Engineered Catalyst Based on MIL-68(Al) with High Stability for Hydrogenation of Carbon Dioxide and Carbon Monoxide at Low Temperature.

Abstract

Today, the importance of decreasing and converting COx gases from the atmosphere into value-added chemicals by catalytic hydrogenation reactions has become one crucial challenge. In the current work, to facilitate the hydrogenation of COx, several mesoporous alumina catalysts with high efficiency and stability were synthesized using the MIL-68(Al) platform, a nanoporous MOF with a high surface area as a precatalyst, encapsulating nickel or nickel-iron nanoparticles (NPs). After removing the organic linker of MIL-68(Al) by calcination in air, two types of catalysts, promoted and unpromoted, were obtained with various loads of nickel and iron. A set of analyses (PXRD, BET-N2, TEM, FE-SEM, ICP-OES, EDX-map, CO2-TPD, H2-TPR, and H2-TPD) were performed to evaluate the physicochemical properties of catalysts. Based on the analysis results, the promoted catalyst had smaller particles and pores due to the effective and uniform distribution of nickel NPs. Also, H2-TPR and CO2-TPD results in samples containing Fe promoter demonstrated the facilitation of the reduction process and the adsorption and activation of CO2, respectively. The results of CO2 methanation indicated an improved catalytic performance for promoted samples, especially at low temperatures (200-300 °C), compared to unpromoted catalysts. 5Fe·15Ni@Al2O3MIL-68(Al) catalyst displayed the best performance compared to other catalysts, with a conversion of 92.4% and selectivity of 99.6% at 350 °C and GHSV = 2500 h-1. Moreover, the 5Fe·15Ni@Al2O3MIL-68(Al) catalyst facilitated the CO2 methanation reaction by reducing the activation energy to 42.5 kJ mol-1 compared with other reported catalysts. Both types of catalysts performed 100% hydrogenation of CO to CH4 with full selectivity at 250 °C and exhibited high stability for at least 100 h at 300 °C. Notably, such high significant catalytic performance is only achieved by the usage of the "MOFs templating strategy" due to the high surface area for the effective distribution of NPs, the strong metal-support interaction, and the formation of nickel aluminate species, preventing the sintering of NPs.