Editorial Cite This: ACS Catal. 2018, 8, 8597−8599
pubs.acs.org/acscatalysis
A Matter of Life(time) and Death
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THE CATECHISM The three “virtues” of catalyst performance are activity, selectivity, and productivity (the last of these being related to catalyst lifetime). In Murzin’s textbook “Engineering Catalysis”,1 they are called the “trinity of catalysis”. Activity is usually the metric of highest interest to academic researchers (although in practice it is often straightforward to compensate for low activity simply by increasing the amount of catalyst in the reactor and/or the reaction temperature). For industry practitioners, the selectivity and productivity metrics are often more important. Low selectivity implies wasted raw materials and the need for costly, energy-intensive product separations, while a need for frequent catalyst replacement/regeneration results in lost production time. Loss of activity is not the only reason to perform catalyst change-out: increased pressure drop due to fines generation or catalyst breakage may necessitate replacement long before a decline in catalyst performance requires it. Lifetimes of months or even years may be required of industrial catalysts operating under normal conditions. Catalyst resiliencethe ability to tolerate fluctuations in feed composition, the occasional introduction of poisons, and changes in reactor conditions, such as intermittent operation is also important to catalyst lifetime. Yet among the three performance metrics, productivity is usually the least explored, and the factors which cause catalysts to die are the least understood, at a fundamental level. ACS Catalysis receives far fewer submissions that focus on this area than those describing catalyst design, catalytic reaction mechanisms, and their impact on activity and selectivity.
supported catalyst may not even be noticed if the molecular fragments also catalyze the desired reaction, and they may reattach to the catalyst when it is isolated at the end of a run. The environment plays a crucial role in catalyst stability. Thus, transformations in an inert atmosphere, in the absence of reactants and/or electrical potential, or without crucial components of the feed such as water, can be very different from those that occur under realistic reaction conditions.4 Stability in the presence of a complex feed that contains a variety of potential catalyst poisons can be much lower than stability in reactions with highly purified feeds or model compounds. Finally, powder catalysts may deactivate very differently from formulated catalysts. For example, the addition of binders such as clays to zeolite methanol-to-hydrocarbon (MTH) catalysts was reported to be as effective in increasing stability toward coking as hierarchical zeolite structuring.5
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WHY IS STABILITY SUCH A NEGLECTED METRIC? In general, research studies should report on all three catalyst performance metrics, particularly in support of any claims to have developed “better” catalysts.6 Why, then, is stability often investigated only superficially? First, it can be difficult to mimic realistic large-scale operating conditions in a lab-scale study. Catalyst lifetime is highly dependent on reaction conditions, including the operating temperature and pressure, reactor configuration, impurity profile of the feed, and so on. Second, time scales can vary widely, from less than a second to many months, and may be very different from the time scale that is most convenient for the activity tests. An abundance of patience is required to conduct very long experiments on valuable equipment. Accelerated aging methods involving high temperatures and/or high concentrations of catalyst poisons can sometimes be used to acquire deactivation data more quickly, but the methods require extensive validation.7 Finally, the resulting insights are often empirical rather than fundamental, making the findings more difficult to publish. By the current metrics for scientific research, investigating catalyst productivity is considered unproductive.
