Alkaline and Alkaline-Earth Ceramic Oxides for CO2 Capture, Separation and Subsequent Catalytic Chemical Conversion

The amounts of anthropogenic carbon dioxide (CO2) in the atmosphere have been raised dramatically, mainly due to the combustion of different carbonaceous materials used in energy production, transport and other important industries such as cement production, iron and steelmaking. To solve or at least mitigate this environmental problem, several alternatives have been proposed. A promising alternative for reducing the CO2 emissions is the separation and/ or capture and concentration of the gas and its subsequent chemical transformation. In that sense, a variety of materials have been tested containing alkaline and/or alkaline-earth oxide ceramics and have been found to be good options.


Introduction
The amounts of anthropogenic carbon dioxide (CO 2 ) in the atmosphere have been raised dramatically, mainly due to the combustion of different carbonaceous materials used in energy production, transport and other important industries such as cement production, iron and steelmaking. To solve or at least mitigate this environmental problem, several alternatives have been proposed. A promising alternative for reducing the CO 2 emissions is the separation and/ or capture and concentration of the gas and its subsequent chemical transformation. In that sense, a variety of materials have been tested containing alkaline and/or alkaline-earth oxide ceramics and have been found to be good options.
The aforementioned ceramics are able to selectively trap CO 2 under different conditions of temperature, pressure, humidity and gas mixture composition. The influence of those factors on the CO 2 capture (physically or chemically) seems to promote different sorption mechanisms, which depend on the material's chemical composition and the sorption conditions used. Actually, this capture performance suggests the feasibility of these kinds of solid for being used with different capture technologies and processes, such as: pressure swing adsorption (PSA), vacuum swing adsorption (VSP), temperature swing adsorption (TSA) and water gas shift reaction (WGSR). Therefore, the fundamental study regarding this matter can help to elucidate the whole phenomena in order to enhance the sorbents' properties.

CO 2 capture by different alkaline and alkaline-earth ceramics
Among the alkaline and/or alkaline-earth oxides, various lithium, sodium, potassium, calcium and magnesium ceramics have been proposed for CO 2 capture through adsorption and chemisorption processes [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. These materials can be classified into two large groups: dense and porous ceramics. Dense ceramics mainly trap CO 2 chemically: the CO 2 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 (Li 5 AlO 4 and Na 2 ZrO 3 , 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 CO 2 (between 150 and 800 °C), as well as their high CO 2 capture capacity.
In these ceramics, the CO 2 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, CO 2 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: 5  Na ZrO + CO Na CO + ZrO ¾¾ ® (2) 2 3 CaO + CO CaCO ¾¾ ® 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 CO 2 chemisorption, through the particle bulk. Some of the diffusion processes correspond to the CO 2 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 CO 2 chemisorption capacities (mmol of CO 2 per gram of ceramic) for the most studied alkaline and alkaline-earth ceramics. As it can be seen, metal oxides (Li 2 O, MgO and CaO) are among the materials with the best CO 2 capture capacities. Nevertheless, Li 2 O 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 CO 2 capture is produced in the presence of steam. Under these conditions the sodium phases may produce sodium bicarbonate (NaHCO 3 ) as the carbonated phase, which is twice the amount of CO 2 could be trapped in comparison to the Na 2 CO 3 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][23][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].  + M III )]. Its value is commonly between 0.2 and 0.33, i.e., the M II /M III molar ratio is in the range of 4 -2 [25], but this is not a limitation ratio and it depends on the M II and M III composition [27][28][29].
Among various CO 2 mesoporous adsorbents, LDH-base materials have been identified as suitable materials for CO 2 sorption at moderate temperatures (T ≤ 400 °C) [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] due to their properties such as large surface area, high anion exchange capacity (2-3 meq/g) and good thermal stability [37][38][39][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 threedimensional 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 physisorption instead of chemisorption [30,42]. For instance, the thermal evolution of the Mg/ Al-CO 3 LDH structure is considered to be crucial in determining the CO 2 adsorption capacity, so there are several studies about this issue [42][43][44].
Reddy et al. [43] studied the effect of the calcination temperature on the adsorptive capacity of the Mg/Al-CO 3 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 CO 2 adsorption according to the following reaction: 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 CO 2 sorption applications, where acidic CO 2 molecules interact with the basic sites on the derived oxide. These characteristics make the LDH-materials acceptable CO 2 captors [43,45]. However, the CO 2 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 CO 2 capture properties. The Mg 3 /Al 1 -CO 3 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 CO 3 2-ions are occupying the basic sites. The obtained samples calcined at 400 °C have the maximum BET surface area of 167 m 2 /g compared with samples calcined at lower temperatures. Moreover, during the calcination of the LDH at higher temperatures (T > 500 °C), most of the CO 3 2-decompose to release some basic sites for CO 2 adsorption. However, the final amount of basic sites decreases with the subsequent crystallization of the MgO and spinel (MgAl 2 O 4 ). 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 CO 2 separation applications as described later..
As mentioned, the thermal evolution of the layered structure has a great influence on the CO 2 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 CO 2 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 CO 2 sorption capacity. Yong et al. [47,48] studied the factors which influence the CO 2 capture in LDH materials, such as aluminum content, water content and heat treatment temperature. Regarding the M/Al-CO 3 LDHs (M = Mg, Ni, Co, Cu or Zn), the best CO 2 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-CO 3 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 CO 2 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 CO 2 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 CO 2 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 CO 2 adsorption [48].
