Using nature's blueprint to expand catalysis with Earth-abundant metals

Abstract

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.

Fig. 1.. Definition of different groups of transition metals.

Platinum group metals (PGMs) include Ru, Rh, Pd, Os, Ir, and Pt. The broader term, preciousmetals, includes PGMs along with Re, Au, and Ag. Earth-abundant metals (EAMs), sometimes referred to as base metals, include all other transition metals.(Tc is shown but is radioactive and unstable.) The height of the pillar for each metal indicates its crustal abundance on a log scale; the values range from 5.6% (Fe) to ~0.001 ppm (Rh, Ir). The black bar on each metal shows (also on a log scale) the relative amount of CO₂ produced through mining and purification for each metal (20), which is markedly larger for PGMs than for EAMs.

Fig. 2.

Many of the transformations carried out by enzymatic EAM catalysts are replicated in the chemical industry by means of PGM catalysts.

Fig. 3.

Physical properties of EAMs versus PGMs, illustrating substantial differences that lead to divergent reactivity that can be exploited in catalysis. Data are from (1, 38, 124).

Fig. 4.

The utility of enzymatic catalysis can be enhanced by expanding active-site reactivity to abiotic substrates, minimizing the enzymatic scaffolding, and enabling operation in nonphysiological reaction environments. Images were obtained from PDB code 1W0E, cytochrome P450.

Fig. 5.. EAM enzymes provide the blueprint for molecular EAM catalyst design.

The example shown is [Fe-Fe]-hydrogenase (center; PDB code 5LA3). (A) Proton relays positioned proximate to EAM activesites (blue highlight) are deployed in molecular catalysts for hydrogen production (125). (B) Multimetallic cluster active sites catalyze energy conversion reactions (95). (C) Transport to active sites via enzyme channels can be mimicked in porous molecular materials (126). (D) The density of available electronic states is increased through redox-active ligands that can steer reactivity in synthetic systems (90). Me, methyl; ^tBu, tert-butyl; ⁱPr, isopropyl; Ph, phenyl.

Fig. 6.. EAM sites in enzymes such as nitrogenase provide the blueprint for heterogeneous EAM catalyst design.

(A and B) Multimetallic cooperativity in nature [green in (A)] can guide the design of mixed metal-oxide oxygen evolution catalysts (B) (106). (C) The more covalent metal-ligand bonding in natural systems [Fe/Mo in (A)] parallels the more covalent chalcogenide (108) and graphitic carbon host lattices (127) in synthetic catalysts. (D) The function of the fine-tuned catalyst microenvironments in enzymes can be replicated in synthetic catalysts through micro- and mesostructuring (128).