crystallites
of the CuO/ZnO catalyst increases tremendously, leading to a
decrease in active surface area and, thus, to a loss of activity.
Even if no water is loaded into the reactor, the water formed
during the hydrogenolysis of glycerol causes the deactivation
of the catalyst. Increasing the reaction temperature has no
significant influence on the loss of active surface area and cannot,
therefore, be the reason for the deactivation of the catalyst.
According to the copper surface areas determined by N O
2
chemisorption for the CuO/ZnO-OG catalyst after being used in
the hydrogenolysis of glycerol at different temperatures (Fig. 4),
the temperature seems to influence the growth of the copper
particles. It is expected that the decrease in the copper surface
area correlates with the amount of water produced during the
◦
reaction. However, at a reaction temperature of 210 C, the
Acknowledgements
The authors are grateful to the FAUDI-Stiftung (project 73) for
financial support of this work.
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