Adsorptive And Oxidative Removal Of Sulfur-Containing Compounds From Oils On Carbon-Based Materials

The utiliztion for fuel oils has caused severe environment impact all over the world due to the fact that the sulfur-containing organic compounds in fuel oils result in the severe corrosion of reactors and equipment in the oil processing step, as well as SO_X, resulting from the combustion of these sulfur compounds, is one of the main sources for acid rain. Therefore, desulfurization has become a necessary step in the oil processing technology. Of various desulfurization methods, adsorptive desulfurization has received much attention due to the fact that it can be operated at room temperature and under atmospheric pressure without using hydrogen or any other reactive gases. In this thesis, the possibility of using porous carbon materials as adsorbents to remove sulfur-containing species in oils was investigated.Commercial coconut-based activated carbon (AC) can efficiently adsorbs the bigger size sulfur compounds in model oils and the removal efficiency for the sulfur compounds follows the order of 4,6-DMDBT＞DBT＞BT＞T. The experimental data and adsorptive isotherms of T and DBT molecules on the as-received AC fit the Freundlich model and equation well. Both T and DBT adsorption are a spontaneous and decreasing entropy process. Nevertheless, T adsorption is an exothermal process, in which T molecule lies on the surface of the AC adsorbent by a vertical adsorption mode, while DBT adsorption is an endothermal process, in which DBT molecule lies on the surface of the AC adsorbent by a flat adsorption mode.HNO₃ is believed to be a good reagent to control or tune the pore structure and surface chemical characteristics of the AC adsorbent. Oxidation with HNO₃ at ambient modification conditions removes inorganic components or ashes of ca. 50% in the AC sample, produces carboxyl functional groups on the AC surface, as well as introduces new micropores centering around 0.54 nm to a great degree. The results for the adsorptive capability of the modifided AC adsorbents for the thiophenic sulfur compounds show that the AC adsorbents modified at 30, 60, and 90℃can adsorb more T or BT molecules, but this is not the case for the removal of the bigger size sulfur compounds such as DBT and 4,6-DMDBT.The possible mechanism in the adsorptive process for removing the thiopheic sulfur compounds is proposed and discussed. The results show that the porous structure of the adsorbents is prerequisite for the removal of thiophenic sulfur compounds in the oils, nevertheless, the leading factor determining the adsorptive capability of the carbon adsorbents differs from case to case. In the case of the adsorptive removal of T molecule, the carboxyl groups deriving from CO₂ desorption and the micropores similar to the critical diameter of T molecule play important roles simultaneously. The carboxyl groups and the porous structure are responsible for the adsorptive removal of BT molecule. For removal of DBT and 4,6-DMDBT molecules, the porous structure is the determining factor. The frontier orbital energy gap shows that the AC adsorbents mainly interact with the sulfur compounds by LUMO of the AC and HOMO of the sulfur compounds and the interaction intensity between the adsorbent and the adsorbate increases with the molecular size and the carboxyl groups on the AC surface.A newly magnetic carbon-encapsulated iron nanoparticle adsorbent (Fe@Cs) has been successfully developed from biomass starch and iron nitrate by carbonization in flowing hydrogen. It has been found that the iron species, the core/shell shape, and the diameter of the Fe@Cs samples as well as the surface chemical properties and the porous structure in carbon shell of the Fe@Cs samples, can be tuned to some degree by changing carbonization temperature. Fe@C-900 sample at 900℃reveals a unique bcc-Fe core and porous carbon shell structure and the diameters in a range of 30～50 nm. The adsorptive capability of the Fe@C-900 sample for T molecule in model oils is 2 times higher than that of the commercial AC adsorbent and the micropores with a size similar to the T molecule size are found to be of benefits to removing the T molecule. The magnetic bcc-Fe core of the Fe@Cs materials makes it possible to be used as a magnetically separable adsorbent for the removal of sulfur compounds from liquid oils. This makes it easy to recover the adsorbents after the adsorption is finished.Based on the results presented above, a simple method for synthesis of the magnetic Fe@Cs adsorbents has been successfully developed using the AC sample as carbon source. The comparative study indicates that the pore structure of the carbon substrate is one of the crucial factors for the formation of the Fe@AC sample. The preliminary experiments demonstrate that the adsorptive capability of the Fe@AC adsorbent for T molecule in the model oils is higher than that of the commercial AC.The feasibility of oxidative removal of T molecule over fresh and pretreated AC-based iron-group catalysts has been explored. Of the catalysts tested, Fe₂O₃/AC-H exhibits the highest catalytic activity for the oxidative removal of T molecule, implying that it will be of potential as catalyst for oxidative removal of T molecule.