title: 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 with13C 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 observed13C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the13C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of13C 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.
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Salim
authors.initials: S
authors.lastname: Ok
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: David W
authors.initials: DW
authors.lastname: Hoyt
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Amity
authors.initials: A
authors.lastname: Andersen
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Julie
authors.initials: J
authors.lastname: Sheets
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Susan A
authors.initials: SA
authors.lastname: Welch
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: David R
authors.initials: DR
authors.lastname: Cole
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Karl T
authors.identifier: 0000-0001-9609-9516
authors.initials: KT
authors.lastname: Mueller
authors.affiliation: School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States.|Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States.
authors.forename: Nancy M
authors.identifier: 0000-0002-9643-6794
authors.initials: NM
authors.lastname: Washton