GB407335A - Improvements in or relating to the manufacture of aliphatic anhydrides - Google Patents

Abstract

Aliphatic anhydrides such as acetic and methoxy-acetic anhydride are produced by thermal decomposition of the corresponding acids by means of salts of acids of phosphorus of lower degrees of oxidation than phosphoric acid, such as phosphites and hypophosphites. Suitable salts are calcium and magnesium phosphites and potassium and chromic hypophosphites or, according to the Provisional Specification, manganous phosphite and nickel pyrophosphite. Carriers may be used such as kieselguhr, carborundum, asbestos, silica gel and pumice. The catalyst may be molten or may be used in a molten bath, which may be a metal such as lead or a mixture of metals or tellurium as described in Specification 387,692. Alternatively the catalyst may be suspended in the acid, which is then injected into the reaction zone. Oxidizing gases such as air or oxygen are preferably excluded, but inert gases such as nitrogen or reducing gases such as carbon monoxide and hydrogen may be present. The acid may be preheated under pressure or not, and the preheating may be effected as described in Specifications 387,692 and 407,065. The reaction chamber may be made of or lined with a metal such as copper, nickel, iron, nickel-chromium steel, fireclay or fused silica, and the anhydride may be separated from the reaction vapours as described in Specifications 256,663, 289,972, 298,667, 303,772, and 310,863, [all in Class 2 (iii)]. In an example, acetic acid vapour is preheated to 450 DEG C. and passed through a fireclay tube containing calcium phosphite on silica gel or pumice at 700 DEG C, in the absence of air. Specification 407,367 is disclaimed.