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311
type and equal Si/Al ratio but with a different counterion (i.e.,
Na or H) were used (NaY and USY(2.6), NaHBeta and HBeta).
To gain information on the effect of the porous structure, ze-
olites with similar Si/Al ratios but different framework types
were used (USY(15), HBeta). Finally, two crystalline microp-
orous aluminophosphates (AlPO-5 and VPI-5) were selected to
explore the catalytic behaviour of this class of frameworks char-
acterised by alternating AlO4 and PO4 units and large microp-
ores [24]. All of the chosen materials have pores of sufficiently
large size to allow fast diffusion of the reagents and of the epox-
ide.
The epoxidation activity of the catalysts was tested using two
molar ratios of hydrogen peroxide and cyclooctene (2 and 10).
The conversion of cyclooctene was higher when a higher ox-
idant/substrate ratio was used with all of the catalysts except
VPI-5, which showed constant low activity in both cases (Ta-
ble 1). The yields of epoxycyclooctane also improved with the
higher oxidant/substrate ratio, with the exception of USY(15)
and VPI-5 (Fig. 2 and Table 1). The highest yields in the
epoxide were found with the ordered mesoporous materials,
Ga-MCM-41 and Al-MCM-41, and with zeolite USY(2.6) (Ta-
ble 1, entries 2, 4, and 8). The better catalytic results obtained
with Ga-MCM-41 compared with Al-MCM-41 are in agree-
ment with those found for the respective oxides [10]. Remark-
ably, these catalysts are not those that give the highest conver-
sion of cyclooctene, but rather those that convert cyclooctene
to epoxycyclooctane with high selectivity. This result can be
explained in terms of the acid strength of the Al and Ga cen-
tres. Zeolite USY(2.6) has milder acid sites compared with other
zeolites with higher Si/Al ratios [25–27]. Mesoporous alumi-
nosilicate MCM-41 has fewer and weaker Brønsted acid sites
compared with zeolites with the same Si/Al ratio [19,28,29].
Consequently, it can be concluded that acid sites with moderate
strength are needed for the efficient epoxidation of cyclooctene.
These sites are suitable for the formation of surface hydroper-
oxides and are of sufficiently mild acidity to prevent further
reaction of the epoxide, which would cause a decrease in the
selectivity of the process [14,18]. In principle, the epoxidation
activity of USY(2.6) could be related also to extra-framework
octahedral aluminium species present in varying amounts in
USY zeolites [27]. However, Al-MCM-41 has only a very small
amount of octahedral aluminium compared with USY(2.6), as
shown by 27Al MAS NMR analysis (Fig. 3) [27,30]. Because
these two catalysts are both active in the epoxidation of cy-
clooctene with H2O2, we can infer that the active sites are tetra-
hedral aluminium species and that extra-framework aluminium
plays a minor role, if any, in the catalytic behaviour of zeolite
27
Fig. 3. Al MAS NMR spectra of Al-MCM-41 (black) and USY
(grey).
(2.6)
The peak around 60 ppm originates from tetrahedral aluminium located in the
framework of the aluminosilicates. The peak around 0 ppm belongs to octahe-
dral aluminium species, which in the case of aluminosilicates are identified as
extra-framework species. The peak around 30 ppm, present only in the spectrum
of USY
, is assigned either to pentacoordinated or to distorted tetrahedral
(2.6)
species [27,30].
of these considerations, the poor activity of the microporous
aluminophosphates AlPO-5 and VPI-5 can be ascribed to their
negligible Brønsted acidity.
When zeolites with Si/Al ꢀ 6 (i.e., with stronger acid sites)
were used, high conversions of cyclooctene but low yields in
epoxycyclooctane, and thus low selectivities, were observed
(Table 1, entries 9–16). Along with the epoxide, the main prod-
ucts were 1,2-cyclooctanediol, 1,2-cyclooctanediol diacetate,
and 5-hydroxy epoxycyclooctane, that is, products of the fur-
ther reaction of epoxycyclooctane, which is catalysed by strong
acid sites. This behaviour is particularly evident when compar-
ing the results obtained with the USY zeolites with different
Si/Al ratios (Table 1, entries 8, 10, and 12); the cyclooctene
conversion increased with the value of Si/Al, whereas the se-
lectivity toward the epoxide decreased dramatically. The higher
reactivity observed with zeolites with higher Si/Al ratios also
explains the decreased yield in the epoxide measured with
USY(15) when the oxidant/substrate ratio is increased from 2
to 10. For the catalytic tests performed using zeolites with Si/Al
ꢀ 6, it was not possible to obtain a good mass balance, in con-
trast with the total mass balance found with all of the other
catalysts used in this work (Table 1). This effect was more pro-
nounced for the catalysts that showed the highest cyclooctene
conversion (e.g., 12), suggesting that in the presence of strong
acid sites, polymeric species are produced, which might cause
blocking of the zeolite pores [31]. This is in agreement with
the GC–MS analysis of the reaction products of catalysts 9–16,
which, along with the byproducts mentioned above, showed the
formation of dimeric species.
USY(2.6)
.
Zeolite NaY showed much lower activity compared with the
corresponding zeolite in the H form, USY(2.6) (Fig. 2). The cat-
alytic results of NaHBeta and HBeta followed the same trend,
although the difference is less marked, probably due to the
presence of both Na+ and H+ counterions in the NaHBeta ze-
olite. These results confirm the role of Brønsted acid sites as
catalytic active centres in the epoxidation of alkenes with hy-
drogen peroxide, in agreement with the mechanism proposed
for the epoxidation catalysed by aluminium oxide [10]. In view
Cis-cyclooctene was used as substrate in all of the catalytic
tests reported here [10,12]. Generally, the epoxidation of cy-
clooctene is a stereoselective reaction even when using the
strained trans-cyclooctene as substrate [32,33]. Nevertheless,
when the epoxidation was performed in the presence of zeolite
beta or USY, a product assigned to trans-epoxycyclooctane on