Co-feeding acetaldehyde (1-4%, on a carbon basis) with methanol/dimethyl ether on HZSM-5 at 673 K and 110 kPa total feed pressure resulted in a monotonic increase in ethene (9.6 to 14.9%) and methylbenzene (5.3 to 7.8%) selectivity (Figure 1). The mechanistic basis for the observed increase in ethene and methylbenzene selectivity is acetaldehyde undergoing aldol-condensation on Brønsted acid sites to form higher homologues (e.g., crotonaldehyde, sorbaldehyde, etc.) that can eventually undergo ring-closure followed by dehydration to form aromatics. Co-feeding acetaldehyde, therefore increases the concentration of aromatics inside the zeolite pores, which enhances the propagation of the aromatics-based methylation/dealkylation cycle and consequentially results in higher ethene selectivity. Ethene/(2-methyl-2-butene + 2-methylbutane), a descriptor of the relative extent of propagation of the aromatics-based catalytic cycle with respect to the olefins-based methylation/cracking cycle, increased monotonically from 1.3 to 2.5 in the presence of ~4% acetaldehyde (Figure 1). In an isotopic experiment, where 13C2-acetaldehyde (~4%, on a carbon basis) was co-reacted with unlabeled methanol/dimethyl ether on HZSM-5 at 673 K and 110 kPa total feed pressure, ethene observed in the effluent was enriched in 13C2-ethene, suggesting that at least some ethene is being produced directly from acetaldehyde or its homologues. Similar to acetaldehyde, other oxygenates like acetone, propanal, or furan, when co-reacted with methanol/dimethyl ether in small quantities (&lt;5%, on a carbon basis) systematically enhance the selectivity towards ethene and propene. Co-feeding oxygenates with methanol/dimethyl ether, therefore, represents a simple strategy to control light olefin selectivity in methanol-to-hydrocarbons conversion.

Enhancing light olefin selectivity in methanol-to-hydrocarbons conversion by co-feeding oxygenates

251st National Meeting (2016)

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