Process for the preparation of dialkyl esters of maleic acid

The prepn. of maleic acid dialkyl esters (I) comprises reacting maleic acid, maleic acid anhydride or maleic acid monoalkyl esters with 1-8C alcohols in the presence of an acid catalyst and at the boiling point of the mixt and passing the water/alcohol/vapour mixt. formed opt. together with its condensed phase, as the feed to a hydrophilic membrane which removes the water and then recycling the dewatered alcohol to the reaction mixt. Pref. the alcohol is one which forms an azeotrope with water, esp EtOH, and the starting material is maleic acid anhydride. The membrane is made of cellulose diacetate, cellulose triacetate, polyvinyl alcohol or polyamide or is a pore-free layer formed by plasma polymerisation. Esp. the membrane is a composite membrane and is in the form of a coiled module. The molar ratio alcohol to acid function to be esterified is 10:1 to 1:1, esp 1.1:1 to 1.4:1. The amt. condensed phase in the feed if 5-90 (esp 5-50, esp. pref. 10-20) wt%.