Main-Product

Benzenedicarboxylic acid is the systematic name for a benzene ring carrying two carboxylic acid (–COOH) groups, with molecular formula C₆H₄(COOH)₂ or C₈H₆O₄ and molecular weight of 166.13 g/mol. As with xylene, three positional isomers exist, but the term is most commonly used industrially to refer to terephthalic acid (the 1,4-isomer), which is by far the most commercially significant.

All three share the identical molecular formula C₈H₆O₄ and differ only in the relative positioning of the carboxylic acid groups, mirroring the substitution pattern of their xylene precursors.

All three phthalic acid isomers are produced industrially by catalytic liquid-phase air oxidation of the corresponding xylene isomer, using a cobalt/manganese/bromide catalyst system dissolved in acetic acid solvent at elevated temperature (roughly 150–200°C) and pressure (~20–30 bar). Terephthalic acid comes from oxidizing para-xylene, isophthalic acid from meta-xylene, and phthalic acid from ortho-xylene, with each methyl group on the ring being oxidized stepwise to a carboxylic acid group.

Phthalic acid is somewhat unusual among the three in that it is more commonly produced (and sold commercially) as its dehydrated form, phthalic anhydride, via catalytic oxidation of either ortho-xylene or naphthalene over a vanadium pentoxide catalyst at 340–385°C, with the anhydride subsequently hydrolyzed back to the free acid if needed. In all cases, the crude oxidation product undergoes crystallization, filtration, and purification steps to remove water, unreacted xylene, and byproducts such as benzoic acid before reaching commercial purity.