Article pubs.acs.org/IECR
H2SO4‑Catalyzed Hydrothermal Pretreatment of Triploid Poplar to Enhance Enzymatic Hydrolysis Haiyan Yang,† Kun Wang,† Feng Xu,† Run-Cang Sun,*,†,‡ and Yubin Lu§ †
Institute of Biomass Chemistry and Technology, College of Material Science and Technology, Beijing Forestry University, Beijing 100083, China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China § Chunlei Industrial Group Co., Ltd. Hebei 054001, China ABSTRACT: Sealed structure of plant biomass resists assault on cellulose from enzymatic deconstruction. In this study, pretreatments of triploid poplar were conducted with 0.5 wt % H2SO4 at various temperatures (100−200 °C) to loosen the intricate structure. The effect of temperature on cellulose structure and enzymatic digestibility was evaluated. The results indicated that the effective removal of hemicelluloses and amorphous cellulose incurred the increment of crystalline indices of residues. Companied with the degradation of carbohydrates, inhibitory compounds were accumulated with the increasing severity. On the whole, 120 °C was considered as the optimum temperature by taking the balance between substrate digestibility and recovery into account.
1. INTRODUCTION To meet increasing energy demands and to mitigate climate change, biosourced material process has come to the forefront of sustainable engineering research.1,2 The term of biorefinery is converting biomass into coproduction of transportation biofuels, bioenergy, and marketable chemicals and materials.3−6 Among all the biomass, agricultural and forestry residues are widely used due to their abundance, renewability, ready availability, and low cost. Woody residues mainly comprise three carbon-based polymers: cellulose, hemicelluloses, and lignin. Cellulose is a linear polymer of glucose, in which the strong hydrogenbonding network between and within molecules results in various conformations.7 Hemicelluloses, incorporated a range of different sugars, embed in between the cellulose elementary fibrils.8 Lignin, as a complex of cross-linked polymers with unique chemical and physiological functions, surrounds and protects the cellulose and hemicelluloses.9,10 Plants hold onto these chemicals with a tight grip, incurring the recalcitrance of the lignocellulosic biomass. The highly sealed structure is responsible for the desirable mechanical properties but makes extracting the energy-containing molecules challenging.10,11 Hence, a pretreatment process is crucially required to break down the macroscopic rigidity of biomass and decrease the physical barriers to mass transport for fuel production.10 Basically, the pretreatment processes can be categorized as biological, chemical, physical, and thermal approaches. For the cellulose-to-ethanol process, dilute sulfuric acid hydrolysis is considered as an attractive technique. Although acidic pretreatment can promote the hemicellulose hydrolysis and cellulose digestibility, there is a risk of forming and generating a range of toxic compounds from hemicelluloses and lignin decomposition.12 These compounds include formic, acetic, and levulinic acids, furfural, 5-hydroxymethylfurfural (5-HMF), and phenols. In general, the toxicity of each of the compounds for bacteria and yeast is directly related to hydropobicity.13 Different © 2012 American Chemical Society
inhibitory mechanisms have been proposed for weak acids, furfural, and 5-HMF. Uncoupling and intracellular anion accumulation is contributed to the inhibition of weak acids.14 Furfural and HMF reduction to alcohol occurred during fermentation.15,16 However, the toxicity of the lignocellulosic hydrolyzate resulted from the aggregation of several toxic compounds but not an individual compound.17 To minimize the formation of inhibitors, the conditions for pretreatment should be optimized. In this work, dilute sulfuric acid was used to prepare substrates from triploid Populus tomentosa Carr. In order to optimize the condition, pretreatments were conducted at different temperatures. The effects of temperature on the weight losses, inhibitors formation, cellulosic fibrillar structure, and cellulose digestibility were comparatively investigated.
2. MATERIALS AND METHODS 2.1. Raw Materials and Pretreatment. Chips of triploid poplar (3 years old) were obtained from Shandong Province, China. They were ground to 0.18−0.25 mm powder and dewaxed with ethanol/toluene (1:2, v/v) for 8 h. The main chemical components of the dewaxed wood were determined as 49.5% cellulose, 21.9% xylan, and 24.1% lignin (standard deviation < 2%). The pretreatments were performed in an autoclave. Three grams of sample was immersed in 30 mL dilute sulfuric acid (0.5% v/v). The residence time was fixed at 2 h. Samples were held in an oil batch at 100, 110, 120, 130, 140, 150, 160, 180, and 200 °C, respectively. At the end of the incubation, the samples were separated into solid and liquid fractions by filtration. The solid residues were washed with 100 mL hot Received: Revised: Accepted: Published: 11598
April 4, 2012 July 6, 2012 August 21, 2012 August 21, 2012 dx.doi.org/10.1021/ie300895y | Ind. Eng. Chem. Res. 2012, 51, 11598−11604
Industrial & Engineering Chemistry Research
Article
Table 1. Yield and Crystalline Indices of the Original Material and Cellulosic Residues residuesa R0 b
yield (%) CrIc
43.5
R
R
92.4 44.4
92.4 43.9
100
110
R
R
R
R
84.4 44.4
69.1 47.8
71.0 47.7
64.2 48.0
120
130
140
150
R
R
R
62.9 47.7
60.6 49.3
41.2 32.1
160
180
200
a
R0, R100, R110, R120, R130, R140, R150, R160, R180, and R200 represent raw material and the residues collected from the dilute sulfuric acid pretreatment at 100, 110, 120, 130, 140, 150, 160, 180, and 200 °C, respectively. bStandard deviation
