A Highly Tunable Approach to Enhance CO2 Capture with Liquid

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Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

A Highly Tunable Approach to Enhance CO2 Capture with Liquid Alkali/amines Xingguang Xu† and Colin D. Wood*,† †

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CSIRO, Energy, Australian Resources Research Centre, Kensington, Western Australia 6151, Australia ABSTRACT: A diverse range of alkali/amine infused hydrogels (AIHs) were generated by incorporating the liquids into a hydrogel particle for carbon capture application. As a consequence, the CO2 uptake was significantly enhanced owing to the increased contact area. This AIHs technique was highly tunable as it could be applicable to varying species of alkali chemicals and it was found that their molecular structure and architectures could impact the CO2 uptake. Compared to stirred bulk alkali/amine solutions, the CO2 absorption capacity of AIHs was increased by 400% within 30 min with a low hydrogel loading (10 w/w%). In addition, the recyclability of various AIHs was assessed and was found to be extremely encouraging. The effect of salinity on the performance of AIHs was also investigated and high salinity was found to have a minimal effect on CO2 absorption. Most importantly, the preparation of AIHs is fast and straightforward with few wastes and byproducts formed in the preparation process. In all, extensive investigations were presented and the AIHs were found to be a highly tunable and effective approach to enhance CO2 capture with liquid alkali/amines.

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INTRODUCTION The escalating level of carbon dioxide (CO2) in the atmosphere is perhaps the one of the greatest environmental concerns in our age as it is closely associated with the global warming that has been attracting widespread public attention over the past few decades.1−3 It is well accepted that the alarming rise in atmospheric CO2 concentration is largely the consequence of massive consumption of fossil fuels including coal, crude oil, and natural gas.4 For this reason, one of the most appealing strategies to address the global warming is to capture the CO2 released from power plants and natural gas wells, and then permanently store the absorbed CO 2 underground.5−7 The urgent need for carbon capture that helps reducing the CO2 concentration in the atmosphere greatly prompts the investigation into the CO2 uptake material and technology. To date, the so-called “wet scrubbing” that applies the aqueous solution of monoethanolamine (MEA) is the most commercially mature and well-established technology.8−10 It possesses numerous advantages including high reactivity, high selectivity and reasonable uptake capacity and is seen as a promising technique for carbon capture in industrial scale. However, wet scrubbing by MEA suffers from multiple inherent shortcomings including low absorption efficiency, solvent loss, and high energy demand.11−13 In an attempt to overcome the drawbacks of the MEA scrubbing technique, new CO2 absorbents and CO2 capture technologies have been extensively developed over the past few decades. Among them, solid absorbents, for example metal− organic frameworks (MOFs) or porous organic polymers are seen as a promising replacement owing to their outstanding Published XXXX by the American Chemical Society

absorption capacity, high reactivity and the acceptable recyclability.14−16 Nevertheless, both high price and chemical complexity hamper their application in carbon capture plants. Besides, considerable operational pressure and sensitivity to water vapor that always exists in pipelines also greatly reduce their feasibility. Other emerging CO2 uptake materials include ionic liquid,17,18 nanoparticle-supported amines12,19and “dry” base or alkanolamines,20,21 but they all display intrinsic limitations. Ionic liquids often suffers from complex synthesis and low absorption rate, the nanoparticle-supported amines always undergo substantial mass loss during regeneration process and can use expensive perfluorinated reagents, while the dry base exhibits unfavorable volumetric density and poor stability. Hydrogels are cross-linked polymeric materials with threedimensional networks. Due to their exceptional capability to absorb and retain liquids, wide availability and environmentfriendliness, hydrogels have been used in a variety of fields including biomedical engineering,22−24 separation technology,25,26 agriculture industry,27−29 cosmetic industry,30 enhanced oil recovery31 and so forth. In our previous work, we briefly reported a novel platform to capture CO2 using hydrogels.32 Our strategy was to use hydrogel particles as microreactors where the inorganic alkali or organic amine solutions were retained (Figure 1). Accordingly, this new Received: May 17, 2018 Revised: August 6, 2018 Accepted: August 13, 2018

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DOI: 10.1021/acs.est.8b02641 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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Figure 1. Mechanism of CO2 capture by AIHs and features with length scales spanning from centimeter to micrometer.

