Abstract
Two novel protocols for the chemical valorization of polyhydroxybutyrate (PHB) were developed, aiming at the production of two bio-based molecules: methyl 3-hydroxybutyrate (MHB) and methyl 3-methoxybutyrate (MMB). Optimized reaction conditions were applied to pure PHB and PHB inclusions inside bacterial cells as starting materials. MHB was synthesized through a single-step catalytic methanolysis, while MMB was synthesized through a three-step process: thermolytic distillation to give crotonic acid (CA), esterification to give methyl crotonate (MC), and oxa-Michael addition of MeOH. The obtained MHB and MMB were tested as solvents for the recovery of PHB itself both from freeze-dried single strain cultures (SSC) and mixed microbial cultures (MMC) with low to medium contents of PHB (22–57 wt %). High PHB recovery was achieved: up to 96 ± 1% through MHB and up to 98 ± 1% through MMB. Extraction from MMC slurry (with a PHB content of 39% on dry weight) was also performed, recovering 77 ± 2% using MHB and 92 ± 2% using MMB. High purities and excellent molecular weights and polydispersity indexes of extracted PHB were obtained with both MHB and MMB. Solubility in water, octanol/water partition coefficients (log Kow), and aerobic ready biodegradability of both solvents were also evaluated.
Highlights
The increasing global consumption of plastics demands more sustainable materials, produced from renewable resources, biodegradable, and/or efficiently recyclable, to reuse them or obtain new substances
Microwave-assisted PHB degradation in alkaline MeOH has been proposed as a sustainable way to produce PHB monomers,[29] while the use of acidic or basic functionalized ionic liquids in methanol leads to the synthesis of methyl 3-hydroxybutyrate (MHB).[30,31]. Starting from these seminal approaches, the aim of this work is to merge the optimization of the chemical recycling route to the innovation in the extractive step, transforming PHB into valuable C4-building blocks that can be used as solvents for PHB extraction from single strain cultures (SSC) and mixed microbial cultures (MMC) (Figure 1)
The solvents have been tested in PHB extraction from both single strain and mixed microbial cultures
Summary
The increasing global consumption of plastics demands more sustainable materials, produced from renewable resources, biodegradable, and/or efficiently recyclable, to reuse them or obtain new substances. Polyhydroxyalkanoates (PHAs) are bio-based and biodegradable polyesters of hydroxy acid monomers biosynthesized by different kinds of bacteria through the aerobic conversion of various feedstocks[2] and potentially capable of replacing fossil-based plastics thanks to similar mechanical and physical proprieties.[3] At present, the main bottleneck for the production and application of PHA at a large scale is their high production cost (2.2−5.0 €/kg), which is at least three times higher than the main fossil-based polymers that cost less than 1.0 €/kg.[4] This is due to the fact that the most common way to produce PHAs at the industrial scale is through the use of single strain cultures (SSC) of natural or genetically engineered bacteria that need a sterile environment and selected/purified carbon feedstock. Several alternative methods to the wellknown use of toxic chlorinated solvents (e.g., chloroform or dichloromethane)[8] or oxidants (NaClO)[9] are proposed for the recovery of PHAs from SSC
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