Implications of Cofeeding Acetaldehyde on Ethene Selectivity in Methanol-to-Hydrocarbons Conversion on MFI and Its Mechanistic Interpretation

Cofeeding acetaldehyde (1-4 C%) with dimethyl ether (DME) and methanol (DME:methanol ∼9:1, on a carbon basis) on two MFI-type zeolites: a conventional (Conv) MFI zeolite (SiO₂/Al₂O₃ ∼80, diffusion length ∼250 nm) and a self-pillared pentasil (SPP) MFI zeolite (SiO₂/Al₂O₃ ∼150, diffusion length ∼1.5 nm) at 673 K resulted in a monotonic increase in selectivity toward ethene (from 9.3 to 15 C% on Conv MFI and from 1.4 to 6.4 C% on SPP MFI) and methylbenzenes (from 4.9 to 7.8 C% on Conv MFI and 2.6 to 5.3 C% on SPP MFI). The mechanistic basis for this increase in ethene and methylbenzene (MB) selectivity is acetaldehyde undergoing multiple aldol-condensation reactions to form higher homologues (e.g., sorbaldehyde) that subsequently undergo ring-closure followed by dehydration to form aromatics (e.g., benzene). Cofeeding acetaldehyde, therefore, increases the concentration of aromatics inside the zeolite pores, which in turn enhances the propagation of the aromatics-based methylation/dealkylation cycle and consequentially results in higher ethene production. In an isotopic experiment where ¹³C₂-acetaldehyde (∼4 C%) was coreacted with unlabeled DME and methanol (DME:methanol ∼9:1, on carbon basis) on Conv MFI and SPP MFI at 673 K, ethene present in the effluent was enriched with two ¹³C labels and the net ¹³C-content in ethene (11-12% on Conv MFI and 45-52% on SPP MFI) was higher than the ¹³C-content in MBs (5-6% on Conv MFI and 9-17% on SPP MFI). Ethene, therefore, besides being formed via aromatic-dealkylation, is also being produced from acetaldehyde or its aldol-condensation products via a direct synthesis route.