Multi-level Modeling of Silica–Template Interactions During Initial Stages of Zeolite Synthesis

Abstract

Zeolite synthesis is driven by structure-directing agents, such as tetrapropyl ammonium ions (TPA ? ) for Silicalite-1 and ZSM-5. However, the guiding role of these organic templates in the complex assembly to highly ordered frameworks remains unclear, limiting the prospects for advanced material synthesis. In this work, both static ab initio and dynamic classical modeling techniques are employed to provide insight into the interactions between TPA ? and Silicalite-1 precursors. We find that as soon as the typical straight 10-ring channel of Silicalite-1 or ZSM-5 is formed from smaller oligomers, the TPA ? template is partially squeezed out of the resulting cavity. Partial retention of the template in the cavity is, however, indispensable to prevent collapse of the channel and subsequent hydrolysis.

Advances in theory and their application within the field of zeolite chemistry

“…Some of the observations made by Lewis et al 74 were later confirmed by Verstraelen et al 82 in a multi-level modeling study on the silica-template interactions during the initial stages of zeolite synthesis. Both static ab initio and classical MD studies were performed to obtain insight into the interactions between the tetrapropylammonium ions (TPA+) and silicalite-1 precursors.…”

Simple synthesis recipes of porous materials

“…In one class of models, the precursor nanoparticle framework is unrelated to that of the final zeolite. The group of Lobo proposed a detailed model in which nanoparticles consist of an unstructured silica-rich core surrounded by a TPA-rich shell [15][16][17]. This would imply a rather unspecific template role limited to change of solvent properties.…”

“…These observations also were corroborated by theoretical studies where a clear preference of the template molecule towards Silicalite-1 fragments was derived. [16] Such a scenario implies the TPA has, asides the solvent structuring at early stages in the synthesis, also a direct structure directing effect on molecular level.…”

“…Alternatively, the observations could be explained by aggregation of already structured nanoparticles into larger entities [18]. Several studies suggested that such an aggregation process is activated, imposed by repulsive colloidal interactions [16].…”

“…Interestingly, molecular simulations indicate that also during the oriented aggregation step the template might play an important role in aligning approaching species. Template molecules situated between two zeolite fragments can interact with both, orienting the fragments before fusion can occur [16].…”