Pore Size Engineering in Zeolites

Zeolite science and technology continue to develop in a rapid way. The use of zeolites has been explored in many scientific disciplines: modern inorganic and organic chemistry, physical chemistry, mineralogy, etc. and in all types of chemical engineering process technology. The wide variety of applications includes separation and recovery of gases, liquids and ions, catalysis (isomerisation-, alkylation-, hydrogenation-, cracking-, etc. reactions) and ion exchange. The main purpose of this book is to give a review on the different modification techniques in order to vary in a controlled way the porosity of zeolites and related materials. As D.E.W. Vaughan states in Zeolites: Facts, Figures, Future, Part A, the pores in zeolites range from 2.2A for the silica polymorphs tridymite and cristobalite to about 10A for VPI-5, in terms of molecular accessiblitiy from He to triethyl-benzene. Crystallographically, the diffusion pathways increase in descrete steps from 6-rings to 8-rings, 10-rings, 12-rings and 18-rings, however, an important influence can be observed from the configuration of the rings, the localisation and number of exchangeable cations and the increasing vibrational amplitudes of the structures with temperature or the chemical modification of the zeolite framework, so that the effective ring diameters can be changed in a contineous way from <2A tot 15A In order to tune the zeolitic pore system in a controlled way, various modification techniques will be discussed such as (a) a modification by a cation exchange process, (b) a modification by a preadsorption of polar molecules and (c) a modification of the zeolitic framework. Depending on the modification method, both the molecular sieving and selectivity of zeolites can be altered in a controlled way, which is an important tool in the separation, encapsulation and catalyst technology. In the last decade, pore size engineering in zeolites and related materials (sepiolite, montmorillonite, silica, alumina, etc.) was intensively studied in the Laboratory of Inorganic Chemistry at the University of Antwerp (UIA) in order to tune the sorbent specificities of various zeolites and to increase the shape and product selectivity of zeolitic catalysts. Therefore, the author wishes to thank his research team, namely Dr. G. Peeters, Dr. A. Thys, Dr. J. Philipaerts, Dr. Yongan Yan, Dr. P. De Hulsters and C. Vanhoof for their excellent research work. My thanks also to Mrs. M. Stalmans for her patiently typing skills during the preparation of the manuscript. Also the author wants to thank the N.F.W.O. (National Science Foundation of Belgium), I.W.O.N.L. (Instituut voor Wetenschappelijk Onderzoek in de Nijverheid en Landbouw), The Commission of the European Communities, the I.U.A.P. (interuniversitaire Attractiepool) for their financial support during the research activities.

Table of Contents:
Part 1 Introduction: zeolite architecture. Part 2 Pore size engineering in zeolites: modification by a cation exchange process; modification by a preadsorption of polar molecules; modification of the zeolitic framework; pore size modification by crystallographic changes; internal and external modification of the zeolite structure - silanation, disilanation, boranation, implantation of boran-nitrogen compounds, modification by inorganic acids and their salts; external surface modification of the zeolite crystals. Part 3 Applications: encapsulation of gases in zeolites; separation of gases by modified zeolites.