BIFUNCTIONAL PATHWAYS IN CATALYSIS BY SOLID ACIDS AND BASES

Chemical reactions catalyzed by solid acids and bases often require th at reactants, intermediates, or activated complexes interact with seve ral surface functions. Concerted and sequential bifunctional pathways also occur in homogeneous and enzyme catalysis. Hydrogenation and dehy dration reactions require acid-base site pairs of intermediate strengt h, because such sites can form, stabilize, and discard adsorbed interm ediates during a catalytic turnover. Deuterium exchange and H-H dissoc iation reactions also occur on acid-base pairs present in single-compo nent or binary oxides and in supported oxide clusters. Hydrogenation o f aromatic acids, dehydration of alkanols and methanolamine, condensat ion of alcohols, and deuterium exchange provide specific examples of b ifunctional acid-base catalysis. Dehydration and dehydrogenation react ions of alkanols, widely used as probes of acid or base sites, probe i nstead the density and chemical properties of acid-base site pairs. Co ncerted bifunctional pathways require that sites co-exist within molec ular distances. On surfaces, the inappropriate location of these sites can prevent concerted interactions, but rapid transfer of intermediat es via surface or gas phase diffusion leads to kinetic coupling betwee n distant sites and to sequential bifunctional pathways. These bifunct ional sequences overcome proximity requirements by equilibration of ad sorbed species throughout surface regions containing several types of sites. Diffusion of alkenes in the gas phase couples dehydrogenation a nd acid sites during n-alkane isomerization on bifunctional tungsten c arbides modified by chemisorbed oxygen. These bifunctional surfaces fo rm Bronsted acid sites by surface migration of H adatoms from WC to WO x sites. Acid (or base) sites and H-2 dissociation sites on surfaces i nteract via surface diffusion of H adatoms. This leads to bifunctional alkane and alkanol reactions via kinetic coupling of C-H or O-H bond activation and hydrogen adsorption-desorption steps. Propane dehydroge nation on H-ZSM5 modified by exchanged cations, n-heptane isomerizatio n on ZrO2 doped with WOx and Pt, and alcohol condensation on Cu-promot ed Mg5CeOx oxides illustrate the role of kinetic coupling mediated by migration of hydrogen adatoms. In each example, metal clusters or isol ated cations increase the rate of acid or base catalysis by providing a 'porthole' for hydrogen adsorption and desorption.