Changes in lignocellulosic supramolecular and ultrastructure during dilute acid pretreatment of Populus and switchgrass Marcus Foston, Art J. Ragauskas* BioEnergy Science Center, School of Chemistry and Biochemistry, Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th St., Atlanta, GA 30332, USA article info Article history: Received 23 March 2009 Received in revised form 17 July 2010 Accepted 23 July 2010 Available online 22 September 2010 Keywords: Dilute acid pretreatment Cellulose Supramolecular structure Populus Switchgrass abstract Dilute acid pretreatment (DAP) is commonly employed prior to enzymatic deconstruction of cellulose to increase overall sugar and subsequent ethanol yields from downstream bioconversion processes. Typically optimization of pretreatment is evaluated by deter- mining hemicellulose removal, subsequent reactivity towards enzymatic deconstruction, and recoverable polysaccharide yields. In this study, the affect of DAP on the supramo- lecular and ultrastructure of lignocellulosic biomass was evaluated. A series of dilute acidic pretreatments, employing w0.10e0.20 mol/m 3 H 2 SO 4 at w160e180  C, for varying residence times were conducted on both Populus and switchgrass samples. The untreated and pre- treated biomass samples were characterized by carbohydrate and lignin analysis, gel permeation chromatography (GPC) and 13 C cross polarization magic angle spinning (CPMAS) NMR spectroscopy. GPC analysis shows a reduction in the molecular weight of cellulose and change in its polydispersity index (PDI) with increasing residence time, indicating that pretreatment is actually degrading the cellulose chains. 13 C CPMAS and non-linear line-fitting of the C 4 region in the carbon spectrum of the isolated cellulose not only showed that the crystallinity index increases with residence time, but that the lateral fibril dimension (LFD) and lateral fibril aggregate dimension (LFAD) increase as well. ª 2010 Elsevier Ltd. All rights reserved. 1. Introduction Lignocellulosic biomass may be used as a potential source of renewable energy via biochemical conversion of cellulose to second generation biofuels like cellulosic ethanol. Many lignocellulosic raw materials, such as Populus and switch- grass, have been evaluated for their potential bioconversion as biomass energy crops. Cellulose can effectively be decon- structed by enzymatic hydrolysis into its constituent monomer and fermented to ethanol. The structure of ligno- cellulosics occurs as a complex microstructure system composed of lignin and hemicellulose matrix encapsulating and supporting cellulose fibrils packed into bundles. The cellulose fibril themselves are a mixture of ordered and unordered regions [1,2]. Adding to the structural complexity, X-ray and neutron diffraction studies have provided infor- mation suggesting multiple crystalline allomorphs such as cellulose I a and I b exist in native cellulose [3,4]. The economics of biofuel production is very dependent on the overall sugar yields and energy cost associated with biomass deconstruction. The very properties that make cellulose so useful in nature as a structural biopolymer in the cell walls of plants makes it difficult to deconstruct. The mechanisms of efficiently overcoming recalcitrance are therefore important to understand. Typically, prior to enzy- matic hydrolysis and fermentation, biomass is subjected to * Corresponding author. Tel.: þ1 404 894 9701; fax: þ1 404 894 4778. E-mail address: Art.Ragauskas@chemistry.gatech.edu (A.J. Ragauskas). Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe biomass and bioenergy 34 (2010) 1885 e1895 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.07.023