CHAPTER 1: Layered Double Hydroxides-derived Intermediate-temperature CO₂ Adsorbents

Carbon dioxide (CO₂) is a major greenhouse gas that has triggered global warming and climate change.^1–3  Over the past two centuries, its concentration in the atmosphere has rapidly increased, mainly because of human activities such as fossil fuel burning.^4–9  However, it is predicted that this trend of increasing atmospheric CO₂ concentration with not be altered within the next several decades, because fossil fuels will still be the dominant energy source. Moreover, the energy demand will increase further by 53% by 2030.¹⁰  So development of technologies for CO₂ capture and storage will become increasingly important.¹¹ 

Concerns about the increasing concentration of greenhouse gases in the atmosphere have stimulated the study of CO₂ capture by using solid adsorbents.^12,13  LDHs-derived mixed oxides are considered to be one of the most promising adsorbents for CO₂ capture at an intermediate-temperature range (200–400 °C), which require less energy in regeneration and show superior multicycle stability.^14,15  In addition, they show fast adsorption/desorption kinetics and good performance in the presence of water,^16–18  making them very attractive not only for pre-combustion CO₂ capture but also for applications involving CO₂ equilibria such as sorption enhanced water gas shift (SEWGS) and sorption enhanced steam reforming (SESR).^19–22 

LDHs, also known as hydrotalcite-like compounds or anionic clay, are composed of stacked positively-charged brucite-like layers with interlayer spaces containing charge-compensating anions and water molecules. The metal cations occupy the centers of octahedra whose vertexes contain hydroxide ions. The octahedra are connected by edge sharing to form an infinite sheet.²³  It has the composition [M_1−x²⁺M_x³⁺(OH)₂][Aⁿ⁻]_x/n·ZH₂O, where M²⁺ and M³⁺ are divalent and trivalent cations, respectively, and Aⁿ⁻ is the anion that is intercalated in the interlayer for charge compensation.^24–28  The general structure of LDHs is shown in Figure 1.1. Because the chemical composition of both the inorganic layers and the interlayer gallery anions can be precisely controlled, LDHs possess highly tunable properties and potentially can be used in a wide range of applications, such as for catalysts, fire retardant additives, polymer–LDH nanocomposites, CO₂ capture, luminescent materials, and magnetic materials.^29–35  LDHs have been investigated for decades as adsorbents as well.^36–38  Owing to their high surface area and abundant basic sites at the surface, LDHs are considered to be favorable for adsorbing acidic CO₂.^10,39  Fresh LDHs themselves do not possess any basic sites. However, upon thermal treatment, a LDH gradually loses interlayer water; with the temperature increasing, it dehydroxylates and decarbonates to a large extent, leading to the formation of a mixed oxide with a 3D network, with a larger surface area and good stability at high temperature.^18,40  Therefore, LDHs are significantly more active for CO₂ removal after thermal decomposition, being transformed into basic mixed oxides (LDOs).¹¹ 

Starting from the natural mineral (MgAl–CO₃), a lot of work has been done on LDHs-derived CO₂ adsorbents, which includes (1) influence of the chemical composition of LDHs, (2) influence of the synthetic conditions and method, (3) LDH-based composites, (4) influence of doping alkali metals, (5) influence of other co-existing gases, (6) adsorption mechanism and kinetics study, and (7) techno-economic assessment etc. Although several recent review papers have provided prolific insights into the progress in this area, most previous reviews have only discussed LDHs-derived CO₂ adsorbents as a part of the papers. A detailed review focusing only on LDHs-derived compounds as CO₂ adsorbents, addressing their pros and cons, and potential applications in industry is still needed.

In this regard, the objective of this chapter is to review the currently available literature on adsorption of CO₂ on LDHs-derived compounds, focusing on the aforementioned seven parts. In addition, based on the overview, the closing section will suggest future research efforts.

As mentioned in the introduction, LDHs are composed of stacked positively-charged brucite-like layers with interlayer spaces containing charge-compensating anions and water molecules.^41,42  They are also very open to physicochemical manipulation, numerous combinations of structural cations (Mg²⁺, Co²⁺, Cu²⁺, Ca²⁺, Fe³⁺, Al³⁺ etc.) and interlayer charge-compensating anions (Cl⁻, NO₃⁻, CO₃²⁻, HCO₃⁻, SO₄²⁻ etc.).⁴³  With a proper understanding of their composition–structure–property relationships, LDHs may conceivably be synthesized and tailored as effective high temperature adsorbents for CO₂ separation and capture. Plenty of work has been done to study how substitution of cations and interlayer charge-compensating anions affects the CO₂ adsorptive capacity of LDHs.^43,44  Yong et al.⁴⁵  first investigated the effect of Al content in commercial LDHs on their CO₂ capture capacity. The adsorption capacities of three kinds of commercial LDHs from CONDEA Chemie Gmbh company (PURAL MG30 (70% Al₂O₃), MG50 (50% Al₂O₃), and MG70 (30% Al₂O₃)) with different aluminium contents were tested at 300 °C and 1 bar. The results showed that the amounts of adsorbed CO₂ increased with a decrease of aluminium content from 70% (MG30) to 50% (MG50). However, the amount of adsorbed CO₂ on MG50 is slightly higher than that on MG70 although MG50 has a higher aluminium content. The reason given was that incorporated aluminium has two functions: (1) the density of the layer charge in LDHs increases with increasing aluminium content, which is favorable for the adsorption of CO₂, (2) increasing the aluminium content leads to a decrease in the interlayer spacing of LDHs and a reduction in the number of high-strength CO₂ adsorption sites in the LDHs. Later, Wang et al.⁴⁶  synthesized various X_nY_3−nAl-LDHs by a constant pH co-precipitation method with the (X²⁺ + Y²⁺)/Al³⁺ molar fixed at 3.0. Besides the CO₂ capture capacity of different LDHs, the structural effects of cations on LDHs and derived oxides, the thermal decomposition behaviors of LDHs precursors, as well as the modeling of CO₂ adsorption on mixed oxides have also been studied. The results revealed that the precursors MgAl–CO₃ and CoAl–CO₃ LDHs exhibit the typical X-ray diffractogram of LDHs, and slight differences were observed for samples CoMgAl–CO₃ and CaCoAl–CO₃ LDHs compared with MgAl–CO₃ and CoAl–CO₃ LDHs with the corresponding diffraction peaks being weaker, as well as the fact that MgCaAl–CO₃ LDHs contains an impure phase CaCO₃. However, after calcination, the precursor LDHs were decomposed, leading to various oxide derivatives e.g. MgAlO and MgCaAlO etc. The group found that the introduction of Co into LDHs promotes the formation of a spinel phase after calcination at 550 °C. The thermal decomposition behaviors of LDH precursors were tested during heating in air. Then, it was concluded that the thermogravimetric loss during heating involves two or three steps. The first stage occurs at 50–200 °C, mainly ascribed to the loss of surface-adsorbed and interlayer water molecules. Subsequently, the second stage takes place at 200–500 °C, including the dehydroxylation of layer hydroxyl groups and decomposition of interlayer carbonate, causing a collapse of the layer structure. The second stage for MgAl–CO₃ LDHs occurs at 441 °C. When Co partially or fully replaces the Mg in LDHs, this stage takes place at much lower temperatures (337 and 247 °C for CoMgAl–CO₃ LDHs and CoAl–CO₃ LDHs, respectively). In contrast, the replacement of partial or all Mg with Ca leads to one more event at a much higher temperature. Furthermore, the CO₂ capture capacity of the various LDHs-derived adsorbents were tested. The results showed that the CaCoAl system has the highest CO₂ adsorption capability, which can reach 1.39 mmol g⁻¹ of CO₂ (i.e., 6.12 wt%) from a gas mixture (8% CO₂ in N₂) at 350 °C and 1 bar in a fixed-bed reactor within 20 min. Commonly, it is considered that CO₂ adsorption on these oxides is more likely a chemisorption, which involves a number of steps, such as CO₂ diffusing into the surface and pores, reacting with the active sites, and forming a product layer on the surface in the carbonate form. With the adsorption going on, it becomes more difficult for CO₂ to diffuse into the inner pores/surface or penetrate the product layer to be adsorbed. The adsorbent is gradually deactivated. Therefore, the deactivated models have also been used in their study. The result showed that for all adsorbents except for CaCoAlO, both the initial adsorption rate constant (k₀) and the deactivation rate constant (k_d) are quite similar, with the average values of k₀ and k_d being 165 cm³ min g⁻¹ and 0.57 min⁻¹, respectively. However, the constant k_d of CaCoAlO (212 cm³ min g⁻¹) is much larger than the average (165 cm³ min g⁻¹), which is consistent with its highest adsorption capacity for CO_2.

Moreover, Lwin et al.⁴⁷  studied the CuAl system for CO₂ capture and tested the influence of the Cu–Al molar ratio, the influence of the adsorption temperature, as well as the adsorption kinetics. They synthesized CuAl–CO₃ LDHs with a Cu–Al molar ratio between 1 and 3. The physically adsorbed moisture, intercalated water, bonded hydroxyls, and intercalated CO₂ of LDHs were removed by thermal decomposition with heating to 600 °C, before the samples were subjected to CO₂ flow. It was noticed that the adsorption capacities are high at low temperatures (100–400 °C) and decrease as the temperature increases. And the adsorption capacities are in the order Cu₁Al₁ > Cu₂Al₁ > Cu₃Al₁, with similar adsorption capacities for Cu₁Al₁ and Cu₂Al₁. However, above 400 °C, the adsorption capacity of Cu₂Al₁ slightly exceeds that of Cu₁Al₁. Thus, the optimum Cu–Al molar ratio for maximum adsorption capacity probably lies between 1 and 2. Moreover, the initial rates of adsorption on the fresh adsorbents at 600 °C were also calculated, which showed that the initial specific adsorption rate of Cu₂Al₁ is significantly higher than those of the other two. As per the result of the specific rates of CO₂ adsorption on the fresh Cu₂Al₁ adsorbent at different temperatures, it can be seen that at low temperatures (<400 °C), the rate of adsorption only steadily increases with temperature. However, above 400 °C, the adsorption rate increases rapidly with temperature. Furthermore, through the calculating of activation energy values, they suggested that the physical adsorption dominates at low temperatures (<400 °C) and the chemisorption dominates at high temperatures (400 °C).

Later on, the influence of trivalent cations of LDHs on CO₂ capture was systematically investigated by Wang et al.⁴⁸  Four kinds of trivalent cations were investigated, which include Al, Fe, Ga, and Mn in the Mg-M-CO₃ LDHs (M is the trivalent cation). They first examined the structural evolution of the four synthesized LDHs by thermal treatment. The results indicated that the dehydration and the dehydroxylation/decarbonation properties are highly dependent on the type of trivalent cation. The first stage (30–190 °C), which corresponds to the desorption of water, follows the order Mg₃Al₁ > Mg₃Fe₁ > Mg₃Ga₁ > Mg₃Mn₁. This suggests that the interlayer water has the highest stability in Mg₃Al₁, while it is the least stable in Mg₃Mn₁. However, at the second stage (200–500 °C), it showed a different order, which suggested that the dehydroxylation and decarbonation follows the order Mg₃Al₁ > Mg₃Ga₁ > Mg₃Fe₁ > Mg₃Mn₁. To get a deeper insight into the structure evolution of LDHs, the calcined samples were characterized by XRD analyses. The structural evolution for all LDHs also showed two stages. During the first stage, LDH structures gradually decomposed and transformed into amorphous phases, showing a decrease of basal spacing as the temperature was elevated. The upward shift of the d₀₀₃ peak is mainly caused by desorption of interlayer H₂O, leading to a shrinkage of the LDH layers. The temperature at which the LDH structure just disappeared was defined as T_ct. T_ct followed the order Mg₃Al₁ (400 °C) > Mg₃Ga₁ (300 °C) ≈ Mg₃Fe₁ (300 °C) > Mg₃Mn₁ (250 °C). In the second stage, the amorphous phase was gradually transformed into periclase MgO and spinel oxides. With increasing temperature, the two peaks of MgO gradually appeared in all four samples. They found that the two peaks became narrower at high temperatures for Mg₃Al₁ and Mg₃Ga₁, without the formation of any spinel oxides up to 700 °C. However, for Mg₃Fe₁ and Mg₃Mn₁, although there was only one phase at 400 °C (periclase MgO), a further increase of temperature led to the formation of MgFe₂O₄ for Mg₃Fe₁ and Mg₂MnO₄ for Mg₃Mn₁. Therefore, it can be concluded that the tendency of converting periclase MgO into spinel oxides follows the order of Mg₃Mn₁ > Mg₃Fe₁ > Mg₃Ga₁ > Mg₃Al₁. Because of the different thermal stabilities of each Mg-M-CO₃ LDH, the calcination temperature effect on CO₂ capture capacities was studied. CO₂ adsorption experiments were performed by exposing the calcined LDHs to pure CO₂, at 1 bar and 200 °C for 2 h. The optimal calcination temperature (T_oc) is different for each LDH as shown in Figure 1.2. It indicates that the optimal calcination temperature is different for each LDH, following the order Mg₃Al₁ (400 °C) > Mg₃Ga₁ (350 °C) > Mg₃Fe₁ (300 °C) > Mg₃Mn₁ (250 °C). This phenomenon can be explained by the difference in their thermal stabilities. When the calcination temperature is too low, the LDH structures are not completely decomposed; when the calcination temperature is too high, MgO or even spinel oxides are formed. Also, the T_oc value of each LDH was very close to the T_ct, implying that the quasi-amorphous phases obtained by heating at the lowest temperature (T_ct) have the highest CO₂ capture capacities. Furthermore, the maximum CO₂ capture capacity of each LDH was Mg₃Al₁ (0.41 mmol g⁻¹), Mg₃Fe₁ (0.46 mmol g⁻¹), Mg₃Mn₁ (0.42 mmol g⁻¹), and Mg₃Ga₁ (0.27 mmol g⁻¹). It is verified that calcination temperature is critical for each LDH. A very different calcination temperature is needed for each LDH to get the maximum CO₂ capture capacity. Then, the effect of the adsorption temperature on CO₂ capture capacity was further measured, as shown in Figure 1.3. It can be seen that low temperatures favor CO₂ adsorption and the maximum CO₂ capture took place at 200 °C for all LDHs. In Figure 1.3, it is clearly shown that the CO₂ capture capacities decrease with an increase of adsorption temperature when the calcination temperature is not too high. However, the CO₂ capture capacities increase with increasing adsorption temperature when the calcination temperature is sufficiently high (i.e., 550 °C for Mg₃Ga₁ and Mg₃Fe₁, and 400 °C for Mg₃Mn₁). This phenomenon is again related to the thermal stabilities. Keeping in mind the above, low-temperature calcination of LDHs leads to amorphous phases, on which the CO₂ is mainly adsorbed physically. The adsorption strength is weak and, therefore, only a small amount of CO₂ can be adsorbed at a high temperature, while samples calcined at a high temperature were already transformed into the MgO phase and chemical adsorption of CO₂ takes place mainly. Therefore, all of the above have shown that trivalent cations in LDH can influence their CO₂ capture capacity, which actually determines the structure evolution of LDH derivatives under thermal treatment.

