Rehydroxylation-Enhanced Amine Loading in SBA-15 for Direct Air Capture of CO ₂

Maximizing amine loading remains a central challenge in improving the performance of amine-grafted adsorbents for the direct air capture (DAC) of CO 2 . Herein, we report a series of NH 2 -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 NH 2 -X-SBA-15-AT samples. Among the samples, NH 2 –Al-SBA-15-AT exhibited a breakthrough time exceeding 200 min and a dynamic CO 2 capacity of 0.67 mmol/g, corresponding to a more than 3-fold increase over conventionally grafted NH 2 –SBA-15. Rehydroxylation increased the surface hydroxyl density, thereby providing more anchoring sites for amine grafting and increasing the density of the effective CO 2 adsorption sites. In situ DRIFTS indicated the formation of ammonium carbamate species during CO 2 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.