Influence of silicon precursors for Li4SiO4 based carbon capture

Wang et al. [20] selected two kinds of RHA samples as the silicon precursors for the preparation of Li₄SiO₄ material, which contained the SiO₂ contents of 94.71 and 98.84 wt.%, respectively. HCl aqueous solution was used to pretreat the two RHA samples, then Li₄SiO₄ materials were synthesized by the solid-state reaction method with Li₂CO₃. The employment of RHA produced a smaller particle size, larger pore volume, and surface area compared with pure Li₄SiO₄ material. They reported a weight gain of nearly 135 % over 15 cycles, which was much higher than that of pure Li₄SiO₄ material. Furthermore, Wang et al. [58] pretreated rice husk samples at 600 and 1000 °C, respectively, and cyclic performances of the two RHA-synthesized Li₄SiO₄ materials pretreated at 1000 °C achieved better CO₂ absorption performance, which was similar to that of the RHA-derived Li₄SiO₄ material mentioned above. To study the effects of RHA as the silicon precursor on the CO₂ absorption properties of Li₄SiO₄ material, Wang et al. [59] selected RHA and two kinds of nanosilica (Aerosil and quartz) to prepare Li₄SiO₄ materials by solid-state reaction method, and SEM images and BET analysis indicated that RHA-synthesized Li₄SiO₄ material possessed higher surface area and larger pore volume. Furthermore, the weight gain of RHA-synthesized Li₄SiO₄ material was higher and faster than that of the two nanosilica-synthesized Li₄SiO₄ materials, and its cyclic CO₂ absorption capacity reached nearly 30 wt.% over 15 cycles. The authors ascribed this phenomenon to the almost unchanged surface morphology of Li₄SiO₄ material prepared from RHA over multiple absorption/regeneration cycles. Qiao et al. [60] also noted that RHA-derived Li₄SiO₄ material could enhance the yield of H₂ and reduce the energy consumption in the process of sorption-enhanced steam ethanol reforming.

Fly ash (FA) is a kind of hazardous mineral residue released from coal-fired power plants, and it accounts for approximately 88% in the total coal ash content, which contains a high silicon content, thus it has been used to fabricate useful materials [61,62]. Therefore, Li₄SiO₄ materials can also be prepared from FA as a silicon precursor. Olivares-Marín et al. [47] fabricated Li₄SiO₄ material from Li₂CO₃ and three kinds of FA, and the samples were doped with several amounts ranging from 5 to 40 mol% of K₂CO₃. The cyclic CO₂ absorption capacity of one of the doped FA-Li₄SiO₄ was approximately 100 mg/g over 10 cycles, which was far below the theoretical absorption capacity of Li₄SiO₄ material synthesized from pure SiO₂, but it was relatively stable over multiple cycles. Sanna et al. [63] synthesized Na/Li-FA Li₄SiO₄ material with different molar ratios of Li₂CO₃, FA, and Na₂CO₃, and the material was doped with K₂CO₃. They reported that the CO₂ absorption capacity of the obtained Li₄SiO₄ material was approximately 50 mg/g in low CO₂ concentration in the presence of water vapor, and water vapor had no effect on the cyclic CO₂ absorption capacity.

Shan et al. [64] selected diatomite as silicon precursor, containing the SiO₂ content of approximately 75% [65], and zeolite was also chosen as precursor for comparison. Li₄SiO₄ was synthesized by the solid-state reaction method. Li₄SiO₄ synthesized from diatomite showed higher CO₂ absorption capacity. Li₄SiO₄ material synthesized from diatomite achieved better CO₂ absorption performance than that synthesized from pure SiO₂ because of the higher specific surface area of the former [66]. In order to determine the optimum molar ratio of Li₂CO₃ to SiO₂, Shan et al. [65] prepared a series of Li₄SiO₄ containing the molar ratios of Li₂CO₃ to SiO₂ ranging from 2.0 to 2.8 and their CO₂ absorption capacities carbonated under 50 vol.% CO₂ at 620 °C for 30 min were shown in Table 3. “