Barrierless methane-to-methanol conversion: the unique mechanism of AlO+

Abstract

The kinetics of AlO⁺ + CH₄ are studied from 300–500 K using a selected-ion flow tube. At all temperatures the reaction proceeds near the Langevin–Gioumousis–Stevenson collision rate with two product channels: hydrogen atom abstraction (AlOH⁺ + CH₃, 86 ± 5%) and methanol formation (Al⁺ + CH₃OH, 14 ± 5%). Density functional calculations show the key Al–CH₃OH⁺ intermediate is formed barrierlessly via a mechanism unique to aluminum, avoiding the rate-limiting step common to other MO⁺. The reaction of Al₂O₃⁺ + CH₄ follows a similar mechanism to that for AlO⁺ through to the key intermediate; however, the conversion to methanol occurs only for AlO⁺ due to favorable energetics attributed to a weaker Al⁺–CH₃OH bond. Importantly, that bond strength may be tuned independent of competing product channels by altering the acidity of the Al with electron-withdrawing or donating groups, indicating a key design criteria to develop a real world Al-atom catalyst.