SUPPORTED TRANSITION METAL CARBONYLS AS A NEW CLASS OF HETEROGENEOUS CATALYSTS

A broad study (covering all elements having stable carbonyls, including both cluster and mixed-metal complexes) has been made of a new class of catalysts, transition metal carbonyl complexes directly deposited on high surface area inorganic supports (primarily (gamma)-Al(,2)O(,3)). These catalysts have the potential of combining some of the advantages of homogeneous catalysts (by virtue of the use of molecular precursors) with the high stability of traditional heterogeneous catalysts. The technique of temperature programmed decomposition (TPDE) was developed to efficiently characterize these materials during catalyst synthesis. TPDE involves continuously monitoring gas evolution, primarily carbon monoxide and hydrogen, as a physisorbed carbonyl is activated by linearly ramping the temperature upward. Hydrogen is evolved during a TPDE from a redox reaction between the initially zero valent metal of the carbonyl and the surface hydroxyl groups of the support. Thus, a TPDE chromatogram contains information on the temperature regions at which a complex first begins to decompose and bind to the support, the existence of zero-valent subcarbonyl species, and the temperature and extent of catalyst oxidation. For the group VIB hexacarbonyls additional analysis of the peak shapes have yielded information on the kinetics of the decomposition and novel subcarbonyl species on (gamma)-Al(,2)O(,3), SiO(,2), and 13 X molecular sieve have been identified.; All carbonyls became oxidized during TPDE to 600(DEGREES)C. The extent of oxidation can be controlled by varying the temperature of activation as well as the hydroxyl content of the support. In general, oxidation dramatically lowers catalyst activity. However, some loss of carbon monoxide (to develop coordinative unsaturation) is necessary to produce an active catalyst. Thus, the information in a TPDE chromatogram allows one to predict the synthesis most likely to produce the most active catalysts. These ideas led to a generally applicable theory of the relation between the activity of a catalyst and its structure.; A number of supported metals, especially molybdenum and tungsten, are difficult to reduce. For these metals appropriate synthesis from the carbonyls can lead to zero valent supported catalysts. The activities of these new catalysts for the hydrogenation of ethylene and methanation are about 100 fold higher than their traditional analogues. Similar rate enhancements are expected for other reactions involving "soft" reactants (as isomerization and Fischer-Tropsch syntheses) and other difficult to reduce metals (such as vanadium and manganese). In a number of cases, high activities can be achieved at rather low temperatures of activation and thus carbonyl derived catalysts can have much higher dispersions than their traditional analogues. The most dramatic case is for iron catalysts derived from the carbonyls which are over 10 fold more dispersed than traditional iron catalysts.