A Study on Acid-catalysis by Silver-exchanged Zeolites

固体酸催化反应的速率被系统中存在的氢大大提高。因此，预还原的银交换Y型分子筛（AgY）对二甲苯的异构化和异构化的催化活性比氢Y型分子筛高出几倍，而且仅在气态氢存在下。氢气对1-丁烯异构化和醇脱水反应也有促进作用。在预还原态的十二钨磷酸银催化剂上，氢对甲醇转化为烃类和1-丁烯异构化反应的催化活性也有类似的增强作用，在过渡金属复合催化剂上，甲醇转化率有显著的增强作用虽然这些催化体系用途广泛，但有一些共同的特点：这种增强作用只发生在酸催化的反应中。增强效应是可逆的。氢在催化剂上的吸附量小。这些结果表明，气相中的分子氢与表面上的质子发生了相互转化，而且气态氢诱导的质子活性远高于沸石中OH基团或母体杂多酸的质子活性。

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.