Class I Amine-Functionalized Sorbents
Many works have been presented in the literature about this class of sorbents. Polyethylenimine (PEI) loaded onto a mesoporous silica-based molecular sieve was suggested by Xu et al. [
62] for CO
2 capture from the air. It is evident that PEI is the most important amine considered for this class; it can have a linear or branched structure. Moreover, PEI has attracted the attention of researchers due to its high amine content, low volatility and commercial availability [
63].
An overview of different works related to class I amine-functionalized sorbents investigated in the literature is shown in
Table 2.
An amine-functionalized sorbent made by PEI impregnation onto alumina has been studied in the work of Sakwa Noval et al. [
78]. An adsorption capacity of 0.75 molCO
2/kg at a high amine loading, with 303 K and simulated ambient air, was evaluated for a larger extruded monolith sorbent, and it was found to be higher than that of an alumina powder sorbent (0.7 molCO
2/kg). In addition, the authors found that, for the first sorbent, adsorption capacities for two to five cycles were stable but slightly lower than that for one cycle due to minor PEI leaching or rearrangement in the monolith, which has been observed for powder sorbents. The same amine, PEI, was impregnated with a loading up to 21 %wt onto a microporous polymer (PIM-1); an uptake of 0.2 molCO
2/kg at 308 K for simulated air and 1 molCO
2/kg for a flue gas with 14% of CO
2 were measured [
80]. However, at a higher amine loading, the CO
2 adsorption capacity was reduced due to the inaccessibility of the amine sites.
Goeppert et al. [
74] evaluated how different kinds of PEI impregnated onto fumed silica (linear, branched, with low and high molecular weights, respectively, of 1800 and 25,000 Da) influenced adsorption capacity and found the best uptake for the branched sample with a low molecular weight at a loading of 33 %wt (56 mgCO
2/g). At a higher amine loading (50 %wt), higher adsorption capacities were obtained of up to 107.3 mgCO
2/g for the PEI with an 800 Da molecular weight. However, PEI of lower molecular weight can be more prone to leaching than homologues with higher molecular weights. This could lead to a loss of capacity over time and the possible contamination of downstream equipment.
Chen et al. [
79] reported the impregnation of PEI onto non-polar resin HP20, measuring the adsorption capacity for pure CO
2 and simulated air with a CO
2 concentration of 400 ppm. The values for the investigated parameter were, respectively, 4.122 molCO
2/kg and 2.26 molCO
2/kg. This material exhibits promising potential for CO
2 capture from ambient air. Moreover, the authors found that a pore size of 43–68 nm is responsible for diffusion/adsorption phenomena, using a non-local density functional theory. On the other hand, they verified that temperature has a negative effect on adsorption due to exothermic reactions.
Amine mixtures have been investigated. In this context, Brilman and Veneman [
64] analyzed the adsorption capacity of a sorbent obtained by the impregnation of silica and polymer supports with tetraethylenepentamine (TEPA) and PEI at different temperatures and CO
2 concentrations. At 308 K and 400 ppm, they measured an adsorption capacity of 2.5 molCO
2/kg, using temperature swing adsorption for regeneration. With a sensitivity study, it was found that regeneration temperatures depend on CO
2 purity and that for the capture of CO
2 from the air a regeneration temperature above 125 °C is needed in order to maintain a reasonable cyclic capacity (above 1.5 molCO
2/kg).
An amine mixture consisting of PEI and poly(ethylene glycol) (PEG) impregnated onto a fumed silica support has been reported in Meth et al. [
71] as allowing good CO
2 uptake and at higher temperatures. The experimental results showed that the outside surface area of the support was more important than the inside porosity, offering a more effective surface area and morphology for the distribution of the amine for CO
2 capture without the negative side-effects of agglomeration and stickiness.
Better efficiencies were reported for an amine mixture of PEI and PEG200 loaded onto SBA-15 by Sakwa-Novak et al. [
73]; the CO
2 uptake was 0.79 molCO
2/kg. The authors found that the mixture of the two amines provided a way of tuning the sorbent performance and reducing capital costs (via increased amine efficiency), though with increased operating costs.
