Research Article pubs.acs.org/journal/ascecg
Physiochemical Characterization of Lignocellulosic Biomass Dissolution by Flowthrough Pretreatment Lishi Yan,†,‡ Yunqiao Pu,§ Mark Bowden,∥ Arthur J. Ragauskas,§,⊥ and Bin Yang*,† †
Bioproducts, Sciences and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, Washington 99354, United States ‡ School of Chemical Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, People’s Republic of China § Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ⊥ Department of Chemical and Biomolecular Engineering; Department of Forestry, Wildlife, and Fisheries, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: Comprehensive understanding of biomass solubilization chemistry in aqueous pretreatment such as water-only and dilute acid flowthrough pretreatment is of fundamental importance to achieve the goal of valorizing biomass to fermentable sugars and lignin for biofuels production. In this study, poplar wood was flowthrough pretreated by water-only or 0.05% (w/w) sulfuric acid at different temperatures (220−270 °C), flow rate (25 mL/min), and reaction times (8−90 min), resulting in significant disruption of the lignocellulosic biomass. Ion chromatography (IC), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, and solid state cross-polarization/magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy were applied to characterize the pretreated biomass whole slurries in order to reveal depolymerization as well as solubilization mechanism and identify unique dissolution structural features during these pretreatments. Results showed temperature-dependent cellulose decrystallization in flowthrough pretreatment. Crystalline cellulose was completely disrupted, and mostly converted to amorphous cellulose and oligomers by water-only operation at 270 °C for 10 min and by 0.05 wt % H2SO4 flowthrough pretreatment at 220 °C for 12 min. Flowthrough pretreatment with 0.05% (w/w) H2SO4 led to a greater disruption of structures in pretreated poplar at a lower temperature compared to water-only pretreatment. KEYWORDS: Flowthrough pretreatment, Poplar wood, Cellulose, Hemicellulose, Lignin, Enzymatic hydrolysis, Depolymerization, Decrystallization
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INTRODUCTION The decreasing reserves of fossil fuels and the growing greenhouse gas emissions have caused an increase in bioenergy and bioproducts research in recent years. Lignocellulosic biomass having a complex cell wall structure that typically contains cellulose, hemicellulose, and lignin represents a promising carbon-based alternative as an energy source and a sustainable feedstock.1 Pretreatment is a vital step to overcome the recalcitrance of lignocellulosic feedstocks, including: (1) hemicellulose, lignin and other compounds coating the surface of the cellulose microfibrils, and (2) the crystalline nature of the cellulose structure, for the biochemical conversion of lignocellulosic biomass into ethanol or other chemicals.1,2 Therefore, efforts have been devoted to develop a cost-effective pretreatment system with viable technology and to understand the fundamental principles that contribute to the reduced recalcitrance by pretreatment. Characterizing the chemical © XXXX American Chemical Society
transformations of the pretreated lignocellulosic biomass could provide significant insights into the mechanism of pretreatment that overcomes the lignocellulosic biomass recalcitrance for the viable production of bioethanol or other chemicals.3 Among various pretreatment technologies, acidic hot water pretreatment with water-only or very dilute acid (95% glucose yield based on original glucan content in raw biomass. This indicated that, besides glucose and soluble glucose oligomers, the remainder of removed cellulose could exist in the form of cellulosic fractions with high digestibility. Determination of DP of Sugar Oligomers. Flowthrough pretreatment with water-only or very dilute acid resulted in significant biomass removal (∼98%) under selected conditions. Apart from the mono sugars (glucose and xylose), large fractions of removed carbohydrates were in the form of soluble oligomers. The analysis of DP distribution of these soluble oligomers appeared to be important in understanding the effects of these pretreatment approaches on sugar generation. It was previously reported that IC is an effective instrument for characterizing the DP of the released sugar oligomers from acidic flowthrough pretreatment.12 The DP distribution data for both xylooligomers and glucose oligomers analyzed by IC is summarized in Figure 1. Figure 1a revealed that the DP of released xylooligomers was predominately >6 under lower temperatures (220 and 230 °C). The DP distribution of xylooligomers shifted significantly toward to the lower DP as the temperature increased. When the temperature reached 270 °C, 21.1% xylose and 45.3% short chain xylooligomers (DP 2− 6) were obtained whereas the xylooligomers with DP > 6 were also observed 31.6%. Adding very dilute acid (0.05 wt % H2SO4) significantly reduced the DP of the xyooligomers. The yield of xylose plus short chain xylooligomers (DP 2−6) increased from 35.2%−52.1% to 67.1%−87.5% when 0.05% (w/w) H2SO4 was added under identical temperature range (220−240 °C). Figure 1b shows the DP distribution of removed cellulose after flowthrough pretreatment. It was found that less than 15% soluble cellulosic fractions including glucose and shorter chain glucose oligomers (DP 2−6) were observed at relatively lower temperatures (220−230 °C), along with around 12% released insoluble longer chain cellulosic fractions (DP > 6) and substantial amount of cellulose residue (∼75%). Continuous increasing temperatures to 240 °C enhanced the yield of soluble glucose and shorter chain glucose oligomers
Figure 1. DP distribution and polysaccharides mass balance for wateronly and 0.05 wt % H2SO4 hydrolysis of poplar wood. (a) Xylan hydrolysis (b) cellulose hydrolysis. (a1) 220 °C, water-only, 30 min; (a2) 230 °C, water-only, 16 min; (a3) 240 °C, water-only, 12 min; (a4) 270 °C, water-only, 10 min (b1) 210 °C, 0.05 wt % H2SO4, 30 min; (b2) 220 °C, 0.05 wt % H2SO4, 12 min; (b3) 240 °C, 0.05 wt % H2SO4, 8 min.
