Among the porous systems for CO2 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 TO4 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 CO2 from other gases via molecular sieving [67-69]. 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 CO2/N2 [69]. In the same sense, T zeolite membranes exhibited very high selectivity, of about 400, for CO2/CH4 and 104 for CO2/N2. The high selec­tivity of CO2/CH4 exhibited by T zeolites is due to the small pore size of about 0.41 nm, which is similar in size to the CH4 molecule but larger than CO2 [69]. Table 1 shows the kinetic diameter of various molecules that are present in CO2 containing gas mixtures such as flue and natural gas [70].

Deca-dodecasil 3R (DDR) (0.36 nm x 0.44 nm), and pseudo-zeolite materials like silicoaluminophosphate (SAPO)-34 (0.38 nm) also show
high CO2/CH4 selectivities due to narrow molecular sieving, which con­trols molecular transport into this material [69, 71-73]. For example, Tomita et al. [74] obtained a CO2/CH4 separation factor of 220 and CO2 permeance values of 7 x 10-8 mol m-2 s-1 Pa-1 at 28 °C on a DDR membrane [75].

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).

J = фqsDcL1 + bPf1 + bPp or J = фqsDcL1 — 0p1 + 0f

where J is the permeation flux, ф is a geometric correction factor that in­volves both membrane porosity and tortuosity, Dc is the corrected dif — fusivity of the permeating species, L is the membrane thickness, Pf and Pp represent the feed and permeate pressure respectively and 0f and 0p 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).

F = фqsDcLDcLK

where F is the permeance and K = qsb is the adsorption equilibrium con­stant. Therefore, from Eq. (5) it can be concluded that permeance is deter­
mined by both diflusivity (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 CO2 and other molecules such as N2 by promoting preferential CO2 adsorption [63, 77]. It is well known that zeolites show affinity for polar molecules, like CO2, 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 [78-80]. Lara-Medina et al. [77] carried out separation studies of CO2 and N2 with a silicalite-1 zeolite membrane prepared via hydrothermal synthesis and subsequently modified by using lithium solutions in order to promote preferential CO2 adsorption and diffusion. CO2/N2 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 Clinoptilo — lite 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 CO2 and N2 molecules have similar sizes (Table 1), the dif­ference in diffusivity is not a strong controlling factor in determining selectivity.

Modified y-Al2O3 mesoporous membranes have been also reported as a means for CO2 separation [64]. Transport mechanisms in porous mem­branes have the contribution of different regimes. An overview of the dif­ferent mechanisms is given in Table 2.

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, y-Al2O3 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.

F = 2epr3RTL8RTnM0.5 + 2epDsrA0NavdxsdP

where r is the mean pore radius, p 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, Ao is the surface area occupied by a molecule, Ds is the surface diffusion coefficient, Nav is Avogadro’s constant and Xs 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 CO2/N2 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.

aCO2N2 = MCO2MN2

Based on the aforesaid, CO2/N2 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 pm thickness) y-Al2O3 and CaO — or SiO2-modified y-Al2O3 membranes for CO2 separation at temperatures between 25 and 400 °C. Impregnation of membranes with SiO2 or alkaline CaO was done in order to improve the CO2/N2 selectivity by promoting adsorption between CO2 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 CO2/N2 separation fac­tor increased from 1.0 to 1.38 at 25 °C after modification of the y-Al2O3 with SiO2. On the other hand, CaO impregnated membranes showed a separation factor of 0.98, which is even lower than that of the unmodified y-Al2O3. The same behavior has been reported by Uhlhorn et al. [82-83]. They reported MgO modified y-Al2O3 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 y-alumina surface give more strong
base sites than those promoted by silica. Therefore, it promotes a strong bonding of CO2 on the alumina surface, causing CO2 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 CO2 permselective membranes. In these cases ceramic materials were chosen because of their well-known properties of physisorption of CO2 at low and intermediated temperatures.

Kusakabe et al. [84] prepared both pure and modified BaTiO3 CO2 permselective membranes via the alkoxide based sol-gel method; im­pregnation and calcination at 600 °C. In order to establish the effects of CO2 partial pressure, temperature and influence of the secondary oxide presence (CuO, MgO or La2O) on the CO2 adsorption properties of the membranes, pure and modified barium titanate powders were first evalu­ated by thermogravimetry and chromatography techniques. Dynamic CO2 absorption was evaluated by applying the impulse response method, wherein the BaTiO3 powder was packed in a separation column. The re­sults suggested that the CO2 molecules adsorbed on the BaTiO3 powder are mobile at temperatures about 500 °C. Therefore, this membrane ex­hibits CO2 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 CO2/N2 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 Li4SiO4-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 Sili- calite-1 (MFI zeolite), a zeolite thin film was first prepared on the support by following the dry gel conversion technique. Then, the prepared Sili — calite-1 layer was impregnated via dipping into a slurry containing lithium and silica fumed reactants (Li:Si = 4:1) and subsequently into a Li2CO3- K2CO3 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 Li4SiO4 formed membrane. A similar procedure of coating and calcination was carried out to prepare high quality membranes starting from mesopo — rous silica sources with pore sizes of 1.8-12.8 nm. Precursors react to form a Li4SiO4 membrane of 2-5 pm thickness that exhibits an N2 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 Li2CO3- K2CO3carbonate. Due to the fact that the membrane operates in a rich CO2 atmosphere, carbonates do not decompose even at temperatures of 600 °C. The maximum CO2/N2 permeance ratio was 0.85. The separation factor was higher than that for the Knudsen diffusion. Therefore, it can be conclude that Li4SiO4 layer was selective to CO2 over N2 at high tempera­ture of 600 °C.

Nomura [86] reported a two -stage approach for the preparation of Li — 4SiO4-CO2 selective membranes that involves the fabrication of a support­ed Li4SiO4 membrane and its subsequent modification by using a chemi­cal vapor deposition (CVD) method. First, for the preparation of a thin Li4SiO4 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 CO2 over N2, 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. N2 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 CO2/N2 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 CO2/N2 selectivity. The aforesaid is the result of the CO2 chemisorption on the membrane. Lithium ortho­silicate reacts with CO2 to form lithium carbonate and lithium metasilicate (Li2SiO3) as products, as indicated by Eq. (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 CO2 to form carbonates, not only preferential adsorption of CO2 molecules over N2 occurs, but CO2 chemisorption and reaction.. Therefore, it is mandatory to establish the operational temperature within a range where CO2 selective adsorption on the membrane layer promotes the separation process without reaction.