Jackson, David Shaun Frederick; (2018) Building a Synthetic Pathway For Nylon precursor Biosynthesis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Biorefineries allow for the sustainable production of higher value products from biomass. In addition to bioethanol, they can produce added value chemicals and pharmaceutical intermediates from isolated component compounds such as sugars. Sugar beet pulp (SBP) is a high volume, low value by-product from sugar beet processing with a low lignin and a high carbohydrate content, making it an attractive biomass feedstock for biorefinery processing. The pectin fraction of SBP can be isolated via steam explosion, which, after complete acid hydrolysis, gives a hydrolysate rich in monosaccharides: primarily L-arabinose (Ara) and D-galacturonic acid (GA), with some D-galactose (Gal) and L-rhamnose (Rha). Isolation of these sugars is therefore a critical step in realising an integrated, whole crop biorefinery. Currently, little work has been reported on the separation and utilisation of SBP hydrolysates. The aim of this thesis is to establish novel, scalable separation processes for the isolation of the component monosaccharides from crude hydrolysed sugar beet pulp pectin. Centrifugal partition chromatography (CPC) is a liquid-liquid separation technique with no solid stationary phase and offers an alternative to traditional resin-based chromatographic techniques. As such it can more easily cope with crude feedstreams such as hydrolysates. Hydrophilic ethanol : ammonium sulphate two-phase systems were examined based on monosaccharide partition coefficients and phase settling times. An ethanol : aqueous ammonium sulphate (300 g L-1 ) (0.8:1.8 v:v) system was chosen for CPC separations of the crude SBP hydrolysate and was shown to be capable of removing the coloured contaminants and isolating three sugar fractions in a single step: Rha, Ara and Gal, and GA. The separation was optimised and the throughput was increased by maximising the sample loading. Operation in an elution-extrusion mode allowed for reproducible separations in 100 min without additional column regeneration. The process was scaled up from a 250 to a 950 mL column providing a final throughput of 1.9 gmonosaccharides L -1 column h -1 using the crude SBP. The following purities and recoveries of the three main fractions were achieved: Rha at 92% purity and 93% recovery; Ara at 84% purity and 97% recovery; and GA at 96% purity and 95% recovery. Simulated moving bed (SMB) allows for continuous chromatographic separations using multiple columns, improving separation performance and throughputs. Isolation of Ara from the neutral sugars Gal and Rha was performed with resins and conditions screened on single columns leading to the selection of a Dowex 50W X8 resin in the Ca2+ form. SMB separation using 8 columns was performed in the 4-zone and 3-zone setups and achieved 94% purity with 99% recovery at a throughput of 4.6 gmonosaccharides L -1 column h -1 with a synthetic mixture of the neutral sugars (Ara, Gal and Rha). However, equivalent separations could not be achieved using the crude SBP hydrolysate which needed pretreatment before SMB. Decolourisation with activated carbon was able to remove 97% of the coloured contaminants with sugar losses of 15% (w/w) in a batch process demonstrated to 50 mL scale. Anion exchange chromatography using a Dowex 1x8 resin was then found to be capable of isolating GA from a synthetic crude mixture of GA and neutral sugars with a dynamic binding capacity of 1.31 mmol mL-1 resin. However, further work is needed to enable this anion exchange step to achieve satisfactory separations with the decolourised crude hydrolysate. The isolated neutral sugars, after GA removal, can be processed on the SMB with comparable separation performance and throughput to a mixture of neutral sugars prepared without GA. In summary, this thesis presents two possible process paths each with their own benefits and drawbacks. CPC is capable of processing the crude SBP hydrolysate directly, isolating the sugars and removing the coloured contaminants in a single step. However, Ara co-elutes with Gal providing a stream that is only 84% pure. In SMB, the potential throughputs and separation performance are higher, however, this could only be experimentally demonstrated with synthetic crude mixtures of sugars and not with the crude SBP hydrolysate. Further pretreatment or SMB method development would be required in order to process the crude hydrolysate, and the resulting multistep processes may reduce the overall viability. Overall this thesis demonstrates two feasible approaches to the preparative scale separation of SBP pectin hydrolysates and supports development of an integrated SBP biorefinery.

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