Catalyst and process optimization for petrochemical applications of stereoselective olefin metathesis

The class of chemical processes known as olefin metathesis involves rearrangements of bonds between two unsaturated reacting molecules, termed olefins, or alkenes, to form two new product olefin molecules. Even if such rearrangements at first glance may seem trivial, olefin metathesis is a very important reaction, which is not least true for the petrochemical industry, for which metathesis is used, in large scale, to make more valuable olefins from cheaper ones. Olefin metathesis requires the presence of a catalyst in order to proceed. Whereas standard olefin metathesis catalysts give product olefins of two different structural forms, termed cis and trans, respectively; the main goal of the project has been to develop, via detailed control of the catalyst molecular structure, efficient catalysts and processes predominantly giving the cis form. The cis form is often the only intended product, in the case of which the trans form is an unwanted byproduct, so catalysts giving a high content of cis-product simplify or eliminate the subsequent, often costly, product separation. The most general and important result of the project is that we have shown how to make cis-selective catalysts (that is, catalysts giving predominantly cis-products) starting from almost any existing, non-selective catalysts based on the metal ruthenium, the only metal that so far has been used in molecular olefin metathesis catalysts for industrial processes. Many such non-selective ruthenium-based catalysts have distinct advantageous properties and have been commercialized with the aim to use these properties in particular applications. The fact that cis-selective versions of most or all of these commercially available catalysts can now be made, in most cases in a single reaction step, greatly increases the chances of identifying cis-selective catalysts suitable for the individual metathesis processes. In other words, cis-selective catalysts with particular properties and for particular chemical processes can be tailor-made. For example, whereas catalysts based on metals such as ruthenium are often fragile and do not tolerate water, air, or impure solvents or starting materials, one such tailor-made catalyst is remarkably robust and can be used in air and in the presence of water, which is highly useful for practical applications. Other examples of our new, tailor-made catalysts handle, as the only cis-selective metathesis catalysts, reactions where all the reactants are so-called carboxylic acids, and some of the catalysts are also very efficient in the synthesis of large ring molecules often found in pharmaceutical drugs. Whereas the above catalysts are discrete molecules with well-defined structures, most industrially used catalysts are solid materials and are called heterogeneous catalysts. The main advantage of heterogeneous catalysts is that they can be separated from the reaction medium after the process by simple filtration. Conversely, the main disadvantage of the heterogeneous catalysts is that it is difficult to control the molecular structure of the catalytically active species on the surface of the solid material. In other words, it is difficult to control and tune the properties of the heterogeneous catalysts, and this lack of control is the reason why the many varieties of well-defined molecular (as opposed to heterogeneous) metathesis catalysts, for the production of everything from pharmaceuticals and pheromones to polymers, hardly have heterogeneous counterparts. Thus, to improve upon the lack of modern, tunable heterogeneous catalysts, another project goal has been to create heterogeneous versions of the above-described cis-selective molecular metathesis catalysts. The first examples of cis-selective heterogeneous olefin metathesis catalysts have been achieved by first equipping the solid support material with specially designed chemical groups, called linkers, and next by binding these linkers to discrete catalyst molecules.

The first examples of predominant formation of cis-olefinic products using functional group tolerant homogeneous organometallic catalysts have recently been demonstrated in our laboratory. Although they are currently mainly being developed for ring-closin g metathesis in natural product synthesis, they have already shown very promising catalytic activities and selectivities in a broad range of olefin metathesis reactions. The present proposal aims at taking advantage of the remarkable properties of the new stereoselective catalysts and the competitive advantage they offer by launching a project to further develop and optimize these catalysts and their use in specific processes for production of high-value, targeted alkene stereoisomers and polymers with sp ecific cis/trans ratios. For the application in (large-scale) production of short and intermediate-length olefins heterogeneous versions of the new catalysts will be developed and they will be thoroughly tested and tuned for use in fixed-bed flow reactors . For the application in ring-opening metathesis polymerization (ROMP) target specialty polymers with expected unique and desired properties due to high cis/trans ratios will be identified and catalysts and process conditions (reactor types, solvent, temp erature, etc.) for such ROMP-based production will be optimized.