Liquid Phase Adsorptive Desulfurization using Modified Zeolites for Transportation and Fuel Cell Applications

Sulfur removal from transportation fuels is essential for maintaining a pollutant-free environment and ensuring a healthy life. Adsorptive desulfurization using zeolites is an attractive desulfurization method, because of its low energy and cost requirements compared with conventional hydrodesulfurization. However, diffusion limitations within the micropores of zeolites can reduce their adsorption capacity, especially when refractory sulfur compounds are present. Moreover, the coexistence of aromatics in fuels exacerbates selective adsorption of sulfur. These challenges may be alleviated through careful physical and chemical modification, without compromising important structural properties of the zeolite. The introduction of mesoporosity via the surfactant-assisted method creates well-ordered mesopores that allow refractory sulfur compounds to access the internal adsorption sites, thus overcoming diffusion limitations. The incorporation of Cerium (Ce) and/or Copper (Cu) via ion-exchange enhances the binding strength of the metal-adsorbate interaction through multiple adsorption configurations. Using a fixed-bed adsorption column, the resulting ion-exchanged mesoporous Y zeolites were tested for their sulfur adsorption capabilities. Breakthrough curves show that mesoporosity increases the sulfur capacity by allowing refractory sulfur compounds to access internal adsorption sites. Metals significantly increase the selectivity towards sulfur compounds. The adsorption mechanisms of sulfur compounds were further studied, at the molecular level, using in-situ Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS). From DRIFTS studies, it was shown that the metals display high affinity for sulfur, via either π-complexation or direct σ-bond interaction. A reduction in capacity was realized when aromatics were added to the model fuel. Further efforts were made to investigate the role of metal composition and configuration on the selective adsorption of sulfur. It was demonstrated that 2%Cu10%CeSAY exhibits the highest adsorption capacity in the presence of aromatics. Similar adsorption capacities were obtained after two regeneration cycles. To fundamentally understand the adsorption mechanism from a theoretical perspective, density functional theory (DFT) calculations were performed using a two-layer ONIOM cluster. Subsequently, natural bond orbital (NBO) analysis was used to demonstrate the energetic importance between molecular orbitals and further identify correlations between electron transfer patterns and adsorption enthalpies. Finally, these DFT and NBO results were used to explain adsorption behavior from experimental results.