(381ab) Investigation of the Sulfur Adsorption Capability of Ion-Exchange Y Zeolites Using Density Functional Theory and IR Spectroscopy

Investigation of the Sulfur Adsorption Capability of Ion-Exchange Y Zeolites using Density Functional Theory and IR Spectroscopy

Kevin X. Lee, Stavros Caratzoulas, George Tsilomelekis and Julia A. Valla

Department of Chemical & Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, CT 06269-3222, USA,

Phone: +1-860-486 4602, e-mail: ioulia.valla@.uconn.edu

The detrimental effect of sulfur emissions on human health and on the environment has been a growing concern. To overcome this problem, research has been focused on identifying adsorbents for the removal of sulfur molecules from hydrocarbons fuels. Specifically, there is a growing interest on fundamental studies of sulfur adsorption at the molecular level to obtain a better understanding of how sulfur species interact with metal cations in Y zeolites.^1,2 Experimental studies have shown that the incorporation of metals (e.g. Cu and Ce) plays an important role in enhanced adsorption capacity of refractory sulfur compounds such as dibenzothiophene (DBT) and alkylated-DBTs.³ Cu⁺ has an electronic configuration of 1s²2s²2p⁶3s²3d¹⁰4s⁰, which is highly desired for Ï€-complexation, whereas Ce⁴⁺ is an f-block element with high positive charge and polarizability, which prefers to interact with sulfur via the direct S-M bond. While Cu⁺ exhibits a higher sulfur capacity, aromatic compounds with similar shape and structure with sulfur molecules may also interact with the metal via the same adsorption mode. A better candidate for higher sulfur selectivity may be Ce⁴⁺, but the trade-off is the low sulfur capacity. One way to overcome the individual limitations is to exploit the synergistic effects of both metals by ion-exchanging them into the Y zeolite. Spectroscopic studies have shown that CuCeY can adsorb sulfur compounds via both Ï€-complexation and direct S-M adsorption modes, resulting in a higher sulfur capacity and selectivity.⁴ The desorption temperature of CuCeY was higher compared to CuY and CeY, suggesting that the bond energies are stronger than the individual counterparts. To further understand the role of bond strength on the adsorption of sulfur, density functional theory (DFT) calculations were performed on sulfur and metal-modified Y zeolites. Here we will present the calculated binding/adsorption energies and the results of the natural orbital (NBO) analysis, which give further information on the adsorption modes of sulfur on the metal exchanged zeolites.

References

1. Hernández-Maldonado, A. J. & Yang, R. T. Desulfurization of Diesel Fuels by Adsorption via π-Complexation with Vapor-Phase Exchanged Cu(I)−Y Zeolites. J. Am. Chem. Soc. 126, 992–993 (2004).
2. Hernández-Maldonado, A. J. & Yang, R. T. New Sorbents for Desulfurization of Diesel Fuels via π-Complexation. AIChE J. 50, 791–801 (2004).
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4. Lee, K. X., Tsilomelekis, G. & Valla, J. A. Removal of benzothiophene and dibenzothiophene from hydrocarbon fuels using CuCe mesoporous Y zeolites in the presence of aromatics. Appl. Catal. B Environ. 234, 130–142 (2018).