Valorisation of Waste Lignin for Phenol Derivative Compounds Using Hydrothermal Liquefaction
Marais, H.B., Marx, S., Venter, R.J.
Approximately 95% of global phenolic compounds (such as catechol, guaiacol, syringol, and vanillin) used in the pharmaceutical, fragrance and industrial industries are produced from petroleum-based phenolics such as benzene. Lignin waste, a by-product from industrial processes such as paper mills, is rich in bio-based phenolic groups that could be utilised to replace or supplement fossil-based products. Generally, lignin valorisation can be divided into three categories, power/fuel, macromolecules and aromatics. The highly aromatic nature of lignin makes lignin the highest value-added valorisation constituent of biomass and research is needed to improve lignin valorisation processes for commercial application. A promising technology able to extract phenolic derivatives from lignin at moderate temperatures and pressures is hydrothermal liquefaction (HTL). The effect of HTL on the yield of phenolic derivatives using a South African waste lignin in the aqueous product phase has not yet been investigated in detail. This research is necessary to develop a more economically viable HTL process as biochar and bio-oil alone could not justify an economic option for the HTL process. Thus, other streams produced during the HTL process must be developed to produce a more economical process through value added chemicals from lignin. In this study the effect of hydrothermal liquefaction on the phenolic derivatives in the aqueous phase was investigated through using a two-level factorial design approach. This was done as a first step to identify possible interactions between process variables and to identify the bio-products obtainable during the HTL of South African sodium lignosulfonate. Experiments were conducted in a SS316 high-pressure autoclave of 0.954 L with a heating mantle of 6 kW. All HTL products were quantified and analysed. Main phenolic derivatives in the aqueous phase were vanillin (0.67 g/kg biomass), guaiacol (3.74 g/kg biomass), p-cresol (1.72 g/kg biomass), and catechol (2.18 g/kg biomass). Compared to other research obtaining vanillin with a relative yield of 33% at 250.C, and guaiacol of 19% using supercritical H2O:CO2 as solvent vanillin production was lower, however guaiacol production was higher and other value added chemicals such as p-cresol were produced with the use of simple untreated biomass and water HTL, simplifying the process and lowering costs. Also, selective conversion of lignin is a challenge due to the complex nature of lignin. However, in this study it was proven that the manipulation of HTL parameters can favour the production of specific phenolic compounds. This proves that waste lignin can not only be depolymerised into economically value-added phenolic products but can also lessen the reliance on fossil fuels to produce phenolic compounds for the pharmaceutical, fragrance and industrial industries and may contribute to a more economically HTL process.
lignin, waste, hydrothermal liquefaction (HTL), phenolic compounds, experimental design approach
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