Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The Appeal by Hoffmann, Schleyer, and Schaefer opens up a key point of debate concerning the meaning that chemists give to the word stable. They have addressed stability in the context of being able to synthesize molecules, and introduce the terms "viable" versus "fleeting". As noted by Koch, different communities use the term stability to describe different things, and this can cause immense problems when manuscripts are refereed. I would like to widen the debate into the area of application of molecules and in particular to the field of heterogeneous catalysis, although the comments can be made equally for any molecules that find applications, e.g in sensors. I am, initially, reminded of a similar debate in the late 1980’s concerning the meaning of the term "yield" in the hotly researched field of methane oxidative coupling, where yield was used in so many different ways, and this led to great confusion. However, this Appeal is totally correct to pinpoint the meaning of stability as a matter of concern as this has far wider implications. In designing heterogeneous catalysts we use theoretical and model surface science studies to help pinpoint the types of structures we should be synthesizing. However, once fabricated the real catalyst has to be tested for activity, and it is here that stability has different meanings to groups of researchers, very much along the lines of "viable" versus "fleeting" as suggested by Hoffmann, Schleyer, and Schaefer. For example, when designing catalysts for a new reaction one wishes to observe catalysis at some level to reach the proof of concept stage. Hence the observation of a fleeting number of catalytic turnovers is encouraging and permits more detailed investigation to try to design a viable system that can achieve this catalysis for longer periods of time. Of course it is well known that catalysts deactivate, they can be poisoned during use, or can leach active components, or can be structurally modified; in this sense molecules used in such applications are inherently unstable, but they are still useful. It is all a matter of how they are engineered when used commercially. For example the zeolite catalyst for generation of gasoline by the FCC process exhibits stable catalysis for a few seconds whereas ammonia synthesis catalysts last for years; both are useful.
Heterogeneous catalysts are investigated by chemists and chemical engineers and these two communities can have distinctly different views of the meaning of stability, very much along the lines of fleeting and viable. These differences can come to the fore in refereeing of papers. For chemists, stability in terms of a heterogeneous catalyst typically means that the material does not leach into the reaction medium and hence can be reused and can, therefore, yield a substantial number of catalytic turnovers. For a chemist such a catalyst is in the viable category. Prior to this they might have observed poorer activity with limited re-use and this proof of concept stage could be considered to be in the fleeting stage. For chemical engineers, stability in heterogeneous catalysis means that the material can be used in an industrial style reactor for several thousands of hours; only then in the eyes of many in that community can it be considered viable. All other demonstrations of catalysis can be considered fleeting. Hence, the concepts of "viable" versus "fleeting" can have ambiguity when considering applications of molecules, but is fine when considering the synthesis of molecules, since then we are only trying to get some into a bottle.
So I find it refreshing that the various meanings of how we use the term stability are being debated. Perhaps we should be encouraged to describe, briefly, in what sense we are using stability when we draft papers as this will aid both the referee and the wider audience we wish to reach.
Graham J. Hutchings
Cardiff University (UK)