Organic amine, templating agent

Since then, organic amines, quaternary-ammonium bases, metal complexes, and other compounds have been extensively used in zeolite synthesis, acting as space fillers with low specificity, structure-directing agent, or true templates ... 

The extensive size of organic amine as structure-directing templates or pore filling agents, coupled with a new gel chemistry resulted in the discovery of a third generation of zeolites containing Al3+ and P5+ as lattice atoms (1982). These aluminophosphate materials are a family of molecular sieves as shown in figure 7.10. 

From the very low catalytic activity of the SAPO-8 samples, it can be concluded that Si for P substitution does not occur in SAPO-8, and probably neither in its precursor, Si-VPI-5. A plausible reason is that in the absence of organic amines in the micropores and in the absence of alkali cations, no species are available to play the role of charge compensating agent. Si for P substitution in AIPO4 molecular sieves requires filling of the micropores with template [4,6]. 

In the course of synthesis, the template molecules act as structure-directing agents. Size and shape of the template determines the geometry of the pore. During crystallization, the amphiphilic nature of the template (mainly organic amines) allows for molecular interaction between the aqueous silicate solution and the growing hydrophobic silica framework. Thermodynamic analyses suggest that the kinetics of nucleation are the most important factors in the crystallization process. 

A key enabler often employed in the synthesis of zeolites is the template, often called an organic directing agent. The template type is frequently different for microporous zeolites, mesoporous materials, and macroporous materials. The template can be an individual molecule (e.g., quaternary salts or linear amines), in-situ formed micelle clusters, or preformed structures (e.g., polyethylene spheres). 

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