Temperature Programmed Surface Reaction studies of the methanol to gasoline (MTG) conversion over ZSM-5

The methanol to gasoline conversion over ZSM-5 has been studied by the Temperature Programmed Surface Reaction (TPSR) technique. The technique is able to delineate the two steps in the process: the dehydration of methanol to dimethyl ether and the subsequent conversion of dimethyl ether to hydrocarbons. The activation barriers associated with each step was evaluated from the TPSR profiles and are 108 kJ/mol and 195 kJ/mol respectively. The methanol desorption profile shows considerable change with the amount of methanol molecules adsorbed per Bronsted site of the zeolite. The energy associated with the desorption process, (CH3OHH+-ZSM-5-->(CH3OHH+-ZSM-5+CH3OH, shows a spectrum of values depending on n. The spectrum is not continuous and has a gap for n=1. The trend is similar to that for the corresponding gas-phase reaction, (CH3OHH+-->(CH3OHH++CH3OH. The present investigation highlights the importance of the protonated methanol cluster, (CH3OHH+-ZSM-5, in the primary step of the MTG process, the formation of dimethyl ether. The reaction is compared with the corresponding gas phase ion-molecule reaction of CH3OH2+.