material was referred to as alkali/amine infused hydrogels (AIHs). They could be readily regenerated using a standard convective oven (Figure 2). However, the investigated amines

Figure 2. Illustration of the regeneration process of AIHs.

in that work were very limited. In other words, how the chemistry of amines may influence the performance of AIHs is unclear so far. Besides, the understanding on effects of amine concentration and hydrogel loading is still lacking. In the present work, we take a significantly further step trying to fill those knowledge gaps and explore more features of AIHs that may attract wide attentions from carbon capture industry. In particular, this study validates the high tunability of the AIHs technique for a wide range of alkali chemicals. We believe that the extensive investigations on AIHs in this work will provide valuable insight into the design and fabrication of green and functional materials for carbon capture industry.

Figure 3. Chemical structures of the investigated organic amines. The primary, secondary, and tertiary amine groups are highlighted with blue, green, and red, respectively. The pKa values presented here are in water.

respectively. Yates hydrogels which are able to absorb up to 400 times its weight in water are a commercial product and are supplied by DuluxGroup Pty. Ltd. It is a potassium salt and primarily composed of polyacrylamide. High purity (99.0%) of carbon dioxide (CO2) is purchased from BOC Gases. Ultrapure water applied in the solution preparation is produced by a Synergy Water Purification System. CO2 Uptake Experiments. The experimental procedure is analogous to that proposed by Cooper.20 2g of alkali or amine aqueous solution with known concentration is added into a 6 mL sample vial sealed by a screw cap with rubber septa. A specific amount of hydrogel is then added to the solution. Afterward, the mixture is stirred for 10 s and left for 5 min, enabling the formation of AIHs with varying species and concentrations of amines. Next, CO2 gas is injected into a balloon equipped with a needle, therefore the vial and the CO2-inflated balloon can be connected through the needle that penetrates the rubber septa. A second needle that also pierces the septa is used as a vent, by doing so the CO2 gas can readily

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EXPERIMENTAL SECTION Materials. All chemicals and materials are obtained from commercial sources and are used as supplied. Inorganic alkali K2CO3, alkanolamine N-methyldiethanolamine (MDEA), sterically hindered amine 2-Amino-2-methyl-1-propanol (AMP), cyclic amine pyrrolidine, diamine pentaethylenehexamine (PEHA) and polyamine polyethylenimine (PEI, branched, average Mw ∼ 800), aluminum pellet (purity 99.0%), and KCl are obtained from Sigma-Aldrich. The molecular structure of organic amines are shown in Figure 3. The boiling points of MDEA, AMP, pyrrolidine, PEHA, and PEI are around 210 °C, 150 °C, 87 °C, 300 °C, and 260 °C, B



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Figure 4. CO2 uptake of various AIHs (blue line) and bulk alkali/amine solution (red line). The alkali/amine concentration is 30 w/w%. 0.3 g of hydrogel is used in AIHs.

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flow through the spaces between the prepared “liquid” amine particles. The mass difference of AIHs before and after CO2 absorption is recorded at regular time intervals until absorption equilibrium is established. All the experiments are performed under ambient conditions. AIHs Recyclability Assessments. Regardless of the nature and class of CO2 absorbents, materials for CO2 capture are always regenerated in real world applications in an attempt to reduce the material costs and improve the overall economics.33 In this study, the cyclic capacity of AIHs is evaluated via the classic temperature swing approach.34,35 In a typical cycle, fresh AIHs first undergoes the CO2 uptake experiments as stated above, then the CO2-loaded sample is placed in a BINDER convective heating oven for 30 min. Subsequently, a specific amount of distilled water is added into the heated sample to make up the evaporative water loss. Afterward, the sample is cooled to ambient temperature before the CO2 uptake measurement initiates again. It should be noted that the regeneration temperature is closely associated with the species of the applied alkali or amines.

