Historical and Philosophical Remarks on Ziegler-Natta Catalysts

Introduction

In the first part of the present essay, the history of Ziegler-Natta catalysts will be presented in a minimal form, with a choice of topics and argumentation aimed to give an acceptable description of the complex, parallel evolution of the catalysts themselves, and of the problems connected to them. In the second part of the essay, the discourse will proceed with a more philosophical allure. As a general method of inquiry, I will analyze chemical language and chemical discourse,[1] in order to make clear which actual epistemology is used by chemists, and what is the ontological status of catalysts. I may add that my philosophical inquiry does not need any sophisticated preliminary notion, because it is almost a common sense reflection on chemistry. Anyway, after a discussion of several contrasting definitions of ‘catalyst’[2], and an analysis of the catalysts chemical properties, I will venture to propose a suitable metaphor for the catalytic activity of a material. Afterwards, the metaphor will be ‘applied’ to different reactions of the olefins, in order to illustrate some critical features of industrial catalysts (including the Ziegler-Natta catalysts). In the last two sections, I will regard intellectual attitudes to industrial catalysis, and, eventually, conclude with a glance on the makeup of industrial catalysis as an academic discipline.
 
 
 

1. Historical Sketch

1.1 Cameo portraits

Before 1945 Ziegler had obtained important results in several connected fields: free-radicals chemistry (1923-35); polymerization of butadiene by alkali metals (1928-34); new synthesis of organolithium compounds (1930); large-ring compounds, dilution principle (1933); synthesis of cantharidine and ascaridole (1942-4). The last theme of research (developed during the war!) was originated by his interest in many-members rings; not only synthesized Ziegler the two natural compounds, he also developed two reactions of broad utility, allylic bromination with N-bromosuccinimide and 1,4-addition of oxygen to 1,3-dienes (Elsch 1983, p. 1011). The obvious comment on this type of scientific interests is that Ziegler was fundamentally an organic chemist of the classical German school. It can be added that, as a German professor, his Institute was practically an extension of himself (McMillan 1979, p. 62).

If we follow an analogous ‘biographical’ scheme for Natta, we see that his principal achievements until 1945 were essentially in industrial catalysis: industrial synthesis of methanol (1928, with Montecatini); industrial synthesis of formaldehyde and its polymerization (1932, with Montecatini); synthesis of butadiene from alcohol and its separation from butenes (1942, with Pirelli); oxosynthesis (1945, with BDP). A couple of comments may be useful. In his researches as student, Natta worked on X-ray structure determination, and his first industrial synthesis was the result of a careful study of the relationship between the crystal structure of the catalytic oxides and their activity. The second noteworthy point is Natta’s continuous connection with the most important Italian chemical companies. His success as industrial chemist was so evident that already in 1938 he was considered the most important Italian chemist, at least concerning the autarchy research policy. From that point of view, the contrast between the research fields Ziegler and Natta during the war is strident; while Ziegler worked on an antihelminthic (ascaridole), Natta worked on the intermediates of the synthesis of Buna.
 
 

1.2 A meeting in Frankfurt, and its consequences

Ziegler considered the so-called Aufbau reaction an important discovery, and he was diligent in publishing and patenting the results. The international chemical community was not at all excited, however. Eventually something happened when Ziegler gave a lecture at a meeting of the Gesellschaft Deutscher Chemiker in Frankfurt (19 May 1952). Among the audience, there were Natta and his assistant, Piero Pino, an organic chemist. While the lecture did not cause any general stir, Natta and Pino were stirred. Since Natta’s research budget was tight, he convinced Giustiniani, an important Montecatini manager, to invite Ziegler to Milan. At the Milan meeting an agreement was signed, whereby Montecatini purchased rights for industrial developments of Ziegler’s discoveries in Italy, and Natta obtained access to Ziegler’s studies, in a field badly defined as "transformation of olefins" (McMillan 1979, p. 54). As a consequence of the agreement, Italian chemists would work in Mülheim, so that in February 1953 three young researchers arrived at Ziegler’s institute.

Since the end of 1952, a doctoral candidate, Holzkamp, has been working on growth reaction with ethylene and ethylaluminum in a steel pressure vessel (100°C, 100 atm). In a routine experiment he was surprised to get almost only 1-butene very fast. After a "strenuous investigation" (Ziegler 1968, p. 11) Holzkamp discovered that the catalytic effect was due to nickel present in the reaction vessel. A systematic search for substances having effects similar to nickel began. In June 1953, they investigated chromium, whose compounds gave some butene but also a small amount of material with high molecular weight. This result was encouraging. At the end of October, Breil, another of Ziegler’s collaborators, came to zirconium: a rapid and complete polymerization occurred. Moreover, the infrared spectra demonstrated that the polymer was linear. When the turn of titanium came up, the result was again striking. The reaction was so fast that the vessel became hot, and the product was partially decomposed. Thus, the problem was passed to Heinz Martin, who was looking for the mildest possible conditions of polymerization. Since it was apparent that the system Ti/Al-alkyl was very active, Martin tried the simplest possible conditions: no higher pressure at all and no external heating (Ziegler et al. 1955b, 543-544). The result of the trial was that Martin burst in Ziegler’s office waving a glass flask and crying: "Es geht in Glass!" (McMillan 1979, p. 67).[6]

In Milan, Natta was constantly informed about Ziegler’s progress by his young researchers, but he and his principal collaborator, Piero Pino, were more interested in synthetic rubber than in plastic, so propylene was the monomer of election. On 11 March 1954, Paolo Chini fractionated the reaction product by boiling solvent extraction. He obtained three fractions, the last of which was a highly crystalline, high melting, white powder. The very next day Paolo Corradini obtained a diffraction pattern from a sample stretched to five times its length, and the pattern confirmed a high degree of crystallinity (Pino & Moretti 1987, p. 689; McMillan 1979, p. 96). The most extraordinary aspect of the new polymer went to light when "the X-ray diffraction spectra were satisfactorily interpreted assuming that all the asymmetric carbon atoms of the main chain had, at least for long chain sections, the same steric configuration" (Pino & Moretti, loc. cit.).

