Title: Novel metathesis strategies for sustainable chemistry
Other Titles: Nieuwe metathese strategieën voor duurzame chemie
Authors: Dewaele, Annelies
Issue Date: 27-Oct-2016
Abstract: World population growth, together with the increasing industrialization of developing countries, is associated with an expansion of raw materials exploitation to meet the demand for fuels, energy, and chemicals. As these occurrences put an immense pressure on the Earth and its resources, there is an ever-growing need for the development of sustainable chemical processes. Introducing catalysis is an important step towards sustainability.
Olefin metathesis reactions are amongst the most essential catalytic processes in organic chemistry and they have paved the way for new developments in polymer chemistry and the synthesis of fine and commodity chemicals. This powerful technique allows the efficient reorganization of olefin fragments by scission and regeneration of new carbon-carbon double bonds, catalyzed by (ruthenium-based) organometallic complexes. Notwithstanding the fact that olefin metathesis can create new molecules with high atom efficiency under mild reaction conditions with low catalyst loadings, its sustainability character can still be improved.
Previous research in metathesis chemistry has mostly focused on the transformation of petrochemical feedstocks in presence of homogeneous and heterogeneous catalysts. The latter are usually synthesized through elaborate and expensive procedures with high loss of the original catalytic activity. In this doctoral research, it was therefore decided to focus on the development of novel sustainable metathesis strategies, both in terms of the choice of feedstock (biomass derived molecules) and the type of catalyst (practical synthesis of heterogeneous catalysts). After comprehensive study of the literature, several issues have been recognized, and they formed the basis of the research objectives of this dissertation.
A first research objective explored the practical and efficient immobilization strategy of ruthenium-based catalysts. More specifically, non-covalent immobilization of second-generation Hoveyda-Grubbs (HG2) complexes on mesoporous siliceous supports was applied according to a recent soft-immobilization invention of KU Leuven. In contrast to tedious covalent heterogeneous approaches, the proposed soft-immobilization strategy avoids complicated modifications of both the support and the catalytic complex, and is very practical to accomplish. The intense exploration revealed several important criteria with regard to the characteristics and properties of the silica support and metathesis complex for designing stable and active heterogeneous metathesis catalysis. In short, immobilization involves physical interaction of the support with the organometallic complex, through interaction with the silanols. Silica with high surface contents of silanols are therefore required. Adsorption isotherms showed a high affinity of the silica for the complex, preferably the presence of germinal silanols was beneficial. Water removal is essential as it fills the pores, hampering diffusion of organic molecules to the active supported complex, but the water removal procedure (thermal) should be carried out with care. Too high temperature treatments generate reactive siloxanes, which react with the organic complex and deactivate the complex.
Metathesis reactions are usually very fast, and therefore care has to be taken not to end up in the diffusional regime during catalytic testing. The pore structure of the silica indeed was found to have a great impact on the reaction rate. Very porous silica with 3D accessibility showed the best performance, and are able to carry high metathesis complex loadings without compromising the mass transport of molecules to and from the active sites. These micro- and macrokinetic aspects, though common in heterogeneous catalysis, have been overlooked in state-of-the-art studies on heterogeneous catalysis with immobilized complexes, but this study shows a huge impact on both observed selectivity and activity when reactions occur in the different (diffusional and chemical) regime.
After designing the optimal catalyst-support system, the heterogeneous catalyst is evaluated in several metathesis reaction types. Among them, there is one innovative synthesis of novel functional chemicals and materials from alternative feedstocks. Real compounds derived from lignin biopolymers were therefore explored to synthesize less toxic bio-based alternatives for the traditional alkylphenols. As said, methoxylated alkylphenols with an unsaturated alkyl chain (MAPs) were synthesized by cross metathesis of lignin-derived eugenol and olefins in presence of the supported metathesis catalyst. Whereas the additional presence of the methoxy group is proposed to reduce the toxicity of the alkylphenol drastically, the additional unsaturation could benefit the biodegradability of the MAP. High product yields were obtained by cross metathesis of eugenol with internal olefins, starting from a high olefin-to-eugenol reactant ratio.
Metathesis, whether applied homogeneously or heterogeneously, is an elegant reaction type to create new bio-based molecules from the carbohydrate fraction. Whereas recent literature proposed the synthesis of methyl vinyl glycolate (MVG) (a new platform molecule) from sucrose, this dissertation explored the reactivity of the vinyl substituent in this highly functional molecule for the synthesis of new C6-diacids. Self-metathesis of MVG for instance occurred in almost quantitative yields, even in absence of solvent. Hydrolysis of the MVG dimer led to the unsaturated acid 2,5-dihydroxy-3-hexenedioic acid (DHHDA), while subsequent hydrogenation gave access to 2,5-dihydroxy adipic acid. The potential of the unsaturated diacid DHHDA as a polymer building block was demonstrated by co-polymerization with lactic acid, resulting in cross-linked polylactic acid (PLA) with an improved thermal stability, compared to pure PLA. Besides, DHHDA was successfully used for the synthesis of bio-based nylon-6,6 polyamide analogs by co-polymerization with hexamethylenediamine.
The selectivity of metathesis reactions using heterogeneous catalysis is thus related to the mass transport issues and the constraints of the active site. Also the ligand structure around the metal active site may affect the selectivity outcome of the catalysis. This has been nicely demonstrated in the valorization of unsaturated polymers using metathesis as a sustainable solution to manage (industrial) waste streams. 1,4-Polybutadiene, an omnipresent polymer, was recognized here as an excellent substrate for the synthesis of large unsaturated macrocycles (≥ C16) by cyclo-depolymerization in presence of homogeneous Grubbs catalysts. Key criteria to attain high ≥ C16 cyclic oligomer yields are a low polymer concentration (< 0.2 M), a high-molecular weight polymer without vinyl impurities, but most of all the type of catalyst. First-generation Grubbs catalysts selectively synthesize the large cyclic oligomers ≥ C16, whereas second-generation Grubbs catalysts convert these primary products to the thermodynamically favored C12 cyclic oligomer. Unraveling of the reaction mechanism and study of the electronic properties of these two types of catalysts indicated a dominant effect of the electronic properties of the catalyst ligands on product selectivity, rather than catalyst deactivation. These results nicely show the potential of product selectivity control by the ruthenium ligand structures in homogeneous reactions.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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