Evolution of Coke Deposit and Its Effect on Product Selectivity for Methanol-to-Olefin Reaction in Fluidized Bed

The coke deposition on the catalyst is known to be crucial to the methanol-to-olefin conversion reaction. The effect of coke deposition level on the product selectivity for a SAPO-34-containing catalyst was investigated. The test results obtained from a fluidized bed reactor indicated that ethylene selectivity increased faster than that of propylene with increasing coke deposition up to about 6 wt % whereas the selectivity to butenes decreased. The combined selectivity of ethylene plus propylene peaked to 74% when the total coke level accumulated on the working catalyst reached 6.3 wt % at which stage methanol conversion of 99.4% and minimum coking rate of 1.4 wt % were also recorded. The coke species deposited on the catalyst continued to evolve with increasing reaction time. Carbons of alkyl types dominated in early reaction time with minor amount of aromatic carbons. As reaction progressed, more cokes of aromatic in nature were formed as evidenced by the NMR results. Methanol conversion and olefin selectivity began to decline more significantly with increasing aromatic hydrocarbon deposition on the working catalyst.