Article pubs.acs.org/IECR
Effects of Incubation Time on the Fractionation and Characterization of Lignin During Steam Explosion Pretreatment Kun Wang,† Jian-Xin Jiang,† Feng Xu,† and Run-Cang Sun*,†,‡ †
Institute of Biomass Chemistry and Technology, Beijing Forestry University, Beijing 100083, China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China
‡
ABSTRACT: A two-stage process based on steam explosion pretreatment and alkaline solution post-treatment was applied to fractionate Lespedeza cyrtobotrya stalks into cellulose, hemicelluloses, and lignin. The characteristics of the lignin fractions permit the determination of the influence of incubation time (2−10 min) on the clean fractionation of lignocelluloses. The results indicated that steam explosion at 2.25 MPa for 2−5 min significantly enhanced the lignin fractionation in alkaline solution from 0.47% (raw material) to 16.36−17.13%. However, the repolymerization reactions were extensively conducted at oversevere conditions (6 and 10 min) and partially led to the lower yield of lignin fractions. The cleavage of β-O-4 linkages was determined by 1H NMR as increasing incubation time from 2 (11.1%) to 10 min (7.3%). However, the molecular weight of the lignin fractions was gradually increased from 1185 g/mol to 1816 g/mol. These phenomena demonstrated that depolymerization reactions were accompanied with comprehensive repolymerization reactions with the severity in the steam explosion process. It was also found that steam explosion pretreatment at low severities increased the surface area of the isolated lignin fraction. Consequently, this simple and effective process is useful for the further utilization of lignin to produce high-value chemicals.
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INTRODUCTION The slow decline in available oil reserves during the early 21st century prompts the conversion of biomass resources for making industrial products throughout the world. Bioethanol has already been introduced on a large scale in Brazil, the United States, and some European countries, and it is expected to be one of the dominating renewable biofuels in the transport sector within the coming 20 years.1 Thereby, lignin as a nonnegligible part of lignocelluloses is currently of great interest to the specialist in various fields of science and industry searching for new practical application. Lignin is the second only to cellulose in mass of natural polymer formed per annum and nature’s most abundant aromatic (phenolic) polymer,2 which is an integral part of the cell wall structure and chemically linked to hemicelluloses.3 In addition, it is also the most easily accessible at the lowest production cost. There are many potential applications of lignin in materials as a structural element, again a role close to its natural function, which can be subdivided into specific subsectors, such as phenolic resins, epoxies, adhesives, polyolefins, and others.4 For example, the plywood board produced with acetosolv lignin in phenol-formaldehyde resin revealed better knife-test results than that obtained with a commercial phenol-formaldehyde resin.5 The addition of considerable lignin (
