Oxidative Dehydrogenation Of Propane To Propylene In The Presence Of HCl Catalyzed By CeO₂-based Nanocrystals

Propylene is an important chemical raw material.Currently,it mainly comes from cracking of naphtha.With the continuous depletion of petroleum resources in recent years,the development of new technology for propylene production has become a research hotspot,especially directly produced from propane.The propane conversion of propane dehydrogenation is limited by the thermodynamic equilibrium,the catalyst deactivation fast and need frequent regeneration;Propane oxidation dehydrogenation can achieve efficient conversion of propane,but propylene is more active than propane and easy to deep oxidation.Therefore,how to achieve high selectivity propylene at high propane conversion to be a research focus.This dissertation focuses on studies of oxidative dehydrogenation of propane to propylene in the presence of HCl(oxychlorination of propane reaction)catalyzed by CeO2-based nanocrystals.The relationship of structure and catalytic performance of catalysts for this reaction were investigated in detail.Active sites and reaction mechanism were also studied.CeO2 was found to be an efficient catalyst for the conversion of propane to propylene by(O2 + HCl).At 773 K,the conversion of propane was 29%,and the C3H6 selectivity was 61%.The selectivity of chlorinated hydrocarbons(i.e.C3H5Cl and C3H7Cl)in the product was less than 3%.Also,we found that the reaction was structure sensitive and the catalytic behaviors depended on the exposed facet of CeO2 nanocrystals.The CeO2 nanorod with {110} and {100} facets exposed showed the highest activity,whereas the nanocube enclosed by {100} facets was the most selective for propylene formation.The catalytic performance of oxide modified CeO2 nanorods was further investigated.The results showed that the modification of CeO2 nanorods by NiO significantly improved both propane conversion and propylene selectivity.A propylene selectivity of 80%was obtained at propane conversion of 69%over an 8 wt%NiO-CeO2 catalyst at 773 K,offering a single-pass propylene yield of 55%.The catalyst was very stable and no significant deactivation was observed in 100 h of reaction.The in-depth studies reveal that the concentration of surface oxygen vacancies and the capability of catalyst to chemisorb HCl are two crucial factors on controlling the activity and selectivity.We speculate that the oxygen vacancy contributes on the conversion of C3H8,and the active chlorine species generated by HCl activation account for the selective formation of propylene.The covering of catalyst surfaces by chemisorbed chloride species may favor propylene selectivity by suppressing the reactivity of oxygen species for over-oxidation.The kinetic behaviors show that the activation of C3H8 is the decisive step of the oxychlorination of propane.The modification of NiO promotes the activation of HCl and reduces the activation energy of the oxychlorination of propane.