On the other hand, Qian et al. [50] studied the effect of the charge compensation anions (A -= CO 3 -2 , NO 3 -1 , Cl -, SO 4 -2 and HCO 3 -1 ) on the structural properties and CO 2 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-Abased on the surface area of each calcined LDH. Then the CO 2 adsorption capacities of calcined LDH were tested at 200 °C. Mg 3 /Al 1 -CO 3 showed the highest CO 2 adsorption capacity (0.53 mmol/g). This value was much higher than those obtained for calcined Mg 3 /Al 1 -NO 3 > Mg 3 /Al 1 -HCO 3 , Mg 3 Al 1 -Cl, and Mg 3 / Al 1 -SO 4 ( ≈ 0.1 mmol/g). The results indicated that BET surface area of calcined LDHs seems be the main parameter that determines the CO 2 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 CO 2 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 CO 2 capture decrease as a function of the anion size, which promotes a larger interlayer spacing and the higher charge: Fe(CN) 6 4-(1.5 mmol/g) > CO 3 2-(0.5 mmol/g) > NO 3 -(0.4 mmol/g) > OH -(0.4-0.25 mmol/g). The reason is that Fe(CN) 6 4-and CO 3 2-, because they have more void space in the interlayer due size, and are able to accommodate higher CO 2 quantities. Calcined layered double hydroxide derivatives have shown great potential for high temperature CO 2 separation from flue gases. However, the presence of SOx and H 2 O from flue gases can strongly affect CO 2 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 CO 2 and H 2 O. 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 CO 2 by about 25%, from 0.4 mmol/g to 0.5 mmol/g.
Ding et al. [31] studied CO 2 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 CO 2 wet-gas streams. LDH samples were calcined at 400 °C [43] before measuring CO 2 sorption at 200 °C. The gas streams used were CO 2 , CO 2 + H 2 O, flue gas (14% CO 2 , 4% O 2 and 82 % N 2 ) +12% H 2 O.
For a pure CO 2 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 + H 2 O were used to remove the H 2 O 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 CO 2 sorption by 0.42% as compared to dry CO 2 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 CO 2 , Reddy et al. [43] tested a sample in both, wet and dry CO 2 stream conditions. The experiments showed that the same quantity of CO 2 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 CO 2 capture from flue gas was higher than in a pure stream of CO 2 might have been because the polluted gas was diluted in the stream. The presence of the water does not enhance de CO 2 capture; the maximum CO 2 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 cyclability in LDH materials disclose that as function of the temperature the CO 2 capture time can vary. This can be attributed to CO 2 chemisorbed during each cycle [54] and/or to the formation of spinel-based aluminas, such as γ-Al 2 O 3 (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 CO 2 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 CO 2 flows. As previously discussed, the presence of steam in the flow gas improves the CO 2 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-impregnated mixed oxides have a better CO 2 capture capacity due to the addition of K alkaline-earth element that improves the chemical affinity between the acidic CO 2 and alkaline surface of the sorbent material [32,36,[55][56]. Additionally, it has been proposed that K-impregnation reduces the CO 2 diffusion resistance in the material. [57]. Hufton et al. [58] showed that the K-impregnation increases the CO 2 capture, but there is an optimal quantity of K to reach the maximum capture. Qiang et al. [50] tested an Mg 3 /Al 1 -CO 3 (pH = 10) impregnated with 20 wt.% K 2 CO 3 . The CO 2 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 (K 2 CO 3 /LDH ratio between 0 and 1). Three Mg/Al-CO 3 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 K 2 CO 3 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 CO 2 adsorption capacity improved from 0.1mmol/g to 0.95mmol/g with K 2 CO 3 /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, CO 2 chemisorption can occur and the sorbed CO 2 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 K 2 CO 3 and Na 2 CO 3 (1 M) solutions. The re-crystallized materials were tested as CO 2 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 re-crystallized 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 m 2 /g.
Other alkaline elements such as cesium have been studied as reinforcement. Oliveira et al. [55] tested commercial Mg 1 /Al 1 -CO 3 and Mg 6 Al 1 -CO 3 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 CO 2 partial pressure (total pressure 2 bar). The LDH with the highest sorption capacity was Mg 1 /Al 1 -CO 3 -K with 0.76 mmol/g at 403 °C. Among the Cs impregnated samples, the Mg 6 Al 1 -CO 3 -Cs presented the highest capacity with 0.41 mmol/g, while the commercial LDH samples presented CO 2 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 CO 2 molecules with active basic sites promoted by the alkaline element addition.
These results suggest there is an optimum amount of K 2 CO 3 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 CO 2 capture capacity. The influence of potassium is currently not clear and the relevant research is still ongoing. Finally, CO 2 adsorption capacity on the synthesized 20 wt.%K 2 CO 3 /Mg 3 /Al 1 -CO 3 (pH = 10) probably could be further increased in the presence of steam.

Ceramic oxide membranes as an alternative for CO 2 separation
Membrane-based processes, related to gas separation and purification, have achieved an important level of development for a variety of industrial applications [60]. Therefore, the use of separation membranes is one of the promising technologies for reducing the emissions of greenhouse gases such as CO 2 . The term membrane is defined as a permselective barrier between two phases, the feed or upstream and permeate or downstream side [61]. This permselective barrier has the property to control the rate of transport of different species from the upstream to the downstream side, causing the concentration or purification of one of the species present in the feed gas mixture.
Membrane-based processes offer the advantage of large scale application to separate CO 2 from a gas mixture. Figure 2 schematizes the process where concentrated CO 2 is selectively separated from flue gas that is mainly composed of nitrogen and carbon dioxide along with other gases such as water vapor, SO x , NO x and methane. Subsequent to the membrane process, concentrated CO 2 obtained at the permeate side can be disposed or used as raw material for the synthesis of several chemicals such as fuel and value-added products [62]. Of course, the rate of transport or permeation properties of a particular gas through a given membrane depend on the nature of the permeant gas, as well as the physical and chemical properties of the membrane.
Inorganic membranes are more thermally and chemically stable and have better mechanical properties than organic polymer membranes; ceramic membranes offer both the advantage of large scale application and potential for pre-and post-combustion CO 2 separation applications, where membranes systems would be operating at elevated temperatures of 300-1000 °C [63].
