The influence of tungsten on structure and activity of Mo–V–W-mixed oxide catalysts for acrolein oxidation

For the partial oxidation of acrolein to acrylic acid, a series of catalysts with the general formula Mo8V2WcOx and a varying amount of tungsten (0 ≤ c ≤ 5) was investigated. Catalysts were prepared starting from aqueous solutions of ammonium heptamolybdate, metavanadate and metatungstate, which were dried, employing spray drying and crystallisation followed by calcination. The resulting precursors and catalysts were characterised by XRD, SEM and TG/DTA–MS. Additionally, kinetic experiments were applied. Spray drying results in the formation of amorphous structures, whose temperature stability shows a tendency to rise with an increasing amount of tungsten. Spray dried samples in general show a better catalytic performance than crystallised ones. In TP-reactions the spray dried mixed oxides were investigated with regard to their catalytic properties: conversion of acrolein, yield and selectivity regarding the partial oxidation to acrylic acid. The best performance could be assigned to the minor tungsten doped catalysts. The oxygen pathways were investigated with the so-called steady state isotopic transient kinetic analysis (SSITKA). After the replacement of 16O2 against 18O2 in the feed gas, the rate of oxygen incorporation into the reactants and the isotopic distribution of the oxidation products were analysed depending on temperature and tungsten content. Tungsten doping has a significant influence on the amount of 16O removed from the catalyst under 18O2. The amount of oxygen made available by the catalysts correlates with the catalysts’ activity.