(DP 2−6) to 21.0%. The yield of glucose and shorter chain glucose oligomers (DP 2−6) reached 50.1% as temperature was elevated to 270 °C. Compared to increasing temperature, adding dilute acid appeared to have a more significant impact on the DP distribution of the recovered cellulose. The yields of soluble glucose and lower DP glucose oligomers (DP 2−6) obtained by dilute H2SO4 flowthrough pretreatment were 67.0% and 86.3% at 220 and 240 °C, respectively, demonstrating that the majority of removed cellulose was obtained in the soluble form. FTIR Analysis of Pretreated Whole Biomass Slurries. FTIR spectroscopic analysis was employed to investigate the structural alteration of pretreated whole slurries (including solid residues and hydrolysate) of poplar wood. The FTIR spectra of untreated and flowthrough pretreated poplar wood under selected conditions are displayed in Figure 2. The peak around 3348 cm −1 has been previously assigned to the OH stretching,24 and the samples pretreated at temperatures above 240 °C for water-only (Figure 2a3,4) or with 0.05 wt % H2SO4 (Figure 2b3) had a remarkable reduction in the intensity of this peak. The peak at 2900 cm−1 is ascribed to C− H stretching.25 Acidic hot water flowthrough pretreatment for poplar wood under tested conditions produced a negligible D
DOI: 10.1021/acssuschemeng.5b01021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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which could be the result of the relatively severe modification of the lignin. XRD Analysis of the Pretreated Whole Slurries. Results (Effects of Reaction Severity on the Removal of Poplar Wood and the Corresponding Sugar Recovery) showed that almost all cellulose (higher than 93%) was removed during flowthrough pretreatment when the reaction temperature was elevated to 270 °C for water-only and 220 °C or above for 0.05 wt % H2SO4 operations. Figure 3 shows the XRD patterns for
Figure 2. FTIR spectra of untreated poplar wood (control) and flowthrough (25 mL/min) pretreated poplar wood samples with water-only (a) and 0.05 wt % H2SO4 (b). (a1) 220 °C, 30 min; (a2) 230 °C, 16 min; (a3) 240 °C, 12 min; (a4) 270 °C, 10 min (b1) 210 °C, 30 min; (b2) 220 °C, 12 min; (b3) 240 °C, 8 min. The region of FTIR spectra is 3500−1000 cm−1.
