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.