- Email: [email protected]
Reaction: Opportunities for Sustainable Catalysts
CATALYSIS
Reaction: Opportunities for Sustainable Catalysts Patrick L. Holland1,* Patrick Holland pursued doctoral training at the University of California at Berkeley with Robert Bergman and Richard Andersen and postdoctoral training with William Tolman at the University of Minnesota. As a faculty member at the University of Rochester, he began his independent research in 2000 by focusing on the study of three-coordinate Fe-Co complexes. Since then, his research group has addressed Fe-N2 chemistry, reactive metal-ligand multiple bonds, engineered metalloproteins, redox-active ligands, solar H2 production, and the mechanisms of organometallic transformations at base-metal complexes. In 2013, he moved to Yale University, where he is a professor of chemistry. To some chemists, ‘‘sustainability’’ might seem like the latest fad, and to others sustainability research might seem to have been appropriated by our colleagues in environmental science. However, sustainability is of vital importance to the viability of our society and must start with the development of new chemistry. Why? For all practical purposes, we live in a closed system where recycling of everything is necessary, because over time unused waste will build up, and all non-renewable resources will become depleted. Long-term sustainability requires a cyclic series of reactions, in which waste is recaptured to become a resource for the next cycle. This is where catalysis plays an important role, because studies in catalysis uncover new reaction pathways for chemical transformations of both known resources and unknown resources (including waste).
As Ludwig and Schindler explain in their excellent Catalysis piece in the March issue of Chem, catalysts based on precious metals are currently dominant. This is ironic, given that many important catalytic transformations (e.g., C–C cross-coupling, hydroformylation, and hydrosilylation) were initially discovered with the use of base catalysts of abundant metals such as iron, cobalt, and nickel. Thus, the current emphasis on base-metal catalysts can be considered a renaissance in the literal sense. But why were the base metals left behind? I would argue that in the early days of homogeneous catalysis, the science of designing supporting ligands was at a rudimentary stage, and precious metals worked better with the simple supporting ligands that were available. Now, after decades of development on methods for molding new coordination environments, we have the tools to use more sustainable metal choices. In my opinion, the goal of new design studies should not primarily be to create base-metal complexes that mimic what precious metals can do. It is likely that the need for ‘‘precious-metal reactivity’’ and for redox-active ligands has been overemphasized in the recent literature; as support, notice that many of the recent catalytic achievements with base metals have used systems with high-spin electronic configurations rather than low-spin complexes that mimic precious metals.1,2 It has been known for some time that systems in which multiple spin states are close in energy can switch between these spin states to lower reaction barriers and/or change selectivity.3 This potential advantage of base-metal catalysts is relinquished if one attempts to mimic precious-metal behavior by maximizing the ligand field. Thus, studies on base metals are leading to new conceptual models (particularly about shifting spin states) that are distinct from the conceptual models used for
precious metals (such as the ‘‘18-electron rule’’). Using these new concepts, chemists will discover unprecedented catalytic transformations that lead to a diverse set of materials that can come from (and be degraded into) simple materials. This will move us closer to the cyclic resource utilization that is required for a sustainable civilization. The raw materials for this cycle will be abundant and non-toxic molecules such as N2, CO2, and hydroxylated hydrocarbons rather than the dominant reduced hydrocarbons in use today. In this way, we would do well to move away from mimicking precious metals (which have proven to be excellent for petroleum feedstocks, as described by Ludwig and Schindler) into new areas of chemical space: instead of trying to beat precious metals at their own game, it would be smarter to change the game! One difficulty with base-metal catalysts (and in general complexes that have nearby spin states) is that they are often sensitive to O2; conversely, preciousmetal complexes have less rapid O2 reactions and greater robustness. Although sensitivity to air does add an extra complication to reactions, the process of air removal is relatively straightforward. One way that this issue has been addressed is through the use of air-stable halide or carboxylate complexes that are converted into the active catalyst under catalytic conditions.4 Other methods of in situ activation are certainly possible, and imaginative new solutions are likely in the future. One other thought for the future: with the current emphasis on using abundant, cheap, and sustainable metals, chemists often forget that it is not only the metal that should be cheap and sustainable. The supporting ligand should have these qualities as well! Therefore, I think that one of the frontiers for the future will be to use abundant metals in combination with supporting ligands that are easily degradable into recyclable materials. One way to do this is to form supporting
Chem 2, 443–447, April 13, 2017 ª 2017 Elsevier Inc.
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ligands from amino acid components, which are typically biodegradable. Peptide-based supporting ligands also benefit from natural chirality and from our ability to use the tools of molecular biology and directed evolution to prepare and design them.5 More generally, degradable ligands are useful, and creative new ways to recycle the organic fragments in supporting ligands are therefore needed. Through such advances in both the metal choices and the ligand choices, we can make truly sustainable and adaptable catalysis a reality.