change of this peak for each substrate, which implied that few methyl and methylene portions of poplar wood were disrupted. The flowthrough pretreated biomass shows considerable alterations in the region 1800−800 cm−1 of FTIR spectra. The band position at 1040 and 1060 cm−1 was associated with C O stretch, and 1120 cm−1 was attributed to the ring stretching frequency of cellulose.26 The increased intensity of these peaks with elevated temperatures or the addition of 0.05 wt % H2SO4 indicated the exposure of cellulose possible via intermolecular hydrogen bond cleavage.15 At the same time, the peak intensity around 1730 cm−1 assigned to CO in hemicellulose27 decreased with increasing temperature or the addition of 0.05 wt % H2SO4, which revealed the degradation of hemicellulose. The peak intensities in the range of 1250−1760 cm−1 were ascribed for lignin regions.28 In regard to the pretreated samples, subsequent changes in the band position at 1250, 1330, and 1500 cm−1 that were assigned to guaiacyl (G)/CO stretch, syringyl (S)/resinol linkages, and aromatic skeletal vibration, respectively,15 implied the modification and/or degradation on lignin units. The increased temperature (e.g., 240 °C above) or the addition of 0.05 wt % H2SO4 resulted in the relatively significant reduction of peak around 1330 cm−1,
Figure 3. XRD pattern of untreated poplar wood (control) and flowthrough (25 mL/min) pretreated whole slurries with water-only (a) and 0.05 wt % H2SO4 (b). (a1) 220 °C, 30 min; (a2) 230 °C, 16 min; (a3) 240 °C, 12 min; (a4) 270 °C, 10 min (b1) 210 °C, 30 min; (b2) 220 °C, 12 min; (b3) 240 °C, 8 min.
samples obtained under various pretreatment conditions (Effects of Reaction Severity on the Removal of Poplar Wood and the Corresponding Sugar Recovery). The untreated poplar wood (control) exhibited several broad diffraction peaks that matched those for cellulose I, and particularly cellulose Iβ, in the database published by the International Centre for Diffraction Data. The pattern also agreed well with the one calculated from the cellulose Iβ crystal structure determined by Nishiyama and Langham29 and contains a strong peak at 22.5° (2θ) corresponding to the distance between hydrogen-bonded sheets. The samples pretreated with water-only at 220 and 230 °C had nearly identical diffraction patterns, showing that no significant structural changes occurred at these temperatures. It E
DOI: 10.1021/acssuschemeng.5b01021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering was interesting that when the reaction temperature was elevated to 240 °C or above (270 °C), the cellulose (from whole slurries, including solid residues and released fractions) peaks significantly reduced in intensity and a new broad peak near 20° (2θ) emerged. This peak and the reduction in cellulose I peaks suggested that the majority of the cellulose in these pretreated whole slurries samples was amorphous. Such structure changes could account for the significant cellulose removal. It was reported that parallel chains in crystalline cellulose I are stacked through interactions such as van der Waals forces,30 which were calculated to exceed the hydrogenbonding interactions.31 Such attractions were considered as the main factor to resist enzymatic hydrolysis by cellulases. It can be anticipated that transformation of crystalline cellulose to an amorphous form could accelerate the enzymatic hydrolysis of cellulose. The XRD pattern for total cellulosic fractions from the very dilute acid flowthrough pretreated cellulose under tested conditions is presented in Figure 3b. No significant change from the untreated control sample was observed following pretreatment at 210 °C, but at higher temperatures (220 and 240 °C) the intensity of diffraction peaks reduced significantly and was replaced by a broad featureless pattern, suggesting the complete disruption of the crystalline cellulose. Because lignin and hemicelluloses lack of regular crystal structure, the interpretation of the crystallinity of cellulose through XRD analysis is possibly hampered by the hemicellulosic and lignin fractions in the pretreated poplar wood. In this regard, the microcrystalline cellulose (Avicel PH-101) was pretreated in the flowthrough system under identical conditions for poplar wood to investigate further the mechanism of the cellulose structural change under acidic conditions at elevated temperatures (210−270 °C). Results (Supporting Information Figure S1a,b) revealed that the transformation of crystalline cellulose to an amorphous structure for microcrystalline cellulose (Avicel PH-101) occurred at identical conditions as those for poplar wood. Solid State NMR Analysis. The CP/MAS 13C NMR spectra of raw poplar wood (control) and flowthrough pretreated poplar slurries are shown in Figure 4 (a, wateronly; b, 0.05 wt % H2SO4). The major resonances in the spectrum of poplar wood are assigned to CH3 in acetyl groups in hemicellulose and lignin (21 ppm), methoxy groups in lignin (56 ppm), C6 carbon atoms in cellulose (63 ppm), C2, C3, and C5 carbon atoms in cellulose (73−75 ppm), C4 carbon atoms in amorphous carbohydrates (84 ppm) and crystalline cellulose (89 ppm), C1 carbon atoms in cellulose (105 ppm), unsubstituted aromatic carbon atoms in lignin (103−122 ppm), quaternary aromatic carbon atoms (127−143 ppm), oxygen-substituted aromatic carbon in lignin guaiacyl (G) units (148 ppm) and syringyl (S) units (153 ppm), esters and carboxylic acids (173 ppm) including carbonyl carbon in acetyl groups. It should be noted that the poplar spectra also contained resonances of hemicellulose and aliphatic carbons in lignin side chains in the region between 50 to 102 ppm. These hemicellulose signals and lignin side chain carbon signals were not resolved in the NMR spectra due to an overlap with the resonances of the cellulose fraction. In addition, the resonances from various lignin structure units (i.e., syringyl, guaiacyl, and phydroxybenzorate) were not well resolved in the solid state 13C NMR spectra of raw poplar wood and the pretreated samples. The CP/MAS NMR spectrum of untreated poplar wood (control) was dominated by resonances of cellulose and lignin. It was found that both the water-only and dilute sulfuric acid
Figure 4. Solid state NMR spectra of untreated poplar wood and flowthrough pretreated whole slurries with water-only (a) and 0.05% (w/w) H2SO4 (b). (a1) 220 °C, 30 min; (a2) 230 °C, 16 min; (a3) 240 °C, 12 min; (a4) 270 °C, 10 min (b1) 210 °C, 30 min; (b2) 220 °C, 12 min; (b3) 240 °C, 8 min. Ctl, unpretreated control; S, syringyl; G, guaiacyl.