Inorganic ceramic membranes can be classified as porous and nonporous or dense. These differ from each other not only in their structures but also in the mechanism of permeation. In porous membranes, the transport of species is explained with the pore-flow model, in which permeants are transported by pressure-driven convective flow through the pore network. Separation occurs because one of the permeants is excluded (molecular filtration or sieving) from the pores in the membrane and remains in the retentate while the other permeants move towards the downstream side. On the other hand, in nonporous membranes, separation occurs by solutiondiffusion, in which permeants dissolve in the membrane material and then diffuse through the bulk membrane by a concentration gradient [60].

Porous membranes based on alkaline and alkaline-earth ceramic oxides for CO 2 separation
Among the porous systems for CO 2 separation, both microporous (carbon, silica and zeolite membranes) and modified mesoporous membranes have been reported [63][64].
Zeolites are hydrous crystalline aluminosilicates that exhibit an intracrystalline microporous structure as a result of the particular three-dimensional arrangement of their TO 4 tetrahedral units (T=Si or Al) [65]. Zeolite membranes are commonly prepared as thin films grown on porous alumina supports via hydrothermal synthesis and dry gel conversion methods [66]. Zeolite membranes of different structures have been developed to separate CO 2 from other gases via molecular sieving [67][68][69]. For example, membranes prepared with the 12-member ring faujasite (FAU)-type zeolite show high separation factors of 20-100 for binary gas mixtures of CO 2 /N 2 [69]. In the same sense, T zeolite membranes exhibited very high selectivity, of about 400, for CO 2 /CH 4 and 104 for CO 2 /N 2 . The high selectivity of CO 2 /CH 4 exhibited by T zeolites is due to the small pore size of about 0.41 nm, which is similar in size to the CH 4 molecule but larger than CO 2 [69]. Table 1 shows the kinetic diameter of various molecules that are present in CO 2 containing gas mixtures such as flue and natural gas [70].
As discussed, one of the most important factors controlling permeation through microporous membranes is the restriction imposed by the molecular size of the permeant. However, the transport mechanism in microporous systems is more complex than just size exclusion and the permeation and selectivity properties are also affected by competitive adsorption among perment species that produce differences in mobility [76].
Thus, the diffusion mechanism for gas permeation through microporous membranes can be characterized by two modes: one controlled by adsorption and a second one where diffusion dominates [63]. In the case of adsorption-controlled mode with permeating gases having strong affinity with the membrane, a gas permeation flux equation is obtained by assuming steady-state single gas permeation, a constant diffusivity and a single gas adsorption described by a Langmuir-type adsorption isotherm, as in Eq. (5).
where J is the permeation flux, ϕ is a geometric correction factor that involves both membrane porosity and tortuosity, D c is the corrected diffusivity of the permeating species, L is the membrane thickness, P f and P p represent the feed and permeate pressure respectively and θ f and θ p represent the relative occupancies.
Furthermore, if the adsorption isotherm of the permeating gas is linear (1 >> bP), then flux permeation is described by Eq. (6).
where F is the permeance and K = q s b is the adsorption equilibrium constant. Therefore, from Eq. (5) it can be concluded that permeance is determined by both diffusivity (Dc) and adsorption (K). Based on the above, an interesting option to enhance membrane properties is to intercalate zeolite membranes with alkaline and alkaline-earth cations. Zeolite intercalation can enhance the separation between CO 2 and other molecules such as N 2 by promoting preferential CO 2 adsorption [63,77]. It is well known that zeolites show affinity for polar molecules, like CO 2 , due to the strong interaction of their quadrupole moment with the electric field of the zeolite framework. In this sense, the adsorption properties of zeolites can be enhanced by the inclusion of exchangeable cations within the cavities of zeolites where the adsorbent-adsorbate interactions are influenced by the basicity and electric field of the adsorbent cavities [78][79][80]. Lara-Medina et al. [77] carried out separation studies of CO 2 and N 2 with a silicalite-1 zeolite membrane prepared via hydrothermal synthesis and subsequently modified by using lithium solutions in order to promote preferential CO 2 adsorption and diffusion. CO 2 /N 2 separation factor increases from 1.46 up to 6 at 25 psi and 400 •C after lithium modification. An et al. [79] studied a series of membranes prepared starting from natural Clinoptilolite zeolite rocks. Disk membranes were obtained by cutting and polishing of the original minerals, which were subsequently chemically treated with aqueous solutions containing Li, Na, Sr or Ba ions. Ionic exchanged membranes showed better permeation properties due to the presence of the extra framework cations.
Although zeolite membranes offer certain advantages in comparison with polymer membranes, such as chemical stability, the main issues are related to the selectivity decrease as a function of the permeation temperature. This is explained in terms of the contribution of the adsorption to the separation, which decreases sharply as temperature increases. At high temperature, physical adsorption becomes negligible and permeation is mainly controlled by diffusion [63,76]. Additionally, due to the fact that CO 2 and N 2 molecules have similar sizes (Table 1), the difference in diffusivity is not a strong controlling factor in determining selectivity.
Modified γ-Al 2 O 3 mesoporous membranes have been also reported as a means for CO 2 separation [64]. Transport mechanisms in porous membranes have the contribution of different regimes. An overview of the different mechanisms is given in Table 2.
Affects the total flow resistance of the membrane system.

Molecular diffusion >10 μm
Knudsen diffusion 2 -100 nm Occurs when the mean free path of the molecule is much larger than pore radius of the membrane.
Shows selectivity based on molecular weights.

Surface diffusion
Shows selectivity due to interaction of molecules with membrane walls.
