Modeling the Role of Excluded Volume in Zeolite Structure Direction

We investigate the formation of zeolite structures in replica-exchange Monte Carlo simulations of a reactive model of silica polymerization. The simulations incorporate hard spheres to model the effect of excluded volume caused by structure-directing agents (SDAs). We focus on modeling the formation of cage-type zeolite frameworks SOD and LTA. Our model predicts that a relatively wide range of SDA sizes could be used to construct SOD, whereas a narrower range will work for constructing LTA. We also predict that there is potential benefit of including multiple SDAs in each zeolite unit cell, and in the case of LTA with both small and large cavities, there is a strong potential benefit using both small and large SDAs that match the cavities' sizes. We hypothesize that the volume exclusion reduces the configuration space available to the assembling silica units, making it easier for the system to find ordered structures with quasi-spherical cavities.