RESULTS AND DISCUSSION CO2 Uptake of AIHs and Alkali/amine Solutions. In this subsection, the CO2 absorption kinetics and capacity of AIHs were investigated and compared to that of aqueous alkali or amine solutions with a concentration of 30 w/w%. The AIHs sample was prepared by simply “locking” 30 w/w% alkali/amine solution in 0.3 g hydrogel, so that the only difference in these two absorbents was whether hydrogels were used. It should be noted that bulk alkali/amines solutions were stirred to promote the CO2 solubility which, otherwise, would be extremely low. Inorganic alkali K2CO3 and organic amines MDEA, AMP, pyrrolidine, PEHA and PEI were selected, as they represented a wide range of alkali absorbents that were commonly applied for carbon capture. This experimental approach allowed the broad applicability of the AIHs technique to be demonstrated. It was also noted that the hydrogel alone hardly bound any CO2,32therefore the variation in CO2 uptake mostly lay in the way how CO2 interacted with alkali/amines in both scenarios. The CO2 uptake of AIHs and stirred alkali/amine solutions are shown in Figure 4. The inorganic alkali K2CO3 is environmentally benign, inexpensive, nonvolatile and has a C



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outperformed the mass transfer in AIHs at the beginning considering the blocking effect of polymeric chain in hydrogels. Nonetheless, as the absorption proceeded, more and more carbamate salts accumulated in the bulk solution, leading to a remarkable rise in the apparent viscosity,21 therefore the AIHs that had enlarged contact areas surpassed the bulk pyrrolidine solution in the absorption kinetics. As was seen in Figure 4(d), compared to other alkali/amines applied in this study, the variation in the equilibrium CO2 uptake of both absorbents was the least, which might also be the result of the superb rate constant of pyrrolidine. This finding suggests that the advantage of AIHs in the absorption rate could somewhat be diminished if amines with substantially high reactivity were used in AIHs, but it could capture greater amount of CO2 than the stirred amine solution in a long time despite of the slower kinetics in first few minutes. Finally the CO2 capture performance of two distinctive AIHs prepared from diamine PEHA and polyamine PEI were assessed. Both PEHA and PEI were widely used in the preparation of solid absorbents for CO2 capture as they were high molecular weight solvents with negligible volatility and thus mass loss in regeneration process could be minimized.45−47 More importantly, they have favorable CO2 loading capacity as their molecules contained multiple nitrogen atoms that could work as reactive centers. However, Figure 4(e) and (f) demonstrated that high CO2 absorption capacities were not obtained in the form of stirred PEHA or PEI solution. This was mainly attributed to the adverse mass transfer effect in the bulk solution. In fact, the apparent viscosity of PEI solution was so high that it could not be effectively stirred. Surprisingly, neither PEHA-AIHs nor PEI-AIHs displayed as high CO2 loading capacity as expected, with the maximum CO2 uptake only 79.5 mg CO2/ g absorbent and 71.3 mg CO2/ g absorbent, respectively. We speculated that the relatively low capacity could be the consequence of large sizes of PEHA and PEI molecules. CO2 first penetrated the hydrogel skins and reacted with the PEHA and PEI molecules at the outer part of AIHs. Due to the pronounced steric hindrance of the generated carbamate salts, the free CO2 molecules would encounter significant barriers before interacting with the inner amines, leading to the dramatic decline in absorption kinetics after 10 min, as shown in Figure 4(e) and (f). Consequently, a portion of the loaded amines in AIHs might not contact with CO2 gas, and that was why the absorption efficiency was negatively affected. But it must be noted that the PEHA and PEI in the form of AIHs still bound more CO2 than the counterpart stirred solutions at the end of the uptake experiment, indicating the functionality of AIHs to diamines and polyamines. As can be seen, one of the most appealing features of AIHs is its general applicability to various species of alkali chemicals allowing for fine-tuning of the performance. Regardless of the applied alkali/amine species, AIHs had great advantage over the stirred bulk solutions in the CO2 absorption kinetics due to the favorable mass transfer. Meanwhile, AIHs always exhibited higher CO2 uptake within the experimental time scale (30 min). However, the loading capacity of various AIHs was found to be species dependent under the experimental conditions. As illustrated in Figure 4, based on the identical amine weight percentage, the increasing order of uptake values of each AIHs was as follows: K2CO3 < PEHA < MDEA < PEI < Pyrrolidine < AMP, indicating the magnitude of the uptake enhancement of individual alkali/amine by AIHs was clearly

