Demonstration of Polymorphic Spacing Strategy against Sintering: Synthesis of Stabilized Calcium Looping Absorbents for High-Temperature CO2 Sorption

To decrease the sintering deterioration of CaO sorbents in multiple CO2 capture and release cycles, we synthesized a series of stabilized CaO sorbents incorporated with silica through freeze-drying and heat-drying, the latter of which was referred to as benchmark. The ratio of Ca and Si precursors was varied to control the reactive loadings of CaO (from 70 to 100 wt %) and the fraction of spacers in the sorbents. The characterization results show that the freeze-drying method produces sorbents with higher specific area and larger pore volume than the heat-drying method. Moreover, the stability test of over 30 cycles demonstrated that the freeze-dried samples exhibited better performance with higher stability and total CO2 uptake. The high-resolution transmission electron microscopy image shows that Ca2SiO4 crystallites as spacers are distributed within the matrix of CaO crystallites. The optimal spacer loading was determined to be similar to 10 wt %, and the optimal reaction temperature was found to be 700 degrees C. Finally, the best sorbent was tested under harsh conditions and maintained a stable capture capacity with a CO2 uptake of 0.21 g of CO2 g(-1) of sorbent even at the 30th cycle. The performance of the sorbent in this work was then systematically compared to those reported in the literature. The use of a Si-based spacer and freeze-drying have significant potential to enhance the stability of CaO sorbents.