The Catalyst Review May 2024 - 13

These mechanistic findings prompted the authors to carry out CO reduction under high-pressure conditions in KHCO3
(Figure 2), resulting in a high methanol FE of 84% with a partial current density of more than 20 mA cm−2
electrolyte
at −0.98 V versus the
reversible hydrogen electrode (RHE). Based on these findings, the electrochemical CO-to-methanol conversion represents a selective
process viable for practical application. Jing L, Bo S, Yuanzuo G, et al. (2023) Nature Synthesis, https://doi.org/10.1038/s44160-02300384-6
The
Elements of a Green Catalyst: Making Homogeneous Catalysis Greener is About More than Just
Swapping Out Precious Metals for Earth-Abundant Ones
In the chemical manufacturing sector, reducing reliance on precious metal catalysts (PMCs) based on precious metals such as palladium and
rhodium is critical in achieving a sustainable future. This goal is especially important in the pharmaceutical industry, which relies on using PMCs
to enable cross-couplings and other critical organic reactions needed to synthesize drug molecules and their precursors. Earth-abundant firstrow
transition metals represent green alternatives to precious metals, and researchers such as Professor Paul Chirik of Princeton have been
working on coaxing metals such as cobalt, iron and nickel to do reactions that have been dominated by PMCs (Figure 1). However, first-row
metals do not always react by the predictable two-electron-transfer mechanisms characteristic of the well-known precious-metal-catalyzed
reactions. To address this issue, developing new ligands tailored to the needs of first-row metals is an ongoing area of research. One strategy
involves using redox-active ligands that provide an electron reservoir to get metals that prefer one-electron chemistry to participate in twoelectron
chemistry (Figure 2).
Figure 1. Paul Chirik's group is one of many academic labs
working on expanding first-row metal catalysis to do things
that precious metals struggle with, including cross-coupling
reactions between sp2 and sp3 carbons. Iron catalysts are
particularly attractive because the metal is particularly low
cost and abundant, and it has a low carbon footprint.
Figure 2. A copper-catalyzed amination reaction is used in
the early stages of the synthesis of AbbVie's dasabuvir, an
antiviral used to treat hepatitis C.
Other efforts to enhance the " greenness " of PMCs include process optimization and reduction in the amounts of PMCs used in catalytic
transformations. Concerning process optimization, many companies are making use of life-cycle assessment (LCA) metrics, which determine
a process's environmental, health and safety impacts. These metrics include everything from the raw materials used in manufacturing to how
the product is disposed of at the end of its life. LCA also considers how each stage of making and distributing the product fares in terms of
carbon emissions, energy use, water use and other criteria. With respect to developing strategies for reducing the amount of PMCs used in
a given transformation, the choice of reaction solvent plays a critical role. For example, it has been found that using water as a solvent and
very small quantities of well-known palladium catalysts is the greenest way to achieve cross-coupling.
In conclusion, earth-abundant metal catalysts are part of the sustainability-conscious synthetic chemistry toolbox, along with bio- and photocatalysts.
Their use, coupled with process interventions such as carrying out reactions in flow systems, reducing catalyst loadings and using
greener solvents, will lead to beneficial environmental outcomes. Expanding that tool kit is why collaborations between academia and
industry and partnerships, such as the GCI Pharmaceutical Roundtable, are so valuable. Barbu B (2024) Chemical and Engineering News,
Volume 102, Issue 5
The Catalyst Review
May 2024
13

The Catalyst Review May 2024

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