flowthrough pretreatments led to structural changes in the poplar wood to various extents that were dependent upon the pretreatment method and its severity. After the flowthrough pretreatment, the signal intensity of lignin guaiacyl units in poplar wood started to increase and became resolved from adjacent overlaps with syringyl lignin signals. Compared to the untreated poplar (control), the ratio of signal intensity of lignin syringyl to guaiacyl units (i.e., S/G ratio) decreased for the water only flowthrough pretreatment especially when the temperature reached 270 °C, indicating a favorable disruption F
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digestibility for glucose generation. The enzyme loading employed in this study was 100 mg protein CTec2 with 20 mg HTec2/g glucan + xylan. Figure 5a shows the enzymatic glucose yields obtained from water-only flowthrough pretreated poplar wood after enzymatic hydrolysis.
of syringyl lignin during the pretreatment. This was in agreement with previous studies in which the lignin syringyl units were shown more susceptible to degradation under acidic conditions such as hot water and dilute acid pretreatment. The water-only flowthrough pretreatment also resulted in a significant decrease in signal intensity of amorphous domains centered at 84 ppm (Figure 4a1−3). This was primarily attributed to the susceptible degradation of amorphous cellulose as well as syringyl lignin and hemicellulose. Compared to the untreated poplar, dramatic changes were observed in the polar pretreated at 270 °C by water-only flowthrough treatment, in which the spectrum was primarily dominated by lignin resonances with the signals of carbohydrates being significantly decreased and no apparent crystalline cellulose observed. The spectra of poplar wood treated at lower temperatures (i.e., from 220 to 240 °C) was dominated by both lignin and carbohydrates resonances with the presence of crystalline cellulose even with longer pretreatment time. It appeared that the temperature had a crucial impact on the structural changes of the poplar wood as evidenced in the CPMAS spectra results, especially complete disruption of crystalline cellulose. No significant changes were observed for acetyl groups signals (CH3 carbon at ∼21 ppm and carbonyl carbon at ∼173 ppm) after the water-only flowthrough pretreatment. The CP/MAS NMR spectra demonstrated that the flowthrough pretreatment with 0.05 wt % H2SO4 (Figure 4b) resulted in structural changes of poplar in a similar way like the water-only pretreatment while at lower temperature. For pretreatment at 210 °C, similar to water flowthrough pretreatment, the peak at ∼84 ppm showed a reduction in signal intensity due to the degradation of amorphous structures which primarily consisted of amorphous cellulose, lignin and hemicellulose. When the temperature reached 220 °C and above, the polysaccharides signals in NMR spectra were dramatically decreased and lignin resonances became dominant. In agreement with the XRD analysis, the signals of crystalline cellulose were completely diminished after the dilute H2SO4 flowthrough pretreatment at 220 and 240 °C. Similarly, the NMR spectra also showed a substantial increase of relative signal intensity of lignin guaiacyl units at temperature of 220 °C and above, indicating a decrease in lignin S/G ratio due to the favorable degradation of syringyl units under these pretreatment conditions. The solid state CP/MAS NMR analysis revealed that the water-only flowthrough pretreatment resulted in the most dramatic structural changes of poplar at 270 °C where lignin became dominant signals in the spectra and no crystalline cellulose were observed; whereas for the flowthrough pretreatment with 0.05% (w/w) H2SO4, this type of structural changes was observed at lower temperature when pretreatment temperature reached 220 and 240 °C. It was reported that water-only pretreatment was carried out under mild acidic conditions largely due to the release of organic acids from biomass components and a decrease in the pKw of water at the elevated temperature. The results demonstrated that the more acidic conditions in the flowthrough pretreatment with 0.05 wt % H2SO4 led to a significant degradation of structures in pretreated poplar at lower temperature as compared to wateronly pretreatment. Enzymatic Hydrolysis of Pretreated Whole Slurries. Pretreated whole biomass slurries, which contain residues and released fractions, were hydrolyzed by enzymes to test their
Figure 5. Enzymatic hydrolysis of untreated poplar wood (control) and flowthrough (25 mL/min) pretreated whole slurries with wateronly (a) and 0.05 wt % H2SO4 (b).
