Facile and efficient synthesis of ordered mesoporous MIL-53(Al)-derived Ni catalysts with improved activity in CO₂ methanation

A novel and feasible way to obtain MIL-53(Al)-derived Ni catalysts for CO2 methanation is presented. The sacrificial MIL-53(Al) was obtained by a less time and energy-consuming hydrothermal synthesis, at 190 degrees C, 12 h (compared to 220 degrees C, 72 h), at no expense on the structural and textural properties. The as-synthesized (as) and activated (lt) forms of MIL-53(Al) were derived at 600 degrees C to give amorphous mesoporous MIL-53(Al)-derived alumina, AlMIL-53(as) and AlMIL-53(lt), respectively. The subsequent Ni(10 wt%) catalysts obtained by impregnation were investigated by XRD, N-2 physisorption, TPR, TEM, H-2-TPD, and CO2-TPD. It was found that Ni/AlMIL-53 catalysts have an ordered mesoporous structure (5-10 nm), uniform distribution of round-shaped Ni nanoparticles (4-5 nm), aligned within the pores of the MIL-53(Al)-derived alumina, and an enhanced H-2 and CO2 adsorption capacity compared to Ni/Al-(com). Thus, Ni/AlMIL-53(as) reveals a H-2 adsorption capacity 2 times larger than Ni/AlMIL-53(lt), and 4 times larger than for Ni/Al-(com). This was explained by the largest dispersion of small Ni nanoparticles (similar to 4.2 nm), but also by an additional amount of H-2 available on the catalyst, as revealed by control TPD tests. Also, Ni/AlMIL-53(as) showed a judicious distribution among weak and medium basic sites. These features made Ni/AlMIL-53(as) the best performing catalyst in CO2 methanation (200-500 degrees C, CO2/H-2/Ar=1/4/1, 36 Lg(-1)h(-1)), with 70% CO2 conversion and 95% CH4 selectivity, at 400 degrees C. All catalysts showed stable CO2 conversion and CH4 selectivity over 24 h time on stream, with no evident deactivation due to Ni sintering or C deposition, the catalytic performance decreasing in the series Ni/AlMIL-53(as)>Ni/AlMIL-53(lt)>Ni/Al-(com).