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

초록

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 theoretical 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.

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