Design of cooperative acid-base catalysts for aldol condensations

As fossil resources are depleting due to an increasing world population and the associated energy and chemicals demand, it is of strategic importance to search for renewable alternatives. Lignocellulosic biomass is a promising renewable resource which can be converted into liquid hydrocarbon fuels via routes which includes aldol condensations. At present, at the industrial scale, aldol condensations are typically catalyzed by homogeneous base catalysts. However, in a pursuit of more sustainable chemical processes, heterogeneous catalysts are desired. In this work, the possibility to efficiently increase the aldol condensation rate using heterogeneous cooperative acid-base catalysts has been investigated. These materials are studied experimentally as well as by (kinetic) modeling. The mechanism by which aldol condensations are carried out on this type of materials is further elucidated. Also insights in the catalyst properties which determine the reaction, such as the spatial positioning of the acid function with respect to the base function, the type of acid and base and the hydrophilic or hydrophobic character of the support, have been obtained. In addition, the properties of the reactants and the reaction environment are investigated via the presence of an electron withdrawing group on the aromatic ring of the reagents and the amount of water in the reaction mixture. The research eventually led to a proposal for an optimal acid-base catalyst for aldol condensations.

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