A comprehensive evaluation of amine-impregnated silica materials for direct air capture of carbon dioxide

The potential of amine-impregnated silica materials for Direct Air Capture (DAC) of CO2 was investigated. A commercially available mesoporous silica was impregnated with tetraethylenepentamine (TEPA) and polyethylenimine (PEI) with molecular weights of 600, 1200, and 1800, each using amine loadings of 10–50 wt%. The materials were screened for equilibrium CO2 uptake, amine efficiency, and adsorption kinetics at temperatures of 25–35 °C. For all loadings, the CO2 uptakes followed the order of TEPA > PEI-600 > PEI-1200 ≈ PEI-1800. Also, for all amines, the materials synthesized with 50% loading achieved the highest equilibrium CO2 uptakes despite suffering from low amine efficiency and slow adsorption kinetics due to diffusional limitations. For these materials, increasing the adsorption temperature from 25 to 30 and 35 °C enhanced equilibrium CO2 uptake and amine efficiency, indicating kinetically controlled adsorption. In contrast, for all amines, the materials synthesized with 40% loading achieved a combination of high equilibrium CO2 uptake, good amine efficiency, and fast adsorption kinetics; therefore, they were selected to undergo 50 successive adsorption–desorption cycles. Despite an initially high CO2 uptake, TEPA suffered from low thermal stability due to its volatility, resulting in an amine loss of 55% and CO2 uptake loss of 29%. In contrast, bulkier amines such as PEI-600 outperformed TEPA, showing high thermal stability (1.4% amine loss) and low CO2 uptake loss (0.8%). Selected materials underwent column-breakthrough experiments in the presence of 400 ppmv CO2 in N2. Under dry conditions, TEPA achieved a longer breakthrough time than PEI-600 (13 versus 4 min). Under humid conditions, the breakthrough time for PEI-600 increased from 4 to 15 min, confirming previous reports that humidity boosts the CO2 uptake of amine-modified materials.