Chapter 2 The Nature of Active Sites in Zeolites: Influence on Catalyst Performance

Abstract

This chapter describes the different types of active sites, their formation, characterization and catalytic reactions associated with hydrocarbon transformations in the general field of fluid catalytic cracking (FCC). Compared to other types of catalysts, zeolites are extremely active, especially in hydrocarbon conversion reactions, and their regular pore dimensions make them selective as to which molecules are adsorbed or converted. A simple correlation between the number, type and strength of the acid sites in zeolites and their catalytic activity is the ultimate goal of refiners. While such a goal has yet to be fully achieved, considerable advances have been made in the area of quantifying and characterizing the active acid sites in zeolites, as well as correlating the data with catalytic performance. It is now possible to directly determine all the aspects of zeolite acidity by a multiplicity of techniques. However, in most cases, the conditions under which the measurements are made are far removed from the conditions experienced in a real life situation, such as an FCC unit. Thus, only general trends can be realized from the data. Consequently, indirect methods—such as model reactions using probe molecules, for example, n-hexane/heptane, cumene, ethyl benzene or xylenes, under conditions closer to those of a real FCC or isomerization unit—play an important role in relating acidity to activity.