Gas phase synthesis of MTBE on dodecatungstosilicic acid as the catalyst

Catalytic synthesis of methyl-tert-butyl ether (MTBE) in gas phase on solid H4SiW12O40 was studied at 40 and 80 degrees C using differential constant flow reactor, Independently sorption of substrates, methanol and isobutene, as well as the product MTBE was studied using sorption balance. From the f act that methanol was easily sorbed by the whole volume of the solid (depen ding on pressure its uptake at 40 degrees C reached up to 12 CH3OH molecule s per 1 Keggin unit (KU)) while isobutene remained only adsorbed at the sur face of heteropolyacid both pristine and saturated with methanol it was con cluded that catalytic reaction occurs at the surface of H4SiW12O40 and reac tion scheme has been proposed including the formation of tert-butyl carbeni um ion with the participation of protons supplied by the catalyst. The assu mption that reaction of this carbocation with methanol supplied also from t he bulk or next-to-surface layer is the rate determining step led to the ki netic equations enabling to interpret the observed negative reaction order with respect to methanol at the steady state at 40 degrees C but about one at initial period of the run. Reaction was characterised by the apparent ac tivation energy as low as 25 kJ mol(-1). At 80 degrees C reaction order wit h respect to isobutene was one but that with respect to methanol decreased to about 0.5 indicating that reaction was diffusion controlled. (C) 1999 El sevier Science B.V. All rights reserved.