Probing Pore Size and Connectivity in Porous Silicas Using ¹³C MAS NMR Spectroscopy of Supercritical Methane

Abstract

Measuring the pore size and pore-size distributions and exploring the fluidexchange dynamics between different types of pores in porous materials remains a significant experimental challenge but is critical to understanding catalysis, chromatography, nutrient cycling, and a whole range of geochemical phenomena, including shale gas and tight gas extraction. Here, we present the results of 1D 13 C NMR and 2D exchange spectroscopy (EXSY) NMR investigations of a porous silica using supercritical methane (scCH 4 ) as a direct probe of pore size and fluid exchange between pore types. The results show that the 13 C chemical shift of scCH 4 adsorbed in nanometer-scale silica pores becomes more negative with increasing pore diameter, in agreement with trends reported for gas hydrates, zeolites, MOFs, and clays and other microporous (<10 nm pores) geochemical materials. These 13 C chemical shifts follow a natural log−linear trend with pore size in the vacuum-dried porous silicas studied here, allowing one to predict pore size based on the 13 C chemical shift in dry silica nanopores. The EXSY and 1D 13 C NMR results both show that CH 4 exchanges between pore and bulk fluid environments over rate scales from 0.01 to 1 kHz in vacuum-dried samples and that exchange dynamics must be considered when interpreting 13 C NMR of pore-adsorbed CH 4 . Introducing H 2 O to the system causes the 13 C chemical shift to become more negative with increasing H 2 O content as a result of H 2 O preferentially filling small pores, forcing CH 4 to occupy larger pores that are better connected to the bulk environment. Likewise, the 1D 13 C data show a decreased distribution of exchange rates between pore-adsorbed and bulk CH 4 in the presence of H 2 O.

Chemistry and Dynamics of Supercritical Carbon Dioxide and Methane in the Slit Pores of Layered Silicates

“…MD-derived plots showing the number of H 2 O (black) and CO 2 (red) per unit cell in simulations of hectorite with different F/OH ratios in the framework as a function of the basal spacing (an approximate basal spacing of 10 Å gives the gallery height). Reproduced with permission from ref . Copyright 2019 American Chemical Society.…”

“…The earliest work using these high-pressure tools examined CO 2 reactions with nonporous minerals in basalt, such as the forsterite/fayalite continuum, since one potential CO 2 sequestration approach is to trap CO 2 via reaction with basaltic glass. ,,− These studies provided key insights into the reactions that occur as well as their mechanisms, rates, and the factors that control them. Other pioneering work examined supercritical CH 4 (scCH 4 ) behavior in model cylindrical silica pores, establishing the mechanism behind the 13 C chemical shift changes for confined scCH 4 , how it varies with pressure, and a relationship between the 13 C chemical shift and the mean dynamic pore size. − This Account discusses another issue in CO 2 sequestration and shale-gas/tight-gas extraction, namely, understanding the interactions of variably wet scCO 2 and scCH 4 with smectite minerals and their slit-shaped micro- and mesopores. Smectite minerals are layered silicates (phyllosilicates) with a permanent negative charge due to isomorphic substitutions in the smectite framework (e.g., Fe 2+ , Li + for Al 3+ or Mg 2+ , respectively).…”

Surface Interactions and Nanoconfinement of Methane and Methane plus CO2 Revealed by High-Pressure Magic Angle Spinning NMR Spectroscopy and Molecular Dynamics

“…The 13 C NMR chemical shift position became more positive when the accessible physical area of the clay for CH 4 was decreased. Bowers et al [28] focused on probing the dimension and connectivity of engineered nanoporous silica with pore sizes of 2.5, 5.0, 10.0, and 20.0 nm by applying high-pressure 13 C MAS-NMR and using supercritical CH 4 . When the pore diameter was increased, the 13 C shift of supercritical CH 4 adsorbed in the nanoporous silica materials became more negative.…”

“…In contrast, we confined methane into synthetic engineered proxies of nanoporous silica and varied the pressure. Compared to Bowers et al [28], we utilized white powder nanoporous silica down to a pore diameter of 1.5 nm with relatively higher surface areas than porous monolith disks. In the present contribution, we tested the influence of CO 2 in gaseous and supercritical states along with computational efforts, which are not explored in the reports by Bowers et al [27,28].…”