Hydrodesulfurization Of Thiophene Over Gold-based Bimetallic Catalysts

Reduction of sulfur content in fuels is a pressing environment concern. Among the numerous desulfurization methods, hydrodesulfurization (HDS) is still the main technique nowadays. New HDS catalysts studies and new HDS engineering designs must be developed to fulfill the more stringent fuel specifications. A new direction in new catalysts studies is gold catalysts. For example, there have already been studies of Au-Pd and A-uCo bimetallic catalysts.In this article, A series of Au-M/SiO₂ bimetallic catalysts are prepared by consecutive impregnation method from HAuCl4 and metallic salt solutions of M. M is representative for the active component in conventional sulfide catalysts, such as Ni, Co, Mo. Their HDS activities of thiophene at atmospheric pressure are investigated. The catalysts are characterized by means of N2 physical adsorption, XRD, TPR and XPS. The synergetic effect between Au and M and their active sites are studied. The results are as follows:1. The HDS activities over AuNi bimetallic catalysts are markedly changed by the activation method, the catalysts actived by sulfidation show better activity. The activities of sulfurized Au-Ni/SiO₂ catalysts increase with Au loading in the range of our experiment; and their activies also increase with Ni loading when Ni loadings are low, reaching highest activity at Ni loading 5%, then slightly lower HDS activities are obtained when Ni loadings increase further.The HDS activities of sulfurized catalysts are well in line with their TPR characterization. From the experiment, we learn that Au and sulfur vacancies are the active phases of this reaction, and bulk nickel sulfides are detrimental to the enhancement of activity. Combining TPR characterization with XPS characterization, we find when Ni loading doesn't exceed 5%, Au promotes the formation of sulfur vacancies which are active sites; however, the presence of gold is favorable for the formation of bulk sulfides when Ni loading gets higher, because in the process of catalysts'preparation we firstly load Ni, then gold, which will result in Ni covered by Au and then make Ni less mobile.In the range of our experiment, the activities of sulfided Au-Ni/SiO₂ bimetallic catalysts are remarkably higher than Au monometallic catalysts, and distinctly higher than monometallic Ni catalyst when Ni loading isn't very high. So the synergetic effect between gold and nickel species is preferable when gold and nickel loading are low and the synergetic effect is mainly produced by hydrogen spillover.2. The influences of sulfidation and reaction condition on HDS activity of Au-Ni/SiO₂ bimetallic catalysts are investigated. The results show that: When Au-Ni/SiO₂ is sulfided at 400℃for 2 h by CS2 and reacted under reaction temperature 400°C, LHSV 1.8～3.6 h^-1, hydrogen flow rate 60ml/min, it exhibits better performance towards reaction of thiophene HDS. From the TPR characterization of the sulfided catalysts, we learn that a higher sulfidation degree results in a better HDS activity.3. The addition of gold promotes the sulfidation of Ni, Co, Mo and improves their HDS activity. For AuNi, AuCo, AuMo bimetallic catalysts, their activity decreases as follows: Au-Co/SiO₂ > Au-Ni/SiO₂ > Au-Mo/SiO₂. Their activities have relation with the dispersion degree of active components. Therefore, the activity of AuMo catalyst is relativity low because Mo is poorly dispersed on support; for AuCo catalyst, because both Au and Co are highly dispersed on SiO₂, it shows better activity.