A Decarboxylation Route from Sugars to Hydrocarbons via Polyhydroxybutyrate

The chemical transformation of biomass-derived intermediates into hydrocarbon fuel components is a topic of considerable interest to many research groups.  A potential route is the conversion of polyhydroxyalkanoates (PHA) to alkenes that would be intermediates to hydrocarbon fuels.  This appears promising as there are several microorganisms which accumulate  high levels of PHA as a form of energy storage molecule.  Thermal breakdown of PHA proceeds via an intermediate carboxylic acid, which can then be decarboxylated to an alkene.  Oligomerization by well known commercial technologies would permit production of a range of hydrocarbon fuels.   Polyhydroxybutyrate (PHB) can be produced in Cupriavidus necator on a variety of carbon sources including glucose, fructose and glycerol with PHB accumulation reaching 75% of dry cell mass.  We have demonstrated the thermal breakdown of polyhydroxybutyrate to 2-butenoic acid (crotonic acid, CA) and the thermal decarboxylation of CA to propene at yields approaching 70% at 400 °C in 15 min.  Combining depolymerization and decarboxylation steps we have demonstrated that PHB can be directly converted to propene and carbon dioxide under similar conditions and in similar yields.  PHB containing cell mass from C. necator has also been directly converted without prior separation of PHB from the cell mass.  The kinetics of the depolymerization and decarboxylation reactions have been studied and both could be described using first-order reaction kinetics.  As expected, decarboxylation was found to be the rate-limiting step, the decarboxylation of CA being 500 to 900 times slower than the depolymerization of PHB.