In Mülheim laboratory, the attention remained concentrated for a while on ethylene polymerization (a topic of enormous economic relevance), and in Milan laboratory the research was focused on the polymerization of various monomers, including styrene. In the following months Natta’s laboratory was very busy, as we can read in a personal account by Pino (ibid.). Since June many patents were filed, and in December 1954, Natta presented the principal results at the Accademia dei Lincei in Rome, and sent a short letter to the Journal of the American Chemical Society. The letter was published in the 20 March 1955 issue of the American journal; the unexpected result of stereoregularity was stressed and a new, relevant term was coined: "We propose to designate as ‘isotactical chains’ […] the polymer chains having such exceptionally regular structure, containing series of asymmetric carbon atoms with the same steric configuration" (Natta et al. 1955, p. 1709). However, a few scientists had yet received the pre-print from Milan. P. Flory wrote Natta a letter (dated 21 January 1955) in which we can read: "The results disclosed in your manuscript are of extraordinary interest; perhaps one should call them revolutionary in significance. The possibilities opened up by such asymmetric polymerizations are of the utmost importance, I am sure" (Pino & Moretti 1987, p. 683).

Probably Ziegler was shocked and hurt by the flow of discoveries, papers and patents from Milan, but his reaction was slow (Ziegler et al. 1955a; arrived at the Angewandte Chemie on 21 July 1955), and only in September 1955 the new process of polyethylene synthesis was fully described (Ziegler et al. 1955b).
 
 

1.3 Four or more generations of catalysts for a-olefins polymerization

The first commercial catalysts resulted from the industrial extension of Natta’s pioneering work on the relationship between the crystal structure of titanium chlorides and the overall activity and selectivity of the catalysts. Natta and his collaborators discovered that TiCl₃ was more stereoselective and that only three structural modifications, out of the four possible ones for TiCl₃, were highly stereoselective. The active modifications (named a, g, d) had a deep purple color and a layer lattice structure, whereas b-TiCl₃ was brown and had a chain like structure. It is to be stressed that Natta had on the TiCl₄/TiCl₃problem the same epistemological position, which had led him to study the crystal structure of methanol catalysts thirty years before. Another important feature of the first generation catalysts was the use of diethylaluminum as cocatalyst. In Table 3, I have emphasized the contrast between activity and stereoselectivity for the three ethylaluminum derivatives, which could be present in the catalytic material.
 

Since 1964 a Lewis base was added to the catalyst, essentially in order to improve the stereoregularity of polymers. Probably Montecatini took the first patent for a catalytic system composed of TiCl₃/AlEt₃ and pyridine (Tait 1986, p. 218). The addition of a Lewis base was a typical ‘chemical move’, based on knowledge from coordination chemistry. However, the higher stereospecificity did not correspond to a similar increase in activity (Crespi & Luciani 1981, p. 459). Thus, a variety of complexing agents have been tried, and "most commercial companies have their own particular recipe for catalyst modification with Lewis bases" (Tait 1986, p. 216; my italics [9]). A real progress was obtained only later, when Solvay introduced the second-generation catalysts. In this case, the crucial move towards increasing activity was made following a solid state chemistry procedure: the transformation of the brown b-TiCl₃ into the stereoselective d-TiCl₃ at low temperature (<100 °C) in presence of TiCl₄, which acts as a catalyst (!) for the change of phase. The lowering of temperature from 160-200 to 65 °C prevented the catalyst particles from growing (Goodall 1986, p. 194). This type of innovation increased the activity of catalysts by a factor of 5, and its stereospecificity too, so that the removal of the atactic fraction from the final product was spared. It is important to stress that the success of the catalyst of the second generation was due to the new morphology of the catalyst particles. They were smaller, and because of a pre-treatment with ether (for extracting AlCl₃), they had a porous and weakly bonded matrix.

The morphology of the Ziegler-Natta catalyst particles is a very sensitive topic from the industrial point of view (as well as from the philosophical one [10]). In fact, the morphological properties of the resulting polymer particles depend on those of the catalyst particles. It is a real process of replica: a spherical catalyst particle provides a (much larger) spherical polymer particle (Galli et al. 1981).

The prehistory of the third generation began in 1960, when Shell patented a catalyst for propylene polymerization that used TiCl₄ supported on MgCl₂. A decisive progress was achieved in 1968, when Montecatini and Mitsui independently patented catalysts prepared from TiCl₄, MgCl₂ and an electron donator, and activated by a mixture of trialkyl-Al with another electron donator. Industrial plants based on this type of catalysts went on stream at the beginning of the 1980s. The actual preparation of the catalysts is complex, and the same use of a support gives a decisive role to materials science, a role enhanced by the crucial discovery of the process of replica by Galli. The third generation of catalysts brought a 50-fold increase of activity, so that the removal of the residual catalyst from the final product was no more necessary.

The fourth generation of catalysts, based on metallocene compounds, is now evolving towards an industrial success. Their origin is very interesting, because it was ‘accidental’. Kaminsky has described the incident with these words: "An accident in our laboratory in 1976 brought about equimolecular amounts of water into the system compared to the trimethylaluminum, and, surprisingly, an unusual high polymerization activity of ethylene was observed" (Kaminsky 1986, p. 257). Kaminsky and Sinn suspected the formation of methyl aluminoxane (MAO), and, on following their conjecture, they discovered that MAO-activated homogeneous metallocene catalysts were capable of polymerizing propene and higher olefins (Brintzinger et al. 1995, p. 1146). In a review on the most promising developments of chiral metallocene catalysts, an important group of researchers wrote: "In the evolution of Ziegler-Natta catalysts, an empirical approach has proven highly successful", but they (and many other scientists) felt that the discovery by Kaminsky and Sinn offered a unique occasion in the search for a single-site catalyst.[11] Additionally it requires the "application of rational conceptional models to the design of new metallocene structures and catalyst activators".[12] However, just the morphology of the polymeric product had demanded a deviation from this completely ‘rational’ route. In fact "[p]ractical application of metallocene catalysts requires their preadsorption on solid supports such as alumina or silica gels", so "[d]etailed guidelines have been developed for the selection of supports", and "advanced protocols" have been proposed (Brintzinger et al. 1995, pp. 1143, 1163; my italics). It seems to me that these ‘protocols’ are not too far from the preceding ‘recipes’, as it may be read in the same above quoted text.[13]

I conclude the historical profile of the evolution of Ziegler-Natta catalysts with a note on the development of the reactor granule technology. In the words of one of the creators of this technology, the new catalysts "have disclosed a new dimension in catalysis: the domain of the polymer’s shape and morphology. It is this control of the architecture or three dimensional structure of the catalyst that has enabled the polymerization reaction not only to reproduce the shape of the catalyst […], but also to generate a solid particle with a controlled reproducible porosity" (Galli 1995, p. 19; bold type in the original text). The metaphor of the reactor granule is revealing a new understanding and will-to-control of the polymerization reaction, whose practical result is "opening the way to the creation of new and completely revolutionary families of materials" (ibid., p. 25). Other important authors have spoken of "microreactors […] fabricated by immobilizing different types of single-site metallocene catalysts" (Brintzinger et al. 1995, p. 1164; my italics), or have described "molecularly engineered mordenite that acts as a shape-constraining ‘molecular reactor’" (Cusumano 1995, p. 962). Thus, it is not surprising that this type of "catalyst particle microreactor" has been studied in considerable details, because "each particle can be regarded as an expanding microreactor with its own energy and material balance" (Tait et al. 1995, pp. 133-4; my italics).[14] The metaphor of the reactor granule ‘moves’ the ‘chemical reactor’ reference from the macroscopic world of engineers to the meso-world of materials scientists.
 
