A Study on Acid-catalysis by Silver-exchanged Zeolites

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The rate of solid-acid-catalyzed reactions are greatly enhanced by hydrogen present in the system. Thus, the catalytic activities of prereduced silver-exchanged Y-zeolites (AgY) for the disproportionation of ethylbenzene and the isomerization of o-xylene are several times higher than those of hydrogen Y- zeolites, the only in the presence of gaseous hydrogen. The enhancing effect of hydrogen was also observed for isomerization 1-butene and dehydration of alcohols over AgY. The similar enhancing effects of hydrogen on the catalytic activities are observed also in the conversion of methanol into hydrocarbons and the isomerization of 1-butene over prereduced form of silver dodecatungstophosphate.A dramatic enhancement was observed in the methanol .conversion over the combination catalysts of transition metal (Pd or Pt)-heteropoly acid supported on silica.Though the catalytic systems are versatile, there are some common features: The enhancement occurs only for acid-catalyzed reactions. The enhancing effect is reversible. The adsorption amount of hydrogen on the catalysts is small. These findings indicate that there is interconversion of molecular hydrogen in the gas phase and protons on the surface, and the activities of the protons induced from gaseous hydrogen are far higher than those of the protons from OH groups in zeolites or from parent heteropoly acids.