Amine-Functionalized Layered Double Hydroxides for Direct Air Capture of CO2

【Research Background】

Direct air capture (DAC) based on adsorption is becoming a technically feasible negative emission technology. DAC can capture distributed carbon emission sources without location restrictions for capture facilities. Currently, DAC technology has reached level 7 (demonstration level) in terms of technological maturity. The absorption-type DAC pilot plants using alkali metal hydroxide solvents have lower operating costs (94~232 $/t_CO2), but face serious issues such as high water consumption and high regeneration temperatures. In contrast, recent studies have shown that large-scale deployment of DAC using adsorption methods is both technically and economically feasible, with the potential to achieve the ambitious goal of capturing 1% of global annual CO₂ emissions. The energy consumption of adsorbent-based DAC systems can reach 0.113~0.145 MJ/mol_CO2, with capture costs ranging from 60 to 190 $/t_CO2. Further reducing the cost of DAC can be achieved by employing efficient adsorbent materials. The development of low-cost adsorbents with high CO₂ adsorption capacity, fast adsorption kinetics, and good adsorption-desorption stability is crucial for the large-scale deployment of DAC processes.

【Results Summary】

Recently, Assistant Professor Zhu Xuancan from the School of Mechanical and Power Engineering at Shanghai Jiao Tong University, along with corresponding authors Professors Ge Tianshu and Wang Ruzhu, reported in Cell Reports Physical Science a class of amine-functionalized, organic solvent-treated Mg_xAl-CO₃ layered double hydroxide (LDH) nanosheets for rapid CO₂ capture from air. The linear triamine grafted onto the highly dispersed LDH nanosheets exhibits a high adsorption capacity of 1.05 mmol/g at 25 °C and 400 ppm CO₂ concentration, which is 30% higher than that of amine-functionalized SBA-15. More importantly, its CO₂ adsorption capacity can reach 70% of the maximum capacity within 30 minutes, while the polyamine-impregnated materials require twice the time. These adsorbents can be regenerated at 80 °C and recover 80% of their maximum CO₂ adsorption capacity. The highly dispersed LDH nanosheets provide excellent thermal, hydrothermal, and chemical stability: the adsorption performance shows negligible decay after 50 adsorption-desorption cycles. Considering the potential cost-effectiveness and scalable production processes, the single molecular layer amine-functionalized organic solvent-treated LDH-derived nanosheets have significant application potential in DAC cycles based on rapid temperature swing adsorption.

This research work was funded by the National Natural Science Foundation of China Youth Project (52006135) and the Shanghai Science and Technology Innovation Action Plan Project (20160712800). This work was also guided by Professor Dermot O’Hare from the Department of Chemistry at the University of Oxford, with authors Wu Junye, Yang Fan from Shanghai Jiao Tong University, and Lyu Meng, Chen Chunping from the University of Oxford participating in the research.

【Article Introduction】

1. Material Synthesis and Characterization Results

A series of Mg_xAl-CO₃ LDH nanosheets with varying sizes and thicknesses were prepared, with the Mg:Al molar ratio (x) varying from 0.55 to 3. Transmission electron microscopy (TEM) images (Figure 1A-F) show that hexagonal platelet morphology and LDH with a particle size of 2 μm can be formed using a hydrothermal urea method (referred to as Mg_xAl-urea). When using the co-precipitation method and controlling the synthesis process’s pH to 10, the lateral size of the LDH was observed to decrease to the nanoscale (referred to as Mg_xAl-w). However, the strong interlayer hydrogen bond network led to significant stacking of the LDH-derived nanosheets, particularly pronounced at low Mg/Al ratios. AMOST treatment allows the AMO organic solvent to replace the interlayer water of the LDH, enabling effective (though not complete) exfoliation of the LDH into nanosheets after vacuum drying. For example, the lateral size of the acetone-washed Mg_xAl-CO₃ LDH (referred to as Mg_xAl-a) is 20-100 nm, and the layer thicknesses of Mg₂Al-a and Mg_0.55Al-a reach 5.26 and 2.50 nm, respectively. The extremely fine and thin size of Mg_0.55Al-a nanosheets is partly due to the low Mg:Al ratio forming Al(OH)₃ impurities that suppressed further growth of the LDH. Moreover, rapid co-precipitation followed by acetone washing (referred to as Mg_xAl-a-F) can further reduce the lateral size of the LDH. However, these LDH-derived nanosheets are more prone to fragmentation and stacking, possibly due to insufficient crystallization under variable pH conditions. Scanning electron microscopy (SEM) images also confirm the flower-like morphology of the LDH and the abundance of slit-like mesopores (Figure 1G). These exfoliated LDH-derived nanosheets are expected to expose a large number of hydroxyl groups suitable for amine grafting.

