Gas phase synthesis of MTBE on dodecatungstosilicic acid as the catalyst

Abstract

Catalytic synthesis of methyl- tert-butyl ether (MTBE) in gas phase on solid H 4SiW 12O 40 was studied at 40 and 80°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 fact that methanol was easily sorbed by the whole volume of the solid (depending on pressure its uptake at 40°C reached up to 12 CH 3OH molecules per 1 Keggin unit (KU)) while isobutene remained only adsorbed at the surface of heteropolyacid both pristine and saturated with methanol it was concluded that catalytic reaction occurs at the surface of H 4SiW 12O 40 and reaction scheme has been proposed including the formation of tert-butyl carbenium ion with the participation of protons supplied by the catalyst. The assumption that reaction of this carbocation with methanol supplied also from the bulk or next-to-surface layer is the rate determining step led to the kinetic equations enabling to interpret the observed negative reaction order with respect to methanol at the steady state at 40°C but about one at initial period of the run. Reaction was characterised by the apparent activation energy as low as 25 kJ mol −1. At 80°C reaction order with respect to isobutene was one but that with respect to methanol decreased to about 0.5 indicating that reaction was diffusion controlled.