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

Cofeeding acetaldehyde (14 C%) with dimethyl ether (DME) and methanol (DME:methanol similar to 9:1, on a carbon basis) on two MFI-type zeolites: a conventional (Conv) MFI zeolite (SiO2/Al2O3 similar to 80, diffusion length similar to 250 nm) and a self-pillared pentasil (SPP) MFI zeolite (SiO2/Al2O3 similar to 150, diffusion length similar to 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 C-13(2)-acetaldehyde (similar to 4 C%) was coreacted with unlabeled DME and methanol (DME:methanol similar to 9:1, on carbon basis) on Conv MFI and SPP MFI at 673 K, ethene present in the effluent was enriched with two C-13 labels and the net C-13-content in ethene (1112% on Conv MFI and 4552% on SPP MFI) was higher than the C-13-content in MBs (56% on Conv MFI and 917% 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.