Catalytic CO2 hydrogenation using mesoporous bimetallic spinel oxides as active heterogeneous base catalysts with long lifetime

CO2 hydrogenation using H-2 was investigated using mesoporous bimetallic aluminum spinel oxides (MAl2O4, where M was Mg, Co, Cu, or Zn) as heterogeneous base catalysts. They could catalyze CO2 hydrogenation without additional catalytic metal components. Most catalysts produced CO as the major product ( > 80%) and relatively small amounts of CH4 and CH3 OH. However, when CoAl2O4 was used as catalyst, CH4 was the major product ( > 80%). A systematic relationship was discovered between the amount of strong basic sites and CO2 conversion. As the amount of strong basic sites increased, the CO2 conversion increased with a linear relationship over all the tested reaction temperatures (300-400 degrees C). However, no systematic relationship was observed between the CO2 conversion and total amount of basic sites, which suggested that only the sufficiently strong basic sites could activate CO2 efficiently. In addition, owing to the high surface area of the mesopores, the catalytic activity of the spinel oxides increased because of the facile diffusion of molecules through their mesoporous channels. Assuming that the strong basic sites were the actual catalytic sites, the apparent activation energies of all catalysts were derived. The results indicated that CuAl2O4 having the highest amount of strong basic sites presented the lowest activation energy of 13.0 kJ mol(-1). This reemphasized the idea that the strong basic sites were useful for the effective activation of CO2 with small energy input. Accordingly, these spinel oxides could be further used for designing multifunctional catalysts supporting catalytic metal nanoparticles on their mesopore walls, which is under investigation.