Cycling of potassium–carbonate co-substituted hydroxyapatite compositions for improved carbon dioxide capture at 500 °C

There is an immediate need to reduce CO2 emissions if the impacts of climate change are to be mitigated. Whilst carbon capture & storage offers such a route, costs associated with this technology necessitate that more efficient means of capture be developed—including studying materials that may have improved performance and/or end-use application as functional materials post-capture. Stoichiometric hydroxyapatite and potassium–carbonate co-substituted apatites were subjected to carbonation–regeneration cycles at 500 °C to explore their applicability for carbon capture, with chemical characterisation performed after each carbonation/regeneration. The CO2 carrying capacity increased alongside the degree of co-substitution; the material with the highest as-prepared potassium/carbonate content exhibited a carrying capacity of 0.57 mmol g−1 sorbent. This was more than seven times that of stoichiometric hydroxyapatite (0.08 mmol g−1 sorbent) and is the first demonstration showing that the CO2 carrying capacity can be increased via ionic substitution mechanisms. It was hypothesised that this improved performance was due to co-substitution having generated additional structural sites for interchange with CO2. In an extended study, the CO2 carrying capacity of one composition was observed to improve following an initial drop after the first cycle. This is significant when compared to calcium oxide, whose reactivity towards CO2 declined by 42% between the first and second cycles alone. FTIR spectroscopy suggested that the apatite material functioned by way of the simultaneous substitution of carbonate ions onto hydroxyl and phosphate sites during carbonation followed by the subsequent loss of these ions when the material was heated in air. Data from this study supports the concept that these materials could play a role as CO2 sorbents for industrial carbon capture and justifies further studies in more complex systems that could evaluate real-world application.