Effect of tertiary amine selection on CO2 to formic acid hydrogenation with Au-np catalyst

The influence of the tertiary amine on CO2 conversion and product selectivity for the direct hydrogenation of CO2 towards formic acid is experimentally studied in batch and packed bed reactors using a Au/TiO2 catalyst. A wide range of tertiary amines were selected, based on their boiling point, kinetic diameter, pKa and solubility with alcohol-based solvents and formic acid. Following these criteria, 15 amines were selected having a wide range of pKa values and molecule sizes. To allow for an initial screening of kinetic rates, formic acid decomposition reactions were performed in batch reactors using an Au/TiO2 catalyst. Following the selection of the most promising amines in terms of CO2 conversion rates, high pressure fixed bed experiments were executed. Steric hindrance and the pKa of the tertiary amine were identified as key parameters influencing both the observed kinetic rates as well as the CO2 conversion. The usage of solventless polar amines such as diethylethanolamine allowed for FA productivity up to 5x that of the benchmark triethylamine system. Catalyst deactivation of the Au/TiO2 catalyst was observed for all amines studied within this work, and the deactivation mechanism was shown to be sintering of the Au nanoparticles with no significant leaching, morphological changes or oxidation of the Au species observed.