Valorization of Wheat Straw Using a Recyclable Hydrotrope at Low Temperatures (≤90 °C)

Abstract

This study evaluated the potential of an acid hydrotropic process at low temperatures for on-farm valorization of wheat straw by producing ligocellulosic nanofibrils (LCNFs), lignin nanoparticles (LNPs), and furfural. p-Toluenesulfonic acid (p-TsOH) was used to fractionate wheat straw under a range of conditions below 90 °C at low to moderate concentrations, between 15 and 60 wt %, for up to 2 h. p-TsOH fractionated wheat straw into a cellulose-rich water-insoluble solid (WIS) fraction and a spent liquor stream that contained dissolved lignin and xylan. Various degrees of delignification and hemicellulose dissolution were obtained and were correlated with a combined delignification factor (CDF) and a combined hydrolysis factor (CHF), respectively. A low p-TsOH concentration of 15 wt % can be used to obtain the desired degree of delignification for producing LCNFs directly from wheat straw. Films made of wheat straw LCNFs, with lignin contents of 12–22%, had excellent mechanical properties, with specific tensile strength over 120 kN·m/kg. The dissolved xylan in the spent liquor was directly dehydrated into furfural catalyzed by the p-TsOH in the spent liquor without an additional catalyst. The dissolved lignin was easily precipitated as LNPs through dilution using water. p-TsOH, as a solid catalyst, can be reused after the steps of lignin precipitation, reconcentration, and dehydration of xylose into furfural. The low-temperature fractionation process could substantially reduce capital and operating costs for on-farm applications.