Correlating phase behaviour and diffusion in mesopores: perspectives revealed by pulsed field gradient NMR

Abstract

Porous solids represent an important class of materials widely used in different applications in the field of chemical engineering. In particular, mesoporous hosts attract special attention due to their fascinating match of transport, geometrical and chemical properties. Not only a very high specific surface area, accessible for adsorption and heterogeneous catalysis, but also their efficient transport properties are the key factors determining optimal use of these materials. Therefore, a fundamental understanding of the correlations between the phase state of confined fluids, their transport properties and the geometrical features of confinement are of particular importance. Among the different analytical techniques, nuclear magnetic resonance (NMR) is especially suited to cover various crucial aspects of the highlighted issues. In this work, we provide a short overview of recent advances related to the interrelations of phase behaviour and diffusion in mesoporous materials studied using various NMR techniques.

Structural modelling of silicon carbide-derived microporous carbon and its application in CO2 capture and separation of volatile gases from moist streams

“…Moreover, according to Karger and Ruthven [85], intracrystalline self-diffusivities can demonstrate different patterns of concentration dependence, ranging from constantly descending or ascending self-diffusivities to a pattern yielding a self-diffusivity variation with a maximum, which occurs in the presence of heterogeneity. Such diffusion patterns are thoroughly discussed by Keil et al for Zeolites [86]. Heterogeneity of SiC-DC structure is evident in our simulation results of the isosteric heat of adsorption, shown in …”

“…Here, T is temperature, [ ( )] is the intrinsic Helmholtz free energy functional and ( ) is the external solid-fluid potential imposed by the pore walls [86].…”

The Impact of Mesopores on Mass Transfer in Nanoporous Materials: Evidence of Diffusion Measurement by NMR

“…This concept is used to illustrate the potentials of PFG NMR for exploring the impact of the mesopores on diffusion in hierarchical porous media. Further details of the measuring technique are given in the relevant literature [25,[40][41][42][43]49]. …”

“…Among the various techniques of microscopic diffusion measurement, the pulsed field gradient technique of NMR (PFG NMR -sometimes also referred to as PGSE (pulsed gradient spin echo) NMR technique) is distinguished by its versatility and robustness [25,[39][40][41][42][43]. These characterisitcs make it a promising method for the observation of mass transfer phenomena in hierarchical pore spaces and for their assessment with reference to their exploitation for the fabrication of transport-optimized materials.…”

“…As a main prerequisite of such techniques the observed mass transfer phenomena have to be determined by the pore space of the adsorbent particles/crystallites rather than by bed effects or by resistances on their surface operating in addition to the resistance of the inner pore space [17][18][19]. Measurements of this type may be performed by Quasi-Elastic Neutron Scattering (QENS) [20][21], by Pulsed Field Gradient NMR (PFG NMR) [22][23][24][25], by micro-imaging by interference microscopy (IFM) [26], and by IR microscopy (IRM) [27].…”