Inhibition of Propylene Oxidation to Acrylic Acid by Amorphous Overlayers on MoV(Nb)TeO Based M2 Catalysts

One of the most effective catalysts for the selective oxidation of propane to acrylic acid (AA) and ammoxidation to acrylonitrile (AN) is the MoV(Nb,Ta)(Te,Sb)O system, comprised of two phases, with M1 the major catalyst and M2 the co-catalyst in symbiosis with M1, converting intermediately formed propylene to the respective desired end products. An improvement in either phase should enhance the overall desired yields of a combined M1/M2 conglomerate. The current study concentrates on the M2 phase (variously substituted and/or doped) where depending on composition and preparation technique crystalline materials or crystalline materials with amorphous overlayers are obtained. Crystalline M2 catalysts without amorphous overlayers are vastly superior to those encumbered with overlayers. In a comparative study the former give a maximum AN yield of ~45% in propylene ammoxidation, the latter ~24%. In the selective oxidation of propylene to acrylic acid, the difference in performance between the two types of catalysts becomes enormous: The crystalline M2 catalyst gives a maximum AA yield of ~34% while the Te–molybdate overlayered M2 a meager ~3%. Doping of crystalline M2 phase with P significantly enhances AA yields over the base and should be seriously considered in future attempts to improve M1/M2 propane/propylene catalyst systems.