Generally, silica materials, such as commercial silica, fumed silica, SBA-15, silica fiber and mesocellular foam (MCF), have been widely investigated due to their large surface areas and high pore volumes [
72]. In addition, non-silica supports, such as mesoporous carbon and γ-alumina, have been studied, and other interesting supports have been taken into account. Fer example, PEI was loaded onto Mg–Al–CO
3 layered double hydroxide-derived mixed metal oxides (MMOs) in Zhu et al. [
72]. At the same amine loading (67%
w/
w), the proposed PEI/MMO sorbent showed a higher CO
2 uptake (2.27 molCO
2/kg) than that obtained with PEI/SBA-15 (1.92 molCO
2/kg) due to the ideal morphologies and nanostructures of their supports. Kinetics and stability were also better for the former, making this sorbent attractive for ultra-dilute CO
2 capture.
Another comparison between two different supports with PEI amines has been reported in Chaikittisilp et al. [
68], where SBA-15/PEI and γ-alumina/PEI sorbents were evaluated. The results showed that, under dry conditions at 298 K, the second provided higher CO
2 adsorption capacities and amine efficiencies, making alumina-supported amine materials promising for direct air capture technology. Moreover, the alumina-supported amine sorbent was more stable in short multicycle temperature swing tests and was more robust upon direct contact with steam.
The sorbent SBA-15/PEI has also been investigated by Wang et al. [
70], who reported a CO
2 uptake of 0.77 molCO
2/kg at 298 K. However, the best adsorption capacity was obtained at 348 K, with a breakthrough and saturation capacity of 63.1 and 66.7 mg CO
2/g, respectively. The sorbent can be easily and completely regenerated under mild conditions (50–110 °C) and is stable in cyclic operations for at least 20 cycles.
Class I sorbents have poor stability over repeated adsorption/desorption cycles. In order to improve the stability of basic PEIs and CO
2 adsorption kinetics, modified PEIs have been analyzed in Choi et al. [
65], who considered 3-aminopropyltrimethoxysilane (A-PEI/silica) and tetraethyl orthotitanate (T-PEI/silica) sorbents in addition to the traditional one, for which there is a stronger bond with silica surfaces. The authors found that this allowed higher adsorption capacity, stability and kinetics until after four temperature swing cycles. In the first cycle, CO
2 uptake was 2.36 molCO
2/kg, 2.26 molCO
2/kg and 2.19 molCO
2/kg for the traditional PEI/silica, A-PEI/silica and T-PEI/silica sorbent, respectively. However, after four cycles, the values for this parameter were, respectively, 1.65 molCO
2/kg, 2.05 molCO
2/kg and 2.16 molCO
2/kg.
In addition to PEI, other amines have been studied. In Sarazen et al. [
81], a branched poly(propylenimine) (PPI) was synthetized via cationic ring opening polymerization of azetidine using various acid initiators (HBr, HClO
4, HCl and CH
3SO
3H). The results showed that a CO
2 uptake of 0.31, 0.25, 0.15 and 0.17 molCO
2/kg was present for the sorbents prepared using HClO
4, HBr, HCl and CH
3SO
3H, respectively. However, HBr and HClO
4-initiated composites also showed slightly higher oxidative resistances than those initiated with HCl or CH
3SO
3H.
Pang et al. [
87] showed that PPI in a linear and dendritic form and supported in silica has better performance (in terms of CO
2 adsorption capacity) than PEI-based sorbents, a higher resistance to oxidative degradation and that it is less sensitive to oxygen during regeneration, allowing a longer life. The increased adsorption capacity is due to the increased basicity of the amines in PPI and decreased nearest-neighbor interactions. On the other hand, secondary amines linked by propylene spacers still have much of their CO
2 capture capability even after exposure to oxidation conditions relevant to carbon capture, making them less sensitive to regeneration conditions.
