(506b) Reliable Textural Characterization of Solvated Nanoporous Materials By NMR Relaxometry – Experiment and Molecular Dynamic Simulation

In the past, major progress has been made concerning the textural characterization of porous materials in the gas/dry state [1]. However, these methods are not sufficient for the characterization of wet materials utilized in liquid phase processes, e.g. catalysis and liquid chromatography, thus providing no direct information concerning the state of the nanoporous materials immersed in a liquid phase. This is particularly important for functionalized nanoporous materials, where the interaction of surface functional groups with the chosen liquid may affect the effective surface area and pore characteristics accessible during their applications. NMR relaxometry is a promising method for assessing textural properties of porous solids in the liquid phase. Although NMR relaxometry has been used for surface area (e.g. [2-5]) and pore size (e.g. [6]) analyses, a systematic and rigorous assessment of the applicability of NMR relaxometry for reliable surface and pore size characterization utilizing materials with true surface area and pore size benchmark data has not been reported. The application of NMR relaxometry is based on the fact that the relaxation signal of a liquid in contact with a surface behaves differently from that of the bulk phase (or “free” liquid). Hence, a material with a higher surface area has a shorter relaxation time, as more fluid is bound to the surface.
In this work, we present a systematic study in which we assess the applicability of NMR relaxometry for reliable surface area assessment utilizing for the first time true surface area benchmark data based on argon 87 K adsorption on nonporous and nanoporous model adsorbent materials (silica and carbon black) coupled with the development of an advanced methodology including the investigation of the choice of probe molecules and the effect of the pore network accessibility [7]. Our results confirm that NMR relaxometry provides fast and reliable surface areas for these model materials. In addition, our work clearly demonstrates the potential of NMR relaxometry for the targeted surface area assessment of defined pore classes using probe molecules with varying kinetic diameters. Furthermore, we suggest a novel and fast molecular sieving methodology for the assessment of the sizes of pore entrances (pore window size). Complementary to textural characterization, NMR relaxometry (based on the ratio of spin-lattice to spin-spin relaxation time) may be a valuable tool for the characterization of wetting characteristics and surface chemistry [8]. Our results show that the T1/T2-ratio of functionalized nanoporous materials can be related to the contact angle of adsorbates on surfaces and based on that could be developed into a unique tool for assessing their wetting characteristics. We systematically investigate the effect of confinement and pore network characteristics on NMR relaxation using well-defined meso- and microporous model materials. This work will be complimented by a systematic molecular dynamics (MD) simulation study on how the confinement affects NMR relaxation times. MD simulations have been already used to study NMR relaxation behavior of pure bulk water and hydrocarbons [9], mixtures of hydrocarbons and polymers [10] and non-wetting fluids confined in silica pores [11]. However, a systematic theoretical study on how the pore size affects NMR relaxation of water confined in silica model pores has not been presented.

This study is a first step towards enabling the application of NMR relaxometry for reliable pore size analysis. In addition, it will also allow us to investigate to what extent confinement and the pore size has to be considered for the application of NMR relaxometry for reliable surface area assessment [7].

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