(669d) Combining Novel Methodologies Based on NMR Relaxometry and Gas Adsorption for a Reliable Surface Area Analysis of Nanoporous Materials

In the past, major progress has been made concerning the textural characterization of porous materials in the gas/dry state. The main drawback of these methods stems from their inability to test wet materials (i.e., in presence of the liquid/mobile phase), thus providing no direct information concerning the state of the nanoporous materials immersed a liquid phase, which is important for many applications (e.g. chromatographic operations). This is particularly important for functionalised nanoporous materials, where the interaction of surface functionals with the chosen liquid may affect the effective surface area and pore characteristics for nanoparticle separation. Hence, for the optimization of processes in the liquid phase (e.g. liquid chromatography) characterization only based on gas adsorption and liquid intrusion techniques (e.g., mercury porosimetry) is not sufficient. However, far fewer advances have been made in assessing textural properties in the liquid phase.
One of the important challenges in liquid phase characterisation is a reliable and fast assessment of surface area. For this, the application of NMR relaxation measurements has recently received a lot of attention. The NMR relaxation measurement method is based on the fact that the liquid in contact with a surface behaves differently from that of the bulk phase (or “free” liquid) in response to a changing magnetic field, i.e., a material with a high surface area has a shorter relaxation time, as more fluid is bound to the surface. Despite the fact that the application of NMR relaxometry for textural characterization of various material types (including silicas and titania, carbons and even metal organic framework materials (MOFs)) has been reported in the literature, a rigorous and systematic validation of the method utilizing materials with true surface area benchmark data has not yet been reported. One problem is that many of the materials used in the reported studies contain a significant amount of microporosity (e,g,, MOFs) and here the Brunauer-Emmett-Teller (BET) method cannot be applied in a straightforward way for determining reliable, probe accessible surface area data. Additional uncertainties result from the choice of nitrogen as the probe molecule, leading to errors in assessing surfaces up to 20-30 % ; due to its quadrupole moment, the orientation of a nitrogen molecule is very much dependent on the surface chemistry of the adsorbent.

In this work the surface area of nonporous and mesoporous silica model adsorbent materials immersed in a liquid phase is investigated by methodologies based on NMR relaxometry and compared with benchmark adsorption data (by applying procedures based on argon adsorption). We further investigate the effect of confinement and pore network characteristic on NMR relaxation and consequently on a reliable assessment of surface area, but also explore the potential of NMR relaxation for pore size analysis.