 
 

2. Philosophical Remarks

2.1 Classical chemistry, increasing complexity

In Natta’s laboratory, a similar mixture of attitudes was at work. On the one hand the first sample of crystalline polypropylene was obtained by extraction with boiling solvents, "a fractionation method little used in Polymer Science at that time as it is not efficient in fractionating polymers having similar structure, according to their molecular weight", but which "was adopted in Natta’s laboratory from 1953 to separate the low molecular weight products from the solid hydrocarbons in the mixtures obtained on polymerizing ethylene with aluminum alkyls" (Pino & Moretti 1987, p. 689). In his personal account Pino speaks also of "some fortuitous events", e.g. "the solvent chosen for the fractionation of the polymers happened to be extremely selective" (ibid., 689-690). On the other hand, since his original training as chemical engineer Natta had used the most advanced physical techniques, and it was the X-ray structure determination by Corradini which gave a firm foundation to the discovery of stereopolymers, "revolutionary in significance" in Flory’s words.
 
 
 

When we consider the long history of Ziegler-Natta catalysts, probably the most important remark is that it is the narrative of an increasing complexity of the catalytic material. Table 4 refers again to the laboratory context; there I have collected the essential features of the three ‘steps’ from the discovery of the growth reaction to that of the propylene polymerization. We see two parallel processes of increasing complexity, one of the products and the other of the catalysts. The linear and stereo-polymers were a completely new type of molecules, and a result of unusual catalysts. It is to be noted that, while Ziegler referred to the nickel and titanium compounds as cocatalyst, Natta considered the transition metal compounds as catalysts, and the organometal compounds as cocatalyst. This lexical and semantic shift was not at all casual, if we look at the scientific background of the two scientists.

Let us now change the context from laboratory to industrial plant, and regard again the long industrial evolution of Ziegler-Natta catalysts in Table 2. The innovations of the first generation, the introduction of the third component, and the second generation principally aimed at the control of the process in order to improve the molecular quality of the product, i.e. its stereoregularity, with a bonus for catalyst activity in the second generation. The catalysts of the third generation gained in complexity with the use of activated MgCl₂, and the control of the process was extended to the non-molecular realm of the polymer morphology. By then the catalytic system included a catalyst, a cocatalyst, additional internal and external electron donors, and an active support. The laboratory development of chiral metallocene catalysts originated the hope of a ‘simplified’ industrial application of the fourth generation catalysts, but the meso-world of polymer morphology has demanded the use of a support, with the ensuing entanglement.

Finally, from the epistemological viewpoint, the developments towards the ‘reactor granule’ appear exciting, because they seem to be the consequence of a new holistic approach to catalysis. However, these ‘fabricated microreactors’ would deserve an additional analysis, which probably should consider the parallel developments in supramolecular chemistry.
 
 

2.2 Catalysts – substances or materials?

Considering the authors whom I read for the present paper, a small list of similar, more or less formal definitions may be presented in chronological order. Benson (1976, p. 6) wrote, "Catalysts may be looked upon as substances that perturb the populations of chemical intermediates"[16]. Mills & Cusumano (1979, p. 16) wrote about "an unusual class of substances known as catalysts"; in the context of a book for chemical engineers Satterfield (1980, p. 8) emphasized the term: "A catalyst is defined as a substance; the acceleration of a rate by an energy-transfer is not regarded as catalysis by this definition". A few years later, in a volume of the Ullmann’s Encyclopedia, Farkas (1986, p. 314) stated that: "Catalysis is the acceleration of a chemical reaction by a small quantity of substance, the catalyst, the amount and nature of which remain essentially unchanged during the reaction".

At that time, in my small collection of definitions the key term for ‘catalyst’ shifted from substance to material, i.e. from a term that has a precise meaning to another term, which is a "nonspecific term used with various shades of meanings in the technical literature". Obviously there are many ‘intermediate’ definitions; I report only one of this type, which will be recalled in the following discussion on *catalyticity [17]: "A catalyst is a substance, or a mixture of substances, which increases the rate of a chemical reaction by providing an alternative, quicker reaction path, without modifying the thermodynamic factors" (Cavani & Trifirò 1994, p. 11, my italics). Anyway, later in their text, Cavani and Trifirò write: "A catalyst used in heterogeneous catalysis is a composite material" (ibid., p. 13). A few other quotations may give an idea of the new linguistic fashion.

Rabo (1993, p. 2) defines ‘catalyst’ in these terms: "a foreign material [which] can greatly accelerate chemical reactions without itself change (ultimately) in the process". According to Cusumano (1994, p. 962) certain "selective oxidation catalysts" are "materials [which] can now be used for the peroxide oxidative conversion of large and bulky organic molecules", and in the same page he discusses several examples taken out from "a new arsenal of safe, selective materials for acid catalysis". Maxwell (1996, pp. 2, 5) speaks of "new catalytic processes in the refining area based on novel shape selective microporous materials", and states that "new materials are also finding application in the area of catalysis related to the chemical industry". Maxwell was speaking at the 11th International Congress of Catalysis, and in the same official, public and highly professional context Burwell (1996, p. 63) adopted the ‘new’ term in a conscious and explicit manner: "Adequate descriptions of many catalysts will require a large number of bits of data since they are usually rather complicated materials rather than single chemicals".

It is possible that I have observed a bias that depends on my reading of certain (few) sources. However, the linguistic shift goes simultaneously with the rise in prestige of materials science.[18] In the context of our philosophical inquiry, that is important because it shows a more explicit attention towards the industrial practice. To be sure, in organic laboratory practice many catalysts are in use for preparative or synthetic aims. However, papers on catalysis rarely contain a single reference to the laboratory use. Overall, the large community of scientists working on catalysis seems to be committed to understand, improve or discover only industrial catalysts.

The actual connotation of the term ‘catalyst’ is much more complex than conveyed by single terms like ‘substance’ or ‘material’, plus a couple of chemical properties (changing the reaction rate; being unchanged at the end of reaction). In order to appreciate some traits of the connotation of ‘catalyst’ in an explicit industrial context we may refer to Table 5, where the principal properties of catalysts are reported, as they have been explicitly listed by representative authors.
 
