Hydrotropic agents are water soluble organic substances that have both hydrophilic and hydrophobic parts in their structure and at certain concentrations, high enough to form molecular aggregates, are capable of improvement of aqueous solubility of lignin. Hydrotropic treatment attracted a great attention in 1950s-1980s as a new cooking method that would be an alternative to the existing pulping technologies. Research at that time focused in altering cooking conditions, using different hydrotropic agents, using other solvent media instead of water and modification of hydrotropic cooking solutions with various additives. Most of the research was made with the focus on manufacturing paper grade pulps, and despite all the efforts, generally the process did not become superior to the sulphite and the alkaline pulping methods with respect to pulp properties. As a consequence the process was practically forgotten and limited to publications in Russian language. Recently, the driving interests towards advanced biomass fractionation in biorefinery applications motivate to revisit hydrotropic pulping as method to fractionate lignin and cellulose. In this work, we present a new process to extract lignin from wood biomass using a hydrotrope and addition of hydrogen peroxide or formic acid in pulping. Our process is faster than conventional hydrotrope pulping and leads to extraction of nearly 70% of total lignin present in wood. The resulting lignin after precipitation in water was characterised using 13C, 1H and 31P NMR, SEC, FTIR, elemental analysis and residual amounts of carbohydrates by GC-methanolysis. Our hydrotropic lignin was practically sulphur and carbohydrate free, had a G/S ratio of 0.44 and molar mass of 3400 Da. The higher heating value of our lignin was measured using an adiabatic combustion calorimeter and combustion bomb and was higher in comparison to other sulphur-free lignins. Prospective applications of our lignin to chemo and thermo conversion are presented and discussed.

New method of isolation of hydrotropic lignin from Birch wood

247th National Meeting (2014)

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