Characterization of lignin-rich residues remaining after continuous super-critical water hydrolysis of poplar wood (Populus albaglandulosa) for conversion to fermentable sugars Sun-Joo Moon a , In-Yong Eom a , Jae-Young Kim a , Tae-Seung Kim a , Soo Min Lee b,1 , In-Gyu Choi a , Joon Weon Choi a,⇑ a Department of Forest Sciences and Research Institute for Agriculture and Life Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-921, Republic of Korea b Division of Wood Chemistry and Microbiology, Korea Forest Research Institute, 57 Hoegi-ro, Dongaemun-gu, Seoul 130-712, Republic of Korea article info Article history: Received 6 December 2010 Received in revised form 22 February 2011 Accepted 22 February 2011 Available online 25 February 2011 Keywords: Super-critical water Poplar wood lignin Acid catalyst GPC DFRC abstract Poplar wood flour (Populous albaglandulosa) was treated with sub- and super-critical water (subcritical: 325, 350 °C; super-critical: 380, 400, 425 °C) for 60 s at 220 ± 10 atm. Hydrochloric acid (0.05% v/v) was added to samples as acidic catalyst. The final products were separated into water soluble fraction and undegraded solids. The yields of undegraded solids were thoroughly dependent on temperature severity and mainly composed of lignin fragments. Average molecular weights of the lignins were between 1500 and 4400 Da, which was only 1/3–1/8-fold of poplar milled wood lignin (13,250 Da). DFRC (Derivatization Followed by Reductive Cleavage) analysis revealed that C6C3 phenols (coniferyl and sinapyl alcohol) were rarely detected in the lignins, indicating occurrence of two probable lignin reactions during SCW hydro- lysis: lignin fragmentation via splitting of b-O-4 linkage and loss of propane side chains. These results were also confirmed by 1 H and 13 C NMR spectroscopic analysis. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Lignocellulosic biomasses have enormous potential as replace- ments for fossil fuels due to their abundance and chemical compo- sitions. In general, all kinds of lignocellulosic biomass consist of three major chemical components: cellulose, hemicellulose and lignin. Since cellulose and hemicellulose, which amount to ca. 70% of the total biomass available, are sugar-based macromole- cules, they could serve as raw materials for bioethanol production in place of starch-based biomass. Lignin is synthesised in nature and makes up 20–25% of plant cell walls with three phenol precur- sors (p-coumaryl (H), coniferyl (G) and sinapyl alcohol (S)) via oxi- dative coupling reactions in the plant cell wall in the presence of peroxidase and H 2 O 2 (Demirbas et al., 2009). Vast quantities of lignin are produced as waste materials in the pulp and paper industries worldwide (Pye, 2008). So far, lignin is still dealt with as waste material due to lack of proper utilization. If ethanol industries based on lignocellulosic biomass grow in the near future, enormous amounts of lignin byproducts will be dis- charged as phenol wastes (Kim et al., 2009). Recently, hydrolysis of lignocellulosics with sub- and super- critical water (SCW) was developed as a replacement for routine technologies, such as acid hydrolysis and enzymatic hydrolysis, for fermentable sugar production (Ehara and Saka, 2002, 2005). The SCW method has the advantages of short reaction times and chemical-free hydrolysis (Matsumura et al., 2006). According to a previous report (Kim et al., 2010), more than 80% of lignocellulosic biomass is hydrolyzed to low molecular weight components, such as glucose, xylose, and several phenolic compounds, during SCW hydrolysis. Cellulose and hemicelluloses are easily hydrolyzed to monomeric sugars by SCW. The highest yield was ca. 23% for a 60 s reaction time based on the oven-dried biomass weight in the presence of 0.05% hydrochloric acid. The SCW hydrolysis pro- cess for lignocellulosic biomass could be very complicated and oc- cur at random patterns. Actually, lignin is much more difficult to degrade than cellulose or hemicelluloses due to its three-dimen- sional network structure and complex chemical linkages, such as carbon–oxygen and carbon–carbon linkages. Lignin degradation can be mainly accomplished via cleavage of carbon–oxygen (ether) linkages by SCW. Some researchers have studied the behavior of lignin in SCW including liquefaction (Qian et al., 2007; Xu and Etcheverry, 2008), gasification (Lu et al., 2008; Yoshida et al., 2004) and pretreatment of ethanol production (Schacht et al., 2008; Zhao et al., 2009). According to Saisu et al. (2003)) and Okuda et al. (2004), lignin decomposes to low molecular 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.02.091 ⇑ Corresponding author. Tel.: +82 2 880 4788; fax: +82 2 873 2318. E-mail address: cjw@snu.ac.kr (J.W. Choi). 1 Tel.: +82 2 961 2745; fax: +82 2 961 2788. Bioresource Technology 102 (2011) 5912–5916 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech