Opposite trends in acetylene/ethylene selectivity change after organic functionalization of Na+- and Cs+-MOR zeolites

Abstract

We developed a novel adsorbent with significant adsorption capacity and selectivity for acetylene(C₂H₂) in an acetylene/ethylene (C₂H₂/C₂H₄) mixture, through post-synthetic functionalization of thiolbenzene onto the micropore walls of MOR zeolite via chemical bonding. To systematically investigate the effects of cation type and the thiolbenzene group on the adsorption properties of the zeolite for these two adsorbates, pristine MOR zeolite and its functionalized counterparts in two different cation forms (Na⁺- and Cs⁺-forms) were analyzed, obtaining sorption isotherms for each adsorbate. In addition, Ideal Adsorbed Solution Theory (IAST) calculation was used to evaluate the theorical selectivity of acetylene/ethylene. The pristine samples in both cation forms show acetylene-preferred sorption ability (IAST selectivity: 12.89 for Na⁺- and 11.60 for Cs⁺-forms) under 100 kPa of gas mixture (C₂H₂: C₂H₄ = 1:1). However, notably, the selectivity of Na⁺-MOR somewhat decreased (50 % decrease to 6.481) after the organic functionalization, while Cs⁺-form increased further (55 % increase to 17.95). Density Functional Theory(DFT) calculations for the binding of two adsorbates on MOR zeolite surfaces explained that the specific adsorption sites for C₂H₂ and C₂H₄, their number, and adsorption energy were different with respect to the presence of thiolbenzene and the type of cations. The additional interaction between thiolbenzene and both adsorbates (C₂H₄, C₂H₂) stabilizes adsorption in Na⁺-form MOR zeolite and Cs⁺-form, leading to the observed decrease and increase in C₂H₂/C₂H₄ selectivity, respectively. This study demonstrates that the co-existence of organic groups and inorganic cations within the micropores synergistically impacts the adsorption properties of zeolites, enabling the fine-tuning of the separation performance of these materials.