CATALYSIS BY HETROPOLY COMPOUNDS .26. GAS-PHASE SYNTHESIS OF METHYL TERT-BUTYL ETHER OVER HETEROPOLYACIDS

Gas phase synthesis of methyl tert-butyl ether (MTBE) from methanol an d isobutylene has been studied with several heteropolyacids at 303-383 K. It was found that a Dawson-type heteropolyacid, H6P2W18O62, was mu ch more active than Keggin-type heteropolyacids, H(n)XW(12)O(40) (X=P, Si, Ge, B, and Co), and other solid acids such as SO42-/ZrO2, SiO2-Al 2O3 and H-ZSM-5 at 323 K. Since the acid strength of H6P2W18O62 was we aker than H3PW12O40 and H4SiW12O40, factors other than the acid streng th are important for the catalytic activity. Pseudoliquid phase behavi or was demonstrated for H6P2W18O62 and H3PW12O40 by the measurements o f the absorption of methanol during the reaction and by the unique pre ssure dependencies of the rate of synthesis. From the absorption data (the amount and rate), it is concluded that the high catalytic activit y of H6P2W18O62 is brought about by a high-activity state of the pseud oliquid phase in which controlled amounts of molecule are absorbed and the absorption-desorption is rapid. On the other hand, the pseudoliqu id phase of H3PW12O40 is in a low-activity state absorbing excessive a mounts of molecule.