Cross-Linking Amine-Rich Compounds into High Performing Selective CO2 Absorbents

Abstract

Amine-based absorbents play a central role in CO2 sequestration and utilization. Amines react selectively with CO2, but a drawback is the unproductive weight of solvent or support in the absorbent. Efforts have focused on metal organic frameworks (MOFs) reaching extremely high CO2 capacity, but limited selectivity to N2 and CH4, and decreased uptake at higher temperatures. A desirable system would have selectivity (cf. amine) and high capacity (cf. MOF), but also increased adsorption at higher temperatures. Here, we demonstrate a proof-of-concept where polyethyleneimine (PEI) is converted to a high capacity and highly selective CO2 absorbent using buckminsterfullerene (C60) as a cross-linker. PEI-C60 (CO2 absorption of 0.14 g/g at 0.1 bar/90°C) is compared to one of the best MOFs, Mg-MOF-74 (0.06 g/g at 0.1 bar/90°C), and does not absorb any measurable amount of CH4 at 50 bar. Thus, PEI-C60 can perform better than MOFs in the sweetening of natural gas.

Pentaethylenehexamine-C60 for Temperature Consistent Carbon Capture

“…It is immediately evident that the rate of CO2 sorption and the resulting CO2 capture capacity after about 5-h exposure are higher at increased temperature. This type of temperature dependence is common in sorbent materials where the kinetics of sorption is limited by slow CO2 mass transport in the bulk of the absorbent, as previously seen and discussed for PEI-C60 [14]. This result is somewhat surprising for PEHA-C60 since linear PEHA is about one hundred times smaller than branched PEI (232.37 g/mol and 25,000 g/mol, respectively), and PEHA has only 2 ending primary amino groups per molecule compared to about 140 for PEI (assuming a primary:secondary:tertiary amino groups ratio of 1:2:1).…”

“…This result is somewhat surprising for PEHA-C60 since linear PEHA is about one hundred times smaller than branched PEI (232.37 g/mol and 25,000 g/mol, respectively), and PEHA has only 2 ending primary amino groups per molecule compared to about 140 for PEI (assuming a primary:secondary:tertiary amino groups ratio of 1:2:1). It appears that two possible factors might be affecting the temperature dependence of CO2 capture of polyaziridine materials like PEHA and PEI: (1) each terminal primary amino group of PEHA can react with a different C60 molecule to form a bridge and then consequently a cross-linked network, as already found for PEI [14], and (2) there might be an activation energy barrier to the successful sorption of CO2, a barrier that is easier to overcome at higher temperature, although purely speculative since we provide no direct evidence for this consideration. In any case, a CO2 sorption plateau is achieved only for the sorption at 90.2 °C in about 5 h (Figure 1), at lower temperature more time is needed.…”

“…The isolated dry product was a compact brown cake easily reduced to fine powder when rubbed between fingers. These product and synthetic procedure are notably different to those of PEI-C60 [14], since the thick molasses-like consistence of PEI requires the use of solvents to allow for an intimate dispersion and contact of fullerene and amine, and the resulting PEI-C60 has a characteristic rubber consistence not evident in the case of PEHA-C60.…”

“…Within this research effort, our group has focused its attention on the development of new materials made from the immobilization of amine compounds on different types of carbon nanomaterials. Building upon the work of Barron et al, on the covalent functionalisation of nanocarbons with amines [12,13], we have developed fullerene-cross-linked polyethyleneimine (PEI)-based sorbents and characterized their CO2 capture performance [14][15][16][17][18]. In particular, PEI-C60 achieved very high CO2 capture selectivity and a maximum sorption capacity of 200 mg/g (mg CO2/g sorbent) [14].…”

“…Building upon the work of Barron et al, on the covalent functionalisation of nanocarbons with amines [12,13], we have developed fullerene-cross-linked polyethyleneimine (PEI)-based sorbents and characterized their CO2 capture performance [14][15][16][17][18]. In particular, PEI-C60 achieved very high CO2 capture selectivity and a maximum sorption capacity of 200 mg/g (mg CO2/g sorbent) [14]. We recently demonstrated that this high performance originates from a non-affinity hydrophobic-hydrophilic "repulsive" effect between C60 and amino groups that forces them to be actively exposed to CO2 [18].…”