Lithium orthosilicate (Li4SiO4) was synthesized by three different techniques:  the solid-state reaction, precipitation, and sol−gel (using a microwave oven) methods. The better results were obtained by the two first methods. In the third case, pure Li4SiO4 could not be obtained, because the microwaves produced the lithium sublimation. The samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption, and thermogravimetric analysis under a flux of CO2. Different particles sizes were obtained as a function of the method of synthesis, and the CO2 sorption analyses gave different results. The particle size modified the stability of the Li4SiO4 during the CO2 sorption/desorption cycles, due to lithium sublimation, as Li2O. Conversely, the isothermal study allowed measuring the kinetic parameters for the chemisorption and diffusion processes, as a function of the particle size. As could be expected, the activation energies obtained, for the small particles, were smaller than those obtained for the large particles. These results were explained in terms of reactivity, for the chemisorption process, and in terms of geometry, for the diffusion process.

Solid solutions of lithium and sodium orthosilicate (Li 4-x Na x SiO 4 ) were synthesized by coprecipitation. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, N 2 adsorption, and thermogravimetric analyses (dynamic and isothermically). Results showed that the solubility limit of sodium into Li 4 SiO 4 is 0.1, Li 3.9 Na 0.1 SiO 4 . Sodium additions, higher than 0.1, produced the formation of secondary phases. Thermal analyses into a CO 2 flux showed that Li 4-x Na x SiO 4 solid solutions present a much better CO 2 absorption than that observed for pure Li 4 SiO 4 . Isothermal analyses were performed to the samples in order to obtain kinetic information. These data were adjusted to double exponential model as there are two different processes taking place, the CO 2 absorption and diffusion processes. Nevertheless, in some cases, it was detected the presence of a third process, desorption. Those experiments were fitted to a triple exponential model. Finally the enthalpy activation energies for the different processes were calculated using the Eyring's model.

The behavior of lithium oxide (Li2O) under an atmosphere of CO2 was studied using thermogravimetric analysis, scanning electron microscopy, and X-ray diffraction techniques. Results show that Li2O can be used for CO2 retention. It is a better absorbent than other lithium ceramics. A chemisorption topochemical reaction is proposed to explain the sorption of CO2. The kinetic parameters were obtained. First, the CO2 sorption is controlled by a first-order reaction. Later, CO2 sorption depends on the diffusion of lithium through the formed shell of Li2CO3, with an extraordinary increase of the diffusion at 600 °C, due to an unusual increase of lithium mobility.

Lithium orthosilicate (Li(4)SiO(4)) was synthesized by solid-state reaction and then its CO(2) chemisorption capacity was evaluated as a function of the CO(2) flow rate and particle size. Initially, a Li(4)SiO(4) sample, with a total surface area of 0.4 m(2)/g, was used to analyze the CO(2) chemisorption, varying the CO(2) flow between 30 and 200 mL/min. Results showed that CO(2) flows modify the kinetic regime from which CO(2) capture is controlled. In the first moments and at low CO(2) flows, the CO(2) capture is controlled by the CO(2) diffusion through the gas-film system, whereas at high CO(2) flows it is controlled by the CO(2) chemisorption reaction rate. Later, at larger times, once the carbonate-oxide external shell has been produced the whole process depends on the CO(2) chemisorption kinetically controlled by the lithium diffusion process, independently of the CO(2) flow. Additionally, thermokinetic analyses suggest that temperature induces a CO(2) particle surface saturation, due to an increment of CO(2) diffusion through the gas-film interface. To elucidate this hypothesis, the Li(4)SiO(4) sample was pulverized to increase the surface area (1.5 m(2)/g). Results showed that increasing the surface particle area, the saturation was not reached. Finally, the enthalpy activation (DeltaH(double dagger)) values were estimated for the two CO(2) chemisorption processes, the CO(2) direct chemisorption produced at the Li(4)SiO(4) surface, and the CO(2) chemisorption kinetically controlled by the lithium diffusion, once the carbonate-oxide shell has been produced.

