Low‐Field NMR Relaxation Analysis of High‐Pressure Ethane Adsorption in Mesoporous Silicas

Abstract

Understanding the behaviour of short‐chain hydrocarbons confined to porous solids informs the targeted extraction of natural resources from geological features, and underpins rational developments in separation, storage and catalytic conversion processes. Herein, we report the application of low‐field (12.7 MHz) 1H nuclear magnetic resonance (NMR) relaxation measurements to characterise ethane dynamics within mesoporous silica materials exhibiting mean pore diameters between 6 and 50 nm. Our measurements provide NMR‐based adsorption isotherms within the range 25–50 bar and at ambient temperature, incorporating the ethane condensation point (40.7 bar at our experimental temperature of 23.6 °C). The quantitative nature of the acquired data is validated via a direct comparison of NMR‐derived excess adsorption capacities with ex situ gravimetric ethane adsorption measurements, which are demonstrated to agree to within 0.2 mmol g−1 of the observed ethane capacity. NMR T2 relaxation time distributions are further demonstrated as a means to decouple interparticle and mesopore dominated adsorption phenomena, with unexpectedly rapid relaxation rates associated with interparticle ethane gas confirmed via a direct comparison with NMR self‐diffusion analysis.

NMR Relaxation of Gas Adsorbed in Microporous Material

“…The other is then the CH 4 in the intercrystal space, which has a T 2 shorter than the bulk gas presumably due to the fast diffusion of gas molecules in the internal gradient. 59 As 1/ T 2 ad = R 2 G + R 2 S and R 2 G can be calculated using eq 1 if the adsorption capacity is determined, the surface relaxation rate R 2 S can be obtained and used to calculate the self-diffusivity by combining eq 3 and eq 4. The results show good correspondence with the pulse field gradient NMR (PFG NMR) data measured at 298 K 11,60 and the quasi-elastic neutron scattering (QENS) data which are measured at 200 K and extrapolated to 300 K, 61 as shown in Figure 3a.…”

“…One of the intrinsic T 2 is longer than the bulk gas, so it is designated to be the adsorbed CH 4 . The other is then the CH 4 in the intercrystal space, which has a T 2 shorter than the bulk gas presumably due to the fast diffusion of gas molecules in the internal gradient . As 1/ T 2 ad = R 2 G + R 2 S and R 2 G can be calculated using eq if the adsorption capacity is determined, the surface relaxation rate R 2 S can be obtained and used to calculate the self-diffusivity by combining eq and eq .…”

Unravelling the structure of CO2 in silica adsorbents: an NMR and computational perspective

“…Owing to the sensitivity to probe adsorbate-adsorbate and adsorbate-adsorbent interactions, NMR relaxation methods have been used to quantify adsorbed gases on solids enabling to register adsorption isotherms. [109][110][111][112][113][114][115][116][117][118] In general, this approach covered a wide range of materials including zeolites, mesoporous silica, activated carbons and MOFs which were probed for the adsorption of CO 2 , CH 4 , C 2 H 6 , C 3 H 8 and also vapours of MeOH, PrOH and H 2 O for gas capture/separation and other emerging applications. In most cases, these experiments were limited to the use of low-field magnets and carried out ex situ.…”

“…One alternative was found by integrating NMR with either a sorption analyser or a homemade gas dosing system. [115][116][117][118][119] For instance, the adsorption of MeOH vapours was studied in the sorption apparatus coupled with the lowfield NMR spectrometer providing simultaneous in situ and in operando measurements. Information on pore filling, structural heterogeneity and host phase transition was obtained.…”

“…Crystal-like silica models. The intercalation of organic/inorganic groups into the pore walls of crystalline-like materials, such as PMO, is more attractive for computational studies due to the easiness in modelling their structures, thereby rendering them suitable for the generation of both periodic 46,116,118 and cluster models 123,125 of the surface PMO walls. Two types of periodic models of PhPMO material were constructed for the first time by Martinez and Pacchioni 122 to study the interaction between their surface and CO 2 and CH 4 molecules.…”