Biobased chemicals from polyhydroxybutyrate

Currently, most chemicals and materials are obtained from fossil resources. After use, these chemicals and materials are converted to CO2. As discussed in chapter 1, this causes a build-up of CO2 in the atmosphere, the main driving force of global warming. In order to reach a sustainable system, biomass could be used as a resource for chemicals and materials instead. A biorefinery approach, where all parts of biomass are used to its full potential is essential. Taking this into consideration, wastewater streams of current biobased processes could be an excellent source for chemicals and materials. However, wastewater is often dilute and heterogeneous of nature. To overcome these challenges, wastewater rich in carbon can be processed by microorganisms to obtain a biodegradable polyester, polyhydroxyalkanoate (PHA). However, the mechanical properties of this polymer make it unsuitable as polymeric material. Moreover, processing of PHA is challenging. To circumvent these issues, we propose a conversion of the inferior PHA to methyl acrylate and propylene (Figure 7.1) which can be used in current processing infrastructure. PHA rich cells are obtained from the purification of wastewater. The PHA obtained can be purified and converted to MC (Figure 7.1, chapter 2) or the PHA rich cells can be used directly (Figure 7.1, chapter 3). For the second step, the conversion of methyl crotonate (MC) to methyl acrylate and propylene, the catalyst was immobilised (Figure 7.1, chapter 4). The current state of ethenolysis reaction on biomass was reviewed (Figure 7.1, chapter 5). The conversion of PHA to methyl acrylate and propylene enables the use of carbon from wastewater streams without the disadvantages related to the direct use of PHA. In chapter 2, the first step of the conversion of PHA to methyl acrylate and propylene was investigated. Since PHA obtained from wastewater exists mostly as polyhydroxybutyrate (PHB), this was chosen as a starting material for our studies. It was shown that PHB could be converted to MC using methanol at 200 °C.. MC has the advantage of being immiscible with water, which aids its separation. In chapter 2, the pathway of the reaction was clarified, which was subsequently used to optimise the conditions of this conversion. The conversion of PHB to MC proceeds via a thermolysis to crotonic acid (CA), which is followed by an esterification to MC. The formation of CA is the rate determining step below 18 bar, where above 18 bar this changes to the esterification to MC. A selectivity of 60% to MC is obtained with a full conversion of PHB with 18 bar being the optimal pressure for the conversion. Microorganisms produce PHA within their cells, which poses challenges to the downstream processing of PHA as the material has to be isolated from within the cells and dried. The isolation and drying of PHB is costly and is responsible for a large part of the production costs of PHA. In order to reduce the costs of PHA for the production of biobased chemicals, the conversion of PHA to MC was tested using whole cells. In chapter 3, PHA rich cells were directly converted to MC using the optimised conditions found in chapter 2. The influence of fermentation salts, water and the presence of valerate monomers in the PHA were studied. It was found that the valerate monomers have no influence on the conversion. Fermentation salts do influence the conversion depending on the salt. Magnesium hydroxide catalyses the conversion of PHB to MC, where magnesium sulphate catalyses the formation of methyl 3-hydroxybutyrate as side product. The reaction tolerates up to 20% water, which means that the drying step in the downstream processing of PHA can be significantly reduced. The second step of the conversion of PHA to methyl acrylate and propylene involves an ethenolysis, a cross metathesis of MC with ethylene. This ethenolysis reaction requires a homogeneous catalyst. One of the most active catalysts for this conversion is the ruthenium based Hovey-Grubbs 2nd generation. However, the required high loading of this catalyst makes it an expensive part of the conversion. In order to enable reusing of the catalyst, immobilisation of the Hovey-Grubbs catalyst was investigated in chapter 4. The catalyst was immobilised inside a metal organic framework (MOF). For this purpose MIL-101-NH2(Al) was used for its large cavities connected by small openings. This allows the catalyst to reside inside the cavities, while the small openings prevent it from leaching out. The catalyst was successfully immobilised using a mechanochemical approach. This method can be applied on other catalysts as well, which was shown by the immobilisation of Zhan catalyst. Both immobilised catalysts show metathesis activity for multiple reaction cycles. It was found that the MOF, MIL-101-NH2(Al), partially undergoes a structural change to form MIL-53-NH2(Al). When MIL-53-NH2(Al) was used as starting MOF the catalyst was trapped but inactive. It was concluded that when starting from MIL-101-NH2(Al), the catalyst trapped in the parts of the material that was converted to MIL-53-NH2(Al) are catalytically inactive. To investigate the current state of the art of the use of ethenolysis on biomass, a literature review was performed in chapter 5. The results of the ethenolysis of methyl oleate (MO) were compared in order to investigate the most important parameters. It was found that the purity of the ethylene feed has the biggest influence on the turn over numbers (TONs) and that a higher purity ethylene has shown a larger impact on the ethenolysis of MO than the development of novel catalysts. When electron poor substrates are used, the highest TONs are obtained with the less stable Hoveyda-Grubbs 2nd generation. However, no studies were performed on the influence of ethylene purity on these reactions and higher TONs may be achieved using a higher purity ethylene. In chapter 6, the results and conclusions of the thesis are summarised. The implications of these findings are discussed and suggestions for further research within the field are given.