Quantitative 29Si NMR spectroscopy of ordered mesoporous silicas: revisiting Q3/Q4 ratios and surface hydroxylation in SBA-15 and MCM-41

Abstract

This study highlights a method for analyzing the structural features of mesoporous silicates (MCM-41, MCM-48, SBA-15, and SBA-16) using ²⁹Si NMR spectroscopy, with a focus on the distribution of Q_n (n = 0–4) silicon environments. This approach not only provides quantitative insights into the condensation and hydroxylation levels within the silica frameworks but also allows for a re-evaluation of previous results and understanding of the material properties. For example, the method reveals that SBA-15 has a higher proportion of fully-condensed Q₄ sites, challenging earlier assumptions about its surface hydroxylation. By employing advanced deconvolution techniques, the study offers a more accurate characterization of these materials, particularly in terms of their surface chemistry, rehydration behaviour, and functionalization potential. This revised understanding is crucial for improving predictions of material behaviour and for optimizing its use in applications such as catalysis and gas separation. The proposed method thus enables a rethinking of the properties of mesoporous materials, offering more profound insight into their structure–function relationships.