Among the alkaline and/or alkaline-earth oxides, various lithium, sodium, potassium, calcium and magnesium ceramics have been proposed for CO2 capture through adsorption and chemisorption processes [1-20]. These materials can be classified into two large groups: dense and porous ceram­ics. Dense ceramics mainly trap CO2 chemically: the CO2 is chemisorbed. Among these ceramics, CaO is the most studied one. It presents very in­teresting sorption capacities at high temperatures (T > 600 °C). In addition to this material, alkaline ceramic oxides have been considered as possible captors, mostly lithium and sodium based ceramics (Li5AlO4 and Na2ZrO3, for example). In these cases, one of the most interesting properties is re­lated to the wide temperature range in which some of these ceramics trap CO2 (between 150 and 800 °C), as well as their high CO2 capture capacity.

In these ceramics, the CO2 capture occurs chemically, through a che­misorption process. At a micrometric scale, a general reaction mechanism has been proposed, where the following steps have been established: Ini­tially, CO2 reacts at the surface of the particles, producing the respective alkaline or alkaline-earth carbonate and in some cases different secondary phases. Some examples are:

FIGURE 1: Theoretical CO2 capture capacities for different alkaline and alkaline-earth ceramics. In the Li8SiO6 (labeled as *) and Li4SiO4 (labeled as +), the maximum capacity can depend on the CO2 moles captured in each different phase formed (Li8SiO6 + CO2 ^ Li4SiO4 + CO2 ^ Li2S2O3 + Li2CO3).

Li5AlO4 + 2CO2 ^ 2Li2CO3 + LiAlO2 (1)

Na2ZrO3 + CO2 ^ Na2CO3 + ZrO2 (2)

CaO + CO2 ^ CaCO3 (3)

The above reactions show that surface products can be composed of carbonates, but as well they can contain metal oxides or other alkaline/al­kaline-earth ceramics. The presence of these secondary phases can modify (improve or reduce) the diffusion processes described below [1].

Once the external carbonate shell is formed, different diffusion mecha­nisms have to be activated in order to continue the CO2 chemisorption, through the particle bulk. Some of the diffusion processes correspond to the CO2 diffusion through the mesoporous external carbonate shell, and some others such as the intercrystalline and grain boundary diffusion pro­cesses [1, 18, 21].

Figure 1 shows the theoretical CO2 chemisorption capacities (mmol of CO2 per gram of ceramic) for the most studied alkaline and alkaline-earth ceramics. As it can be seen, metal oxides (Li2O, MgO and CaO) are among the materials with the best CO2 capture capacities. Nevertheless, Li2O and MgO have not been really considered as possible options due to reactivity and kinetics factors, respectively. On the contrary, CaO is one of the most promising alkaline-earth based materials, with possible real industrial ap­plications. Other interesting materials are ceramics with lithium or sodium phases, which present better thermal stabilities and volume variations than CaO. In addition, the sodium phases may present another advantage if the CO2 capture is produced in the presence of steam. Under these condi­tions the sodium phases may produce sodium bicarbonate (NaHCO3) as the carbonated phase, which is twice the amount of CO2 could be trapped in comparison to the Na2CO3 product.

Other ceramics containing alkaline-earth metals are the layered double hydroxides (LDH) or hydrotalcite-like compounds (HTLc). LDHs, also called anionic clays due to their layered structure and structural resem­blance to a kind of naturally-occurring clay mineral. These materials are a family of anionic clays that have received much attention in the past decades because of their numerous applications in many different fields, such as antacids, PVC additives, flame retardants and more recently for drug delivery systems and as solid sorbents of gaseous pollutants [22-24]. The LDH structure is based on positively charged brucite like [Mg(OH)2] layers that consist of divalent cations surrounded octahedrally by hydrox­ide ions. These octahedral units form infinite layers by edge sharing [25]. Due to the fact that certain fraction of the divalent cations can be substi­tuted by trivalent cations at the centers of octahedral sites, an excess of positive charge is promoted. The excess of positive charge in the main lay­ers of LDHs is compensated by the intercalation of anions in the hydrated interlayer space, to form the three-dimensional structure. These materials have relatively weak bonds between the interlayer and the sheet, so they exhibit excellent ability to capture organic or inorganic anions. The ma­terials are easy to synthesize by several methods such as co-precipitation, rehydration-reconstruction, ion exchange, hydrothermal, urea hydrolysis and sol gel, although not always as a pure phase [26].

