CO2 capture properties of lithium silicates with different ratios of Li2O/SiO2: an ab initio thermodynamic and experimental approach

Abstract

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO2 capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li2O/SiO2 ratios (Li2O, Li8SiO6, Li4SiO4, Li6Si2O7, Li2SiO3, Li2Si2O5, Li2Si3O7, and α-SiO2) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li2O/SiO2 ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li2O/SiO2 ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO2 absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li2O/SiO2 ratio when going from Li2Si3O7 to Li8SiO6, the corresponding lithium silicates have higher CO2 capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO2 chemisorption by lithium silicates, we found that the CO2 capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li2CO3 and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO2 into the shell to continue the CO2 chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li2O and SiO2, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO2 sorbents to fit practical needs.

ab initio Thermodynamic Study of the CO2 Capture Properties of M2CO3 (M = Na, K)- and CaCO3-Promoted MgO Sorbents Towards Forming Double Salts

“…Adding these "strong" CO 2 sorbent into relatively "weak" MgO sorbent, the thermodynamic behaviors of the mixed sorbent are usually located between those of strong and weak sorbents. Similar concepts were applied to decrease the CO 2 capture temperature of the "strong" sorbent which acts as the effective CO 2 capture component while the "weak" part acts as a stabilizer to lift reaction free energy up (less negative), such as the Li 2 O + SiO 2 and Li 2 O + ZrO 2 systems (Duan, 2013;Duan et al, 2013). In this study, however, we use the "weak" MgO as the active capture component and want to increase its CO 2 capture temperature, the "strong" part (Na 2 CO 3 , K 2 CO 3 , CaCO 3 ) involved in the formation of double salt to bring thermodynamic properties (ΔH and ΔG) of mixed system more negative, and in turn, increase the maximum CO 2 capture temperatures T 1 and T 2 .…”

On the Atomic Kinetic Energies in Ceramic Oxides

“…It may be noted that the heavier the element, the larger part it shares in the low phonon states while on going to higher phonon states the situation reverses. Here too, the silica behaves differently, where along the low frequency phonons, up to ~400 cm [28] titania [29] (rutile) and hafnia [3]. Figure 2.…”

“…Figure 2. Ratio between the simulated VDOS of the X element (X = Si, Ti, Hf) and of the oxygen atom in α-silica [28] (blue), titania (rutile) [29] (red) and hafnia [3] (green). The discontinuity between 800 and 1020 cm −1 in silica is due to the fact that no modes occur along this energy band (see Figure 1).…”

Environmental Benign Synthesis of Lithium Silicates and Mg-Al Layered Double Hydroxide from Vermiculite Mineral for CO2 Capture

“…When the Li/Si molar ratio was three, a small portion of Li 2 SiO 3 was detected. Li 2 SiO 3 has no CO 2 sorption ability at high temperatures, which explains the relatively low CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-3 [46]. When the Li/Si molar ratio reached four and five, relatively pure Li 4 SiO 4 was synthesized.…”

“…In particular, at Li/Si = 10, the characteristic peaks of Li 2 O and Li 8 SiO 6 became more obvious, thus explaining the high CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-10. According to the literature, the regeneration performance of Li 2 O and Li 8 SiO 6 is very poor, which means that the sample is not promising for practical applications [46]. In order to obtain the Li 4 SiO 4 -based CO 2 sorbent that has a high CO 2 sorption capacity and good cycling performance, the Li 4 SiO 4 -(EX-VMT)-4 sample was selected as the optimal sample for further study.…”