Role of Liquid vs Vapor Water in the Hydrothermal Degradation of SBA-15

The hydrothermal stability of mesoporous silica is critical for applications including catalytic processing of biofuels due to the presence of significant amounts of water. We have combined neutron diffraction intensity analysis with NLDFT analysis of nitrogen sorption isotherms to characterize the spatial distribution of the secondary pore network in SBA-15 following postcalcination hydrothermal treatment in both liquid and vapor phase water at temperatures from 115 to 155 °C under autogenous pressure. The results are consistent with a degradation mechanism in which silica dissolves from regions of small positive curvature, e.g., near the entrance to the secondary pores, and is redeposited deeper into the framework. Pore volumes decrease fastest for the micropores and more slowly for larger secondary mesopores. Under water treatment at 115 °C, the mesopore diameter increases and the intrawall void fraction decreases significantly. The behavior is similar for steam treatment but occurs more slowly. Differ...