ACKNOWLEDGMENTS I thank the National Science Foundation (CHE-1465017) for supporting our work on sustainable catalysis and thank Daniel Kim, Daniel DeRosha, and Danie¨l Broere for critical comments. 1. Hoyt, J.M., Sylvester, K.T., Semproni, S.P., and Chirik, P.J. (2013). J. Am. Chem. Soc. 135, 4862–4877. 2. Daifuku, S.L., Al-Afyouni, M.H., Snyder, B.E.R., Kneebone, J.L., and Neidig, M.L. (2014). J. Am. Chem. Soc. 136, 9132–9143. 3. Poli, R. (1996). Chem. Rev. 96, 2135–2204. 4. Docherty, J.H., Peng, J., Dominey, A.P., and Thomas, S.P. (2017). Nat. Chem. Published online January 9, 2017. http://dx.doi.org/10. 1038/nchem.2697. 5. Jeschek, M., Reuter, R., Heinisch, T., Trindler, C., Klehr, J., Panke, S., and Ward, T.R. (2016). Nature 537, 661–665. 1Department
of Chemistry, Yale University, New Haven, CT 06511, USA
Frontier Research Center funded by the Department of Energy. He is a fellow of the AAAS, American Chemical Society, and Royal Society of Chemistry. He received his PhD while working for Prof. Charles Casey at the University of WisconsinMadison and worked as a postdoc for Prof. Jack Norton at Colorado State University. He edited the book Catalysis Without Precious Metals (WileyVCH, 2010). He has been a cheapskate for a long time and is glad to have an excuse to practice that habit in his research. Precious metals are the workhorses of catalysis—they play a key role in refining crude oil into fuels and transform organic starting materials into high-value pharmaceutical products and agrochemicals. Rhodium, ruthenium, rhenium, palladium, and platinum are more than the workhorses— having contributed enormously to the development of modern chemical processes that improve our lives in many ways, they are the heroes of catalysis. As noted by Ludwig and Schindler in the March issue of Chem, however, precious (noble) metals also present some drawbacks: high cost, low abundance, and toxicity. Earth-abundant metals are much more sustainable and offer appealing attributes that address the disadvantages of precious metals.
*Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2017.03.017
CATALYSIS
Reaction: Earth-Abundant Metal Catalysts for Energy Conversions R. Morris Bullock1,* Morris Bullock is a laboratory fellow at Pacific Northwest National Laboratory and director of the Center for Molecular Electrocatalysis, an Energy
444
Catalysis is carried out industrially on scales that vary by orders of magnitude. The scale of pharmaceutical manufacturing is dwarfed by the scale of refining fossil fuels. Yet at all scales, there are advantages to replacing precious metals with earthabundant metals. The high value of pharmaceutical products might suggest that using precious metals is not a drawback, given that the cost of chiral phosphine ligands can easily exceed that of the rhodium or ruthenium in the catalyst. Nevertheless, using precious metals is not sustainable in comparison with using earth-
Chem 2, 443–447, April 13, 2017 ª 2017 Elsevier Inc.
abundant metals that are available in orders-of-magnitude higher quantities. Moreover, cost savings from using abundant metal catalysts instead of familiar precious-metal catalysts can still be substantial, considering the billions of dollars spent annually in the manufacture of pharmaceuticals. The scale needed for energy conversions worldwide, however, is enormous, and thus the motivation to focus on abundant metal catalysts is compelling. Solar and wind are sustainable, renewable energy sources, but their intermittency is a drawback, because power is generated only when the sun is shining or the wind is blowing. The mismatch between the demand for energy and the time at which it is generated leads to the requirement for large-scale energy storage. There is also a spatial mismatch: much of the energy is needed in locations remote from those where it originates. Both the temporal and the spatial mismatches of energy supply and demand indicate that energy storage is necessary to facilitate the increasing usage of renewable energy. When excess electrical energy is generated, conversion of that energy to a fuel stores the energy in chemical bonds. We intuitively recognize that large amounts of energy can be stored in chemical bonds. Every time we drive a car with an internal combustion engine, chemical energy stored in C–H and C–C bonds of hydrocarbons is converted into energy, but the greenhouse gas CO2 is also produced. Carbonneutral, renewable energy sources are appealing alternatives. Intense efforts worldwide have focused on the design and development of electrocatalysts for the interconversion of electrical energy and chemical energy. Recent discoveries have shown that essentially all of the critical reactions needed for a sustainable-energy future can be accomplished with
Reaction: Opportunities for Sustainable Catalysts Patrick L. Holland1,* Patrick Holland pursued doctoral training at the University of California at Berkeley with Robert Bergman and Richard Andersen and postdoctoral training with William Tolman at the University of Minnesota. As a faculty member at the University of Rochester, he began his independent research in 2000 by focusing on the study of three-coordinate Fe-Co complexes. Since then, his research group has addressed Fe-N2 chemistry, reactive metal-ligand multiple bonds, engineered metalloproteins, redox-active ligands, solar H2 production, and the mechanisms of organometallic transformations at base-metal complexes. In 2013, he moved to Yale University, where he is a professor of chemistry. To some chemists, ‘‘sustainability’’ might seem like the latest fad, and to others sustainability research might seem to have been appropriated by our colleagues in environmental science. However, sustainability is of vital importance to the viability of our society and must start with the development of new chemistry. Why? For all practical purposes, we live in a closed system where recycling of everything is necessary, because over time unused waste will build up, and all non-renewable resources will become depleted. Long-term sustainability requires a cyclic series of reactions, in which waste is recaptured to become a resource for the next cycle. This is where catalysis plays an important role, because studies in catalysis uncover new reaction pathways for chemical transformations of both known resources and unknown resources (including waste).
