Kinetic Monte Carlo simulation of the synthesis of periodic mesoporous silicas SBA-2 and STAC-1: Generation of realistic atomistic models

SBA-2 and STAC-1 are two related periodic mesoporous silicas (PMSs) that have regular networks of spherical, interconnected pores; the pores are similar in the two materials but the networks differ in their symmetry. The nature of the interconnected network of pores in these materials gives rise to interesting properties related to their potential use in separation processes. In this work, we extend a kinetic Monte Carlo (kMC) technique, originally derived for MCM-41, a simpler PMS, and apply it to mimic the condensation, aggregation, deformation, and calcination stages of the synthesis of SBA-2 and STAC-1. Our simulated synthesis results suggest that the pores are connected through windows formed during micelle aggregation because of the close packing of the spherical micelles and the presence of water molecules at the silica-micelle interface. The simulated materials were validated by comparing properties such as unit cell size, pore size, pore shape, and wall density to results from experimental X-ray diffraction (XRD), transmission electron microscopy (TEM), density measurements, and ²⁹Si NMR. Quantitative agreement between simulated and experimental nitrogen isotherms was achieved demonstrating the realism of the pore models obtained by the kMC simulations. Our results highlight the importance of a realistic, rough pore surface for the prediction of adsorption at low pressures in these materials.