Subsequently, Huang et al.³⁴  first reported an investigation into the performance of LiAl₂ LDHs intercalated with various anions, including CO₃²⁻, NO₃⁻, or Cl⁻, for CO₂ capture. Both co-precipitation and gibbsite intercalation methods were used for the synthesis of LiAl₂ LDHs in their study. The results of XRD validated that only the Li₁Al₂–CO₃ LDH synthesized by the co-precipitation method was a pure LDH phase without any impurities. The other ones synthesized by the co-precipitation method showed the existence of a bayerite impurity, and all the LDHs synthesized by the gibbsite intercalation method showed the existence of gibbsite. Then, the effect of interlayer anions on the CO₂ capture capacity of LiAl₂ LDHs-derived adsorbents was examined. The CO₂ capture capacity of LiAl₂ LDHs synthesized by the co-precipitation method was in the order of LiAl₂–CO₃ (0.44–0.51 mmol g⁻¹) > LiAl₂–NO₃ (0.16 mmol g⁻¹) = LiAl₂–Cl (0.16 mmol g⁻¹). While for the LDHs synthesized by gibbsite intercalation, the CO₂ capture capacity was 0.45, 0.41, and 0.30 mmol g⁻¹ for calcined LiAl₂–CO₃, LiAl₂–NO₃, and LiAl₂–Cl, respectively. It can be seen that the co-precipitation method resulted in a slightly higher performance for CO₂ capture than the gibbsite intercalation method for LiAl₂–CO₃ LDHs.

Besides the effects of interlayer anions and the synthesis method, the effect of the Li–Al molar ratio in the preparation solution was also investigated. LiAl₂–CO₃ was synthesized by the co-precipitation method with the Li–Al ratio from 1 : 3 in the preparation solution. The LiAl₂–CO₃ LDH synthesized with a preparation Li–Al ratio of 3 : 1 showed the highest CO₂ capture capacity of 0.51 (mmol g⁻¹), which is comparable to the most commonly used Mg₃Al–CO₃ LDH (0.50 mmol g⁻¹). Moreover, the results in their study also indicated that LiAl₂ LDHs-derived compounds can be used as CO₂ adsorbents over a wide temperature range (60–400 °C), with a CO₂ capture capacity of 0.94 and 0.51 mmol g⁻¹ at 60 and 200 °C, respectively. Different cations in LDHs and their CO₂ capture performance are recapitulated in Table 1.1.

In addition, the influence of charge-compensating anions in LDHs on CO₂ capture capacity was also investigated. Yong et al.⁴⁵  profoundly studied the effect of anion type on the CO₂ capture capacity of commercial LDHs (EXM696, EXM701, and EXM911 were provided by Süd-Chemie AG Company). In their study, they found that the amounts of adsorbed CO₂ on LDHs containing CO₃²⁻ (EXM samples) were higher than those on LDHs containing OH⁻ (MG samples). The main reasons for this are that (1) the carbonate ion CO₃²⁻ is larger than the hydroxide ion OH⁻, having a larger interlayer spacing (0.765 nm) compared with that of OH⁻ (0.755 nm), and (2) the charge of the CO₃²⁻ is higher than that of the OH⁻. Then, the effect of the adsorption temperature of LDHs was also discussed. Two LDHs were used in their study to capture CO₂ at room temperature (20 °C) and higher temperatures (200 and 300 °C), respectively. The results showed that the capture capacities at the three temperatures of both LDHs follow the order Q(300 °C) > Q(20 °C) > Q(200 °C). Furthermore, the decarbonation behaviors of LDHs at various temperatures were also discussed. When heated to 200 °C, LDHs become dehydrated, and the product retains a layered structure. A significant rearrangement of the octahedral brucite-type layer occurs with the migration of the M³⁺ cation out of the layer to tetrahedral sites in the interlayer, and the d spacings decrease progressively with increasing temperature. Because of the decrease in the d spacings, the LDHs have less void space in their interlayers and can accommodate less carbon dioxide gas. Meanwhile, the amounts of adsorbed CO₂ on the surface of LDHs decrease with increasing temperature. In particular, the low-strength basic sites (bicarbonate) disappeared above 100 °C. Therefore, the adsorption capacities of LDHs at 200 °C are lower than those at 20 °C. At temperatures above 300 °C, the above unfavorable mechanism for CO₂ adsorption is still present. However, the dehydroxylation between OH groups of contiguous layers and the decarbonation processes occur above 300 °C. The former produces a structure modification of the layers consisting of a change in the M³⁺ cation environment from an octahedral to tetrahedral coordination. The latter removes the carbonate anion as CO₂ from the LDHs, which could partially destroy the layers, and increase the surface area and pore volume of the products. The evolution of the LDHs' structure described above is illustrated in Figure 1.4. The heat treatment process of the LDHs has two functions. One is micropore formation with the decomposition of the LDHs, which is intended to favor CO₂ adsorption. The other has the opposite effect, such as causing the d spacings and the amounts of adsorbed CO₂ on the surfaces of the LDHs to decrease with increasing temperature. Subsequently, Hutson et al.⁴³  investigated the effect of divalent cations and interlayer charge-compensating anions on the CO₂ adsorption capacity of LDHs. They found that the MgAl–CO₃ sample showed the highest initial adsorption rate and the highest CO₂ adsorption capacity of 0.62 mmol g⁻¹ at the end of time interval. MgAl–Fe(CN)₆ had an initial adsorption rate close to that of MgAl–CO₃ but had a lower total adsorption capacity than CaAl–CO₃ at the end of the experiment. And MgAl–ClO₄ had the lowest adsorption rate and total adsorption capacity of 0.11 mmol g⁻¹, despite having the largest interlayer spacing and second largest average pore diameter. They also proved that the dominant means of physical adsorption involves interaction of CO₂ with basic sites created by the decomposition of the LDH structure. There appears to be no relationship between the surface basicity and the chemisorption capacity. The irreversible chemisorption appears to be a carbonation/mineralization of the M²⁺ cations.

Moreover, Wang et al.⁴⁹  reported a detailed investigation into how charge-compensating anions and synthesis pH affect the structural properties and CO₂ adsorption capacity of MgAl LDHs. A series of Mg₃Al₁-A (A = CO₃²⁻, NO₃⁻, Cl⁻, SO₄²⁻, and HCO₃⁻) LDHs were synthesized and investigated as CO₂ adsorbents. The influence of the synthesis pH on the chemical composition and CO₂ adsorption capacity of Mg₃Al₁–CO₃ was also studied. Among various LDHs, Mg₃Al₁–CO₃ showed the highest CO₂ capture capacity (0.53 mmol g⁻¹), which was much higher than other LDHs with HCO₃⁻, NO₃⁻, SO₄²⁻, and Cl⁻ anions (∼0.2 mmol g⁻¹). The results indicated that the BET surface area of calcined LDHs seems be the main parameter that determines the CO₂ adsorption capacity. Because the surface area of calcined Mg₃Al₁–CO₃ (239 m² g⁻¹) was much higher than the other calcined LDHs (<140 m² g⁻¹), Mg₃Al₁–CO₃ derivative was supposed to provide more adsorption sites for CO_2. Besides, they considered that the morphology of the synthesized LDHs might also affect CO₂ adsorption. The Mg₃Al₁–CO₃ formed a spheroidal “sand rose” morphology. The size of the “flower ball” was approximately 400–450 nm, and the thickness of the petals was about 24–25 nm, which accounted for about 30–32 brucite-like sheets. However, all other LDHs formed a “stone”-like morphology, with particle sizes ranging from several to several-tenths of micrometers. The abundance of “sand rose”-like LDHs possesses an abundance of pores between “petals”, and thus favors CO₂ diffusion as well as adsorption. However, “stone”-like LDHs were very big in size and were non-porous. Moreover, they proved that the anions have a great effect on the thermal stability, morphology, as well as the surface area of LDHs; they have a significant influence on the CO₂ adsorption capacity of calcined LDHs. Furthermore, Mg₃Al₁–CO₃ was studied using various synthesis pH values from 6.5 to 14. The finding was that the Mg–Al molar ratio increased from 0.5 to 3.1 with an increase in pH from 6.5 to 10. A further increase in pH up to 14 has little effect on the Mg–Al molar ratio. The BET surface area and pore size distribution were also influenced by synthesis pH. The average pore size increased with increasing pH values. Mg₃Al₁–CO₃ synthesized at pH 10–12 showed the best performance for CO₂ capture at high temperature. Therefore, it can be inferred that the anions affect the thermal stability and morphology, as well as the surface area of LDHs, consequently influencing the CO₂ capture capacity.

Wang et al.⁵⁰  first synthesized LDHs intercalated with long-carbon-chain organic anions (e.g. stearate) for CO₂ capture. Compared with traditional LDHs, stearate-intercalated LDHs have a higher CO₂ capture capacity, which can be meliorated to 1.25 mmol g⁻¹. It is 2.5 times higher than that of traditional LDH-based adsorbents (0.5 mmol g⁻¹). In order to know the reason, the XRD patterns of Mg₃Al₁–CO₃ and Mg₃Al₁-stearate were compared (see Figure 1.5(a)). It can be seen that the characteristic diffraction peaks of LDHs were observed for both samples. By replacing CO₃²⁻ with C₁₇H₃₅COO⁻ anions, the 003 peak shifted to a much lower value, from 11.3° for Mg₃Al₁–CO₃ to 2.4° for Mg₃Al₁-stearate. This means that the interlayer distance increased from 0.78 nm for Mg₃Al₁–CO₃ to 3.54 nm for Mg₃Al₁-stearate. After being thermally treated at 400 °C, both Mg₃Al₁–CO₃ and Mg₃Al₁-stearate LDHs were transformed into amorphous mixed oxides (MgAlO_x), with the features of periclase MgO (see Figure 1.5(a)). For Mg₃Al₁-stearate, the characteristic peaks of periclase MgO are broader and the intensities are much weaker than those derived from Mg₃Al₁–CO₃. This suggests that the decomposition of C₁₇H₃₅COO⁻ anions could result in a much lower degree of crystallinity and smaller particle size of MgAlO_x. They also compared the thermal stabilities of the Mg₃Al₁–CO₃ and Mg₃Al₁-stearate LDH (see Figure 1.5(b)). Figure 1.5(b) illustrates that both LDHs show a typical two-stage weight loss. At 700 °C, the weight loss of Mg₃Al₁-stearate is ca. 79.8 wt%, which is much higher than that of Mg₃Al₁–CO₃ (43.2 wt%) because the molecular weight of C₁₇H₃₅COO⁻ is much higher than that of CO₃²⁻. Therefore, it is believed that the improved CO₂ capture capacity is due to the following reasons: (1) the decomposition of long-carbon-chain anions leads to cracks and splits in the LDH plates and creates more surface basicity (O₂⁻) sites; (2) the mixed metal oxide mixture produced has a lower degree of crystallinity; and (3) the quasi-amorphous structure formed is more stable than that from Mg₃Al₁–CO₃. The schematic illustration of the structural changes and the CO₂ capture by Mg₃Al₁–CO₃ and Mg₃Al₁-stearate is shown in Figure 1.6.

Later on, Qin et al.⁵¹  devised a systematic investigation on the promoting effect of the carbon chain length of the intercalated carboxylic anions for the CO₂ capture performance of MgA-organo LDHs. A series of organo-LDHs were successfully synthesized. Since the thermal stabilities of odd-carbon-chain monocarboxylic acids are relatively poor, only carbon-chain monocarboxylic acids, including acetic acid (C2), butyric acid (C4), caproic acid (C6), octanoic acid (C8), capric acid (C10), dodecanoic acid (C12), tetradecanoic acid (C14), and palmitic acid 69 (C16), were intercalated into LDHs in their work. For the synthesis of monocarboxylate-anion- intercalated LDHs, two methods, including co-precipitation and calcination–rehydration, were used. For short-chain anions with less than eight carbons, the calcination–rehydration method was used, while for long-chain anions with more than eight carbons, the co-precipitation method was used. XRD and ATR-FTIR ratified the successful synthesis of the organic-anion-intercalated LDHs. Before the CO₂ capture capacity test, the specific surface areas and pore structures of both fresh and calcined organo-LDHs were analyzed. The results showed that the carbon chain length of the intercalated monocarboxylic acids slightly affects the specific surface area, pore size, and pore volume of fresh Mg₃Al₁ organo-LDHs. The organo-LDHs intercalated with short-carbon-chain monocarboxylic anions (LDH-C2 to LDH-C10) showed a similar BET specific surface area and pore volume of 8.8–16.9 m² g⁻¹ and 0.03–0.08 cm³ g⁻¹, respectively, while LDHs intercalated with long-carbon-chain organic anions (LDH-C12 to LDH-C16) showed a higher specific surface area (20.9–32.7 m² g⁻¹) and pore volume 220 (0.22–0.36 cm³ g⁻¹). After calcination at 400 °C, a drastic increase in surface area was observed for all Mg₃Al₁ organo-LDHs, with an average value of 330–340 m² g⁻¹. The calcination process may create more surface basicity (O²⁻) sites favorable for CO₂ capture. Then the effect of interlayer anions on the CO₂ capture capacity of LDHs-derived adsorbents was examined at 200 °C. The results showed that no obvious difference in CO₂ capture capacity was observed (0.45–0.61 mmol g⁻¹) when the carbon chain is less than 10. However, with an increase in the carbon chain from 10 to 16, the CO₂ capture capacity gradually increased from 0.61 to 0.91 mmol g⁻¹. The highest CO₂ capture capacity (0.91 mmol g⁻¹) was achieved with the adsorbent derived from Mg₃Al₁–C16 LDH. The XRD data of structural changes upon thermal treatment at 400 °C showed that Mg₃Al₁ organo-LDHs intercalated with the long carbon chains of organic anions (C number >10) exhibit a lower degree of crystallinity than those with the short carbon chains of organic anions (C number <10). The decomposition of long carbon chains of organic anions could result in a much lower degree of crystallinity and smaller particle size of amorphous mixed oxides. And the memory effect (structure reconstruction) of the organo-LDHs was also tested. The results suggest that the stability of LDOs is associated with the carbon chain length of the monocarboxylic acid counter anions. It is noteworthy that the quasi-amorphous structures formed from LDHs intercalated with long-carbon-chain organic anions (C number >10) are more stable than those from LDHs intercalated with short-carbon-chain organic anions (C number <10). Since it is well known that a quasi-amorphous structure gives the highest CO₂ capture capacity, this may be a significant reason for the enhanced CO₂ capture capacity of LDHs intercalated with long-carbon-chain organic anions.

