Surface Structure and Acidity Properties of Mesoporous Silica SBA-15 Modified with Aluminum and Titanium: First-Principles Calculations

摘要

Ordered mesoporous silica materials are considered promising supports for the development of novel hydroprocessing catalysts. Specifically, the mesoporous silica SBA-15 exceeds because of high specific surface area, wall thickness, and pore size distribution, features that make this material more stable at environments at which hydroprocessing reactions take place. However, the SBA-15 lacks strong Bronsted acid sites and this fact still hinders its widespread commercial use. In this work we report density functional theory analyses of the structure and acidity properties of the SBA-15 surface. Periodic boundary models are used and the temperature dependence of the Silanol surface density is taken into account. Surface modification by isomorphic substitutions with aluminum and titanium is investigated. It is found that aluminum substitution favors creation of bridging-hydroxyl groups. However, surface modifications with aluminum and titanium also create local structural distortions inducing H-bond interactions which improve the acidity properties of hydroxyl groups on the surface. Calculation of vibrational frequencies of O-H bonds are used to quantify the surface acidity properties.