Depending on the particular system, permeability of a membrane can involve several transport mechanisms that take place simultaneously. Considering no membrane defects and pore sizes in the range of 2.5-5 nm, γ-Al 2 O 3 based membranes theoretically have two transport regimes: Knudsen diffusion and surface diffusion. Eq. (7) describes the permeability of a membrane by taking into consideration the Knudsen and surface diffusion.
where r is the mean pore radius, μ is a shape factor, R is the universal gas constant, T is the temperature, P is the mean pressure, M is molar mass of the gas, A o is the surface area occupied by a molecule, D s is the surface diffusion coefficient, N av is Avogadro's constant and X s is the percentage of occupied surface compared with a monolayer [81].
For the cases when Knudsen diffusion dominates, selectivity can be correlated to the molecular weights of the permeating gases by the so called Graham's law of diffusion, which establishes that the transport rate of any gas is inversely proportional to the square root of its molecular weight. The CO 2 /N 2 separation factor considering pure Knudsen diffusion is given by Eq. (8) and has a value of just 0.8. Therefore, Eq. (8) clearly shows that separation via Knudsen is limited for systems where species are of similar molecular weight.
Based on the aforesaid, CO 2 /N 2 separation factor can be better enhanced by promoting the surface diffusion mechanism (second term on the right hand side of Eq. (7)). Surface diffusion involves the adsorption of gas molecules on the surface of the pore and subsequent diffusion of the adsorbed species along the surface by a concentration gradient. Then separation properties of a membrane can be improved by generating such an interaction between one component of the feed gas mixture with the membrane; one option being via a chemical modification.
Cho et al [81] prepared a series of thin (2-5 μm thickness) γ-Al 2 O 3 and CaO-or SiO 2 -modified γ-Al 2 O 3 membranes for CO 2 separation at temperatures between 25 and 400 °C. Impregnation of membranes with SiO 2 or alkaline CaO was done in order to improve the CO 2 /N 2 selectivity by promoting adsorption between CO 2 gas molecules and the membrane pore wall. Although this kind of chemical modification of the membrane surface and the pore walls is able to activate the surface diffusion mechanism, an interesting behavior was observed. The CO 2 /N 2 separation factor increased from 1.0 to 1.38 at 25 °C after modification of the γ-Al 2 O 3 with SiO 2 . On the other hand, CaO impregnated membranes showed a separation factor of 0.98, which is even lower than that of the unmodified γ-Al 2 O 3. The same behavior has been reported by Uhlhorn et al. [82][83]. They reported MgO modified γ-Al 2 O 3 membranes which did not show significant enhancement in the permeation and selectivity properties as a result of the modification process. This fact was explained in terms of the surface diffusion mechanisms. As discussed, it is expected that physicochemical modifications of the membrane can enhance preferential adsorption of the gas species in the feed. Impregnations with alkaline oxide such as calcium oxide or magnesia on the γ-alumina surface give more strong base sites than those promoted by silica. Therefore, it promotes a strong bonding of CO 2 on the alumina surface, causing CO 2 molecules to lose mobility, resulting in a smaller contribution of surface diffusion to the total transport mechanism.
There is another kind of membrane where alkaline and alkaline-earth ceramic oxides have been used for the fabrication of CO 2 permselective membranes. In these cases ceramic materials were chosen because of their well-known properties of physisorption of CO 2 at low and intermediated temperatures.
Kusakabe et al. [84] prepared both pure and modified BaTiO 3 CO 2 permselective membranes via the alkoxide based sol-gel method; impregnation and calcination at 600 °C. In order to establish the effects of CO 2 partial pressure, temperature and influence of the secondary oxide presence (CuO, MgO or La 2 O) on the CO 2 adsorption properties of the membranes, pure and modified barium titanate powders were first evaluated by thermogravimetry and chromatography techniques. Dynamic CO 2 absorption was evaluated by applying the impulse response method, wherein the BaTiO 3 powder was packed in a separation column. The results suggested that the CO 2 molecules adsorbed on the BaTiO 3 powder are mobile at temperatures about 500 °C. Therefore, this membrane exhibits CO 2 permeation due to surface diffusion mechanism. Even though the prepared membranes showed selectivity, the Knudsen diffusion still has an important contribution to the gas transport due to the presence of membrane defects. The maximum separation factor of CO 2 /N 2 through the membranes was estimated as 1.2. Therefore, further improvement of the permeation properties of this kind of membrane requires obtaining pinhole-free membranes.
Based on the same criteria, Nomura et al. [85] prepared Li 4 SiO 4 -based thin membranes on porous alumina supports. Membranes were obtained by the thermal treatment of different silica containing porous materials (Silicalite-1 and mesoporous silica) impregnated with lithium compounds. The authors called this method solid conversion. The use of different silica porous sources was proposed in order to enhance the reaction rate of Si and Li on the porous support at relatively low temperature, avoiding the reaction between the Li and alumina support itself. In the case of Silicalite-1 (MFI zeolite), a zeolite thin film was first prepared on the support by following the dry gel conversion technique. Then, the prepared Silicalite-1 layer was impregnated via dipping into a slurry containing lithium and silica fumed reactants (Li:Si = 4:1) and subsequently into a Li 2 CO 3 -K 2 CO 3 slurry. The membrane was finally calcined at 600°C for 2 h. It is believed that carbonate melts to fill the cracks and the pinholes of the Li 4 SiO 4 formed membrane. A similar procedure of coating and calcination was carried out to prepare high quality membranes starting from mesoporous silica sources with pore sizes of 1.8-12.8 nm. Precursors react to form a Li 4 SiO 4 membrane of 2-5 μm thickness that exhibits an N 2 permeance of 1.8 x 10 -9 mol m -2 s -1 Pa -1 at 400 °C. This suggests there are no big defects after impregnation of the membrane with the binary mixture of Li 2 CO 3 -K 2 CO 3 carbonate. Due to the fact that the membrane operates in a rich CO 2 atmosphere, carbonates do not decompose even at temperatures of 600 °C. The maximum CO 2 /N 2 permeance ratio was 0.85. The separation factor was higher than that for the Knudsen diffusion. Therefore, it can be conclude that Li 4 SiO 4 layer was selective to CO 2 over N 2 at high temperature of 600 °C.