Kumar et al. [
82] studied TEPA loaded onto an SBA-15 support, an amine-modified mesoporous silicate, and found an adsorption capacity of 3.6 molCO
2/kg. However, the sorbent has problems in terms of degradation over repeated cycling and amine deactivation in the presence of NO
x, SO
x, O
2 and CO.
N,N′-dimethylethylenediamine (mmen) was impregnated onto a MOF, M
2(dobpdc) in the work of McDonald et al. [
84], showing an excellent CO
2 capture capacity at low concentrations. A CO
2 uptake of 2 molCO
2/kg was measured at 0.39 mbar in dry conditions. Dynamic gas adsorption/desorption cycling experiments demonstrated that mmen-Mg2(dobpdc) can be regenerated upon repeated exposures to simulated air, with cycling capacities of 1.05 mmol/g after 1 h of exposure.
Poly(allylamine) (PAA) is an alternative to PEI due to the high number of nitrogen groups that are present. In particular, compared to PEI, PAA has a higher density of primary amines and an extra carbon atom in the building unit, which may offer better thermal stability for CO
2 capture. Chaikittisilp et al. [
66] compared a sorbent made of silica mesocellular foam (MCF) impregnated with PAA and two other sorbents with branched and linear PEI. The results showed that at a low amine loading, PAA and branched PEI had an adsorption capacity higher than that of linear PEI in the simulated ambient air due to the very small primary amine loading in linear PEI. For the sorbent with PAA, branched PEI and linear PEI, CO
2 uptakes were, respectively, 0.63 molCO
2/kg, 0.61 molCO
2/kg and 0.44 molCO
2/kg. The authors also found that at higher loadings of PAA the amine sites did not seem to be as accessible, such that CO
2 capacities and amine efficiencies decreased.
Many studies have considered the effect of moisture on adsorption capacity. In Goeppert et al. [
69], adsorption capacities of 1.7 molCO
2/kg (dry condition) and 1.41 molCO
2/kg (humid condition) were measured at 298 K with a PEI/fumed silica sorbent. They considered that the adsorbed water blocked access to amine groups, thereby reducing CO
2 capacity. However, an opposite trend was registered for a lower amine loading, achieving a CO
2 uptake of 1.77 molCO
2/kg at 67% relative humidity. The different behavior was due to gas diffusion into the adsorbents. With a lower amine loading, the amine was better dispersed on the support, allowing easier access to amino groups for the incoming gases. On the other hand, at a higher amine loading, the amino group might not be as accessible due to a poorer dispersion on the support’s surface and agglomeration of the coated particles.
The positive effect of humidity on CO
2 capture from the air for other sorbents has been reported in several other works. Wang et al. [
76] considered a mesoporous carbon support with polyethylenimine in a fixed bed column and found that the CO
2 adsorption capacity was higher with moisture and that an excellent stability was present at a low CO
2 concentration under temperature swing adsorption/desorption cycles, measuring only a 2–5% drop in the sorption capacity after 10 cycles. In fact, 2.25 molCO
2/kg was obtained at an ambient temperature and in dry conditions, while 2.58 molCO
2/kg was measured at 80% relative humidity. On the other hand, after reducing the CO
2 concentration to 5000 ppm, in dry conditions, the adsorption capacity increased to 3.34 molCO
2/kg. The authors developed, also, a first-order kinetic deactivation model based on experimental breakthrough curves in order to study the influence of CO
2 diffusion on the adsorption kinetics.
The positive effect of moisture on adsorption capacity has also been reported in the work of Kwon et al. [
75], who considered a PEI-functionalized hierarchical bimodal meso/microporous silica support. At 323 K, they measured 2.6 molCO
2/kg in dry conditions and 3.36 molCO
2/kg at 303 K with 19% relative humidity. This adsorption capacity value is the highest CO
2 uptake value recorded for class I sorbents, to the best of the authors’ knowledge, and shows how the presence of water vapor mitigates the kinetic limitations of aminopolymer deposits.
