TRANSFER OF CONCEPTS FROM HOMOGENEOUS TO HETEROGENEOUS CATALYSIS, USEOF HAMMETT RELATIONSHIPS TO ASSESS REACTION-MECHANISMS AND NATURE OF ACTIVE-SITES IN REACTIONS CATALYZED BY SULFIDES, METALS, CLAYS AND ZEOLITES

The application of Hammett or Hammett-type relationships to a series o f various reactions catalyzed by clays, zeolites, sulfides and metals is reviewed in this paper. Such relationships are, of course, of great interest for a better knowledge of the reaction mechanisms over solid catalysts and on the nature of the active species, Among the differen t data obtained in the laboratory over those catalytic systems, it app ears that Hammett slope rho values for the acid-catalyzed reactions in volving Bronsted species in the rate-determining step (hydration, hydr olysis, isomerization, electrophilic aromatic substitution) are of the same order of magnitude for both homogeneous and heterogeneous reacti ons, whereas deviations are more frequently observed for reactions inv olving hydrogen.