SCIENCE & TECHNOLOGY
THE TURNOVER FALLACY Chemists debate the best way to use reaction data to compare CATALYST EFFICIENCIES STEPHEN K. RITTER, C&EN WASHINGTON
place as gaseous reactants flow across or through a fixed bed of a solid, or heterogeneous, catalyst at high temperature. The relatively low value and high volume of the products means a long-lasting catalyst with a high turnover number is often more costeffective than a short-lived catalyst with a high turnover frequency. FOR HIGH-VALUE specialty chemi-
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cals, such as pharmaceuticals, reactions typically take place in batch reactors using CATALYSTS ARE AT the heart of most manjudging individual catalysts. But chemists soluble, or homogeneous, catalysts at low made and natural chemical reactions. Metal still lack a general way to compare different temperatures. In this case, a short reaction catalysts serve as the linchpin for producing catalysts or different classes of catalysts. time is more valuable to the manufacturer many of the chemicals we use on a daily baTwo chemists are calling for a rethink of than high production volume, so high turnsis, such as gasoline, fertilizer, plastics, and these commonly used measures. In a viewover frequencies are preferred and turnmedicines. And enzyme catalysts, which are point paper and follow-up discussion in the over numbers are less important. increasingly being harnessed for chemical journal ACS Catalysis, theoretical chemists Turnovers are generally useful for asprocessing, help optimize the biochemical Sebastian Kozuch and Jan M. L. Martin of sessing the efficiency of a single catalyst factories in cells that produce the energy Weizmann Institute of Science, in Israel, or enzyme, Kozuch and Martin note, but and biomolecules required for life. review the misconceptions and pitfalls of not for comparing different catalysts or All these catalysts speed up reactions by using turnover frequencies and turnover families of catalysts. They suggest it could providing alternative pathways with lower numbers. They propose redefining the be possible to make an apples-to-apples energy barriers. But comparison of difwhen it comes to pickferent catalysts by APPLES VERSUS ORANGES Chemists struggle with how to consistently ing the best catalyst for normalizing catalyst determine efciencies of catalysts, such as acetylcholinesterase (left), a gold a given reaction, chemdata to standardized cluster supported on cerium oxide (center), and an iridium complex (right). ists need a consistent reaction conditions. way to compare the Inspired by classiefficiency of different cal thermodynamic candidates. It’s no easy functions such as task: Metal, nonmetal, the Gibbs activation N R and enzyme catalysts energy of reactions at Ir require vastly different standard conditions R N reaction conditions of temperature, presR and are used on differsure, and concentraent scales. That means tions, Kozuch and R = pinacolborane trying to compare difMartin propose calcuferent catalysts is often lating standard-state an apples-versus-oranges affair. two measures to provide a better way to turnover frequencies and turnover numWhen judging the ability of individual judge catalytic efficiency (DOI: 10.1021/ bers. They set the standard conditions at 1 M catalysts to carry out a mission, chemists cs3005264 and 10.1021/cs4000415). concentration of reactants and products, or have settled on two measures: the turnover Kozuch and Martin say they hoped their 100 kilopascal pressure in the case of gases, frequency and turnover number. proposal would stimulate further discussion and a temperature of 273.15 K (0 °C). Turnover frequency is the amount of in the community—and it has. Chemists In a comment responding to Kozuch product formed in a catalytic reaction responding to the new proposal agree the and Martin’s proposal, chemical kinetics divided by the amount of catalyst and the current turnover definitions aren’t ideal. specialist Gábor Lente of the University of reaction time. It tells researchers how fast But they also say Kozuch and Martin’s paDebrecen, in Hungary, agrees that, despite and furious a catalyst works. Turnover freper gets the debate off on the wrong foot, the utility and common use of turnovers, quencies are calculated in different ways, because it assumes that a standard measure the concept is still not well defined and depending on how qualitative or quantitacan actually be achieved. In reality, a stanoften leads to confusion. However, he tive the scientist or engineer needs to be in dard measure of catalyst efficiency may be questions the value of using turnovers at his or her approach. experimentally impossible. all. Lente argues that more scientifically Turnover number refers to the total The problems with defining a standard rigorous rate equations and rate constants, number of catalytic cycles the catalyst measure of catalyst efficiency can be highwhich include details about the reaction performs during its lifetime. It provides a lighted by considering two main types mechanism, should be used (ACS Catal., measure of catalyst robustness. of catalytic reactions. In high-volume DOI: 10.1021/cs300846b). These metrics, coupled with reaction chemical production industries such as Chemists draft rate equations on the bayields and catalyst selectivity, work well for petroleum refining, reactions typically take sis of the experimentally observed kinetics WWW.CEN-ONLINE.ORG
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of a reaction. Rate equations depend on the concentrations of the reactants and products and on several rate constants. A rate constant is a coefficient that provides the rate for a single step in a multistep reaction. It is independent of the concentrations. With a detailed rate equation, chemists can extrapolate the data for a catalyst to different concentrations and reaction conditions. Lente emphasizes that turnover frequencies are often incorrectly considered to be reaction rates or rate constants when they are determined from yield data of a single experiment rather than from detailed kinetics studies. This is a problem, particularly in homogeneous catalysis, because the turnover frequency changes during a reaction as the concentrations of reactants decrease. In that regard, the turnover frequency is an average rate for the reaction, rather than the maximum rate for a catalyst. OVERALL, Lente thinks Kozuch and Mar-
tin accurately describe the reservations kineticists nurture about the concept of turnovers. He applauds their attempts to suggest ways to resolve the problems.
Many of their recommendations are actually about taking more extensive kinetics measurements, most of which are exactly the ones that are necessary to establish a rate equation, he says. “What I hope chemists can take away from these discussions is more motivation to meet higher scientific standards in their research,” Lente says. “In my view, characterization of catalyst efficiency is a problem for which traditional chemical kinetics already offers a time-tested solution. There is no need to create a new one.” Donna G. Blackmond of California’s Scripps Research Institute, an authority on catalysis and kinetics, agrees. One problem she points out with the new proposal is that the standard concentration and temperature suggested are rarely encountered in catalytic reactions. “Setting up an unrealistic standard state to compare catalysts sheds no light on the problem of definitions and only serves to confuse researchers further,” Blackmond says. “The need to understand each case separately—each catalyst, reaction, set of conditions—remains
and will not be aided by a new definition.” Avelino Corma, a heterogeneous catalysis expert at the Institute of Chemical Technology, in Valencia, Spain, agrees that accurate comparisons demand a full kinetic study. But that is easier said than done, he says. In practice, when a series of catalysts have to be compared, it can become a time sink to measure enough reaction kinetics to get the full rate equation for every single reaction. It’s more realistic to determine turnover frequencies and compare the values in a relative manner, Corma says. John F. Hartwig, a homogeneous catalysis expert at the University of California, Berkeley, suggests chemists should take Kozuch and Martin’s proposal and ensuing discussion in the spirit that everyone should remember the difference between turnover frequencies, turnover numbers, rate equations, and rate constants. “Everybody has their own needs to consider when judging what is the best way to compare catalysts,” Hartwig says. “We just have to live with the fact that we can’t distill catalyst efficiency down to a single comparative number.” ◾
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