(520a) Advances in Surface Chemistry Assessment of Adsorbents By Combining Adsorption, Liquid Intrusion and NMR Spectroscopy

In this work, we focus on the development of a comprehensive strategy/methodology for assessing the effective surface chemistry of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques and solid-state NMR spectroscopy.

For our study we have chosen the highly ordered mesoporous silica molecular sieve SBA-15 and SBA-15 silica functionalized with methyl groups (methylation with hexamethyldisiazane) as model materials allowing for a surface characteristics with different degree of surface functionalization. A full textural characterization of the materials could be obtained by high-resolution argon and nitrogen adsorption experiments at 87 K and 77 K, respectively, coupled with the application of dedicated methods based on non-local-density functional theory.

The applied strategy for surface chemistry characterization is based on the use of adsorptives exhibiting different wettability such as argon adsorption at 87 K, which leads to a complete wetting adsorbate (effective contact angel is zero) independent of the effective surface chemistry, and water adsorption at various temperatures (273 K - 323 K), which is sensitive to the underlying surface chemistry. Depending on the effective hydrophilicity/hydrophobicity of the adsorbent surface, water may adsorb at pressures P smaller than the bulk saturation pressure P₀ (i.e., at P/P₀ £ 1) in case the water adsorbate is completely or at least partially wetting (i.e. effective contact angle < 90°). In case of a fully methylated surface, argon at 87 K still forms complete wetting films, while water adsorption leads to a non-wetting adsorbate (effective contact angle > 90°) indicative of pore filling occurring at pressures larger than the bulk saturation pressure (i.e., at P/P₀ > 1). In the latter case we investigate the pore filling of water at pressures > P₀ by the application of novel water intrusion/extrusion experiments.

Coupled with detailed knowledge about the pore structure (textural characterization) and isoteric heats of adsorption data (obtained from argon and water adsorption measurements at various temperatures), the obtained results allowed us to develop a methodology for quantifying the effective hydrophilicity/hydrophobicity (e.g., hydrophilicity index) and the corresponding effective contact angles.

By coupling these results with advanced, dedicated ¹H, ¹³C and ²⁹Si MAS NMR studies, we are able to assess the chemical nature and surface density of different functional groups, as well as to determine the location of the functional groups (outer surface vs. mesopores or micropores).

Our results and the developed strategy/methodology are essential for the surface characterization of siliceous nanoporous materials for applications in heterogeneous catalysis and separation such as chromatography.