Selective product crystallization as a new catalyst recycling tool in homogeneous transition metal catalysis exemplified by the conversion of oleochemicals

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New and sustainable ways to biopolymers are in focus of attention in times of global raw material change. In particular, raw materials from natural fats and oils have structural properties that make them interesting for the production of certain polymers that were previously petrochemically accessible only. The conversion of these oleochemicals into the necessary polymer precursors is particularly efficient with the aid of homogeneous transition metal catalysts. The many advantages of this form of catalysis, such as high selectivity, mild reaction conditions and reaction control in a homogeneous phase, are countered by the critical task of efficient separation of products and catalyst in the downstream. However, separation is crucial with regard to an ecological and economical viable application: On the one hand, to obtain products free of heavy metal contamination. On the other hand, to enable recycling of the usually expensive precious metal catalysts. However, this task has not yet been solved for the very interesting class of polymer precursors from oleochemicals produced via homogeneous catalysts. In principle, processes for the separation of homogeneous mixtures are based on the different molecular properties of the substances contained, e.g. boiling points (distillation), relative solubilities (extraction), or molecular sizes (nanofiltration). For the discussed substance class, however, there are no sufficient differences between catalyst and product in their properties used in the established separation processes, which is why the desired separation efficiency is not achieved. As a result, scaling to industrial scale presently fails due to the unsolved separation task, although great interest and potential has already been shown. Interestingly, a separation due to different melting points has not yet been considered.The aim of the present research project is therefore to use crystallization for the first time as a separation and recycling process for homogeneous catalysts and thus to fundamentally demonstrate its potential for the existing separation task by means of a model reaction. An interdisciplinary approach is required in order to optimally adapt the respective parameters of reaction and crystallization to each other and to detect mutual interactions as early as possible. By doing so, the combination of homogeneous catalysis and cooling crystallization succeeds to selectively separate the product as a pure solid from the reaction mixture. At the same time, the homogeneous catalyst remains in solution and can be reused after solid/liquid separation has been completed, thus achieving its recycling. These fundamental investigations serve the long-term goal of integrating the homogeneous catalytic synthesis of these valuable, sustainable polymer precursors into an efficient and industrially applicable process.