Design, Synthesis And High-temperature CO₂ Adsorption Capacities Of Lithium Zirconates Ceramics

Direct capture and recovery of high-temperature CO2 are effective approach to decrease the CO2 emmission with great commercial value, especially for coal-burning power plants and hydrogen production by stream methane reforming process. Although the lithium based ceramics such as lithium zirconates presented high CO2 adsorption capacity, matched working temperature and stable multi-cycle adsorption-desorption performance, the major problem was the dramatically reduced adsorption capacity as the CO2 partial pressure (Pco2) decreased to 0.1 bar, which is close to the situation of CO2 produced in the industrial processes. Thus, this paper was focused on the design and synthesis of LixZryOz ceramics with improved CO2 adsorption property promising for the industrialization of CO2 high-temperature adsorption.Firstly, five kinds of lithium zirconates including tetragonal/monoclinic phase Li2ZrO3 (t/m-Li2ZrO3), triclinic/monoclinic phase LieZr2O7 (tri/m-Li6Zr2O7) and rhombohedral phase LigZrO6 (r-Li8ZrO6) were synthesized by solid-state reaction method under common experimental conditions, and the mechanism of the synthesis for each product was discussed. The CO2 adsorption capacities for the synthesized LixZryOz ceramics were studied. In pure CO2 atmosphere, t-Li2ZrO3 presented about 20 wt% capacity of CO2 adsorption at 848 K, while only about 7 wt% for m-Li2ZrO3 under the same conditions; m-Li6Zr2O7 and r-LisZrO6, which were the first time to be reported as effective high-temperature CO2 adsorbents up to date, shown about 11.2 wt% and 53.98 wt% CO2 adsorption capacities at 1023 K and 998 K; However, as the Pco2 decreased to 0.1 bar, about 3.5%,86.7% and 100% capacities of CO2 adsorption were preserved for t-Li2ZrO3, m-Li6Zr2O7 and r-Li8ZrO6, respectively. Although the CO2 adsorption capacities for m-Li6ZrO7 and r-Li8ZrO6 were decreased by 17.6% after 7 cycles and 55% after 11 cycles, respectively, due to the agglomeration of the adsorbents under high temperature, the CO2 adsorption capacity on lithium based ceramics were strongly related with the lithium contents in the LixZryOz adsorbents, especially for the case in low CO2 partial pressure atmosphere, the richer lithium content in the lithium based ceramics, the higher capacities of CO2 adsorption. Further, a double exponential model was employed to simulate the CO2 adsorption processes on m-Li6Zr2O7 and r-Li8ZrO6.Secondly, several mesoporous ZrO2 with high thermal stability were synthesized, and further used as hard-template to produce mesoporous Li2ZrO3. Although the mesoporous structure of ZrO2 precursor was destroyed during the synthesis process of LiZrO3 because of the high calcination temperature and excessive Li+embedded in the mesoporous wall of ZrO2 precursor, a series of Li2ZrO3 materials with different particle sizes (20-250 nm) were produced using mesoporous ZrO2 and LiOH under suitable reaction conditions, and the relations between the particle size and CO2 adsorption capacity were also discussed. The synthesized nano-sized Li2Zr03 (20-30nm) could achieve about 19 wt% CO2 adsorption capacity in the atmosphere of Pco2 at 0.1 bar under 773 K within 30 min, which could be completed desorbed by desorbed at 823 K within 25 min under N2 atmosphere. The capacity could be completely preserved after 13 cycles of adsorption-desorption, indicating very desirable properties of CO2 adsorption from the flue gas of coal-burning power plants or SMR process.Thirdly, the diffusion of O2- was pointed out as the limiting step during the process of CO2 adsorption on Li2ZrO3 because of the largerer ionic radius. By doping with Y2O3 (0.625%-2.5% molar ratio) or CaO (about 2% molar ratio), the produced ZrO2 during the process of CO2 adsorption on Li2ZrO3 presented cubic phase with rich oxygen vacancies, which could facilitate the diffution of O2- ions and result in very fast CO2 adsorption rate. Within only 10 min, about 20 wt% capacity of CO2 adsorption could be achieved in 10% CO2 atmosphere at 773 K, which was the best result of CO2 adsorption on pure or modified Li2ZrO3 up to date. The research results developed the synthesis method of Li2ZrO3, and provided a credible principle for the doping and modification of of Li2ZrO3.