Cyclotrimerization of aliphatic aldehydes catalysed by Keggin-type heteropoly acids and concomitant phase separation

Abstract

The acid-catalysed cyclotrimerization of aliphatic aldehydes has been examined through comparison of heteropoly acids with other acid catalysts. A Keggin-type heteropoly acid such as phosphomolybdic acid catalyses the cyclotrimerization of aldehydes, such as ethanal, propanal, butanal, 2-methyl-propanal, 2,2-dimethylpropanal, hexanal, octanal, and decanal, to produce the respective 2,4,6-trialkyl-1,3,5-trioxanes in high yields. Catalyst turnover number of the heteropoly acid is more than 10000 for propanal cyclotrimerization. In addition to the high catalytic activities, the reaction mixture spontaneously separates into two phases, a product phase and a catalyst phase, at high conversions of aldehyde. For propanal cyclotrimerization, the reaction mixture separates into two liquid phases, and the recovered catalyst phase may be repeatedly applied to the reaction without additional care in isolation of the catalyst. The phase separation phenomenon has been concluded to be caused by the insolubility of the heteropoly acid coordinated with propanal in the product 2,4,6-triethyl-1,3,5-trioxane.