https://doi.org/10.1039/D1MA01072G
“Promising candidates to simultaneously access these adsorption phenomena are solid cross-linked polyamine-based materials. Supported cross-linked polyamine CO2 adsorbents have been reported. Polyethyleneimine (PEI) has been cross-linked with an epoxy resin and coated on a glass fibre matrix;32 or cross-linked with glyoxal, oxalic acid, 1,3-butadiene diepoxide and epichlorohydrin, and supported on a mesostructured cellular silica foam.33,34 A recent study demonstrated the formation of a new polyamine via the cross-linking of 1,3,5-tris(bromomethyl)benzene with ethylene diamine, which was then impregnated onto mesoporous silica SBA-15. Under air capture conditions, the material of 60 wt% organic loading adsorbed up to 1.9 mmol CO2 g−1 SiO2, equating to 0.75 mmol g−1 adsorbent.35
In their unsupported form, cross-linked polyamine adsorbents constitute a relatively new family of solid amine adsorbents, which has been significantly advanced in recent years by several groups, including ours.36,37 One particular advantage of these versatile materials over their supported polyamine counterparts is in their economy of mass: they are composed purely of amine-containing polymers that are cross-linked into solid structures. This negates the need for a support, enabling them to be lighter and potentially more easily regenerated and processed.37
Such adsorbents have been synthesised by Andreoli et al. using C60,38–41 and carbon nanotubes,42 to cross-link PEI or polypropylenimine. PEI (Mw 25 000 Da) cross-linked with C60 showed excellent CO2 adsorption of 0.140 g g−1 (3.18 mmol) at 90 °C under 0.1 bar CO2, with high selectivity for CO2 over CH4 and N2.43 We recently swapped C60 for the epoxy resin bisphenol A diglycidyl ether as a more economical cross-linker to give an adsorbent with a CO2 uptake capacity of 0.101 g g−1 (2.30 mmol g−1) at 90 °C, 0.1 bar CO2.36 Hwang et al. have reacted PEI (Mw 25 000 Da) with glutaraldehyde via an inverse emulsion technique.44 The adsorbent cPEI-GA36 had a surface area of 10 m2 g−1 and displayed the fastest adsorption and highest CO2 capacity of 2.18 mmol g−1 at 75 °C (1 atm CO2). Thompson et al. used polyaldehyde phosphorus dendrimers to cross-link PEI (Mw 600).45 At 65 °C, under 1 atm 30 mL min−1 CO2/60 mL min−1 He, PEI cross-linked with hexakis(4-formylphenoxy)cyclo(triphosphazene) (1-G0/600PEI) adsorbed 13.6 wt% CO2, (3.32 mmol g−1), reducing to 4.1 wt% (0.93 mmol g−1) at 25 °C. Hydrogel beads have been synthesised by Xu et al. by cross-linking PEI (Mw 25 000 Da) with epichlorohydrin.46 Although CO2 uptake of the dry adsorbent was minimal, adsorption by hydrated PEI HB-4.0% EPC (under 15% CO2/N2 at ambient temperature) was 0.0602 g g−1 (1.37 mmol g−1).
As indicated in Table 1, these adsorbents are mostly prepared with a higher molar ratio of amine to cross-linking reactive site, therefore having plentiful free primary and secondary amines remaining within PEI for the chemisorption of CO2. They are most effective at elevated temperatures, or under hydrated conditions, due to the high flexibility and self-affinity of the cross-linked polymeric networks, resulting in insufficient gas diffusion at lower adsorption temperatures. This effect has been demonstrated by Yoo et al. who synthesised macroporous adsorbents from PEI (Mw 750 000 Da) cross-linked with poly(ethylene glycol) diglycidyl ether via an ice-templating method.37 The optimum temperature of adsorption was dependent on the ratio of cross-linker used during preparation. Under 10% CO2/He, 1 atm, the adsorbent with the lowest cross-linker volume (E50-FZ) adsorbed the least CO2 at 25 °C, but the highest at 75 °C, at 3.00 mmol CO2 g−1. Meanwhile, the adsorbent with the highest cross-linker volume (E200-FZ) adsorbed 2.01 mmol CO2 g−1 at 25 °C, but less than 0.6 mmol g−1 at 75 °C. The latter adsorbent had a smaller average pore size of 13 μm and a larger surface area. In this case, more of the primary and secondary amines are involved in cross-linking, resulting in reduced basicity such that chemisorption is impaired, however this is over-compensated by the increased surface area and better diffusion enabling improved uptake at 25 °C.”