Bimetallic BaFe2MAl9O19 (M = Mn, Ni, and Co) hexaaluminates as oxygen carriers for chemical looping dry reforming of methane

Chemical looping dry reforming of methane allows the continuous production of syngas and CO with decreased carbon footprint from two separated reactors, which provided system flexibility for various downstream applications, such as F-T synthesis or hydrogen purification. A key issue for this process is to find an appropriate oxygen carrier (OC) with high reactivity and recyclability. In this work, bimetallic BaFe2MAl9O19 (M = Mn, Ni, and Co) hexaaluminates were studied as OCs, with Fe-substituted BaFe2Al10O19 and BaFe3Al9O19 for comparison. The influence of Mn, Ni, and Co doping on structure, redox reactivity and stability was investigated by means of XRD, XPS, BET, H-2-TPR, CH4-IR, SEM, CO2-TPO and fixed-bed experiments. Pure Fe-substituted OCs presented the coexistence of beta-Al2O3 and magnetoplumbite (MP) hexaaluminate phases, which transformed from MP to beta-Al2O3 during cyclic CH4/CO2 operation. Bimetallic BaFe2MAl9O19 (M = Mn, Ni, and Co) only crystallized in beta-Al2O3 hexaaluminate due to the presence of +2 valent Mn, Ni, and Co, and the beta-Al2O3 structure still remained stable during the CH4 reduction step. Mn doping inhibited the release of lattice oxygen, while Ni doping initiated significant CH4 pyrolysis. Among all dopants, the BaFe2CoAl9O19 OC exhibited good reactivity for syngas production with high CH4 conversion, high syngas yield, desirable H-2/CO ratio (similar to 2), and stable regenerability by CO2, taking advantages of the enhanced oxygen-donation ability and the preservation of hexaaluminate phase during successive CH4/CO2 redox cycles.