Multicomponent catalysts for the oxidation of propylene to acrolein: Fe 2(MoO 4) 3 doped with Bi or Te

Abstract

Multicomponent molybdate catalysts consisting of Fe 2(MoO 4) 3 with different amounts of Bi or Te were used for the oxidation of propylene to acrolein and characterized by means of DT, TG, and DSC analyses, surface area measurements, X-ray diffraction, and ir, Raman, diffuse reflectance, and X-ray photoelectron spectroscopies. The effects of Bi and Te levels were investigated over a wide concentration range from 0.000064 to 0.256 g atoms of Bi or Te per mole of Fe 2(MoO 4) 3. Experimental evidence showed that: (i) Bi, Te, and Mo can enter the Fe 2(MoO 4) 3 structure in the interstitial sites; (ii) at low concentrations Bi and Te tend to concentrate at the catalyst surface; (iii) dopants on the catalyst surface show oxidation states above III for Bi and above IV for Te; (iv) the localization of Bi, Te, and Mo in the interstitial sites, together with the stabilization of the higher oxidation states of Bi and Te, prevents catalyst deactivation; (v) Bi 3(FeO 4)(MoO 4) 2 and MoO 3 are formed in the catalysts with high amounts of Bi and Te, respectively. The catalytic and analytical data lead to the following conclusions: (1) selectivity is related to surface properties such as the presence of surface BiOMOBi (or BiOBi) and TeOMOTe (or TeOTe) chain bonds and controlled valence conductivity involving two-electron transfer processes; (2) activity is related to bulk properties such as oxygen mobility and bulk conductivity that can change with the presence of Bi, Te, or Mo in the Fe 2(MoO 4) 3 interstitial sites; (3) the formation of Bi 3(FeO 4)(MoO 4) 2 and MoO 3 is not directly responsible for the catalytic properties observed.