Insight into the Coke Precursor in the Process of the Methanol-to-Olefins Reaction

In this work, formaldehyde formed in the methanol-to-olefins (MTOs) reaction was found to be a coke precursor on SAPO-34 and the amount of formaldehyde during the entire MTO reaction was measured accurately by the acetylacetone spectrophotometric method. The mechanism of formaldehyde formation, decomposition, and coking in the SAPO-34 molecular sieve in the methanol-to-olefins reaction was investigated. It was found that in the MTO reactor, the formed formaldehyde further condensed into coke on SAPO-34 where it was created and then traveled onto the downstream SAPO-34 and condensed into coke there. The higher the amount of formaldehyde in the MTO reaction, the higher the coking rate and the shorter the catalytic lifetime of SAPO-34. Physical mixing of MgO and SAPO-34 can slow down the coking rate and prolong the catalytic lifetime of SAPO-34 by decomposing the formaldehyde into CO, CO2, and H-2 on MgO.