Amine-Functionalized Layered Double Hydroxides for Direct Air Capture of CO2

Figure 1. A) TEM images of Mg₂Al-urea, B) Mg₂Al-w, C) Mg₂Al-a, D) Mg₂Al-a-F, E) Mg_0.55Al-w, F) Mg_0.55Al-a; G) SEM images of Mg₂Al-a-F and H) TRI-Mg₂Al-a-F; I) Energy spectrum scanning results of Mg₂Al-a-F, N: Mg: Al atomic ratio is 1.86:14.82:7.38.

3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane (TRI) reacted with LDH in toluene for 16 hours at a temperature of 85 °C. The TRI-functionalized LDH nanosheets were then centrifuged and washed successively in toluene, n-hexane, and methanol, followed by vacuum drying at 60 °C. Notably, complete consumption of the free alkoxy ligands of the support can only occur in the presence of water, achieving full surface coverage and multilayer amine grafting. The TRI-functionalized LDH (denoted as TRI-y, y = support) retained the flower-like morphology (Figure 1H), but EDS energy spectrum results indicate that N atoms were uniformly distributed throughout the sample (Figure 1I). Fourier-transform infrared spectroscopy (FTIR) shows that the prepared support has typical LDH spectral bands (Figure 2A), including O-H stretching vibrations of interlayer water and surface hydroxyl groups (3450 cm^–1), C-C stretching (2925 cm^–1) and bending vibrations (1490 cm^–1) of alkyl groups, symmetric and asymmetric stretching vibrations of carboxyl groups (1635, 1355~1410 cm^–1), and lattice vibrations of M-O (445 cm^–1). The enhancement of C-C stretching and bending vibrations, along with the appearance of NH₂ bending (1560 cm^-1) and Si-O-M stretching (1022 cm^-1) vibrations, confirms the successful grafting of TRI molecules (Figure 2B). Additionally, the spectral fluctuations in the 1260~1500 cm^-1 region can be attributed to the ammonium carbamate ions formed by exposure to air and subsequent adsorption of CO₂.

Figure 2. A) FTIR results of LDH; B) TRI-grafted LDH; C) N₂ adsorption-desorption isotherms and DFT pore size distribution of LDH and TRI-grafted LDH at 77 K.

N₂ adsorption/desorption at 77 K can measure the specific surface area and pore structure of the samples. As shown in Figure 2C, the LDH support exhibited a type IV isotherm with an H3 hysteresis loop, indicating that the LDH-derived nanosheets formed slit-like mesopores. The pore size distribution of LDH is much wider compared to ordered mesoporous silica supports (such as SBA-15), with pore volume inversely proportional to the size and thickness of the LDH nanosheets. After grafting, TRI molecules cover the mesopores to a depth of 5-15 nm (Figure 3.2D). The retention of large pore structures further substantiates the single molecular layer dispersion of TRI on the LDH surface. Textural property analysis of the samples indicates that TRI-Mg_0.55Al-a retains a high specific surface area (162 m²/g) and pore volume (0.553 cm³/g), surpassing TRI-SBA-15 (96 m²/g and 0.231 cm³/g) and significantly exceeding PEI-impregnated SBA-15 (PEI-SBA-15) and calcined Mg_0.55Al-a (PEI-Mg_0.55Al-a (c)). The ideal morphology, abundant slit-like mesopores, and large specific surface area and pore volume of TRI-LDH provide optimal amine active sites for CO₂ adsorption.