The same effect was found by Sehaqui et al. [
83]: the CO
2 adsorption capacity increased from 0.5 molCO
2/kg to 2.2 molCO
2/kg for air with 20% and 80% relative humidity, respectively, for a sorbent composed of oxidized nanofibrillated cellulose (NFC) and a high-molar mass PEI. The authors underlined that the presence of water is also advantageous, as it stabilizes the PEI-based sorbent by inhibiting the formation of urea species.
The effect of steam exposure time was studied by Sakwa-Novak and Jones [
77], who examined a PEI-impregnated mesoporous γ-alumina sorbent for which an adsorption capacity of 1.71 molCO
2/kg was measured at 303 K and 50% relative humidity. The authors found that CO
2 adsorption can be reduced by increasing the time of steam exposure due to PEI leaching; after 24 h of steam exposure, a capture reduction of 61.5% was obtained. However, the formation of boehmite on the sorbent surface was not significant with respect to amine efficiency.
Adsorption capacities can be improved by tuning the support, as in Kuwahara et al. [
67], where Zr was incorporated onto SBA-15 to this end, changing the acid–base properties of the support. This study showed that Zr also improves regenerability and stability over continued recycling in capturing CO
2 from flue gas and air. The authors reported that an adsorption capacity of 0.85 molCO
2/kg was obtained and verified that the acid–base properties of the support have a critical, previously unrecognized role in creating more efficient adsorbents, improving thermal stability and adsorbent longevity.
Class II Amine-Functionalized Sorbents
An overview of class II amine-functionalized sorbents is shown in
Table 3 [
88,
89,
90,
91,
92,
93,
94,
95,
96,
97,
98,
99,
100,
101,
102]. Among class II sorbents, Moschetta et al. [
97] considered a new synthesis to transform an alkyl halide-functionalized SBA-15 silica containing Br, Cl and I into an amine-functionalized SBA-15 silica through a procedure involving gas-phase post-grafting, using NH
3 at high pressure. The results showed that this route could increase amine loading in aminosilica sorbents without sacrificing amine efficiency and ensuring a CO
2 adsorption capacity of 0.1 molCO
2/kg at 303 K.
The same support was grafted with low amounts of 3-aminopropyltrimethoxysilane in Stuckert and Yang [
94], achieving an adsorption capacity of 0.14 molCO
2/kg. Compared with other zeolite sorbents, the authors founds that this amine-functionalized sorbent was able to work in wet conditions but with a lower space velocity of 1500 1/h due to slower uptake rates.
A novel amine-based nanofibrillated cellulose (NFC) sorbent was investigated by Gebald et al. [
91]. This support was functionalized by 3-aminopropylmethyldiethoxysilane (APDES), allowing a high CO
2 uptake (2.13 molCO
2/kg) with humidity. The same support was impregnated with N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AEAPDMS) in Ng et al. [
92], where an adsorption capacity of 1.39 molCO
2/kg was measured at 298 K with 40% relative humidity and the kinetics of CO
2 and H
2O co-adsorption were analyzed. The system showed a good stability over 100 cycles, with a reduction in CO
2 uptake of less than 5%.
The positive effect of moisture has been verified, as well, in Wurzbacher et al. [
90], who demonstrated experimentally the stability of temperature vacuum swing adsorption/desorption cycles in a packed bed with diamine-functionalized silica gel sorbents. The CO
2 uptake at 40% relative humidity was 0.44 molCO
2/kg, while in dry conditions it was 0.4 molCO
2/kg.
Higher efficiencies were measured, also, in the work of Didas et al. [
93]. Three sorbent materials were obtained by grafting a primary aminosilane (3-aminopropyltrimethoxysilane, APS) to silica SBA-15, varying the degree of surface coverage: SBA-APS-low (a submonolayer surface coverage of amines), SBA-APS-medium (an approximately monolayer coverage of amine) and SBA-APS-high (a dense, multilayer array of amines) [
93]. The results showed that the CO
2 adsorption capacity could be increased by increasing the partial pressure of CO
2 and the coverage of amines. At 400 ppm, CO
2 uptakes were, respectively, 0.12, 0.24 and 0.8 molCO
2/kg in dry conditions. However, at low relative humidity, the authors found that the formation of bicarbonate species with low surface amine coverage allowed an increase in amine efficiency. This was the first time that bicarbonate formation, which is known to occur in liquid aqueous amine solutions, was observed for sorbents capturing CO
2 from the air.