Nomura [86] reported a two -stage approach for the preparation of Li 4 SiO 4 -CO 2 selective membranes that involves the fabrication of a supported Li 4 SiO 4 membrane and its subsequent modification by using a chemical vapor deposition (CVD) method. First, for the preparation of a thin Li 4 SiO 4 membrane the so called solid conversion method described before was used, which is based on the reaction between a porous silica source and a lithium containing solution coated on a porous alumina membrane support. Although the formed membranes showed certain selectivity due to the preferential adsorption of CO 2 over N 2 , the presence of pinholes and cracks caused low separation factors. Therefore, the membrane defects were fixed by using the counter diffusion CVD method to form a silica coating that fills the gaps between the lithium orthosilicate particles that make up the membrane. N 2 permeance was reduced about three orders of magnitude after CVD modification. Nitrogen permeance before and after the CVD treatment was 3.4 x 10 -6 mol m -2 s -1 Pa -1 and 1.2 x 10 -9 mol m -2 s -1 Pa -1 respectively. In the same sense, the CO 2 /N 2 permeance rate increased from 0.7 to 1.2 at 600 °C. Some issues related with this system are the chemical and structural stability of the membranes observed during the permeation tests at elevated temperature. The membranes were broken when permeation tests were carried out at temperatures higher than 700 °C, with the consequent decrease in the CO 2 /N 2 selectivity. The aforesaid is the result of the CO 2 chemisorption on the membrane. Lithium orthosilicate reacts with CO 2 to form lithium carbonate and lithium metasilicate (Li 2 SiO 3 ) as products, as indicated by Eq. (9). Li SiO + CO Li CO + Li SiO ¬¾ ® (9) Thermodynamically, this reaction is prone to occur at temperatures between room temperature and about 700 °C. However, experimentally it has been observed that reaction kinetics sharply increase above 550 °C. At these temperatures, the formation of carbonates involves an important change in volume that ends in the membrane's rupture.
Therefore, one of the issues related to the development of this kind of inorganic membrane is the thermochemical stability. Due to reactivity of alkaline and alkaline-earth ceramic oxides with CO 2 to form carbonates, not only preferential adsorption of CO 2 molecules over N 2 occurs, but CO 2 chemisorption and reaction.. Therefore, it is mandatory to establish the operational temperature within a range where CO 2 selective adsorption on the membrane layer promotes the separation process without reaction.

Nonporous membranes based on alkaline and alkaline-earth ceramic oxides for CO 2 separation
Some researchers have proposed the use of alkaline and alkaline-earth ceramic oxides to prepare membranes that are able to separate CO 2 at high temperatures via a different transport mechanism than those observed on porous membranes. Li 2 ZrO 3 and Li 4 SiO 4 based membranes are examples of the aforesaid. Permselectivity of CO 2 through these membranes takes place not only due to the selective CO 2 adsorption properties of ceramic phases but also via a mechanism of gas separation that involves the transport of CO 3 2-and O 2-ionic species through the electrolytes (carbonate-metal oxide) phases formed by the reaction of the membrane with the CO 2 [87][88][89].
Kawamura et al. [87] fabricated and characterized a membrane for CO 2 separation at high temperatures. The membrane was made of lithium zirconate (Li 2 ZrO 3 ), an alkaline ceramic oxide that reacts with CO 2 to produced Li 2 CO 3 and ZrO 2 . These two reaction products are electrolyte materials produced in-situ when the membrane is exposed to the rich carbon dioxide atmosphere. The electrolytes formed thus are capable to transport both CO 2 and O 2 across the membrane via a dual ion conduction mechanism. The prepared membrane exhibited a separation factor of 4.9 between CO 2 and CH 4 gas molecules at a temperature of 600 °C. The obtained separation factor is higher than the Knudsen diffusion limit, 0.6. Therefore, the results clearly suggest the potential use of this kind of membrane system for CO 2 separation such as the case of CO 2 removal from natural gas.
Yamaguchi et al. [88] investigated the concept of the dual-ion conduction facilitated mechanism previously observed for the case of Li 2 ZrO 3 membranes by focusing their efforts on the preparation of a CO 2 permselective membrane based on lithium orthosilicate (Li 4 SiO 4 ). The supported membrane was prepared via a dip coating technique by using Li 4 SiO 4 suspensions. The coating process was repeated several times before impregnation of the membrane with a Li 2 CO 3 /K 2 CO 3 carbonate mixture and final sintering at 750 °C. In this membrane system, Li 4 SiO 4 reacts in-situ with CO 2 to form Li 2 CO 3 and Li 2 SiO 3 .
Gas separation studies were performed by using CO 2 /N 2 mixtures as feed gas. The observed CO 2 permeance values were about 1 x 10 -8 mol m -2 s -1 Pa -1 in the temperature range of 525-625°C . The CO 2 /N 2 separation factor was estimated between four and six. Figure 3 shows a scheme of the dual-ion conduction mechanism explained as follows. In the feed side, carbon dioxide dissolves in the material and diffuses as carbonate ions through the molten carbonate electro-lyte due to a concentration gradient. Then, in the downstream side of the membrane, the formation of gaseous CO 2 implies the formation of oxygen ions which must diffuse back to the feed side across the membrane and apparently through the formed Li 2 SiO 3 skeleton to obtain the charge balance. The proposed transport mechanisms supports the higher selectivity values observed in the permeation test for both systems, Li 2 ZrO 3 and Li 4 SiO 4 . Figure 4 shows the separation factor values (CO 2 /N 2 ) obtained for different ceramic membranes described in the present report. The pure Knudsen value is written as baseline and separation factor of nonporous Li 4 SiO 4 for comparison purposes. However, it is important to mention that the oxygen ion diffusion process is not totally clear. Indeed, there is no experimental study regarding the oxygen ionic conductivity properties of Li 2 SiO 3 phase. On the other hand, pure ZrO 2 exhibits poor bulk oxygen ion conductivity. In fact, good conduction properties are observed only in acceptordoped ZrO 2 based materials with oxygen vacancies being the predominant charge carriers [90]. Therefore, oxygen ion conduction through the membrane must be related to different transport paths, such as grain boundaries and interfacial regions formed between the ceramic and molten carbonate on the membrane.