In addition, Othman et al.⁵²  synthesized MgAl LDHs following a combustion–recrystallization–impregnation procedure using aluminium, magnesium nitrate precursors and carbonate solutions for CO₂ capture. Different types and amounts of fuels, different amounts of carbonate and different synthesis temperatures were tested during LDH synthesis in order to understand their roles in forming the structures and CO₂ adsorption levels of LDHs. Saccharose, fructose, glucose and glycine were used as fuels in the study. The results showed that the adsorption capacity follows the order of saccharose > fructose = glucose > glycine. The recrystallized LDH adsorbed the highest CO₂ when 20 wt% of saccharose was used as the fuel to facilitate chemical reaction on combustion at 650 °C, which was possibly due to the higher content of C and H in order to form complexes with the metal ions. However, when the sample was reinforced with additional K and Na at 18.5% and 1.5%, respectively, the adsorption capacity surged from 0.688 to 1.21 mmol g⁻¹, even after five cyclic adsorptions and desorptions. Different anion-intercalated LDHs and their CO₂ capture performances are summarized in Table 1.2.

In general, LDHs can be synthesized by co-precipitation of the corresponding divalent and trivalent cations in an alkali solution. Co-precipitation is a well-known method that is often used. The other well-known synthesis method is the hydrothermal method. Another method is urea hydrolysis, from which one can obtain LDHs after slow and homogeneous precipitation of metal cations as a result of the slow hydrolysis of urea at certain temperatures. Although NaOH can be used to replace urea, the use of urea is better since it progresses slowly, which leads to a low degree of super-saturation during precipitation.⁵³  Moreover, the combustion synthesis can save energy and time, because it involves a very rapid chemical process.⁵⁴  This method is based on the explosive decomposition of some organic fuels such as urea or glycine.⁵⁵  In addition, ion exchange,⁴²  sol–gel,⁵⁶  microwave-assisted methods⁵⁷  and solvothermal methods⁵⁸  etc. are often used to synthesize LDHs. The synthetic conditions and methods may influence the morphologies and specific surface areas of LDHs. Therefore, it is widely accepted that the synthesis method is one of the key influencing parameters for LDHs-derived CO₂ adsorbents.⁵⁹ 

Yang et al.²⁴  prepared LDHs by two different methods, the low supersaturation method and the high supersaturation method, which varied the precipitation rate during the preparation of the LDH precursor, and they studied the influence of the Mg–Al molar ratio. The results ratified that the LDH with a Mg–Al molar ratio of 2 adsorbed the largest amount of CO₂ among LDHs with Mg–Al molar ratio higher than 2. As the ratio of Mg–Al increased to 5, the adsorption capacity of CO₂ decreased. However, as the Mg–Al molar ratio increased further, the LDH adsorbed more CO₂, and the amount of CO₂ adsorption by the LDH with a Mg–Al molar ratio of 10 became almost equal to that of the LDH with a Mg–Al molar ratio of 2 (see Figure 1.7(a)). This is because when the molar ratio of Mg–Al equaled 2, the higher layer charge density of the LDH due to the larger Al content enabled a higher adsorption capacity of CO₂. As the content of Al decreased with the increasing Mg content, the adsorption capacity of CO₂ decreased due to the loss of the layer charge density. In other words, as the Mg²⁺ ion is substituted by a trivalent cation having a similar radius, like Al³⁺, the positive charge density of the layer increases. In order to maintain the electrical neutrality, more anions occupy the interlayer positions where water crystallization also takes place. Thus, the space of the interlayer increasing with Al content was due to the water evaporating during the calcination procedure. Therefore, the LDH with Mg–Al = 2 is most effective in CO₂ adsorption. On the other hand, the number of basic sites increased with the increase in Mg content, resulting in increased CO₂ adsorption on the LDH with Mg–Al = 10. Both LDHs prepared by the different methods showed almost the same adsorption capacity of CO₂; however, the LDH prepared by the high supersaturation method had the higher effective adsorption capacity (see Figure 1.7(b)). This is because the LDH prepared by the high supersaturation method is known to have a low crystallinity, many crystalline nuclei and a high surface area due to its smaller particle size. The surface area of the LDH prepared by the high supersaturation method was 2.46 times larger than that of the LDH prepared by the low supersaturation method. Even with the higher surface area, the average pore radius of the LDH prepared by high supersaturation was larger. The higher surface area and the larger pore seem to be attributed to the better desorption adsorption behavior of the LDH prepared by the high supersaturation method. Then, K₂CO₃ was used to improve the CO₂ adsorption capacity of LDHs.

Furthermore, Wang et al.⁶⁰  demonstrated that the synthesis pH plays a crucial role in the morphology and pore structure as well as the chemical composition of LDHs. Firstly, a series of LDHs was synthesized at various pH values ranging from 6.5 to 14. The results showed that the product is mainly γ-AlOOH coexisting with a certain amount of MgAl–NO₃ and/or MgAl–HCO₃ LDH at pH < 9. And the product is a “stone-like” LDH with a composition of Mg_2.3Al₁–HCO₃ at pH = 9. Since the LDHs were derived from amorphous γ-AlOOH, the pore size of the synthesized LDHs should be similar to that of γ-AlOOH precursor. In these conditions, the interlayer charge-compensating anions were NO₃⁻ and/or HCO₃⁻. However, at pH 10, Al³⁺ and Mg²⁺ had the same chance to be precipitated as Al(OH)₃ and Mg(OH)₂ nanoparticles at the same time, which then immediately converted into Mg₃Al₁–CO₃ LDHs. In the next stage, because the surface of the primary Mg₃Al₁–CO₃ nanoparticles is electrically neutral (pH = isoelectric point (IEP)), the growth of the LDH was relatively slow and preferentially along the 001 plane. The 001 plane has the lowest surface charge density and thus is stable under the synthesis conditions. The growth of the primary nanoparticles into nano-sheets finally resulted in a rosette morphology (see Figure 1.8). With the pH increasing, at pH 11–14, Mg²⁺ precipitated first as Mg(OH)₂, and then was immediately converted into Mg₃Al₁–CO₃ nanoparticles. Although the formation of the LDHs was very fast under these basic conditions, their growth was inhibited. According to the IEP theory, the LDH surface was negatively charged at a pH higher than its IEP. Thus, the interaction between LDH nanoparticles and Al(OH)₄⁻ (and/or CO₃²⁻, OH⁻) was not favored due to the repulsive force. Consequently, mesoporous Mg₃Al₁–CO₃ LDHs were obtained under these conditions (see Figure 1.8). Then, the CO₂ capacities of Mg₃Al₁–CO₃ LDHs synthesized at pH 6.5–14 were tested. The fresh LDHs were calcined using two different protocols: (1) in situ calcination at 400 °C for 1 h; and (2) ex situ calcination at 400 °C for 5 h, immediately followed by in situ calcination at 400 °C for 1 h. After pretreatment, the samples were cooled down to the adsorption temperature (200 °C) and held at that temperature for 0.5 h for stabilization purposes. The feed gas was then switched to CO₂ for adsorption. The results indicate that the CO₂ adsorption capacities were very low when the pH was lower than 10 (see Figure 1.9). Particularly, at pH 6.5 and 7, although the BET surface areas were very high (ca. 165–190 m² g⁻¹), the Mg–Al molar ratio was too low to have sufficient Mg–O sites for CO₂ adsorption. Only a very small amount of CO₂, ca. 0.06–0.08 mmol g⁻¹, was adsorbed on these samples. All LDHs synthesized at pH 10–14 showed a much better performance for capturing CO₂, which might be attributed to the good formation of Mg₃Al–CO₃. The substitution of Mg²⁺ by Al³⁺ creates plenty of active Mg–O sites on the surface. The highest CO₂ capture capacity was observed at pH 12. After being pre-calcined at 400 °C for 1 h and 6 h, the CO₂ capture capacities were 0.83 and 0.58 mmol g⁻¹, respectively. The experiments indicated that further increasing the pre-calcination time has no influence on the CO₂ capture capacity. At pH 13 and 14, the CO₂ adsorption capacity slightly decreased, which might be due to the decrease of the surface area. Subsequently, by employing the IEP concept, Wang et al.⁶¹  synthesized nano-sized spherical Mg₃Al₁–CO₃ LDHs, with an average particle size of ca. 20 nm. According to the IEP mechanism, the net charge of the particles is affected by the pH of their surrounding environment. It can be positively (pH < IEP), neutrally (pH = IEP) or negatively (pH > IEP) charged depending on the relationship between the IEP and the pH. Therefore, in preparation, the surface of the LDH is negatively charged when the synthesis pH is 12. Under such basic conditions, the rate of LDH formation is so fast that the growth in all directions is equal and so spherical nanoparticles form. Furthermore, a unique mesoporous LDH sample composed of uniform LDH nanospheres could be obtained by the aggregation of the above nanoparticles. The pore size and pore shape were analyzed. The results indicated that LDH (IEP) shows a H1 type hysteresis loop, indicating that the pores are produced by the aggregation of uniform spheres and an average pore size of ca. 18 nm. However, LDH synthesized by a conventional co-precipitation method (CC) shows a H3 type hysteresis loop, suggesting that the pores are produced by the aggregation of plate-like particles. The specific surface areas of LDH (IEP) and LDH (CC) are 103 and 114 m² g⁻¹, respectively. In order to confirm the results of the BET, the morphology of the two kinds of LDHs were investigated by SEM and TEM (see Figure 1.10). It can clearly be seen that LDH (CC) exhibited a spheroidal “sand rose” morphology. The size of the “flower ball” is approximately 400–450 nm, and the thickness of the petals is about 24–25 nm, accounting for about 30–32 brucite-like sheets. However, LDH (IEP) contained aggregates of uniform nanospheres with an average size of ca. 20 nm. This is also explained by the IEP. The formation of the “sand rose” LDH is related to its IEP, which is around 10 for Mg₃Al–CO₃. Under this condition, although the formation of the primary particles is fast, the growth is slow because the surface of the initially formed primary particles is electrically neutral (pH = IEP). Consequently, growth of the LDH particles is preferred along the 001 plane, where the surface charge density is low, resulting in a “sand rose” morphology. At pH 12, although the formation of the LDH was very fast under these basic conditions, its growth was inhibited. According to the IEP theory, the LDH surface is negatively charged at pH values higher than its IEP. Thus, the interaction between LDH NPs and Al(OH)₄⁻ (and/or CO₃²⁻, OH⁻) is not favored due to repulsive forces. Consequently, LDH nano-spheres with an average diameter of 20 nm were synthesized. During the washing and drying of the LDH nano-spheres, they stuck together forming novel mesoporous Mg₃Al–CO₃ LDHs. Although the nanospherical LDH showed only a slight increase in its CO₂ capture capacity (0.58 mmol g⁻¹) compared to the traditional “sand rose” Mg₃Al₁–CO₃ LDH (0.53 mmol g⁻¹), it is clear that the mesoporous structure and big pores of nanospherical LDHs are favorable for the dispersion of doped K₂CO₃ species and lead to their superior CO₂ capture capacity.

In addition, Zhang et al.⁵⁹  systematically investigated the influence of synthesis methods on the CO₂ adsorption capacities of Mg₃Al–CO₃ LDH-derived adsorbents. Six different synthesis methods were evaluated in their paper. The XRD analyses confirmed that different synthesis methods resulted in different crystalline and interlayer distances (interlayer anions). And, different synthesis methods can result in different specific surface areas and morphologies. They found that the LDH samples synthesized from Na₂CO₃ as a precipitating agent had much higher CO₂ adsorption capacities than those from urea. The samples synthesized from Na₂CO₃ showed great CO₂ adsorption capacities of 0.73–0.75 mmol g⁻¹, and the aging conditions (e.g., room temperature, hydrothermal, or microwave, etc.) had little effect on the final CO₂ adsorption capacity. However, the samples synthesized from urea exhibited much lower CO₂ capture capacities of 0.3, 0.42 and 0.44 mmol g⁻¹. After hydrothermal or microwave aging, their CO₂ adsorption capacities increased slightly. This phenomenon can be generally explained by the fact that a higher temperature accelerates the decomposition of urea. With the decomposition of urea, more CO₃²⁻ anions could be intercalated into the LDH interlayers, leading to a better CO₂ capture capacity. The effect of adding a surfactant, such as cetyl trimethylammonium bromide, during the synthesis of LDH was also studied. The results indicated that more regular plate-like nanoparticles with a unique lateral size of ca. 200–300 nm were formed. Although different LDH samples possessed different morphologies, no correlation can be found between morphology, specific surface area and CO₂ capture capacity. It was confirmed that the difference in interlayer anions in the final synthesized LDH is the key reason why different synthesis methods can lead to different CO₂ capture capacities. With the same precipitation agent (e.g., Na₂CO₃ or urea), the preparation method has little effect on CO₂ capture capacity.

