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Theoretical Studies on the Oligomerization of Silicate Species in Basic Solution.

The possible reaction pathways of silicate species to linear- and ring-structure oligomers up to silicate hexamers in the basic medium have been studied using the density functional theory. The calculations were performed at the ωB97XD/6-31+G(d,p) level, and the integral equation formalism polarizable continuum model was employed to simulate the solvent effects, and it was found that they are appropriate in exploring the reaction mechanism of silicate species condensation. There are two steps in the anionic silicate condensation reactions: the SiO-Si bond formation step and the dehydration step. Moreover, the latter is the rate-limiting step for most of the reaction pathways except for the cyclization reaction of the linear pentamer to the 5-ring. The short linear oligomers would be likely formed from the reaction between monomers and oligomers, while the longer ones are easily formed through the reactions between short oligomers. The 4-ring and branched 5-ring oligomers are found to be formed very favorable both in kinetic and thermodynamic and could have great influences on the initial stage of zeolite synthesis. The intramolecular and intermolecular hydrogen bond effect of silicate species is an important factor affecting the reaction mechanism.