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.

Elucidating the Role of Tetraethylammonium in the Silicate Condensation Reaction from Ab Initio Molecular Dynamics Simulations

“…For the Ow–Ow RDF, the first peak is at 2.8 Å, which is in good agreement with the experimental data and earlier simulations. 63 , 64 This implies that the presence of TEA + and silicate has no effect on the water structure. The first peak of Si–Ow RDF is located at 3.75 Å for TEA + –water, which is very similar to that of TMA + –water.…”

“…The reaction mechanism of silicate condensation is very similar to that reported in earlier studies. 28 , 63 As described in Scheme 1 , the first reaction step is to form SiO–Si bond resulting in a fivefold silicate intermediate. The second step is the removal of the water to form the dimer product.…”

“…It is protonated either directly by another silicate hydroxyl group or via a proton transfer chain mediated by one or more water molecules. 63 , 65 The presence of ODSAs such as TEA + , TMA + , or TPA + seems to have a minor effect on the water removal step. 30 , 63 This is in consistent with previous studies showing that this step is more determined by the hydrogen-bonding network between silicate and water.…”

“…This trend is opposite to the case of TMA + , where the formation of the linear tetramer has the lowest overall activation barrier among the linear species. 63 …”

“…Details of this ring closure reaction were elucidated in earlier computational studies. 28 , 63 Figure 5 shows snapshots of the calculated 3-ring reaction pathway in the presence of cation TEA + . In the first step, the active oxygen atom from one end of the linear tetramer binds to the Si atom at the other end, yielding an intermediate ring.…”