Comparing Alkene-mediated and Formaldehyde-mediated Diene Formation Routes in Methanol-to-Olefins Catalysis in MFI and CHA

Abstract This work uses DFT to investigate diene formation during methanol-to-olefins (MTO) via alkene-mediated and CH2O-mediated pathways in MFI and CHA zeolites. The rate controlling hydride transfer reactions were investigated with C1–C4 surface-bound alkyls present during MTO. In the CH2O-mediated pathway, rate-determining hydride transfer between surface-bound alkyl and CH3OH occurs most favorably with tert-butyl where rates of the hydride transfer exceed that of methyl at C4:C1 alkyl ratios as low as 10−10 in MFI and 10−6 in CHA. In the alkene-mediated route, hydride transfers proceed most favorably between C4H8 and secondary alkyls to form butadiene, where propyl--C4H8 (in MFI) and sec-butyl--C4H8 (in CHA) hydride transfers occur over methyl--C4H8 when their relative surface concentrations are 10−10 and 10−5 times that of methyl, respectively. Generally, once formed, C2+ surface alkyls are better hydride acceptors than CH3–Z, suggesting deactivation may be driven by larger surface-bound alkyls, formed by MTO products.