Studies on Molecular Sieve Crystallization and Heteroatomic Substitution

Molecular sieves have been scientifically and industrially important materials since their inception. These porous, crystalline networks of tetrahedrally coordinated atoms bridged by oxygen are known for their extremely high surface area and low density, making them excellent systems for catalysis. The framework of the AlPO4 family of molecular sieves is itself non-catalytic, but through heteroatomic substitution these materials can diversify into the catalytically active silicoaluminophosphate (SAPO) and metalloaluminophosphate (MAPO) families of heterogeneous acid and redox catalysts. Research into the methods by which these substitutions occur, and the effects therein on the crystallization of the sieves, is of great importance, as discoveries can lead to refined control over material property and function. To that end, this thesis is concerned with heteroatomic substitution in AlPO4 molecular sieves. SAPO-5 and MAPO-5 were chosen as model systems. Both the dry-gel conversion and hydrothermal synthesis methods were used to produce crystallization intermediates for study. Solid state NMR spectroscopy was used as the primary method of investigation. The SAPO-5 study determined that the dry-gel conversion methods of steam assisted conversion and vapour phase transport follow similar reaction pathways. After an initial divergence caused by the lack of organic structure directing agent in the gel in the VPT method, crystallization is similar between methods, and silicon incorporation results in two populations of silicon sites. One is well incorporated into the framework, while the other either represents reagent silica or dense silicon islands within the framework. The vapour phase silicon uptake method provides a contrast to this data, producing a uniquely well-incorporated silicon population (albeit with significantly less substitution). The MAPO-5 study, looking at manganese and iron substitution, showed that metal centers coordinate with phosphorous very early in the reaction, and are incorporated into the developing sieve before any channels or framework structure are formed. It showed that the use of the hydrothermal synthesis can be effective in capturing crystallization intermediates, and that the presence of paramagnetic materials does not exclude NMR from being a vital source of information