Browsing by Author "Majeke, Bongo Mount-Batten"
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- ItemThe enzymatic valorisation of technical lignins from different sources to phenols(Stellenbosch : Stellenbosch University, 2021-03) Majeke, Bongo Mount-Batten; Gorgens, Johann F.; Tyhoda, Luvuyo; Stellenbosch University. Faculty of Engineering. Dept. of Process engineering.ENGLISH ABSTRACT: Next to cellulose, lignin is the second most abundant natural polymer in the world. Commercially, it is generated as a by-product of the paper and pulp industry and emerging cellulosic ethanol processes. Both industries mainly incinerate lignin to complement their energy requirements, thereby underutilising this valuable renewable resource. The phenolic nature of lignin makes it attractive for the sustainable production of high-value chemicals. Efficient valorisation of lignin requires its depolymerisation to monomeric phenols. Over the years, several lignin depolymerisation strategies have been developed, and this study mainly focused on the enzymatic approach, which offers advantages such as high product selectivity and environmental friendliness. Among the targeted enzymes was the so-called lignin peroxidase (LiP) based on its ability to oxidise both phenolic and non- phenolic lignin moieties. Before the exploitation of the LiP enzyme, the first objective was to resolve barriers associated with its efficient production through the bioreactor optimisation of a methanol fed- batch culture using a recombinant strain of the methylotrophic yeast Pichia pastoris. The maximum volumetric activity, protein concentration, and specific activity of the LiP enzyme were 4480 U/L, 417 mg/L, and 10.7 U/mg, respectively. Upon the application of LiP to the selected technical lignins, the enzyme preferentially catalysed lignin re-polymerisation, with minimal depolymerisation activity, which is a common characteristic of most enzymes involved in lignin modification. To overcome this catalytic promiscuity, and promote lignin depolymerisation, which is desirable for producing phenolic monomers, quinone reductase (QR) enzyme was produced to work cooperatively with LiP (Objective 2). The synergistic application of LiP and QR enzymes achieved the desired lignin depolymerisation and thereby reduced the molecular weight (MWs) of soda-anthraquinone (SAQ) lignin, steam explosion (S-E) lignin, and two sulphite lignins (NaE and NaPE) by 31%, 41%, 34%, and 52%, respectively. However, the desired monomeric phenols from the catalytic activity of the enzyme cocktail (LiP and QR) were still below the quantifiable limit in terms of mass yields. Therefore, enzymatic pre-treatment of lignins followed by pyrolysis was applied to improve the production of monomeric phenol yields (Objective 3). Among the selected lignins, SAQ produced the highest total phenols at 7.84 wt.%, which was within the range (4.59-8.23 wt.%) of pyrolysis catalysed in the presence of inorganic chemicals, applied to a similar lignin.