Finding New Catalysts

Haber achieved excellent results with osmium, but the metal could not be used in large-scale synthesis for two reasons in contact with air it was readily converted to volatile osmium tetroxide (OSO4) more importantly, as already noted, its worldwide supply in 1909 (most of it, as Haber advised, was actually acquired by BASF for 400,000 marks) was limited to some 100 kg, and in October 11, 1910, BASF estimated that this would suffice to produce no more than about 750 t NH3 a year. The other effective catalyst, uranium, was also expensive, but it was, even at that time, obtainable in larger quantities. Tests showed, however, that it too was not a 

Alwin Mittasch (1869-1953) in his BASF office in Oppau in 1929. Courtesy of BASF Un-ternehmensarchiv, Ludwigshafen, Germany. 

Laboratory apparatus designed by Georg Stern and used by Alwin Mittasch for testing catalysts (1910). 

Mittasch eventually concluded that the presence of additional elements, acting as promoters of the reaction, must determine the effectiveness of a metallic catalyst, and he launched a systematic testing of a large number of promoters, both individually as well as in combinations with differing amounts of various ingredients, added to a pure iron catalyst. The first experiments with mixtures were done just before the end of 1909. NaOH and KOH were the first tested additions, and the first patent concerning the mixed catalysts (DRP 249 447) was filed by Bosch, Mittasch, Stern, and Wolf on January 9, 1910.  

Nothing was overlooked in the search for the best possible catalyst all known metals with catalytical properties—Co, Fe, Mn, Mo, Ni, Os, Pd, Pt, U, W—were tested singly in pure form as well as binaries (such as Al-Mg, Ba-Cr, Ca-Ni, Fe-Li, Na-Os, or W-Zr) or ternary elemental catalysts (ranging from Al-Cr-Na to Mn-Si-Na), as well as even more complex mixtures.Soon the testing identified a number of substances that act, even at very low concentrations, as catalytic poisons and whose presence must be strictly avoided, as well as a number of compounds that act as strong promoters of catalysis. Sulfur, phosphorus, and chlorine are the three most common elements in the first category.  

---

The first of the three steps used to find new catalysts was to screen the entire pooled collection of complexes for compatible reaction conditions. This process identified aqueous H202 as an effective oxidant. The second stage of the process was to screen the active metal libraries for epoxidation of frans-P-methylstyrene (TBMS). Activity was determined on a mixture of all 192 ligands, with each indi-... 

Since 1957 and the discovery of the Speir s catalyst H2PtCl6/ PrOH, considerable efforts have been made to find new catalysts with high activity and selectivity. Along with the platinum-based catalysts, the Wilkinson s complex [103] Rh(Ph3P)3Cl is one of the most popular hydrosilylation catalysts. Ruthenium catalysts are also able to promote the addition of silanes to unsaturated carbon-carbon bonds, and several reports have shown during the past decade that the well-defined ruthenium complexes of type Ru(H)(Cl)(CO)L can provide excellent activity and selectivity [104—... 

By a better knowledge of the reactions in the process conditions, to find new catalyst combinations and synergism with noncatalytic processes by a better knowledge of the reactions involved in the process. 

In general, the current approach to finding new catalysts is still rather empirical. Chance and intuition as well as systematic screening play an important role. 

Tlie basic study of intermolecular interactions is facihtated by one-bead-one-stRicture libraries which can be powerful tools for the discovery of hgands to synthetic receptors and vice versa. Encoded combinatorial libraries have been useful for disclosing ligands for well-designed macrocyclic host molecules and to elucidate their specificities for peptide sequences. These studies led via receptors with more flexibility to simple host molecules without elaborate design that ai e accessible to combinatorial synthesis. One application is the development of chemical sensors for analytes that are otherwise difficult to detect or only non-specificaUy detected. Such hbraries have been used to find new catalysts and enzyme mimics. 

A second important project of the Herman Mark group was the investigation of catalysis. Catalytic reactions were the underlying principle of many industrial processes, but not well understood at this time. Therefore empirical methods, a sort of trial and error, were the only way to improve upon and find new catalysts. 

The first hydration of an alkyne was discovered in 1881 by Mikhail Kucherov, a Russian chemist from the Imperial Forestry Institute in St. Petersburg, using mercury(II) bromide as the catalyst [97] producing acetaldehyde. This reaction has been extensively applied in synthesis, although due to the toxicity of mercury compounds and the relatively low turnover numbers (<500), much effort has been done to find new catalysts. Thus, transition-metal-complexes containing Pd (II) [98], Pt(II) [99], Ru(ll) [100], Rh [101], and other metal centers [91] have been used, although in most cases the catalytic efficiency was only moderate. 

---