Dominant reaction pathway for methanol conversion to propene over high silicon H-ZSM-5

The reaction pathway for propene formation in methanol to propene (MTP) process over a high silica H-ZSM-5 catalyst has been investigated in a fixed bed reactor by comparing the experimental results from three kinds of feeding: alkene only, methanol only and mixed alkene and methanol. The results show that alkene methylation with methanol is dominant for the case of methanol and individual C3–C6 alkenes co-feeding, C2= is almost un-reactive. C7= cracks to propene and butene immediately whether co-fed with methanol or not, and C6= cracks to propene readily when reacted alone. Oligomerization occurs but is suppressed by the co-fed methanol for light alkenes of C2–C5. Methylation-cracking has been verified as the main reaction mechanism of a typical MTP process in which recycling of C2= and C4=–C6= to the reactor inlet is required. Based on the relative reactivities of alkenes towards methylation and inter-conversion, a reaction scheme has been presented including a cycle composed of a consecutive methylation from C4= through C5= to C6= and further to C7=, the β-scission of hexene and heptene for propene, and the α-scission of hexene for ethene as well.

► Methylation-cracking is the dominant reaction pathway in a typical MTP process.
► The rank of reactivities of C2-C6 alkenes towards methylation is C5=>C4=>C6=>C3=>C2=.
► Propene is formed mainly from cracking of C6=and C7=via methylation of C4=∼C6=.
► Ethene is formed mainly from C6=cracking.