Solid Amine Sorbents
Solid amine sorbents carry amine groups on a porous solid so that CO2 can be captured chemically without the large heat load of warming an aqueous amine solution during regeneration. The amine may be physically impregnated (for example PEI loaded into a porous support), grafted to an oxide such as silica, or built into a polymer, as in amine-functionalized ion-exchange resins. Each route trades off capacity, diffusion of CO2 to the amine sites, leaching, hydrothermal stability and resistance to degradation. Primary and secondary amines react with CO2 to form carbamic acid or carbamate (and, when water is present, bicarbonate), and these products must be decomposed thermally to regenerate the sorbent. For process design the relevant figure is the working capacity (delta-loading), the difference in loading between the capture and regeneration conditions, rather than the total capacity. See the broader CO2 Capture article.
Supported tertiary amidines on activated carbon (2010)
Alesi, Gray and Kitchin impregnated the bicyclic tertiary amidines DBN and DBU onto activated carbon (up to about 2.7 mol amidine/kg) so that each molecule offered a single, countable basic site (Alesi et al. 2010). In packed-bed tests with roughly 10% CO2, DBN captured up to about 0.86 mol CO2/kg (0.30 mol CO2 per mol amidine) at 29 C and consistently outperformed DBU, with capacity falling as adsorption temperature rose. No CO2 was captured from a dry feed, showing that these amidines bind CO2 only as bicarbonate, and too much humidity (7.4%) cut efficiency in half because the hydrophilic carbon support soaked up water. The study shows that support water uptake, not just amine chemistry, can limit sorbents that rely on bicarbonate formation. Summary: alesi2010.
Characterizing a primary-amine ion-exchange resin (2012)
Alesi and Kitchin characterized Lewatit VP OC 1065, whose amines are bonded to a polystyrene backbone through C-C bonds that should resist the steam hydrolysis seen for Si-O-C linked silica sorbents (Alesi 2012). Its capacity in 10 vol% CO2 was 1.85 to 1.15 mol/kg from 30 to 70 C, its water uptake was low (1.5 mol/kg), and it was stable over 18 cycles. The capture splits into a pressure swing component, attributed to CO2 solubility in the polymer, and a temperature swing component from carbamate chemistry identified by in situ DRIFTS. Isobaric loading curves at 10 and 100 vol% CO2 let the working capacity be estimated for different regeneration choices, showing that regeneration under hot CO2 at 150 C keeps most of the working capacity obtained with inert regeneration. EDS suggested a theoretical amine content of about 6.7 mol N/kg, well above the measured capacities, indicating that some amines are inaccessible. Summary: alesi2012.
Supported amines in a technology-maturity review (2012)
In their review of capture technology, Rubin et al. place supported amines among the laboratory and bench-scale solid sorbent options for post-combustion capture (Rubin et al. 2012). They note the main appeal (amine chemistry without an aqueous solution that must be repeatedly heated and cooled, and lower heat capacity than liquid solvents) along with the challenges that research was then addressing: thermal stability, fouling and degradation in the presence of SO2, plus process issues common to all solid sorbents such as moving heat into and out of the solid, attrition, pressure drop and the choice between fluidized, packed and transport bed contactors. Summary: rubin2012.
Degradation by flue gas impurities (2013)
The follow-up study on the same resin found that 4% O2 was tolerated over 17 cycles but 431 ppm SO2 irreversibly poisoned most of the capacity within a few cycles, and that seven days in air at 120 C oxidized away about 1.1 mol amine/kg (Hallenbeck 2013). Lifetime under real flue gas, not just fresh capacity, therefore limits the economics of these sorbents. See Catalyst Poisoning. Summary: hallenbeck2013.