(340c) Neutron Scattering Characterization of CO2 Sorbent Materials : The Morphology of Polyethyleneimine in Mesorporous Silica and Its Performance Impacts

Composites of polyethyleneimine (PEI) and mesoporous silica are highly effective, reversible adsorbents for CO₂, both from flue gas and in direct air-capture applications.  The morphology of the PEI within the silica pores can strongly impact the overall carbon capture efficiency and rate of saturation. Here, we directly probe the shape of the supported polymer as a function of weight loading through small-angle neutron scattering (SANS). Combined with textural characterization from physisorption analysis, the data indicates that PEI first forms a conformal coating on the pore walls, but at higher loadings the polymer aggregates and forms plugs that span the pore diameter. This model is consistent with the observation of reduced amine-efficiency (CO₂/N binding ratio) at low loadings and points to a tradeoff between achieving high chemical accessibility of the amine binding sites, which are inaccessible when they strongly interact with the silica, and high accessibility for mass transport, which can be hampered by diffusion through PEI plugs at high loading. We illustrate this design principle by demonstrating higher CO₂ capacity and uptake rate for PEI supported in a modified silica, which exhibits repulsive interactions with PEI.