Influence of the reaction temperature on the nature of the active and deactivating species during methanol to olefins conversion over H-SSZ-13

Abstract

The formation of hydrocarbon species during the methanol to olefins (MTO) reaction over zeolite H-SSZ-13 has been systematically studied at reaction temperatures between 573 and 723 K with a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that the applied reaction temperature influences the rate and nature of coke formation as well as the catalyst stability. Correlation between all hydrocarbon species formed inside the catalyst pores with the activity and deactivation of H-SSZ-13 zeolite material was established. By using a multivariate analysis, we found that the nature of the active and deactivating species varies with the reaction temperature. The majority of active hydrocarbon pool species at low reaction temperatures (i.e., 573-598 K) are methylated benzene carbocations, while at high reaction temperatures (i.e., 623-723 K) methylated naphthalene carbocations become dominant. At low reaction temperatures the deactivation occurs because of the pore filling with methylated naphthalene carbocations. In contrast, at higher reaction temperatures the formation of phenanthrene, pyrene carbocations, and highly conjugated polyaromatics during the deactivation increases. This suggests that the formation of highly conjugated polyaromatics on the external surface can play a role in the deactivation of the material by pore blockage.