The extraction and fractionation of waxes from biomass

The aim of this project was to extract and fractionate waxes from abundant and low-cost under-utilised renewable resources using a green alternative technology. Through a review of the literature, the waxes covering agricultural by-products such as straw were identified as a potential source of high value chemicals for a wide range of applications.

Wheat straw waxes were extracted using organic solvents to demonstrate that straw contained high value wax compounds including free fatty acids, fatty alcohols, alkanes, wax esters, sterols, aldehydes and β-diketones. The solvent properties did not affect the composition of the extracts but changed the relative abundance of the different compounds. Linear solvation energy relationship (LSER) was used to model the extraction selectivity relating to total extraction yield and the various wax compounds. Lipophilic and aqueous fractions were separated and LSER results identified that the solvent properties affect only on the quantity of aqueous fraction recovered indicating the selectivity of the solvent.

Extraction of wheat straw wax was carried out using a more environmentally friendly supercritical CO2 extraction. The compositional profiles can be tuned by the manipulation of temperature and pressure and compared with the organic solvent extractions. Optimisation of temperature and pressure was carried out and the total crude yields and wax chemical group yields were modelled using the Chrastil equation to gain a better understanding of conditions required to achieve optimum extraction. The optimisation was used as part of the industrial collaboration scale up with Sundown Products Limited and Evonik Industries where a total of three tonnes of wheat, barley and oat straws were extracted using supercritical CO2 which yielded approximately 60 kg of wax. The three cereal straws were selected based on yield and composition as raw materials for the scale up from the biomass screen of seven different straws using hexane and ethanol extractions.
Economical assessment was carried out based on the scale up trial and it was concluded that currently the cereal straw wax would cost £12 per kg which is about 2 – 3 times higher than commercial waxes. The straw waxes were characterised and physical properties such as melting point were determined and found to be similar to commercial waxes such as beeswax. Fractionation by scCO2, GPC and saponification were used to further separate the wax products for formulation trials and product tests with the project sponsor, Croda. The crude wax products were deeply coloured and highly hydrophobic with no emulsification properties therefore applications such as coatings and polishes were suggested.