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

Reaction–diffusion coupling across the catalyst pore, grain, pellet, and reactor bed has been studied using a particle-resolved transient microkinetic model applied to temperature-programmed desorption and step-response studies of methanol and dimethyl ether conversion over ZSM-5 catalysts, respectively. An evolution of desorption across scales is provided. Five models (coverage, anomalous diffusion, mass transfer, 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 subsequent step-response cycles. The reduction is due to dynamic autocatalytic interconversion across three active site-ensembles. The first active site-ensemble retains the kinetic function of the first step response cycle while the second and third active site-ensembles adopt a new kinetic function mediated by surface methoxy species and adsorbed water. The dynamic site-interconversion mechanism reduces the induction period, increases the reaction efficiency, and describes the formation of primary olefins.