The role of coke deposition in the conversion of methanol to olefins over SAPO-34

Abstract

The conversion of methanol and dimethyl ether (DME) to olefins (MTO and DTO) over SAPO-34 has been studied using a Tapered Element Oscillating Microbalance (TEOM). The effect of coke formation in the MTO reaction was investigated by coking SAPO-34 to desired coke levels using different probe molecules before exposing the catalyst to methanol. Mainly internal coke was obtained when using propene, while mainly external coke was obtained when using a C 4 component too large to enter the pores of SAPO-34. The coke formed from oxygenates, referred to as active coke, promoted olefin formation, while the coke formed from olefins, referred to as inactive coke, only had a deactivating effect. The role of DME during MTO was studied by comparing the MTO and DTO reactions at different coke levels. Apparently, olefins are formed via DME, and the diffusion of DME, which is affected by coke, plays a role in the conversion of methanol to olefins in MTO. The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. This is partly due to promotion by active coke formed initially, while further coke formation led to deactivation, probably by pore blocking.