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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This journal is © The Royal Society of Chemistry 2010