Effect of confinement on the characterization of nanoporous materials by NMR relaxometry

Valid textural characterization is crucial for many applications such as catalysis, separation as well as energy storage/conversion. In that regard, textural characterization in the gas/dry state using gas physisorption and mercury porosimetry is well established, but these methods might not be sufficient for the characterization of wet materials used in liquid-phase processes. Within this context, the applicability of nuclear magnetic resonance (NMR) relaxometry for surface area assessment of nonporous silica/carbon materials has been demonstrated [Schlumberger et al. (2023). https://doi.org/10.1021/acs.langmuir.2c03337]. However, a comprehensive and rigorous assessment of the applicability of NMR relaxometry for surface area and pore size assessment of nanoporous materials coupled with a systematic investigation of how the confinement affects the NMR relaxation behavior is missing so far. Hence, we present here a systematic study based on a series of ordered mesoporous silica model materials exhibiting well-defined pore sizes between approx. 2.5 and 10 nm saturated with a bulk liquid water as well as a bulk water vapor phase. The study suggests that an adaption of the two-fraction-fast-exchange model to account for the pore geometry is necessary for valid surface area assessment as well as pore size analysis of nanoporous silica material particularly for pores smaller than approx. 10 nm.