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.
Bibliographical noteFunding Information:
The authors acknowledge financial support from The Dow Chemical Company and the National Science Foundation (CBET 1055846). The authors also acknowledge Ms. Dandan Xu, University of Minnesota, for the synthesis of SPP MFI zeolite sample.
© 2016 American Chemical Society.
Copyright 2016 Elsevier B.V., All rights reserved.
- acetaldehyde cofeed
- ethene selectivity
- oxygenate cofeed