Plastic materials are an inseparable part of everyday life, but their widespread use comes with significant environmental challenges. The OECD reports that global plastics production has exceeded 400 Mt per year in recent years, and plastic pollution is now detected even in remote ecosystems. Growing interest is therefore directed toward bioplastics that are biologically produced and/or biodegradable. Polyhydroxyalkanoates (PHA) are a family of biodegradable and biocompatible polyesters produced by microorganisms as intracellular storage granules. PHAs consist of 3-hydroxy acid monomer units and can exhibit diverse and attractive material properties depending on their monomer composition. This thesis explores multiple stages of the PHA life cycle – from screening and fermentation to downstream recovery and end-of-life valorization – through the principles of green chemistry and within the broader framework of an integrated biorefinery concept.
At the beginning of the PHA life cycle, or even before production itself, small-scale screening is important for identifying promising bacterial strains and suitable cultivation media for growth and PHA production. This topic was investigated in Paper I, where two bacterial species, Halomonas sp. R5-57, a short-chain-length PHA producer, and Pseudomonas sp. MR4-9, a medium-chain-length PHA producer, were screened in high-throughput mode by cultivation in 96-well plates followed by Fourier-transform infrared spectroscopy (FTIR). Selected results were subsequently quantified and confirmed at a larger cultivation scale. This study demonstrated that high-throughput screening is a powerful tool for evaluating large numbers of cultivation conditions, particularly different carbon sources, and for selecting the most promising candidates for confirmation at a larger scale. From a biorefinery perspective, this approach could also support the rapid assessment of waste streams or renewable raw materials as potential substrates for PHA production.
Upstream processing was further investigated in Paper II, with a focus on lowering feedstock costs by using crude industrial glycerol as an alternative carbon source for Photobacterium ganghwense C2.2 and comparing it with pure glycerol. Crude glycerol supported faster biomass formation but resulted in lower overall PHA production and lower polymer molecular weight, while enabling recovery of fatty acid impurities from the crude feedstock. The use of an inexpensive crude substrate to obtain higher biomass formation and lower-molecular-weight PHA provides a relevant perspective for integrated biomass valorization, especially when such materials can be further converted into higher-value products.
Since downstream processing strongly affects both process economics and sustainability, Paper III evaluated dihydrolevoglucosenone, commercially known as Cyrene, as a biobased and biodegradable extraction solvent, with emphasis on solvent reuse and avoidance of toxic solvent waste. In an integrated process concept, spent biobased solvent that can no longer be reused may also be considered for energy recovery. The extraction process was optimized at small scale using design of experiments and multivariate modeling, targeting extraction yield and polymer molecular weight. Based on the optimized conditions, upscaling experiments in a stirred cell were performed and showed improved extraction performance under scaled conditions.
Finally, PHAs were explored not only from an end-of-life perspective, but also as substrates for valorization and quality restoration. Low-molecular-weight PHA can be depolymerized into monomers or platform chemicals and potentially repolymerized into higher-quality polymers, offering a strategy for upgrading materials such as the PHA obtained from crude glycerol in Paper II. Paper IV investigated the selective conversion of PHB to crotonic acid (CA) in the basic ionic liquid [EMIM][AcO], which acts as both solvent and catalyst. Paper V examined acid-catalyzed hydrolysis using a Brønsted-acid ionic liquid, [ImSO₃H][p-TsO], in a biphasic water/MIBK system, enabling the formation of mainly 3-hydroxybutyric acid (3-HBA), with CA as a byproduct, and facilitating phase separation between the products and the ionic liquid catalyst. In Paper VI, p-toluenesulfonic acid (p-TsOH)-catalyzed alcoholysis of PHB was demonstrated using methanol and ethanol, highlighting process flexibility by enabling subsequent hydrolysis in the same reactor after solvent/reactant exchange.