ChemComm
Communication
In summary, the data reported in the present communication
indicate that CuO/Al2O3 is a very active catalyst for the one-pot
epoxidation of olefins, thanks to the in situ generation of CHP by
reaction of cumene and O2, without the use of a radical initiator. The
results show that the catalyst is essential for both reactions, namely
cumene oxidation and olefin epoxidation, and suggest the existence
of a synergy between the alumina support and the copper phase,
which is crucial for the high catalytic activity observed. The real need
to improve the existing epoxidation routes makes the process
reported here extremely attractive. Therefore, its optimization, in
particular regarding the catalyst stability and recyclability, could be
of significant interest in order to improve the efficiency of the
existing processes based on the use of hydroperoxides, and in
particular CHP, for the synthesis of epoxides, such as PO and others.
The authors thank the Italian Ministry of Education, Uni-
versity and Research for the FIRB Projects ‘‘ItalNanoNet’’
(RBPR05JH2P) and Regione Lombardia for the project ‘‘Accordo
Quadro Regione Lombardia e CNR per l’attuazione di programmi
di ricerca e sviluppo’’.
Fig. 1 Recycling tests of CuO/Al2O3 catalysts after 4 h of reaction. SA is the
specific catalytic activity (molconv (gCu h)À1).
A relevant role may be played by homogeneous species. Some
leaching of Cu from the catalyst was observed, around 4%. The
Sheldon test (Fig. S4, ESI†) showed that the solution is active in the
reaction but with a reaction rate much lower than that observed
without removing the solid catalyst. Moreover when the reaction
was carried out in the presence of an equimolar amount of
Cu(acac)2 the reaction profile was identical to the one observed
in the presence of CuO/Al2O3 (Fig. S5, ESI†). This unequivocally
shows that the amount of Cu leached from the solid is not the only
reason for the activity observed.
It should be outlined that some Cu leached out also from
CuO/SiO2 under the reaction conditions but it did not show any
activity. Therefore we can suggest that only the CuO phase
present on the surface of CuO/Al2O3 is able to promptly
produce a very active species.
The CuO/Al2O3 catalyst shows good stability and reusability
and it can be used at least 3 times, with only a slight decrease in
its catalytic performances in terms of conversion of stilbene
and selectivity towards the epoxide (Fig. 1 and Fig. S2, ESI†). As
a matter of fact a small increase in the specific activity (SA) was
observed. Similar behaviour has already been reported in the
literature16 and in our case could be related with the better
availability and exposure of the copper particles on the support.
HRTEM analysis of the fresh catalyst shows the presence of
uniform and very small copper particles well dispersed on the Al2O3
support. The related histogram of the particle size distribution
(Fig. 2a) indicates copper particles with a mean diameter of 4.0 nm.
On the other hand, the analysis of the recovered catalyst after the
third catalytic run shows again the presence of small particles
(around 4 nm, Fig. 2b). Although some bigger particles (20–50 nm)
are also present (Fig. S3, ESI,† full histogram), this low aggregation
did not significantly affect the catalytic efficiency of the system.
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Fig. 2 Histograms of the particle size distributions of the fresh (a) and the
recycled (b) CuO–Al2O3 system.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 1957--1959 1959