Moreover, Hanif et al.⁶²  investigated the effect of synthetic routes in improving the CO₂ capture capacities of LDH-based adsorbents in the temperature range 300–400 °C. Three synthetic procedures were used, including conventional co-precipitation (LDH-CC), ultra-sonication-assisted co-precipitation (LDH-US) and co-precipitation prior to microwave irradiation of the precipitating gel (LDH-MW). The results showed that the synthetic routes can influence the BET surface area of LDHs, which is related to their CO₂ capture capacity. Thus, LDH-MW has the highest surface area and highest CO₂ equilibrium capacity followed by LDH-US and LDH-CP, respectively. The LDH adsorbents prepared by microwave and ultrasound-assisted synthesis routes showed capacities 3–4 times higher than commercial LDHs in the targeted temperature regime of 300–400 °C. Moreover, the adsorbents are regenerable under evacuation with an inert purge when a small temperature swing up to 450–470 °C is applied.

Although the CO₂ capture capacities of LDHs have been improved by many methods, it is known that LDH pellets are easily pulverized after extensive adsorption–desorption cycles due to impurities and water vapor in the syngas, which has been confirmed by the Energy Research Centre of the Netherlands.⁶³  Recently, several studies have shown that the CO₂ adsorption capacity and multicycle stability of LDHs can be enhanced by using supports such as zeolites, carbon nanofibers (CNFs), multiwall carbon nanotubes (MWCNTs), and graphene oxide (GO) etc.^64–67  Therefore, LDH-based composite materials become promising materials for CO₂ capture. In general, the synthesis method can be divided into two general strategies. One is the “one-pot co-precipitation” method, and the other is the “electrostatically-driven self-assembly” method basing on the electrostatically-driven self-assembly between the delaminated positively charged LDH single sheets and the negatively charged monolayer.⁶⁴ 

Meis et al.⁶⁸  proved that small LDH platelets deposited on a carbon nanofiber (CNF) support can improve the adsorption capacities of an LDH from 1.3 to 2.5 mmol g⁻¹, which is higher than for an unsupported one. They also found that the CO₂ adsorption capacity significantly increased with a decrease in the particle size (see Figure 1.11(a)). The particle size and morphology of the unsupported LDH sample and the supported LDHs were analyzed by SEM and TEM. The results showed that the lateral platelet size of the LDHs prepared using NaOH/Na₂CO₃ as a base related to the aging temperature. The lateral platelet size of the LDHs increased with an increase of the aging temperature (35–300 nm). When urea was used as the precipitant, very large LDH platelets were obtained (2 µm). However, when the LDHs were supported on CNFs, they displayed the smallest crystalline platelets (∼20 nm). Bright- and dark-field TEM micrographs of LDH₁₀-CNF (with the number 10 referring to the weight loading (%) of LDH deposited on CNF) are shown in the insert in Figure 1.11(a) (see inset A of Figure 1.11(a)). It can be seen that LDHs are difficult to distinguish from carbon in the bright-field mode, because of a similarity in density. However, in the dark-field mode (i.e., diffraction contrast imaging), the small LDH crystallites were easier to observe (see inset B of Figure 1.11(a)). The dark-field micrographs revealed more LDHs than could be detected in the bright-field mode. For clarity, the arrows in inset A of Figure 1.11(a) indicate the LDH platelets. The group proposed that adsorption with CO₂ on an edge or corner is more likely to occur (see Figure 1.11(b)). A high increase in the CO₂ capacities of the activated supported LDHs is based on a greater number of defects on the small Mg(Al)O_x crystals. The individual crystals are anchored on the CNF and thus have less flexibility, which might contribute to more defects and low-coordinated oxygen. That is to say that more adsorption sites are formed in the activated supported LDHs [Mg(Al)O_x] upon heat treatment as a result of the limited mobility.

Furthermore, Garcia-Gallastegui et al.⁶⁹  introduced MWCNTs into LDHs to form a composite for CO₂ capture. Specifically, “one-pot co-precipitation” was used, in which LDH nanoparticles were precipitated directly onto oxidized MWCNTs. The oxidation pretreatment of MWCNTs was to ensure a favorable electrostatic interaction and hence a good dispersion. The CO₂ adsorption capacity and stability of LDHs both increased significantly when the LDHs were supported by MWCNTs. The group considered that increases in the effective surface area and the structural disorder of supported materials are the two factors that may contribute towards enhancing the adsorbent/gas contact. Then they also synthesized a LDH/graphene oxide (LDH/GO) composite by the “one-pot co-precipitation” method, whose CO₂ capture capacity was increased by 62% using only 7 wt% GO as the support. They believed that the resultant LDH serves as a spacer to prevent aggregation of individual graphene sheets, particularly as highly oxidized debris in the GO sample is washed out by an aqueous base. Conversely, the GO also supports the LDH, improving the dispersion, and nucleating a more active structure. Therefore, the CO₂ adsorption capacity and multicycle stability of LDHs both increased when the LDHs were supported on GO because of the enhanced particle dispersion.⁷⁰ 

Wang et al.⁷¹  synthesized a LDH and GO composite based on the “electrostatically driven self-assembly” method. First, MgAl-LDH was delaminated into single nanosheets in formamide, while the GO was obtained by the exfoliation of graphite oxide in water through ultra-sonication. Then the MgAl–NO₃ LDH-NS/GO (LDH-NS/GO) composite was immediately formed once the exfoliated MgAl–NO₃ LDH dispersion was added to the GO dispersion (see Figure 1.12). It is considered that the negatively charged GO was complementary to the positive charge of the delaminated LDH sheets, which was likely to contribute to the stabilization of the growing nanocomposite. A series of LDH-NS/GO composites with different GO loadings from 2.9 to 50.0 wt% were prepared and their CO₂ capture capacities were tested. The results demonstrated that the LDH-NS/GO nanocomposite with 6.54 wt% GO showed the maximum adsorption capacity, which is more than twice that of pure LDH. The enhancement in adsorption capacity in the presence of GO can be attributed to the LDH single sheets and their dispersion and stabilization on the support. With 6.54 wt% GO loading, the geometric and electrostatic compatibility between LDH single sheets and GO single sheets appeared to favor heterogeneous nucleation, dispersion, and stabilization. SEM, TEM and BET ratified that a mesopores nanocomposite formed after exfoliated LDH was mixed with GO. After thermal decomposition, mixed metal oxides were obtained with the basic sites required for CO₂ adsorption. Then the calcination and adsorption temperatures were also investigated. It was found that the optimal calcination temperature for the LDH-NS/GO nanocomposite was 400 °C, which is similar to that of pristine LDH. And, the CO₂ adsorption capacity of the LDH-NS/GO nanocomposite increased with a decrease in adsorption temperature. The maximum CO₂ capture capacity was reached at 60 °C (1.0 mmol g⁻¹). However, at the high temperature, the maximum value took place at 200 °C, which indicated that the LDH-NS/GO nanocomposite had good CO₂ capture capacity in a wide temperature range, which shows promise not only for the SEWGS process but also for the post-combustion of flue gases. Finally, the CO₂ adsorption–desorption cycling stability was tested in a typical temperature swing adsorption process. The LDH-NS/GO nanocomposite showed good cycling stability during 22 cycles.

Then, Iruretagoyena et al.^72,73  presented a detailed study of CO₂ adsorption on sodium-free GO-supported LDOs. The CO₂ adsorption capacities of the unsupported and GO-supported LDOs in the first exposure to the adsorptive gas are similar, indicating that GO does not modify significantly the nature of the adsorption sites. They found that the first-contact adsorption data for pure LDO and GO composites are better described by the Freundlich isotherm, but after multiple temperature-swing cycles, the isotherms tend to fit the Langmuir model as the surface becomes more uniform. Multicycle temperature swing and isothermal N₂ purge experiments suggested that the loss of CO₂ capacity after the first cycle is caused by irreversible adsorption over very strong basic sites that are created during the thermal activation. The adsorption capacity of the materials used in temperature-swing cycles decays more gradually, possibly indicating some additional thermal degradation. They also found that the thermal stability of LDO is markedly enhanced by the addition of modest amounts of GO, causing the CO₂ adsorption capacity of the LDO/GO composites to be better preserved over thermal cycling. Most importantly, the presence of moisture had a positive influence on CO₂ removal due to the formation of new adsorption sites as a result of surface rehydration of LDO/GO composites.

Furthermore, Wang et al.⁶⁴  performed a comparative study on the influence of synthesis methods and the chemical composition of LDH/oxidized carbon nanotube (LDH/OCNT) nanocomposites on their CO₂ capture performance. Three types of LDH/OCNT nanocomposites, including MgAl-NO₃-NS/OCNT, MgAl–NO₃/OCNT, and MgAl–CO₃/OCNT, were prepared using “electrostatic self-assembly” and “one-pot co-precipitation” methods. XRD, SEM, and TEM analyses revealed that the synthesis method has little influence on the morphology and structure of the formed nanocomposites. In all three nanocomposites, the OCNT was tightly absorbed and randomly distributed on the surface of LDH nanosheets or interacted with LDH nanosheets, which improves the dispersion of LDH in the nanocomposites. The results showed that both the CO₂ adsorption capacity and the multi-cycle stability of LDH-derived adsorbents were improved by the introduction of OCNT. In particular, the absolute CO₂ capture capacity of MgAl–NO₃ LDH was more than doubled by adding 9.1 wt% OCNT. The results also demonstrated that these LDH/OCNT nanocomposites have good CO₂ adsorption–desorption cycling stability.

Moreover, Zhu et al.¹⁰  synthesized a LDO/activated carbon-based composite adsorbent for H₂/CO₂ separation. The composite is synthesized from commercially available activated carbon, which is loaded with MgAl LDH using the urea or co-precipitation methods. The results revealed that the co-precipitation method is more efficient for the synthesis of the composite adsorbent. As the adsorption temperature increased from 40 to 400 °C, the adsorption capacity of the composite decreased due to losses in the physical adsorption capacity. However, the adsorption performance was better when compared to activated carbon at elevated temperatures (200–300 °C) due to an improvement in the chemical adsorption. The reversible adsorption capacity of the composite adsorbent is approximately 1.25 times that of LDH and 2.13 times that of activated carbon at 1 bar and 200 °C. The adsorption capacity of the composite adsorbent reaches 0.185 mmol g⁻¹ at 200 °C and 1 bar for the second adsorption–desorption cycle, and 86% of the total CO₂ is adsorbed within 20 min. Then, K₂CO₃ was loaded on the surface of the composite adsorbent via impregnation to further promote the adsorption performance. The schematic diagram and the performance expectations of the carbon-based composite adsorbent are shown in Figure 1.13.

Additionally, Bhatta et al.⁷⁴  explored the application of a coal-derived graphitic material (CGM) as an effective support for MgAl LDH, enhancing the CO₂ adsorption capacity. The adsorption capacity of CGM-supported MgAl LDH was found to be further increased upon K₂CO₃ impregnation. And, they proved that the addition of a support material increased the surface area of neat MgAl LDH, leading to enhanced adsorption capacity.

Very recently, Wang et al.⁷⁵  performed a comprehensive and comparative study on the influence of charge-compensating anions of LDHs in LDH-NS/GO nanocomposites for CO₂ capture. Four kinds of different anion-intercalated LDHs, including NO₃⁻, C₂H₄NO₂⁻ (glycine), C₁₂H₂₅SO₄⁻ (DDS), CH₃COO⁻ (acetate), were prepared by hydrothermal and anion exchange methods then exfoliated in formamide, water and butanol. The LDH and GO nanocomposites were synthesized by the “electrostatically-driven self-assembly” method. By varying the key synthesis parameters, a series of LDH-NS/GO nanocomposites with different chemical compositions were obtained and their performance for CO₂ capture was compared systematically. The results showed that the CO₂ adsorption capacity of the LDH was increased when mixed with GO because of enhanced particle dispersion and disordered structure. And, the CO₂ capture capacity of the LDH–NO₃–NS/GO, LDH-glycine-NS/GO, and LDH-DDS-NS/GO can be improved to nearly twice that of neat LDH-NO₃, LDH-glycine, and LDH-DDS, respectively, except LDH-acetate-NS/GO because of the unstable CH₃COO⁻. It was proved that the CO₂ capture capacity of LDH-NS/GO was related to the charge-compensation anions of LDH in the LDH-NS/GO nanocomposite, but it is also influenced by the stability of the charge-compensation anions of LDH. Furthermore, the influence of calcination temperature and adsorption temperature on the CO₂ capture capacity of the LDH-glycine-NS/GO nanocomposite was also investigated. Moreover, 0.92 mmol g⁻¹ was the maximum CO₂ adsorption capacity, which took place at 60 °C by calcination at 400 °C. But at high temperature, the maximum value was 0.46 mmol g⁻¹, which was obtained at 200 °C by calcination at 400 °C. Finally, the recycling performance was tested, which demonstrated that these LDH-NS/GO nanocomposites have good CO₂ adsorption–desorption cycling stability.

Except for carbon based LDH composites, the other materials for supported LDH composite were also studied. Qiao et al.⁷⁶  fabricated a multilayer LDH/MgCO₃ composite-based thin film on an aluminium substrate using a urea hydrolysis method, in which aluminium foil/mesh was used as the substrate as well as the sole source of aluminium. By varying the recrystallization time, recrystallization temperature, and the Mg–urea ratio, the formation process of the MgAl–CO₃ LDH thin film was investigated in detail. The results showed that when the recrystallization time was 48 h, the recrystallization temperature was 140 °C, the Mg–urea ratio was 1 : 1, and the synthesis time was 12 h, the composite showed a CO₂ adsorption capacity as high as 0.56 mmol g⁻¹ (see Figure 1.14), which is comparable to that of the conventional MgAl–CO₃ LDH powder-derived adsorbent. The adsorption–desorption cycling test showed that the adsorption capacity only slightly decreased during the first cycle and then leveled off from the 15th cycle onwards with a value of 0.39 mmol g⁻¹ (see Figure 1.14(e)), which indicated that it has good CO₂ adsorption–desorption cycling stability and potential for practical applications. Both the Al-foil- and Al-mesh-based LDH thin film can be further fabricated into certain shapes for practical applications, as proposed in Figure 1.14(f). For instance, the Al-foil can be made into any shape, such as pellets, for practical use. Before growing the LDH and MgCO₃ film on its surface, some holes can be made on the Al-foil, which creates more gas diffusion pathways, which will enhance gas diffusion within the solid (Figure 1.14(f, left)). After growing the LDH and MgCO₃ films on the surface of an Al-wire, it can be packed into an Al-mesh (Figure 1.14(f, right)). In this way, the surface-formed LDH and MgCO₃ films can be separated by the Al-foil and Al-wire substrates, preventing them from sintering and pasting.

