Quantitative ²⁹Si NMR Spectroscopy of Ordered Mesoporous Silicas: Revisiting Q₃/Q₄ Ratios and Surface Hydroxylation in SBA-15 and MCM-41

Abstract

Nuclear Magnetic Resonance (NMR) spectroscopy, particularly involving the ²⁹Si nucleus, is a key tool in understanding the structure and reactivity of silicon-based materials. This study presents a refined quantitative analysis of Q₂, Q₃, and Q₄ environments in the most representative ordered mesoporous silicas – SBA-15, SBA-16, MCM-41, and MCM-48—using solid-state ²⁹Si MAS (Magic Angle Spinning) NMR spectroscopy in combination with advanced deconvolution strategies. The findings of this study, which correct previous overestimations of surface silanol content, are crucial for a more accurate understanding of these materials. Our results reveal that SBA-15 has a higher degree of siloxane network condensation (dominance of Q₄ sites) and a significantly lower surface silanol (Q₃) content than previously reported. This refined interpretation of ²⁹Si spectra indicates that the surface of SBA-15 is less hydroxylated, leading to reduced hydrophilicity, diminished hydrogen bonding potential, and altered acid-base characteristics. These structural insights have direct implications for the behavior of these materials in post-synthetic functionalization, adsorption, catalysis, and rehydration processes, enhancing our understanding of their practical applications. This study underscores the critical importance of precise spectral deconvolution in assessing the surface chemistry of mesoporous silicas. It also highlights the necessity of reevaluating earlier data, as demonstrated by our findings. By correlating spectral parameters with structural features, we provide a more accurate and realistic understanding of silicon-oxygen network organization in mesoporous frameworks. These findings not only open new directions for designing and optimizing functional silicate-based materials but also emphasize the need for a critical reevaluation of existing data in the field.