phthalic anhydride

Currently, phthalic anhydride is mainly produced through catalyzed oxidation of o-xylene. A variety of metal oxides are used as catalysts. A typical one is V₂O₅ + TiO₂/Sb₂O₃. Approximate conditions for the vapor-phase oxidation are 375–435°C and 0.7 atmosphere. The yield of phthalic anhydride is about 85%:

Liquid-phase oxidation of o-xylene also works at approximately 150°C. Cobalt or manganese acetate in acetic acid medium serves as a catalyst.The major by-products of this process are maleic anhydride, benzoic acid, and citraconic anhydride (methylmaleic anhydride). Maleic anhydride could be recovered economically.  Phthalic anhydride’s main use is for producing plasticizers by reactions with C₄–C₁₀ alcohols. The most important polyvinyl chloride plasticizer is formed by the reaction of 2-ethylhexanol and phthalic anhydride:

Phthalic anhydride is also used to make polyester and alkyd resins. It is a precursor for phthalonitrile by an ammoxidation route used to produce phthalamide and phathilimide. The reaction scheme for producing phthalonitrile, phthalamide, and phathilimide is shown in Figure 1.1.

Figure 1.1. The reaction scheme for o-xylene to phthalonitrile.