 
 

The three lists agree in giving the greatest evidence to the fundamental characteristics of activity, selectivity, and stability. Farkas (1986, p. 322) closes his list at this point, even if in the course of the article he treats many other properties. The other authors list the remaining "factors" (Mills & Cusumano 1979, p. 20) or "properties" (Cavani & Trifirò 1994, p. 12) as elements which determine the choice of a particular catalyst. In this modal and pragmatic context, the cost of a catalyst becomes a significant property: it is also significant that these authors agree on the regenerability of catalysts, in spite of the usual definition, which denies any chemical change.[19] Cavani and Trifirò, the most recent authors quoted in Table 5, list also "toxicity" – a homage to the environmental correctness. Mills and Cusumano explicitly list the "physical suitability", however the Italian authors treat this and other important properties in the text following their first list, and I will examine the content and the structure of their text because it presents a meaningful hierarchy of concepts/properties.

The paper of Cavani and Trifirò is dedicated to the classification of industrial catalysts. Hence, it is extremely rich of implicit suggestions about the different industrial relevance of catalysts. They catalogue these relevant characteristics of a "composite material" used in heterogeneous catalysis: "i) the relative amount of several components, ii) its shape, iii) its size, iv) its pore distribution and v) its surface area". This list is impressive if one reads it having in mind the distinction between ‘primary’ and ‘secondary properties’ (Schummer 1998, p. 132), because terms such as ‘shape’, ‘size’, ‘pore’, ‘area’, seem to point in no way at chemical properties. In fact in the industrial context, they often pertain to a meso-world, somewhere in between the micro- and macro-worlds.[20] If we return to the article of the Italian authors, we see that they use the rhetoric device of successive expansion of the text (Cavani & Trifirò 1994, pp. 13-14), an expansion which may be summarized in this way:
 

Their emphasis of the role of supports is noteworthy. On the one hand, they confirm the nature of materials for catalysts; on the other hand, they stress the fact that the catalyticity of a material results not only from the active species and from the chemical promoters, but also from the many co-operative functions of the support. In the following parts of the article, I will use the principal results of this section: the nature of materials of catalysts, the role of their secondary qualities, the functional meaning of the support.
 
 

2.3 A suitable metaphor

I agree with Mills and Cusumano (1979) in judging selectivity as the first and essential property of a working catalyst. The metaphor which I will ‘construct’ is principally aimed for a description of this particular property. However, before proposing the metaphor it is necessary to look closer at the meaning of the phrase ‘chemical property’. Schummer has proposed a typology of scientific material properties, and he has given the following definition: A material property is reproducible behavior within certain reproducible contextual conditions (Schummer 1998, p. 133; emphasis in the original text). The context which the property is referred to has in Schummer’s (and my) opinion the crucial role of distinguishing several types of properties, and in this way of permitting a classification of properties. In this vein, the contextual factors, which characterize the chemical properties of a certain substance, are other chemical substances. A slightly different way of exploring some shades of the pragmatic field [22] of ‘chemical property’ may be the analysis of the properties, which we usually attribute just to a chemical substance.

Only certain properties of a chemical substance are intrinsic, i.e. properties of a system made up only by this particular substance. Many physical properties of the substance of properties belong to this class, as well as a crucial chemical property: stability, in respect to a more or less canonical structure of its molecules. From this point of view, the concept of stability is tightly connected on one side with that of purity, with all its fascinating difficulties (Schummer 1998, pp. 136-143), on the other side with the physical conditions of the system (temperature, pressure, and electromagnetic fields). It has to be noted that specific electromagnetic fields may induce electronic excitation of the molecular system to states with features (geometry, electronic densities, reactivity) greatly different from the corresponding ones of the ground state. An important consequence is that "when the reaction is taking place on an excited electronic potential energy surface[, t]he topology of the excited surface can be completely different from the ground-state one" (Levine & Bernstein 1987, p. 121), and unusual reaction paths become accessible. A whole discipline, photochemistry, is concerned with the particular reactivity of excited molecules. The same use of the fundamental unity of measure of quantity of substance, the mole, for photons as well as for chemical substances, testifies to the fact that in a photochemical system the photons are the particles of a very special chemical substance, the light. Thus, at least some spectroscopic properties of a substance are at the border between physical and chemical (reactive) properties.

In particular circumstances, the molecules of a substance may react with themselves through the process of self-ionization; e.g., in ‘pure’ water different molecular species exist, which may behave as Brønsted acid and base. In other circumstances the molecules of a substance may tautomerize, isomerize, simply decompose, or – as we have seen at length in this paper – the molecules of certain substances may polymerize. It is obvious that at the beginning of a chemical reaction, by definition (Schummer 1998, p. 134), at least two different chemical substances are present in the system; thus, the ‘solitariness’ of the substance disappears, and with it, the concept of ‘intrinsic’ properties fades. But a point important for my argument is that almost all the intrinsic properties are virtual, i.e. they need a particular context in order to become ‘explicit’, ‘active’, etc. A simple example is just that of spectroscopic properties which became ‘actual’ through the interaction of molecules of the substance with suitable electromagnetic fields.

The virtuality of most chemical properties is evident when we describe its reactivity. Here it is appropriate to refer to the detailed protocols, which have to be followed in order to use correctly a substance as a reagent. In the protocols, we find the instructions for (re)creating the right context for the use of the reagent. In many cases the immediately available information is less detailed and specialized. However the simple fact that a substance is, for example, a ketone suggests that in the appropriate conditions we could make sense of its ‘ketonicity’ by realizing any of those many reactions which qualify the assignment of that particular ketonic substance to a specific position inside the complex network of chemical relations (Schummer 1998, p. 135).

Following this train of thought we see that the property of being a catalyst is exceedingly virtual. Certain cases of catalytic activity seem almost scandalous. I think of the Orito reaction in which platinum, modified by the presence of preadsorbed cinchonidine, becomes active for the stereoselective hydrogenation of a a-ketoester. Since 1989 several groups have studied this type of reactions; e.g. Thomas (1994, p. 922) explored by computer graphics the way in which this stereoselective process might proceed; since 1991 another group of researchers worked on the cinchona-Pt question using quantum chemistry and molecular mechanics (Baiker 1996, pp. 55-59). Baiker’s group was able to gain the knowledge necessary to identify new, simpler and more stereoselective (commercial) substances, but overall a crucial aspect of the problem remained unsolved, just the source of the catalytic activity. On this topic the words of Burwell (1996, p. 68) at the 11th International Congress of Catalysis are illuminating: "Presumably, adsorption of cinchonidine on Pt generates an optical active catalyst, but why should the rate be 30x that of the same catalyst without cinchonidine?" Anyway, also in less exotic cases than stereoselective modification, the catalyticity of a substance/material is a property, which (by definition) needs the context of other reacting substances in order to be expressed. This statement is useful because it draws our attention to the context of reacting substances, which are necessary in order that an (in general)[23] extraneous substance may act as catalyst; consequently we have to choose an appropriate general way to consider reacting systems.

