Importance of amine in carbon dioxide conversion to methanol catalyzed by Ru-PNP complex

Abstract

The theoretical insight with DFT calculations was carried out to explore the mechanism of CO2 conversion to methanol. The tandem catalysis with the Ru-Macho-BH complex and ethylenediamine (EDA) was investigated. Three possible routes, Route A, B, and C, were realized for the process. Route A (δE > 60 kcal mol−1) consists of three steps, formic acid formation, formamide formation, and methanol formation (from alkanolamine). Route B (δE = 35.07 kcal mol−1) contains four steps, formic acid formation, formamide formation, formaldehyde synthesis, and methanol formation (from formaldehyde). Route C (δE = 39.14 kcal mol−1), a non-amine route, involves three steps, formic acid formation, formaldehyde dehydration, and methanol formation. Route B has the lowest energetic span and, thus, we propose it as a favorite route for CO2 conversion to methanol. Although Route C has a lower activation barrier, it has a larger δE. The exogenic reaction of the formamide formation step in Route B can affect lower energy requirements in further methanol formation. In the absence of EDA, CO2 hydrogenation to formic acid cannot produce, because the formate form is trapped by the catalyst. Thus, it can explain why CO2 conversion to methanol is impossible without the presence of amines.