Study of coke deposition phenomena on the SAPO_34 catalyst and its effects on light olefin selectivity during the methanol to olefin reaction

Abstract

In the current contribution a combined measurement of product selectivity with coke deposition has been performed in order to study the deactivation of SAPO_34 during methanol conversion to light olefins, emphasizing the effects of coke formation on the product selectivities as well as investigating the parameters affecting coke formation. The shape selective effect of coke formation favors lighter hydrocarbon products and, depending on the reaction conditions, 14 wt% to 23 wt% coke content in a zeolite bed is suggested to be an appropriate range in terms of light olefin production. Comparing hydrogen transfer index (defined as [alkane/(alkene + alkane)] for C_n species) as an indicator of the aromatic formation with coke content of the catalyst led to a meaningful relationship in which the coke deposition behavior could be predicted through the hydrogen transfer index change during the methanol to olefin reaction. Analysis of the coke compounds revealed that, by increasing the temperature and as the time-on-stream went on, the relative distribution of soluble/insoluble coke shifted towards insoluble coke. Depending on the species formed at different reaction temperatures, the coke species removal by air treatment varies between the temperatures of 550–930 K. A mathematical relationship between the initial reaction conditions and time-on-stream with the average coke content is proposed as a deactivation model and verified.