Study on the Roles of Water in Solid Amine-Based Direct Air Capture of CO 2

. Climate change triggered by the increasing amount of CO 2 released into the atmosphere has become a global priority. Direct Air Capture (DAC) of CO 2 has been regarded as one of the disruptive technologies to realize negative emission. Among the multiple strategies employed, adsorption on solid amines is a promising choice owing to its low energy requirements. Since the water vapor ubiquitously exists in ambient air, understanding the role it plays in DAC by the amine-functionalized solid sorbents is of vital importance. Due to the diversity in amine and support features, both promotion and deterioration of water impact on CO 2 adsorption were revealed. Herein we present a multifaceted review of the trends and innovations in DAC by solid amines under humid conditions. The effect of water was discussed over four groups of materials, based on the support categories, i.e. , (a) mesoporous silica, (b) mesoporous alumina, (c) mesoporous carbon and other inorganic supports, and (d) cellulose and other organic supports.


Introduction
A series of problems caused by the continuous acceleration of global warming have aroused great concern of the whole society about anthropogenic CO 2 emissions, and promoted the development of carbon capture, utilization, and storage (CCUS) technologies. Controlling temperature rise below 1.5°C by the end of this century requires the participation of negative emission technologies, including direct air capture with the lowest land footprints and flexible deployments. The major differences between capture capture from concentration sources (e.g., flue gas or industrial exhaust gas) and ambient air lies in the CO 2 partial pressure. The widely-used technology of CO 2 absorption by alkanolamine solutions like monoethanolamine was proven as less effective because of 0.04% CO 2 in air and low liquid-gas ratio. The alkali metal hydroxide-based absorption, owing to its high affinity, low volatility, and process maturity, has been employed, whereas still suffers from high energy requirement in calcination under ~800°C [1].
Compared with absorption, adsorption does not require the high latent heat of solvent, and has certain advantages in reducing equipment corrosion and resistance to degradation. In particular, for air capture scenario with large gas volume and less pollutants, adsorption technology can effectively deal with tricky problems including amine volatilization and water loss. Researchers around the world have carried out extensive and in-depth studies in the field of adsorption-based air capture. In 2010, the first-ever application of solid amine sorbents in DAC was conduct by Sayari et al., showing superiority in capture capacities and energy consumption [2]. Relevant researches were booming in the last decade, with a variety of sorbents proposed which could basically categorized into class 1-3 (Fig. 1).
The class 1 sorbents are prepared by impregnation of porous supports with amine-containing polymers or small molecules including branched/linear poly(ethylenimine) (PEI), poly(propylenimine) (PPI), poly(allylamine) (PAA), and tetraethylenepentamine (TEPA). Both inorganic supports, e.g., silica, alumina, activated carbon, and organic ones, e.g., cellulose, polystyrene (PS) are available. The class 2 sorbents are typically prepared by grafting amine-containing small molecules, mostly aminosilane, onto pre-formed supports. 3aminopropyltrimethoxysilane (APTES or APS), 3-(2-Aminoethylamino)propyldimethoxymethyl silane (ED), and 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane (DT) are the most commonly used. Essential feature of the supports, silica and nanocellulose as the representatives, is the reactive OH groups on the surface. Therefore aminosilane can be attached to the surface via Si-O-Si linkages. High amine loadings can be easily achieved for class 1 sorbents, whereas the weak hydrogen bond involved incurs easily leaching of the amines. Owing to the Si-O-Si bonding, class 2 sorbents are more stable with moderate amine loadings. Class 3 sorbents are originated from the idea that combines the advantages of class 1 and 2 sorbents. By in situ polymerization of reactive amine monomers on and in the silica, polyamine structure can be covalently bonded to the surface [3]. Water ubiquitously exists in ambient air, e.g., the annual average relative humidity in China ranges from 40%-80%. Due to its unique chemical reactivity and physical properties, water shows complex effects on solid amine-based CO 2 capture, depending on the nature of sorbents and separation variables. It is important to clarify the roles of water plays in CO 2 reaction and diffusion, for the effective choice of sorbents and processes. Water effects in CO 2 capture were documented in a few papers. While there is no multifaceted report reviewing that how water influences the air capture performance for the whole types of solid amines. This review aims to fill the gap by discussing both the promotion and deterioration of water on CO 2 uptake, with emphasis on the underlying mechanism.

