Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study

Abstract

Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observedC chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values ofC nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.

The Influence of Nanoporosity on the Behavior of Carbon-Bearing Fluids

“…In the absence of water, CO 2 did enter the NH 4 and Cs clays, but was excluded from the Na clay. A weak intercalation of CH 4 has been observed for smectites, 137 but is more pronounced for 4-nm weakly hydrated silica pores, as demonstrated by Ok et al 138…”

Fluid Behavior in Nanoporous Silica

“…This long range ordered structure is consistent with the single peak (∼4.2 nm) in the pore size distribution plot ( Figure S1) determined from gas sorption analysis. In addition, the TEM imaging of this nanoporous silica shows an ordered arrangement of about 4.0 nm pores in parallel (Ok et al, 2017). Figure S2b shows the five diffraction maxima observed in the TXRD scan of silica-2.5 nm, with d-spacings 3.1, 2.1, 1.9, 1.5, and 1.3 nm, respectively.…”

“…This long range ordered structure is consistent with the single peak (~4.2 nm) in the pore size distribution plot ( Figure S1 ) determined from gas sorption analysis. In addition, the TEM imaging of this nanoporous silica shows an ordered arrangement of about 4.0 nm pores in parallel (Ok et al, 2017 ).…”

Catalyst characterization in the presence of solvent: development of liquid phase structure–activity relationships

“…Recently, customized NMR rotors have been designed to withstand conditions as extreme as 523 K and 200 bar, making solid state NMR a robust technique for the in situ monitoring of reactions. 34 , 35 Additionally, solid state NMR with magic-angle spinning (MAS) is particularly well suited for differentiating substrates that are adsorbed versus those remaining in bulk solution. 6 This is because adsorbed species typically result in significant peak broadening due to the restricted mobility or heterogeneities in the adsorbed structures.…”