1. R.I. Bergman and N.W. Frisch, U.S. Pat. 3,293,268 to Princeton Chemical Research, Inc. (1966).
2. P.N. Rylander and W.J. Zimmerscheid, U.S. Pat. 2,773,921 to Standard Oil Co. (1956).
3. Vanadium-Phosphorus Oxide Catalysts for the Selective Oxidation of C4Hydrocarbons to Maleic Anhydride
4. R.A. Schneider, U.S. Pat. 4,043,943 to Chevron Research Co. (1977).
5. N.J. Bremer and D.E. Dria, U.S. Pat. 4,244,879 to Standard Oil Co. (1981).