As Ludwig and Schindler explain in their excellent Catalysis piece in the March issue of Chem, catalysts based on precious metals are currently dominant. This is ironic, given that many important catalytic transformations (e.g., C–C cross-coupling, hydroformylation, and hydrosilylation) were initially discovered with the use of base catalysts of abundant metals such as iron, cobalt, and nickel. Thus, the current emphasis on base-metal catalysts can be considered a renaissance in the literal sense. But why were the base metals left behind? I would argue that in the early days of homogeneous catalysis, the science of designing supporting ligands was at a rudimentary stage, and precious metals worked better with the simple supporting ligands that were available. Now, after decades of development on methods for molding new coordination environments, we have the tools to use more sustainable metal choices. In my opinion, the goal of new design studies should not primarily be to create base-metal complexes that mimic what precious metals can do. It is likely that the need for ‘‘precious-metal reactivity’’ and for redox-active ligands has been overemphasized in the recent literature; as support, notice that many of the recent catalytic achievements with base metals have used systems with high-spin electronic configurations rather than low-spin complexes that mimic precious metals.1,2 It has been known for some time that systems in which multiple spin states are close in energy can switch between these spin states to lower reaction barriers and/or change selectivity.3 This potential advantage of base-metal catalysts is relinquished if one attempts to mimic precious-metal behavior by maximizing the ligand field. Thus, studies on base metals are leading to new conceptual models (particularly about shifting spin states) that are distinct from the conceptual models used for
precious metals (such as the ‘‘18-electron rule’’). Using these new concepts, chemists will discover unprecedented catalytic transformations that lead to a diverse set of materials that can come from (and be degraded into) simple materials. This will move us closer to the cyclic resource utilization that is required for a sustainable civilization. The raw materials for this cycle will be abundant and non-toxic molecules such as N2, CO2, and hydroxylated hydrocarbons rather than the dominant reduced hydrocarbons in use today. In this way, we would do well to move away from mimicking precious metals (which have proven to be excellent for petroleum feedstocks, as described by Ludwig and Schindler) into new areas of chemical space: instead of trying to beat precious metals at their own game, it would be smarter to change the game! One difficulty with base-metal catalysts (and in general complexes that have nearby spin states) is that they are often sensitive to O2; conversely, preciousmetal complexes have less rapid O2 reactions and greater robustness. Although sensitivity to air does add an extra complication to reactions, the process of air removal is relatively straightforward. One way that this issue has been addressed is through the use of air-stable halide or carboxylate complexes that are converted into the active catalyst under catalytic conditions.4 Other methods of in situ activation are certainly possible, and imaginative new solutions are likely in the future. One other thought for the future: with the current emphasis on using abundant, cheap, and sustainable metals, chemists often forget that it is not only the metal that should be cheap and sustainable. The supporting ligand should have these qualities as well! Therefore, I think that one of the frontiers for the future will be to use abundant metals in combination with supporting ligands that are easily degradable into recyclable materials. One way to do this is to form supporting
Chem 2, 443–447, April 13, 2017 ª 2017 Elsevier Inc.
443
ligands from amino acid components, which are typically biodegradable. Peptide-based supporting ligands also benefit from natural chirality and from our ability to use the tools of molecular biology and directed evolution to prepare and design them.5 More generally, degradable ligands are useful, and creative new ways to recycle the organic fragments in supporting ligands are therefore needed. Through such advances in both the metal choices and the ligand choices, we can make truly sustainable and adaptable catalysis a reality.
