Synthesis of Porous Silicates

Abstract

The issues of importance and future concern in the synthesis of porous silicates and porous materials that contain a large fraction of silica, e.g. zeolites and other crystalline molecular sieves, are reviewed. The thermodynamics of zeolite synthesis are discussed, including a detailed thermodynamic analysis of the synthesis of pure-silica ZSM-5. The kinetics of porous silicate synthesis are reviewed, with particular emphasis on the control of porous structure formation through the use of organic structuredirecting agents. Ordered mesoporous materials are discussed in the context of distinguishing their features from zeolites in order to describe further the unique properties of each class of material. Finally, several unresolved issues in the understanding of the synthesis process are outlined, the resolutions of which would aid in the synthesis of porous silicates by design.

The Chemistry of Form

“…For instance, the vast number of zeolite structures of comparable free energies of formation can be attributed to kinetic selectivity induced by template-directed mechanisms during crystallization. [6] The situation becomes even more complicated with the structural description of natural forms. Although these structures clearly retain a recognizable and reproducible pattern of order at the level of visual perception, they do not fit the crystallographers Weltanschauung for several reasons.…”

Heat capacities, third-law entropies and thermodynamic functions of SiO2molecular sieves fromT= 0 K to 400 K

“…(1) While zeolites are found in nature, novel synthetic methods have been developed that allow one to tailor the size, shape, and chemical reactivity of the pores and channels. (2) These synthetic techniques usually employ an organic structure-directing template that is then burned off at high temperatures, leaving the inorganic framework behind. The extent to which these synthetic techniques rely on thermodynamic stabilization or favorable kinetic effects is not well understood.…”

“…The extent to which these synthetic techniques rely on thermodynamic stabilization or favorable kinetic effects is not well understood. (2) Our research groups from California Institute of Technology, University of California, Davis, and Brigham Young University have collaborated to provide experimental thermochemical and thermodynamic data to shed light on the question of whether there is a thermodynamic driving force that stabilizes these structures. This paper reports heat capacity measurements from 14 (T /K) 400 made at Brigham Young University on four polymorphs of pure SiO 2 which have zeolitic frameworks.…”

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

“…Although zeolite synthesis and catalysis are based on some general features that are well understood (22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32), most zeolites pose complex challenges that are resolved only at the empirical experimental level. The ZSM-5-silicalite series generally is synthesized with sodium and a templating agent, especially a tetrapropylammonium ion, but template-free syntheses are described (28,29,33).…”