The LDH materials are represented by the general formula: [M1-xn MxnI(OH)2]x+[Am-]x/mmH2O where Mn and MnI are divalent (Mg2+, Ni2+, Zn2+, Cu2+, etc.) and trivalent cations (Al3+, Fe3+, Cr3+, etc.), respec­tively, and Am — is a charge compensating anion such as CO32-, SO42-, NO3-, Cl-, OH-, where x is equal to the molar ratio of [MIn/(Mn + MIn)]. Its value is commonly between 0.2 and 0.33, i. e., the Mn/MnI molar ratio is in the range of 4 — 2 [25], but this is not a limitation ratio and it depends on the Mn and MnI composition [27-29].

Among various CO2 mesoporous adsorbents, LDH-base materials have been identified as suitable materials for CO2 sorption at moderate tempera­tures (T < 400 °C) [30-46] due to their properties such as large surface area, high anion exchange capacity (2-3 meq/g) and good thermal stability [37-40]. The LDH materials themselves do not possess any basic sites. For that reason, it is preferred to use their derived mixed oxides, formed by the thermal decomposition of LDH, which do exhibit interesting basic properties. Thermal decomposition of the material occurs in three stages, first at temperatures lower than 200 °C, at which the dehydration of su­perficial and interlayer water molecules takes place on the material. Then the second decomposition stage takes place in the range of 300-400 °C, at which the structure collapses due to a partial dehydroxylation process, typically associated with both the decomposition of Al-OH and the Mg — OH hydroxides. During dehydroxylation, changes occur in the structure. A portion of the trivalent cations of the brucite like layers migrates to the in­terlaminar region, allowing the preservation of the laminar characteristics of the material [41]. Finally, the total decomposition of the material occurs at temperatures higher than 400 °C, when the decarbonation process is completed [42].

Once the temperature reaches about 400 °C, LDH forms a three-dimen­sional network of compact oxygen with a disordered distribution of cat­ions in the interstices, where the cations M+3 are tetrahedrally coordinated (interlayer region) and M+2 are octahedrally coordinated. The compres­sive-expansion stresses associated with the formation of the amorphous three-dimensional networks and their connection to the octahedral layer increases the surface area and pore volume, which can help improve the storage capacity properties, for example for gas sorption related applica­tions, besides decreasing the ability of the Mg+2 cation to favor physisorp — tion instead of chemisorption [30, 42]. For instance, the thermal evolution of the Mg/Al-CO3 LDH structure is considered to be crucial in determin­ing the CO2 adsorption capacity, so there are several studies about this issue [42-44].

Reddy et al. [43] studied the effect of the calcination temperature on the adsorptive capacity of the Mg/Al-CO3 LDH. They found out that the best properties were obtained at calcination temperature of 400 °C, which they attributed to the obtaining of a combination of surface area and the availability of the active basic sites. Actually, at this temperature the mate­rial is still amorphous, which allows having a relatively high surface area. Therefore, there is a high number of exposed basic sites, allowing the re­versible CO2 adsorption according to the following reaction:

Mg-O + CO2 ^ Mg-O… CO2 (ad) (4)

However, if the LDH is calcined under 500 °C, the material is able to transform back to the original LDH structure when it is exposed to a carbonate solution or another anionic containing solution. Finally, if the sample is heated to temperatures above 500 °C, the structural changes become irreversible because of the spinel phase formation [37].

As mentioned, the mixed oxides derived from the LDH calcination possess some interesting characteristics such as high specific surface area, excess of positive charge that needs to be compensated, basic sites and thermal stability at elevated temperatures (200 — 400 °C). Besides these aspects, it is important to consider the advantage of acid-base interactions on the CO2 sorption applications, where acidic CO2 molecules interact with the basic sites on the derived oxide. These characteristics make the

LDH-materials acceptable CO2 captors [43, 45]. However, the CO2 ad­sorption capacity of this material is low compared with other ceramic sor­bents; reaching mean values smaller than 0.1 mmol/g [46]. Nevertheless, many studies suggest that the adsorption capacity of LDH materials can be improved by modifying a factor set such as: composition, improvement of the material’s basicity and contaminant gas stream composition [30-32, 36, 41-45, 47-59].

