Generalized Reaction Mechanism for the Selective Aerobic Oxidation of Aryl and Alkyl Alcohols over Nitrogen-Doped Graphene

Abstract

In this study, an attempt has been made to investigate the mechanistic pathway for the aerobic oxidation of alcohols over nitrogen-doped graphene using density functional theory methods employing a suitable model for graphene. The formation of activated oxygen species (AOS), upon oxidation, by dioxygen has been investigated with the aid of various possible nitrogen-doped models. The detailed reaction mechanism for the oxidation of benzyl alcohol and ethanol by the three AOS obtained in the present study has been unraveled. Results indicate that the ketonic oxygen species oxidizes aromatic alcohol with minimum activation energy of ∼26.5 kcal/mol. On the contrary, the activation energy for the oxidation of alkyl alcohol by AOS present at the center is the lowest, which is also similar to that of ketonic oxygen species. On the basis of the results, a generalized reaction mechanism has been arrived for alcohol oxidation by nitrogen-doped graphene. Findings reveal the valuable lead information for the optimal control over selective oxidation of alcohol by N-doped graphene based on dopant concentration and temperature

Role of Graphitic Nitrogen and π‐Conjugated Functional Groups in Selective Oxidation of Alcohols: A DFT based Mechanistic Elucidation

“…This observation is in agreement with the experimental report that, the alcohols with more than one functional groups shows good selectivity than alcohols with one functional group . This can also be validated with our previous report on oxidation of aliphatic alcohols where, high activation barriers for oxidation of ethyl alcohol are witnessed when compared to the oxidation of benzyl alcohol, and allylic alcohols in the present study …”

“…Because of high tendency for radical reactions, hydroxyl hydrogen (H1) of AA transfers to the O2 of AOS via transition state AT 1C to form O2−H1 bond (intermediate AI 1C ). The activation barrier for the hydrogen hopping reaction is calculated to be 21.3 kcal mol −1 , which is observed to be low when compared to the previously reported values for aliphatic alcohol . Hydrogen bonding interactions between oxygen (O3) of AA and H1 are observed (1.87 Å) in AI 1C along with weak hydrogen bonding interaction (2.52 Å) between O1 of AOS and H2 of AA and O−O bond elongates to 1.42 Å.…”

“…The aerobic oxidation reaction of alcohols is initiated with the activation of dioxygen by N‐doped graphene to form Activated Oxygen Species (AOS). Three possible AOS have been considered for the present study based on our previous reports . They are, SubC‐substrate with N‐doping at the center, SubE‐substrate with N‐doping at the edge and SubK‐ketonic substrate with N‐doping at the edge.…”

“…Hence, by computing pKa values of the intermediates formed elicits the role of additional functional groups. For the better understanding, we have considered the intermediates formed in the second hydrogen transfer step of the aerobic oxidation of alkyl alcohols and benzyl alcohols from our previous reports . The intermediates considered are CH 3 CH 2 O .…”

“…The model system of graphene and activated oxygen species (AOS) were extensively studied in our previous studies, which formed the basis for the current investigation . The models adopted in this study were benchmarked against the periodic zig‐zag nanoribbon and hence proved to be potentially useful in studying the catalytic reactions.…”