Structure and Dynamics of Ethane Confined in Silica Nanopores in the Presence of CO2

Published

February 28, 2020

Author(s)

Tingting Liu, Siddharth Guatam, David R. Cole, Sumant Patankar, David Tomasko, Wei Zhou, Gernot Rother

Abstract

Fundamental understanding of subcritical/supercritical behavior of key hydrocarbon species inside nanoporous matrices at elevated pressure and temperature is less developed compared to bulk fluids, but this knowledge is needed for efficient utilization of the hydrocarbon resources in subsurface energy systems. This study explores in detail the structure and dynamics of ethane (C₂H₆) fluid confined in silica nanopores, with a focus on the effects of pressure and different ratios of C₂H₆ and CO₂ at non-ambient temperature. Quasi-elastic neutron scattering (QENS) experiments were carried out for the pure C₂H₆, C₂H₆: CO₂ =3:1, and 1:3 mixed fluids confined in 4-nm cylindrical silica pores at three different pressures (30, 65, and 100 bar) at 323 K. Two Lorentzian functions were required to fit the spectra, corresponding to fast and slow translational motions. No localized motions (rotations, vibrations) were detected. Higher pressures resulted in hindrances of the diffusivity of C₂H₆ molecules in all systems investigated. Pore size was found to be an important factor, i.e., dynamics of confined C₂H₆ is more restricted in smaller pores. Molecular dynamics (MD) simulations were performed to complement the QENS experiment at 65 bar. The simulations indicate CO_{2 molecules are more strongly attracted to the pore surface compared to C}d2^H₆. The C₂H₆ molecules adsorbing to the pore surface (for pure C₂H₆ system) or the CO_{2 molecules in the mixtures form a dense first layer (L1) close to the pore surface and a second less dense layer extending into the pore center (L2). Both the experiments and simulations revealed the role that CO}d2 molecules play in enhancing C₂H₆ diffusion ('molecular lubrication') at high CO_{2: C}d2^H₆ ratios. The energy scales of the two dynamic components, fast and slow, are in good agreement between experiments and simulations. The simulations also identified the fast component as the main contributor to the dynamics. Molecule motions in the L2 region are mostly responsible for the dynamics (fast and slow) that can be detected by the instrument. The simulation data shows that although the energies involved in the rotational motions of C₂H₆ fall into the energy window accessible to the instrument, the contribution of the rotational motion to the experimental spectra is negligible.

Citation

Journal of Physical Chemistry

Volume

152

Issue

8

Pub Type

Journals

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Keywords

Nanoporous Material, Quasi-elastic neutron scattering, Molecular dynamics

Citation

Liu, T. , Guatam, S. , Cole, D. , Patankar, S. , Tomasko, D. , Zhou, W. and Rother, G. (2020), Structure and Dynamics of Ethane Confined in Silica Nanopores in the Presence of CO₂, Journal of Physical Chemistry, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=928740 (Accessed May 7, 2024)