3 - Towards the rational design of zeolite frameworks

Summary

Introduction

Zeolites are crystalline, microporous materials composed of oxide tetrahedra of silicon and aluminum that are three-dimensionally connected to give an O/[Si + Al] = 2. Each Al⁺³ in the oxide framework requires an extra-framework charge-balancing cation to maintain electroneutrality of the material. The charge-balancing cations for as-synthesized zeolites can be inorganic, organic or organometallic and can be post-synthetically changed to industrially useful cations such as protons and transition metals.

In addition to aluminosilicates, crystalline microporous materials can be phosphate-based. The aluminophosphate (AlPO₄) framework is electroneutral (analogue of SiO₂), and the aluminum and/or phosphorus tetrahedral atoms can be substituted by a number of metal and non-metal atoms that result in producing charged frameworks [1–3], e.g. Si⁺⁴ substitution for P⁺⁵. In addition, numerous other metal oxide, and nitride based microporous materials have been reported recently [4, 5].

To date, approximately 125 different microporous structures have been reported to the International Zeolite Commission (IZC) for structure codes [6] and the interested reader can access this information via the following website: http://www.iza-structure.org. Each microporous material is assigned a threeletter code by the IZC (the IZC code will be used throughout the remainder of this chapter). These microporous materials are distinguished by the connectivity of the framework atoms that determine the size and dimensionality of the pores and/or cages that they possess.