Heterogeneous open porous structure

The behavior of both heterogeneous catalysts is, in general terms, similar. The slightly higher conversion, achieved in second and consecutive rans, with catalyst CXIOx-Rh is probably related to the more open porous structure. In both cases, the heterogenized complex is partially transformed, with reaction time, into a highly dispersed heterogeneous catalyst, which has shown to be very active and recyclable. 

Zeolites are widely used in heterogeneous catalysis. In principal, their highly controllable porous structures have great potential for use as enantioselective catalysts. A considerable research effort has been devoted to the development of chiral zeolites [32]. Only zeolite beta and titanosiUcate ETSIO exist in chiral form [33, 34], although it is very difficult to obtain zeolite in enantiopure form [32]. Zeolites are typically synthesized in the presence of surfactant templates, which are removed by high-temperature calcination, a process that invariably destroys the chiral conformation of such assemblies [32]. Low enantioselectivities attributable to the chiral zeolite framework structure have been observed by Davis and Lobo [35] for the ring opening of trans-stilbene oxide with water. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

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