Catalytic upgrading of ethanol to n-butanol using an aliphatic Mn–PNP complex: theoretical insights into reaction mechanisms and product selectivity

Abstract

The selective and efficient catalytic upgrading of ethanol to n-butanol is much desired as the utilization of n-butanol as a fuel additive has attracted considerable attention due to its advantages over ethanol. However, the lack of understanding of the mechanistic pathway for the efficient conversion and selective upgrading of ethanol into n-butanol catalyzed by an aliphatic PNP based Mn complex has inspired us to carry out a systematic study using density functional theoretical calculations. We find that the dehydrogenation of ethanol into acetaldehyde controls the conversion of the reaction into the product and the product selectivity can directly be controlled by the aldol condensation and hydrogenation of crotonaldehyde into higher carbon products (C₄₊) and n-butanol, respectively. Our study reveals that the N–H functionality of the Mn–PNP complex shows a metal–ligand cooperation (MLC) mechanism during the reaction. It shows an excellent hydrogenation nature toward the conversion of crotonaldehyde to n-butanol over the aldol condensation of crotonaldehyde. Our microkinetic modeling study reports a product selectivity of 99% and 12.6% yield of butanol, which are in excellent agreement with the experimental findings (selectivity of 92% and 9.8% yield of butanol). Our reaction free energies and microkinetic modeling studies predict the most favorable reaction mechanism for the selective upgrading of ethanol to n-butanol, which provides a fundamental role of such catalysts and contributes insights into the development of such bifunctional homogeneous catalysts.