Making chemistry less precious

Much of modern chemistry relies on catalysis by precious metals such as platinum, palladium, and rhodium. By contrast, more abundant metals such as iron and copper suffice in biochemistry. Bullock et al. review the opportunities presented from the study of enzymes to shift the balance in synthetic catalysts further toward the use of these abundant metals. Whether by modifying the enzymes themselves or by designing ligand and support architectures that take advantage of the cheaper metals' characteristic electron transfer properties, recent work points toward substantial progress.

Science, this issue p. eabc3183

Catalysis has had a transformative impact on society, playing a crucial role in the production of modern materials, medicines, fuels, and chemicals. Precious metals have been the cornerstone of many industrial catalytic processes for decades, providing high activity, stability, and tolerance to poisons. In stark contrast, redox catalysis essential to life is carried out by metalloenzymes that feature exclusively Earth-abundant metals (EAMs). The terrestrial abundance of some EAMs is 104 times that of precious metals, and thus their increased use would lead to reduced cost and environmental footprint. In addition to these practical considerations, EAMs display distinct reactivity profiles that originate from their characteristic electronic structure, thermochemistry, and kinetics. The behavior of EAMs provides compelling scientific opportunities for catalyst design. We assert that natures blueprint provides essential principles for vastly expanding the use of EAMs in sustainable catalysis.

Exquisite tuning of the local environment around EAM active sites is key to enabling their use in catalysis. Such control is achieved in enzymatic catalysis by directed evolution of the amino acid environment, resulting in engineered enzymes with extraordinary catalytic performance. Similarly in molecular catalysis, modifying the steric and electronic properties of ligands can lead to some EAM catalysts with performance superior to that obtained from precious metal catalysts. In addition, for heterogeneous catalysts, the local environment and electronic structure of active sites can be modified by bonding to other metals or main-group elements, facilitating reaction pathways distinct from those involving precious metals. Innovations in the design of EAM catalysts demonstrate their potential to catalyze many of the reactions that traditionally relied on precious metals, although further improvements are needed in activity, selectivity, lifetime, or energy efficiency. The characteristics of EAMs point to an overarching need for improved theories and computational methods that accurately treat their multiconfigurational electronic structure.

The remarkable ability of enzymes to catalyze a variety of reactions under mild conditions, using only EAMs, highlights compelling opportunities for the discovery of new catalysis. Although enzymes are versatile platforms for harnessing the properties of EAMs, they are insufficiently robust under the harsh pH, temperature, pressure, and solvent conditions required for some industrial catalytic processes. Thus, systematic strategies are needed for directed evolution to extend the reactivity and persistence of engineered enzymes. For molecular catalysts, the tunability of the ligands provides opportunities for systematically varying the activities of EAMs. Key challenges include enhancing metal-ligand cooperativity, controlling transport to EAM active sites, and mastering the interactions of EAM centers with both metal-based and organic-based redox-active ligands. In heterogeneous catalysis, tuning the lattice environment of EAMs offers new opportunities for catalyst discovery, but for practical applications EAM catalysts should exhibit long-term stability and high active-site density. Thus, advances are needed in the synthesis of materials with tunable phase and nanostructure, as well as insights into how EAM catalysts undergo electronic and structural changes under sustained catalytic turnover. Strategies for controlling EAM reactivity patterns, coupled with advances in synthetic methods and spectroscopic and computational techniques, are critical for the systematic use of EAMs in sustainable catalysis.

Natures blueprint provides the fundamental principles for expanding the use of abundant metals in catalysis by controlling the local environment and electronic structure of metal centers. Examples include nitrogenase-based enzymatic catalysts for N2 reduction, metalloporphyrin-based molecular catalysts for reduction of oxygen and carbon dioxide, and metal chalcogenides in heterogeneous catalysis for hydrodesulfurization and hydrogen evolution reactions.

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 natures 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.

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