Synthesis Of Alumina Supported Catalysts And The Diffusion Kinetics For HDS Reaction

The use of transportation vehicles become more and more common gradually.However,the combustion emission of fossil fuels by vehicles is one of the major sources for the formation of PM2.5,especially in recent years,the total amounts of diesel-powered vehicles increase drastically,and the huge consumption of diesel also aggravates the air pollution problems.It was found that the uncontrolled emission of vehicle exhaust is one of the main sources of fog and haze weather.And the sulfides are believed to be the main species of the pollutants.In order to control and eliminate the environmental pollution derived from automobile exhaust emission,the stringent specifications of high quality transportation fuels were established in many countries of the world.The sulfur compounds?involving DBT and 4,6-DMDBT?in fuels have great damage to the environment and the catalysts for the post treatment of automobile exhaust.Therefore,the ultra-deep desulfurization must be proceed in order to meet the increasingly rigorous requirements of fuel quality.The hydrodesulfurization?HDS?of the sulfur compounds is the key point to produce ultra-clean diesel.Not only open pores and highly dispersed active phase are the desired properties of the catalysts,but also the suitable acidity of the catalyst should be possessed.Therefore,the synthesis of novel support materials with open pores and suitable acidity is the critical point for the development of highly active catalysts.Considering that the characteristics of strong steric hindrances and mass diffusion resistance caused by the small pore sizes,and the difficulty to form the Ni Mo S-II phase with proper stacking derived from the strong metal-support interaction of the traditional ?-Al₂O₃,this research mainly focused on the directional synthesis of novel and cheap support material of Al₂O₃ with appropriate pore sizes and crystal structures,which are favorable to achieve high activity in HDS reactions,moreover,it also aimed at providing a superior alumina-based catalysts for the production of ultra clean diesel.First of all,a series of alumina materials with different specific surface areas,pore sizes and crystal structures were prepared by the high temperature pyrolysis of pseudo-boehmite,including: ?-Al₂O₃,?-Al₂O₃ and ?-Al₂O₃.The sequences of phase transformations and the best roasting conditions of pseudo-boehmite were found as follows: ?-Al₂O₃?550 ?,2 ?/min???-Al₂O₃?900 ?,2 ?/min???-Al₂O₃?1000 ?,2 ?/min?.Then,mesoporous Al₂O₃ materials with different crystal polymorphs were synthesized from the boehmite sol by a hydrothermal synthesis method using Al Cl3·6H2O as the aluminium source and polyethylene glycol as the mesostructure directing agent respectively.A series of Ni Mo supported hydrotreating catalysts with different Al₂O₃ crystal polymorphs were prepared.The typical physico-chemical properties of the series catalysts were characterized by using XRD,BET,UV-Vis,Raman,H2-TPR,XPS and HRTEM characterization methods.The corresponding hydrotreating performances of the catalysts were evaluated using sulfur-containing model compounds as the probe reactant.The corresponding hydrotreating activities were found to decrease as follows: Ni Mo/?-x >Ni Mo/?-x >Ni Mo/?-x.Under the reaction conditions of T=340 ?,H2/Oil=200,P=4.0 MPa and WHSV=20 h-1,the Ni Mo/?-Al₂O₃?aging temperature=50 ??catalysts presented the highest HDS activity,the DBT HDS activity can reach as high as 99.9%,the HDS activity of 4,6-DMDBT can reach as high as 94.3%.Moreover,the DBT desulfurization rate of Ni Mo/?-Al₂O₃?aging temperature=50 ??is greater than 99.6% at different WHSVs.Secondly,the influences of the operation conditions of WHSV?20-150 h-1?,reaction temperature?300-380 ??,reaction pressure?3.0-5.0 MPa?and H2/Oil?100-300?on the HDS performance of sulfur-containing model compounds were studied systematically.And the DBT and 4,6-DMDBT HDS results showed that the HDS activities decreased with the increase of WHSV,and enhanced with the increase of reaction temperature,reaction pressure and H2/Oil.The results also showed that the DBT HDS activities over all catalysts were higher than that in the 4,6-DMDBT HDS,which indicated that the HDS activities decreased with the increase of model sulfur-containing compounds molecule sizes.Thirdly,the HDS activities of model sulfur-containing compounds?DBT and 4,6-DMDBT?on Ni Mo/Al₂O₃ catalysts supported by Al₂O₃ with different crystal polymorphs were investigated to understand the diffusion process by means of kinetics method in order to further study the hindered diffusion process of macromolecular sulfides in the pore channels of catalysts.The variable rules for the catalyst effective factor???and effective diffusion coefficient?De?of model sulfur-containing compounds with different molecule sizes under different HDS reaction conditions were studied systematically.The results showed that the ? values over Ni Mo/?-Al₂O₃ catalyst in the HDS of different reactant molecules were higher than that over Ni Mo/?-Al₂O₃ and Ni Mo/?-Al₂O₃ catalyst owing to the excellent mass transfer ability of the Ni Mo/?-Al₂O₃ catalyst.Besides,the De decreased with the increasing molecule sizes of sulfur-containing model compounds,which provided sound foundation for the diffusion kinetic study of sulfides with different molecule sizes over Ni Mo/Al₂O₃ catalysts supported by Al₂O₃ with different crystal polymorphs and provided the kinetic guidance for the design of industrial ultra-deep HDS catalysts.