Fumaric acid is currently produced by isomerization of maleic acid, which is produced from maleic anhydride. Maleic anhydride, in turn, is industrially produced by catalytic oxidation of suitable hydrocarbons in the gas phase. Benzene used to be the predominant starting material, but oxidation of n-butane or n-butane–n-butene mixtures has become more important in recent years (Lohbeck et al. 1990). The butane oxidation reaction equation to maleic anhydride is: C4H10 + 3.5O2 → C4H2O3 + 4H2O.
In the most common maleic anhydride process, the catalyst is embedded in fixed-bed tubular reactors. The catalysts applied in this process are based on vanadium and phosphorus oxides. In this process, water is formed as a by-product, and a fairly small amount of it can be directly liquefied from the reaction gas by partial condensation. Organic solvents absorb maleic anhydride contained in the reaction gas. More than 98% of the anhydride can be absorbed in this way. The solvent–anhydride mixture is then subjected to fractional distillation to separate maleic anhydride from the solvent, and the latter is returned to the absorption column (Lohbeck et al. 1990). The thus obtained pure maleic anhydride is then hydrolyzed into maleic acid according to the reaction equation C4H2O3+H2O → C4H4O4.
Subsequently, the maleic acid is converted almost quantitatively by thermal or catalytic cis–trans isomerization into fumaric acid (Lohbeck et al. 1990). The most common catalysts used are mineral acids, peroxy compounds, and thiourea. The crude fumaric acid obtained in this way is purified by crystallization from water (Fig. 2). As will be shown later, a comparable purification may be used for fermentative fumaric acid production.