As previously mentioned, Reddy et al. [43] studied the influence of the calcination temperature of LDHs on their CO2 capture properties. The Mg3/Al1-CO3 material was calcined at different temperatures ranging from 200 to 600 °C. The results showed that when the calcination temperatures are under 400 °C, LDH is considered to be dehydrated and materials still keep the layered structure intact, wherein the CO32- ions are occupying the basic sites. The obtained samples calcined at 400 °C have the maximum BET surface area of 167 m2/g compared with samples calcined at lower temperatures. Moreover, during the calcination of the LDH at higher tem­peratures (T > 500 °C), most of the CO32- decompose to release some basic sites for CO2 adsorption. However, the final amount of basic sites decreas­es with the subsequent crystallization of the MgO and spinel (MgAl2O4). Hence, LDH materials obtained at 400 °C have the highest surface area and the maximum quantities of active basic sites exposed. Because of these characteristics, they achieved a total sorption capacity of 0.5 mmol/g [43]. The same researchers observed that 88% of the captured gas can be desorbed and during the material regeneration 98% of the original weight is gained. This is another important property of LDH materials in high temperature CO2 separation applications as described later..

As mentioned, the thermal evolution of the layered structure has a great influence on the CO2 capture. The loss of superficial interlayer water occurs at 200 °C. Then at temperatures between 300 and 400 °C the layer decomposition begins, resulting in an amorphous 3D network with the highest surface area [30], so the adsorption temperature improves the CO2 capture in the order of 400 > 300 > 20 >200 °C [41-42, 47, 52]

Several researchers have investigated a set of different factors to im­prove the CO2 sorption capacity. Yong et al. [47, 48] studied the factors which influence the CO2 capture in LDH materials, such as aluminum content, water content and heat treatment temperature. Regarding the M/

Al-CO3 LDHs (M = Mg, Ni, Co, Cu or Zn), the best CO2 sorption capacity was obtained for the Mg/Al materials degassed at 400 °C and with adsorp­tion conditions of 25 °C. In general, the sorption capacity follows the trend Ni > Mg > Co > Cu = Zn. However, when the degassed temperature is in­creased, the trend is modified to Mg (400 °C) > Co (300°C) > Ni (350°C) > Cu (300°C) >Zn (200°C). These results show that Mg/Al-CO3 is the best composition at the degassing temperature of 400 °C [47]. At this tempera­ture, the material consists of an amorphous phase with optimal properties for use as CO2 captor [42]. Also, the influence of Al+3 has been studied as a trivalent cation at 25 and 300 °C adsorption temperatures, by Yong [41] and Yamamoto [49] respectively. Both samples were degassed at 300 °C and the results showed that the CO2 capture is influenced by the adsorption temperature. At a temperature of 25 °C, the maximum adsorption was 0.41 mmol/g with an Mg/Al ratio equal to 1.5 (x = 0.375) [41] and at 300 °C the amount of CO2 adsorbed was 1.5 mmol/g for a cation ratio of 1.66 (x = 0.4) [49]. The differences between the two capacities can be attributed to the Al content differences. The Al incorporation in the structure has two functions: 1) to increase the charge density on the brucite-like sheet; and 2) to reduce the interlaminar distance and the number of sites with high resistance to CO2 adsorption [48].

On the other hand, Qian et al. [50] studied the effect of the charge compensation anions (A — = CO3-2, NO3-1, Cl-, SO4-2 and HCO3-1) on the structural properties and CO2 adsorption capacity of Mg/Al-A — (molar ra­tio equal to 3). Despite all of the prepared LDH materials showed the typical XRD patterns of LDH materials, slight structural and microstruc­tural differences were observed. In fact, the interlayer distance changed by varying the interlayer anions due to their difference in sizes and carried charges. These differences affect the morphology and the BET surface area of both fresh and heat-treated LDH materials. Additionally, thermal treatments were performed in order to optimize the adsorption capacity of these materials. The optimal temperature treatment was established for each Mg/Al-A — based on the surface area of each calcined LDH. Then the CO2 adsorption capacities of calcined LDH were tested at 200 °C. Mg3/ Al1-CO3 showed the highest CO2 adsorption capacity (0.53 mmol/g). This value was much higher than those obtained for calcined Mg3/Al1-NO3 > Mg3/Al1-HCO3, Mg3Al1-Cl, and Mg3/Al1-SO4 ( ~ 0.1 mmol/g). The results indicated that BET surface area of calcined LDHs seems be the main pa­rameter that determines the CO2 adsorption capacity because the Mg-O active basic site [43, 45].

It has been demonstrated that the quasi-amorphous phase obtained by the thermal treatment of LDH at the lowest possible temperature has the highest CO2 capture capacity. This finding is in line with the fact that high calcination temperature can decrease the number of active Mg-O sites due to the formation of crystal MgO [51].