In addition, Chang et al.⁷⁷  prepared AlOOH-supported CaAl LDH composites by using mesoporous alumina (g-Al₂O₃) and calcium chloride (CaCl₂) in a hydrothermal urea reaction. The nanostructured CaAl LDHs with nanosized platelets (3–30 nm) were formed and dispersed inside the crystalline framework of mesoporous AlOOH (boehmite). After calcination of AlOOH-supported LDHs at 700 °C, the mesoporous CaAl metal oxides exhibited ordered hexagonal mesoporous arrays or uniform nanotubes with a large surface area of 273 m² g⁻¹, a narrow pore size distribution of 6.2 nm, and highly crystalline frameworks. The crystal structure of the calcined mesoporous CaAl metal oxides was multiphasic, consisting of CaO/Ca(OH)₂, Al₂O₃, and CaAlO mixed oxides. The mesoporous metal oxides were used as a solid adsorbent for CO₂ adsorption at high temperatures and reached a CO₂ capture capacity as high as 10.1 mmol g⁻¹ at 650 °C. Furthermore, it was demonstrated that the mesoporous CaAl oxides showed a more rapid adsorption rate (for 1–2 min) and longer cycle life (weight change retention: 80% for 30 cycles) of the adsorbent because of the greater surface area and increased number of activated sites in the mesostructures. The LDH-based composites and their CO₂ capture performances are summarized in Table 1.3.

Although alkali metal carbonates exhibit good CO₂ adsorption capacity, they are not very attractive adsorbents due to low kinetics, higher regeneration energies, difficulty with heat control, and poor durability.^74,78  However, it is well known that alkali metals are good promoters for LDHs to improve their CO₂ adsorption ability.^79–84  It is believed that doping K₂CO₃ on LDHs can increase the adsorption capacity of CO₂ because it changes both the chemical and physical properties of LDHs.²⁴  It is also contemplated that K₂CO₃ impregnation increases the adsorption capacity of LDHs due to an increase in the number of active sites on the surface, despite a reduction in BET area and pore volume.^85,86  To date, much work has been done on alkali-metal-promoted LDHs.

Yang et al.²⁴  found that the CO₂ adsorption capacity of LDHs can be increased by K₂CO₃ impregnation. 20 wt% K₂CO₃ was determined to be the optimal loading, with the highest CO₂ adsorption capacity of 0.77 mmol g⁻¹ at 450 °C and 800 mmHg. However, above the critical amount of K₂CO₃, the adsorption capacity of CO₂ decreased due to the blockage of the pores of LDOs. Subsequently, Oliveira et al.⁸⁷  reported that the adsorbed CO₂ on MG 30 (commercial LDH from Sasol, Germany) increased from <0.1 to 0.76 mmol g⁻¹ after doping with K₂CO₃. The CO₂ adsorption isotherms of MG30-K were determined at temperatures of 306, 403 and 510 °C and CO₂ partial pressures up to 0.40 bar in the presence of steam. The CO₂ sorption capacity at 403 °C in alkali-modified LDHs was higher than those at 306 and 510 °C. In this regard, an adsorption mechanism combining physical adsorption and a chemical reaction was proposed. Based on the proposed mechanism, the bi-Langmuir model was selected to fit the experimental points. This model is considered to fit the general trend displayed by the experimental data of MG30-K samples. Then, a study of the cyclic stability of the MG30-K sample was performed. From the measurement of the adsorption capacity in the first and last CO₂ breakthrough curves and the fact that during the cyclic operation the temperature and concentration curves are similar for all cycles, it can be concluded that there is a small loss (7%) in adsorption capacity after 75 cycles.

Then, Walspurger et al.⁸¹  reported that the total CO₂ capacity of a 11 wt% K₂CO₃-promoted LDH with a Mg–Al molar ratio of 2.9 can reach up to 15.1 mmol g ⁻¹ at 350 °C in the presence of sufficient concentrations of CO₂ and steam. They also investigated the K⁺ promotion mechanism, which suggested that the rearrangement of surface carbonates occurs at high temperatures and that the interaction between aluminium oxide centers and potassium carbonate plays a crucial role in the formation of active sites (strongly basic) for CO₂ capture at 300–500 °C, and were able to reversibly adsorb CO₂ at high temperatures.^81,85  Moreover, Lee et al.⁸⁸  reported that the equilibrium CO₂ uptake of LDHs could even be increased up to 10 times by impregnation with K₂CO₃. They proposed the mechanism of CO₂ adsorption enhancement by K₂CO₃ impregnation. They found that there was an optimal amount of K₂CO₃ for the maximum equilibrium CO₂ adsorption uptake, which was caused by two conflicting effects: enhanced basicity and reduced surface area with increasing K₂CO₃ amount. The results showed that the incorporation of LDHs and K₂CO₃ increased the thermal stability of K₂CO₃ without changing the structure of the LDHs, resulting in both enhanced equilibrium CO₂ adsorption uptake and fast CO₂ adsorption kinetics.

Subsequently, Wang et al.¹³  systematically investigated alkali-metal-carbonate-promoted LDHs as high temperature CO₂ adsorbents. The LDHs were prepared by the co-precipitation method at various pH values (9–14), followed by impregnation with various alkali metal carbonates (Li₂CO₃, K₂CO₃, Na₂CO₃, Rb₂CO₃ and Cs₂CO₃). It was found that the pH values in the alkali-metal-oxide-promoted LDHs had little influence on the CO₂ capture capacity of K₂CO₃/LDHs when they were equal to, or higher than, 10. Different alkali metal carbonates and different solvents were compared. The results showed that with the same weight loading, Li₂CO₃ (1.05 mmol g⁻¹) has a better promotion effect than K₂CO₃ (0.96 mmol g⁻¹), at 400 °C and 1 bar, followed by Na₂CO₃ (0.83 mmol g⁻¹), Rb₂CO₃ (0.79 mmol g⁻¹), and Cs₂CO₃ (0.75 mmol g⁻¹)-doped LDHs. And, different solvents, including water, methanol, ethanol, 2-propanol and 1-butanol, were used, which indicated that the capacity was greatly increased when organic solvents were used. It is believed that organic solvents can probably result in a better dispersion of K⁺ during the impregnation. However, with water as the solvent, the amorphous MgAlO_x mixed oxide was transformed back to the LDH structure again during the preparation. This is due to the presence of both H₂O and CO₃²⁻ in the mixture. In addition, a systematic investigation on the effect of synthesis methods was performed by preparing the same CO₂ adsorbent, 20 wt% K₂CO₃/LDHs (pH = 10), via four synthesis methods including impregnation–filtration (IF), impregnation–rotary evaporation (IR), incipient wetness imprecation (IWI) and ball milling (BM). An impurity phase of K₂CO₃ (H₂O)_1.5 is observed in the sample synthesized by IF, which leads to a relatively poor dispersion of K₂CO₃ and a relatively low CO₂ capture capacity. Among the four adsorbents, K₂CO₃/LDH (IR) shows the highest CO₂ capture capacity of 1.22 mmol g⁻¹. The adsorption capacity of the adsorbents varies with the preparation method and follows the order of IR > IWI > BM > IF.

Furthermore, Li et al.⁸⁹  promoted stearate-pillared LDH by K₂CO₃ as a precursor for CO₂ capture. Experimental results showed a striking CO₂ capacity, up to 1.93 mmol g⁻¹ at 300 °C, 1.7 times higher than that of a conventional K₂CO₃-promoted LDH adsorbent (1.11 mmol g⁻¹), and fast adsorption kinetics for this precursor. For comparison purposes, the K₂CO₃-promoted MgAl–CO₃ LDH (K–MgAl–CO₃ LDH) and K₂CO₃-promoted MgAl-stearate LDH (K–MgAl-stearate LDH) were synthesized using a wet impregnation method. The general schemes for a sequential process are provided by Figure 1.15, including intercalation of stearate instead of carbonate in MgAl-LDH (a-1 and b-1) and K₂CO₃ promotion for various anion-pillared LDH precursors (a-2 and b-2), which evolve into LDO adsorbents after calcination at 450 °C (a-3 and b-3). It fully demonstrates that the K ion may enter the interlayer of a very spacious stearate-pillared LDH interlayer, facilitating a better promotion effect of K by forming weak surface chemical bonds. For a comparison of the K promotion effects between different sized anions (carbonate, 0.234 nm; stearate, 2.219 nm), in K–MgAl-stearate LDH, a greatly enlarged interlayer distance (3–4 nm) pillared by a long-carbon-chain stearate allows for more cation exchange during the promotion process, not only on the external surface of the LDH but possibly on the internal surface of every sheet of the LDH layer. The markedly increased amount of K-element promoters formed on nearly all accessible precursor surfaces is attributed to more dense, well-dispersed basic sites on all surfaces that could possibly react with CO₂ after calcination to form adsorbents. In order to find out the surface layout of K ions on both samples, TEM micrographs of both carbonate and stearate anion-based LDH precursors and adsorbents after calcination on 450 °C at the same zoom-in magnitude are provided in Figure 1.16. From Figure 1.16, it can be easily distinguished that the plate-like LDH stack up one upon another because the TEM facilitates a projection view perpendicular to the surface of the layers. Some small dark scattered parts, which are proposed to be K-containing particles, have been observed randomly among the plates. Then, EDX was employed to confirm that the black spots indicated by red arrows are K-containing particles. For K–MgAl–CO₃ LDH and K–MgAl-stearate LDH, the weight percentages of K in the dark areas (a-1 and b-1) were measured to be 0.9 and 2.7%, respectively (see Figure 1.16). On the contrary, K cannot be detected in normal non-black spot sections, such as a-2 and b-2 (see Figure 1.16). Therefore, they concluded that the dark areas are K promotion areas, which are useful clues for inspecting the extent of K ion dispersion on different anion-based LDH precursors. However, for adsorbents calcined from K–MgAl–CO₃ LDH, dark areas indicating elemental K may not be easily distinguished, but the K weight percentages detected at external (c-1) and internal (c-2) points are 0.6 and 0.0%, respectively (see Figure 1.16). This result reveals that the tendency for K dispersion is likely the same as that for its precursor, where K tends to promote surface adsorption exclusively around the edge of the LDH plate. On the other hand, brucite-like layers of K–MgAl-stearate LDH all collapse and break into smaller-sized non-crystals, with a dispersion of black points where the K weight percentage reaches 3.4% (d-1), and the TEM image vividly shows well-dispersed K ions on the surface of the adsorbent (see Figure 1.16). In conclusion, all the results proved that the enlarged interlayer distance will help the K ions disperse into the LDH interlayer and, thus, create more Al(Mg)–O–K bonds to enhance the wet promotion effect on both the surface and intersurface of brucite-like LDH layers.

Following this, Li et al.⁹⁰  synthesized K₂CO₃ promoted n-caprylate (CH₃(CH₂)₆COO⁻) LDH (K-Mg₃Al-C8) and K₂CO₃ promoted stearate (CH₃(CH₂)₆COO⁻) LDH (K-Mg₃Al-C8) for CO₂ capture, comparing with the K₂CO₃-promoted Mg₃Al-CO₃ (K-Mg₃Al-CO₃). The results revealed that an extended interlayer distance created more Al(Mg)–O–K bonds to enhance the wet promotion effect on the brucite-like LDH layers, giving rise to higher CO₂ capacities of 1.97 mmol g⁻¹ (K–Mg₃Al–C18) and 1.36 mmol g⁻¹ (K–Mg₃Al–C8) at 300 °C and atmospheric pressure. Reversible isotherms (from vacuum to 1000 kPa, 250–400 °C) of all three types of K₂CO₃-promoted LDHs were carried out to demonstrate that the CO₂ capacity increased with increasing the temperature and length of the intercalated anion as well. The isosteric heat of adsorption of these LDHs was between 2.5 and 92.4 kJ mol⁻¹, much smaller than common chemical adsorption. The kinetics of the LDHs truly fitted the Elovich equation, confirming that the CO₂ reaction on the heterogeneous surface of LDHs' basic sites involves chemical adsorption. Novel LDHs disclosed that almost half of the CO₂ capacity was lost after the first isothermal cycle. TPD and FTIR results proved the existence of two types of surface basic sites. One type was a strong-bonded monodentate carbonate, which could only be decomposed by calcination at 700 °C (see Figure 1.17(a)); another type was a relatively weak bidentate carbonate, which could be desorbed by decreasing the partial pressure of the adsorbate (see Figure 1.17(b)). Therefore, bicarbonate was crucial for isothermal cycles for CO₂ capture. Experimental results revealed that the effective capacity of new LDH adsorbents combined the advantages of both chemisorption (chemical bonded and good selectivity) and physisorption (partly reversible only by changing partial pressure), which could be highly probable for future application in pressure swing adsorption (PSA) operations.

Miguel et al.⁴⁴  also expounded gallium-substituted LDH for CO₂ capture, which was prepared by a co-precipitation method and modified by impregnation with an alkali (K and Cs) and alkaline-earth (Sr) metals. The results indicated that the best material, i.e. LDH substituted with gallium and promoted with potassium (LDH-10Ga-20K), showed suitability for use in cyclic operations with regeneration under a low CO₂ pressure (1.08 bar), particularly at 300 °C, where a very good working capacity (1.82 mmol g⁻¹) was obtained. It was established that modification with alkali metals increases the adsorption capacity of original samples (LDH and LDH-10Ga). However, adding K provided higher adsorption values, when compared to Cs, and this should be related to its higher basicity. On the contrary, the adsorption capacity decreased for the sample modified with an alkaline-earth metal (Sr). The adsorption capacity of the materials decreased in general with the increase of their specific BET surface area, which means that the available BET surface area does not play a crucial role on the adsorption capacity; the adsorption capacity is more likely to be related to the chemical nature of the exposed surface. Moreover, a series of adsorption–desorption cycles towards CO₂ under low-pressure conditions (<0.0001–0.15 bar) at 300 °C and 200 °C on LDH-10Ga-20K materials were effectuated. Average working capacities of 0.88 and 0.23 mmol g⁻¹ were obtained after repeated cycles at 300 and 200 °C, respectively. The adsorption kinetics was apprised through uptake measurements, showing two parallel resistances, which were described well by the Freundlich model.

