Modified microporous aluminosilicates as novel solid acid catalysts.

摘要

The primary focus in zeolite research as applied to catalysis has been the synthesis of novel catalytic materials and the understanding of the structure-property relationships of various zeolite catalysts. Despite the tremendous amount of research that has been done on solid acid zeolites, our knowledge about these systems is still fairly limited. We have focussed our attention on three zeolite based catalytic systems.; We have synthesized a zirconium containing microporous silicate by a novel post-synthetic route. Zirconium was incorporated by the a post-synthetic gas phase treatment of the boron form of the zeolite with zirconium (IV) chloride (ZrCl₄). We have shown that the zeolite we have synthesized contains mono-dispersed zirconium and we found no evidence for the presence of any extra framework zirconium oxides. Zirconium is not tetrahedrally coordinated in the zeolite but is rather only partially grafted to the framework through one, two or three Si-O-Zr bonds. It is catalytically active for reactions requiring mild acidity such as the double bond isomerization of 1-butene.; We have investigated the structural and catalytic properties of MCM-56 which is closely related to MCM-22. MCM-56 is indeed made of MCM-22 unit cells which extend along [100] and [010] direction for hundreds of unit cells but extend only a couple of unit cells along the [001] direction. Unfortunately, the sheets of MCM-56 curl-up tightly upon calcination, preventing access to the acid sites on its surface. This leads to lowered catalytic activity involving large organic molecules such as toluene.; The reaction mechanism of the one step oxidation of benzene to phenol over H-ZSM-5 with nitrous oxide (N₂O) as the oxidant, is still not clear. Based on kinetic studies, spectroscopic investigations and DFT calculations, we have shown that Brønsted acid sites play a key role in the reaction chemistry. Though H-MCM-22 and H-Beta are active for the reaction, their catalytic activity is much lower than that H-ZSM-5. We have hypothesized the formation of a hydro-peroxide species upon the decomposition of N ₂O over the Brønsted acid site. We propose that it is this hydro-peroxide species that is responsible for the catalytic activity of the zeolite.