This study reports on a comparative study of acid hydrotropic fractionation (AHF) of birch wood using maleic acid (MA) and p-toluenesulfonic acid (p-TsOH). Under the same level of delignification, lignin dissolved by MA is much less condensed with a higher content of ether aryl β-O-4 linkages. Lignin depolymerization dominated in MA hydrotropic fractionation (MAHF) and resulted in a single lower Mw peak, in contrast to the competitive depolymerization and repolymerization in p-TsOH HF with a bimodal distribution. The less condensed MA-dissolved lignin facilitated catalytic conversion to monophenols. Carboxylation of residual lignin in fractionated cellulosic solids (WIS) enhanced enzymatic saccharification by decreasing nonproductive cellulase binding to lignin. At a low cellulase loading of 10 FPU g^-1 glucan, saccharification of WIS-M_T120 from MAHF at 120 °C was 95% compared with 48% for WIS-P_T85 from p-TsOH HF at 85 °C under the same level of delignification of 63%. Residual lignin carboxylation also facilitated nanofibrillation of WIS for producing lignin-containing cellulose nanofibrils (LCNFs) through an enhanced lignin lubrication effect to substantially decrease fibrillation energy. LCNFs from only one pass of microfluidization of WIS-M_T120 have the same morphology as those from WIS-P_T85 after three passes. MA also has a lower solubility and higher minimal hydrotropic concentration, which facilitated acid recovery. MA is FDA-approved as an indirect food additive (21CFR175-177), affording significant advantages compared with p-TsOH for biorefinery applications.

Junyong Zhu

USDA Forest Service

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