Probing the Distribution\nand Mobility of Aminopolymers\nafter Multiple Sorption-Regeneration Cycles: Neutron Scattering Studies

Solid-supported amines are effective CO<sub>2</sub> adsorbents\ncapable of capturing CO<sub>2</sub> from flue gas streams (10–15\nvol % CO<sub>2</sub>) and from ultradilute streams, such as ambient\nair (∼400 ppm CO<sub>2</sub>). Amine sorbents have demonstrated\npromising performance (e.g., high CO<sub>2</sub> uptake and uptake\nrates) with stable characteristics under repeated, idealized thermal\nswing conditions, enabling multicycle application. Literature studies\nsuggest that solid-supported amines such as PEI/SBA-15 generally exhibit\nslowly reducing CO<sub>2</sub> uptake rates or capacities over repeated\nthermal swing capture-regeneration cycles under simulated DAC conditions.\nWhile there are experimental reports describing changes in supported\namine mass, degradation of amine sites, and changes in support structures\nover cycling, there is limited knowledge about the structure and mobility\nof the amine domains in the support pores over extended use. Furthermore,\nlittle is known about the effects of H<sub>2</sub>O on cyclic applications\nof PEI/SBA-15 despite the inevitable presence of H<sub>2</sub>O in\nambient air. Here, we present a series of neutron scattering studies\nexploring the distribution and mobility of PEI in mesoporous silica\nSBA-15 as a function of thermal cycling and cyclic conditions. Small-angle\nneutron scattering (SANS) and quasielastic neutron scattering (QENS)\nare used to study the amine and H<sub>2</sub>O distributions and amine\nmobility, respectively. Applying repeated thermal swings under dry\nconditions leads to the thorough removal of water from the sorbent,\ncausing thinner and more rigid wall-coating PEI layers that eventually\nlead to slower CO<sub>2</sub> uptake rates. On the other hand, wet\ncyclic conditions led to the sorption of atmospheric water at the\nwall-PEI interfaces. When PEI remains hydrated, the amine distribution\n(i.e., wall-coating PEI layer thickness) is retained over cycling,\nwhile lubrication effects of water yield improved PEI mobility, in\nturn leading to faster CO<sub>2</sub> uptake rates.