Yong [41] and Yamamoto [49] investigated the influence of the sev­eral types of anions. The results suggested that the amounts CO2 capture decrease as a function of the anion size, which promotes a larger inter­layer spacing and the higher charge: Fe(CN)64- (1.5 mmol/g) > CO32- (0.5 mmol/g) > NO3- (0.4 mmol/g) > OH — (0.4-0.25 mmol/g). The reason is that Fe(CN)64- and CO32-, because they have more void space in the interlayer due size, and are able to accommodate higher CO2 quantities. Calcined layered double hydroxide derivatives have shown great potential for high temperature CO2 separation from flue gases. However, the presence of SOx and H2O from flue gases can strongly affect CO2 adsorption capac­ity and regeneration of hydrotalcite-like compounds. Flue gases emitted from power stations contain considerable amounts of water in the form of steam. The percentage of water found in the flue gas emitted from dif­ferent sources varies between 7 and 22%, with the emissions from brown coal combustion having the highest water content [45]. For many other gas adsorption sorbents, steam generally has a negative effect on the adsorp­tion performance because of competition for basic sites between CO2 and H2O. However, the presence of water or steam seems to be favorable for the adsorption capacity onto LDH [31,43,53,54]. This fact is the result of the increasing potential energy that is able to further activate basic sites, possibly by maintaining the hydroxyl concentration of the surface mate­rial and/or preventing site poisoning through carbonate or coke deposition [31]. An example of the above was reported by Yong et al. [47], who found that water or steam can increase the adsorption capacity of CO2 by about 25%, from 0.4 mmol/g to 0.5 mmol/g.

Ding et al. [31] studied CO2 adsorption at higher temperatures (480 °C) under conditions for steam reforming of methane. They found an ad­sorption capacity of 0.58 mmol/g, which was independent of water vapor content in the feed. In turn, Reddy et al. [45] investigated calcined LDHs’ sorption performance influenced by CO2 wet-gas streams. LDH samples were calcined at 400 °C [43] before measuring CO2 sorption at 200 °C. The gas streams used were CO2, CO2 + H2O, flue gas (14% CO2, 4% O2 and 82 % N2) +12% H2O.

For a pure CO2 dry sorption, the maximum weight gain was 2.72% (~0.61 mmol/g) after 60 min, whereas the wet adsorption increased the weight of the calcined LDHs to 4.81%, showing an additional 2.09%, where He and He + H2O were used to remove the H2O water capture. The results showed that the helium has virtually no significant sorption affin­ity for the material, whereas the water-sorption profile of it clearly indi­cates a water weight gain of 1.67%, i. e., the gain was 0.1mmol/g due to steam presence, showing that water has a positive effect, shifting the CO2 sorption by 0.42% as compared to dry CO2 sorption. Also, these results revealed that in all cases about 70% sorption occurs during the first 5 min and reaches equilibrium after around 30 min.

To determine the influence of CO2, Reddy et al. [43] tested a sample in both, wet and dry CO2 stream conditions. The experiments showed that the same quantity of CO2 can be trapped for the solid sorbent after two hours. The presence of water in the stream only affects the kinetics of the process. This result is in agreement with that reported by Ding et al. [31]. On the other hand, the results of the material tested suggest that the fact the CO2 capture from flue gas was higher than in a pure stream of CO2 might have been because the polluted gas was diluted in the stream. The presence of the water does not enhance de CO2 capture; the maximum CO2 adsorbed was 0.9 mmol/g. The differences between Reddy et al. results and the pre­viously mentioned studies can be caused by the use of uncalcined LDHs, which already contain an — OH network.

To apply these materials in industrial processes, it is important to know the times during which each sorbent material can be used. Tests of the cy- clability in LDH materials disclose that as function of the temperature the CO2 capture time can vary. This can be attributed to CO2 chemisorbed dur­ing each cycle [54] and/or to the formation of spinel-based aluminas, such as y-Al2O3 (at temperatures higher than 400 °C). Hibino et al. [52] found that the carbonate content, acting as charge-compensating anion, continu­ously decreases in subsequent calcination-rehydration cycles. Reddy et al.

tested LDH materials during six CO2 adsorption (200 °C)-desorption (300 °C) cycles. The average amount gained was 0.58 mmol/g, whereas 75% of this value is desorbed, reaching desorption equilibrium after the third cycle. This can be attributed to the stabilization of the material phase and basic sites during the temperature swing.

