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M.J. Lippits, B.E. Nieuwenhuys / Journal of Catalysis 274 (2010) 142–149
first heating stage in ethanol dehydrogenation. In the following
cooling stage, much less ethylene oxide was formed, as also was
observed for the gold-based catalyst. On the silver-based catalysts,
no ethylene oxide was detected in the ethanol dehydrogenation
reaction. Measurements with an ethanol/O2 ratio of 1 show ethyl-
ene oxide formation on all three metal-based catalysts. In Table 3,
some results of the catalysts with the best performance in ethylene
oxide production are presented. In all cases, the best selectivity is
achieved with the Li2O-containing catalysts. The Au/Li2O/Al2O3 cat-
alysts show the highest ethanol conversion with good selectivity to
ethylene oxide at 200 °C. At temperatures of 300 °C, the selectivity
to ethylene oxide is the highest on the Au/Li2O/Al2O3 catalyst. At
higher temperatures, the selectivity to ethylene oxide drops on
all catalysts but remains the highest for Ag/Li2O/Al2O3.
With all three metal-based catalysts, the most promising results
are found when Li2O is added, suggesting that the role of Li2O is
very important in the conversion of ethanol into ethylene oxide.
The copper- and gold-based catalysts show some similarity in reac-
tivity and selectivity. The gold-based catalyst is the most selective
to ethylene oxide. Both show a maximum selectivity around
200 °C. At higher temperatures, the selectivity decreases with
increasing temperature, as ethylene, CO and CO2 are formed. The
silver-based catalyst also shows high selectivity to ethylene oxide
at 200 °C but at lower conversion than the gold-based catalyst. At
temperatures of 400 °C, the selectivity to ethylene oxide remains at
higher levels, and less CO, ethylene and no CO2 are formed. The sil-
ver-based catalyst also differs from the copper- and gold-based
catalyst as in ethanol dehydrogenation no ethylene oxide is
formed.
contact time, no indications were found of any intermediates. Also,
when an ethylene/O2 flow was used, no ethylene oxide was
detected. The only carbon-containing products were CO and CO2.
Addition of Li species results in a great increase in selectivity to
ethylene oxide. This promoting effect of lithium may be twofold.
First, lithium can act as a structural promoter by influencing the
shape and size, and thus the active sites, of the gold particles
[21]. Second, it lowers the activity of the alumina support by
influencing the acidic sites of the alumina. In this way, it favors
the reaction pathway in which the gold nanoparticles are
involved.
For the role of oxygen, we have made the following observa-
tions: ethanol conversion and ethylene oxide formation start at
lower temperature than O2 conversion. Secondly, we found no
apparent relationship between O2 conversion and ethylene oxide
formation. These observations led us to believe that the main role
of oxygen is to prevent coke formation. In the measurements with
high O2 content, almost no ethylene oxide is formed, but the etha-
nol is further oxidized mainly to CO2. Hence, a low concentration of
oxygen is important for a high ethylene oxide selectivity.
5. Conclusions
Gold-based catalysts are active in ethanol dehydrogenation,
oxidation and dehydration. In a gas flow with a low O2 concentra-
tion, a high selectivity to ethylene oxide can be obtained. The pres-
ence of O2 is very important to prevent carbon deposition. With the
best performing catalyst, Au/Li2O/Al2O3, a selectivity to ethylene
oxide up to 88% is obtained. By improving the oxygen uptake, ceria
makes oxygen available to the catalytic reaction sites. No indica-
tions are found of a combinatorial effect of Li2O and CeOx in these
reactions.
4.6. Role of gold, lithium, cerium and oxygen
The results presented do not result in a complete picture of the
mechanism of ethylene oxide formation from ethanol, but some
annotations can be made. As ethylene oxide formation is only
observed in the presence of gold nanoparticles, it can be concluded
that the gold particles contain active sites needed for the formation
of ethylene oxide. When an ethanol-only flow is used, these sites
are deactivated by carbon deposition. The selectivity then shifts
to the formation of acetaldehyde, which is not observed on the
bare support. Hence, most probably, another active site is present
on the gold, which is active in the formation of acetaldehyde,
and this site is not affected by carbon deposition.
It is unlikely that the formation of ethylene oxide is the result of
a reaction of ethanol on the gold particles with oxygen from the
support as the addition of ceria, which is very capable of supplying
oxygen [19,32], would then increase the formation of ethylene
oxide and just the opposite is found.
Acknowledgments
The authors thank Dr. J-P. Lange from Shell Global Solutions for
the discussions concerning the results presented in this paper.
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Catalyst
Au/Li2O/
Temperature (°C)
Conversion (%)
Selectivity (%)
c
-Al2O3
-Al2O3
-Al2O3
200
300
400
80
90
100
95
71
10
Ag/Li2O/
c
c
200
300
400
58
90
100
96
54
30
Cu/Li2O/
200
300
400
70
92
100
90
15
4
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