Dynamic site-interconversion reduces the induction period of methanol-to-olefin conversion

Reaction-diffusion coupling across the catalyst pore (active sites, catalyst surface), grain, pellet and reactor bed has been studied using a combination of theoretical concepts, and a particle-resolved transient microkinetic model applied to temperature-programmed desorption (TPD), and step response studies of methanol, and dimethyl ether conversion over ZSM-5 catalysts. TPD profile resolution shows coupling between adsorption, desorption, and diffusion across scales. Six models (coverage, anomalous diffusion, mass transfer of gas, and adsorbed phase, fixed site-interconversion, and dynamic site-interconversion) are investigated to describe the 44-min induction period in the first step-response cycle and the 95% reduction in induction period in subsequent cycles of dimethyl ether conversion. For the first time, we show that the reduction in induction period is not due to a zeolite-occluded pool (or coverage) effect. Dynamic interconversion across three site ensembles allows for the prediction of the induction period of propylene formation. Adsorption, desorption, and surface reaction of dimethyl ether occurs alongside with competitive adsorption of water and methanol leading to stable intermediates that produce propylene. The reduction in induction period is due to an autocatalytic transformation between three different reactive site ensembles. The first site ensemble retains the kinetic function of the first step response cycle while the second and third site ensemble adopt a new kinetic function due to transformation conveyed by surface methoxy species. The dynamic site interconversion model agrees with results from temperature-programmed surface reaction studies and are tenable for understanding the reduction in induction period and increasing the process efficiency particularly during start-up and steady-state conversion of methanol to olefins.