2. CO₂ Adsorption Capacity Evaluation

Figure 3A displays the CO₂ adsorption isotherms of all prepared adsorbents at 25 °C. Although the TRI-grafted bulk and stacked LDH exhibited poor CO₂ capacity under extremely dilute conditions, the TRI-grafted AMO-LDH showed significant improvement. At a CO₂ pressure of 0.4 mbar, the CO₂ adsorption capacity of TRI-Mg_0.55Al-a is 1.049 mmol/g, which is 16.7 times that of TRI-Mg_0.55Al-w (0.063 mmol/g) and 30% higher than TRI-SBA-15 (0.808 mmol/g). The rapid co-precipitation method did not significantly enhance the adsorption performance of TRI-LDH, indicating that the reduction in size and thickness of the LDH and the exfoliated morphological structure play crucial roles in establishing amine grafting sites. The monomer amine-grafted AMO-LDH also exhibited high CO₂ adsorption performance at 0.4 mbar, with the adsorption capacities of APS-Mg₂Al-a at 0.416 mmol/g and APS-Mg_0.55Al-a at 0.778 mmol/g.

Figure 3. A) CO₂ isothermal adsorption lines of amine-grafted samples at 25 °C; B) CO₂ isothermal adsorption lines of TRI-Mg_0.55Al-a at 25, 45, and 65 °C; C) Heat of adsorption of TRI-Mg_0.55Al-a; D) Relationship between CO₂ adsorption capacity and amine loading at 0.4 mbar and 25 °C.

Although the TRI-grafted bulk and stacked LDH exhibited poor CO₂ capacity under extremely dilute conditions, the TRI-grafted AMO-LDH showed significant improvement. At a CO₂ pressure of 0.4 mbar, the CO₂ adsorption capacity of TRI-Mg_0.55Al-a is 1.049 mmol/g, which is 16.7 times that of TRI-Mg_0.55Al-w (0.063 mmol/g) and 30% higher than TRI-SBA-15 (0.808 mmol/g). The rapid co-precipitation method did not significantly enhance the adsorption performance of TRI-LDH, indicating that the reduction in size and thickness of the LDH and the exfoliated morphological structure play crucial roles in establishing amine grafting sites. The monomer amine-grafted AMO-LDH also exhibited high CO₂ adsorption performance at 0.4 mbar, with the adsorption capacities of APS-Mg₂Al-a at 0.416 mmol/g and APS-Mg_0.55Al-a at 0.778 mmol/g.

Considering the low physical adsorption of the LDH support at 25 °C and 0.4 mbar and the reduction in specific surface area after amine grafting, the high CO₂ adsorption capacity of TRI-LDH is primarily attributed to the chemical adsorption of amine groups. The heat of adsorption of the TRI-Mg_0.55Al-a sample was calculated based on the Clausius–Clapeyron equation from the isothermal adsorption lines at 25, 45, and 65 °C (Figure 3B, C). The adsorption heat is underestimated at low CO₂ coverage but quickly reaches 79 kJ/mol, and remains above 65 kJ/mol for adsorption capacities below 1.2 mmol/g, indicating typical chemical adsorption of amines with CO₂. At higher surface coverage, the adsorption heat ranges from 15 to 40 kJ/mol, representing a predominance of physical adsorption.

