Photo-assisted Adsorptive Desulfurization of Hydrocarbon Fuels over TiO2 and Ag-TiO2
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Organic sulfur compounds are one of the most common impurities in crude oil which is widely used as transportation fuels such as gasoline, diesel and jet fuels. Sulfur in liquid hydrocarbon fuels is also considered as one of the major causes of environmental pollution. The sulfur level in transportation fuels is now restricted by severe regulations in many countries for environmental protection. Moreover, ultra-low sulfur fuels are also required by fuel cells because sulfur species can easily poison the catalysts in fuel cell processors. Electrodes can also be damaged by sulfur in hydrocarbon fuels. Thus, desulfurization is essential for the production of transportation fuels as well as fuel cell application area. Catalytic hydrodesulfurization (HDS) is a commercial sulfur removal technology which has been widely used in refining. However, several desulfurization technologies now considered to replace current conventional HDS to produce ultra clean fuels at milder operation conditions and lower cost. Among these alternative processes, adsorptive desulfurization (ADS) process has shown to remove organosulfur compounds efficiently under mild conditions. Recently, Ag/TiO2 and Ag/TiO2-Al2O3 adsorbents with high sulfur removal capacities have been developed by our adsorption group. Sulfur removal pathway using Ag dispersed on TiO2 was then studied in details. Acidic hydroxyl groups on TiO2 surface were considered as the main active sites. As a result, the desulfurization capacity of TiO2 is related to surface acidity. The presented research work focused on TiO2/UV application in adsorptive desulfurization (ADS) to improve TiO2 adsorbents’ sulfur removal performances and mechanistic investigation of adsorptive sulfur removal under UV irradiation using TiO2 adsorbents. In this work, the development of unique UV-irradiated adsorptive desulfurization using TiO2, TiO2-Al2O3, Ag/TiO2 and Ag/TiO2-Al2O3 adsorbents has been presented here along with sulfur removal pathways and possible photo-oxidative reactions on TiO2 surface. Ultra-violet (UV) sources at a long wavelength (λ=365 nm) have been directly applied to dynamic breakthrough process without adding any corrosive oxidative agents. It was observed that UV irradiation before and during breakthrough processes both enhanced TiO2 adsorbents’ sulfur removal capacities. For experiments without UV, breakthrough and saturation capacities of calcined TiO2 were 2.45 mg S/g and 3.90 mg S/g using a model fuel containing 3500 ppmw sulfur as benzothiophene in n-octane. And the capacities increased to 4.05 mg S/g and 5.63 mg S/g for experiments under UV. Moreover, UV-treated TiO2 samples can also achieve high sulfur removal capacities. The performance was persistent after thermal regeneration at 450 °C. The negative effect of H2O on acid based adsorbents has been reported by many studies. The same trend was observed here. However, TiO2/UV system can solve the problem caused by H2O additive. Highest capacities for calcined TiO2 were obtained during photo-assisted ADS using a model fuel containing H2O additive. The desulfurization performance under different conditions, effects of H2O and UV on surface hydroxyl groups, and the thermal stability of photo-activated species on TiO2 surface are all presented in Chapter III. Silver loaded on TiO2 proved beneficial to desulfurization performance when compared to calcined TiO2 support. The performance of Ag/TiO2 (4 wt%) obtained from adsorptive desulfurization under UV, the effect of H2O, as well as chemical state change of silver ions, were studied and discussed in Chapter IV. Ag/TiO2 showed a different behavior from calcined TiO2 during UV-irradiated breakthrough experiments when using a model fuel containing no H2O. The sulfur removal capacities of Ag/TiO2 were observed to decrease dramatically due to the formation of silver metal under UV which can be demonstrated by XPS results. Adding H2O into the model fuel could prevent silver oxides turn into Ag metals under UV which further improved desulfurization performance of Ag/TiO2. Highest capacities for Ag/TiO2 were obtained during UV-assisted ADS using a model fuel containing H2O additive. Having confirmed that UV irradiation can improve desulfurization performance of TiO2 adsorbents, several studies were carried out to determine the effect of UV and H2O on active surface sites (Chapter V). A relationship between surface acidity and sulfur removal capacities for TiO2 and Ag/TiO2 was observed by our group. And acidic -OH groups have been postulated to be the main active sites on TiO2 surface. Thus, the sulfur removal capacities were related to the total number of surface hydroxyl groups based on previous studies. Effects of UV and H2O on surface hydroxyl groups were carried out via in situ IR and XPS. The number of isolated hydroxyl groups Ti(IV)-OH increased while bridged and H-bonded hydroxyl groups were removed after the exposure to UV based on infrared and XPS spectra. The effect of H2O on Ti(IV)-OH groups on TiO2 treated with or without UV was also studied in details. Possible pathways of forming terminal hydroxyl groups from bridged -OH and bridged O sites were discussed in Chapter V. Surface hydroxyl radicals were observed using fluorescence spectroscopy (Chapter V). These radicals were considered as main active species on TiO2 surface under UV which can act as strong oxidants during desulfurization process. Organosulfur removal mechanisms under dark and UV were both investigated in Chapter VI after characterizing active sites on TiO2 surface. Isolated -OH groups contributed in the removal of sulfur aromatic molecules, as observed by in situ IR. Thiophenic compounds were considered to be removed via π-interactions between surface hydroxyl groups and thiophene ring as well as aromatic ring under both dark and UV conditions. However, photo-oxidation and ring opening reactions were also observed on the TiO2 surface under UV irradiation. The resulting oxidative products were also adsorbed via π-interactions onto the TiO2 surface. Sulfur removal mechanisms under different conditions were illustrated in details in Chapter VI. No photo-oxidative products were detected in the liquid phase as confirmed by GC-PFPD and GC-FID results.