that this compound is not stable under the reaction conditions
in the presence of Au/CeO2 catalysts.
When both the metal oxide support and the gold nanoparticles
actively promote a reaction, it is often observed that there is an
optimum amount of gold loading on the support. Comparing
the performance of a series of Au/CeO2 samples in which the
loading of Au has been systematically increased from 0.007
to 1.5 wt%, it can be clearly demonstrated that the increased
presence of gold nanoparticles gradually increases the reaction
rate, with the samples containing gold giving higher conversion
of PC compared to ceria.
Although the kinetics of Fig. 1 clearly shows the promoting
effect of gold, the overall yields shown in Table 1 do not
surpass 35%. In this context, it should be mentioned that
although in the chemical literature there are catalysts with
similar performance, the temperature of the reported reactions is
typically in the range 150–170 ◦C that is significantly higher than
the reaction temperature employed in this work. We also note
that some uncatalyzed reaction with low selectivity takes place at
170 ◦C.
After having found that gold promotes that catalytic activity
of CeO2 for transalkylation of PC by MeOH to form DMC,
we also tested the applicability of Au/CeO2 to catalyze PC
transalkylation with other alcohols, in order to widen the scope
of the reaction. As it can be seen in Table 1, the presence of gold
in low weight percentages also increases the catalytic activity of
CeO2 for the formation of diethyl carbonate. Similarly, Au/CeO2
also catalyzed the formation of propyl and butyl carbonates
that are formed in lower yields. As can be seen in Fig. 2,
the selectivity towards transalkylation significantly decreases
with increasing alcohol chain length, suggesting that steric
reasons make transallylation more difficult and unfavorable
as the number of carbons in the alcohol increases. Thus, PC
decomposition seems to predominate as the number of alcohol
carbons increases.
The influence of the amount of gold on the catalytic activity of
Au/CeO2 catalyst for PC transalkylation to DMC was studied
◦
quantitatively by plotting the initial reaction rate at 140 C vs.
the amount of gold present, maintaining the total amount of
CeO2 constant. As can be seen in Fig. 3, concerning the initial
reaction rate there is an optimum for the activity of Au/CeO2
catalysts for a gold content at 0.08 wt%
Fig. 3 Turnover frequencies (TOF) measured at low PC conversion
vs. the amount of gold loaded on nanoparticulated ceria. Reaction
conditions: PC 10 mmol, MeOH 100 mmol, weight of CeO2 115 mg,
reaction temperature 140 ◦C.
Besides the initial PC reaction rate, the selectivity towards
DMC is also strongly influenced by the gold loading. Thus, as
can be seen in Table 1, DMC selectivity at 6 h reaction time
increases substantially for very low gold loadings. Beyond a
certain gold loading the selectivity towards DMC decreases.
This DMC selectivity decrease with the gold loading beyond
0.5 wt% is due to the decomposition of PC and/or DMC to
CO2. An optimum performance of the promotional influence of
gold is achieved at about 0.5 wt%, at which the balance between
increased PC conversion and high DMC selectivity results in the
highest DMC yield at 6 h reaction time.
In conclusion, it has been shown that ceria nanocrystallites
is a moderately active catalyst for the transalkylation of PC
by methanol, exhibiting low selectivity. The presence of gold
nanoparticles on ceria in appropriate loading significantly
increases the activity and selectivity towards transalkylation.
Fig. 2 Conversion (white columns) and selectivity (black columns) data
for the transalkylation reaction as a function of the alcohol number
of carbons. Reaction conditions: alcohol (100 mmol), PC (10 mmol),
Au/CeO2 (0.5 wt%) (115 mg), reaction time 5 h, reaction temperature
140 ◦C.
Au/CeO2 (0.5 wt%) acts as a heterogeneous catalyst. Thus,
if the reaction was initiated under normal conditions in the
presence of Au/CeO2 (0.5 wt%) and then the solid hot filtered
when the conversion was about 30% and the reaction continued
in the absence of solid, no further conversion was observed
in the supernatant solution. In addition, chemical analysis
by inductively coupled plasma showed that no detectable
amounts of gold were present in the liquid phase after the
reaction.
Notes and references
After the reaction, the solid catalyst Au/CeO2 (0.5 wt%)
can be recovered from the liquid reaction mixture by filtration,
washed with acetone and water (pH 10) and dried before reuse
for consecutive runs. It was observed that the catalytic activity
was maintained in four consecutive recycles. Chemical analysis
of gold in the reused catalyst showed no leaching of gold from
the solid.
1 P. Tundo and M. Selva, Acc. Chem. Res., 2002, 35, 706.
2 M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975.
3 D. Delledonne, F. Rivettia and U. Romano, Appl. Catal., A, 2001,
221, 241.
4 T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365.
5 D. J. Darensbourg, Chem. Rev., 2007, 107, 2388.
6 H. Wang, M. H. Wang, N. Zhao, W. Wei and Y. H. Sun, Catal. Lett.,
2005, 105, 253.
This journal is
The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 949–952 | 951
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