Porous CaO–MgO Nanostructures for CO₂ Capture

Monolithic porous nanostructures of CaO–MgO composites were synthesized by a rapid self-sustained combustion reaction of molded pellets made of a mixture of nitrate salts of calcium and magnesium, urea, and starch. Urea is the fuel, and starch acts as a binder and a removable in situ template leading to porous monoliths. The synthesis is rapid, single-step, and solvent-free. In addition, the products retained a small quantity (1–2%) of carbon formed from starch. Porous monoliths were probed for high-temperature (650 °C) CO2 capture at atmospheric pressure in a 20% CO2 gas stream. While the pristine CaO porous nanostructure captured 76.8 mass % of CO2 initially, it retained only a capture of 22 mass %, equivalent to 28% carbonation efficiency, after 100 carbonation–decarbonation cycles. The CaO–MgO porous nanostructures with varied amounts of MgO (10–40 mol %) exhibit CO2 capture capacities of 67–51 mass % of the sorbent. CaO80–MgO20 porous nanostructures captured 61.6 mass % of CO2 and retained 84.6% (52.1 mass % of CO2) of its initial capacity after 100 carbonation–decarbonation cycles. Thus, the hetero-oxide porous nanostructures exhibit enhanced cycle stability in addition to high CO2 capture capacity, repressing the sintering-induced limitation of porous CaO. The high carbonation efficiency and cycling stability of the porous nanostructures as CO2 sorbents are attributed to the synergistic combination of large surface area, a porous network, and an inert MgO stabilizer.