Kinetic Monte Carlo Simulation of the Synthesis of\nPeriodic Mesoporous Silicas SBA‑2 and STAC-1: Generation of\nRealistic Atomistic Models

Abstract

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