A Grand Canonical Monte Carlo Study of the Adsorption of Methane, Ethane, and Their Mixtures in One-Dimensional Nanoporous Materials

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In an effort to understand the adsorption and transport of substrates in one-dimensional nanoporous materials, we use grand canonical Monte Carlo simulations to study the adsorption of methane, ethane, and their mixtures in AlPO4-5. For pure component adsorption we show that the energetically favorable ethane molecules are preferentially adsorbed at low loadings at a constant temperature. An increase in loading leads to a reversal of the adsorption preference as the entropic advantage of the smaller methane molecules begins to dominate at higher loadings. We observe a similar effect in the adsorption behavior as a function of temperature. At low temperatures the energetically favorable ethane dominates the adsorption process. However, an increase in temperature reduces the energetic advantage of ethane as more molecules explore the energetically less favorable regions, and methane is preferred at high temperatures. This reversal occurs at lower temperatures as the chemical potential of the species increases. We show that the reversal in the species preferentially adsorbed does not occur in the binary mixtures. Ethane molecules continue to adsorb unaffected by the presence of methane while methane molecules can only adsorb in the sites left unoccupied by ethane. Methane fails to displace the ethane molecules. We also show that the ethane selectivity is a nonlinear function of the chemical potentials of the two species. The selectivity is a function of the behavior of the mixture in both the bulk and the adsorbed phases. An increase in the ethane chemical potential leads to a monotonic increase in the ethane mole ratio in both the phases. The selectivity then is governed by the relative changes in these two ratios. An increase in the temperature causes a decrease in the ethane selectivity at a given chemical potential. This decrease can be attributed to the decrease in the energetic advantage of the ethane molecules as more molecules explore the energetically less favorable states at higher temperatures. The selectivity approaches a constant value at high temperatures. This high-temperature selectivity depends only on the entropic factor.

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