low binding energy with CO2, making it a favorable chemical for large-scale carbon capture.36 Typically, “promoters” such as MEA, glycine and piperazine (PZ) are used along with K2CO3 to promote its extremely slow kinetics,37 but these additives will also accelerate the corrosion of carbon steel. As could be seen in Figure 4(a), without the addition of any promoters, the CO2 absorption kinetics of AIHs was notably improved compared to that of the stirred bulk solution. This was attributed to the enlarged contact area available for CO2 binding, so the uptake was increased. Meanwhile, it was found that CO2 uptake of AIHs nearly reached the plateau (60.1 mg CO2/ g AIHs) in 20 min, while that of the bulk K2CO3 solution was only 10.6 mg CO2/ g solution when the measurement was terminated. It is worth noting that the 3D networks of the hydrogels may slow the CO2 movement (barrier effects) and affect the CO2 absorption rate,38 but the mass transfer effect significantly outweighs the barrier effect in AIHs, which leads to its enhanced CO2 uptake in comparison with the bulk solution. As a tertiary alkanolamine, MDEA reacts with CO2 in equimolar ratio via the base-catalyzed hydration mechanism, leading to a desired CO2 loading capacity of 1.0 mol CO2/mol amine. In particular, MDEA exhibits lower absorption heats and slower degradation rates compared to that of its primary and secondary counterparts, making it a fairly competitive candidate for CO2 capture.39 However, the reactivity of MDEA to CO2 is considerably low. In fact, the reaction rate constant was found to be less than 18.2 m3/s/kmol at 25 °C,40 which could be substantiated by the poor CO2 uptake of bulk MDEA solution in Figure 4 (b). On the other hand, it was observed the CO2 absorption of MDEA-AIHs increased rapidly over time, suggesting the reaction kinetics had been noticeably promoted by the more frequent contact between CO2 and MDEA molecules. As a consequence, the maximal CO2 uptake in 30 min was increased from 31.6 mg CO2/ g absorbent (stirred MDEA solution) to 78.3 mg CO2/ g absorbent (MDEA-AIHs). The organic amine AMP represented a class of promising solvents for CO2 absorption, namely sterically hindered amines.41,42 Like MDEA, it also had a high thermodynamic capacity of 1 mol CO2/mol amine with lower regeneration energy demand as well as weaker corrosivity than that of the commonly applied MEA. Although its reactivity to CO2 is superior to that of MDEA,43 which could be seen in Figure 4 (b) and (c), the kinetics of its bulk solution was not sufficiently fast. Therefore, activators such as diethanolamine (DEA) are typically used in real world applications. However, when it came to AMP-AIHs, activators were not required. As illustrated in Figure 4 (c), in comparison with the stirred AMP bulk solution, both the CO2 absorption rate and capacity were greatly improved if AMP-AIHs is applied as the CO2 absorbent. Similar to K2CO3-AIHs and MDEA-AIHs, AMPAIHs almost obtained its maximal CO2 uptake in 15 min, but its value (102.9 mg CO2/ g absorbent) was the highest among the three absorbents. Interestingly, the five-membered cyclic amine pyrrolidine displayed remarkably fast kinetics. In particular, the CO2 uptake rate of its aqueous solution was even faster than that of AIHs in first few minutes. Conway et al.44 claimed pyrrolidine as an “ideal” solvent for carbon capture with regards to its exceptionally high reaction rate constant that was associated with its high electronic density. Therefore, in this scenario, the mass transfer in stirred pyrrolidine solution D