In 1981, G.M. Schwab, in a note on the history of concepts in catalysis stressed that during several decades many authors had proposed a more or less generally valid ‘theory of catalysis’, but that "with time their concepts have gone astray more and more, and have grown rather in number than in validity". He added that "an older concept had seldom been totally replaced or refuted by a newer one, but that usually an older concept [had] been represented in new or modern scientific language". Schwab regarded this last fact as a proof "that the mechanisms of catalysis are so manifold that many fields of thought must be employed – although the notion of catalysis is very unitary from the viewpoint of thermodynamics and kinetics" (Schwab 1981, p. 11). It is exactly from this point of view that I intend to discuss the catalytic activity and selectivity of a substance/material.

We can consider a closed chemical system consisting of a set of chemical substances each with a certain number of moles. At any value of temperature and pressure, the evolution of the system must obey the laws of thermodynamics. However, in many actual chemical systems the variation of the Gibbs free energy may be negative for a large set of reactions. Thus, we may set a thermodynamic framework given by all the DG <  0 corresponding to possible reactions inside the system, such that the substances are connected through the usual stoichiometric equations. Nevertheless, as we know, the evolution of the system does not necessarily evolve towards the lowest value of the Gibbs energy, principally because of the kinetic aspect of the reactions. Speaking in terms of activation energy, at the given temperature the molecules of the virtually reacting system may or may not have enough energy to climb the energy passes which separates/unites the states before and after the permitted reactions. When the temperature rises, the number of different molecular species in the system may boost, because of the increasing probability of successful attacks on molecular stability. At a given temperature and pressure the most evident feature of the thermodynamic framework is built up by the minima corresponding to all the possible mixtures of products. These minima are connected by the relative maxima corresponding to the ambiguous substances named ‘activated complexes’. On this supporting structure of the classical thermodynamic framework, chemical kinetics introduces/reproduces an essentially kinetic feature, the reaction rates, with their empirically determinate, non-stoichiometric, orders of reaction, and their reference to the macroscopic world of chemical substances. At this point, statistical thermodynamics and quantum chemistry try to describe, qualitatively and quantitatively, the molecular dynamics, which rules the transformation of molecular systems at the microscopic level of reality. We may say that classical and statistical thermodynamics and quantum chemistry give us a vantage point from which we enjoy a good view on the valleys, passes, and saddles of the potential energy surface. These valleys and passes constitute the ‘natural’ kinetic landscape of any chemical system. The metaphor I am proposing is to consider a catalyst as a contrivance to modify the kinetic landscape of a chemical system. In Sect. 2.4 I will ‘apply’ this metaphor to three cases of industrial catalysis, but before I support the choice of the word ‘landscape’ with a brief comment.
 
 

2.3.1 A gloss on ‘landscape’ and landscapes

2.4 Kinetic landscaping

2.4.1 Burning butenes

Products and reagents are involved in more than one reaction, each occurring with its own rate and stoichiometry; thus, we can describe the rate of any appearing and disappearing substance in this form:

r_i = S_ja_ijr_j           i = 1,… 6;  j = 1,… 5

It is to be appreciated that in this vast kinetic landscape the most crowded resorts could be the minima of free energy corresponding to the combustion end products, carbon dioxide, and water. As a matter of fact, the ignition temperatures of the four butene isomers (listed at the beginning of this sub-section) are 384, 325, 325, and 465 °C, respectively. However, in 1985 there were three plants on stream, in Germany and in Japan, in which n-butenes and mixed butenes were oxidized to maleic anhydride over V₂O₅/P₂O₅ catalyst, with selectivity of about 50-60 mol % (Obenaus et al. 1985, p. 486).

A similar kinetic landscape (at a lower temperature) may be modified by catalyst containing vanadium pentoxide, along with a variety of other oxides (titanium, zinc, aluminum or antimony oxides). At 200-320 °C, it is possible to control the oxidation of n-butenes to form acetic acid in this way :
 
 
 

If we now write three overall oxidation reactions of butenes in the simplest stoichiometric way, we see at a glance the permanent identity of reagents and the enormous difference of products in the making of maleic anhydride, the production of acetic acid, and the simple combustion:
 
 

C₄H₈  +  3 O₂   ®    C₄H₂O₃  +   3 H₂O

C₄H₈  +  2 O₂   ®    2 C₂H₄O₂

C₄H₈  +  6 O₂   ®    4 CO₂  +  4 H₂O

Returning now to the industrial process for obtaining maleic anhydride, the feedstock was the C₄ product from a steam cracker, consisting essentially of butenes and butadiene plus smaller amounts of butanes. Under optimum operating conditions butanes remained without reaction, isobutene burnt to carbon oxides and water, and the n-butenes and butadiene were converted to maleic anhydride (Satterfield 1980, p. 204). I conclude this point stressing that the C₄ components followed three very different paths: no reaction, completely oxidized, and partially oxidized (in the wanted measure and structural position).
 
 

2.4.2 Making nitriles

CH₂=CH-CH₃  +  NH₃  +  1½ O₂   ®    CH₂=CH-CN  +  H₂O

The process of ammoxidation is obviously connected with the previously discussed processes of oxydehydrogenation and oxidation. It is to be expected that ammoxidation which is a 6 electrons oxidation is more difficult and demanding process than oxidation strictu sensu (4 electrons) and oxydehydrogenation (2 electrons). As it was pointed out by Grasselli (1986, p. 217), "effective ammoxidation catalysts are multifunctional in nature. They must perform a complex sequence of [thirty-two] bond-breaking and bond-making processes […] and must provide a facile pathway to the desired intermediates since, thermodynamically, undesirable waste and by-product formation (i.e., CO, CO₂, HCN) is more favorable than the formation of the desired product". The sequence of thirty-two microscopic processes discussed by Grasselli is a good example of the (tentative) mechanistic understanding of heterogeneous catalytic reactions. Nevertheless, I have mentioned Grasselli’s knowledge exploit also for other reasons.

