(466d) High-Pressure CO2 and CH4 Transport in Smooth Crystalline Silica Mesopores:a Molecular Dynamics Study

The knowledge about gas transport under nano-confinement plays crucial roles in diverse engineering applications, such as catalytic reactions and high-performance membranes for gas separation, etc. In particular, high-pressure CO₂ and CH₄ transport in silica mesopores are important for geological carbon sequestration (GCS) and subsurface energy extraction in shale/tight reservoirs, as silica is one of the most common minerals in subsurface formations.

In general, high-pressure gas transport is dominated by convection (driven by pressure gradient) and regarded as continuum flow, which can be described by the hydrodynamic Hagen-Poiseuille (HP) equation with no-slip boundary condition, assuming homogeneous fluid distributions. However, Holt et al. (Science 2006, 312 (5776), 1034-1037) reported that fluid flow enhancement over the HP equation with no-slip boundary condition can be several orders of magnitude in sub-2 nm carbon nanotubes (CNTs). Such enhancement is attributed to specular reflections thanks to the smooth CNT surfaces, which is confirmed by molecular dynamic (MD) simulations. The general understanding is that gas molecules display significant slippage on smooth crystalline surfaces (such as graphite and quartz), but much-dampened slippage on rough amorphous surfaces (such as kerogen). In addition, Firouzi and Wilcox (The Journal of chemical physics 2013, 138, 064705) reported that CO₂ shows less significant slippage compared to CH₄ in carbon nanopores, which is attributed to stronger gas-surface interactions. Recently, Qian et al. (Applied Surface Science 2023, 611, 155613) observed that specular reflections on the smooth crystalline surface are also dependent on fluid molecular characteristics. They found that gas molecules with more complex geometries and configurations incline to have more diffuse reflections. On the other hand, Qian et al. (Carbon 2021, 180, 85-91) reported that even on smooth surfaces, a subtle distinction in surface atomic-level roughness can result in remarkable differences in helium transport. In fact, it has been illustrated that fluid molecules have favored orientations and configurations even on smooth crystalline surfaces (such as silica, MgO, alumina, graphene and boron nitride). Phan et al. (ACS nano 2016, 10 (8), 7646-7656) claimed that on crystalline surface as CH₄ molecules might be “stuck” in some favorable locations, which influences its transport. Thus, it is intriguing to study the influence of favored orientations and configurations near smooth crystalline surfaces on gas transport. Although the effects of surface and fluid molecular characteristics on gas transport have been investigated before, how specific surface and gas molecular characteristics collectively influence gas slippage behaviors on smooth crystalline surfaces still remains elusive.

In this study, we use MD simulations to study pressure-driven CO₂ and CH₄ transport as well as their slippage behaviors in β-cristobalite mesopores (with a width of 5 nm) under a typical reservoir condition (323 K and 20 MPa). Both CO₂ and CH₄ have an apparent adsorption layer on pore surface, but while gas slippage is obvious in CH₄ flow, it is negligible in CO₂ flow. On the other hand, the linear molecular structure of CO₂ allows it to align perpendicularly to the surface, even penetrating into the gaps among surface O atoms. Notably, this perpendicular orientation of CO₂ molecules is energetically favored near the hexagonal center of the surface structure. In contrast, the symmetric molecular structure of CH₄, coupled with its larger size, prevents its penetration into surfaces. As a result, despite smooth crystalline surfaces, CO₂ topological accessible planes are much more curved than that of CH₄, which significantly hinders CO₂ slippage. In addition, CO₂ topological accessible planes over β-cristobalite in this work may be more curved than those over more compact crystal surface in the study of Qian et al. (Applied Surface Science 2023, 611, 155613), indicating that CO₂ has more degree of freedom for its orientations, consuming more kinetic energy during gas-surface collisions and impeding specular reflection (thus, slippage). This study elucidates the importance of collective effects of specific fluid and surface characteristics as well as their specific interactions on gas slippage behaviors on smooth crystalline surfaces, which provides important insights into gas sequestration and extraction in subsurface formations, and optimization of functional materials for gas separation.