Hufton et al. [54] studied a LDH material during several cycles in dry and wet CO2 flows. As previously discussed, the presence of steam in the flow gas improves the CO2 adsorption. However, after 10 adsorption cy­cles, the capture decreased 45%. The same behavior was observed in the dry gas flow. However, the final capture was similar to the wet gas stream, in agreement with Reddy et al. [43].

Recent studies have demonstrated that K-impregnated LDH or K-im- pregnated mixed oxides have a better CO2 capture capacity due to the addition of K alkaline-earth element that improves the chemical affinity between the acidic CO2 and alkaline surface of the sorbent material [32, 36, 55-56]. Additionally, it has been proposed that K-impregnation re­duces the CO2 diffusion resistance in the material. [57]. Hufton et al. [58] showed that the K-impregnation increases the CO2 capture, but there is an optimal quantity of K to reach the maximum capture. Qiang et al. [50] tested an Mg3/Al1-CO3 (pH = 10) impregnated with 20 wt.% K2CO3. The CO2 adsorption capacity was increased between 0.81 and 0.85 mmol/g in the temperature range of 300-350 °C. This adsorption capacity is adequate for application in water gas shift reactions (WGS).

Lee et al. [59] tested the behavior of three commercial LDHs impreg­nated with K (K2CO3/LDH ratio between 0 and 1). Three Mg/Al-CO3 LDH with different contents of magnesium were used. Results indicated that the sorption capacity of the LDH is improved by about 10 times with the optimal K2CO3 additions. Additionally, it was observed that impregnation is not the only factor that influences the adsorption but the composition too. The best value was obtained when the content of divalent cation was reduced and therefore, the material had a composition with the maximum trivalent cation content. The CO2 adsorption capacity improved from 0.1mmol/g to 0.95mmol/g with K2CO3/LDH weight ratio equal to 0.35 at 400 °C. After determining the optimal alkaline source/LDH ratio, a set of samples was evaluated as a function of the temperature and the results showed a maximum of 1.35 mmol/g, at 50 °C. In the impregnated materi­als, CO2 chemisorption can occur and the sorbed CO2 can be further stored as metal carbonate forms.

Other alkaline elements can be used to improve the sorption capac­ity of materials. Martunus et al. [46] studied the impregnation of LDH with Na and K. The LDH samples were thermally treated at 450 °C for 5 min then calcined samples were re-crystallized in K2CO3 and Na2CO3 (1 M) solutions. The re-crystallized materials were tested as CO2 captors and the capture was maximum with LDH-Na (0.688 mmol/g) > LDH — K (0.575 mmol/g) at 350 °C after five cycles. Finally, the re-crystallized material with the highest capture was calcined at 650 °C for 4 h and re­crystallized with a solution containing the appropriate quantities of K and Na to achieve alkaline metal loading up to 20%. When the sample was Impregnated with additional K and Na at 18.4% and 1.6%, respectively, the adsorption capacity rose from 0.688 to 1.21 mmol/g. This capacity increase was achievable despite the relatively low BET surface area, equal to 124 m2/g.

Other alkaline elements such as cesium have been studied as reinforce­ment. Oliveira et al. [55] tested commercial Mg1/Al1-CO3 and Mg6Al1-CO3 impregnated with K and Cs carbonates. The materials were evaluated in the presence of steam (26% v/v water content) gas at different temperatures (306, 403 and 510 °C) at 0.4 bar of CO2 partial pressure (total pressure 2 bar). The LDH with the highest sorption capacity was Mg1/Al1-CO3-K with 0.76 mmol/g at 403 °C. Among the Cs impregnated samples, the Mg6Al1-CO3-Cs presented the highest capacity with 0.41 mmol/g, while the commercial LDH samples presented CO2 sorption capacities around 0.1 mmol/g.

The results suggest the existence of a sorption mechanism combining physical adsorption and chemical reaction. First the maximum physical adsorption is reached, then the chemisorption begins, but there is an op­timal temperature. If the temperature is too low, the chemisorption does not happen, but with higher temperatures the loss of porosity impedes the contact of CO2 molecules with active basic sites promoted by the alkaline element addition.

These results suggest there is an optimum amount of K2CO3 to impreg­nate the LDH that achieves a balance between the increase in the basicity of the sorbent material and its reduction of surface area, associated with

CO2 capture capacity. The influence of potassium is currently not clear and the relevant research is still ongoing. Finally, CO2 adsorption capacity on the synthesized 20 wt.% K2CO3/Mg3/Al1-CO3 (pH = 10) probably could be further increased in the presence of steam.