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

Abstract

Selective oxidation of alcohols to aldehydes is challenging reaction due to its accessibility to overoxidation. In this study, we have made an attempt to unravel the mechanistic aspects of selective oxidation of allyl alcohols that contain multiple functional groups catalyzed by N-doped graphene. The role of graphitic nitrogen and the presence of π-conjugated functional groups are demonstrated using the state-of-the-art density functional theory calculations. The detailed reaction mechanism for aerobic oxidation of allyl alcohol (AA) and cinnamyl alcohol (CA) are investigated. The formation of activated oxygen species (AOS) over N-doped graphene (NG) has been adopted from our previous report. The results revealed that ketonic AOS oxidizes allyl alcohols into aldehydes selectively with a relatively lower activation barrier of 20.1 kcal mol-1 . The oxidation of alcohols with the AOS formed at the edge results in high activation barriers owing to its high thermodynamic stability. Similarly, AOS formed at the center leads to the formation of H2 O2 along with high activation barriers. As a consequence, AOS formed at the center is less active when compared to ketonic AOS. The overoxidation of aldehyde is only possible due to the formation of H2 O2 . However, it is unlikely to happen due to unfavorable ambient conditions. The presence of multiple π-conjugated functional groups is responsible for the significant reduction in the activation barriers of the second hydrogen transfer step due to the stabilization of intermediate by increasing the acidic nature of the intermediates. On the basis of the results, a generalized reaction mechanism has been proposed. These results would definitely shed light on the effective fabrication of catalysts for oxidation of alcohol and sustainable energy.