Catalytic utilization of transition, post-transition and rare earth metal-substituted brownmillerite-related solids for the continuous-flow hydrogenations of CO2 and furfural into synthetic fuels

Abstract

Brownmillerite-type perovskites (or called dicalcium ferrite derivatives, Ca₂Fe_1.4M_0.6O_x), as excellent candidates for Earth-abundant catalysts, were tested in gas- (CO₂) and liquid-phase (furfural) hydrogenations for the first time. Preparation of precursor forms of catalysts using a variety of metals M(III) = Al, Sc, Cr, Mn, Fe, Co, Ga, Y, Rh, Sb, In, La, Yb and Bi was attempted, and the results show that incorporation of metals larger than Sc(III) was not possible without the formation of extralattice phases. Dicalcium ferrite derivatives formed by calcination of hydrocalumite-based layered double hydroxides, interestingly, the appearance of the layered double hydroxides correlated well with the successful formation of brownmillerites in most cases. In situ reduced forms (containing Fe(0) and iron carbide active phases) of most materials performed well with and without impregnation of nickel nanoparticles in thermocatalytic CO₂ reduction. High CO selectivities between 60 and 99 % were measured, resulting from the combination of the following reaction pathways: hydrogen-assisted CO₂ dissociation, thermal decomposition of formyl and formate intermediates and reverse water gas shift reactions of carboxylate products. Ex situ reduced brownmillerites (or even hydrocalumites) with M = Fe and Rh possessed outstanding and robust activity in furfural reduction (by simultaneous direct and transfer hydrogenation), and furfuryl alcohol selectivity was consistently full in the scale-up tests. In these hydrogenations to sustainable fuels, performance of brownmillerite-based catalysts has been able to outperform many of the transition (Co, Ni, Cu) and noble metal (Ru, Pd, Pt) catalysts known from the literature, highlighting the potential of these cheap and readily producible materials.