Doping with K₂CO₃ can improve the CO₂ capture capacity of LDH-derived compounds. However, what about the stability of K₂CO₃ doping LDH derived compounds? In this regard, Huang et al.³⁴  used K₂CO₃ to promote LiAl₂–CO₃ LDH and tested it at different temperatures. The results showed that the CO₂ adsorption capacities of LiAl₂–CO₃ LDH with 20 wt% K₂CO₃ at 60, 100, 150, 200, 300, and 400 °C were 1.27, 0.98, 0.83, 0.83, 0.63, and 0.68 mmol g⁻¹ respectively, which are all higher than neat LiAl₂–CO₃ LDH. Moreover, they also compared the stability of neat LiAl₂–CO₃ LDH and K₂CO₃-promoted LiAl₂–CO₃ LDH using a typical temperature swing adsorption process. During the 22 cycles, both results demonstrated that the CO₂ adsorption capacity in cycle 2 was lower than that in cycle 1, which was due to some irreversible chemisorption. For neat LiAl₂–CO₃, the capture capacity became stable from cycle 2, which was ca. 0.36–0.37 mmol g⁻¹ during the next 20 cycles; while for the K₂CO₃-promoted LiAl₂–CO₃ LDH, the CO₂ capture capacity became stable from cycle 8, which was ca. 0.52 mmol g⁻¹ during the next 14 cycles. Although the CO₂ capture capacity of K₂CO₃-promoted LiAl₂–CO₃ LDH dropped more than neat LiAl₂–CO₃ LDH, its stable adsorption capacity was still much higher.

Likewise, Meis et al.⁹¹  first compared the CO₂ capture capacities of supported LDHs and unsupported LDHs with doping K₂CO₃. The K₂CO₃-doped-supported LDHs adsorbed 1.7–2.2 mmol g⁻¹ CO₂ at 250 °C, which exceeded the capacity of unloaded supported LDHs and K-loaded unsupported LDHs under identical conditions (see Figure 1.18(a)). The key for the increase in capacity in alkali-loaded LDHs is a close contact, at least at a mesoscopic level, between the LDHs and the alkali metal carbonate. The alkali metal carbonate could be successfully introduced either by impregnation of a K₂CO₃ solution on as-synthesized LDHs or by leaving residual K (or Na), from the synthesis, in the final material. The latter method is advocated since it omits a washing step after precipitation. After loading the alkali carbonate on unsupported and supported LDHs, their capture capacities were then comprehensively investigated. Loading the unsupported LDHs with an alkali carbonate metal increased the capacity 210–280% at 250 °C (see Figure 1.18(a)). On the other hand, the capacity of the supported LDHs, after loading with the alkali metal carbonate, increased only 30–70% at 250 °C. Such an effect can be tentatively explained by the higher concentration of defects (low-coordination oxygen sites) on the activated supported LDHs and the low amounts of defects on activated unsupported LDHs. Thus, it is more likely that there is a higher increase in edges and corners on an unsupported LDH after loading with an alkali carbonate metal than on alkali-carbonate-supported LDHs (see Figure 1.18(b)). The increase in capture capacity on alkali-loaded supported LDHs is attributed to a higher concentration of defects on the surface of the alkali-loaded LDHs caused by the presence of Na⁺/K⁺ on the surface of Mg(Al)O_x (Figure 1.18(b)). The group proposed a tentative mechanism: K⁺ substitutes Mg²⁺ and additional oxygen vacancies at the surface are created. Possibly, more oxygen atoms in the vicinity of K⁺ adsorb CO₂ (Figure 1.18(b)). Wang et al.⁷¹  studied the CO₂ capture capacity of LDH/GO composites with K₂CO₃ doping. First, a series of MgAl–NO₃ LDH-NS/GO composites with K₂CO₃ loadings of 5, 10, 15, 18 and 20 wt% were prepared using the incipient wetness impregnation method. Before doping with K₂CO₃, the sample was calcined at 400 °C for 5 h. Next, the K₂CO₃ solution was dropped into the calcined sample with constant milling. The mixture was dried at 60 °C in an oven, and re-calcined at 400 °C for 5 h before CO₂ capture tests. The results showed that the CO₂ capture capacity was increased with the increase of K₂CO₃ loading from 5 to 15 wt%. The highest CO₂ capture capacity of ca. 0.6 mmol g⁻¹ was obtained with 15 wt% K₂CO₃, which was 2.5 times that of pure LDH (0.24 mol g⁻¹). The CO₂ capture capacity started to drop with a further increase of K₂CO₃ loading to 20 wt%, which might be due to pore blockage from excess K₂CO₃. Additionally, Zhu et al.¹⁰  loaded K₂CO₃ onto the surface of a layered double oxide/activated carbon-based composite to promote the adsorption performance. The adsorption isotherms of activated carbon (AC), K₂CO₃-modified activated carbon (AC-K) and the K₂CO₃-modified layered double oxide/activated carbon composite (AC-LDH_co-K) were tested under the following conditions: 200 °C and 0–3 MPa. The results are shown in Figure 1.19. As noted, the adsorption capacity of the K₂CO₃-modified composite adsorbent in the pressure range of 0–3 MPa was the largest of the three adsorbents. The adsorption isotherm data showed that the K₂CO₃-promoted composite adsorbent has an adsorption capacity of 1.741 mmol g⁻¹ at 200 °C and 3 MPa, which is higher than that for activated carbon (1.410 mmol g⁻¹) and for the composite adsorbent without K₂CO₃ (1.638 mmol g⁻¹). For the PSA process, the working capacity, Δq, defined as the adsorption capacity difference between the high-pressure value and the low-pressure value, is especially important. The Δq was calculated under the conjecture that the adsorbents adsorbed at 3 MPa and desorbed at 0.5 MPa, and the results showed that the Δq for the K₂CO₃-modified composite adsorbent was 1.226 mmol g⁻¹, which was 1.05 times that of the K₂CO₃-modified activated carbon and 1.23 times that of activated carbon. The result is similar to the CO₂ adsorption capacity of ca. 1.36 mmol g⁻¹ for the industrially available monoethanolamine (MEA) unit. Furthermore, Li et al.⁹²  synthesized a K-promoted γ-alumina/MgAl-LDO composite adsorbent for CO₂ capture. A novel “in situ adhesive” concept was presented. The immediate introduction of a K₂CO₃ solution as an electrolyte into alumina-sol-soaked MG63 paste was first proposed to partly coagulate colloidal alumina sol to form a K₂CO₃-promoted pseudo boehmite in situ during paste mixing and stirring before the pelleting process. The synthesis process of a K-promoted γ-alumina/MgAl-LDO composite adsorbent is shown in Figure 1.20. In situ formation of a K-promoted pseudo-boehmite (PB) in the paste extrusion process allowed the preparation of an adsorbent in extrudate form with a higher CO₂ capacity (0.65 mmol g⁻¹) and radial crushing mechanical strength (75.6 N per pellet) when compared to regular extrusion. Furthermore, the whole extrusion method provided an easy approach, which combined the process of surface modification and the process of pelleting from powders to extruders in a single step. The performance tests results revealed that the CO₂ capacity of MG63 decreased while that of the K-promoted PB increased with the increase of calcination temperature above 400 °C. The K-promoted PB is transformed to K-promoted γ-alumina at about 400 °C, and this process increases the mechanical strength of the adsorbent. The cycling test result showed that the K-promoted γ-alumina/MgAl-LDO composite had good mechanical stability, withstanding various cyclic adsorption–desorption, steam sweeping, pressure change, and wear tests. The adsorbents were kept in the fixed-bed reactor for 21 days for 11 breakthrough tests with regeneration. The pellets remained intact and in shape when taken out (see Figure 1.21).

Copious studies have focused heavily on the use of K₂CO₃ as the precursor for the K promoter. Nevertheless, Wu et al.⁸³  employed potassium nitrate (KNO₃) as the K precursor with a new impregnation method to obtain a K-promoted LDH for CO₂ capture. The CO₂ adsorption isotherms of the material obtained were investigated at 335, 383, and 438 °C, with p_CO2 from 0.05 to 0.5 bar. A bi-Langmuir adsorption isotherm model combining physical adsorption and a chemical reaction was used to fit the equilibrium data. It was perceived that the equilibrium adsorption results can be successfully described by a bi-Langmuir isotherm. The highest CO₂ adsorption capacity can reach 1.13 mmol g⁻¹ at 383 °C with p_CO2 = 0.5 bar under humid conditions. The reason is that the KNO₃ is thermally decomposed into K₂O, NO₂ and O₂. The released NO₂ and O₂ gases have an additional calcination effect on the LDH, and K₂O incorporates better into the LDH, since no needle-like morphology was found in the SEM micrographs. The increased adsorption capacity can be attributed to a better interaction between the K₂O promoter and the LDH adsorbent. Breakthrough curves at different conditions were also investigated experimentally and numerically. A mathematical model was developed to simulate the adsorption and desorption process; the model satisfactorily predicts the breakthrough curves of CO₂ in the column packed with the K-LDH. The stability of the K-LDH was studied with repeated adsorption–desorption cycles. It was found that both the CO₂ adsorption capacity and kinetics were not affected much after 10 cycles. Additionally, CO₂ desorption experiments were also conducted with temperature swing regeneration. The temperature swing regeneration can be achieved within 60 min, which reduces to half the time required for regeneration under isothermal conditions. It is proved that the K-LDH material has very good thermal and cyclic stability.

A recent breakthrough has led to the highest adsorption value of LDHs to be reported in the literature so far. Kim et al.⁹³  synthesized LDHs with Mg–Al molar ratios between 3 and 30 as CO₂ adsorbents by a modified co-precipitation method in the presence of NaNO₃, which markedly improved the CO₂ adsorption capacity. When the Mg–Al molar ratio increased from 3 to 20, the CO₂ uptake at 300 min increased significantly from 0.83 to 9.27 mmol g⁻¹ at 240 °C and 1 bar. Likewise, when the Mg–Al molar ratio was between 12 and 30, they found that the LDHs showed a two-step CO₂ adsorption behavior compared with the normal single-step CO₂ adsorption of the LDH with a Mg–Al molar ratio of 2. In the LDHs with high Mg–Al molar ratios, the CO₂ adsorption in the first 5–15 min is low, and the initial CO₂ loading is even lower than that of the LDH with a Mg–Al molar ratio of 3. After the first plateau, the CO₂ adsorption goes through a slow transition to a second phase in which the CO₂ loading is very high. The results show that the quantity of NaNO₃ is closely related to the CO₂ adsorption of the LDH and the existence of NaNO₃ promotes the CO₂ adsorption. However, when the Mg–Al molar ratio is further increased to 25 or 30, the CO₂ adsorption uptake slightly decreases. The reason for this decrease is thought to be the loss of pores and reduction of surface area resulting from pore blockage caused by deposition of excess NaNO₃. Because of these two conflicting effects of NaNO₃, there is an optimal amount of NaNO₃ for maximum CO₂ adsorption. The plot of CO₂ adsorption uptake versus relative content of NaNO₃ in LDHs clearly shows that the amount of NaNO₃ is a key factor for enhanced CO₂ adsorption on LDHs, and that there exists an optimal amount of NaNO₃ for the highest CO₂ uptake. Then, the effect of temperature on CO₂ adsorption was investigated using thermogravimetric analyzer (TGA) at temperatures ranging from 200 to 330 °C for the LDH with a Mg–Al molar ratio of 20. The result revealed that CO₂ adsorption uptake at 300 min increases as the temperature increases from 200 to 270 °C. The maximum adsorption uptake of 9.27 mmol g⁻¹ is achieved at 240 °C. However, the adsorption uptake sharply decreases to 2.85 and 0.438 mmol g⁻¹ at 300 and 330 °C, respectively. It is proved that the adsorption kinetics also vary depending on temperature. It is thought that the MgCO₃ on LDHs with a high Mg–Al molar ratio is promoted by the existence of NaNO₃ in a manner similar to that of the MgO-based CO₂ adsorbent. Harada and Zhang et al.^94,95  proclaimed that MgO-based CO₂ adsorbents were promoted with an alkali metal nitrate and inferred that the molten salt on the surface of solid MgO intensifies the rapid generation of active MgO sites and carbonate ions required for the formation of MgCO₃ crystals. Ultimately, the cyclic adsorption and desorption test was carried out for a LDH with a Mg–Al molar ratio of 20. The change in CO₂ uptake was recorded over 16 cycles of CO₂ adsorption at 240 °C for 4 h under a flow of CO₂ and desorption at 400 °C for 1 h under a flow of N₂. The results showed that the CO₂ uptake was noticeably reduced during the first five cycles and then became stabilized with a slight decrease and maintained only 25% of the initial CO₂ adsorption uptake after 16 cycles. However, its working capacity is still high compared to other LDHs.

