Computational insight on Hydrocarbon selectivity on Synthesized Zeolite variants in Pressure Swing Adsorption

Synthesizing zeolites from natural kaolin clay presents a sustainable and cost-effective alternative to traditional synthetic precursors. The different variants of zeolites, characterized by their unique Si/Al ratios and pore structures, can significantly affect their selectivity and adsorption behaviors when interacting with various hydrocarbon gases. This study seeks to explore and explain how hydrocarbon gases (CH₄, C₂H₆, C₃H₈) engage with zeolite frameworks on a molecular level. To achieve this, zeolite variants were synthesized by etching kaolin with different concentrations of sulfuric acid (20%, 40%, and 60%). The resulting zeolites were tailored to have adjustable Si/Al ratios and porosity, which were then characterized through BET, SEM-EDX, and XRD techniques. The molecular interactions between the zeolite variants and hydrocarbons were investigated using MEP and NBO analyses to connect the electronic properties with the variations in etching. The findings revealed that the LUMO-HOMO energy gap ranges from 2.396 to 2.944 eV, showing a correlation with the zeolite framework and Si/Al ratio. Moreover, the adsorption behavior exhibited differing selectivity, stabilization energy, and reactivity with the three hydrocarbons, highlighting trends that depend on the framework. Changes in bond lengths within the zeolite-hydrocarbon complexes, spanning 3.107 to 4.196 Å, suggest that adsorption occurs in a less confined environment. This could enhance diffusion and improve the kinetics of adsorption and desorption, offering promising implications for gas purification in pressure swing adsorption (PSA) systems.

The computational analysis and molecular modelling of the zeolite-hydrocarbon gases were carried out using the Gaussian 16.0 and Gausview 6.0 visualization software packages, to present an understanding of how hydrocarbon gases are adsorbed by various commercial and lab-created zeolites via molecular electrostatic potential (MEP) and natural bond orbital (NBO) charge analysis. The DFT was adopted to optimize all zeolite-hydrocarbon complexes described by the HOMO-LUMO energy gap of the hydrocarbon complexes, which were calculated at the (B3LYP-D3/6-311 + + G(d,p)) method. This approach provides an insight into the hydrocarbon gas selectivity of the zeolite surface at the atomic level. The differences in reactivity and electronic stability (energy gap) can be utilized in pressure swing adsorption (PSA) processes to enhance selectivity in refineries and other real-world applications.