The development of simple, cost-effective and sustainable methods to transform CO2 into feedstock chemicals is essential to reduce the dependence on fossil fuels of the chemical industry. Here, we report the selective and efficient catalytic hydrogenation of CO2 to formic acid (FA) using a synergistic combination of an ionic liquid (IL) with basic anions and relatively simple catalysts derived from the precursor [Ru3(CO)12]. Very high values of TON (17000) and TOF have been observed and FA solutions with concentrations of up to 1.2 M have been produced. In this system, the imidazolium based IL associated with the acetate anion acts as precursor for the formation of the catalytically active Ru-H species, catalyst stabilizer and as an acid buffer, shifting the equilibrium towards free formic acid. Moreover, the IL acts as an entropic driver (via augmentation of the number of microstates), lowering the entropic contribution imposed by the IL surrounding the catalytically active sites. The favorable thermodynamic conditions enable the reaction to proceed efficiently at low pressure pressures, and furthermore the immobilization of the IL onto a solid support facilitates the separation of FA at the end of the reaction.

CO2 is selectively hydrogenated to HCO2H or hydrocarbons (HCs) by RuFe nanoparticles (NPs) in ionic liquids (ILs) under mild reaction conditions. The generation of HCO2H occurs in ILs containing basic anions, whereas heavy HCs (up to C21 at 150 °C) are formed in the presence of ILs containing nonbasic anions. Remarkably, high values of TONs (400) and a TOF value of 23.52 h–1 for formic acid with a molar ratio of 2.03 per BMI·OAc IL were obtained. Moreover, these NPs exhibited outstanding abilities in the formation of long-chain HCs with efficient catalytic activity (12% conversion) in a BMI·NTf2 hydrophobic IL. The IL forms a cage around the NPs that controls the diffusion/residence time of the substrates, intermediates, and products. The distinct CO2 hydrogenation pathways (HCO2H or FT via RWGS) catalyzed by the RuFe alloy are directly related to the basicity and hydrophobicity of the IL ion pair (mainly imposed by the anion) and the composition of the metal alloy. The presence of Fe in the RuFe alloy provides enhanced catalytic performance via a metal dilution effect for the formation of HCO2H and via a synergistic effect for the generation of heavy HCs.

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.

The efficient transformation of CO2 into chemicals and fuels is a key challenge for the decarbonisation of the synthetic production chain. Formic acid (FA) represents the first product of CO2 hydrogenation and can be a precursor of higher added value products or employed as a hydrogen storage vector. Bases are typically required to overcome thermodynamic barriers in the synthesis of FA, generating waste and requiring post-processing of the formate salts. The employment of buffers can overcome these limitations, but their catalytic performance has so far been modest. Here, we present a methodology utilising IL as buffers to catalytically transform CO2 into FA with very high efficiency and comparable performance to the base-assisted systems. The combination of multifunctional basic ionic liquids and catalyst design enables the synthesis of FA with very high catalytic efficiency in TONs of >8*105 and TOFs > 2.1*104 h−1.

Dedicated to Prof. Faruk Nome (UFSC-Brazil) in the occasion of his 65 th anniversary.