In analogy to the oxidative degradation of benzene to , lene yields phthalic anhydride:

The original reaction conducted with stoichiometric amounts of oxidizing agents was replaced by BASF in 1916 by an air oxidation with V₂O₅ catalysts. IG Farben produced 12000 tonnes phthalic anhydride as early as 1941.

Two types of processes have been developed for the gas-phase oxidation:
1. Processes with fixed-bed catalyst, which can be subdivided into a low temperature version (350-400°C) using a pure feed, and a high-temperature version (400-550°C) with a lower grade of naphthalene.
2. Fluidized-bed processes at temperatures above 370 °C.

Well-known naphthalene oxidation processes have been developed by BASF, Von Heyden, Koppers, and Sherwin WilliamdBadger. They differ mainly in the type of reactor, catalyst, and phthalic anhydride isolation. Therefore the following description portrays only the reaction principles.

To 1:

The catalyst - usually V₂O₅/SiO₂ with, e. g., K₂SO₄ promoter - is situated in a multitube reactor cooled with a salt melt to remove the substantial heat of reaction. Naphthalene and air are introduced through an evaporator. With fresh catalyst, the reaction occurs at 360°C; the temperature must be slowly increased as the catalyst activity decreases. The reaction gases are cooled rapidly to below 125 °C - the dew point of the anhydride. The crude product, which crystallizes as needles, is completely dehydrated in melting vessels and then distilled. The selectivity is 86-91 % at a naphthalene conversion of about 90%.

The byproducts include 1,4-naphthoquinone, maleic anhydride, and higher molecular weight condensation products.

The high-temperature process was developed in the USA specifically for lower grades of naphthalene. V₂O₅ on a support is also used as a catalyst. Although at the higher temperature of 400-550 °C the catalyst does not lose its activity very rapidly, its selectivity is noticeably lower (60-74 %). Maleic anhydride (6-10%) is obtained as a byproduct along with other compounds.

To 2:

The fluidized-bed manufacture of phthalic anhydride using V₂O₅ catalysts at 350-380°C was first practiced in 1944 by Badger/Shcnvin Williams in the USA. In the commcrical process, liquid naphthalcnc is injected into thc fluidized bcd where prchcated air serves as the vortex gas. The advantages of this proccss are thosc characteristic of all fluidized-bcd processes: uniform temperature distribution in the cntirc catalyst bed, possibility of rapidly exchanging catalyst, and catalyst circulation to remove heat with a secondary current. A high conversion with a largc throughput - i.e., a high space-time yield - is obtained. The selectivity to phthalic anhydride is as high as 74%. Part of the anhydride can bc separated in liquid form.