Differences among the deactivation pathway of HZSM-5 zeolite and SAPO-34 in the transformation of ethylene or 1-butene to propylene

The deactivation of HZMS-5 and SAPO-34 catalysts has been studied in the transformation of ethylene and 1-butene under propylene intensification conditions. The deterioration of spent catalysts' physical properties have been quantified and coke has been characterized by TPO and by several spectroscopic techniques (Raman, ¹³C NMR, FTIR, FTIR-TPO), in order to determine the effect reaction medium composition and the severity of catalyst shape selectivity have on the nature and location of the coke in the porous structure. The results reveal that the mechanism for coke deactivation consists of two steps: one for the formation of alkylated aromatics by oligomerization and another for the coke growth-condensation. The first step is analogous for both catalysts and it principally depends on the catalyst acid strength and acid site density. The second step is different for both catalysts: the microporous structure of SAPO-34, with cavities in the intersections, inhibits the diffusion of alkylated aromatics towards the outside of the structure, thus blocking active acid sites; whereas, HZSM-5 structure, with a high connectivity and without cavities, favors the diffusion of the aromatics that evolve for a longer time outside of the micropores. At process conditions, the results demonstrate that the coke formation is faster from ethylene than from 1-butene, due to the lower reactivity of ethylene for oligomerization-cracking mechanisms as well as its higher capability for coke formation.