Finding Ketenes in the Methanol to Olefins Process

Methanol is an important platform chemical, widely applied in the chemical industry to produce olefins and aromatics. Although the Methanol To Olefins (MTO) process has been utilized for decades, there is still a lack of mechanistic understanding of this process. Especially the induction period, where the first alkenes are produced, is not well understood. Revealing the underlying chemistry aids rational catalyst design to further optimize selectivities and conversion of this process.

In the initiation of MTO, ketenes, such as H₂=C=O, H₃C-HC=C=O and (H₃C)₂C=C=O, were only hypothesized to play a substantial role in forming ethylene and propylene based only on computations. However, ketenes often evade detection due to their high reactivity, which makes fast, sensitive and multiplexed detection techniques compulsory. Operando photoelectron photoion coincidence (PEPICO) techniques with vacuum ultraviolet (VUV) synchrotron radiation from the Swiss Light Source (SLS) have been our tool of choice to shed light on ketenes.

In a study recently published in Angewandte Chemie, led by the Reaction Dynamics group at PSI, ketene (H₂=C=O) was in-situ synthesized from methyl acetate over a zeolite catalyst. Methyl acetate was found as intermediate during the MTO reaction and plays a leading role in the formation of surface acetyl species, which are in equilibrium with ketenes.

The formation of methylketene (H₃C-HC=C=O) via acid-site catalyzed methylation of ketene (H₂=C=O) was evidenced by the detection of its photoion mass-selected threshold photoelectron spectrum (ms-TPES). At the same conditions, ethylene, as the first olefin, is produced via decarbonylation (CO loss) of methylketene. The observation of methylketene and ethylene in concert evidences the computationally predicted ketene-to-ethylene route and fills a long-standing knowledge gap on how the initial C=C bond is made.