Recently, Qin et al.⁵¹  tried to verify the promoting effect of (Li–Na–K)NO₃ on a MgAl–C16 LDH-derived adsorbent. The molar ratio of Li–Na–K was fixed at 3 : 6 : 1 in the (Li–Na–K)NO₃ molten salt. The CO₂ adsorption capacity of 15 mol% (Li–Na–K)NO₃ coated MgAl–C16-LDH with different Mg–Al molar ratios was tested. All samples were tested at 200 °C with a constant flow of high purity CO₂ (40 mL min⁻¹), and the adsorption uptake was measured for 2 h. The results proved that the CO₂ uptake decreased with the increase in Mg–Al molar ratio without alkali nitrate coating. However, 2ating with 15 mol% (Li–Na–K)NO₃, the CO₂ uptake slightly increased when the Mg–Al molar ratio was 5 : 1 to 9 : 1, and significantly increased to 2.03 when the Mg–Al molar ratio was increased to 20 : 1. Then, the influence of alkali metal nitrate molar loading on the CO₂ capture capacity of Mg₂₀Al₁–C16 LDH-derived adsorbents was also studied, as shown in Figure 1.22(a). It can be seen obviously that the CO₂ adsorption capacity first increased from 2.03 to 3.21 mmol g⁻¹ with the increase in alkali nitrate loading from 15 to 55 mol%. Nevertheless, a further increase in the alkali nitrate loading to 70 mol% did not lead to additional CO₂ adsorption capacity. In contrast, the CO₂ uptake finally diminished to 2.68 mmol g⁻¹ when the loading was 70 mol%. Thus, 55 mol% was selected as the best alkali nitrate loading for subsequent studies. Figure 1.22(b) shows a comparison of the CO₂ adsorption performance of Mg₂₀Al₁–C16 LDH and 55 mol% (Li–Na–K)NO₃-coated Mg₂₀Al₁–C16 LDH-derived adsorbents. After coating with 55 mol% (Li–Na–K)NO₃, the CO₂ uptake after 2 h significantly increased from 0.34 to 3.21 mmol g⁻¹. These results indicated that the CO₂ adsorption capacity was dramatically improved by the alkali metal nitrate molten salt coating. The stability of the adsorbents during CO₂ adsorption–desorption cycles was also tested. The adsorption was performed at 200 °C in a high purity CO₂ flow, and desorption was performed at a higher temperature (400 °C) in a high purity N₂ flow for 30 min. Figure 1.22(c) and (d) shows the CO₂ adsorption and desorption performance of (Li–Na–K)NO₃-promoted Mg₂₀Al₁–C16 LDO during 22 cycles. The results verified that there was no deterioration in the CO₂ uptake over 22 cycles. The CO₂ uptake gradually increased within the first few cycles, and eventually became stable thereafter. As shown in Figure 1.22(d), the CO₂ uptake increased from 1.41 mmol g⁻¹ in cycle 1 to 1.90 mmol g⁻¹ in cycle 14. After cycle 14, there was still a slight raise in CO₂ capture capacity, which finally reached 1.92 mmol g⁻¹ in cycle 22. This was compared with monoethanol amine, which is the industrial standard of practical reaction-based CO₂ separation processes. It is clear that the performance of the as-synthesized 55 mol% (Li–Na–K)NO₃-promoted Mg₂₀Al₁–C16-derived LDO adsorbent has a much higher reversible CO₂ capture capacity (1.92 mmol g⁻¹) than the industrial standard (1.36 mmol g⁻¹), suggesting that this novel CO₂ adsorbent is very promising for SEWGS processes.

Moreover, LDHs have also been modified by other materials, e.g. P123, or polyol. Azzouz et al.⁹⁶  synthesized polyol-modified LDHs with optimum affinity towards CO₂ and improved adsorption capacity. Furthermore, Dantas et al.⁹⁷  used P123-modified LDH (LDH-P123), which showed a greater textural area and had a better CO₂ adsorption capacity.

Flue gases emitted from power stations contain considerable amounts of water in the form of steam and SO_x. The percentage of water found in the flue gases emitted from different sources varies between 7–22%, with emissions from brown coal combustion having the maximum percentage of water, which may adversely influence CO₂ capture when using conventional solid adsorbents.¹⁷  SO_x emissions are proportional to the sulfur content of the fuel, and for the majority of coals currently in use, the sulfur content is in the range of 1–3%.⁹⁸  Analyses of flue gases produced by power plants burning coal before desulfurization indicate 0.1–0.2% SO₂ and ∼0.005% SO₃.⁹⁹  And, LDH-derived compounds have potential applications in SO_x adsorption. Therefore, to detect the adsorption of water and SO_x by LDH-derived compounds and its influence on CO₂ adsorption is very important in the context of CO₂ capture from flue gases.^17,98 

Ficicilar et al.¹⁰⁰  disclosed the effect of water vapor on the adsorption rate, as well as on the CO₂ adsorption capacity of LDHs. Their results revealed that in the absence of water vapor, the breakthrough curves obtained between 400 and 452 °C were quite close to each other. A shift of the breakthrough curves was observed with an increase in temperature (especially over 500 °C). This shift corresponds to a decrease in the CO₂ adsorption capacity. The total adsorption capacities of CO₂ obtained in the absence and presence of water vapor are found to be comparable, around 1.10 mmol g⁻¹ below 452 °C. However, the breakthrough capacities, which correspond to the capacities until the effluent concentration reaches 5% of the inlet value, are found to be lower in the presence of excess water vapor. In the presence of excess water vapor, shrinkage of pore mouths due to adsorption of H₂O + CO₂ on the active LDH surfaces near the pore mouths may be more significant, causing partial pore mouth closure and an additional diffusion resistance for the transport of CO₂ to the active sites of the adsorbent. All these results showed that the best temperature range was between 450 and 500 °C for the adsorption of CO₂ on activated LDHs both in the absence and presence of water vapor. The total adsorption capacity was as high as 1.16 mmol g⁻¹ and breakthrough capacity was as high as 0.70 mmol g⁻¹, which is rather attractive for CO₂ recovery from process and fuel gases.

Moreover, Ram Reddy et al.¹⁷  also investigated the influence of water on LDH adsorption performance. They substantiated that the presence of water in the feed did not influence the CO₂ adsorption rate but had a positive impact on the CO₂ adsorption efficiency of LDOs. It was found that the CO₂ adsorption increased from 0.61 to 0.71 mmol g⁻¹, tested under dry and wet-gas conditions, respectively. Water is expected to react with LDO, forming magnesium and aluminium hydroxides, which in the presence of CO₂, form bicarbonates without affecting the net CO₂ adsorption. Regeneration of the compounds at 400 °C brought back more than 90% of the original adsorption capacity. CO₂ adsorption studies conducted using mixed gases (14% CO₂) have shown high adsorption values even though the CO₂ concentration was diluted by almost seven times. LDOs did not show any proportionate decrease in the adsorption capacity with the decrease in CO₂ concentration in the feed gas. In the case of mixed gas, water did not hamper CO₂ adsorption performance. LDOs exhibited highly consistent adsorption–desorption performance even after reducing the cycle time from 30 to 10 min. Temperature cycling in wet conditions demonstrated very high levels of desorption (75%), which reached an equilibrium value (67%) after initial stabilization. Shorter time cycles were found to be more effective in improving the overall efficiency of the process. These findings demonstrate the hydrothermal stability, recyclability, and regenerability of LDOs.

In order to gain insight into the mechanism of CO₂ capture in the temperature range of 300–600 °C and steam partial pressures in the range of 0–4.55 bar, Maroño et al.¹⁰¹  investigated the thermal behaviour and CO₂ capture capacities of three K-promoted LDH-based materials with different Mg–Al molar ratios and potassium carbonate content as well as the effect of calcination temperature, process temperature, system pressure and water content in the feed gas. Thermo-gravimetric analysis of the samples revealed that a pre-calcination temperature above 500 °C guaranteed that the interlayer CO₂ and H₂O were released from the structure and the materials were ready to be effective in the CO₂ capture process. Dynamic and isothermal CO₂ capture tests performed under dry and wet feed-gas conditions showed that the presence of different amounts of steam during the CO₂ capture process has a strong influence on the capture process efficiency and the capture mechanisms involved. Under dry conditions and low water content in the feed gas, the main capture mechanism identified was the formation of K-dawsonite (KAl(CO₃)(OH)₂). However, when the amount of steam available in the reaction system was increased to up to 35% v/v (P_H2O = 4.5 bar), the CO₂ capture capacity of the adsorbents increased dramatically, as high as 9 mol kg⁻¹ MG61-K₂CO₃. It is indicated that the absolute values of P_CO2 and P_H2O and the ratio P_H2O–P_CO2 play a crucial role in the CO₂ capture mechanisms and efficiencies of potassium-promoted LDH-based adsorbents for their application in SEWGS processes.

Recently, Iruretagoyena et al.⁷²  compared the effect of water on the CO₂ capture capacities of unsupported LDO and a LDO/GO composite. It was shown that the first-contact-adsorption isotherms under wet conditions (0.16 mol steam/mol total) for pure LDO and LDO/GO were obtained from breakthrough analysis. Under the operating conditions, no sign of gasification of GO was observed. The presence of water has a beneficial effect on the adsorption properties of both materials, causing an increase in the CO₂ capacity. The adsorption of H₂O and CO₂ was observed to produce an increase in the temperature inside the column (DT = 21 K) that is considerably higher than that under dry conditions (DT = 4 K). Temperature-swing multicycle experiments were also carried out on adsorption in the presence of steam and the regeneration under dry nitrogen showed a loss of capacity similar to that obtained in completely dry cycles.

In addition, Ram Reddy et al.⁹⁸  investigated the effect of SO_x on the performance of LDH-derived CO₂ adsorbents. First, they compared the adsorptions of CO₂ and SO_x on LDO, respectively. The comparison of CO₂ and SO_x adsorption on LDO revealed that CO₂ adsorption is much faster and reached saturation in ∼30 min. On the other hand, SO_x adsorption is relatively slow, but its adsorption levels are much higher than CO₂. The strong adsorption potential of SO_x was exhibited even after 2 h. In order to understand the influence of SO_x on CO₂ adsorption and vice versa, two different experiments were conducted, In the first experiment, CO₂ adsorption was performed for the first 60 min using mixed gas followed by SO_x (0.1% in N₂) for the next 60 min, and the sample was finally regenerated at 400 °C. In the second experiment, SO_x adsorption was carried out for the first 60 min followed by CO₂ (100%) adsorption and regeneration. The results obtained from these two experiments showed that even at low flue gas feed concentrations of SO_x (0.1%), the adsorption values were very high, reaching a maximum adsorption capacity equivalent to 11.04 wt%. Regeneration of LDO in pure helium resulted in regaining up to 58% of its original adsorption capacity. Temperature cycling also revealed the irreversible nature of SO_x adsorption. In addition, regeneration after CO₂/SO_x and SO_x/CO₂ adsorption experiments showed that SO_x replaces CO₂. SO_x adsorption over CO₂ was favored due to the strong acid–base interactions between SO_x and LDO, thus forming sulfites and sulfates. Hence, LDH derivatives for CO₂ capture require a de-SO_x unit operation upstream.

Besides SO_x and water, in an integrated gasification combined cycle (IGCC) power plant, significant amounts of H₂S will be present in the coal syngas ranging from a few hundred ppm to a few %.¹⁰²  Since both CO₂ and H₂S have an acidic character, both may strongly interact with the adsorbent material, which has a basic character and may consequently alter the benefits of the SEWGS process under sour conditions. Therefore, van Dijk et al.¹⁰²  investigated the behavior of K₂CO₃-promoted LDHs adsorption on CO₂ in the presence of a relevant amount of H₂S. The experimental results showed that the sorbent displays reversible co-adsorption of CO₂ and H₂S in a multiple cycle experiment at 400 °C and 5 bar. The CO₂ adsorption capacity of LDHs is not significantly affected compared to sulfur-free conditions. The experimental results were qualitatively explained by the mechanistic model represented in Figure 1.23(a). The model assumes two different sites on the adsorbent interacting with H₂S. Type A sites would allow for reversible competitive co-adsorption of H₂S and CO₂, probably involving surface species such as carbonates (CO₃²⁻and HCO₃⁻), –SH, and thiocarbonate (SCO₃⁻ and HSCO₂). The adsorption would be competitive with CO₂ being able to push H₂S from these Type A sites. Type B sites would exclusively take up H₂S reversibly, involving the formation of a metal sulfide. The metal sulfide would be formed from the metal oxide under H₂S-rich conditions, while the presence of steam and the absence of H₂S during regeneration would result in partial backformation of the metal oxide and release of H₂S according to the reaction in eqn (1.1). The high basicity of MgO suggests it participates in this reversible sulfide formation, although it cannot be excluded that the other metal oxides present, Al₂O₃ and/or K₂O, contribute or that a mixed metal oxide is involved.

In order to further elucidate the interaction mechanism, another experiment was carried out by the same group. In the experiment, steam is omitted from both the adsorption and regeneration feeds. Accordingly, CO₂ and H₂S are dynamically fed in the absence of steam. Figure 1.23(b) shows the results of the responses for He and H₂S for an 18-cycle experiment. It can be seen clearly that all the H₂S is adsorbed by the adsorbent during the first few cycles. However, breakthrough fronts of H₂S are observed although the inlet concentration was not reached during the subsequent cycles. Apparently, under dry conditions, the adsorbent is capable of taking up a significant quantity of H₂S. And when only focussing on cycle 13 and 14, they show the normalized responses for He, CO₂ and H₂S, while steam is shown as a MS intensity. The reversible CO₂ uptake is largely reduced since the CO₂ breakthrough basically coincides with the He transient. Moreover, the evolution of small amounts of H₂O is detected upon exposure of the sorbent to adsorption conditions for every cycle. In summary, the results indicated that SEWGS under sour conditions is capable of simultaneous decarbonation and desulfurization of sour syngas originating from the gasification of coal. It will accordingly produce a H₂-rich product with low contents of CO₂ and H₂S and a CO₂ + H₂S rich product.

Besides studies of the CO₂ capture capacities of LDHs, the adsorption site, adsorption mechanism and kinetics have also been investigated. LDHs have been intensively studied for high-temperature CO₂ capture; however, big differences in the CO₂ capture capacities, ranging from 0.28 to 0.6 mmol g⁻¹, have often been reported for the same MgAl–CO₃ LDH. Furthermore, how the active Mg–O species that are responsible for CO₂ adsorption are formed is of great interest. Therefore, in order to have a clearer understanding of the CO₂ adsorption site and mechanism of formation of LDH-derived metal oxides, a careful examination of the structural changes during thermal treatment and the CO₂ adsorption sites and the formation mechanism of LDH-derived metal oxides were investigated by Gao et al.¹⁰³  using XRD and solid state nuclear magnetic resonance (NMR) analyses. First, the structural changes of Mg₃Al₁–CO₃ LDH in the temperature range of 300–950 °C were examined by XRD analyses, as shown in Figure 1.24(A). During the first stage of calcination (T < 400 °C), the LDH structure gradually decomposes and transforms into an amorphous phase. This is evidenced by a decrease in the basal spacing as the temperature is increased, for example, an upward shift of the d₀₀₃ Bragg reflection of the LDH was observed at 300 °C. It is due to the desorption of interlayer H₂O and the shrinkage of LDH layers. At this stage, since it is still in its layered structure, it is reasonable that the CO₂ capture capacity is low. At 400 °C, it becomes an amorphous mixed phase oxide of nominal composition MgAlO_x. Characteristic Bragg reflections due to periclase MgO are observed, and upon further heating to 800 °C, the MgO Bragg reflections become narrower, and it partially transforms into a MgAl₂O₄ spinel at 950 °C. Figure 1.24(B) shows that the characteristic Bragg reflections of MgO are shifted to higher d-spacings with an increase of temperature from 400 to 800 °C, which may indicate a migration of Al³⁺ cations from the MgO lattice to the surface. Because the ionic radius of Mg²⁺ is bigger than that of Al³⁺, the unit cell of the MgO lattice containing fewer substituted Al³⁺ ions will be larger. This result suggests that the introduction of Al³⁺ into the amorphous-like periclase MgO lattice could produce active Mg–O species for CO₂ adsorption. When the calcination temperature is too high, it tends to order into the MgAl₂O₄ spinel and thus diminishes the surface-active CO₂ adsorption sites.

