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 (scCO(2)) known as sustainable, nonflammable, naturally abundant, and catalytically active solvent. Depending on the synthesis conditions, such as temperature (250, 300, and 350 degrees C) and water-to-scCO(2) 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-scCO(2) ratio. The maximum selectivity toward formation of the specific phenolic products, such as guaiacol and vanillin was observed at the highest H2O:scCO(2) = 1:5 ratio. At 350 degrees C the relative yield of guaiacol was similar to 39% whereas at 250 degrees C vanillin was a dominant phenolic monomer with a relative yield of similar to 33%. Moreover, at the intermediate temperature of 300 degrees C both guaiacol and vanillin were produced with relative yields of similar to 28% and similar to 13%, respectively. The highest amounts of scCO(2) resulted in the highest total relative phenolic yields of similar to 79%, similar to 71%, and similar to 73% at 250, 300, and 350 degrees C, respectively. Demonstrated for the first time, the effect of water-to-scCO(2) 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. (C) 2016 Elsevier Ltd. All rights reserved.