Combined ex and in situ measurements elucidate the dynamics of retained species in ZSM-5 and SAPO-18 catalysts used in the methanol-to-olefins reaction

We have studied the dynamics of the retained species on ZSM-5 and SAPO-18 zeolites using a combination of temperature-programmed desorption/oxidation, ex situ analysis and in situ FTIR spectroscopic measurements over the entire conversion range, using fixed-bed and spectroscopic cell reactors, in continuous and discontinuous mode. Our results point to the appropriateness of the combined methodologies to track the interconversion of active into deactivating species. We found a statistically relevant (supported by linear regression and multivariate analysis) association of our observations using the different complementary methodologies. The kinetics of this interconversion depends on the initial conversion (tuned by acidity and space time) and microporous topology, and involve: (i) in the ZSM-5 catalysts, the diffusion of monocyclic aromatics toward the exterior of the zeolite to form coke and (ii) in the SAPO-18 catalysts, the obstruction of the cages by aromatics that grow up to tetracyclic aromatics islands.