A New Pore Shape Analysis Method Based on the Adsorption-Controlled ¹²⁹Xe-NMR Technique

In crystalline porous materials, pore shapes can be determined using analytical techniques, such as transmission electron microscopy and X-ray diffraction. However, pore shape analysis for amorphous nanoporous materials remains a longstanding challenge. In our previous study, we confirmed that, for microporous activated carbons with slit-shaped pores, the interaction between the pore surface and an adsorbed xenon (Xe) molecule (δ_S) depends only on a parameter related to pore size─the ratio of the pore volume and specific surface area (V/SSA)─and we verified the effectiveness of adsorption-controlled Xe isotope-nuclear magnetic resonance (¹²⁹Xe-NMR) for evaluating the pore size of the slit-shaped porous carbons. In the present study, we examined the potential of this ¹²⁹Xe-NMR technique for pore shape analysis in amorphous nanoporous materials using various carbon- and silica-based nanoporous materials with pores of different shapes ranging in size from micropores to mesopores. By using the inverse pore size factor (SSA/V), we found that nanoporous materials with cylindrical pores exhibited clearly different δ_S-SSA/V plots from those of slit-type nanoporous materials. At the same pore size, δ_S values were higher for cylindrical pores than for slit-shaped pores, consistent with the stronger solid-fluid interaction for the former estimated by integrated Lennard-Jones calculations. Moreover, in δ_S-V/SSA plots, distinct correlations for each nanoporous material series were obtained, corresponding to the pore connectivity gradient. These results demonstrate the potential of adsorption-controlled ¹²⁹Xe-NMR for analyzing the pore shape and connectivity as well as the pore size in various nanoporous materials.