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different. It should be noted that higher and faster CO2 absorption can be realized only if significant amount of amine solutions are retained in AIHs. Considering this, more focus should be placed on the establishment of relationship between the amine chemistry and amine saturation in AIHs in future work. Effect of Alkali/amine Concentrations on the CO2 Uptake of AIHs. As is well understood, more CO2 will be absorbed if greater amount of alkali/amine is used in the absorbent. For this reason, it is critical to identify the highest concentration of a specific alkali/amine that can be retained by given amount of hydrogel in AIHs. Two g of solution with various alkali/amine concentrations was applied along with 0.3 g of hydrogel to investigate possibility of the formation of AIHs. The results are illustrated in Figure 6. Evidently the high ion strength in K2CO3 solution posed negative effect on the absorbing capacity of hydrogels, which was in line with the conclusion from previous research.48 Consequently, K2CO3AIHs was not formed if the solution concentration was above 40 w/w%. By contrast, formations of MDEA-AIHs and AMPAIHs were still realized even if the MDEA and AMP concentration were as high as 60 w/w% and 50 w/w% respectively. As shown in Figure 5, these two AIHs contained the greatest amine content among the developed AIHs. We speculate that the high amine loading was largely attributed to the presence of hydroxyl in their molecules. Besides, it appeared that the number of hydroxyl could have evidently influenced, as more MDEA was retained in hydrogels than AMP. As for PEHA and PEI, their “amine saturation” in hydrogels were found to be 40 w/w%, which was merely greater than that of the inorganic alkali K2CO3. This might suggest that the molecular size might influence the maximal loading of organic amines in hydrogels which is the result of a complex interplay between chemical structure, size, and architecture and would also be affected by the nature of the hydrogel. This demonstrates that there is significant scope to optimize the system based on amine and hydrogel type. Based on the alkali/amine saturation in AIHs obtained above, the effect of alkali/amine concentration on CO2 uptake of AIHs was investigated. As could be observed in Figure 6, the amount of the absorbed CO2 and the K2CO3 concentration in AIHs were nearly linear dependent. This relationship was also observed in the case of MDEA-AIHs and AMP-AIHs. The three alkali chemicals all had relatively small molecular size, so their hindrance to CO2 molecular was negligible, enabling CO2 to readily react with the alkali/amine molecules located in the core of the AIHs. In other words, the retained alkali/amine in these hydrogels could nearly be fully utilized. Accordingly, a linear relationship between CO2 absorption and alkali/amine loading was established. It must be noted that more time was always required to form the AIHs that had higher alkali/amine loading. For example, it nearly took 1 h to form the MDEAAIHs with a concentration of 60 w/w%, so this might be a key issue for industrial application of systems with high concentrations so it would require optimizing. On the other hand, PEHA and especially PEI had relatively large molecular sizes. For this reason, the resulting carbamate salts would severely affect the CO2 interaction with the inner PEHA and PEI, and this hindrance effect became more pronounced if the amine concentration was increased. Hence, the increasing tendency of the CO2 uptake was affected at higher amine loading for PEHA-AIHs and PEI-AIHs. Likewise, longer time

Figure 5. Saturations of differing alkali/amines in AIHs at ambient temperature. 0.3 g of hydrogel is used. The alkali/amine concentration from left to right is 10 w/w%, 20 w/w%, 30 w/w%, 40 w/w%, 50 w/w%, 60 w/w%, and 70 w/w%. The red dotted line indicates the maximum concentration where AIHs can be formed.

Figure 6. Effect of alkali/amine concentration on the CO2 uptake of AIHs. 0.3 g of hydrogel is added into 2 g of alkali/amine solution.

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particle. That was to say, the AIHs was somewhat “diluted”, and fewer PEHA or PEI molecular was included in an individual particle. Therefore, the hindrance effect by their large size molecules was mitigated. It must be noted that, regardless of the alkali/amine species, higher hydrogel loading could also enable the AIHs to form in a more rapid manner, but costs on materials would accordingly increase, so identifying an optimal hydrogel loading that might balance the CO2 uptake performance of AIHs and economics was critical. In this work, it appeared that the optimal hydrogel loading for most investigated alkali/amine was 0.2 or 0.3 g per 2g of solution. Therefore, the low hydrogel loading which only accounts for 10% of the total AIHs mass is another competitive advantage, so materials costs will be greatly reduced. Effect of Salinity on the CO2 Uptake Performance of AIHs. The pronounced effect of the abundant presence of K+ on the absorbing capability of hydrogels in previous section demonstrates that the salinity might also impact on the CO2 absorption of the organic amine-based AIHs. To investigate this effect, 0.3 g of hydrogels was used to absorb organic amine solutions with varying salinities (KCl). It was found that AIHs could easily be formed in the testing range of salinity (1000− 50 000 ppm), irrespective of the species of organic amines. The CO2 loading capacities of each AIHs as a function of the salinity are illustrated in Figure 8. It appeared that the CO2

was also required to form the AIHs with greater content of PEHA and PEI. Effect of Hydrogel Amount on the CO2 Uptake of AIHs. As stated earlier, the effectiveness of AIHs contributed to the enlarged contact area available for CO2 absorption. Therefore, it is essential to identify the lowest hydrogel loading that could completely retain the applied alkali/amine solution, so that a large contact area was created while material cost was minimized. In this subsection, varying amounts of hydrogel were added into 2 g of solution with an alkali/amine mass concentration of 30%, and their capacities to capture CO2 were evaluated. The results are given in Figure 7. It was observed

