CO{sub 2} capture properties of lithium silicates with different ratios of Li{sub 2}O/SiO{sub 2}: an ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO{sub 2} capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li{sub 2}O/SiO{sub 2} ratios (Li{sub 2}O, Li{sub 8}SiO{sub 6}, Li{sub 4}SiO{sub 4}, Li{sub 6}Si{sub 2}O{sub 7}, Li{sub 2}SiO{sub 3}, Li{sub 2}Si{sub 2}O{sub 5}, Li{sub 2}Si{sub 3}O{sub 7}, and a-SiO{sub 2}) 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 Li{sub 2}O/SiO{sub 2} ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li{sub 2}O/SiO{sub 2} ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO{sub 2} absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li{sub 2}O/SiO{sub 2} ratio when going from Li{sub 2}Si{sub 3}O{sub 7} to Li{sub 8}SiO{sub 6}, the corresponding lithium silicates have higher CO{sub 2} capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO{sub 2} chemisorption by lithium silicates, we found that the CO{sub 2} capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li{sub 2}CO{sub 3} and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO{sub 2} into the shell to continue the CO{sub 2} chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li{sub 2}O and SiO{sub 2}, 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 CO{sub 2} sorbents to fit practical needs

CO2 absorption studies on mixed alkali orthosilicates containing rare-earth second-phase additives

© 2015 American Chemical Society. Lithium silicate containing eutectic orthosilicate mixtures developed by a solid-state route displayed excellent characteristics as carbon dioxide absorbents at elevated temperature, showing absorption capacity of 256 mg g-1. Incorporation of second-phase materials was investigated as a strategy to enhance the stability of the absorbent materials against agglomeration and sintering during powder processing and high-temperature cyclic absorption/desorption loading. Yttrium oxide, gadolinium oxide, and lanthanum phosphate were added as second phases to the absorbent. It was found that when the composites were rich in absorbents (10:1 and 20:1 absorbent/second phase), the absorption performance was hardly influenced by the type of the second-phase material present. Yttrium oxide or gadolinium oxide additions in large quantities were found to enhance the absorption capacity of the orthosilicate phase. The 2:1 sample containing yttrium oxide gave absorption capacity of 315 mg g-1 of orthosilicate absorbent present in the composite sample. On the basis of the structural and morphological studies, we believe that the nonreactive second-phase components formed a virtual shell against the segregation of absorbent phase, thereby helping to improve their absorption performance. Cyclic studies have supported the superior stability and performance of such composite absorbent materials