New Cs-containing Mo-V4+ based oxides with the structure of the M1 phase -: Base for new catalysts for the direct alkane activation

The high catalytic performance Of Mo3.5V4+W0.5O14 for the selective oxidation of acrolein (ACR) to acrylic acid (AA) is strongly connected to its semi-crystalline tunnel structure of an oxidic bronze. The real structure in the crystallographic (a and b)-plane is characterized by an intergrowth of different channel sizes typical for oxidic bronze structures without long-range order in the (a and b)-plane. The growth direction of the needle-shaped crystals is perpendicular to the (a and b)-plane and parallel to the crystallographic c-axis. In this direction, in contrast to the high disorder of different tunnel motifs in the (a and b)-plane, a strong crystallographic order of corner-shared MO6 octahedra (M = Mo, V, W) is observed. A structural analogue in the family of the block or shear structures is M-Nb2O5. This structure is characterized by an intimate intergrowth of ReO3-blocks with very different block sizes and without any long-range order in the (a and b)-plane, and by a high order of corner-shared NbO6 octahedra in [c]. The semi-crystalline oxidic bronze structure Of Mo3.5V4+W0.5O14 is not unique. Many ternary and quaternary oxides of transition metals, which crystallize at high temperatures with the crystal structure of oxidic bronzes, at low calcination temperatures exhibit a semi-crystalline bronze structure. Mo10V24+Nb2TeO42-x, with the crystal structure of Cs-0.5[Nb2.5W2.5O14] (M1 phase) exhibits its best catalytic properties in the direct oxidation of propane to acrylic acid if no impurity of M2 phase is present. Phase-pure M1 phase can be prepared in a highly reproducible way by avoiding any precipitation by continuous and instantaneous spray-drying of aqueous solutions. The resulting amorphous precursor is calcinated in air (<300 degrees C) and in N-2 (300-650 degrees C) and post-treated with aqueous acids to remove any impurity of undesired phases (e.g. the M2 phase). New Mo-V based oxides with the composition A(0.5)[Mo5-a-bVa4+XbO14] (A = Rb, Cs, X = no element, Nb, Ta, W, Sb, Bi, Se, Te....) and with the structure of the M1 phase can be prepared. Due to the Cs-content, the new compounds exhibit a low catalytic activity in the direct propane oxidation. By treating Cs-0.5[Mo3.7V1.24+Bi0.1O13.9] with HClO4 50% of the Cs could be removed, and the catalytic activity was increased. Therewith the mentioned new preparation concept for tailor-made new Mo-V based M1 phases with limited Cs-content and with flexible composition may enable new and well-defined M1 phases with (e.g. Bi containing) active sites to overcome the catalytic difficulties in the step of the oxidative transformation of propene to acrolein within the direct oxidation of propane to AA. (C) 2006 Elsevier B.V. All rights reserved.