The first reason is that Grasselli’s results let us look closer at the relationship between the chemical reaction and its description in terms of pathways on the free energy surface. The acts of the molecular drama of ammoxidation are actually played on particular sites of the catalyst surface. Our description in terms of quantum chemistry and statistical thermodynamics tries to reproduce the drama on the free energy surface, where any pass or saddle of the reaction path corresponds to an activated complex. Grasselli’s scheme proposed only one type of active site of Bi₂MoO₆ (a model catalyst). This site (rather complicated) is the chemical stage of the thirty-two electronic acts. Our quantum mechanical description should reproduce the sequence on the energy surface, but there (probably) many of the thirty-two bond changes should happen on a different pass or on a different point of a saddle. At any point of interest, a transition state complex may be bigger and looser than the preceding one, or more compact and stiffer, and these different conditions change the frequency of the normal mode which correspond to a passage across the energy barrier (the pass, the saddle) (Benson 1976, pp. 86, 82). Grasselli describes p and s allyl intermediates, and states that "the position of the equilibrium between the p and the s allyl intermediates varies greatly depending on the nature of the catalyst" (molibdate or antimonate) (Grasselli 1986, p. 219). Thus, we see that any catalytic contrivance not only changes the height of the energy barriers, but also modifies the constitution of the activated complexes. The landscaping of the energy surface can (or must) became a more delicate gardening on the top of the energy barriers. As Benson (1976, p. 6) stated: "Catalysts may be looked upon as substances that perturb the populations of chemical intermediates".

A second aspect of the Sohio ammoxidation process is of interest, but not from an epistemological point of view. In the British process, developed by Distillers Ltd., propylene was converted first to acrolein, and then, without any actual treatment of the acrolein, it was converted to acrylonitrile. In the Sohio process, the whole reaction was carried out in a single step. There was a patent suit in the early 1960s between the British firm and the Standard Oil of Ohio, a suit which hinged on whether the Ohio one stage process was an infringement of Distillers’ patents on the two-stage process. The essential question was ‘simple’: was acrolein an intermediate in the one-stage process? Thus the understanding of the reaction mechanism was of more than academic importance; unfortunately the case was settled out of the court, and we cannot read a sentence about the right, actual, pathway from propylene to acrylonitrile (Reuben & Burstall 1973, p. 300).
 
 

2.4.3 Assembling polymers

The landscaping of the Ziegler-Natta catalysts is very particular. The kinetic landscapes corresponding to the polymerization of ethylene or of propylene have thousands of valleys separated by passes of very similar relative height. In this case, the function of the catalyst seems to select a unique pathway, whose crucial feature is not the overall increase of the reaction rate, but the compulsory repetition of the same (microscopic) action, exactly as it happens in a (macroscopic) assembly line.[25] From this point of view, the Ziegler-Natta catalysts are extraordinary contrivances.
 
 

2.5 Intellectual attitudes to industrial catalysis

2.5.0 Industrial catalysis for the less intelligent?

Polanyi – through Wigglesworth’s words – suggests two other themes connected with the opposition basic/applied science: in applied research, a different attitude of mind is necessary, and unexpected rules have to be followed. In the next subsections I will treat these two points, in order to get a better comprehension of industrial catalysis, as both discipline and subject matter.
 
 

2.5.1 A different attitude of mind, hints from philosophy of technology

Sometimes technicians seem to share the same idea, or, better, the same ideology. The reading of a technical paper can help me to explain myself. In 1994, in the final part of a six-day seminar of catalysis, an Italian author was treating the distressing problem of the scale-up of catalyst production. In the published text of the lesson, he wrote, "Procedures for catalyst manufacturing are usually developed in an empirical way, through time-consuming and costly work, though some attempts of a scientific approach begin to appear in the literature". A few lines later he added, "the preparation of catalysts having good industrial performance can show insurmountable difficulties even for catalytic researchers, if not skilled in manufacturing practice. This mainly because even small changes in manufacturing procedure may have large effects on catalytic properties" (Pernicone 1994, p. 388, my italics). There is obviously a derogatory overtone attributed to ‘empirical’. In this text (as well as in many other technical texts) the word ‘empirical’ is used as an antonym of ‘scientific’, while the syntactic and semantic context could bear quite a different couple of epistemic terms: ‘experimental’ vs. ‘theoretical’. The ideology behind this attitude is apparent in the same quoted text, where the author explains that "an empirical way" is time- and money consuming. The pressure towards a more ‘scientific’ industrial catalysis (as academic discipline) has its origin and justification in the high cost of the ‘empirical’ industrial catalysis (as the processes on stream). However, Pernicone gives us a clue for the correct understanding of his own work as a skilled manufacturer of catalyst. In fact, his couple of phrases "small changes"–"large effects" is indicative of the complex nature of industrial catalysis.

A last aspect of Pernicone’s highly technical discourse is pertinent to our inquiry. The quantity of structured/organized knowledge available to a ‘skilled manufacturer’ is enormous. Pernicone quotes in particular 24 different unit operations in catalyst manufacturing (Pernicone 1994, p. 403), each of which has its own rules [27], and more or less approximate laws.[28] Frankly, I think that technology at large, and chemical technology in particular have that explicit theoretical framework required by Carpenter – though not always with a pedigree certified by mathematical physics.

From philosophy of technology, we might draw several other suggestions useful for the present inquiry. For sake of brevity (and, I hope, of clarity) I will mention only one more, which demands of the analyst of technology a shift of attention: from things, made and used, to processes, making and using. Introducing this point of view Mitcham states that "[t]o take process or activity as the fundamental category of technology is characteristic of two different professional groups, engineers and social scientists. Engineers, in focusing on process, place stress on the making aspect, social scientists on using" (Mitcham 1984, p. 308; italics in the original text). The ‘making’ point of view emphasizes the creative technological practices of invention and design; the’ using’ perspective regards the social, economic aspects of production and utilization. Both viewpoints have been used in the preceding parts of this essay, the first one mostly in the historical Part 1, the second one mostly in the present Part 2. The opposition thing/process is actually somewhat blurred when we consider the contextual meaning of the properties which define a thing (cf. Sects. 2.2 and 2.3); however an epistemologically meaningful parallel shift of focus remains:
 
 
 

Mitcham states that "Cybernetics claims to reduce objects to processes; what is important is not what a thing is but how a thing behaves" (Mitcham 1984, p. 316). Probably, as in many other cases, the reductionism of cybernetics is too strong; anyway, in industrial catalysis knowing how a catalytic process runs (at the appropriate scale) is perhaps more important as knowing that a catalyst is such-and-such a thing.
 