Elemental analysis of the samples was conducted to quantitatively determine the amine loading. The N element content of the LDH support was close to zero, indicating that the nitrate was effectively removed during the washing process, ensuring the accuracy of assessing amine content based on the N content in elemental analysis. The influence of the LDH support morphology on the TRI loading can be clearly observed: the stone-like LDH sheets generated by Mg₂Al-urea almost cannot undergo surface reactions with amines, while the exfoliated AMO-LDH exhibits extremely high amine loading, reaching up to 6.399 mmol/g. The low amine loading of the bulk TRI-grafted LDH also indicates that multilayer grafting is unlikely to occur. Comparing the CO₂ adsorption capacity at 0.4 mbar with the amine loading, it can be found that the amine efficiency of TRI-LDH ranges from 0.09 to 0.17 (Figure 3D). Among them, TRI-Mg_0.55Al-a has outstanding amine loading (6.399 mmol/g) and amine efficiency (0.164), surpassing most of the currently reported type 2 adsorbents. Compared to other TRI-LDHs with lower amine loading, TRI-Mg_0.55Al-a exhibits higher amine efficiency, which can be attributed to the high density of grafted amine active sites facilitating the simultaneous binding of two adjacent amine groups to CO₂ molecules. Belmabkhout et al. reported that TRI-PE-MCM-41 has a higher amine loading (7.9 mmol/g). However, in their work, the samples were prepared using a water-assisted process, leading to the stacking of multilayer amines. The resulting cross-linked array is prone to clogging the support’s mesopores, hindering CO₂ diffusion and reducing amine efficiency. In contrast to TRI-grafted samples, APS-Mg₂Al-a and APS-Mg_0.55Al-a containing only primary amines exhibited higher amine efficiencies of 0.183 and 0.212, respectively. Reports of amine efficiencies exceeding 0.2 are rare in the literature, and in most cases, they occur in conjunction with lower amine loadings, making physical adsorption on the support surface likely to contribute significantly to the overall adsorption capacity.

3. CO₂ Adsorption Kinetics Evaluation

The DAC adsorbents exhibit poor kinetics under extremely dilute conditions, with polyamine-impregnated adsorbents typically requiring hours to reach half of their adsorption capacity. In contrast, as shown in Figure 4A, the amine-grafted LDH demonstrates a rapid CO₂ adsorption rate in a nitrogen flow containing 400 ppm CO₂, with the CO₂ adsorption amount within 120 minutes being consistent with the adsorption amount determined by isothermal adsorption lines. Notably, the CO₂ adsorption amount of TRI-Mg_0.55Al-a within 60 minutes even exceeds that of the typical type 1 adsorbent PEI-SBA-15. Normalizing the dynamic CO₂ adsorption amount to the total CO₂ adsorption amount provides a more intuitive comparison (Figure 4B), where both APS- and TRI-Mg_0.55Al-a achieve 70% of their adsorption capacity within 30 minutes, while PEI-SBA-15 requires twice the time to reach the same adsorption amount. The longer diffusion process observed for TRI-SBA-15 further indicates that using LDH with larger pore sizes as a support can effectively avoid amine clogging.

Figure 4. A) Dynamic adsorption amount of amine-grafted samples at 25 °C and 400 ppm CO₂ concentration after 60 minutes of regeneration at 120 °C in pure N₂; B) Normalized dynamic CO₂ adsorption amount within 120 minutes after dividing by the total CO₂ adsorption amount; C) Normalized CO₂ desorption of adsorbents obtained by ramping temperature at a rate of 5 °C/min under saturated conditions of 400 ppm CO₂; D) Results of programmed temperature desorption (TPD) obtained from the differential calculation of C; E) Normalized CO₂ dynamic desorption amount of TRI-Mg_0.55Al-a under pure N₂ conditions and different regeneration temperatures within 60 minutes; F) CO₂ adsorption amount of TRI-Mg_0.55Al-a under 25 °C and 400 ppm CO₂ conditions within 120 minutes after regeneration under pure N₂ conditions and different regeneration temperatures.

When using a heating rate of 5 °C/min, the results in Figures 4 C, D also show that the desorption temperature peak of the amine-grafted LDH (75 °C) is lower than that of PEI-SBA-15 (100 °C). The mild regeneration requirements of amine-grafted LDH provide the opportunity to utilize industrial waste heat below 100 °C for thermal regeneration. For example, TRI-Mg_0.55Al-a retains 80% of its total adsorption amount even at desorption temperatures as low as 80 °C (Figure 4C). Notably, the single molecular layer amines formed by dry grafting on the exfoliated LDH nanosheets can achieve the shortest CO₂ desorption and diffusion paths. In contrast, multilayer amines are prone to thickening and aggregation on the support surface, requiring additional energy to transfer desorbed CO₂ within the amine film. Additionally, TRI-Mg_0.55Al-a exhibits extremely fast CO₂ desorption kinetics, regenerating 90% of the CO₂ adsorption amount within 10 minutes at only 70 °C (Figure 4E). After desorption over a wider temperature range, the next cycle retains rapid adsorption kinetics, with the CO₂ adsorption amount within 120 minutes aligning well with TPD results (Figure 4F).

