(649a) Computational Study on the Thermal Degradation Mechanism and Gas Adsorption Properties of Mesoporous Silica MCM-41 after High Temperature Treatment

Computational modeling is a powerful tool to investigate high temperature processes that are not experimentally facile. In particular, MCM-41-type mesoporous silica, which has potential applications in the capture and storage of gases, was studied here with both Molecular Dynamics (MD) simulation and Monte Carlo (MC) simulation in order to investigate the porous structure transformation under high-temperature treatment up to 5000 K, and its resulting gas adsorption/separation properties. It was found that the process starts with the breakage of Si-O bonds in the solid matrix, followed by reformation at the surface, ultimately leading to pore collapse. This degradation process is dependent on several factors including heating rate, such that complete pore closure is postponed at high heating rates as simulated by MD. By applying the Kissinger equation, a strong correlation between simulated pore collapse temperature and experimental values was found and yielded activation energy of 373.8 kJ/mol for the pore closure. Monte Carlo simulation was then applied to better understand how the original MCM-41 and post-thermally treated structure (2650 K) behave when adsorbing/storing different noble gases at 300 K for pressures varying from 1 kPa to 10000 kPa. Our study revealed the rougher surface and smaller pore size of the 2650 K model can increase the adsorbate-adsorbent interaction due to a curvature effect, and the phenomenon is amplified with the increase of gas molecule size. The selectivity between two different noble gases was then found to be improved by the thermally treated model.