Changes of Surface Properties and Water-Tolerant Catalytic Activity of Solid Acid Cs2.5H0.5PW12O40 in Water

Abstract

Cs2.5H0.5PW12O40, a hydrogen cesium salt of dodecatungstophosphoric acid, exhibited a high catalytic activity for hydrolysis of ethyl acetate in large excess water. Adsorption of water demonstrated that interactions between Cs2.5H0.5PW12O40 and water are weak. This nature and the strong acidity are possible reasons for the high activity. Changes of the catalytic performance, acidic property, and pore structure of Cs2.5H0.5PW12O40 by heat treatments in water were investigated. The decrease in the activity by the treatments was small at 353 K but was appreciable at 393 K. The calorimetry of ammonia adsorption showed that the water treatments reduced the acid amount, whereas the acid strength was little changed, indicating that the decrease in the catalytic activity was due to that in the acid amount. Pore size distribution analysis of Cs2.5H0.5PW12O40 with nitrogen adsorption revealed that the pore became larger by the water treatment. Infrared spectroscopy confirmed that the primary structure (Keggin structure) of the heteropoly compound was retained in hot water. A small amount of heteropoly species was released into water during the treatments, which was detected by UV spectroscopy. It was concluded that the released species were not ionically dissociated but present as heteropoly clusters. The changes of the pore structure are discussed in relation to alteration of the aggregated state of Cs2.5H0.5PW12O40 crystallites.