Effect of Counter Ions on the Silica Oligomerization Reaction

Abstract

The silicate oligomerization reaction is key to sol-gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have addressed the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Here we report a study of a silica condensation reaction in aqueous solution in the presence of counter ions (Li(+) and NH(4)(+)). Ab-initio molecular dynamics simulations have been used to construct reaction energy diagrams including transition state free energies. Contact with Li+ as well as NH(4)(+) increases the activation energies of the dimerization step compared to the situation in the absence of counterions. The presence of NH(4)(+) has no effect on consecutive oligomerization steps. Hence NH(4)(+) will increase the relative formation rate of larger oligomers.