Aqueous Li+/Al3+ alkaline solution for CO2 capture and the massive Li–Al–CO3 hydrotalcite precipitation during the interaction between CO2 gas and the Li+/Al3+ aqueous solution

Abstract

The concentrations of Al³⁺ and Li⁺ in an aqueous alkali affect the ability of the solution to capture CO₂. When CO₂ gas was forced into such a solution, CO₂ was captured via the formation of a carbonate-containing compound. The carbonate-containing compound was Li–Al–CO₃ hydrotalcite (Li–Al–CO₃ LDH) or amorphous aluminium hydroxycarbonate. The former compound contained 2.84–3.22 wt% carbon, while the latter contained less carbon (1.52–1.97 wt%). The Li–Al–CO₃ LDH was porous with nanosized pores and had a BET surface area of 114.8–161.8 m² g⁻¹. The injection of a relatively low CO₂ gas flow rate into the Al³⁺- and Li⁺-containing aqueous solution favoured the production of Li–Al–CO₃ LDH. For example, forcing CO₂ gas into the aqueous solution at a flow rate of 70 mL min⁻¹ produced Li–Al–CO₃ LDH. Injecting CO₂ at a higher flow rate (120 mL min⁻¹) into the same solution produced amorphous aluminium hydroxycarbonate. A CO₂ capture capacity (mmol per gram AlLi IMC) of up to ∼30 mmol g⁻¹ (1.30 kg CO₂ per kg of AlLi) was achieved in 120 s. The fall in the pH of the alkaline solution during CO₂ bubbling was the critical factor that determined whether the final product was Li–Al–CO₃ LDH or amorphous aluminum hydroxycarbonate.