Spectroscopic insight into post-synthetic surface modification of porous glass beads as a silica model system

Applications in catalysis, adsorption and separation require high surface areas as provided by mesoporous materials. Particularly attractive is the class of silica-based mesoporous glasses, which are mechanically and chemically very stable and post-synthetically modifiable allowing specific surface properties to be introduced. One of the catalytically relevant moieties is the sulfonic acid group. To optimize the performance of mesoporous glass systems, analytical methods are required to determine the state of surface modification and its effect on the porosity. To this end, we here propose a specific combination of spectroscopic methods: The porosity during the introduction of thiol functionalities and subsequent oxidation into sulfonic acid groups on the surface of porous micro glass beads is investigated using hyperpolarized ¹²⁹Xe NMR, revealing that during the two modification steps the textural properties are preserved. The grafting mode as well as the surface coverage are determined using ²⁹Si MAS NMR. The oxidation step is demonstrated to be complete as probed by Raman spectroscopy and ¹³C MAS NMR. Our combined analysis demonstrates the successful and complete surface modification as well as the maintenance of the free accessibility of the mesopore system.