| Sulfur (S) and its derivatives present in fuel oils are considered as the major air pollutants producers. With the combustion of fuel oils, it comes out in exhaust gases in the form of sulfur oxides and goes to the atmosphere. Being seriously dangerous to the atmosphere, special attention is needed for the control of these oxides. As a result, environmental regulating authorities are consistently making the permissible limits of S in fuel oils narrower since last two decades. In order to cope with these stringent regulations, several techniques and methods have been attempted for the desulfurization of fuel oils, for example oxidative desulfurization, biodesulfurization and catalytic hydrodesulfurization (HDS). Among these techniques, HDS has earned its position over the years based on its efficiency, diverse nature and flexibility in the process mechanization. In the present work, a novel catalytic HDS process was developed for dibenzothiophene (DBT) through in situ hydrogen production via ethanol and methanol reforming reaction over Ru and Pd promoted (Co/Ni) Mo supported Al2O3 based catalysts.There were two major steps involved in this project. The first step was the preparation of the HDS catalysts while the second was the practical experimental procedure for analyzing their HDS activity toward DBT (as model fuel). High surface area containing Al2O3 was selected as catalytic support, which was successively impregnated with water soluble salts of Mo, Co, Ni, Ru or Pd in the mentioned order. The metal loadings were kept constant throughout the process in order to minimize the effect of change in catalytic activity due to the extent of active phase formation. Ru and Pd being very expensive, were used in very low quantity i.e. 1 wt.% and 0.5 wt.% respectively as compared to those reported earlier. Catalytic activity was evaluated in a 250 ml batch autoclave reactor in the complete absence of external hydrogen supply. The hydrogen needed for the HDS reaction was supplied through the reforming reaction of ethanol (in case of Ru and Pd based catalysts) and methanol (in case of Pd promoted catalyst) separately. The in situ generated hydrogen utilization mechanism contributed the most toward the novelty in the current project. A 900 ppm DBT solution in n-octane was used as model fuel. Experiments were carried out in a temperature range of 320-400℃while reaction time was varied from 1-13 h. Apart from this, the effect of eight selected organic additives namely, decalin, tetralin, naphthalene, anthracene, diethylene glycol (DEG), phenol, o-xylene and pyridine was also studied. Each additive based on its chemical nature and structure, presented different effect on activity of the catalysts toward HDS of DBT i.e. some inhibited while some enhanced it. Decalin, tetralin, DEG and phenol type additives were found to be quite effective toward HDS reaction over all types of catalysts. Catalytic activity was measured in terms of reforming reaction of ethanol/methanol and HDS reaction of DBT separately. Reaction products were analyzed using HPLC and GC-MS techniques while catalytic samples (fresh and used) were characterized in terms of SBET surface area properties. The results showed that our process based on in situ generated hydrogen for the HDS of DBT is quite promising. It was found that in case of both ethanol and methanol reforming reactions, Ni based catalysts were more active than Co ones. Similar was the case with HDS reaction, whereas the incorporation of a noble metal i.e. Ru or Pd notably increased the catalytic activity. In all sets of experiments performed, for Ru based catalysts, HDS activity followed the order:Ru-Ni-MO/Al2O3> Ni-Mo/Al2O3> Ru-Co-Mo/Al2O3> Co-Mo/Al2O3 and for Pd promoted catalysts the order was: Pd-Ni-Mo/Al2O3> Ni-Mo/Al2O3> Pd-Co-Mo/Al2O3> Co-Mo/Al2O3. In case of Pd promoted catalyst, ethanol reforming activity was calculated; while a side reaction i.e. dehydration of ethanol, leading to the production of diethyl ether (DEE), was also confirmed by GC analyses. Similar was the case of methanol reforming, where dimethyl ether (DME) was confirmed as the dehydration product. In case of both the ethanol and methanol reforming reactions, Pd promoted catalysts followed the same order mentioned above. In this series, the increased HDS activity due to the incorporation of Pd was noteworthy, as 0.5 wt.% Pd loading showing 97% DBT conversion at 13 h reaction time and 380℃temperature has not been yet reported elsewhere. The GC-MS analyses indicated that biphenyl (BP) was the major product while bicyclohexane (BCH) or cyclohexylbenzene (CHB) were completely absent, revealing that direct desulfurization (DDS) pathway was followed, which is a trademark of HDS reaction at as high temperature as 380℃. Moreover, it was confirmed that the in situ HDS process is similar in its approach with the ex situ hydrogen utilization based process.Mild operating conditions, cost effectiveness, low metal loadings, reasonably high catalytic activity and utilization of in situ generated hydrogen proved the present process quite fruitful and superior to the currently in service conventional catalytic HDS process. Based on these results, the current process might be applied as an alternative approach toward HDS of DBT on industrial scale.
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