(228b) Effect of Confinement on the Characterization of Nanoporous Materials By NMR Relaxometry

During the last decades, major progress has been made concerning the synthesis of nanoporous materials allowing for the custom design of nanoporous materials for targeted applications in various areas such as separation, catalysis, gas and energy storage. Enhancing the efficiency of these processes requires the tuning of the selectivity of the porous material to certain compounds of interest. Textural properties, such as the specific surface area and pore (entrance) size may affect the process efficiency. Hence, valid textural characterization methodologies are essential for surface area assessment as well as pore size analysis. Within this context, gas adsorption is widely used; however, its application is limited to dry materials, i.e., gas adsorption cannot determine effective textural properties of wet porous materials utilized in liquid phase processes such as chromatography and catalysis.

Characterization in the liquid phase is especially important for functionalized materials, where the interaction of surface functional groups with the chosen liquid may affect the effective accessible surface area and pore structure during their application. Hence, the development of novel methodologies for textural characterization of solvated porous materials is required. Within this context, we have recently shown that NMR relaxometry can be developed into a methodology for surface area assessment of nanoporous materials immersed in a liquid phase [1]. Moreover, it can be a valuable tool for the fast characterization of the wettability of nanoporous materials based on the ratio of spin-lattice to spin-spin relaxation time [2,3]. Beyond this, we have demonstrated that NMR relaxometry shows potential to be developed as novel methodology for fast determination of pore entrance sizes. In this sense, the choice of probe molecules with varying kinetic diameters enables one to get information about the pore entrance size due to their different accessibility to the pore system [1]. Based on that, this study presents further development of applying NMR relaxometry for fast molecular sieving to zeolites.

Moreover, we systematically investigate the effect of confinement and pore network characteristics on NMR relaxation using well-defined meso- and microporous model materials. This 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. The results emphasize that applying the adapted model instead of the original two-fractions-fast-exchange model assuming a planar surface results in a significant improvement of the NMR T2 surface areas as well as pore sizes for the investigated samples.

References:

[1] C. Schlumberger, L. Sandner, A. Michalowski, M. Thommes. Reliable Surface Area Assessment of Wet and Dry Nonporous and Nanoporous Particles: Nuclear Magnetic Resonance Relaxometry and Gas Physisorption. Langmuir 2023, 39 (13), 4611–4621.

[2] C. D’Agostino, J. Mitchell, M.D. Mantle, L. F. Gladden, Chemistry - A European Journal, 20 (40), (2014) p. 13009

[3] C. Schlumberger, C. Cuadrado Collados, J. Söllner, C. Huber, D. Wisser, H-.-F. Liu, C.-K. Chang, S. A. Schuster, M. R. Schure, M. Hartmann, I. Siepmann, M. Thommes, ACS Applied Nano Materials, 7 (2), (2024) p. 1572.