 

2.5.2 Working rules and unexpected rules, hints from a Methodenstreit

In the title of the article that caused Thomas’ indignation, Schlögl rose a simple question: "Heterogeneous Catalysis – Still Magic or Already Science?", which he plainly answers in the conclusion: "if we try to answer the question asked in the title the realistic reply must be – still magic" (Schlögl 1993, p. 383). The three-page essay discusses the relationships between the "catalytic performance" of a solid catalyst and four groups of determining factors (formal kinetics, chemisorptions processes, microkinetics, and microstructure of the catalyst), whose quantitative relations between each other and the mode of catalyst operation must be determined. At the core of the analysis, there is the catalytic reaction.[29] Schlögl’s principal points are:

1. The optimizing of industrial heterogeneous catalysis needs to understand "the operation of a catalytic system at an atomic level", but "it is not adequate to have only a model", because "[e]xtrapolations from the data obtained from the simplified model to the actual catalytic reaction may endanger the relevance of a whole mechanistic reconstruction".
2. "Industrial chemists tend to be rather skeptical towards inductive-scientific strategies in heterogeneous catalysis, since it is evident that our understanding of the relationships between structure and reactivity is inadequate".
3. "[T]he long history of catalyst characterization should have taught the lesson that all analysis should be done in situ", i.e., "a property of a catalyst [has to be] investigated as function of reaction conditions characteristic of the practical application under simultaneous observation of the reaction kinetics".

In the article, written by Sir John Thomas and Kyril I. Zamaraev, the answer to point (b) was oblique, if not devious: "If Schlögl’s claims were true, future employers and boardroom protagonists could well underestimate or misjudge the contributions that the research chemist could make to the future of their industry, and employment prospects for numerous highly trained physical scientists would be bleak" (Thomas & Zamaraev 1994, p. 308). On the crucial point (a), the rebuttals by the two anglo-russian authors were numerous, distributed among "four distinct categories of catalysts". In many cases these were simply statements on the present state of the art concerning the characterization of catalyst by solid-state chemical research: "so well understood is the correlation between crystal structure and selectivity among acidic shape-selective catalysts that computational chemistry has already contributed significantly to the evolution of superior catalyst for a given task" (ibid., p. 309). In at least one case the appeal to a ‘new rationality’ was somewhat whimsical: "The fourth category of catalyst research encompasses advances in the application of rationalized new ideas relating to the design and mode of operation of catalytic reactors" (ibid., p. 310; my italics). In think that it is sufficient to open a textbook on chemical reactors for seeing that chemical engineers have not waited for Thomas’ and Zamaraev’s advice in order to use rationality in their design practice. However, the point more pertinent to our inquiry is on Ziegler-Natta catalysts:

Schlögl admits that "Thomas and Zamaraev give an expert description of recent ingenious developments in heterogeneous catalysis", however he adds that a "proof that these developments are based upon an understanding of the structure-reactivity relationship is lacking" (Schlögl 1994, p. 311). In Schlögl’s reply, precisely on cases discussed by Thomas and Zamaraev, we find a constant reference to "working rules". "The important field of selective oxidation catalysis", in particular, "is characterized by the existence of qualitative concepts […] which summarize the practical experience of researchers over about 25 years"; even if they can not "be used in a quantitative way like a theory", "such working concepts are of great practical value in guiding the chemical intuition required in developing catalytic processes". Schlögl’s epistemological conclusion is that

It is clear from the preceding analysis that Schlögl looks at rules and processes, while Thomas prefers laws and things. However, the discussion has also a meaning that is not only epistemological. It was also a running debate on method, whose sociological meaning is clear: behind any Metodenstreit, there is always a struggle for supremacy within a discipline or among disciplines.[30] This is a general and perhaps unexpected rule of the epistemological discourse, as it may easily be demonstrated through socio-linguistic analysis (Cerruti 1992). However, what is at stake in the Schlögl vs. Thomas case, as well as in other debates on catalysis (as discipline), is not only the hierarchy between academic disciplines or specialties, but also another and more meaningful one: the hierarchy – affirmed and proclaimed – between pure (or basic) research and applied (or industrial) research. Here the question is not about the prestige of academic communities, but it points directly to a central problem of epistemology, whether the value of knowledge depends on its source.

An industrial catalyst is a material which must demonstrate (to a high degree) that all the properties discussed above (Sect. 2.2) are not virtual, but correspond to a complex, actual behavior inside the reactor. The success of a catalyst is the result of a tremendous knowledge effort. If that success makes a material ontologically an industrial catalyst, then we have to attribute the same epistemic value to any single bit of knowledge or of practical action which contributes in a necessary way to the final success. The appeal to a scale of rationality is not simply an ideological move towards an easy gained academic supremacy, but it is a traditional way of affirming a cultural and social hegemony.
 
 

2.5.3 A language game on ‘political’ and ‘rational’

2.6 Epistemic uncertainty and disciplinary openness: a glance on the disciplinary makeup

In his opening lecture, Paolo Corradini, one of the ‘midwives’ of the birth of stereospecific polymerization, speaks of "possible mechanism"[32] and states that "[m]olecular mechanics studies, relative to models for the Ziegler-Natta catalysts stereospecific polymerization, lend support to the hypothesis that the differences in the rates of insertion of units having different configurations are mainly ought to non-bonded interactions at the catalytic site" (Corradini 1995, p. 8). In the succeeding papers of the volume, we find many statements on the ‘certainty’ of the reaction mechanism:

The dialectics between epistemic uncertainty and disciplinary openness is not simple and painless, in particular because of the huge economic interests which press the scientific community working on Ziegler-Natta catalysts and, more generally, on industrial catalysis. In a lecture given at the 10th International Congress on Catalysis, J.A. Rabo said that – in his opinion – "the most important future objective in catalysis science" is the "characterization of catalytic sites at the atomic and molecular level" (Rabo 1993, p. 23; italics in the original text). In the final part of his discourse, entitled "The Discipline of Catalysis", he stressed that "the makeup of the discipline in catalysis is continuously evolving". In particular he referred to "surface science, material science, inorganic synthesis, theory" in the past, and to "biocatalysis and new areas of inorganic synthesis for catalytic materials" for the future (ibid., p. 28).

The important theme of the disciplinary makeup introduced by Rabo was resumed at the 11th International Congress, in two general lectures by Alfons Baiker (ETH, Zürich) and Ian Maxwell (Koninklijke / Shell-Laboratorium, Amsterdam). For sake of compactness, I have listed the components of their ‘recipes’ in Table 6. Both of the lectures were highly interesting (from our point of view, too). At the end of my essay, I mention only two enlightening passages of Maxwell’s opening lecture. He spoke of a "perceived gap between academic basic research and industrial applied research", and of consequent political moves in the United Kingdom and in the Netherlands, in order to foster also in the academic world a "multi-disciplinary approaches to problem solving". (Maxwell 1996, pp. 7-8). After having listed the eight disciplines of Table 6, Maxwell stated that "innovation in this field is therefore very often achieved by lateral thinking across these different disciplines" (ibid., p. 1). I am sure that this lateral thinking is not for the less intelligent.
 
