A/B-site Doping Modification Of LaMnO₃ Chalcogenide And Its Ethane Chemical Looping Oxidative Dehydrogenation Properties

Ethylene is an important raw material for the production of various chemical intermediates and polymers,with a wide range of applications.With the development of shale gas revolution,the technology of producing ethylene from low-concentration ethane in shale gas has attracted much attention.The chemical looping-oxidative dehydrogenation（CL-ODH）of ethane to ethylene has become a research hotspot in ethylene production technology.The CL-ODH technology of ethane is a new ethylene production technology that uses redox catalysts（also known as oxygen carriers）to provide lattice oxygen to convert ethane into ethylene and water.Since the oxygen carrier does not contain gaseous oxygen,it can effectively suppress the deep oxidation of ethane and therefore has higher selectivity.After reacting with ethane,the oxygen carrier is reduced;Then,lattice oxygen can be restored by reacting with air through oxygen carriers,in order to re-enter the next cycle.Therefore,the oxygen carrier is the core of ethane chemical looping-oxidative dehydrogenation process.Among many catalysts,perovskite-type metal oxides are widely used in the chemical looping conversion of carbon-containing fuels due to their unique structure,good thermal stability and high oxygen migration rate,and are considered one of the most promising redox catalysts.This study is mainly based on perovskite oxygen carrier La Mn O₃,using sol-gel method to prepare different La_1-xA_xMn O_3+δand La Mn_1-yB_yO_3+δperovskite oxygen carriers,studying the effect of different A/B site metal substitution on the ethane reaction performance of oxygen carriers,obtaining the optimal reaction performance oxygen carrier,and analyzing the crystal structure,micro morphology,oxygen species content and oxygen migration rate through XRD,SEM-EDS,BET,XPS,O₂-TPD,H₂-TPR and other characterization methods to obtain intrinsic factors affecting reaction performance.In order to test its catalytic activity,Sr,Sm,Pr and Ce metals were doped into A site of La Mn O₃ oxygen carrier.It was found that Ce ion introduction can significantly improve the ethylene selectivity of samples.At 700℃,La_0.7Ce_0.3Mn O3 can obtain optimal performance with 22%ethane conversion rate and 57%ethylene selectivity in 20 cycles of oxidation-reduction test with good stability.The study shows that after Ce substitution,Ce O₂ is introduced into perovskite structure causing distortion of perovskite structure and reducing Mn⁴⁺/Mn³⁺value on sample surface after Ce⁴⁺introduction promoting formation of oxygen vacancy and improving proportion of lattice oxygen species which is the main reason for improving ethylene selectivity.Co and Ni were doped in a certain proportion at the B site of La Mn O₃,and then the performance of ethane oxidative dehydrogenation was tested on a fixed bed reactor.It was found that at 650℃La Mn_0.7Ni_0.3O₃ can increase ethylene selectivity from 23.30%to 54.76%;at 700℃La Mn_0.8Co_0.2O₃ can increase ethylene selectivity from 37.75%to 73.52%.Characterization results show that Co and Ni doping transform rhombic structure of La Mn O₃into cubic structure providing more active sites for reaction by increasing specific surface area and pore volume;partial introduction of Co²⁺and Ni²⁺promotes Mn³⁺to Mn⁴⁺transformation forming oxygen vacancy which is more conducive to lattice oxygen transmission to oxygen carrier surface thereby improving ethylene selectivity and improving its performance.