Katalytische Upcycling van α-Ketoglutaarzuur tot Duurzame Nylon Materialen

The current chemical industry predominantly employs fossil-based carbon feedstocks such as oil, coal, and natural gas for the production of various chemicals, including key building blocks for plastic materials. Although these production processes are economically established and the materials produced serve many applications, their dependence on finite resources, environmental impact, and challenges in recycling contribute to considerable sustainability concerns. Plastics, especially low-quality and difficult-to-recycle polymers like polyethylene, often accumulate in landfills and natural environments, or generate greenhouse gases when incinerated. To address these challenges, a threefold strategy is essential for sustainable chemical production: (1) replacing fossil carbon with renewable sources such as biomass and plastic waste; (2) adopting Green Chemistry principles in production through the use of heterogeneous catalysts and non-toxic reagents; and (3) designing materials that not only outperform conventional alternatives, yet also enable efficient recycling back to their original precursors. Among various approaches, the dual bioprocess-catalysis strategy has gained attention. This approach leverages microorganisms as cell factories to convert renewable or waste feedstocks (e.g., glucose, xylose, glycerol, fatty acids, and n-alkanes) into bio-based platform chemicals. These compounds, often low-value, require catalytic valorisation to generate higher-value chemicals or materials. Heterogeneous catalysts enable active and selective transformations of these bio-based substrates under mild conditions with recyclability of the catalyst and without waste disposal, aligning with Green Chemistry goals. Owing to this approach, several bio-based platform chemicals such as lactic acid, succinic acid, malic acid, itaconic acid, and glutamic acid have already reached industrial levels. This thesis work has evaluated a similar compound, α-ketoglutaric acid (α-KGA), according to the same principles outlined above. A comprehensive literature review highlighted the effective production of α-KGA using the yeast Yarrowia lipolytica with n-alkanes or glycerol as carbon sources. Meanwhile, the catalytic valorisation of α-KGA to commodity chemicals or building blocks for plastics has remained underexplored. Accordingly, this thesis research pursued two main objectives: First, developing a heterogeneous catalytic system for the hydrogenation of α-KGA, a key industrial reaction; and second, integrating the hydrogenated product into novel building blocks for sustainable nylon synthesis. Commercial α-KGA was hydrogenated in MQ water, by applying hydrogen gas, and a commercial catalyst in a batch reactor. By monitoring the concentrations using HPLC analysis, and characterising the catalysts under study (CO-chemisorption, TPR, TPO, TEM, and XPS), the reaction performance was unravelled through a catalyst screening, substrate scope exploration, and kinetic study. Under mild conditions (40 °C, 50 bar), the most suitable catalyst was 5% Pd/C, balancing a high catalytic activity (4.3-7.9 1/s) with complete conversion after 1-2 h. An optimal coverage of Pd(0) sites at the carbon surface (21-28%) together with supplementary Pd(2+) sites (<70%) resulted in high initial activity. Moreover, the oxophobic nature of Pd(0) and the in situ reduction of Pd(2+) prevented inhibition of the product (α-hydroxyglutaric acid, α-HGA), contrary to the more oxophilic metals used for biomass valorisation (Ru and Rh). The weaker α-KGA-Pd(0) interaction was compensated by the electronic effect of the α-COOH group, which enhances the polarity of the C=O reacting moiety. Activation by both the catalyst and the α-COOH group was presumably followed by a proton addition, and a hydride addition (Pd-H) in the rate-determining step, as supported by the kinetic study. Finally, at the reactor level, high yields of α-HGA were obtained (up to 95 mol%) and high volumetric productivity (149 g/L/h). By positioning the α-KGA hydrogenation as a core process, the upstream production of α-KGA and its downstream valorisation into sustainable nylons were successfully carried out. Polyethylene (PE), notoriously present in plastic waste, was chemically recycled via pyrolysis, distillation, and hydrogenation into an alkane-rich pyrolysis oil (typically with 35 wt% yield). These oils were an ideal substrate for Yarrowia lipolytica yeasts to grow (2.35 g DW/L after 3 days), enabling biological production of α-KGA (up to 38 g/L) when co-fed with glycerol. The α-KGA was purified from the broth via liquid-liquid extraction (29 wt% yield with 72% purity). Following the hydrogenation of α-KGA, the α-HGA product was recovered (89 wt%), fully converted into γ-carboxyl butyrolactone at 110 °C, and subsequently esterified into γ-ethoxycarbonyl-γ-butyrolactone (γEC-γBL) with an 88 wt% yield, using an Amberlyst 15® acidic resin. The purified γEC-γBL was polymerised together with hexamethylenediamine to produce α-hydroxy polyamide 65 (α-HPA 65) in 88 wt% yield. Through melt phase polymerisation (under vacuum, 200 °C, 5 h), high number-average molecular weights were obtained (201 kDa) together with interesting thermal properties (glass transition point at 46 °C, melting point at 153 °C, and degradation point at 456 °C). Overall, α-HPA 65 has an improved processing window relatively to conventional nylon 66. Crucially, α-HPA 65 was recycled under ethanolysis conditions (100 °C, 1 M sulphuric acid catalyst) achieving 83 mol% monomer yield after 24 h, whereas nylon 66 recycling proved less efficient under the same conditions (only 24 mol%). In summary, this work establishes a proof-of-concept for upcycling low-value, difficult-to-recycle polyethylene waste into a value-added, high-performance nylon material through an integrated pyrolysis, bioprocessing, and catalytic valorisation. This approach contributes considerably to sustainable production of materials using circular economy principles.