This concept involves the fabrication of nonporous membranes capable of selectively separating CO 2 via its transport, as carbonate ions. Dual phase membranes are made of an oxygen ion conductive porous ceramic phase that hosts a molten carbonate phase. Rui et al. [98] proposed the CO 2 separation by the electrochemical conversion of CO 2 molecules to carbonate ions (CO 3 2-), which are subsequently transported across the membrane. Carbonate ionic species (CO 3 2-) are formed by the surface reaction between CO 2 and oxygen that comes from the ceramic oxide phase (feed side, Eq.(10)) and then transport of CO 3 2-takes place through the molten carbonate.
Once carbonate ions have reached the permeate side, molecular CO 2 is released to the gas phase, delivering O O x species back to the ceramic oxide solid phase. This process takes place due to a chemical gradient of CO 2 in the system ( Figure 5). Here, it is important to emphasize that dual-phase membranes are nonporous and therefore exhibit high separation selectivity as a result of the transport mechanism. Figure 5 also shows the SEM image of the cross section of a ceramic oxide-carbonate membrane prepared by pressing La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ powders and subsequent infiltration of the obtained porous ceramic (bright phase) with carbonate (dark phase). Table 3 summarizes the different studies reported and certain advances that have been achieved so far regarding the dual-phase membrane concept. This table also includes the Li 2 ZrO 3 and Li 4 SiO 4 nonporous membranes previously described. Although the original reports do not clearly explain the operational mechanism [26][27], the dual-phase membrane concept gives a much better idea of the possible phenomenology involved [30,33,36].

Applications of CO 2 permselective ceramic oxide membranes for the design of membrane reactors.
As mentioned, CO 2 can be used as raw material for the synthesis of several chemicals [99]. Moreover, if CO 2 is concentrated or separated by a membrane system exhibiting high CO 2 permeation and permselectivity, this open up the possibility to develop a continuous process of membrane reaction to simultaneously capture and chemically convert CO 2 . For example, if the membrane is able to separate CO 2 at intermediate and even high temperatures, it can be used for the design of a membrane reactor for the production and purification of hydrogen and syngas. Syngas is a gaseous fuel with a main chemical composition of CO, H 2 , CO 2 , and CH 4 . Syngas can be used as feedstock for the synthesis of several other clean fuels such as H 2 , methanol, ethanol, diesel and other hydrocarbons synthesized via the Fischer-Tropsch process [100][101][102][103][104].
Among the different processes for the synthesis of syngas and hydrogen, CO 2 methane reforming Eq. (11) and the water-gas shift reaction (WGS) Eq. (12) are the most promising options.   Figure 6 schematizes the membrane reactor concept considering the two reactions described above. Figure 6A shows a membrane reactor for dry reforming of methane to produce syngas at temperatures between 700 and 800 °C. Figure 6B illustrates the use of ceramic oxide  Table 3. Reported studies on dual-phase and related membranes for CO 2 separation.
CO2 Sequestration and Valorization membranes for hydrogen purification by separating the CO 2 from water-gas shift products at about 550 °C. Additionally, Figure 6B shows the possibility of using a ceramic sorbent to chemically trap the permeate CO 2 and therefore enhance the CO 2 permeation process by reducing the concentration of CO 2 in the permeate side. Figure 6. Schematic representation of the membrane reactor concept using a CO 2 permselective ceramic membrane: (a) CO 2 dry methane reforming and (b) water-gas shift reaction with hydrogen purification wherein CO 2 capture promotes the separation process.

Chemical transformation of CO 2 catalyzed by ceramic materials: the use of new alternatives.
One of the most widely used chemical absorption techniques for carbon capture and storage/sequestration (CCS) is CO 2 adsorption by ceramic materials. Once CO 2 has been captured-fixed, it can be converted into value-added products such as precursors in chemical transformation reactions. CO 2 is extensively used for enhanced oil recovery, as a monomer feedstock for urea and polymer synthesis, in the food and beverage industry as a propellant, and in production of chemicals. Therefore, the capture-fixation of CO 2 would make a system suitable for accomplishing chemical transformation of CO 2 . The utilization of carbon dioxide is also very attractive because it is environmentally benign [105][106][107][108][109][110][111][112][113][114][115]. CO 2 conversion to fuel and value-added products is an ideal route for CO 2 utilization due to the simultaneous disposal of CO 2 and the benefit that many products can be used as alternate transportation fuels [116]. CO 2 chemical transformation methods include (i) reverse water-gas shift, (ii) hydrogenation to hydrocarbons, alcohols, dimethyl ether and formic acid, (iii) reaction with hydrocarbons to syngas, (iv) photoand electrochemical/catalytic conversion, and (v) thermo-chemical conversion . CO 2 can be catalyzed to valuable organic or inorganic compounds, where some basic catalytic materials (containing alkaline or alkaline-earth elements) are used. The activation of CO 2 by alkali metals has received considerable attention in various surface science studies, which have demonstrated the formation of intermediate CO 2 , dissociation of CO 2 and formation of oxalate and carbonate alkali compounds [118][119][120][121]. Carbon dioxide has been identified as one such potential vector molecule (through (A) (B) Figure 6. Schematic representation of the membrane reactor concept using a CO 2 permselective ceramic membrane: (a) CO 2 dry methane reforming and (b) water-gas shift reaction with hydrogen purification wherein CO 2 capture promotes the separation process.

