N-decorated carbon nanotubes as a metal-free heterogeneous catalyst for mild CO2 chemical reduction to methanol

Exploiting carbon dioxide for the production of chemicals and fuels is one of the hottest topics of current sustainable catalysis with a relevant societal impact. Indeed, carbon dioxide is a cheap, non-toxic and renewable C1 carbon source that can be transformed into products of added value with the assistance of a catalyst.[1] On this ground, the conversion of carbon dioxide to methanol is a very important chemical process that allows the production of an alternative energy vector (MeOH). However, the CO2 molecule is extremely stable and its chemical conversion to target chemicals is difficult. Therefore, the preparation of new and efficient catalysts for promoting the process remains a challenging matter. Many reducing agents (hydrogen, hydrosilanes and hydroboranes) in combination with transition metal based heterogeneous catalysts have been employed successfully for the conversion of CO2 into products of added value.[2] Anyway, such a heterogeneous technology faces with economic and environmental impacts linked to the use of metals. On a longer term vision, a sustainable technology for CO2 reduction based on a cheap alternative is offered by metal-free catalysts. Recently, a variety of organocatalysts operating under homogeneous conditions has emerged for CO2 reduction into valuable products.[3,4] Among these, N-heterocycles featured by N-[5] or C-[6] basic sites have been reported as good homogeneous candidates for CO2 reduction to methanol in the presence of various hydroboranes at ambient conditions. Following our experience on metal-free N-decorated carbon nanomaterials (N-CNMs) successfully employed in several key catalytic transformations,[7] carbon nanotubes grafted with pyridine groups have been scrutinized as the first heterogeneous metal-free systems in the challenging CO2 hydroboration to methoxyborane. The prepared materials showed high catalytic activity with high TONs and complete recyclability thus candidating as a heterogeneous benchmark for the process.[8] In addition, on the joint basis of experimental data and ab initio simulations, a reaction mechanism has been proposed suggesting the key role of the carrier as an electronic reservoir for the dangling pyridine active arms.