The ternary Mo-W and V-W mixed oxides are part of the quaternary V-Mo-W catalyst system which is highly active and selective for the important and widely used selective oxidation of acrolein to acrylic acid. Both subsystems are active and selective catalysts for this reaction, varying with composition. The microstructure of the samples has been identified as homogeneously distributed nanocrystalline grains within the shell of the hollow balls that are typical for the spray-drying process. The crystallite sizes differ between the two sample series and depend on the composition as well. The test for the catalytic performance revealed the highest selectivity and yield in acrylic acid production for the V-W catalyst with 70% tungsten content, although tungsten oxide is known to be chemically inactive. The active components vanadium oxide and molybdenum oxide presumably are the most selective for the reaction of acrolein to acrylic acid when their structure and cluster size is stabilized by a stable and inert WO3 matrix. The heating of the samples during the catalytic performance test caused a visible growth of the nanocrystallites which also protruded from the surface of the hollow balls. The structural rearrangements led to an increase of the catalytic performance.
© Oldenbourg Wissenschaftsverlag
