Francis G. Dwyer

ZSM-4 crystallization via faujasite metamorphosis

The crystallization of ZSM-4 via faujasite metamorphosis has been studied by X-ray diffraction and scanning electron microscopy. By the addition of tetramethylammonium (TMA) ions, the well-known transition from metastable faujasite to “P” zeolite was directed to ZSM-4 zeolite instead. The following reaction sequence was observed for these crystallizations: There is no indication of a simultaneous or competitive crystallization since no ZSM-4 is formed until the faujasite crystallization is complete. Also, there is no evidence to indicate that the faujasite reverts to an amorphous material which might serve as a nutrient source for the recrystallization to ZSM-4. Scanning electron microscopic observations are consistent with a mechanism that would include a dissolution of surface faujasite crystals that supplies nutrient for the ZSM-4 crystallization which, in turn, is nucleated by the surface of the same faujasite crystals.

CATALYSIS FOR CONTROL OF AUTOMOTIVE EMISSIONS

Abstract In recent years, concern over our environment has led to substantial action by the executive and legislative branches of government on both the state and federal levels. A major environmental concern is that of air pollution. The sources of air pollution are mainly industrial or stationary sources, which includes a wide variety of manufacturing facilities and power plants, and automobiles. In many urban areas, due to their sheer number, the automobiles are major contributors to air pollution.

Chapter 13 Shape Selectivity in Catalytic Cracking

Publisher Summary This chapter discusses the shape selectivity in catalytic cracking. Shape selective catalysis is a term normally reserved to describe reactions that take place over restricted pore molecular sieves. Shape selectivity in zeolite catalysis is characterized by one or any combination of three primary mechanisms: (1) reactant shape selectivity whereby molecules are sterically discriminated based upon their ability or inability to enter the restricted pores of the zeolite, (2) product shape selectivity whereby bulkier molecules are sterically hindered from leaving the zeolite, and (3) spatioselectivity whereby the formation of molecular transition states is restricted by the confines of the zeolite channels, intersections, or cages. These characteristics of shape selective catalysis have been traditionally applied to restricted pore zeolites, such as ZSM-5. The evidence for zeolite shape selectivity is obtained primarily by examining the relative rates of conversion of isoparaffins to normal paraffins. Because isoparaffins have intrinsically higher cracking rate constants, evidence of selective normal paraffin conversion or higher iso-normal ratios in the product is usually evidence for shape selectivity.

Pore Size and Shape Effects in Zeolite Catalysis

The catalytic pore size of ZSM-5 has been determined on the basis of the sizes of molecules converted during the dewaxing of waxy distillate chargestocks. From conversions of alkane and alkylbenzene molecular classes, ZSM-5 behaves in catalytic conversions as if it has an elliptical pore with the approximate dimensions of 0.55 × 0.7 nm and can in fact discriminate by both molecular size and shape. Other zeolites have been similarly examined: the catalytic pore sizes of mordenite and ZSM-23 are 0.9-1.0 nm and 0.45 × 0.65 nm, respectively. The catalytic pore sizes are always larger than computed crystallographic pore sizes, a phenomenon generally observed in adsorption measurements.

Structure Sensitivity in Zeolite Catalysts

Abstract Almost from its beginning, zeolite catalysis has had a strong dependency upon the catalyst structure. Shape selective catalysis using zeolites has controlled selectivity by constraining the reactants that can be admitted to the catalytic sites, the products that can emit from the zeolite pores and the products that can be formed within the zeolite cavity. Although advances continue to be made in this area, they are mainly subtle refinements of the original concepts. A major challenge is and has been to combine the shape selective constraints with homogeneous and/or enzymatic catalysis. The major obstacle has been the limited size of zeolite pores and cavities to accommodate the host homogeneous or enzymatic agents. With the recent discovery of the 18 membered AlPO 4 , VPl-5/MCM-9, hope for the synthesis of zeolites to accommodate such systems has been renewed.