Effect Of A,B Site Doped On The Catalytic Behaviors For Isobutane Selective Oxidativedehy Drogenation Of LaCrO₃and LaMnO₃Perovskite

In this paper, LaMO perovskites, A site substituted and B site substituted LaMO3were prepared using citric acid complexation method. A series of characterization method such as XRD, IR, SEM, TPR were used to investigate the structure and composition, morphology and the lattice oxygen activity of the perovskite. Furthermore, the as-prepared perovskites were applied to the reaction of the dehydrogenation of isobutane. The effect of the preparation conditions and substitution of the A or/and B site on the performance of the oxidation dehydrogenation of isobutane was systematically studied.The influence of B-site ions on the structure and property of the perovskite was investigated, It was found that the LaMO3catalyst mainly exist as the typical ABO3perovskite. Depending on the B-site ions, the structure and the activity and amount of lattice oxygen can vary significantly. Among the four kind of perovskites, LaCrO3and LaMnO3display better dehydrogenation behavior. At660℃, space velocity1500h-1, i-C4H10:02:N2=20:3:12, the isobutane conversion was8.6%for LaCrO3and8.2%for LaMnO3while the isobutene selectivity were79.6%and76.9%, respectively.The influence of A-site doped alkaline metal on the structure and properties of LaCrO3and LaMnO3were discussed. As for LaCrO3, the influence of alkaline metal ions can be attributed to the phase composition, the ratio of chromium ions with higher valence and lower valence, the content of the defect sites, acid-basic property of the surface and the activity of lattice oxygen. The Mg doped LaCrO3exihibited best performance. At660℃, compared with the un-doped LaCrO3,the conversion reached10.0%, increasing by16.3%, and the selectivity was up to81.9%, increasing by3.0%. For the LaMnO3perovskite, the substitution of lower valence Mg and Sr ions could cause the increase of the valence of part of the B-site ions, transferring Mn3+to Mn4+and lift the reactivity of the lattice oxygen. The Mg doped LaMnO3presented better performance of selective oxidative dehydrogenation. At660℃compared with the un-doped LaMnO3,the conversion reached11.6%,increasing by41.5%,and the selectivity was up to81.5%,increasing by5.7%.The influence of B-site doped transition metal and the co-substitution of A and B-site on the structure and properties of the perovskite were discussed. The characterization results showed that the catalysts formed well-defined perovskite structure. The changes in the size of the BO6unit cell and the bond length greatly influenced the properties of the catalyst. Among all the B-site doped materials, LaCr0.5Mn0.5O3showed the best activity. At660℃, isobutane conversion reached9.6%and the selectivity was81.8%. The doping of the A-site of LaCr0.5Mn0.5O3with alkaline earth metal has obvious effect on the selective oxidative property, which can be attributed to the phase composition, acid-base properties and lattice oxygen. The Mg doped LaCr0.5Mn0.5O3displayed the best performance. At670℃, isobutane conversion of13.4%and isobutene selectivity of85.3%can be obtained.