Doping of MoVNbTeO (M1) and MoVTeO (M2) Phases for Selective Oxidation of Propane and Propylene to Acrylic Acid

Abstract

The effectiveness of MoV(Nb,Ta)(Te,Sb)O catalysts for the selective oxidation of propane to acrylic acid (AA) and ammoxidation to acrylonitrile (AN) is well known and recorded in the literature. One of the best known catalyst systems is comprised of two phases: M1 (orthorhombic) Mo7.8V1.2NbTe0.94O28.9 and M2 (pseudo-hexagonal) Mo4.67V1.33Te1.82O19.82, usually in a 60/40 ratio. The M1 structure performs all of the catalytic functions needed for converting propane to acrylic acid or acrylonitrile, since all key metals having the desired catalytic functions are located at the active center of the catalyst and within bonding distance of each other to perform this complex task. The M2 phase is a co-catalyst in symbiosis with M1, performing a mop-up operation converting free intermediately formed propylene to the respective desired end products (AA or AN). Various attempts have been reported in the literature, with varying degrees of success, to substitute select elements of the two respective structures to enhance the yields of the desired end products. A yield improvement in either phase should theoretically lead to an enhancement of the overall desired yield and of M1/M2 optimal mixtures. Our current study concentrates on the selective doping of the M1, as well as, M2 structure in the selective oxidation of propane and propylene, respectively, to acrylic acid; it centers at doping these structures with low levels of elements having acidic (P, B, W) or redox (Cu) properties. Higher acrylic acid yields were obtained with the doped M1 (2–5%) and M2 (up to 15%) systems. Directed, high throughput methodology was employed as the experimental technique. The study of low doping levels is being extended to include a broader base of elements, as well as, M1/M2 mixtures (optimum compositions) aimed at achieving still higher useful product yields (AA or AN).