Improvement of Li4SiO4 by doping solid and salts for CO2 capture

https://doi.org/10.3390/ijms20040928

“The CO₂ absorption rate of Li₄SiO₄ material is mainly controlled by the diffusion of ions and CO₂. Zhao et al. [36] reported that solid solution usually formed with the doping of Al₂O₃ during solid-state preparation, thus increased oxygen vacancies could promote the diffusion in the product layer. Ortiz-Landeros et al. [37] reported that Al₂O₃ addition and ball milling could extend the range of CO₂ absorption temperature. In addition, Ortiz-Landeros et al. [38] compared Li_4+x(Si_1−xAl_x)O₄ with Li_4−x(Si_1−xV_x)O₄ as the solid solutions, and the results showed that diffusion resistance of CO₂ and ions in Li_4+x(Si_1−xAl_x)O₄ was diminished, while the presence of V was adverse to the diffusion through the product layer.”

“The doping of alkali metals, such as Na and K, could produce a layer of molten salts with low eutectic temperature, which reduced diffusion resistance effectively, thus the limiting step of Li₄SiO₄ material for CO₂ absorption could be resolved. The CO₂ absorption performance of various alkali metal-doped Li₄SiO₄ materials is summarized in Table 2 [39,40,41,42,43,44,45,46].”

“As presented in Table 2, Na and K were the most commonly reported alkali metals to enhance the CO₂ absorption performance of Li₄SiO₄ material. In order to determine the most appropriate doping method for K₂CO₃, Seggiani et al. [39] compared eutectic doping and simple mechanical addition of K₂CO₃, and they found that Li₄SiO₄ particles obtained from mechanical addition were smaller, as shown in Figure 8, so the mechanical doping method may be more appropriate for the doping of K₂CO₃. Olivares-Marín [47] et al. synthesized K₂CO₃-doped Li₄SiO₄ material with fly ash as the silicon precursor, and they reported that the CO₂ absorption capacity of the prepared Li₄SiO₄ material increased with the increase of the dopant amount. It is also worth noting that Zhang et al. [42] reported that the K₂CO₃ doped Li₄SiO₄ material cooperated well with the Ni/γ-Al₂O₃ catalyst in the sorption-enhanced steam methane reforming (SE-SMR) system, and high-purity hydrogen (>95%) could be obtained at lower temperatures ranging from 500 to 550 °C, and the presence of steam in the regeneration atmosphere could improve the reaction rate obviously. Mejía-Trejo et al. [48] prepared Na-doped Li₄SiO₄ material by doping Na₂CO₃ into the starting materials of TEOS and Li₂CO₃ through the co-precipitation route, and they noted that the addition of Na₂CO₃ increased the activity and reduced the equilibrium temperature of Li₄SiO₄ material for CO₂ absorption, and Li_3.85Na_0.15SiO₄ had the highest CO₂ absorption capacity among various Na-doped Li₄SiO₄ materials. Seggiani et al. [40] noted that dopants like K₂CO₃ and Na₂CO₃ could form eutectic mixtures with Li₂CO₃, which melted at high temperatures (>500 °C), so the diffusion of ions and CO₂ was enhanced in the diffusion-controlled stage. Yang et al. [43] reported that orderly crystalline arrangement of Li₄SiO₄ was broken by doped K₂CO₃ and Na₂CO₃ for their different crystal sizes, thus more pores and larger specific surface area were generated.”