Roles of water in direct air capture: brief comments on the amine-CO2-water interaction
The amine-CO 2 -water interaction is vital to CO 2 adsorption by solid amines. Primary and secondary amines can react with CO 2 via the zwitterion mechanism. In the absence of water, two amine moieties and one CO 2 molecule are involved in the reaction, giving one ammonium carbamate. In the presence of water, the maximum molar ratio of CO 2 :amine is 1:1, since water molecule serves as the additional free base instead of amine (Fig. 2a). Tertiary amine only reacts with CO 2 via water-assisting deprotonation, forming bicarbonate or carbonate species in the presence of water. The molar ratio of CO 2 :amine is 1:1 (Fig. 2b). That mechanism is also applicable for primary and secondary amines in aqueous states, whereas routinely neglected for the solid amines considering the slow kinetics [4]. Enhanced CO 2 adsorption under humid conditions can be ascribed to the formation of bicarbonate or carbamate. The co-adsorbed water could also be detrimental as it blocks the access for CO 2 approaching amines. Besides, a few studies revealed that water has positive effect through reduced kinetics restrictions, either acting as a diffusive intermediate to transport CO 2 or increasing the mobility of amine chains via a plasticization effect [5].

Roles of water in direct air capture: the reaction and diffusion aspects
The studies on amine-CO 2 -water interaction suggested that water could have a convolutional influence on direct air capture from reaction and diffusion aspects. To elucidate that, relevant researches were reviewed separately by the differences in supports. Key properties concerning DAC were explained as below: (I) adsorption capacity: the amount of CO 2 adsorbed. (II) amine efficiency: the mole of CO 2 captured per mole of amine functional group. primary, secondary, sterically hindered amines of (a) and tertiary amines of (b).

