Effect of n-Octadecylphosphonic Acid Coating on Zeolite 5A for Adsorptive Separation of Carbon Dioxide and Propylene

Understanding the competitive adsorption mechanism is crucial for the rational design of CO2 adsorbents. In this work, the surface of zeolite 5A was modified with varying concentrations of n-octadecylphosphonic acid (ODPA) to enhance the adsorptive separation of CO2 over C3H6. With a 0.01 mol/L concentration of ODPA, the modified zeolite 5A achieves CO2/C3H6 ideal selectivity over 73 at 298 K, a substantial improvement over the pristine zeolite 5A, which exhibits a selectivity of 6.07. The Sips isotherm model provides an excellent fit to the experimental data, offering insights into the adsorption mechanism, with a calculated enthalpy change of −30.70 kJ/mol for CO2 and −16.54 kJ/mol for C3H6, along with favorable Gibbs free energy changes ranging from −9.00 to −3.54 kJ/mol for CO2 and −4.96 to −2.04 kJ/mol for C3H6 over the temperature range of 298–373 K. Kinetic analysis reveals faster diffusion in pristine zeolite 5A; however, surface modification significantly enhances CO2/C3H6 selectivity while maintaining balanced adsorption capacity. Adsorption uptakes of CO2 and C3H6 in pristine zeolite 5A follow the pseudo-first-order model and pseudo-second-order model, respectively. Pristine zeolite 5A shows rapid adsorption, with a CO2 adsorption capacity of 4.10 mmol/g with a rate constant of 2.60 min−1, and a C3H6 adsorption capacity of 1.99 mmol/g with a rate constant of 0.34 min−1. The modification with ODPA increases adsorption energy barriers, with CO2 activation energy reaching 5.18 kJ/mol and C3H6 activation energy up to 15.63 kJ/mol, while tetrahydrofuran washing restores site accessibility, demonstrating tunable diffusion and adsorption behavior. These findings lay the foundation for designing high-efficiency, and selective adsorbents through targeted surface engineering.