Method for the recycling of a Lewis acid

A process for the recovery of a Lewis acid from a reaction mixture (I) which has been obtained in the hydrocyanation of an olefinically unsaturated compound to a nitrile which has a miscibility gap with water under certain pressure and temperature conditions, in the presence of a catalyst system comprising a Lewis acid and a complex compound comprising a phosphorus-containing compound which is suitable as ligand and a central atom which is suitable for the complex compound, which comprises

• a) removing the complex compound from mixture (I) to give a mixture (II),
• b) adding water to the mixture (II) and placing the mixture (II) under pressure and temperature conditions such that a phase (III) which has a higher content of water than of the nitrile and a phase (IV) which has a higher content of the nitrile than of water are obtained, where phase (III) has a higher content of the Lewis acid than does phase (IV),
• c) adding a liquid diluent (V) that
• c1) does not form an azeotrope with water and whose boiling point under certain pressure conditions is higher than that of water, or
• c2) forms an azeotrope or heteroazeotrope with water under certain pressure conditions,
• to phase (III) to form a mixture of phase III and liquid diluent V,
• d) subjecting the mixture of phase (III) and liquid diluent (V) to distillation under the pressure conditions of step c1 or step c2, giving a mixture (VI) which has a higher content of water than of the diluent (V) and a mixture (VII) which has a higher content of the diluent (V) than of water, where the mixture (VII) has a higher content of the Lewis acid than does the mixture (VI), and
• e) subjecting the mixture (VII) to hydrocyanation of the olefinically unsaturated compound to give the nitrile, in the presence of the catalyst system.