Solid-supported amines are effective CO2 adsorbents
capable of capturing CO2 from flue gas streams (10–15
vol % CO2) and from ultradilute streams, such as ambient
air (∼400 ppm CO2). Amine sorbents have demonstrated
promising performance (e.g., high CO2 uptake and uptake
rates) with stable characteristics under repeated, idealized thermal
swing conditions, enabling multicycle application. Literature studies
suggest that solid-supported amines such as PEI/SBA-15 generally exhibit
slowly reducing CO2 uptake rates or capacities over repeated
thermal swing capture-regeneration cycles under simulated DAC conditions.
While there are experimental reports describing changes in supported
amine mass, degradation of amine sites, and changes in support structures
over cycling, there is limited knowledge about the structure and mobility
of the amine domains in the support pores over extended use. Furthermore,
little is known about the effects of H2O on cyclic applications
of PEI/SBA-15 despite the inevitable presence of H2O in
ambient air. Here, we present a series of neutron scattering studies
exploring the distribution and mobility of PEI in mesoporous silica
SBA-15 as a function of thermal cycling and cyclic conditions. Small-angle
neutron scattering (SANS) and quasielastic neutron scattering (QENS)
are used to study the amine and H2O distributions and amine
mobility, respectively. Applying repeated thermal swings under dry
conditions leads to the thorough removal of water from the sorbent,
causing thinner and more rigid wall-coating PEI layers that eventually
lead to slower CO2 uptake rates. On the other hand, wet
cyclic conditions led to the sorption of atmospheric water at the
wall-PEI interfaces. When PEI remains hydrated, the amine distribution
(i.e., wall-coating PEI layer thickness) is retained over cycling,
while lubrication effects of water yield improved PEI mobility, in
turn leading to faster CO2 uptake rates.