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New one-pot process to produce gasoline-grade biofuel from the bacterial biopolymer PHB

A team from the Hawaii Natural Energy Institute, University of Hawaii at Manoa is developing a new one-pot process to produce gasoline-grade (C6–C18) hydrocarbon oil from polyhydroxybutyrate (PHB)—an energy storage material formed from renewable feedstock in many bacterial species. In contrast to conventional biofuels derived from plant biomass, the resultant PHB oil has a high content of alkenes or aromatics, depending on the catalyst.

PHB has already been identified as having great potential as an intermediate in the production of hydrocarbon fuels. One approach, described by a team from the National Renewable Energy Laboratory (Wang et al.), is thermally to depolymerize and decarboxylate PHB at 400 ˚C to propene, for subsequent upgrading to hydrocarbon fuels via commercial oligomerization technologies.

In the University of Hawaii process, PHB (C4H6O2)n is degraded, deoxygenated and reformed into the oil on the solid catalyst (solid phosphoric acid, SPA) in a one-pot reaction under mild conditions. The reaction, as reported in the team’s latest paper in the journal Fuel, started at 213 °C and completed below 240 °C.

Metabolix and PHB
PHB is a polyhydroxyalkanoate (PHA), a polymer belonging to the class of polyesters of interest as bio-derived and biodegradable plastics. US-based biotech firm Metabolix is developing technology for the production of PHAs, including PHB, at industrial scale.
The company notes that while applications for PHAs have focused mainly on their use as biodegradable bioplastics, these polymers have a number of other unique features that will allow their use in other applications, such as the production of chemical intermediates and use as value-added feeds.
Metabolix is creating proprietary systems to produce PHBs in high quantity in the leaves of biomass crops or seeds of oilseed crops for these multiple applications.
Using tobacco as a demonstration system for proof of concept, Metabolix researchers have published results demonstrating that production of high levels of PHB, up to an average 18% in leaves and 9% in the biomass of the entire tobacco plant, can be achieved. In addition to tobacco, Metabolix is developing different genetic engineering systems for different plant crops including switchgrass, oilseeds and sugarcane.

Catalyst properties affecting the reaction included the calcination temperature in catalyst preparation; the catalyst dosage in the reaction; the total acid and free acid contents of fresh or used catalyst; and the working time when the catalyst was repeatedly reused.

Total mass recovery was 94 wt%. About 80% of oxygen was removed as CO2 via decarboxylation. In addition to the oil, other products included carbon dioxide (CO2), propylene (C3H6), water (H2O) and some char-like solid.

The hydrocarbon oil (30 wt% yield) was separated into two fractions according to their boiling temperature range:

  • a light oil (bp 40–240 °C) with 23 wt% yield; and
  • a heavy oil (bp 240–310 °C) with 7 wt% yield.

The light oil has the same elemental composition and high heating value (HHV 41.4/kg) of a commercial gasoline and the heavy oil (HHV 38.4 MJ/kg) has the similar elemental composition of biodiesel.

The oil, the researchers suggested, is a good source of bio-based alkenes and aromatics that can improve the performance of bio-based fuels in modern engines.



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