Rehydroxylation-Enhanced Amine Loading in SBA-15 for Direct Air Capture of CO₂

Maximizing amine loading remains a central challenge in improving the performance of amine-grafted adsorbents for the direct air capture (DAC) of CO₂. Herein, we report a series of NH₂-X-SBA-15-AT adsorbents (X = Al, Ce, Ti, Zr, Fe, and Cr) prepared via a rehydroxylation strategy that combines heteroatom doping with an acid treatment. Before amine grafting, the acid-treated supports showed negligible breakthrough times (less than 2 min); after amine functionalization, the breakthrough times increased dramatically for all NH₂-X-SBA-15-AT samples. Among the samples, NH₂–Al-SBA-15-AT exhibited a breakthrough time exceeding 200 min and a dynamic CO₂ capacity of 0.67 mmol/g, corresponding to a more than 3-fold increase over conventionally grafted NH₂–SBA-15. Rehydroxylation increased the surface hydroxyl density, thereby providing more anchoring sites for amine grafting and increasing the density of the effective CO₂ adsorption sites. In situ DRIFTS indicated the formation of ammonium carbamate species during CO₂ adsorption and suggested that surface hydroxyls may participate in hydrogen bonding interactions with adsorbed amine-derived species. These interactions may help stabilize carbamate-containing intermediates and, thus, contribute to the observed differences in regeneration behavior among the adsorbents. These results demonstrate that rehydroxylation is an effective route to improve amine utilization on mesoporous silica and provide useful guidance for designing DAC adsorbents with an enhanced capacity and tunable regeneration characteristics.