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Influence of Silica Sources on the Chemical Composition of Aluminosilicate Hydrogels and the Results of Their Hydrothermal Treatment

Among the parameters that influence the outcome of a hydrothermal synthesis of molecular sieve zeolites, the effects of various silica sources are some of the least well understood. Prior studies have noted that some silica sources were “ active, ” or “ inactive, ” and there have been some suggestions that aluminium impurities can contribute to a silica source’ s “ activity” to promote certain zeolite syntheses. However, a silica source’ s activity toward promoting zeolite crystallization has not been shown definitively be linked to any specific impurity in the silica. This study reports on the use of four different silica sources and the corresponding distribution of Na, Al, and Si between the supernatant and amorphous gel phases. It is shown that the distribution of those species was unaffected by the choice of silica source, but did vary with batch composition. Furthermore, the degree of silicate polycondensation (DPS), measured by the molybdate method, did not vary with the choice of the silica source. The observable differences in particulate and structural properties of the products crystallized from the four different silica sources were analysed in terms of the critical processes of zeolite crystallization (gel dissolution, nucleation, crystal growth)

aluminosilicate gels ; precipitation ; silica source ; chemical composition ; hydrothermal treatment ; zeolite ; crystal size

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