The role of protons in acid-base type reactions on heteropolyacid catalysts:: gas-phase MTBE synthesis on H4SiW12O40

Basing on the kinetic and sorption experiments a mechanism of catalytic formation of methyl-tert-butyl ether (MTBE) on H4SiW12O40 was proposed. A reaction intermediate, carbenium ion C4H9+, is forming at the catalyst surface from isobutene supplied from gas phase and protons supplied by the solid. Subsequently C4H9+ reacts with methanol absorbed by the solid heteropolyacid. The model satisfactorily explains the anomalous reaction order with respect to methanol which under the applied conditions is positive at the initial stage of the catalytic process and becomes negative at its steady state.