(349aj) A Multimodal XAFS and DFT Study of the Sulfur Adsorption Sites in Cu and Ce Ion-Exchanged Y Zeolite

Adsorptive desulfurization (ADS) is an effective method for the removal of sulfur compounds from hydrocarbon fuels under ambient conditions. Cu and Ce exchanged Y zeolites have exhibited high capacity and selectivity as adsorbents of thiophenic sulfur compounds from liquid fuels.¹ Previous studies have found that the oxidation state and synergy between Cu and Ce ions play an important role in the desulfurization performance.²

In this study CuY, CeY, and CuCeY zeolites were prepared using liquid ion-exchange of NaY. As the desulfurization performance of CuCeY has been previously reported, this research aims to shed light on the interaction between Cu and Ce cations that leads to increased sulfur adsorption. To do this, Density Functional Theory (DFT) models of the zeolites were optimized and the cation sites were probed using in-situ X-ray adsorption spectroscopy (XAS) at the Cu K and Ce L edges. Previous research has shown XAFS to be a useful tool for examining metal species in zeolites and this, coupled with the DFT model as a starting point for theoretical path fitting, can reveal the local geometry and oxidation states of the Cu and Ce species.^3-5

The DFT model of CuCeY is shown in Figure 1 with Cu and Ce cations located in the six member rings of the zeolite framework. Extended x-ray adsorption fine structure (EXAFS) fitting results confirm this placement of Ce cations, and show evidence of metallic Cu clusters formed during reduction, as has been reported in CuY.^4,5 The x-ray adsorption near edge structure (XANES) analysis confirms the reduction of the metal cations as displayed in Figure 2 for CuCeY. This study of the active sites of reduced CuCeY through XAS and DFT helps to elucidate the mechanism for its superior desulfurization performance, which will help to enable the synthesis of even more effective novel sulfur adsorbents.