Development of high-performance solid catalysts for selective oxidative dehydrogenation of ethane by structural fine tuning with in-situ XRD

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The structure of the SrBi_3O_4Cl_3 catalyst was analyzed by a Rietveld analysis. In the X1X2 structure, metal-oxygen sheets were separated by single chlorine layers and double chlorine layers alternately. SrBi_3O_4Cl_3 crystallized in space group 14/mmm with lattice parameters of a=3.9370(1), c=27.0177(7)Å. The structure showed that between two Bi(1) and Bi(2) sites, the former being of Bi in the metal-oxygen sheet faced the single chlorine sheet and the later faced the double chlorine sheet, strontium ions occupied the Bi(2) site only, and did not occupy the Bi(1) site. Based on this structural materials, much highly selective and active catalysts were developed. 74% yield of ethene has been attained in the oxidative dehydrogenation of ethane at 640℃ over the catalyst having a composition of KSr_2Bi_3O_4Cl_6.The lanthanum-substituted complex metal chloride oxide catalysts were also investigated. With the replacement of lanthanum ion in the cation-oxygen sheet of SrBi_3O_4Cl_3 catalyst … More s, the SrBi_<3-n>La_nO_4Cl_3 (n【less than or equal】0.75) catalyst remained the X1X2 structure, the activity for the oxidative dehydrogenation of ethane were promoted prominently. The Rietveld analysis of the XRD patterns of SrBi_<2.25>La_<0.75>O_4Cl_3 catalyst revealed that the increased coordination number of the metal ion facing the double chlorine sheet from 8 to 9 resulted in a decrease of the thickness of the double chlorine sheet. The interaction between the metal-oxide sheet and the chlorine sheet became stronger, which is supported by a reduced binding energy of chlorine in SrBi_<2.25>La_<0.75>O_4Cl_3. It is suggested that the activation of structural chlorine into a radical form is promoted by the replacement with lanthanum ion, which turns out to be the increase of the oxidation activity of ethane.The structure of the Bi_2SmO_4Cl catalyst was refined by Rietveld method. The space group is P4/mmm, a=3.8938(1)Å, c=8.9691(3)Å. It can be seen that the [Bi_2SmO_4] layer is present, which may be regarded as a'triple' rather than'double' fluorite unit. This cationic layer is balanced by the simplest possible anionic layer, Cl^-, hence the structure has the layer sequence -Cl-[Bi_2SmO_4]- Cl- etc. along the crystallographic c axis. La ion is coordinated only by oxide layers and Bi by both oxide and chloride layers. The Bi_2LaO_4Cl catalysts showed a high activity for the oxidation of ethane, but the selectivity to ethene were low, which can be attributed to the promoted oxidation ability of La-oxygen-La sheets. Less