Effect of water–carbon dioxide ratio on the selectivity of phenolic compounds produced from alkali lignin in sub- and supercritical fluid mixtures

Lignin is considered as the most abundant renewable carbon source after cellulose and non-commercialized waste product with constantly growing annual production exceeding 50 million tons per year. This article is focused on a newly developed selective synthesis of high-value phenolic products from lignin in presence of supercritical carbon dioxide (scCO2) known as sustainable, non-flammable, naturally abundant, and catalytically active solvent. Depending on the synthesis conditions, such as temperature (250, 300, and 350 °C) and water-to-scCO2 ratio (1:5, 1:2, 1:1, and 2:1), high yield of the specific phenolic compounds such as phenol, guaiacols, and vanillin has been achieved. The GCMS analysis reveals the trend of the increased total phenolic yield with temperature and strong dependence of the selectivity on the water-to-scCO2 ratio. The maximum selectivity toward formation of the specific phenolic products, such as guaiacol and vanillin was observed at the highest H2O:scCO2 = 1:5 ratio. At 350 °C the relative yield of guaiacol was ∼39% whereas at 250 °C vanillin was a dominant phenolic monomer with a relative yield of ∼33%. Moreover, at the intermediate temperature of 300 °C both guaiacol and vanillin were produced with relative yields of ∼28% and ∼13%, respectively. The highest amounts of scCO2 resulted in the highest total relative phenolic yields of ∼79%, ∼71%, and ∼73% at 250, 300, and 350 °C, respectively. Demonstrated for the first time, the effect of water-to-scCO2 ratio in the process of alkali lignin liquefaction will have significant impact on selective synthesis of phenolic compounds, their use in synthesis of “green” polymers with desirable properties, and sustainable utilization of both carbon dioxide and lignin waste.

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