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 <i>n</i>-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.