Effect of thermochemical pretreatment on lignin alteration and cell wall microstructural degradation in Eucalyptus globulus: comparison of acid, alkali, and water pretreatments

Abstract

Pretreatment is an essential step to effectively hydrolyze lignocellulosic polysaccharides. In this paper, we investigated the degree of decompositions of lignin and cell wall structure using dilute acid, alkali, and water pretreatments to assess both chemical and ultrastructural alterations during pretreatment. The thioacidolysis method showed that β-O-4 linkages in lignin were mostly cleaved after all pretreatments, in which the highest decrease of β-O-4 units was for NaOH pretreatment, followed by hot water and H2SO4 pretreatments. The amounts of lignin degradation compounds, including vanillin and syringaldehyde, in the supernatant water also differed between the three pretreatments. Field-emission scanning electron microscopy revealed clear differences among the pretreatments in decomposing the ultrastructure of the inner surface of the fiber cell walls. Small pores were formed due to degradation of a part of the warty layer of the innermost surface in H2SO4 pretreatment. The warty layer was more degraded in hot water pretreatment and thus the cellulose microfibrils of the secondary walls were exposed. NaOH pretreatment showed that the warty layer was almost completely decomposed. The comparative study of different pretreatments using chemical methods and microscopic observations led to a better understanding of decomposition of wood cell walls by thermochemical pretreatment.