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

## TL;DR

This paper shows how modifying zeolite 5A with a specific acid improves its ability to separate CO2 from propylene, making it more efficient for adsorption.

## Contribution

The study introduces a surface modification technique using n-octadecylphosphonic acid to significantly enhance CO2/C3H6 adsorption selectivity in zeolite 5A.

## Key findings

- Modified zeolite 5A with 0.01 mol/L ODPA achieves CO2/C3H6 ideal selectivity over 73 at 298 K.
- Surface modification increases CO2 activation energy to 5.18 kJ/mol and C3H6 to 15.63 kJ/mol.
- Tetrahydrofuran washing restores site accessibility, showing tunable diffusion and adsorption behavior.

## Abstract

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.