Drastic-yet-distinct alterations in rarefied gas transport of CO2 and propane in nanochannels by finely-tuning surface characteristics

L Duan and ZH Jin, CHEMICAL ENGINEERING JOURNAL, 514, 162957 (2025).

DOI: 10.1016/j.cej.2025.162957

Rarefied gas transports with similar molecular weights (such as CO2 and propane) render similar Knudsen diffusivity in nanochannels. Nevertheless, by using molecular dynamics (MD) simulations, we find that the Knudsen theory breaks down for rarefied CO2 and propane transport in beta-cristobalite nanochannels with width of 5 nm under ambient conditions (298 K and 1 atm): CO2 self-diffusivity is only half of that of propane. The drastic differences in their self-diffusivity are due to the penetration of CO2 into the three-dimensional hexagonal ring structures on beta-cristobalite surface, resulting in substantial CO2 rotations and curved topological accessible plane, which are detrimental to its diffusion. In contrast, propane cannot penetrate into pore surface. On the other hand, by finely-tuning surface properties (the size of surface oxygen atoms), we observe drastic-yet-distinct alterations in their self-diffusivities: the enhancement in CO2 self- diffusivities is more than 8-fold of that for propane (290 % v.s. 35 %). This is achieved by prohibiting CO2 penetration and consequently limiting its rotations, thereby largely promoting its transport. On the other hand, the bending structure of propane, coupled with its larger size, always prevents its penetration into regular or tuned (pseudo) surface. Our study indicates that the collective effects of fluid and surface characteristics are instrumental to rarefied gas transport in nanochannels which are largely overlooked in conventional diffusion models and previous experimental as well as simulation studies. This work offers novel insights into rarefied gas transport mechanisms and the development and optimization of advanced materials for gas capture and separation.

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