ACKNOWLEDGMENTS I thank the National Science Foundation (CHE-1465017) for supporting our work on sustainable catalysis and thank Daniel Kim, Daniel DeRosha, and Danie¨l Broere for critical comments. 1. Hoyt, J.M., Sylvester, K.T., Semproni, S.P., and Chirik, P.J. (2013). J. Am. Chem. Soc. 135, 4862–4877. 2. Daifuku, S.L., Al-Afyouni, M.H., Snyder, B.E.R., Kneebone, J.L., and Neidig, M.L. (2014). J. Am. Chem. Soc. 136, 9132–9143. 3. Poli, R. (1996). Chem. Rev. 96, 2135–2204. 4. Docherty, J.H., Peng, J., Dominey, A.P., and Thomas, S.P. (2017). Nat. Chem. Published online January 9, 2017. http://dx.doi.org/10. 1038/nchem.2697. 5. Jeschek, M., Reuter, R., Heinisch, T., Trindler, C., Klehr, J., Panke, S., and Ward, T.R. (2016). Nature 537, 661–665. 1Department
of Chemistry, Yale University, New Haven, CT 06511, USA
Frontier Research Center funded by the Department of Energy. He is a fellow of the AAAS, American Chemical Society, and Royal Society of Chemistry. He received his PhD while working for Prof. Charles Casey at the University of WisconsinMadison and worked as a postdoc for Prof. Jack Norton at Colorado State University. He edited the book Catalysis Without Precious Metals (WileyVCH, 2010). He has been a cheapskate for a long time and is glad to have an excuse to practice that habit in his research. Precious metals are the workhorses of catalysis—they play a key role in refining crude oil into fuels and transform organic starting materials into high-value pharmaceutical products and agrochemicals. Rhodium, ruthenium, rhenium, palladium, and platinum are more than the workhorses— having contributed enormously to the development of modern chemical processes that improve our lives in many ways, they are the heroes of catalysis. As noted by Ludwig and Schindler in the March issue of Chem, however, precious (noble) metals also present some drawbacks: high cost, low abundance, and toxicity. Earth-abundant metals are much more sustainable and offer appealing attributes that address the disadvantages of precious metals.
*Correspondence: [email protected] http://dx.doi.org/10.1016/j.chempr.2017.03.017
CATALYSIS
Reaction: Earth-Abundant Metal Catalysts for Energy Conversions R. Morris Bullock1,* Morris Bullock is a laboratory fellow at Pacific Northwest National Laboratory and director of the Center for Molecular Electrocatalysis, an Energy
444
Catalysis is carried out industrially on scales that vary by orders of magnitude. The scale of pharmaceutical manufacturing is dwarfed by the scale of refining fossil fuels. Yet at all scales, there are advantages to replacing precious metals with earthabundant metals. The high value of pharmaceutical products might suggest that using precious metals is not a drawback, given that the cost of chiral phosphine ligands can easily exceed that of the rhodium or ruthenium in the catalyst. Nevertheless, using precious metals is not sustainable in comparison with using earth-
Chem 2, 443–447, April 13, 2017 ª 2017 Elsevier Inc.
abundant metals that are available in orders-of-magnitude higher quantities. Moreover, cost savings from using abundant metal catalysts instead of familiar precious-metal catalysts can still be substantial, considering the billions of dollars spent annually in the manufacture of pharmaceuticals. The scale needed for energy conversions worldwide, however, is enormous, and thus the motivation to focus on abundant metal catalysts is compelling. Solar and wind are sustainable, renewable energy sources, but their intermittency is a drawback, because power is generated only when the sun is shining or the wind is blowing. The mismatch between the demand for energy and the time at which it is generated leads to the requirement for large-scale energy storage. There is also a spatial mismatch: much of the energy is needed in locations remote from those where it originates. Both the temporal and the spatial mismatches of energy supply and demand indicate that energy storage is necessary to facilitate the increasing usage of renewable energy. When excess electrical energy is generated, conversion of that energy to a fuel stores the energy in chemical bonds. We intuitively recognize that large amounts of energy can be stored in chemical bonds. Every time we drive a car with an internal combustion engine, chemical energy stored in C–H and C–C bonds of hydrocarbons is converted into energy, but the greenhouse gas CO2 is also produced. Carbonneutral, renewable energy sources are appealing alternatives. Intense efforts worldwide have focused on the design and development of electrocatalysts for the interconversion of electrical energy and chemical energy. Recent discoveries have shown that essentially all of the critical reactions needed for a sustainable-energy future can be accomplished with