Bifunctional pathways in catalysis by solid acids and bases

Chemical reactions catalyzed by solid acids and bases often require that reactants, intermediates, or activated complexes interact with several surface functions. Concerted and sequential bifunctional pathways also occur in homogeneous and enzyme catalysis. Hydrogenation and dehydration reactions require acid-base site pairs of intermediate strength, because such sites can form, stabilize, and discard adsorbed intermediates during a catalytic turnover. Deuterium exchange and H-H dissociation reactions also occur on acid-base pairs present in single-component or binary oxides and in supported oxide clusters. Hydrogenation of aromatic acids, dehydration of alkanols and methanolamine, condensation of alcohols, and deuterium exchange provide specific examples of bifunctional acid-base catalysis. Dehydration and dehydrogenation reactions of alkanols, widely used as probes of acid or base sites, probe instead the density and chemical properties of acid-base site pairs. Concerted bifunctional pathways require that sites co-exist within molecular distances. On surfaces, the inappropriate location of these sites can prevent concerted interactions, but rapid transfer of intermediates via surface or gas phase diffusion leads to kinetic coupling between distant sites and to sequential bifunctional pathways. These bifunctional sequences overcome proximity requirements by equilibration of adsorbed species throughout surface regions containing several types of sites. Diffusion of alkenes in the gas phase couples dehydrogenation and acid sites during n-alkane isomerization on bifunctional tungsten carbides modified by chemisorbed oxygen. These bifunctional surfaces form Bronsted acid sites by surface migration of H adatoms from WC to WOx sites. Acid (or base) sites and H-2 dissociation sites on surfaces interact 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 dehydrogenation on H-ZSM5 modified by exchanged cations, n-heptane isomerization on ZrO2 doped with WOx and Pt, and alcohol condensation on Cu-promoted Mg5CeOx oxides illustrate the role of kinetic coupling mediated by migration of hydrogen adatoms. In each example, metal clusters or isolated cations increase the rate of acid or base catalysis by providing a 'porthole' for hydrogen adsorption and desorption.