Studies On The Co/Al₂O₃ Catalysts For Catalytic Dehydrogenation Of Propane To Propylene

There is a huge market demand for propylene because it is an important industrial chemical and chemical intermediate.Although lots of catalyst shows well performance in light alkane dehydrogenation,it faces many problems.So we still need to explore other dehydrogenation catalyst which more stable,cheaper and non-toxic.The aim of this thesis is to study the catalytic performance of Co/Al₂O₃ catalyst in the propane dehydrogenation reaction.Improving its catalytic performance by changing the condition of pre-treatment,the preparation method,the calcination temperature and adding complexing agent,etc.Studying the effect of the state of active species on the dehydrogenation performance in Co/Al₂O₃catalyst by XRD,H₂-TPR and other characterization measures.In this thesis,the Co/Al₂O₃ catalyst with a load of 5.7 wt.%was selected as the research object.The effect of pre-treatment condition on the catalytic performance of 5.7 wt.%Co/Al₂O₃ catalyst for propane dehydrogenation shows that metal Co may responsible for the conversion of propane,and the dispersion of Co species on the surface have an remarkable impact on it's dehydrogenation behavior.Specifically,well dispersed metal Co facilitates the dehydrogenation of propane,while poorly dispersed metal Co results in cracking and coking.The pre-treatment protocols have an important influence on the dispersion of Co species on the catalyst surface.The fresh catalyst is mainly large Co₃O₄ particles(D_Co/Al2O3=33.68 nm).After pre-reduction or reaction,the aggregation state of Co species did not changed,and it was reduced to the large metal Co(D_Co/Al2O3-R=24.90 nm),resulting in very low propylene selectivity and serious coking.While re-oxidization after pre-reduction,the cobalt species on the surface got re-dispersed(D_Co/Al2O3-RO=6.07 nm),which shows high selectivity to propylene?⁹3.0%?.Although the well dispersed metal Co shows highly selectivity to propylene,carbon deposition is inevitable.Carbon deposition causes the deactivation of Co/Al₂O₃-RO catalyst after the 30 min reaction.While the regeneration at high temperature?650 ^oC?can refresh most of the activity.The calcination temperature also greatly affect the catalytic performance of the 5.7 wt.%Co/Al₂O₃ catalyst.The interaction between cobalt species and alumina strengthened with the calcination temperature improved,which led a fraction of Co species get into the structure of alumina.These Co species cannot be reduced in the reaction,which improved the dispersion state of surface available cobalt species.Thus restraining the side reactions,like cracking and coking.But further improving the calcination temperature increased the amount of non-reducible cobalt spinel phase,losing the activity and prolonging the induction period.In the preparation of Co/Al₂O₃ catalyst,the introduction of cobalt has a significant impact on the reaction performance of the catalyst.For the sol-gel and citric acid sol-gel methods,the raw materials were mixed and formed homogenous solution in a short time,leading a higher degree of dispersion.The citric acid sol-gel method,further promoted the dispersion of Co species due to the introduction of citric acid,which was beneficial to the dehydrogenation of propane.Introducing the complexing reagent into the impregnation process of Co/Al₂O₃ catalyst can promote the dispersion of the Co species by complexing effect,thus improving the selectivity to propylene.The catalyst obtained with citric acid shows the best reaction performance that the highest conversion of propane was 46.3%and the selectivity to propylene was higher than 90.0%.Moreover,the amount of citric has the optimum value,neither too much nor too less can obtain the best modification effect.