4. Cycling Stability Testing

Considering the potential thermal, hydrothermal, and chemical stability of amine-modified LDH, along with its excellent adsorption capacity and kinetics, this material is expected to have promising applications in rapid temperature swing adsorption (R-TSA) processes to significantly enhance CO₂ yield and reduce DAC energy consumption. To further demonstrate this, Figure 5A shows the results of 50 cycling tests of TRI-Mg_0.55Al-a, with 60 minutes of adsorption conditions at 25 °C and 400 ppm CO₂/N₂, and 15 minutes of desorption conditions at 120 °C and pure N₂.

Figure 5. A) 50 cycling tests of TRI-Mg_0.55Al-a under 25 °C and 400 ppm CO₂/N₂ for 60 minutes of adsorption and 15 minutes of desorption at 120 °C and N₂; B) Comparison of stability of type 2 adsorbents under dry conditions.

In this case, the average CO₂ adsorption amount of TRI-Mg_0.55Al-a is 0.912 mmol/g, maintaining a high stability level throughout the entire cycle. The stability results were compared with existing literature data, named A-X/Y, where A,X, and Y represent the adsorbent, adsorption temperature, and desorption temperature, respectively (Figure 5B). Overall, amine-grafted adsorbents exhibit better cycling stability under dry conditions compared to polyamine-impregnated adsorbents due to lower amine leakage during heating. For example, PEI-SBA-15-50/130 lost over 50% of its adsorption capacity after only 30 cycles. Additional treatments are required to enhance the bonding strength between polyamines and the support, such as adding nano-dispersants or performing surface modifications. In contrast, Sayari et al. indicated that TRI-PE-MCM-41-50/120 exhibited much lower performance decay (14%) after 40 cycles, with the formation of urea-like substances in a CO₂ atmosphere and at high temperatures being the primary decay mechanism. Subsequent studies have shown that secondary amines are more stable than primary amines in CO₂ induced deactivation. In the presence of steam, the deactivation caused by amine groups forming urea can be significantly suppressed, but it requires the support to have high hydrothermal stability. Previous work has shown that amine-functionalized LDH exhibits high stability over 20 cycles under steam purging conditions.

【Conclusion】

This work developed a novel type 2 adsorbent composed of amine-grafted Mg-Al-CO₃ LDH-derived nanosheets. In the first step, we successfully synthesized a series of LDH nanosheets with controllable sizes and thicknesses. These LDH-derived nanosheets can be exfoliated into structures with a wider pore size distribution than ordered mesoporous silica materials. Amine functionalization was conducted under anhydrous conditions, grafting single molecular layer amines onto the exposed hydroxyl groups of LDH through a silylation reaction. The size and thickness of the nanosheets, along with their stacking morphology, determine the maximum amine loading. For instance, the bulk Mg₂Al-urea exhibits lower amine loading, while the extremely fine (20 nm) and ultra-thin (2.5 nm) nanosheets of Mg_0.55Al-a have an amine loading of 6.399 mmol/g. The results indicate that under conditions of 25 °C and 400 ppm CO₂, the CO₂ adsorption capacity of TRI-Mg_0.55Al-a is 1.05 mmol/g, which is 30% higher than that of TRI-modified SBA-15. The APS-grafted Mg_0.55Al-a, containing only primary amines, exhibits higher amine efficiency (0.212).

