Adsorption Selectivity of Thiophene and Benzene on Metal Ion–Modified Nanoporous M-HY Zeolites: A Theoretical Mechanistic Study

Background: The desulfurization of fuel oil is of great importance for environmental protection. A core challenge of adsorptive desulfurization is the difficulty in achieving the highly selective adsorption of trace organic sulfur compounds in real fuel systems characterized by the copresence of diverse aromatics and other competitors. Method: The mechanism of adsorption selectivity between thiophene and benzene on metal ions (M = Al3+, Ti3+, Mn3+, Co3+, Cu2+, and Zn2+)-modified M-Hydrogen Y zeolite (M-HY zeolites) was systematically elucidated using our own N-layered integrated molecular orbital and molecular mechanics (ONIOM) method. Results: All metal ions could be incorporated into the zeolite framework without markedly disrupting the pore structure. Brønsted acid strength followed the order of Ti-HY > Mn-HY > Al-HY > Co-HY > Zn-HY ≈ Cu-HY, and both thiophene and benzene were adsorbed by forming π-complexes with the acid sites. The energies of thiophene and benzene adsorption on Al-, Ti-, Mn-, and Zn-HY zeolites were comparable (70–79 kJ/mol), indicating poor selectivity. In contrast, the energies of benzene adsorption on Co-HY and Cu-HY zeolites were 39.66 and 36.74 kJ/mol higher than those of thiophene, respectively, implying the preferential adsorption of benzene. Conclusions: Adsorption performance was governed by acid strength, framework structure, and the intrinsic properties of the adsorbate molecules; selective adsorption could not be achieved by simply increasing the number of acid sites. This study provides a theoretical basis for the design of highly selective desulfurization adsorbents and highlights the need to move beyond the conventional π-complexation mechanism toward enhancing specific S-M interactions.