Establishing ultraporous permanently polarized hydroxyapatite as a green and highly efficient catalyst for carbon dioxide conversion in continuous flow under mild conditions†

We present the use of an ultraporous permanently polarized hydroxyapatite (upp-HAp) catalyst for continuous and highly efficient production of formic acid (predominant) and acetic acid using wet CO₂ (i.e. CO₂ bubbled into liquid water) as a reagent. In all cases, reactions were conducted at temperatures ranging from 95 to 150 °C, using a CO₂ constant flow of 100 mL s⁻¹, and without applying any external electric field and/or UV radiation. Herein, we study how to transfer such a catalytic system from batch to continuous reactions, focusing on the water supply (proton source): (1) wet CO₂ or (2) liquid water in small amounts is introduced in the reactor. In general, the reduction of CO₂ to formic acid predominates over the C–C bond formation reaction. On the other hand, when liquid water is added, two interesting outcomes are observed: (1) the yield of products is higher than in the first scenario (>2 mmol g_c⁻¹·min⁻¹) while the initial liquid water remains largely available due to the mild reaction temperature (95 °C); and (2) a high yield of ethanol (>0.5 mmol g_c⁻¹·min⁻¹) is observed at 120 °C, as a result of the increased efficiency of the C–C bond formation. Analysis of kinetic studies through temporal and temperature dependence shows that CO₂ fixation is the rate limiting step, ruling out the competing effect of proton adsorption on the binding sites and confirming the crucial role of water. The activation energy for the CO₂ fixation reaction has been determined to be 66 ± 1 kJ mol⁻¹, which is within the range of conventional electro-assisted catalysts. Finally, mechanistic insights on the CO₂ activation and role of the binding sites of upp-HAp are provided through isotopic-labeling (¹³CO₂) and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies.