Understanding the competitive adsorption mechanism is essential for the development of adsorptive separation of ethylene (C
2H
4) and ethane (C
2H
6). In this work, density functional theory calculations and molecular dynamics simulations were employed to investigate the
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Understanding the competitive adsorption mechanism is essential for the development of adsorptive separation of ethylene (C
2H
4) and ethane (C
2H
6). In this work, density functional theory calculations and molecular dynamics simulations were employed to investigate the adsorption of C
2H
4 and C
2H
6 in two LTA-type zeolites, ITQ-29 and 5A. The results show that the adsorption energies of the gas molecules in zeolite 5A are more negative than in ITQ-29, and the difference in adsorption energy between C
2H
4 and C
2H
6 in zeolite 5A is significantly larger than in ITQ-29, 13.3 versus 6.2 kJ/mol. Zeolite ITQ-29 demonstrates high C
2H
4/C
2H
6 ideal selectivity (43.5 at 5 ns) while exhibiting slow C
2H
4 uptake efficiency due to the small pore windows, hindering C
2H
4 diffusion (1.05 × 10
−10 m
2/s at 298 K). In contrast, zeolite 5A facilitates the faster diffusion of C
2H
4 molecules (3.25 × 10
−9 m
2/s at 298 K) and exhibits a modest C
2H
4/C
2H
6 selectivity of 1.11 at 5 ns in single-gas adsorption and 2.72 in equimolar binary mixture adsorption. To enhance C
2H
4/C
2H
6 selectivity, methyl phosphonic acid is introduced onto zeolite 5A to add a sieving layer that enables the C
2H
4 molecules to preferentially permeate, and the optimal coverage of methyl phosphonic acid is 50%, yielding a C
2H
4/C
2H
6 selectivity of 17.5 at 5 ns in mixture adsorption and preserving the C
2H
4 uptake efficiency. The insights into the competitive diffusion of molecules in the coating layer and inside the zeolites provide a theoretical basis for the rational design of high-performance adsorbents.
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