Mesoporous silica
Mesoporous silica is comprised of honeycomb-like structures with mesoporous channels that enables impregnating or tethering active groups onto internal surfaces. Questioning on how water influences the CO 2 adsorption emerged with the first proposal of PEIimpregnated mesoporous silica for flue gas CO 2 capture. Up to 50% increase in CO 2 capacity were achieved by introducing water content (0-15.5%) in the feed gas of CO 2 (12.6%-14.9%). Branched PEI reacted with CO 2 in the presence of water to form carbamate and bicarbonate species but not synchronously because of quicker CO 2 uptake than water. The CO 2 capacity enhancement was ascribed to the water-involved 1:1 stoichiometric adsorption, especially for the tertiary amine groups whose reactivity was endued by water [6].
The effect of moisture on the CO 2 adsorption was related to multiple parameters, including amine structure, amine loading, support hydrophilicity, CO 2 /H 2 O content, and temperature. The 10% CO 2 uptakes of functionalized silica with the same loading of linear or branched PEI were similar at dry condition, e.g., 1.36 and 1.45 mmol/g for linear and branched PEI, respectively at 25°C. While promoting function of water on CO 2 capacity was found more significant for the linear PEI than the branched one (195% v.s. 108%).Under ultra-high humidity, the branched PEI suffered from CO 2 capacity decrease, in contrast to its linear counterpart which remained being facilitated probably due to more hydrophobic nature [7].
Using PEI-impregnated CARiACT G10 silica, Monazam et al. reported a linear increase of 10% CO 2 uptake with humidity at 60°C. 85.6% increase was due to lower crosslinking degree of amine, and bicarbonate formation either from carbamate hydrolysis or direct reaction of PEI with CO 2 . Humid CO 2 adsorption on the prehydrated sorbents followed a two-step mechanism, i.e., a rapid regime of chemisorption on surface and a slow regime of bulk diffusion within aminopolymer. The equilibrium time prolonged with increasing water content in the gas stream, as deeper penetration into PEI layer comes along with larger proportion of slow regimes [8].
Zhang et al. investigated the CO 2 /H 2 O adsorption on linear and branched PEI functionalized silica (Fig. 3). The CO 2 capacity of linear PEI-loaded silica was improved by 2-fold at medium humidities. The enhancement was more pronounce at higher amine loadings (>44.5%) and lower temperature (<70°C), which implies that the major contribution of water lies in mass transfer. Water promotion nearly changed as CO 2 concentration lower from 10% to 1%, despite a little more competitive adsorption at high humidity [9]. In more dilute cases like 0.5%CO 2 in the feed gas, CO 2 uptake could still increase from 1.33 to 2.28 mmol/g by introducing 90%RH at 25°C [10]. Difference in CO 2 partial pressure between flue gas capture and DAC cases caused a significant reduction of CO 2 /H 2 O ratio, whereas positive effect of water did not seem to change. Sayari et al. examined the 400 ppm CO 2 uptake by PEI impregnated MCM-41 sorbent under a wide range of humidity (0-88%RH). CO 2 capacity steadily increased from 2.19 to 2.95 mmol/g. 35% increase in CO 2 capacity was associated with partial change from carbamate to bicarbonate formation. Plateau in enhancement arose when RH approaching 60%, and noticeable increase in CO 2 capacity did not occur at higher RHs [11]. Using PEI-functionalized cellulose acetate/silica fiber sorbent, in the presence of 385-395 ppm CO 2 with 85% RH at 35°C, Jones et al. reported 78% and 171% increases in breakthrough and pseudoequilibrium capacities, respectively compared to dry condition. The more than doubled CO 2 capacity suggested the release of PEI reactivity, as water not only enhances the mobility and flexibility of molecular chain to expose more amine sites, but also facilitates carbonate species formation [12].
The hierarchical silica with bimodal meso-/macroporosity was proposed as substrate to achieve a high PEI loading of 72.4%. Moisture (19%RH) increased the 400 ppm CO 2 uptake from 2.6 to 3.4 mmol/g at 30°C. It was suggested that water could enhance the aminopolymer chain mobility by weakening inter or intramolecular hydrogen bonds and dipole−dipole interactions. Besides, bicarbonate formation in the moist air might not only increase the CO 2 capacity, but also facilitate mass transfer due to less aminopolymer chain cross-linking compared to carbamate formation [13].Water-improved amine accessibility was also found in PEI impregnated MCFs. With 2% water content in feed gas, the highest enhancement factor was 1.53 at 33°C as CO 2 uptake increasing from 1.54 to 2.36 mmol/g. Temperature rise caused enhancement factor decline to 1.22 at 58°C [14]. Moisture higher than 3% appeared to be less effective and even detrimental to CO 2 adsorption, as it might present additional diffusion barrier to amines especially at high amine loadings [15].
Considering higher CO 2 affinity, the all primary amine-comprised PAA-impregnated MCF was proposed and achieved capacity of 0.86 mmol/g for 400 ppm CO 2 , with amine loading of 7.24 mmol/g at 25°C [16]. A further increase of amine loading to 12.2 mmol/g, whereas resulted in negligible CO 2 uptake in dry air. That sorbent could be activated by moisture, as the dry and prehumidified sample achieved CO 2 capacity of 11.8 and 12.6 mg/g, respectively at 60%-70%RH. Densely loaded sorbents usually suffers from high diffusion resistance, and water might act as a plasticizer, loosening the polymer network, and allowing for penetration of CO 2 [17].
