PHB into PHB: Recycling of polyhydroxybutyrate by a tandem “thermolytic distillation-microbial fermentation” process

The recycling of polyhydroxybutyrate (PHB) via depolymerization-polymerization is here proposed and assessed for the first time. Effective depolymerization of PHB to crotonic acid through a thermolytic distillation at mild conditions (170 °C and 150 mbar), and without the need of any catalyst, gives a crotonic acid-enriched condensate (94% yield and 98% selectivity towards trans-crotonic acid) that was used without any further purification as a substrate for producing renewed PHB with a culture of Cupriavidus necator. The yield of PHB (0.6 g PHB per g of crotonic acid), final PHB content (63%), and PHB-molecular weight (1.5 MDa) confirmed the technical feasibility of this tandem approach, with a 55% overall yield. A preliminary evaluation of the energy consumption of various depolymerization approaches indicated that the thermolytic distillation consumes 20–25% less energy than previously reported methods; the energy requirement for producing crotonic acid from PHBwaste with 50% moisture has a still acceptable energy demand (4 MJ/kgcrotonic acid), comparable to the energy required for obtaining fermentable sugars (4.4 MJ/kgsugar) to be used for feeding bacteria that accumulate PHB.