Single-event kinetic model for methanol-to-olefins (MTO) over ZSM-5: Fundamental kinetics for the olefin co-feed reactivity

Methanol-to-olefins (MTO) is an alternative pathway to selectively produce lower olefins on demand. Acid zeolites like ZSM-5 convert methanol to dimethyl ether (DME) and water, followed by the formation of olefins as well as of paraffins and aromatics as side products. In this study, butene was used as model compound for the recycle in the industrial methanol-to-propylene (MTP) process and co-fed with methanol. During these kinetic experiments, both methanol and butene inlet partial pressures were varied as well as the total volumetric flow rates. Temperatures between 708 and 788 K were applied as the aim of this work is to model the fundamental kinetics of the MTO chemistry at olefin co-feed conditions. For the kinetic model, the single-event methodology is used in order to reduce the number of estimated parameters while depicting each surface reaction. Olefin interconversion as well as olefin methylation and oxygenate interconversion steps are all covered by the model. Only the formation of aromatics is described in a simplified and thus not fundamental way. Over 4000 reaction steps are modeled using only eleven estimated parameters. The resulting high numeric significance of the activation energies allows a mechanistic analysis of the different reaction pathways and an assessment of the most important propene production steps. This shows high temperatures to be advantageous for fast carbon transfer to the olefin hydrocarbon pool and subsequent cracking of mainly hexenes and heptenes to propene.