Owing to the co-existence of primary and secondary amine groups in TEPA and its structural simplicity, the mechanism of humid CO 2 adsorption on TEPAimmobilized silica was explored by in situ IR. CO 2 was adsorbed on the primary amine sites as ammonium carbamate, and on the more hydrophilic secondary amine sites as carbamic acid. Water adsorbed could displace the amine from the surface silanol groups to release the reactivity. Moreover, the formation of carbamic acid on the secondary amine could be facilitated by proton transfer from ammonium ion to carbamate via neighboring water molecules, and thus the 1:1 stoichiometric reaction could contribute to CO 2 capacity increase in the presence of moisture [18].
Sorbents containing single amine types or sterical hindrance structure were studied with spectroscopic and molecule scale method deep into amine-CO 2 -water interaction. For APS-based sorbents in dilute CO 2 capture. Carbamate species were the main product at dry and humid condition, as evidenced by in situ IR spectra. The predominance was even more explicit for densely-loaded amines. In the absence of moisture, Danon et al. reported that silylpropylcarbamate species formed when free hydroxyls exist on the APS-grafted silica surface, and it was the only possible reaction for low amine loadings, in which neighboring amines were isolated. For higher spatial dispersion of amines, the CO 2 chemisorption may not occur due to the very limited amine interaction. Besides, the forming of surface-bonded carbamate were far slower than free carbamate. In the presence of moisture, the boned carbamate could hydrolysis to liberate the hydrogen-bonded amines, and thus contributed to higher moist CO 2 uptake than dry condition [19].
Existence of carbamic acid products were suspected in studies decades ago, although it was until the usage of time-resolved IR that carbamic acid got confirmed in CO 2 reacting with some n-propylamine modified bicontinuous silicas [20]. Foo et al. revealed for APS grafted SBA-15 silicas, carbamic acids were stabilized by hydrogen bond with surface hydroxyls, amine dimers or neighboring NH groups. Carbamic acid was not preferable at high amine loadings and condensed slowly with silanol groups to form bonded carbamate. Similar to surface bonded carbamate, carbamic acid stability reduced with humidity, due to water-mediated transition to carbamates [21].
Sayari's group pioneered DAC research with aminegrafted sorbents in the presence of moisture. The uptake of 0.03% CO 2 by triamine-grafted pore-expanded MCM-41 silica could increase from 0.90 to 1.19 and 1.40 mmol/g for 27%RH and 64%RH, respectively at 25°C, as seen in Table 1 [22]. Yang et al. examined the DAC performance of APS and DT-grafted SBA-15 silicas, and found a similar 40% increase of CO 2 capacity in 60%-80%RH [23]. With difference FT-IR spectra, the mechanism of CO 2 /H 2 O sorption on APS-modified silica considering the amine coverage was revealed. Moisture was found always advantageous to CO 2 adsorption, while became less effective as amine coverage decreases, especially at ultralow pressure. The only product species detected were carbamates for sorbents with high amine loading, under both dry and humid conditions. Facilitated CO 2 adsorption by moisture was related to the reduced kinetic restrictions. Only at long time scale and low amine loading were the bicarbonate suspected, suggesting the poor kinetics of its formation and less contribution to the capacity [24]. In stark contrast to primary or secondary amine, tertiary amine itself was considered as non-reactive in dry CO 2 since it could not be deprotonated. Therefore formation of bicarbonate is the exclusive route for tertiary amine. Small IR peaks (1645, 1360 cm -1 ) related to bicarbonate was found when tertiary amine grafted SBA-15 reacts with dry CO 2 due to the residual physisorbed water. Studies showed that the bicarbonate product was usually missed by the conventional 13 C CPMAS, while Bloch decay could detect a single narrow resonance assigned to bicarbonate for tertiary amine-modified SBA-15 [21].
Lee et al. studied the CO 2 chemisorption of sterically hindered amines (a primary or secondary amine with a secondary or tertiary α-carbon) modified silicas in the presence of water. In situ FTIR revealed that both the sterically hindered and unhindered amines could form carbamate or carbamic acid under dry condition, despite weaker chemisorbed species of the hindered one. Under humid condition, the sterically hindered amines are prone to form bicarbonate, as unfavorable steric interaction between the COO − group and the methyl/methylene substituents on the α-carbon attributing to the carbamate's poor stability. The preferential formation of bicarbonate might also due in part to kinetic factors, since CO 2 diffusion to the amine sites could be blocked by steric hindrance on the α-carbon (carbamate or carbamic routes), and the nitrogen was more likely protonated by water (bicarbonate route). When compared to the solution of which ionic species could be stabilized through solvation or hydrogen bond, the extent of bicarbonate formation on solid amines was much lower [25].

