Methanol to olefins (MTO) process is more and more promising as it can produce petrochemicals by using coal, natural gas and biomass beyond petroleum, thus mitigating the oil crisis. Though several commercial units have been operated recent years, the reaction mechanism and kinetics are controversial. Nearly all the current available kinetic models were proposed based on the hydrocarbon pool mechanism, in which the light olefins were produced through several parallel pathways. However, recent studies revealed that olefin methylation-cracking is the dominant pathway over HZSM-5 under the operation condition of industrial process, where methanol is co-reacted with olefins.
In this study, the pathway of the co-reaction of methanol and olefin as well as olefin transformation alone over a high-silicon HZSM-5 at 400-490ºC was investigated. The results showed that methanol converts mainly through the methylation reactions of C3-C6 olefins and rarely through that of ethylene. The interconversion pathway of olefins is more complex: C7= converts into C3= and C4= dominantly through a monomolecular cracking, whereas C4= to C3= + C5= and C3= to C4= + C5= through a bi- and trimolecular cracking, respectively, and both mono- and bimolecular cracking for C5= and C6=, resulting in various olefin species. The observed activation energy for both bi- and trimolecular cracking was negative, which can be well interpreted by the combination of the reaction and adsorption steps on the catalyst surface.
On the basis of the proposed reaction pathway, a kinetic model was developed, with the corresponding parameters correlated using the experimental data. A fairly satisfactory agreement between the calculated and experimental data was achieved over the test temperature arrange of 400-490 ºC.