Molecular design of heterogeneous catalysts:: the case of olefin metathesis

Within the context of sustainable technology, catalysis is a means to more efficient processes ( lower energy demands, highly selective, waste-free), thereby providing a better use of raw materials. Industry relies heavily on heterogeneous catalysis to perform chemical transformations, but the development of these systems can be slowed down by the difficulty in understanding them, since they are often ill-defined and may contain several types of active sites (which can also be detrimental to selectivity). More recently, homogeneous catalysis has emerged, and it is probably because of a molecular understanding of chemical phenomena with well-defined systems that new processes have been rapidly set up. Our approach, called Surface Organometallic Chemistry (SOMC), has been to bring these two fields together: the result is a molecular approach to the design of heterogeneous catalysts. Within this short review, our strategy is delineated and the case of olefin metathesis is used to exemplify this approach. The design, the preparation and characterisation of a well-defined rhenacarbene are described. Based on a combined use of mass balance analysis, IR spectroscopy, advanced NMR techniques and EXAFS, it has been shown that [Re(=CHtBu)(equivalent toCtBu)(CH(2)tBu)(2)] reacts with a partially dehydroxylated silica at 700degreesC to give a single surface complex: <LF>syn-[(equivalent toSiO)Re(=CHtBu)(equivalent toCtBu)(CH(2)tBu)(equivalent toSiOSiequivalent to)] (1). This surface complex 1 catalyses olefin metathesis at low temperatures and without co-catalyst. The structure and the activity of this type of catalyst are discussed and compared with other existing homogeneous and heterogeneous catalysis systems.