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 molecular weight peak, in contrast to the competitive depolymerization and repolymerization in p-TsOH AHF with a bimodal distribution. The less condensed MA-dissolved lignin facilitated catalytic conversion to monophenols. Carboxylation of residual lignin in fractionated cellulosic water-insoluble solids (WISs) enhanced enzymatic saccharification by decreasing nonproductive cellulase binding to lignin. At a low cellulase loading of 10 FPU g-1 glucan, saccharification of WIS-MT120 from MAHF at 120 °C was 95 % compared with 48 % for WIS-PT85 from p-TsOH AHF 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, which substantially decreases fibrillation energy. LCNFs from only one pass of microfluidization of WIS-MT120 have the same morphology as those from WIS-PT85 after three passes. MA also has a lower solubility and higher minimal hydrotropic concentration, which facilitated acid recovery. MA is U.S. Food and Drug Administration (FDA)-approved as an indirect food additive, affording significant advantages compared with p-TsOH for biorefinery applications.
Keywords: acid recovery; cellulose nanofibrils; esterification; lignin; maleic acid.
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