The impact of single molecular layer amine grafting on the support morphology is minimal, alleviating amine clogging and retaining the high specific surface area (162 m²/g) and pore volume (0.553 cm³/g) of the support. Therefore, these ideal CO₂ diffusion channels ensure rapid adsorption kinetics. Both APS- and TRI-Mg_0.55Al-a achieve 70% of their adsorption capacity within 30 minutes and can regenerate 80% of their adsorption amount at temperatures as low as 80 °C. Furthermore, the strengthened chemical bonds between amines and LDH, as well as good steam resistance, provide excellent thermal, hydrothermal, and chemical stability. In 50 rapid adsorption/desorption tests, the CO₂ working amount of TRI-Mg_0.55Al-a is 0.912 mmol/g. Considering the low-cost and scalable production processes, the single molecular layer amine-grafted LDH nanosheets are highly attractive for constructing new R-TSA processes for DAC.

【Team Introduction】

Zhu Xuancan, first author, Assistant Professor at the School of Mechanical and Power Engineering, Shanghai Jiao Tong University. Engaged in the synthesis of new adsorbents, carbon capture and hydrogen production processes, theoretical and applied research on negative emission technologies, having published over 30 papers related to CO₂ adsorption and separation in journals such as Progress in Energy and Combustion Science. Received honors such as the Excellent Innovation Achievement of Young Talents (2020), Shanghai Super Postdoctoral (2019), and Tsinghua University Excellent Doctoral Dissertation (2019), and leads projects funded by the National Natural Science Foundation of China (2021), the China Postdoctoral Science Foundation General Fund (2019), and the Postdoctoral Innovation Talent Support Program (2019).

Ge Tianshu, corresponding author, Professor at the School of Mechanical and Power Engineering, Shanghai Jiao Tong University. Mainly engaged in research on solid dehumidification air conditioning and heat pumps, coupled heat and mass transfer and efficient utilization of low-grade energy, advanced water/carbon direct capture technologies, etc. He has led over 20 projects funded by the National Natural Science Foundation, the National Major Instrument Development Special Subproject, and the Ministry of Education Doctoral Fund, and published over 70 SCI papers, with over 3900 citations. He has received funding from the National Natural Science Foundation Excellent Young Scientist Fund, was selected for the Ministry of Education Changjiang Scholars Award (Young Scholars), and received the Willis H. Carrier Young Scholar Award from the International Institute of Refrigeration, as well as nominations for the National Top 100 Doctoral Dissertations.

Wang Ruzhu, corresponding author, Chair Professor at the School of Mechanical and Power Engineering, Shanghai Jiao Tong University. Received the International Refrigeration J&E Hall Gold Medal awarded by the British Institute of Refrigeration (2013), the Asian Refrigeration Academic Award jointly awarded by China, Japan, and South Korea (2017), the Nukiyama Thermal Science Memorial Award awarded by the Japan Society of Heat Transfer (2018), the highest academic award of the International Institute of Refrigeration, the Gustav Lorentzen Medal (2019), and the Peter Ritter von Rittinger International Heat Pump Award from the International Energy Agency (2021). The achievements led by Professor Wang have also received the National Natural Science Second Prize (2014), the National Technology Invention Second Prize (2010), and the National Teaching Achievement Second Prize (2009).

The ITEWA interdisciplinary innovation team at Shanghai Jiao Tong University (Innovative Team for Energy, Water & Air): Created by Professor Wang Ruzhu in 2018, dedicated to solving cutting-edge fundamental scientific problems and key technologies in the fields of energy, water, and air, aiming to achieve overall solutions at the material-device-system level through interdisciplinary collaboration, promoting breakthrough progress in related fields. In the past three years, nearly 20 academic papers have been published in top interdisciplinary journals, including Joule (4 papers), Energy & Environmental Science, Advanced Materials, Angewandte Chemie, ACS Energy Letters, ACS Central Science, ACS Materials Letters, Energy Storage Materials, Nano Energy, and Water Research.

Zhu, X.; Lyu, M.; Ge, T.; Wu, J; Chen, C.; Yang, F.; O’Hare, D.; Wang, R., Modified layered double hydroxides for efficient and reversible carbon dioxide capture from air. Cell Reports Physical Science 2021. DOI: 10.1016/j.xcrp.2021.100484

【Carbon Neutrality】CO₂ conversion into key solvents for lithium batteries, the market in various fields is expected to reach nearly 10 billion in the future!

2021-07-10