Catalyst development approaches

It is beyond the scope of this book to go though all the specifics of catalyst testing and to discuss all pitfalls that may arise. Instead we list the Ten Commandments for the Testing of Catalysts. This is a set of guidelines that have been provided by experts of a company called Catalytica [F.M. Dautzenberg in Characterization of Catalyst Development An Iterative Approach (Eds. S.A. Bradley, M.J. Gattuso, R.J. Ber-tolacini), ACS Symposium Series, Vol. 411 (1989)]. 

Please note that the approach taken to find a solution for carbon deposition started with leads from surface science and computational chemistry and led to the straightforward development of a new catalyst a good example of rational catalyst development based on fundamental knowledge ... 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

This chapter will primarily cover combinatorial and parallel efforts to develop transition metal complexes as new catalysts. The considerable effort to use combinatorial approaches to make new inorganic materials will not be addressed (15-17). Additionally, the efforts to develop screens for combinatorial approaches to catalyst development will be addressed only in situations where they are used in the development of new catalysts. 

Since better selectivity was obtained when the ligands were removed from the dihydropyran-derivatized polymer support, after synthesis they were cleaved from the polymer and used in catalysis without purification. Comparable selectivities were obtained with ligands that were used directly without purification and selected examples where the ligands were purified prior to use. For such an approach to be useful in catalyst development, it is critical that the material from the synthesis be of sufficient purity to be used without purification. 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

The work discussed above by Snapper, Hoveyda, and co-workers (27) illustrates the power of a parallel approach to catalyst development. The authors took a basic ligand type that had been reported by Inoue and co-workers (25) for the catalysis of cyanohydrin synthesis and optimized the system for two other reactions and a number of substrates. 

The last work pertaining to the discovery of new catalysts is perhaps the most novel approach to be reported thus far. In one of the earliest approaches taken toward catalyst development, Menger et al. (61) attempted to find catalysts for phosphate ester hydrolysis. A series of eight functionalized carboxylic acids were attached to polyallylamine in various combinations. Each of these polymers were then treated with one of three metals, Mg2+, Zn2+, or Fe3+. The different members of each library were identified by the relative percentages of each carboxylic acid attached to the polyamine. For example, one polymer possessed 15% Oct, 15% Imi, 15% Phe, and 5% Fe3+. There is no attempt to identify the location of the various carboxylic acids in a given polymer. This approach is novel since each system consists of an ensemble of different ligands with the carboxylic acids positioned in various locations. Each polymer within a given ratio of carboxylic acids consists of a combinatorial library of potential catalysts. 

Franklin, N.L., Pinchbeck, P.H., and Popper, F. (1956), A Statistical Approach to Catalyst Development Part I The Effect of Process Variables on the Vapour Phase Oxidation of Naphthalene, Trans. Instn. Chem. Engrs, 34, 280-293. 

The following sections are organized into two parts. The first presents a brief overview of several approaches to preparing nanostructured supports and catalysts that are under current development. Approaches that incorporate building blocks in either part of the system (catalyst or support) will be highlighted. The second part describes a new approach to preparing nanostructured catalysts that we are currently developing in our laboratories. 

Chiral carbo- and heterocycles are widespread structural motifs in biologically active compounds. The cycloisomerization of 1,6-dienes (A) offers an elegant and atom-economic [56] approach to five- or six-membered carbo- or heterocycles [57]. Metal complexes based on Pd [58], Ni [59], Rh [60], Ru[61], and Ti[62] have been identified as promising lead structures for catalyst development. Some of the reported systems are highly chemo- and regioselective toward the formation of the individual five-membered ring compounds B-D (Scheme 2.1.5.2). Enantioselec-tive cycloisomerization, however, has been assessed only sparsely so far, and remains a challenging task [46, 63]. 

Combinatorial approaches to catalyst development with multichannel detectors... 

If one is confronted with expensive manual work with low complexity, a typical industrial approach would be to automate such a standard manual procedure. This will not only reduce costs of staff but will also increase the reproducibility of the coating procedure, which is an extremely important fact in catalyst development. 

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