Other supports have been studied. In Didas et al. [
95], in particular, a comparison between a primary (3-aminopropylsilyl, APS), a secondary (N-methyl-3-aminopropylsilyl, MAPS) and a third (N,N-dimethyl-3-aminopropylsilyl, DMAPS) amine-functionalizing mesocellular foam (MCF) has been reported. The results showed that the primary amine had the greatest potential for CO
2 capture, which at low pressure (100 ppm) was 1 molCO
2/kg. In fact, primary amines possess both the highest amine efficiency for CO
2 adsorption as well as enhanced water affinity (which improves CO
2 adsorption from the air) compared to other amine types.
A mesoporous alumina support functionalized by APS has been analyzed in Potter et al. [
98] and a CO
2 uptake in the range of 0.15–0.75 molCO
2/kg, according to the amine loading, was measured.
On the other hand, good results were obtained in the work of Lu et al. [
96], whose sorbent has great potential to be used in direct air capture technology. The sorbent was PPN-6-CH
2DETA, with a CO
2 uptake of 1.04 molCO
2/kg and an extraordinarily high CO
2 selectivity (3.6 × 10
10). The authors affirmed that low costs could be achieved with this kind of sorbent due to its particular properties.
High CO
2 adsorption capacities were also obtained in the work of Belmabkhout et al. [
88], where a TRI-PE-MCM-41 sorbent was suggested (MCM-41 silica with the surface functionalized by a triaminesilane (TRI-PE)). CO
2 was removed from dry and humid air at different CO
2 concentrations. At 298 K, at 400 ppm and 300 ppm of CO
2, adsorption capacities were, respectively, 0.98 molCO
2/kg and 0.9 molCO
2/kg. Moisture had a positive effect on capture. In fact, for the last concentration, a relative humidity of 26% and 67% allowed uptakes of 1.19 molCO
2/kg and 1.4 molCO
2/kg, respectively.
The same support functionalized by a diethylenetriamino organosilanes (DT) amine was used in Wagner et al. [
89], allowing, for the simulated air, capacities up to 0.61 molCO
2/kg at 303 K, while lower temperatures ensured an uptake of 1.16 molCO
2/kg. Moreover, in their experiments, the authors found that a decrease in the sorbent’s capacity was due to the formation of urea groups, while reactions with atmospheric trace gases or oxygen were neglected.
MOF supports, highly crystalline materials with large surface areas and diverse structures, have also been investigated. In Lee et al. [
99], Mg
2(dobpdc) functionalized with ethylenediamine achieved a CO
2 uptake of 2.83 molCO
2/kg. A good stability was obtained during adsorption/desorption cycles; this sorbent is promising for CO
2 capture, with an ultrafast CO
2 adsorption rate when compared with porous materials. The results showed that the adsorption of CO
2 onto this amine led to the formation of carbamic acid and not carbamate.
In Liao et al. [
100], the previous amine was replaced by hydrazine (H
2N
4), increasing the adsorption capacity to 3.89 molCO
2/kg. This high CO
2 adsorption capacity, even at low pressure, was due to the ultrahigh concentration of amine groups and the chemisorption of CO
2 associated with carbamic acid formation. Moreover, these good performances could be conserved under mixed gas kinetic conditions and with humidity.
Another MOF, Mg/DOBDC (magnesium dioxybenzenedicarboxylate), modified with ethylenediamine (ED), has been analyzed in Choi et al. [
102], ensuring a CO
2 uptake of 1.5 molCO
2/kg after four regeneration cycles. This value was constant after adsorption/desorption cycles.