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STABILITY WITH RESPECT TO...WHAT? Deactivation can arise due to loss of intrinsic (per-site) activity, or a decrease in the number of active sites, or increasingly restricted access to the active sites. It can include changes in selectivity as well as activity over time. (This can be a desired outcome. For example, the in situ coking of Pd increases its selectivity, at the expense of activity, in alkyne semihydrogenation.)2 Since deactivation processes vary widely, it is necessary to describe the type of stability being observed as precisely as possible. Thermal, mechanical, and chemical deactivation phenomena are all possible, encompassing mechanisms as varied as ligand modification or loss, changes in oxidation state, poisoning, fouling, precipitation, volatilization or leaching of key components, sintering or redispersion of the active phase, new phase formation and phase segregation, attrition, fracturing, and pore collapse.3 Some, but not all, of these transformations can be reversed. Very different mechanisms may be involved in slow degradation versus catastrophic failure, but even these phenomena can be difficult to distinguish. For example, in continuous flow reactor tests, volatile or leached sites from a solid catalyst bed may (at least initially) be recaptured downstream, delaying the onset of observable deactivation. In batch reactor tests, detachment of active sites from a © XXXX American Chemical Society
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ASSESSING STABILITY
When researchers do attempt to assess catalyst stability, it is important that they conduct experiments which provide useful information. Common but incorrect assessments include showing that a recycled catalyst from a batch reactor repeatedly achieves high yields (Figure 1), and demonstrating that a flow reactor can be operated long-term at full (or equilibrium) conversion (Figure 2). The large number of published papers that report invalid assessments has caused an unfortunate proliferation of these practices. Since deactivation is a kinetic phenomenon, it must be studied under kinetically controlled conditions, just like activity. This issue has been amply discussed,8,9 but is still often overlooked in noncatalysis-
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DOI: 10.1021/acscatal.8b03199 ACS Catal. 2018, 8, 8597−8599
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Editorial
irrelevant.) Although enzymes often have high activity, their individual stability (and therefore productivity) is often low. Living systems manage deactivation by continuous replacement. A related property has been invoked for inorganic catalysts which spontaneously reassemble from solutions of the constituent ions,11 and for metal nanoparticles which redisperse by ingress and egress of metal ions from a complex oxide host.12 More typically, the heterogeneous catalyst and the reactor are designed to allow for periodic or continuous regeneration, for example, by calcination or washing. It can be even more effective to use the insight from deactivation studies for prevention. Understanding the rates and mechanisms of deactivation processes, as well as the tolerance (sensitivity) of catalysts, can lead to the design of more robust materials and to more effective strategies to prolong catalyst lifetimes. Finally, understanding complex deactivation mechanisms can lead to new, fundamental insight into how catalysts function, and the ability to dramatically alter their performance. A recent, elegant example is the Y2O3scavenging of formaldehyde formed during MTH to prevent zeolite catalyst deactivation.13
Figure 1. Assessment of catalyst stability under batch conditions: (a) invalid assessment by recycle: the amount of catalyst may be far more than needed to achieve full conversion in the allotted time, masking the presence of deactivation; (b) valid assessment by rates: the change in the apparent rate constant in the consecutive experimental runs 1− 3 serves to quantify the extent of deactivation.
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WHAT ACS CATALYSIS WANTS ACS Catalysis considers rigorous, fundamental studies of catalyst stability and deactivation mechanisms to be valuable components of the portfolio of papers we seek to publish. Understanding the structural changes that occur in catalytic materials during extended operation and as a result of temperature cycling (especially when thermal regeneration is practiced), as well as chemical changes due to side-reactions, exposure to contaminants in the feed, or the presence of sideproducts generated in the reaction, are important contributions to catalysis science. Since such studies are currently underrepresented in the catalysis literature, our science will advance, and our community will benefit from increased emphasis on the productivity metric. Susannah L. Scott, Associate Editor University of California, Santa Barbara
Figure 2. Assessment of catalyst stability under flow conditions: (a) invalid assessment at full (or equilibrium) conversion: the amount of time elapased before a decrease in conversion is observed does not necessarily reflect stability. The rate may be far higher than is needed to achieve full conversion in the reactor, masking the initial presence of deactivation. In the example shown, the slopes are the same after the conversion decreases below 100%, suggesting that the two catalysts are actually deactivating at the same rate; (b) valid assessment at intermediate conversion: the rate of change in conversion over time, under kinetically controlled reaction conditions, reflects the rate of deactivation. In the example shown, the two catalysts are deactivating at dif ferent rates.
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AUTHOR INFORMATION
ORCID
focused journals.10 Correct methods are described in most catalysis textbooks, to which authors are encouraged to refer.
