Exploration of unprecedented catalytic dehydrogenation mechanism of methylamine-water mixture in presence of Ru-pincer complex: A systematic DFT study

Abstract

A mechanistic approach to explore the complete free energy profile for the production of three molecules of H2 from methylamine-water mixture promoted by Ru-pincer complex has been proposed in this article. The first cycle of the catalytic pathway deals with the conversion of methylamine to methyleneamine followed by the release of first H2 from hydrogenated pincer. Among all four hydrogen release processes revealed here, acid-based catalyzed H2 elimination is found to be kinetically the most favored pathway. The first and second cycles are connected through an intermediate catalytic cycle, in which methyleneamine is catalytically hydrolyzed to aminomethanol/formaldehyde. After the formation of aminomethanol, two pathways occur in parallel; in one pathway formamide is directly generated from aminomethanol via hydrogen abstraction and in another pathway tautomeric derivative of formamide is produced, which is readily transformed to formamide in presence of water. The generation of hydrogenated pincer further allows the release of second molecular H2 following the similar H2 elimination pathways. In the third catalytic cycle, the hydrogen abstraction from formamide by pincer leads to produce iminomethanone along with the hydrogenated pincer, which further releases the third molecule of H2. In presence of water, iminomethanone undergoes dissociation to produce CO2 and NH3. Apart from the direct H2 generation pathway, we have also identified an alternative pathway, where the formaldehyde formation pathway acts as the bridge between direct and alternative pathways. The catalytic hydrolysis of formaldehyde to methanediol followed by the dehydrogenation to formic acid opens up the possibility of releasing second molecular H2 alternatively. Afterwards, the dehydrogenation of formic acid to CO2 is an alternative pathway for releasing third molecule of H2. Overall, the entire study gives an imperative idea about the catalytic performance of Ru-pincer complex in the dehydrogenation of methylamine-water mixture.