In Darunte et al. [
86], a MIL-101(Cr) material functionalized with tris (2-amino ethyl) (TREN) has been evaluated to capture CO
2 from simulated air. The results showed a CO
2 uptake of 0.35 molCO
2/kg; the sorbent is not suitable for direct air capture systems. Similar results were obtained by Hu et al. (2014) using MIL-101(Cr) loaded with smaller amine molecules, such as DETA and DADPA. The authors found that these sorbents are more suitable for CO
2 capture from flue gas or for CO
2/N
2 separation, thanks to the mild regeneration energy.
In order to improve performances, Li et al. [
101] developed a new method based on the Brønsted acid–base reaction to tether alkylamines into Cr-MIL-101-SO
3H for CO
2 capture. The new sorbent was functionalized with tris(2-aminoethyl)amine (TAEA), allowing a CO
2 uptake of 1.12 molCO
2/kg.
An interesting study has been carried out by Elfving et al. [
36], considering a polystyrene sorbent functionalized with a primary amine. The analysis was developed in real conditions, such as low partial pressure, low temperature (similar to temperatures in Finland, 273 K and 263 K) and presence of humidity. The results showed that the highest CO
2 uptake (1.06 molCO
2/kg) was achieved at 263 K with humidity.
Class III Amine-Functionalized Sorbents
A few works about class III amine-functionalized sorbents have been published, as shown in
Table 4 [
18,
103,
104,
105]. Among these, Chaikittisilp et al. [
104] studied, for the first time, a poly(l-lysine) brush–mesoporous silica hybrid to capture CO
2 from the air, under dry conditions at ambient temperature. Comparing the results with other functionalized amine sorbents, they found that the investigated sorbent had a CO
2 capture capacity of 0.6 molCO
2/kg, higher than that of PEI–SBA-15 (0.32 molCO
2/kg) and dimethylamine (DMA)–SBA-15 (0.19 molCO
2/kg), with a slightly higher amine loading. The sorbent was robust during short regeneration tests.
Good performances have been reported in the work of Choi et al. [
103], considering hyperbranched aminosilica (HAS) sorbents, for a simulated humidified air with 400 ppm of CO
2. The authors found that, by increasing the amine loading from 2.3 mmolN/g to 9.9 mmolN/g, the CO
2 adsorption capacity increased from 0.16 molCO
2/kg to 1.78 molCO
2/kg. CO
2 was captured reversibly without a significant degradation of performances in short, multi-cyclic operations.
Functionalized polysilsesquioxane-based hybrid silica materials were studied by Abhilash et al. [
105], showing an adsorption capacity of 1.68 molCO
2/kg. Moreover, the authors found a good recycling capability without efficiency losses after 50 cycles of adsorption/desorption in ambient air.
Other sorbents have been proposed and investigated for class III. Diethylenetriamine (DETA) supported on CB-N-g-PCMS-OH
− was obtained from the polymerization of nitrene and showed a CO
2 uptake of 0.14 molCO
2/kg with 95% relative humidity [
18].
Faster absorption and desorption rates were obtained using a CB-g-xPCMS-OH
− support (8.6 × 10
−3 m
−1), although the CO
2 adsorption capacity was the same [
18]. These two previous supports were in carbon black functionalized with atom transfer radical polymerization (ATRP) initiators.
Inverse templating is another method used to prepare highly porous polymeric structures [
18]. PS-CC and colloidal crystals were functionalized by DETA with a CO
2 uptake of 0.57 molCO
2/kg and 0.36 molCO
2/kg, respectively.
High internal phase emulsion (HIPE)-based materials have also been investigated, achieving an adsorption capacity of 0.5 molCO
2/kg and 0.72 molCO
2/kg [
18].
An alternative sorbent of class III has been proposed by Lunn and Shantz [
106]. The material was based on the polymerization of Z-protected L-lysine N-carboxyanhydride on an aminopropyl-functionalized SBA-15 support. This approach resulted in polymer brushes tethered to the oxide surface within the mesoporous silica network, yielding a hybrid aminosilica rich in primary amine groups.