Critical Role of Amine-Surface Hydroxyl Interactions in Direct Air Capture over Amine-Impregnated Sorbents

Polyethylenimine (PEI)-containing aminosilica adsorbents have been evaluated extensively as candidates for direct air capture, and the co-impregnation of polyethylene glycol (PEG) is a commonly used approach for improving amine utilization. Trends in amine utilization as a function of PEI/PEG loading have thus far been rationalized on the basis of changes in binding chemistry, amine-amine cooperative interactions, diffusion limitations, and amine distribution. We present herein evidence for the role of amine-surface hydroxyl interactions in controlling CO₂ sorption behavior in both PEI and PEI-PEG sorbents, and the corresponding disproportionate favorable impact of PEG addition specifically at low PEI loadings. PEG addition appears to disrupt surface hydroxyl-amine interactions that otherwise lower amine utilization, and improvements in capture performance resulting from its incorporation wane as the fraction of amines devoid of surface interactions increases with PEI loading. Amine-surface hydroxyl interactions that are deleterious to adsorption performance can alternatively be mitigated through surface silylation with hexamethyldisilazane (HMDS). Wider consideration of such interactions in evaluating adsorption data will likely provide a more sound footing for the rational design of aminosilica sorbents for low-pressure CO₂ adsorption applications.