Phthalic Anhydride (99,5%) - China

Brief Overview

Phthalic anhydride, with the chemical formula C₆H₄(CO)₂O, acts as the anhydride of phthalic acid and represents a significant commercial form of this acid. It stands as a pioneering example of the widespread use of a dicarboxylic acid anhydride, playing a crucial role in industrial chemistry. Particularly, it is instrumental in the extensive production of plasticizers for plastics, contributing to the estimated global production volume of about 3 million tonnes annually in 2000.

Manufacturing Process:

Discovered by Auguste Laurent in 1836, contemporary synthesis of phthalic anhydride employs various methods, including the oxidation of naphthalene or ortho-xylene. These processes involve vanadium pentoxide (V2O5) as the active oxidant, playing a pivotal role in multiple steps and being regenerated by molecular oxygen. Starting from o-xylene, the oxidation reaction is run at about 320–400 °C and has the following stoichiometry:

C₆H₄(CH₃)₂ + 3 O₂ → C₆H₄(CO)₂O + 3 H₂O

The reaction proceeds with about 70% selectivity. About 10% of maleic anhydride is also produced: C₆H₄(CH₃)₂ + 7.5 O₂ → C₄H₂O₃ +4 H₂O + 4 CO₂

Phthalic anhydride and maleic anhydride are recovered by distillation by a series of switch condensers.

The naphthalene route (the Gibbs phthalic anhydride process or the Gibbs–Wohg naphthalene oxidation reaction), a process whose use has declined in compared to the o-xylene route, has the following mechanism:

Phthalic anhydride can also be prepared from phthalic acid by simple dehydration.

Phthalate esters plasticizers:

The primary application of phthalic anhydride lies in serving as a precursor to phthalate esters, essential plasticizers for vinyl chloride. The production of these esters, derived from alcoholysis reactions with phthalic anhydride, reached an annual scale of approximately 6.5×109 kg in the 1980s, showcasing a continuous increase, all originating from phthalic anhydride. Additionally, it is extensively used in manufacturing polyester resins and finds minor applications in alkyd resins for paints and lacquers, specific gums, insect repellents, and urethane polyester polyols.

Precursor to dyestuffs

In the dyestuff production sector, phthalic anhydride plays a significant role, notably contributing to the synthesis of the anthroquinone dye quinizarin through reactions with para-chlorophenol followed by chloride hydrolysis. The well-known compound phenolphthalein is synthesized by condensing phthalic anhydride with two equivalents of phenol under acidic conditions, giving it its name.

Pharmaceuticals

In the pharmaceutical industry, the treatment of phthalic anhydride with cellulose acetate produces cellulose acetate phthalate (CAP), a common excipient used for enteric coatings. CAP has demonstrated antiviral activity, and phthalic anhydride itself is a degradation product of CAP.