Mechanism of nonionic surfactant-assisted Fe3O4@UIO-66-NH2 in high-solid enzymatic hydrolysis

Abstract

To improve the production of fermentable sugars from FeCl₃-catalyzed ethanol pretreated wheat straw, the compound of magnetic metal–organic frameworks (Fe₃O₄@UIO-66-NH₂) and nonionic surfactant were introduced to the high-solid enzymatic hydrolysis. The combination of Fe₃O₄@UIO-66-NH₂ with the nonionic surfactant PEG 8000 at 20 %, 26 %, and 32 % (w/v) solids resulted in the release of 155.54, 172.39, and 182.07 g/L glucose after 120 h, respectively, which were higher than those of without PEG 8000 (146.63, 160.28 and 177.40 g/L) and without Fe₃O₄@UIO-66-NH₂ and PEG 8000 (135.14, 163.92 and 162.18 g/L), attributing to the reduction of ineffective adsorption of cellulase and increment in cellulase activity. Fluorescence spectroscopy indicated that PEG 8000 caused the formation of complex between cellulase and Fe₃O₄@UIO-66-NH₂ to build a more compact structure, which led to the reduction of structural flexibility and fluidity, thereby improving the stability and activity of cellulase. In addition, the binding energy of PEG-assisted Fe₃O₄@UIO-66-NH₂ to lignin was significantly higher than that of PEG 8000 or Fe₃O₄@UIO-66-NH₂ alone, which was mainly due to the formation of strong hydrogen bonding between PEG and lignin, whereas Fe₃O₄@UIO-66-NH₂ formed electrostatic interactions with lignin through charge transfer on the carboxyl group. The formation of hydrogen bonds between lignin and PEG 8000 prevented unproductive adsorption between cellulase and lignin. The interaction of PEG 8000 and Fe₃O₄@UIO-66-NH₂ with lignin formed a more stable system, weakening the possibility of cellulase-lignin interaction. The efficient combination of magnetic metal organic framework and surfactant provides a potential way towards economically feasible sugar production from wheat straw.