(118c) Critical Role of Surface Hydroxyl-Amine Interactions in Direct Air Capture Using Single Component and Composite PEI-Silica Sorbents

Polyethyleneimine (PEI)-impregnated silica adsorbents have been widely explored as candidates for direct air capture, and the co-impregnation of polyethylene glycol (PEG) is a commonly used approach for improving amine efficiency. Improvements in amine utilization from PEG addition were previously rationalized mainly from changes in CO₂ binding chemistry, amine-amine cooperative interactions, and the mitigation of diffusion limitations. We present herein evidence for the determinative role of amine-surface hydroxyl interactions in controlling CO₂ sorption behavior in the PEI-silica system. PEG addition appears to disrupt surface hydroxyl-amine interactions to release “inaccessible” surface-bounded amine sites to free PEI. Unfavorable surface hydroxyl-amine interactions can also be mitigated through surface silylation with hexamethyldisilazane (HMDS) which is, however, not effective when PEG is present, suggesting the analogous role of PEG co-impregnation and HMDS-capping in PEI population transfer for improving amine utilization. Thermal degradation behaviors and FTIR spectra also supports the change of PEI morphology after PEG introduced. Insights gained into the role of amine-surface hydroxyl interactions provide guidelines for the advanced design of aminosilica sorbents for direct air capture applications.