Chemical transformation of CO 2 catalyzed by ceramic materials: the use of new alternatives.
One of the most widely used chemical absorption techniques for carbon capture and storage/ sequestration (CCS) is CO 2 adsorption by ceramic materials. Once CO 2 has been captured-fixed, it can be converted into value-added products such as precursors in chemical transformation reactions. CO 2 is extensively used for enhanced oil recovery, as a monomer feedstock for urea and polymer synthesis, in the food and beverage industry as a propellant, and in production of chemicals. Therefore, the capture-fixation of CO 2 would make a system suitable for accomplishing chemical transformation of CO 2 . The utilization of carbon dioxide is also very attractive because it is environmentally benign [105][106][107][108][109][110][111][112][113][114][115]. CO 2 conversion to fuel and value-added products is an ideal route for CO 2 utilization due to the simultaneous disposal of CO 2 and the benefit that many products can be used as alternate transportation fuels [116]. CO 2 chemical transformation methods include (i) reverse water-gas shift, (ii) hydrogenation to hydrocarbons, alcohols, dimethyl ether and formic acid, (iii) reaction with hydrocarbons to syngas, (iv) photo-and electrochemical/catalytic conversion, and (v) thermo-chemical conversion . CO 2 can be catalyzed to valuable organic or inorganic compounds, where some basic catalytic materials (containing alkaline or alkaline-earth elements) are used. The activation of CO 2 by alkali metals has received considerable attention in various surface science studies, which have demonstrated the formation of intermediate CO 2 , dissociation of CO 2 and formation of oxalate and carbonate alkali compounds [118][119][120][121]. Carbon dioxide has been identified as one such potential vector molecule (through reduction to syngas, methanol, methane, formic acid, formaldehyde, dimethylether (DME) and short-chain olefins) [117][118][120][121][122]. CO 2 is a kinetically and thermodynamically stable molecule, so CO 2 conversion reactions are endothermic and need efficient catalysts to obtain high yield. CO 2 conversion to carbon monoxide (CO) looks like the simplest route for CO 2 reduction [121]. CO is a feedstock or intermediate product for the production of methanol and hydrocarbon fuels via Fischer-Tropsch synthesis of CH 4 /CO 2 reforming to form syngas (CO/H 2 ) [122]. CO 2 reforming with CH 4 is an example of CO 2 being used as a soft oxidant, where the dioxide is dissociated into CO and surface oxygen, and oxygen abstracts hydrogen from methane to form water via the water-gas shift reaction (WGS) (Eqs. 11 and 12) [100][101][102][103]121]. The catalytic chemistry of the reverse watergas shift reaction and the following transformation to methanol/DME (or hydrocarbons via Fischer-Tropsch synthesis), and the subsequent production of gasoline (methanol-to-gasoline or diesel via hydrocracking of the alkanes produced in the Fisher-Tropsch process) are well established [102,[117][118][119][120][121][122]. On the other hand, methanol can be produced directly from carbon dioxide sources by catalytic hydrogenation and photo-assisted electrochemical reduction. A wide variety of CO 2 photo-reduction methods have been performed to oxygenate products, including formic acid (HCOOH) and formaldehyde (HCHO). HCOOH and HCHO are the simplest oxygenates produced from the reduction of CO 2 with H 2 O (or proton solvents) [121]. Furthermore, CO 2 can be utilized as a monomeric building block to synthesize various valueadded oxygen-rich compounds and polymers under mild conditions. As an example, chemical conversion of CO 2 through C-N bond formation can produce value-added chemicals such as oxazolidinones, quinazolines, carbamates, isocyanates and polyurethanes [105]. These commodity chemicals have been synthesized from green methods and have important applications in the pharmaceutical and plastic industries. The chemisorption of CO 2 based on C-N bond formation could be one of the most efficient strategies, utilizing liquid absorbents such as conventional aqueous amine solutions, chilled ammonia, amino-functionalized ionic liquids, and solid absorbents including amino-functionalized silica, carbon, polymers and resins. The processes by which chemicals for CO 2 capture are manufactured should also be considered in terms of their energy requirements, efficiencies, waste products, and CO 2 emissions [105,123]. In that sense, dimethyl carbonate (DMC) is a promising target molecule derived from CO 2 catalyzed by inorganic dehydrating agents such as molecular sieves [107]. Dimethyl carbonate has received much attention as a safe, non-corrosive, and environmentally friendly building block for the production of polycarbonates and other chemicals, an additive to fuel oil owing to its high octane number and an electrolyte in lithium batteries due to its high dielectric constant. It can be synthesized through a two-step transesterification process utilizing CO 2 as raw material [105,107].
Among the alkaline earth metal oxides, CaO is a promising basic heterogeneous catalyst for synthesizing biodiesel at mild temperatures (below the boiling point of methanol, MeOH) and at atmospheric pressure due to its plentiful availability and low cost, but it is rapidly hydrated and carbonated upon contact with room temperature air. CaO is the most widely used catalyst for transesterification and produces a high yield of 98% of fatty acid methyl esters (FAME) during the first cycle of reaction [130]. Granados et al. [142] used activated CaO as a solid base catalyst in the transesterification of sunflower oil to investigate the role of water and carbon dioxide on the deterioration of the catalytic performance upon contact with air for different periods. The study showed that CaO was rapidly hydrated and carbonated in air. Consequently, the reusability of the catalyst for subsequent steps is a big question mark. Di Serio et al. [170] reported a 92% biodiesel yield with MgO catalyst, using 12:1 methanol to oil molar ratio with 5.0wt% of the catalyst at methanol supercritical condition for 1 h. Wen et al. [171] carried out transesterification from waste cooking oil with methanol at 170 °C for 6 h with 10wt % of MgO/TiO 2 and 50:1 M ratio of MeOH and oil. Guo et al. [172] studied the methyl ester yield produced via transesterification of soybean oil using sodium silicate as a catalyst. Sodium silicate was an effective catalyst for the microwave-irradiated production of biodiesel and hydrothermal production of hydrogen from by-product glycerol combined with Ni catalyst. The optimum reaction conditions obtained were 7.5:1 M ratio of alcohol/oil, 3wt% catalyst amount, 1 h reaction time and 60 °C reaction temperature. The FAME yield was ~100%. On the other hand, microwave-assisted transesterification of vegetable oil with sodium silicate is an effective and economical method for the rapid production of biodiesel. The reused catalyst after transesterification process for four cycles was recovered. Overall, sodium silicate was fully used in biodiesel production and glycerol gasification, and this co-production process provided a novel green method for biodiesel production and glycerol utilization [172].
