Ketenes in the Induction of the Methanol‐to‐Olefins Process

Abstract

Ketene (CH 2 =C=O) has been postulated as a key intermediate for the first olefin production in the zeolite-catalyzed chemistry of methanol-to-olefins (MTO) and syngas-to-olefins (STO) processes. The reaction mechanism remains elusive, because the shortlived ethenone ketene and its derivatives are difficult to detect, which is further complicated by the low expected ketene concentration. We report on the experimental detection of methylketene (CH 3 À CH=C=O) formed by the methylation of ketene on HZSM-5 via operando synchrotron photoelectron photoion coincidence (PEPI-CO) spectroscopy. Ketene is produced in situ from methyl acetate. The observation of methylketene as the ethylene precursor evidences a computationally predicted ketene-to-ethylene route proceeding via a methylketene intermediate followed by decarbonylation.

On the origin of initial hydrocarbons in methanol-to-hydrocarbons reaction

“…As a result, only ethylene and propylene were observed at the beginning, while other olefinic species were absent. This further confirms the experiments from Wu et al 47 Although oligomerization of ethylene and propylene resulted is generally considered contributor to the alkenes-based cycle 8 and the origin of the first aromatics, we have identified more feasible routes to form the first aromatic compound, toluene, as well as other aromatic species. This was done by a thorough analysis of various gaseous species and their variation with the reaction temperature.…”

“…Both methyl radical-and oxygenate-driven pathways were found to be important for hydrocarbon generation in MTH. Just recently, using methyl acetate, as excellent precursors for ketene, the MTH chemistry was mimicked by Wu et al 47 to investigate the olefin formation with PEPICO spectroscopy. The detection of ketene species before any olefins strongly suggests that ketene is an important early intermediate.…”

“…This can be further rationalized by the detection of a product of ethenone protonation and methylation 47 , i.e., acetone (CH3COCH3, m/z 58, IE ~9.70). Note that we did not observe any other acetyl-containing derivatives, such as acetaldehyde (CH3CHO, m/z 44), acetic acid (CH3COOH, m/z 60) or methyl acetate (CH3COOCH3, m/z 74).…”

“…Analogously to acetone, butanone (C2H5COCH3) can be derived from protonation and methylation of methyl-ketene (CH3CHCO), which is the precursor to ethylene after decarbonylation. Similarly, pentanone (C3H7COCH3) is formed from protonation and methylation of dimethyl-ketene ((CH3)2CCO, the precursor to propylene) 47 . These observations support the crucial role of ketenes in the generation of the first olefins.…”