Nanoconfinement is known to alter thermodynamic and transport properties of fluids. Using molecular dynamics (MD) simulations coupled with neutrons scattering, we have explored the adsorption, structure, and diffusion of water, aqueous electrolytes, pure hydrocarbons and mixtures of hydrocarbons and other substances in slit-shaped pores at various conditions of temperature, pressure and surface features. Our results showed, e.g., that preferential adsorption of CO2 to the pore surface can compete with the adsorption of hydrocarbons, lower the activation energy for diffusion, and enhance hydrocarbon mobility resulting in “molecular lubrication”. Similarly, water can preferentially adsorb on silica-based pore surfaces. Building on prior results, we explored whether the preferential adsorption of either CO2 or H2O on the pore surface can affect the equilibrium conversion of CO2 to CH4. CO2 + 4H2 = CH4 + H2O The system we considered is meant to represent a nanoporous matrix in contact with a larger micro-fracture. Water, hydrogen, carbon dioxide and methane are free to exchange between the nanopores and the micro-fracture and to react within the various environments. The micro-fracture is large enough to provide a ‘bulk’ system, in which classic thermodynamics holds. Because we are mostly interested in the thermodynamic equilibrium of this Fischer-Tropsch reaction (not yet on the reaction mechanism), we implemented the reactive ensemble Monte Carlo method. This method has been used extensively, and only requires information regarding the thermodynamic properties of reactants and products (not of the intermediates) at ideal gas conditions. To explicitly consider the effect of confinement, we generated slit-shaped pores (1 and 2 nm) carved out of silica, constructed from β–cristobalite crystals. Indeed, our preliminary results show that preferential adsorption of the fluids in the narrow pores can affect equilibrium. When water, one product, is preferentially adsorbed, “the position of equilibrium moves to partially reverse the change”, yielding more water, and consequently more methane, thus enhancing the overall reaction conversion. The effects of pore size, pore chemistry and pore morphology were investigated. All conditions that enhance water adsorption (i.e., increasing pore hydrophilicity or roughness) result in an enhancement of reaction yield.

Reactivity of C-O-H fluids in nanoporous regimes

2019 Spring National Meeting

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