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Preliminary Study on Converting Hybrid Poplar to High-Value Chemicals and Lignin Using Organosolv Ethanol Process Da-Eun Kim and Xuejun Pan* Biological Systems Engineering, UniVersity of WisconsinsMadison, 460 Henry Mall, Madison, Wisconsin 53706
The organosolv ethanol process was adapted for converting hybrid poplar (Populus nigra L. × P. maximowiczii) into organosolv lignin and saccharide-derived chemicals, such as levulinic acid (LA), hydroxymethylfurfural (HMF), and furfural. The effect of process conditions (temperature, ethanol concentration, sulfuric acid dosage, and reaction time) on product yields was investigated using an experimental matrix designed with response surface methodology (RSM) and small Hartley composite design. The conditions ranged over 173-207 °C, 15-66 min, 2.3-5.7% H2SO4 (SA) on oven-dry wood (w/w), and 33-67% ethanol concentration (v/v). Results indicated that temperature, sulfuric acid loading, and their interaction had a significant effect on the yields of lignin and saccharide-derived chemicals. Comparative investigation of the organosolv process and acid process indicated that ethanol not only enhanced the delignification and production of organosolv lignin, but also improved the conversion yields of pentoses to furfural and hexoses to HMF and LA. 1. Introduction The utilization of lignocellulosic biomass for chemicals, fuels, and materials is now the subject of increasing interest and awareness because of the concerns of environment and oil shortage.1,2 Biomass has been applied in different energy sectors. For example, heat, power, and fuels can be produced from biomass through thermochemical routes (direct combustion, gasification, pyrolysis, and liquefaction).3 Biogas and bioethanol can be made from biomass through anaerobic digestion and enzymatic saccharification follwed by fermentation, respectively.4 Converting biomass carbohydrates to chemicals, such as furfural from pentoses and hydroxymethylfurfural (HMF), levulinic acid (LA), and formic acid from hexoses, is another area drawing increasing attention. Furfural is a key derivative of pentoses like xylose and has a broad spectrum of industrial applications. For example, it has been used as solvent in lubricants, coatings, adhesives, and furan resin.5,6 Furfural is also a starting point for a large family of derivative chemical and polymer products.7 A few industrial processes for producing furfural from biomass have been reviewed.6,8,9 Typically, the production of furfural involves two steps. The primary reaction is acidic hydrolysis of pentosans to pentoses. The next step is dehydration of the pentoses to furfural. Levulinic acid (LA) has been proposed as a platform chemical for producing a wide range of value-added products, such as dyestuffs, polymers, pharmaceutically active compounds, flavor substance, plasticizers, solvents, and fuel additives.10,11 Many methods for producing LA have been developed; the most widely used approach is dehydration of carbohydrates with acid.12 HMF is a versatile biomass-derived platform compound for synthesizing a broad range of chemicals that are currently derived from petroleum.13 It can also be transformed to gasolinelike fuels through aldol condensation and hydrogenation.14 Generally, HMF is produced via acid-catalyzed dehydration of hexoses. The simplest process is to treat sugars or biomass in H2SO4 solution, but HMF yield is low. Many studies were * To whom correspondence should be addressed. E-mail: xpan@ wisc.edu.