In order to obtain a more in-depth understanding of the structural changes that occur during calcination, ²⁷Al solid state NMR analysis was performed. Figure 1.24(C) shows the ²⁷Al solid state NMR spectra of Mg₃Al–CO₃ LDH calcined at different temperatures ranging from 25 to 800 °C. As anticipated, the spectrum of fresh Mg₃Al–CO₃ LDH exhibited a single resonance near 9.4 ppm, which can be assigned to octahedrally coordinated Al,¹⁰⁴  which confirmed that the LDH structure is formed by metals coordinated to six hydroxyl groups with octahedral geometry. However, when the samples were calcined, a new signal at around 76–84 ppm was observed, which can be attributed to the formation of tetrahedrally coordinated Al.^105,106  This suggests that after calcination, the Al coordination has been partially changed from six-coordinated to four-coordinated. It was proposed by Bellotto et al.¹⁰⁷  that the Al atoms have diffused out of the octahedral brucite layers and are located in the interlayer. They are tetrahedrally coordinated to three oxygens of the layer and one apical oxygen of the interlayer. The sites formerly occupied by Al are left vacant, and the dimensions and geometry of the octahedral layer are maintained. It is proposed that such a structural change gives rise to Al³⁺ cation vacancies in the layer, which can subsequently induce the formation of active Mg–O species as well. Consistent with the XRD analysis, MgAl₂O₄ was formed when the calcination temperature was too high (800 °C), at which point the octahedral Al signal shifted to 84 ppm.

Keeping the discussion above in mind, two mechanisms for the formation of active Mg–O species were proposed when LDH is optimally calcined. One mechanism is shown in Figure 1.25(a), which considered that the active Mg–O species can be generated by substitution of Mg²⁺ by Al³⁺ in the periclase MgO lattice. In order to compensate for the positive charge generated by Al³⁺, the adjacent oxygen anions will become coordinatively unsaturated. When one Al³⁺ is inserted into the periclase MgO lattice, there will be generation of two active Mg–O species. The other mechanism is shown in Figure 1.25(b), which considered that a portion of the inserted Al³⁺ might diffuse out of the octahedral sites and become tetrahedrally coordinated in the interlayer. The site formerly occupied by Al³⁺ is left vacant, which consequently produces three active Mg–O species around it. Both of the mechanisms demonstrated that the calcination temperature is one of the key parameters that determine the number of active Mg–O species. If the calcination temperature is too low, the Mg–O bonds cannot be broken, and the hydroxide phase remains; if the calcination temperature is too high, the Mg and Al will start to react and form the MgAl₂O₄ spinel oxide. Since each LDH has a different thermal stability, their optimal calcination temperatures are different. Likewise, this explains why the quasi-amorphous phase obtained by thermal treatment at the lowest possible temperature gives the highest CO₂ capture capacity.

This is compelling evidence that points toward the fact that the actual mechanism of uptake and release of CO₂ by LDHs is complex.^{21,23,108–110}  In addition, the regenerability of the material and the reversibility of the adsorption are also critical for ensuring efficient use of the adsorbents, as well as for designing industrial adsorption units.^111,112  Ram Reddy et al.¹⁸  found that reversible and irreversible CO₂ adsorption were determined to be about 88% and 12% of the total CO₂ adsorption, respectively. Regeneration restored the Mg–Al mixed oxide to 98% of its initial CO₂ adsorption after several cycles of CO₂ adsorption testing. Subsequently, Ebner et al.¹⁶  developed a non-equilibrium kinetic model to describe the reversible adsorption and desorption behavior of CO₂ in a K-promoted LDH. The model consisted of three reversible reactions. Two of the reactions were of the Langmuir–Hinshelwood type, with slow and intermediate kinetics, and one was a mass-transfer-limited chemisorption process with very fast kinetics. To calibrate and test this model, a K-promoted LDH was synthesized and studied to determine its dynamic behavior during CO₂ adsorption and desorption cycles carried out at 400 °C. Then, a long cycle time adsorption (700 min) and desorption (700 min) experiment was carried out with a sample activated at 400 °C for 12 h in helium. With this experiment approaching equilibrium at the end of each step, it proved that the adsorption and desorption behavior of CO₂ in K-promoted LDH was completely reversible. Moreover, the effect of the activation time (8, 12, 16, and 20 h) was also studied, with samples cycled twice with a 45 min half-cycle time. The eight-parameter non-equilibrium kinetic model was fitted successfully to the long cycle time adsorption (700 min) and desorption (700 min) experiment. The model predicted successfully the dynamic and cyclic behavior of both the much shorter cycle time experiments and the different activation time experiments. The same model accurately simulated the reversible adsorption and desorption behavior of the very fast, intermediate, and slow kinetic processes, the approach to periodic behavior during cycling, and the independence between the CO₂ working capacity and activation time. Finally, they also proved that the adsorption and desorption behavior was due to a combination of completely reversible adsorption, diffusion, and reaction phenomena. Subsequently, Du et al.¹¹³  extended this reversible non-equilibrium kinetic (RNEK) model to account for changes in temperature by making each of the three reactions temperature dependent. This temperature dependent version of the RNEK model was successfully validated against experimental cycling data obtained over a wide range of temperatures from 300 to 500 °C for both long and short cycle times. The same model provides a much deeper understanding of the dynamic behavior of CO₂ in K-promoted LDH. The RNEK model, which describes the adsorption and desorption behavior of CO₂ in K-promoted LDH, was further modified to account for different CO₂ partial pressures by Du et al.¹¹⁴  The results verified that the RNEK model fitted the experimental data very well over the wide ranges of pressure.

Isa et al.¹¹⁵  reported the rates of adsorption of CO₂, which is an important aspect in determining the viability of the removal of CO₂ with respect to the modification of LDH's surface area. In order to investigate changes in the physical structure of LDHs with CO₂ adsorption and heat treatment, SEM analysis was used. The results proved that the particles were in a rearranged structure with modified surfaces after fresh LDH reacted with CO₂ at different temperatures (32, 450, and 550 °C), which indicated that the LDH particles are not morphologically stable and undergo changes in particle structures and at all temperatures when CO₂ is adsorbed. From the experimental observations, pellets of 20 mm diameter used at a temperature of 32 °C gave better rates of adsorption compared with smaller sized pellets. Then, a close examination of the morphology of LDHs after adsorption of CO₂ showed changes in the structure and morphology, which were somewhat prominent, as observed using SEM. Disintegrated or deformed particles were observed in the morphologies of LDH samples after adsorption of CO₂ at all temperatures. This behavior is found to be common because the LDH structures consist of layered double hydroxides where positively charged (cations), a hydroxide layer (brucite sheet) and charged balancing anions exist with an interlamellar space. When CO₂ is adsorbed on a LDH, the CO₂ seems to break down the layered structures, leading to changes in the morphology. This shows that with the progression of adsorption of CO₂, the morphology, the structure and the area could change leading to changes in the available equivalent adsorption area for CO_2. This could happen when CO₂ is adsorbed, the particles disintegrate and the internal surfaces are exposed further, facilitating adsorption. Finally, an extended Langmuir model, which incorporated surface modifications with adsorption, was developed based on the Langmuir kinetic principle. The extended model was found to fit the experimental data satisfactorily with good correlation coefficients (R²) around 0.81 to 0.92.

For CO₂ adsorbents, besides the development of efficient CO₂ capture, another important and urgent issue is their techno-economic assessment in real applications.¹²  LDHs are synthesized using relatively cheap precursors and simple methods compared with alkaline ceramics.^7,53  LDH-derived CO₂ adsorbents are being investigated for their application in the SEWGS process, which is a promising CO₂ capture process for pre-combustion technology. However, SEWGS is an innovative technology for CO₂ capture. In this process, CO₂ capture or removal of CO₂ from the products of the WGS reaction is expected to shift the reaction to the desired direction for H₂ production and CO reduction.¹¹⁶  SEWGS has not been used in real applications yet, with only a few papers discussing the technical and economic assessments of this process. Gazzani and Manzolini et al.^117,118  evaluated the thermodynamic performance and the economic assessment of CO₂ capture in natural gas combined cycles with SEWGS. In their study, a post-combustion scheme based on amine scrubbing and a pre-combustion lay-out with MDEA is taken as a reference. The former is close to commercialization, while the latter is more similar to SEWGS schemes. The SEWGS working conditions were optimized in terms of the carbon capture ratio and the purity of the CO₂ separated out as well as the number of vessels adopted. Moreover, two different types of adsorbents—adsorbent alfa and adsorbent beta—were considered in order to evaluate the impact of the adsorbents' cyclic capacity on system performances. The results showed that SEWGS with adsorbent alfa could avoid 91% of CO₂ emissions and reduce the efficiency penalty of amine scrubbing technologies from 8.4% to 7.2%. However, no significant impact of CO₂ purity on system performances was determined, while for the adoption of adsorbent beta, which has an improved capacity of 60% compared to adsorbent alfa, further reduction in specific primary energy consumption for CO₂ avoidance together with the vessel number was observed. The best overall performance in terms of specific primary energy consumption for CO₂ avoided (SPECCA) had a net electrical efficiency of 51.93% and CO₂ avoidance of 86%. Moreover, the result of the economic assessment showed that with reference adsorbent performances, the calculated cost of CO₂ avoided is about 58 €/t_CO2, which is lower than reference MDEA (64 €/t_CO2) but higher than MEA (48.5 €/t_CO2). The adoption of an adsorbent with improved performances brings down the cost of CO₂ avoided down to 49 €/t_CO2, which is comparable to post-combustion technology. This is a consequence of reforming section costs, which penalizes pre-combustion technologies; specific investment costs for SEWGS cases are 15% higher than MEA. Finally, as far as SEWGS working conditions are concerned, the optimal CO₂ capture rate depends on the adsorbent cyclic capacity ranging from 90% to 95%, while the selected CO₂ purity is 99%. Finally, the advantage of SEWGS was summarized compared to reference cases: (i) higher efficiency, (ii) lower CO₂ emissions, (iii) lower SPECCA, and (iv) no environmental issues. From an economic point of view, the cost of CO₂ avoided is comparable to amine scrubbing technology although pre-combustion technologies have higher investment costs. They considered that future work will show that SEWGS has further advantages when applied to coal base power plants.

Clearly, LDH-derived mixed oxides as CO₂ adsorbents have been investigated over a broad range, both theoretically and experimentally. Based on a critical analysis of all findings discussed above, LDH-derived compounds have a good ability to capture CO₂, especially with good stability and easy regenerations. However, in order to further understand the mechanism of CO₂ capture by LDH-derived compounds for application on an industrial scale, the following recommendations are made for further research.

The studies reviewed above were carried out with a single-component system, i.e., CO₂. Only a few papers have discussed the influence of SO₂ and water etc. The other gases in the matrix that can potentially compete for CO₂ capture have not been discussed as yet; for example, the reactive gases (NH₃, NO_x, and CO) present in industrial flue gas.¹¹⁹  Therefore, future research must consider the other gases that may exist in a real CO₂ capture system.

Although a few modeling studies have been reviewed above, most of them are experimental studies. In order to understand the mechanism and develop LDH- derived compounds as CO₂ adsorbents under real conditions, future research must focus on developing molecular models and force fields based on actual flue gas conditions, which could be a new solution to make economic evaluations of LDH- derived compounds.

LDHs are very interesting and marvelous adsorption materials for CO₂, especially in SEWGS and SESR, which have good prospects for economic developments and social effects. In this current chapter, the research progress of LDHs as CO₂ capture materials has been thoroughly reviewed. The contents mainly consist of seven parts, including: (1) the influence of chemical composition (substituting divalent/trivalent cations or inter-layer anions), (2) the influence of synthetic conditions and methods (including tuning the pH and temperature as well as the different synthetic methods: co-precipitation, hydrothermal, urea hydrolysis, ion exchange methods etc.), (3) the LDH-based composite (including LDH/GO, LDH/CNT, LDH/Al etc.), (4) the influence of doping with alkali metals (including K₂CO₃, KNO₃ and NaNO₃ etc.), (5) the influence of other co-existing gases, (6) the adsorption mechanism and kinetics studies, and (7) the techno-economic assessment etc. It is indicated that chemical composition, synthetic conditions and methods influence CO₂ capture capacities. Furthermore, LDH-based composites not only improved CO₂ capture capacity, but also enhanced the mechanical strength. Doping alkali metals on LDHs is a vital way to increase the adsorption capacity of LDHs. In addition, the presence of water in the feed gas had a positive effect on the CO₂ adsorption capacity, whereas the presence of SO₂ had a negative effect. Furthermore, the adsorption mechanism and kinetics were discussed. Finally, the techno-economic assessment of the SEWGS process was explored.

All results have proved that LDHs are very promising and necessary to integrate CO₂ adsorbents into current operating systems, either for decreasing the energy penalty or for capturing CO₂.

This work was supported by the National Natural Science Foundation of China (51622801, 51572029, and 51308045), Beijing Excellent Young Scholar (2015000026833ZK11), and the Scientific Researching Fund Projects of Yunnan Provincial Department of Education (2017ZZX137).