 
 

2.7 Conclusions

As a conclusion, it seems to me that this long inquiry supports the following picture of industrial catalysis as academic discipline. The professional standpoint is relevant to the understanding, description, and control of the chemical process (catalysis), while the actual performance of an industrial catalyst tests the efficacy of the professional understanding. Along with a constant and common reference to this economic level of reality (the industrial production), scientists select their epistemic arguments both for a better understanding of the microscopic level of reality (the chemical process), and a higher, personal status at the social level of reality (the chemical community).
 
 
 

Notes

[2]   Reference to a word as a dictionary lemma is indicated by single quotation marks.

[3]   For a more meaningful portrait of Karl Ziegler, see Ziegler 1968 , Wilke 1975, McMillan 1979, Elsch 1983.

[4]   On Giulio Natta see McMillan 1979, Carrà et al. 1982, Cerruti 1989.

[5]   A very readable history was written by Frank M. McMillan (1979); other sources are in Seymour & Cheng 1986; some essays by industrial researchers are somewhat unfair, and in this sense particularly interesting. I will say nothing on the question of priority, important from the economic point of view, but tedious in history and dull in epistemology; moreover, on this question I completely agree with the conclusions of Piero Pino (Pino & Moretti 1987).

[6]   Until then polyethylene had been produced by a very high-pressure process discovered in the 1930s at ICI (Ballard 1986). The ICI polyethylene resulted from a chain radical reaction, and it had molecules with branched chain. Ziegler’s polymer had a higher density than ICI polymer, so the two materials were named ‘high density polyethylene’ (HDPE) and ‘low density polyethylene’ (LDPE), respectively.

[7]   Table 2 refers to catalytic materials functioning in industrial plants, whose high cost and low flexibility limits the quick renewal of processes.

[8]   Vide infra, Sect. 2.6.

[9]   The term ‘recipe’ sounds ironic. The study of the actual microscopic function of ‘internal’ and ‘external donors’ is now a very active research field.

[10]   Vide infra, Sect. 2.2, and Note .

[11]   A single-site catalyst is something as a Holy Grail in the catalysis world.

[12]   I will treat at length the opposition empirical/rational in Sect. 2.5.2.

[13]   "Detailed guidelines have been developed for the selection of supports with optimal composition, particle size, pore size distribution, and surface OH group density, and for their treatment with various alkyl aluminum and aluminoxane activators" (Brintzinger et al. 1995, p. 1163).

[14]   The microreactor metaphor is in use at least since 1988; see reference in (Karol 1995, p. 574).

[15]   ‘Musterung’ is used also in the military register, for inspection of troops ‘von Truppen’, or for the medical examination for military service ‘von Rekruten’.

[16]   This is a very interesting definition, and I will return later on to its usefulness.

[17]   The star is used here for ‘catalyticity’ in order to pay attention to a possible non-grammaticality of the term. However, in the rest of the present essay I will use ‘catalyticity’ without any star.

[18]   See, for example, a recent ‘explanation’ of this exploit in Kelly 1998.

[19]   Just before the quoted list Cavani and Trifirò (1994, p. 11) wrote, "The catalyst remains, in general, unaltered at the end of the catalytic process".

[20]   This is an important ontological point, which regards the level of reality pertaining to the catalyst performance. We have seen above that the morphology of the Ziegler-Natta catalyst particles rules the morphology of the resulting polymer particles. Thus, a molecular process is controlled by geometrical factors and reproduces the same factors at a larger scale; see for example the figure in (Whiteley et al. 1992, p. 501).

[21]   This term is particularly curious because "[t]hese ‘dormant’ species might still be quite reactive, however, with regard to other important transformations, such as chain termination or catalyst deactivation reactions" (Brintzinger et al. 1995, p. 1165, note 34).

[22]   Here I am referring to a pragmatic (from prâgma: fact, action) field in order to characterize an exploration which continuously looks at events outside the intermediate world of language; only in this last realm a ‘pure’ semantic analysis is possible.

[23]   The important case of autocatalytic systems falls within my discourse, with simple ‘grammatical’ modifications.

[24]   Personally I have an analogous aesthetic (sometimes ecstatic) and emotional attitude towards the best results (and descriptions) of several scientific disciplines, including classical and quantum mechanics. I invite the readers of this footnote to look quietly at the figure on p. 286 of the splendid book of Levine and Bernstein (1987). The figure shows on a time scale of 10^-14 s the wave-packet evolution for the collinear H + H₂ (n = 1) collision.

[25]   The term ‘dormant’ is used in the specialized register on Ziegler-Natta catalysts for the ionic species inactive concerning the chain growth; ‘dormant’ brings several unusual semantic traits into the microscopic description. The traits are of teleological, anthropomorphic, estimative sort; see also the preceding Note .

[26]   Vincent Brian Wigglesworth dedicated his research life to the study of insect physiology, and discovered the juvenile hormone and its function in the control of growth and form in insects. During the war he was Director of the Agricultural Research Council Unit of Insect Physiology.

[27]   E.g., for extrusion: "Water content if often exceptionally critical: a change as small as 1% (in the usual range 30-50%) could be sufficient to make extrusion impracticable" (Pernicone 1994, p. 397).

[28]   E.g., for precipitation the supersaturation dependence on concentration, temperature and pH (Perego & Villa 1994, p. 27), and for tableting the "very crude relationship […] between ductility, melting point, elastic modulus and Mohr’s scale of hardness" (ibid., p. 38).

[29]   The position of the catalytic reaction is also graphically central in fig. 1 of Schlögl 1993.

[30]   In the 1890s, the methodological struggle that was dubbed Methodenstreit was fought by Karl Lamprecht against the political history of G. von Below and of the majority of German historians.

[31]   My suspects on a too liberal use of ‘reason’ grew many years ago, when I read a splendid essay on the history of the word ‘razza’ by the great linguist Leo Spitzer (Spitzer 1948, pp. 147-169); in fact Spitzer had demonstrated the connection between the Italian ‘razza’ and the Latin ‘ratio’. From Italy ‘razza’ was exported and adapted in other European countries, so that the English ‘race’, the French ‘race’, and the German ‘Rasse’ derived from the Italian word.

[32]   All the italics in this subsection are mine.

[33]   The same title of this paper is interesting.
 
 
 

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