Pyridine-decorated carbon nanotubes as heterogeneous metal-free catalysts for mild CO2 reduction to methanol with hydroboranes

Exploiting carbon dioxide for the production of chemicals and fuels is one of the hottest topics of current sustainable catalysis. On this ground growing interest is devoted to the conversion of carbon dioxide to methanol regarded as a valuable alternative energy source. Unfortunately, CO2 chemical inertness and thermodynamic stability make its reduction difficult. Many reducing agents in combination with transition metal-based heterogeneous catalysts have been employed successfully for the conversion of CO2 into products of added value but, on a longer term vision, a sustainable, cheap and environmentally benign alternative is offered by metal-free catalysts. Recently, a variety of organocatalysts operating under homogeneous conditions has emerged for CO2 reduction into valuable products. In particular, N-heterocycles featured by N- or C- basic sites have shown remarkable activity in CO2 hydroboration to methanol. Following our experience on metal-free N-decorated carbon nanomaterials (N-CNMs) successfully employed in several key catalytic transformations, pyridine-decorated carbon nanomaterials have been tested for chemical reduction of CO2 to methanol with different hydroboranes. Catalyst recycling tests have also been run showing complete recovery of its performance even after several runs. With turn-over numbers close to those claimed for other metal and metal-free homogeneous catalysts of the state-of-the-art and its effective re-use in catalysis, pyridine-decorated carbon nanomaterial candidates as a heterogeneous benchmark for this process. In addition, a hydroboration mechanism has been proposed on the joint basis of experimental data and ab initio simulations, suggesting the key role of the carbon nanotube carrier as an electronic reservoir for the dangling pyridine active arms.