Surfactant-assisted ionic liquid fractionation of spruce produces a low molar weight and less condensed lignin

Surfactant-assisted ionoSolv fractionation of spruce sawdust was investigated with the aim of improving the fractionation performance and the quality of the recovered fractions. Anionic surfactants sodium dodecyl sulfate (SDS) and linear alkylbenzenesulfonate (LAS) were added to ionic liquid (IL) water mixtures of N,N -dimethylbutylammonium hydrogen sulphate ([DMBA][HSO 4 ]) containing 20–50 wt% water to process spruce sawdust at 150 and 170 °C for 30–60 min. 1 H NMR spectroscopy established that the processing conditions partly hydrolysed SDS whilst leaving LAS untouched. The use of LAS surfactant decreased the fractionation time but did not encourage the use of a higher water content. IonoSolv fractionation employing 2–10 wt% LAS and 20 wt% water resulted in faster delignification, decreasing the reaction time by 50 % at 150 ºC, with small improvements in cellulose recovery (87.6 % with 2 wt% LAS vs 83.7 % without LAS at 150 ºC). Post-treatment lignin analysis showed that the lignin structure was less condensed when LAS was present during the fractionation. Addition of surfactant had a lightening effect on the colour of the recovered lignin fractions, which is a key feature for its use in applications such as coatings, cosmetics or sunscreen additives. A proportion of LAS surfactant precipitated with the lignin fraction and was found to be mainly physically associated to lignin, although traces of condensed lignin-LAS were also detected. Whilst the extent to which surfactants may assist ionoSolv fractionation was limited, the study highlights the importance of characterising both lignin and cellulose chemical compositions.