Lignin, the most abundant aromatic biopolymer on earth, remains vastly underutilized as only 2% finds its way into value-added products such as thermosets, additives, binders, and dispersants, while the rest is predominantly used for low-value heating purposes. Given its structural heterogeneity and inherent recalcitrance, lignin has been subjected to various chemical modifications to improve its reactivity. One of the most effective approaches is increasing the phenolic OH content of lignin through phenolation, which typically involves condensing phenol with lignin in an acid or alkali medium. This is because the phenolic OH groups remain the most reactive functional groups and control the chemical reactivity of lignin. Lignin is mainly isolated from black liquor, a by-product of the alkaline pulping process, including the predominant kraft pulping process. In this study, we have developed a novel method for modifying lignin in black liquor from alkaline pulping through phenolation. We have synthesized phenolated lignins from kraft black liquor using both conventional oil baths and microwave heating methods. The dissolved lignin to phenol molar ratio varied from 1:1 to 1:3. After the reaction, the phenolic OH contents were quantified using 31P NMR, and the molecular weights of the samples were measured using GPC. Our preliminary results indicate that the phenolation of lignin in black liquor at 160oC for 2h under oil bath heating results in a more than threefold increase in phenolic OH content from an initial value of around 2 mmol/g to around 6.5 mmol/g. Similar modifications using microwave-assisted heating are conducted in a much shorter time with the increase of reaction temperature to 190oC. Our future work will include optimizing the reaction parameters with both modes of heating. We will also conduct advanced analysis of the samples using 2D HSQC NMR to detect the inter-unit linkages of the lignin samples and evaluate the potential of the phenolated lignins in value-added applications.