Several techniques have been investigated for the transesterification reaction using heterogeneous catalysts for biodiesel production, as follows: transesterification via radio frequency microwaves, alcohol reflex temperature, alcohol supercritical temperature and ultrasonication [127,[173][174][175][176][177]. Recently, the use of ultrasonic irradiation has gained interest in biodiesel production [173][174][175][176][177]. Ultrasonic energy can emulsify the reactants to reduce the catalyst requirement, methanol-oil ratio, reaction time and reaction temperature and also provides the mechanical energy for mixing and the required activation energy for initiating the transesterification reaction [173][174][175][176]. The ultrasound phenomenon has its own physical and chemical effects on the liquid-liquid heterogeneous reaction system through cavitation bubbles, according to the following principles [175]: (1) the chemical effect, in which radicals such as H + and OHare produced during a transient implosive collapse of bubbles (in a liquid irradiated with ultrasound), which accelerates chemical reaction in the bulk medium; and (2) the physical effect of emulsification, in which the microturbulence generated due to radial motion of bubbles leads to intimate mixing (homogenizing the mixture) of the immiscible reactants. Accordingly, the interfacial region between the oil and alcohol increases sharply, resulting in faster reaction kinetics and higher conversion of oil and biodiesel yield [127]. In 2000, the ultrasonication reactor was first introduced by Hielscher Ultrasonic GmbH for biodiesel production. Nishimura et al. [175] studied the transesterification of vegetable oil using lowfrequency ultrasound ). An excellent yield (~98%) was obtained at a 28 kHz ultrasound while a significant reduction of reaction time was obtained by using 40 kHz ultrasound. Salamatinia et al. [176] used ultrasonic assisted transesterification to improve the reaction rate. In this study, they used SrO and BaO as heterogeneous catalysts in the production of biodiesel from palm oil. The results showed that the basic properties of the catalyst were the main cause for their high activity. The low-frequency ultrasonic assisted transesterification process had no significant mechanical effects on SrO, but BaO catalyst study confirmed that the ultrasound treatment significantly improved the process by reducing the reaction time to less than 50 min at a catalyst loading of 2.8wt% to achieve biodiesel yield higher than 95%. Another study of alkali earth metals was carried out by Mootabadi et al. [177]. They reported the effect of ultrasonic waves at 20 kHz and 200W on the regenerated catalyst and compared mechanical stirring and ultrasonic irradiation. They investigated the optimum conditions, using palm oil for biodiesel production with catalysts such as CaO, SrO and BaO. They concluded that catalyst leaching was the main cause for the catalyst inactivity in the case of the re-used catalyst. BaO catalyst was found to be stable during the leaching. At the optimized condition, 95.2% yield was achieved with 60 min of reaction time for both BaO and SrO catalysts. For CaO catalyst, 77.3% yield was achieved with the same conditions. The use of ultrasound showed great enhancement of the reaction parameters in terms of the obtained yield and reaction time. The obtained yields were 30 to 40% higher in comparison to the corresponding results obtained using a conventional stirring reactor system without ultrasonication. Deng et al. [178] prepared nano-sized mixed Mg/Al oxides. Due to their strong basicity, the nanoparticles were further used as catalyst for biodiesel production from jatropha oil. Experiments were conducted with the solid basic catalyst in an ultrasonic transesterification reaction. Under the optimum conditions, biodiesel yield was 95.2%. After removing the glycerol on the catalyst surface, the nano-sized mixed Mg/Al oxides were reused eight times. The authors concluded that calcination of hydrotalcite nanocatalyst under ultrasonic radiation is an effective method for the production of biodiesel from jatropha oil. The activity of base solid catalysts is associated to their basic strength, such that the most basic catalyst showed the highest conversion. In another work, Deng et al. [179] reported optimum conditions for biodiesel production in the presence of base solid catalysts. They studied BaO and Ca-Mg-Al hydrotalcite (the most effective). The 95% biodiesel yield from jatropha oils and Ca-Mg-Al hydrotalcite was established with 30 min of reaction time. Ca-Mg-Al hydrotalcite could be reused twelve times after washing of the adsorbed glycerol from the catalyst surface with ethanol. Other types of heterogeneous catalysts under ultrasonic irradiation were used for transesterification by Georgogianni et al. [114]. They studied a wide range of catalysts including Mg-MCM-41, Mg-Al hydrotalcite and K + -impregnated zirconium oxide. They mixed frying oils, methanol and the catalyst in a batch reactor with mechanical stirring for 24 h and with ultrasonication for 5 h. The results suggested that the basic strength was the cause of the good activity of the catalysts. Mg-Al hydrotalcite achieved the highest reaction conversion of 87% at a reaction temperature of 60 °C. Overall, ultrasonic irradiation significantly enhanced the reaction rate, causing a reduction in reaction time, and the biodiesel yield increased [114]. Consequently, a better understanding of the use of ultrasonic sound waves to accelerate the transesterification process could lead to substantial future improvement of both batch and continuous production systems, to obtain a more sustainable biodiesel production process [127].