(453g) A Fundamental Rietveld Refinement and DFT Study on the Sulfur Adsorption Sites of Cu­­Ce-Y Zeolite

Ion-exchanged Y zeolites have proven to be effective adsorbents for adsorptive desulfurization (ADS) of thiophenic compounds from hydrocarbon fuel. Compared with conventional hydrodesulfurization (HDS), ADS can operate at ambient conditions, while producing sulfur-free fuels. HDS requires extremely intense energy conditions to break down refractory sulfur compounds, but ADS can mitigate this challenge by utilizing mesoporous zeolites. Mesoporosity grants sulfur with high kinetic diameter access to internal active sites, thereby eliminating diffusion limitations. Recently, the combination of transition metals, such as Cu, and rare-earths, such as Ce, have shown promising synergy in increasing both the capacity and selectivity for sulfur.^1–3 In fact, promising ADS results using CuCeY zeolites have been well-established in the literature.^3–5
This particular study focuses on the fundamental investigation of sulfur adsorption at the molecular level. Specifically, Rietveld refinement was performed on an x-ray diffraction (XRD) zeolite pattern to determine structural parameters, atomic composition and location of cations in CuCeY. Figure 1 shows the observed, calculated and differential XRD patterns of CuCeY. The difference curve indicates minimal discrepancy between the calculated and observed XRD patterns, leading to a relatively low residual error of R_wp = 0.09 and goodness of fit (GOF) = 3.35. Table 1 shows information regarding the final structure of CuCeY, which indicates that there are approximately 8 Ce⁴⁺ cations in the sodalite cage and 9 Cu²⁺ in the supercage, consistent with our XRF results.
Using a zeolite model based on the results of the refinement, density function theory (DFT) studies were carried out to investigate the energetics of metal-sulfur interaction. Figure 2 shows the two-layer ONIOM cluster that was used to study the adsorption mechanism of various sulfur compounds (e.g. thiophene) on CuCeY. The computational results show strong adsorption of sulfur compounds on CuCeY and indicate selectivity towards sulfur over aromatic molecules. Subsequently, natural bond orbital (NBO) analysis was carried out on the high-theory QM layer. Both DFT and NBO analyses show significant charge transfer between the outer-shell orbitals of the cation and the sulfur molecule, as a result of σ-donation from the π-bond of the sulfur compound to the unoccupied orbitals of Cu, and the simultaneous backdonation of electron density from the filled 3d orbitals of Cu to the antibonding π* orbitals of the adsorbate. The combination of fundamental studies on CuCeY using Rietveld refinement and DFT to support experimental findings has been helpful in elucidating the role of bimetals on the adsorption of sulfur. These techniques will help guide the synthesis of more effective ion-exchanged zeolites as novel sulfur adsorbents.

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. Velu, S., Ma, X. & Song, C. Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite-Based Adsorbents. Ind. Eng. Chem. Res. 42, 5293–5304 (2003).
3. 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).
4. Lee, K. X. & Valla, J. A. Investigation of metal-exchanged mesoporous Y zeolites for the adsorptive desulfurization of liquid fuels. Appl. Catal. B Environ. 201, 359–369 (2017).
5. Lee, K. X., Wang, H., Karakalos, S., Tsilomelekis, G. & Valla, J. A. Adsorptive Desulfurization of 4,6-Dimethyldibenzothiophene on Bimetallic Mesoporous Y Zeolites: Effects of Cu and Ce Composition and Configuration. Ind. Eng. Chem. Res. 58, 18301–18312 (2019).