Study On The Li-based Catalysts For Oxidative Dehydrogenation Of Propane To Olefins

Known as the vital chemical monomer,ethylene and propylene is in great demand.But the traditional steam tracking process is hard to meet the increasing market requirement of propylene for the limitation of its feedstock and its low ratio of C₃H₆/C₂H₄.The new process,catalytic dehydrogenation of propane to propylene,though has high selectivity to propylene,it has its own defectiveness especially in energy consumption,carbon deposition and propane conversion.Thus oxidizing agent was introduced to solve the three problems.Unfortunately,the study of the catalyst,especially for the V and Mo catalysts obeying redox mechanism,is now facing the scabrous puzzle of week activity or bad olefin selectivity,so we chosen the Li/MgO catalyst,which possesses both high activity and excellent olefin selectivity,as the object for study.The reaction properties combined with series of catalyst characterization methods,such as XRD,N₂-adsorption and desorption,TG-DTA,were researched to explain the the reaction mechanism.MgO was proven to be the best support among various supported Li catalysts in oxidative dehydrogenation of propane.And the performance of Li/MgO was deeply affected by the loading amounts of Li.As the Li capacity increased,the conversion of C₃H₈ and the selectivity of C₂H₄,C₂H₆,CH₄ and CO_x.increased at first,and decreased afterwards,yet the selectivity of C₃H₆ showed a completely opposite trend.As the Li content was 3 mol%,the conversion of C₃H₈ reached the peak value 53.6%and the ratio of C₃H₈/C₂H₄ fell to the least value 0.67.It was attributed to the competitive adsorption between two types of H in C₃H₈.When the active Li⁺O^-sites dispersed separately,the competition was gentle,for the bond breaking energy between the secondary carbon and hydrogen was much lower,it boosted the selectivity of C₃H₆.But when the dispersion of the Li⁺O^-site was poor,the competition became intense,since the steric hindrance was much lower as the H in the first carbon was adsorbed by the catalyst,it led to higher selectivity to C₂H₄ and other products.The synthesis condition and the synthesis method have significant impact on the catalytic performance of Li/MgO.For catalysts with Li loading below（0.6Li/MgO）and above（10Li/MgO）monolayer coverage when based at 600 ^oC,the selectivity of C₃H₆increased with calcination temperature raising from 600 ^oC to 750 ^oC,while the conversion of C₃H₈ and the selectivity of C₂H₄,CO_x,CH₄ and C₂H₆ decreased.It was revealed that the BET area decreased and the Li species ran off markedly with increasing calcination temperature,as a consequence,the active sites decreased and its dispersion became much better.To investigate the influence of synthesis method,Li/MgO catalysts with two different Li contents（8 mol%and 15 mol%）were prepared by citric acid sol-gel method.The results showed that the catalyst synthesized by citric acid sol-gel method do have higher specific area,better Li distribution and nicer catalytic performance than impregation method.Particularly,the ratio of C₃H₈/C₂H₄ increased from 0.7 to 1.3 and the yield of olefins augmented from 30.1%to42.3%when the synthesise method changed at Li content in 8 mol%.The reaction mechanism of propane dehydrogenation over Li/MgO catalysts was convinced by the study of the reaction condition influences.It was proved that the decreasing C₃H₈/O₂ ratio,increasing temperature and enlarging particle size could facilitate the propane conversion,yet decrease the the ratio of C₃H₆/C₂H₄.The catalysts was tested in a continuous-regenerated reaction.The conversion declined from 48.3%to 44.0%at the first36-hour-reaction,and it decent rapidly from 54.3%to 36.2%at the second 36-hour-reaction after a 2-hour-regeneration.Combing the catalysts characterization results with the reaction mechanism,it was speculated that the inactivation of Li/MgO catalysts was caused by carbon deposit,sintering,the loss of active Li species and the occupied active Li⁺O^-site by H.