Molecular Sieves for Use in Catalysis

Abstract

Zeolites and molecular sieves are finding applications in many areas of catalysis, generating intense interest in these materials in industrial and academic laboratories. As catalysts, zeolites exhibit appreciable acid activity with shape-selective features not available in the compositionally equivalent amorphous catalysts. In addition, these materials can act as supports for numerous catalytically active metals. Major advances have occurred in the synthesis of molecular sieve materials since the initial discovery of the synthetic zeolite molecular sieve types A, X, and Y, and a great number of techniques have evolved for identifying and characterizing these materials. Added to an extensive and ever growing list of aluminosilicate zeolites are molecular sieves containing other elemental compositions. These materials differ in their catalytic activity relative to the aluminosilicate zeolites and may have potential in customizing or tailoring the molecular sieve catalyst for specific applications. Elements isoelectronic with Al+3 or Si+4 have been proposed to substitute into the framework lattice during synthesis. These include B+3, Ga+3, Fe+3, and Cr+3 substituting for Al+3, and Ge+4 and Ti+4 for Si+4. The incorporation of transition elements such as Fe+3 for framework Al+3 positions modifies the acid activity and, in addition, provides a novel means of obtaining high dispersions of these metals within the constrained pores of industrially interesting catalyst materials.