Mesoporous silica
Alumina supports are more stable than silica when exposed to steam, therefore amine functionalized alumina was preferred when steam was used in regeneration. Studies of water-related issue focus on the stability as DAC sorbents treated with steam.. PEI impregnated alumina and silica underwent 24h steam treatment at 105°C, and alumina supported material turned out to be much more stable with 74.8% of its CO 2 capacity at 400 ppm retained, in contrast to 18.7% for silica supported one. The discrepancy was related to mild structural change of PEI impregnated alumina and drastic collapse of mesostructured of silica, although the supports have comparable stability exposed to steam [26]. The oxidation stability of alumina supported amine sorbents in the presence of moisture was investigated by Bali et al.
When treated with humid flow of 21% oxygen at 110°C for 20 h, the 10% CO 2 capacity of PEI impregnated silica drastically decreased by 70% from 1.87 to 0.56 mmol/g, whereas only 7.5% reduction to 1.73 mmol/g relative to fresh sample was found in PAA impregnated silica [27].

Mesoporous carbon and other inorganic supports
Mesoporous carbon functionalized with PEI was employed in dilute and ultra-dilute CO 2 capture, the CO 2 capacity gains due to 80%RH for 5000 ppm and 400 ppm CO 2 were 21.3% and 14.7%, respectively. Less profound improvement at 400 ppm was ascribed to the abated CO 2 /H 2 O selectivity [28]. By loading 67% branched PEI onto Mg-Al-CO 3 layered double hydroxides (LDHs), the sorbent with abundant mesopores and broad pore size distribution could have adsorption capacity of 1.16 mmol/g, at 25°C under 400 ppm CO 2 . Up to 50% increase of CO 2 uptake by the PEI-impregnated LDHs were found with humidified CO2 stream, higher than the similar LDHs grafted with TRI (36.7%). The change of product to ammonium carbonates and the LDHs' prevention of amine access blockages contributed to the waterfacilitated CO 2 adsorption. In contrast to a 14% loss of CO 2 capacity after 20 cycles under dry condition, the amine modified LDHs were stabilized when regenerated using humid nitrogen purge [29].

Cellulose and other organic supports
The employment of nanofibrillated cellulose (NFC) as support in amine-based CO 2 sorbent was suggested by Steinfeld et al. The 400 ppm CO 2 capacity of APS-grafted NFC varied from 0.36 to 0.65 mmol/g for temperature range of 10-30°C and relative humidity range of 20%-80%, with evident promotion of water on CO 2 capacities revealed. Compared to water adsorption, both the capacity and kinetics of CO 2 adsorption were much lower, indicating rate controlling mechanism determined by diffusion and surface reaction with amine moieties [30]. Mutual interactions between CO 2 and water during coadsorption on APS-grafted NFC were investigated. The 10% and 400 ppm CO 2 capacity increased from 2.26 and 1.11 mmol/g to 2.54 and 2.13 mmol/g, respectively, in the presence of 2.55 kPa water vapor (90%RH) at 23°C. The more significant role water plays under low partial pressure of CO 2 was presumably ascribed to differed diffusion resistances. Besides, the water capacities were merely affected by the presence of 45 Pa CO 2 , as abundant adsorption sites for water, e.g., silanol group, free cellulose hydroxyl group, existed apart from amineassociated sites [31]. The 40% PEI-immobilized poly(methyl methacrylate) (PMMA) sorbents exhibited uptake of 2.50 mmol/g in pure CO 2 stream at 40°C, and further increased to 3.60 mmol/g as 2% water vapor (25.5%RH) involved [32]. In ultra-dilute case, the saturation capacity for 400 ppm CO 2 of 50% PEI-loaded macroporous PMMA resins was 1.96 mmol/g. Relative humidity of 10% enhanced the CO 2 capacity by 61.2% while excess humidity could have a negative influence. The inverted-V trend of CO 2 uptake response to humidity was similar under elevated CO 2 concentration, e.g., 5000 ppm, with the optimal RH and enhancement factor shifting to 40% and 75.6%, respectively. Compared to cellulose, a stronger competitive adsorption on PMMA-based sorbents was found where CO 2 and water share the same adsorption sites [33].

Conclusions
Water ubiquitously exists in the ambient air. Understanding the role water plays in solid amine-based CO 2 capture was important for developing DAC materials and processes. The CO 2 adsorption is generally facilitated by water, making solid amines one of the most promising DAC sorbents under real conditions. In the presence of moisture, CO 2 reacton with solid amines containing primary or secondary amine groups could shift from carbamate (CO 2 :N=0.5) to bicarbonate formation (CO 2 :N=1), resulting higher capacities. Moreover, "inactive" tertiary amines could only react with CO 2 in humid conditions. The enhance CO 2 diffusion was also revealed as water could increase the mobility of aminopolymer chain and acting as a diffusive intermediate. The negative influence of water usually emerged at high humidity, and it may stem from competitive CO 2 /H 2 O adsorption or the blockage of CO 2 access to amines.
Having obtained the water effect on direct air capture under complex variables including amine/support structure, amine loading, CO 2 /H 2 O content, and temperature, choices of sorbent, either from reaction or diffusion perspectives, are easier to be made. It also sheds light on the development of novel materials based on the underlying amine-CO 2 -water mechanism. Another waterrelated issue which does not addressed in this review, i.e., the energy penalty of moisture removal in regeneration, is also important in large-scale demployment of DAC and can be further discussed.