Figure 7. Effect of hydrogel loadings on the CO2 uptake of AIHs. 2g of alkali/amine solution with a mass concentration of 30% is applied.

that 0.1 g of hydrogel could only absorb a small portion of K2CO3 solution, with the formed AIHs suspending in the remaining solution. As a result, CO2 interacted with K2CO3 to a limited extent and thus the absorption capacity was poor. When greater amount of hydrogel was used (0.2 and 0.3 g), more and more K2CO3 solution was locked in the hydrogel particles, which resulted in the improved contact area and overall CO2 uptake. In fact, the applied K2CO3 solution was nearly fully retained by 0.3 g of hydrogels. Then the hydrogel loading was further increased to 0.4, 0.5, and 0.6 g. On one hand, the absolute mass of the absorbed CO2 would rise to a slight extent. On the other hand, the overall mass of the absorbent was also increased. Consequently, the CO2 loading capacity (mg CO2/g absorbent) was found to slightly decline. When it came to MDEA and AMP, similar trends were observed, except that 0.2 g of hydrogels seemed to be sufficient to effectively absorb the amine solutions and create interstitial voids between the particles that increase the contact area for interaction between the gas and the liquid. This again might be closely associated with the hydrophilic hydroxyl in the molecules of MDEA and AMP. Similarly, their uptake capacities were also found to slightly decrease if the hydrogel loading was above 0.5 g. Conversely, the CO2 uptake of PEHA and PEI, whose molecular size were relative large compared to other applied alkali/amines, was found to be positively correlated with the hydrogel loading. First, as stated early, higher content of hydrogels could lead to better absorption of PEHA and PEI solutions. More importantly, with the identical overall amine content in 2g of solution (30 w/w%), greater hydrogel loading reduced the amine content in each hydrogel

Figure 8. Effect of salinity on the CO2 uptake of AIHs. 0.3 g of hydrogel is added into amine solutions with a mass concentration of 30%.

uptake was barely affected until the salinity reached 50 000 ppm, indicating the salinity had little influence on the absorbing capability of hydrogels in the range of 1000− 10 000 ppm. In other words, the amine solution was well retained in the hydrogel without free liquid draining from the particles and collecting between them. When the salinity was increased up to 50 000 ppm, the CO2 loading of all AIHs only decreased by 5% on average. This finding has implications for industrial application as it might considerably reduce the overall operation costs for carbon capture. In the carbon capture industry, it was economically unacceptable to always use distilled water in the preparation of AIHs. However, if other sources of water such as fresh water (salinity 1000 ppm), seawater (salinity 10 000 ppm) or even wastewater (salinity 50 000 ppm) were adjacent to the operation site, then supply and transport of distilled water was not indispensable. This is of great significance because the material supply costs would be significantly reduced. It should be noted that the salinity should not be extremely high in order to enable the formation F



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Figure 9. Recyability of various AIHs. Regeneration temperatures of K2CO3-AIHs, MDEA-AIHs, AMP-AIHs, PEHA-AIHs, and PEI-AIHs are 120 °C, 70 °C, 75 °C, 150 °C and 170 °C, respectively. The various regeneration temperatures are the consequence of different decompsition temperatures for corresponding bicarbonmate salts or carbamate salts.

