Barrierless methane-to-methanol conversion: the unique mechanism of AlO+. Issue 26 (26th June 2020) - Sweeny, Brendan C.
Abstract : Alcohol for free? Aluminum can: aluminum enables oxygen transfer to methane via a unique, barrierless formation of R-Al–MeOH. The key Al–MeOH bond strength is tunable by controlling the acidity of the Al via electron-donating or withdrawing R groups. 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 + + CH3 OH, 14 ± 5%). Density functional calculations show the key Al–CH3 OH + intermediate is formed barrierlessly via a mechanism unique to aluminum, avoiding the rate-limiting step common to other MO + . The reaction of Al2 O3 + + 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 + –CH3 OH 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.