Mesoporous nickel-alumina catalysts derived from MIL-53(Al) metal-organic framework: A new promising path for synthesizing CO2 methanation catalysts

Abstract

A new synthesis route for the preparation of highly efficient and stable porous Ni-based alumina catalysts for CO2 methanation is presented. It is based on the use of MIL-53(Al), an Al-containing metal-organic framework (MOF) with high surface area, as sacrificial support. A series of Ni-Al2O3 powder samples with Ni loadings ranging from 5 to 20 wt% was thus obtained. Their properties were thoroughly characterized by a set of complementary techniques including N2-sorption, X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), CO2 adsorption, temperature programmed reduction (H2-TPR) and transmission electron microscopy (TEM). After nickel impregnation and thermal assisted organic ligands elimination, the resulting Ni-Al2O3 materials appear as interwoven alumina nanosheets in which Ni cations are intimately mixed forming NiAl2O4 spinel nanophases dispersed within amorphous alumina. This nanosheet morphology is preserved after the reduction of the Ni cations that leads to Ni0-Al2O3 catalysts composed of homogeneously and highly dispersed Ni0 nanoparticles, even at the highest 20 wt% Ni content. As a result, the activity in CO2 methanation, evaluated between 250–450 °C under atmospheric pressure, using a constant gas hourly space velocity of 68,900 h−1 and a molar reactant ratio H2/CO2 of 4, increased proportionally with respect to the Ni loading. On the most active catalyst, the selectivity to CH4 was always excellent (between 96 % and 100 %) and the obtained CH4 yield (∼70 % at 300 °C) was about two times higher than on a commercial Ni-based Al2O3 catalyst containing 25 wt% of Ni. The catalytic performances were also better than those of the already reported porous catalysts Ni/USY, Ni/SBA-15 as well as a Ni-Al2O3 synthesized by a EISA one-pot procedure, tested under the same reaction conditions for comparison. In this work the utilization of MIL-53(Al) as starting material for the synthesis of Ni-Al2O3 catalysts was responsible for a peculiar improvement of the metallic dispersion due to the high surface area of this MOF and of the metal-support interaction likely due to the existence of remaining NiAl2O4 at the metal-support interface after reduction. Sintering and agglomeration (the main cause of deactivation) were therefore limited, thus boosting the catalytic performance (activity, selectivity and stability).