bicarbonateas primary product.42,51 Therefore, from the perspective of energy consumption, they should be first considered in real carbon capture scenario. Nevertheless, there was an noticeable trade-off between the regeneration energy demand and the cyclic capacity. As could be seen in Figure 9 (b) and (c), the CO2 uptake of their AIHs were 93.2% and 91.6% of the initial value over ten cycles. The relativel lower loading capacity was mostly attributed to their higher volatilities, so a small portion of MDEA and AMP evaporated in each cycle. In fact, the high volatility of organic amines is not a major issue for the stripper column as they could completely be recovered, but it must be seriously considered in the absorber, that was why devices such as waterwashes were employed in the conventional MEA scrubbing technique. Likewise, appropriate equipment should be designed and installed on the top of absorbers if MDEAAIHs and AMP-AIHs were used for carbon capture in industrial scale. Unsurprisingly, due to their extremely low volatility, PEHA-AIHs, and especially PEI-AIHs had encouraging CO2 loading capacity over ten absorption−desorption cycles. However, their regeneration temperatures were the highest among the investigated alkali/amines (150 °C for PEHA-AIHs and 170 °C for PEI-AIHs), which posed heavy energy burden for the carbon capture process. Besides, it must be noted that certain amount of water was always added to the heated sample to make up the evaporative loss of water in each cycle. Generally, K2CO3-AIHs, PEHAAIHs, and PEI-AIHs required longer time to absorb the added water, whereas MDEA-AIHs and AMP-AIHs required significantly shorter period of time, which might be attributed to the varying molecular chemistry of the invesigated

of AIHs. For example, in previous subsection, it was verified that 40 w/w% K2CO3 solution (salinity 400 000 ppm) was not completely absorbed by hydrogels, leading to poor CO2 uptake performance. In this study, we mainly took into account the effect of overall salinity without considering the ion type, but more work regarding this aspect will be conducted in the future. Regeneration Evaluation. To lower the material costs and promote the operation economics, any CO2 absorbents that may find their applications in industry must be easily regenerated with an exceptional cyclic capacity.49 Therefore, we assessed the recyclability of various AIHs using a conventional convective oven. The CO2-loaded sample was first heated to a certain temperature to enable the sufficient deposition of bicarbonmate salts or carbamate salts. Afterward, the CO2-free sample was cooled to ambient temperature before its CO2 loading capacity was remeasured and compared to the initial capacity. Ten cycles of CO2 absorption and desorption were performed and the results were illustrated in Figure 9. As for K2CO3-AIHs, its absorption capacity only dropped by less than 2% after ten cycles. Generally, the loss of the loading capacity mainly arose from the evaporation of alkali chemicals,50 but K2CO3 was an inorganic salt and its content in AIHs hardly changed during the heating process. Therefore, most of its CO2 uptake capacity was maintained over multipule cycles. However, its relatively high regeneration tempertaure (120 °C) and the corresponding energy input should be considered. In this sense, MDEA and AMP displayed a competitive advantage over K2CO3 as their AIHs could be regenerated at lower temperatures (70 and 75 °C respectively) as a result of the lower reaction heat in the formation of G



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chemicals. In real industry, the productivity is also a crucial factor. For this reason, the time required to form AIHs must be considered in screening suitable alkali/amines. In this sense, MDEA and AMP were the top candidates to prepare AIHs that might be deployed in power plants or natural gas processing stations. It is also worth noting that the degradation of the hydrogel must to considered particuarly in the presence of the amine so long-term studies are underway to investigate this. The AIHs were stable over the period of this study but some shrinkage of the polymer was observed but the performance of was not compromised but further research is required.Lastly, the “liquid lock” capability of AIHs was briefly invesigated. Aluminum pellets were immersed in bulk alkali/amine solutions and AIHs, as illustrated in Figure 10. After a certain

Figure 10. Appreance of aluminum pellet after the immersion in bulk alkali/amine solution and AIHs for 10 h.

period of time, their appearances were recorded and compared. The results demonstrated that AIHs was capable of firmly retain the alkali/amine solution in hydrogel particles therefore a surface with remarkable dryness was obtained. In other words, there was minimal direct contact between amines and surface of pellets, while on the other hand, aqueous alkali/ amine solutions evidently affected the appearance of the aluminum pellets and caused corrosion. This charcteristic along with the easy fabrication, high performance, costeffectiveness and wide applicability to various types of alkali/ amines and possibly other chemicals make AIHs a viable and robust approach for carbon capture.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Colin D. Wood: 0000-0001-6160-0112 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support and technical assistance from CSIRO through the Office of the Chief Executive (OCE) postdoctoral funding program.

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REFERENCES

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A Highly Tunable Approach to Enhance CO2 Capture with Liquid

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