Catalysis by hetropoly compounds. Part XXVI . Gas phase synthesis of methyl tert-butyl ether over heteropolyacids

Abstract

Gas phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene has been studied with several heteropolyacids at 303–383 K. It was found that a Dawson-type heteropolyacid, H 6P 2W 18O 62, was much active than Keggin-type heteropolyacids, H n XW 12O 40 (X = P, Si, Ge, B, and Co), and other solid acids such as SO 2− 4 ZrO 2 , SiOAl 2O 3 and H-ZSM-5 at 323 K. Since the acid strength of H 6P 2W 18O 62 was weaker than H 3PW 12O 40 and H 4SiW 12O 40, factors other than the acid strength are important for the catalytic activity. Pseudoliquid phase behavior was demonstrated for H 6P 2W 18O 62 and H 3PW 12O 40 by the measurements of the absorption of methanol during the reaction and by the unique pressure dependencies of the rate of synthesis. From the absorption data (the amount and rate), it is concluded that the high catalytic activity of H 6P 2W 18O 62 is brought about by a high-activity state of the pseudoliquid phase in which controlled amounts of molecule are absorbed and the absorption-desorption is rapid. On the other hand, the pseudoliquid phase of H 3PW 12O 40 is in a low-activity state absorbing excessive amounts of molecule.