The role of a structure directing agent tetramethylammonium template in the initial steps of silicate oligomerization in aqueous solution.

Abstract

The understanding of the formation of silicate oligomers in the initial stage of zeolite synthesis is of fundamental scientific and technological importance. The use of different organic structure directing agents is known to be a key factor in the formation of different silicate species, and the final zeolite structure. Tetramethylammonium (TMA(+)), for example, is indispensable for the formation of the LTA zeolite type. However, the role of a TMA(+) template has not yet been elucidated at the molecular level. In this study, ab initio molecular dynamic simulations were combined with thermodynamic integration to arrive at an understanding of the role of TMA(+) in the formation of various silicate species, ranging from dimer to 4-ring. Free energy profiles show that trimer and 3-ring silicate are less favourable than other oligomers such as linear tetramer, branched tetramer and 4-ring structures. TMA(+) exhibits an important role in controlling the predominant species in solution via its coordination with silicate structures during the reaction process. This can explain that formation of D4R·8TMA crystals, as observed in experiment, is controlled by the single 4-ring formation step.