conducted to improve the selectivity of hexoses into HMF. For example, the organic solvent dimethyl sulfoxide (DMSO) was used to replace water, and the water-free environment promoted the dehydration of glucose into HMF.15 In another study, methyl isobutyl ketone (MIBK) was added as extraction solvent to collect HMF in situ when it was generated.16 The organosolv ethanol process was first developed as a pulping process for hardwoods.17-19 The process uses a blend of ethanol and water with a small amount of mineral acid to delignify the wood chips for chemical pulp production. Recently, the organosolv ethanol process has been modified into a biorefining platform for fuel ethanol production.20-23 It was demonstrated that the process not only produced readily digestible cellulose substrate for ethanol production, but also generated high-value byproducts including high-quality lignin and chemicals derived from hemicellulose.19,20 When pretreating lodgepole pine using the organosolv process in our previous work,22 we found that, at certain conditions of high severity (high temperature and acid loading), the overall sugar recovery was very low. Mass balance analysis indicated that a significant amount of cellulose was converted into levulinic acid during the pretreatment (data not published). This finding triggered the present research. In this study, the organosolv ethanol process was adapted for converting biomass to furfural, HMF, LA, and organosolv lignin. Although the organosolv process worked well with different types of biomass,20-22 this study will focus on hybrid poplar, an important fast-growing energy crop in the northern part of the United States. The specific objective of the study is to optimize organosolv ethanol process conditions (temperature, ethanol concentration, sulfuric acid dosage, and reaction time) to maximize the yields of lignin and saccharide-derived chemicals. 2. Materials and Methods 2.1. Materials. F1 hybrid poplar (Populus nigra L. × P. maximowiczii) was used as a feedstock in this study. The poplar chips were generously provided by the USDA Forest Service, Forest Product Laboratory (Madison, WI). After being air-dried for 3 days, the poplar chips were ground using a Wiley mill (IKA, Wilmington, NC) passing through a 1 mm screen. The
10.1021/ie101671r 2010 American Chemical Society Published on Web 10/26/2010
Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010
resulting wood meals were stored in sealed plastic bags at room temperature. Chemical reagents used in this study were purchased from Fisher Scientific (Pittsburgh, PA) and SigmaAldrich (St. Louis, MO) and used as received. 2.2. Analysis Methods. Moisture content was determined by drying the materials to constant weight at 105 °C in a convection oven. Ash and Klason lignin of hybrid poplar chips were determined according to TAPPI (Technical Association of Pulp and Paper Industry) standard methods T211 om-93 and T-222, respectively. Water and ethanol extractives were determined according to the ASTM Standard Test Method E169001. The hydrolysate from Klason lignin determination was retained for analysis of monosaccharides and acid-soluble lignin. Acid-soluble lignin was determined using a UV-visible spectrophotometer (CARY 50 BIO, VARIAN Inc., Palo Alto, CA) at the wavelength of 205 nm with 3% H2SO4 as reference.24 Structural monosaccharides were determined using a high performance liquid chromatograph (HPLC, Dionex ICS-3000) equipped with dual pumps, a postcolumn pump, an autosampler, and an electrochemical (EC, integrated amperometry) detector (Dionex, Sunnyvale, CA). The saccharides were quantified with reference to sugar standards. Standards were autoclaved at 121 °C for 1 h prior to analysis to compensate for sugar destruction during heating. The HPLC method for structural carbohydrate determination is as follows: PA1 analytic column and PA1 guard column (Dionex), column temperature 20 °C, eluent flow rate 0.7 mL/min with gradient (0 f 25 min, 100% water; 25 f 35 min, 40% water and 60% 0.1 M NaOH; 35 f 40 min, 100% water), and postcolumn eluent (0.5 M NaOH) flow rate 0.3 mL/ min for maintaining EC cell pH >12.5. Saccharide-derived chemicals (LA, HMF, and furfural) were determined using the same Dionex HPLC system above but with a UV-vis detector and the following method: Supelcogel C-610H analytic column (30 cm × 7.8 mm) and Supelguard C-610H guard column (Supelco, Bellefonte, PA), column temperature 20 °C, eluent (0.1% H3PO4) flow rate 0.7 mL/min, and UV-vis detector at 210 nm. 2.3. Organosolv Ethanol Process. Hybrid poplar powder was treated with aqueous ethanol containing sulfuric acid (catalyst) using a microwave reactor (MARS Xpress, CEM Corp., Matthews, NC) equipped with a high-pressure reaction vessel (xp-1500). A flowchart summarizing the organosolv ethanol process is shown in Figure 1. Specifically, 4 g of airdried poplar powder was treated in each batch. After reaction, the vessel was cooled to room temperature and the original spent liquor was sampled immediately for the determination of sugars, LA, HMF, and furfural. The mixture was filtered using a Bu¨chner funnel to separate the substrate (solid residue) and the reaction spent liquor. The residue was washed with about 30 mL of warm ethanol (same concentration as for the reaction) and then washed with acetone until no color remained in the filtrate. The solid residue was then washed with water and ovendried to determine yield and chemical composition. The filtrate (spent liquor + ethanol and acetone washings) was collected into a round-bottom flask and concentrated using a rotary evaporator. The concentrated filtrate was poured into about 400 mL of water with stirring. A small amount of acetone was used to transfer the lignin on the flask wall. The precipitated organosolv lignin was collected on Whatman No. 1 filter paper by filtration, washed thoroughly with water, and air-dried. 2.4. Experiment Design and Statistical Analysis. Experiments were designed using response surface methodology (RSM). Data were analyzed using SAS 9.1 (Statistical Analysis System, SAS Institute Inc., Cary, NC) for Windows. Analysis
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Figure 1. Flowchart of the organosolv ethanol process.
of variance (ANOVA) was performed, and the p-value (
