A catalyst recycling method in the Koch reaction has been investigated using hydrogen fluoride and sulfuric acid as catalyst.
From the experimental results, the following has been proposed as a possible procedure:
Using an anhydrous hydrogen fluoride catalyst, carboxylation was conventionally carried out which was followed by hydrolysis using HF-H₂O in place of H₂O. On subsequent distillation, the hydrogen fluoride catalyst was recovered almost quantitatively as distillate and the RCOOHHF complex was obtained as residue. Treatment of the residue with an equimolar amount of water resulted in the formation of two layers. Carboxylic acid was obtained from the upper layer and the lower layer (HF-H₂O) was recycled to the hydrolysis step.
In the case of the sulfuric acid process, the carboxylic acid produced was effectively extracted with a halogenated hydrocarbon solvent without diluting the sulfuric acid catalyst. The solvent was separated from the carboxylic acid by distillation. The raffinate (H₂SO₄) was reused for carboxylation without reconcentration.