Susannah L. Scott: 0000-0003-1161-0499
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Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
THE GOAL: MITIGATING DEACTIVATION The main reason for studying deactivation is to understand enough about it to slow it down. The use of a guard bed in front of a catalyst bed can protect the latter from poisons; addition of an inert diluent can minimize thermal excursions in hot spots; stripped catalyst components can be captured and recycled; and a temperature ramp can increase overall productivity (reminding us of the importance of a wide range in thermal stability). In rare cases, such as metallocene polymerization catalysts, an extremely high productivity during a very short active lifetime (typically, a few minutes) renders catalyst stability on longer time scales unimportant. More often, irreversible deactivation of catalysts represents one of the most important limitations on their practical use. For molecular catalysts, this can be much more serious than the difficulty in separating them from the reaction mixture. (Supported versions of molecular catalysts are likewise usually not regenerable, making the ability to recover them largely
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REFERENCES
(1) Murzin, D. Yu. Engineering Catalysis; De Gruyter: Berlin/Boston, 2013. (2) Borodziński, A.; Bond, G. C. Selective Hydrogenation of Ethyne in Ethene-Rich Streams on Palladium Catalysts. Part 1. Effect of Changes to the Catalyst During Reaction. Catal. Rev.: Sci. Eng. 2006, 48, 91−144. (3) Argyle, M. D.; Bartholomew, C. H. Heterogeneous Catalyst Deactivation and Regeneration: A Review. Catalysts 2015, 5, 145− 269. (4) Castanheira, L.; Silva, W. O.; Lima, F. H. B.; Crisci, A.; Dubau, L.; Maillard, F. Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Effect of the Carbon Structure, the Degradation Protocol, and the Gas Atmosphere. ACS Catal. 2015, 5, 2184−2194. (5) Michels, N.-L.; Mitchell, S.; Pérez-Ramírez, J. Effects of Binders on the Performance of Shaped Hierarchical MFI Zeolites in Methanol-to-Hydrocarbons. ACS Catal. 2014, 4, 2409−2417. 8598
DOI: 10.1021/acscatal.8b03199 ACS Catal. 2018, 8, 8597−8599
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(6) Armor, J. N. Do You Really Have a Better Catalyst? Appl. Catal., A 2005, 282, 1−4. (7) Ramanathan, K.; Oh, S. H. Modeling and Analysis of Rapid Catalyst Aging Cycles. Chem. Eng. Res. Des. 2014, 92, 350−361. (8) Jones, C. W. On the Stability and Recyclability of Supported Metal−Ligand Complex Catalysts: Myths, Misconceptions and Critical Research Needs. Top. Catal. 2010, 53, 942−952. (9) Bligaard, T.; Bullock, R. M.; Campbell, C. T.; Chen, J. G.; Gates, B. C.; Gorte, R. J.; Jones, C. W.; Jones, W. D.; Kitchin, J. R.; Scott, S. L. Toward Benchmarking in Catalysis Science: Best Practices, Challenges, and Opportunities. ACS Catal. 2016, 6, 2590−2602. (10) Schüth, F.; Ward, M. D.; Buriak, J. M. Common Pitfalls of Catalysis Manuscripts Submitted to Chemistry of Materials. Chem. Mater. 2018, 30, 3599−3600. (11) Costentin, C.; Nocera, D. G. Self-healing catalysis in water. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 13380−13384. (12) Zhu, X.; Li, K.; Neal, L.; Li, F. Perovskites as Geo-inspired Oxygen Storage Materials for Chemical Looping and Three-Way Catalysis: A Perspective. ACS Catal. 2018, 8, 8213−8236. (13) Hwang, A.; Bhan, A. Bifunctional Strategy Coupling Y2O3Catalyzed Alkanal Decomposition with Methanol-to-Olefins Catalysis for Enhanced Lifetime. ACS Catal. 2017, 7, 4417−4422.
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DOI: 10.1021/acscatal.8b03199 ACS Catal. 2018, 8, 8597−8599
