Lattice Monte Carlo Simulations in Search of Zeolite Analogues: Effects of Structure Directing Agents

Abstract

We use parallel tempering Monte Carlo simulations to search for crystalline states of a lattice model of silica polymerization in the presence of structure directing agents (SDAs). Following previous work where we have discretized continuous space into a body-centered cubic (bcc) lattice, we have modeled tetrahedral molecules (T(OH)4) as corner-sharing tetrahedra on a bcc unit cell. The SDAs were represented as quasi-spherical species with diameters of 6.4 and 10.4 Å to study the effect of SDA size on the resulting crystal structures. Our parallel tempering Monte Carlo simulations produce fully connected crystalline structures finding the emergence of 3D microporous materials with SDAs occupying the pore spaces and 2D layered materials with SDAs occupying the gallery space in between layers. We have found that the strength of SDA–oxygen attraction plays a significant role in directing final micropore structures. For relatively strong attractions (>1.2 kcal/mol SDA–oxygen contacts) we have found only 2D layered materials; for attractions below this cutoff we observed 3D microporous crystals; and for no attraction—modeling the SDA as a quasi-hard sphere—we again found only 2D layered materials. In the space of 3D microporous crystals, we have also found that using larger SDAs or a lower concentration of a given SDA generate crystals with larger rings.