Lithium zirconates, Li 2 ZrO 3 and Li 6 Zr 2 O 7 , were synthesized by solid-state reaction. The thermal analyses of Li 6 Zr 2 O 7 showed a continuous decomposition process due to lithium sublimation. However, the thermal behavior of this compound changed slightly when different gas environments were used. If nitrogen was used, Li 6 Zr 2 O 7 decomposed in a mixture of Li 2 ZrO 3 , ZrO 2 , and Li 2 O (g) . Nevertheless, air environment produced a different and more complex decomposition mechanism at high temperatures. In this case, lithium reacted with the oxygen from the air to produce Li 2 O at the surface, producing a temporary increase of the total weight. Subsequently, Li 2 O and some oxygen, from the Li 6 Zr 2 O 7 structure, sublimed to produce Li 2 ZrO 3 and ZrO 2 . The CO 2 absorption capacity of both zirconates was studied. The materials absorbed CO 2 at around the same temperature, 450-650 °C. Still, Li 6 Zr 2 O 7 absorbed 4 times more CO 2 than Li 2 ZrO 3 . Furthermore, the CO 2 sorption rate of Li 2 ZrO 3 was much slower than that of Li 6 Zr 2 O 7 at short times. Apparently, at the beginning of the absorption process, there was more lithium available to react with CO 2 in the Li 6 Zr 2 O 7 sample, as expected, although the sorption rates of both ceramics became similar after long times. A correlation is established between the lithium and CO 2 diffusion through the Li 2 CO 3 produced on the surface of the particles. The best temperature for the CO 2 absorption on Li 6 -Zr 2 O 7 was 550 °C. Finally, XRD analyses, after the CO 2 absorption, and cyclic thermogravimetric analyses showed that Li 6 Zr 2 O 7 was not regenerated. In all cases, the final product was Li 2 ZrO 3 .

Lithium aluminates (LiAlO(2) and Li(5)AlO(4)) were synthesized, characterized, and tested as possible CO(2) captors. LiAlO(2) did not seem to have good qualities for the CO(2) absorption. On the contrary, Li(5)AlO(4) showed excellent behavior as a possible CO(2) captor. Li(5)AlO(4) was thermally analyzed under a CO(2) flux dynamically and isothermically at different temperatures. These results clearly showed that Li(5)AlO(4) is able to absorb CO(2) in a wide temperature range (200-700 degrees C). Nevertheless, an important sintering effect was observed during the thermal treatment of the samples, which produced an atypical behavior during the CO(2) absorption at low temperatures. However, at high temperatures, once the lithium diffusion is activated, the sintering effect did not interfere with the CO(2) absorption. Eyring's model was used to determine the activation enthalpies of the CO(2) absorption (15.6 kJ/mol) and lithium diffusion (52.1 kJ/mol); the last one is the limiting process.

The kinetics analysis of carbon dioxide (CO 2 ) chemisorption on sodium zirconate (Na 2 ZrO 3 ), which implies a sodium diffusion process, was investigated. Initially, Na 2 ZrO 3 was analyzed by X-ray diffraction, scanning electron microscopy, and N 2 adsorption, to characterize the material. Finally, the material was thermally analyzed under a CO 2 flux. Later, different isothermal experiments were performed under a CO 2 flux to study kinetically the CO 2 chemisorption on Na 2 ZrO 3 . Results showed that there is a sintering effect of the sample during the heating process. This effect produced, at low temperatures, a decrease in the CO 2 chemisorption efficiency. However, at high temperatures, once the sodium diffusion was activated, the sintering effect did not interfere with the CO 2 chemisorption process. Modeling the CO 2 chemisorption on Na 2 ZrO 3 (in terms of a double process: chemisorption and sodium diffusion) allowed us to estimate the activation energy for these processes, 33 866 J/mol (chemisorption) and 48 009 J/mol (diffusion), which demonstrated that the diffusion process is the limiting step.

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