The enzymatic glucose yield from untreated poplar wood was merely 18.9% within 4 h, and the highest enzymatic glucose yield was observed 26.3% over 120 h. The efficacy of glucose generation from poplar wood pretreated at 220−240 °C with water-only appeared to be enhanced to 49.7%, 53.0%, and 69.5% at 4 h, respectively. The enzymatic glucose yield increased constantly as the reaction time prolonged, and around 90% enzymatic glucose yield was obtained after 120 h of enzymatic hydrolysis. Poplar wood pretreated at 270 °C seemed to result in much faster glucose generation compared with that pretreated at lower temperatures (220−240 °C), and the enzymatic glucose yields obtained were 76.0% and 95.4% after 4 and 120 h enzymatic hydrolysis, respectively. Such elevated efficacy of enzymatic hydrolysis could be predominately attributed to the diminished crystalline cellulose after pretreatment, as discussed in XRD Analysis of the Pretreated Whole Slurries. Figure 5b reveals that the glucose yield profile for 0.05 wt % H2SO4 treatment at 210 °C produced a much lower glucose yield than those at higher temperatures (220− 230 °C). It was found that the enzymatic glucose yields from 0.05 wt %% H2SO4 pretreated poplar at 220 and 240 °C reached 87.4% and 93.6% after 4 h of enzymatic hydrolysis, as G
DOI: 10.1021/acssuschemeng.5b01021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering compared to 57.2% glucose yield at 210 °C pretreatment. This elevated efficiency of glucose generation from the dilute sulfuric acid flowthough pretreatment at 220 and 240 °C could be attributed to the structural changes of poplar wood such as the complete disruption of the crystalline cellulose in the pretreated poplar wood.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01021. XRD pattern of untreated Avicel (control) and flowthrough (25 mL/min) pretreated Avicel samples (PDF).
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REFERENCES
(1) Yang, B.; Wyman, C. E. Pretreatment: the key to unlocking lowcost cellulosic ethanol. Biofuels, Bioprod. Biorefin. 2008, 2, 26−40. (2) Yang, B.; Dai, Z.; Ding, S. Y.; Wyman, C. E. Enzymatic hydrolysis of cellulosic biomass. Biofuels 2011, 2, 421−449. (3) Cao, S.; Pu, Y.; Studer, M.; Wyman, C.; Ragauskas, A. J. Chemical transformations of Populus trichocarpa during dilute acid pretreatment. RSC Adv. 2012, 2, 10925−10936. (4) Bobleter, O.; Binder, H.; Concin, R.; Burtscher, E. The conversion of biomass to fuel raw material by hydrothermal pretreatment. In Energy from Biomass; Applied Science Publishers: London, 1981. (5) Mok, W. S. L., Jr.; Antal, M. J. Uncatalyzed solvolysis of whole biomass hemicellulose by hot compressed liquid water. Ind. Eng. Chem. Res. 1992, 31, 1157−1161. (6) Liu, C.; Wyman, C. E. The effect of flow rate of very dilute sulfuric acid on xylan, lignin, and total mass removal from corn stover. Ind. Eng. Chem. Res. 2004, 43, 2781−2788. (7) Lora, J. H.; Wayman, M. Delignification of hardwoods by autohydrolysis and extraction. Tappi J. 1978, 61, 47−50. (8) Bouchard, J.; Nguyen, T. S.; Chornet, E.; Overend, R. P. Analytical methodology for biomass pretreatment. 2. Characterization of the filtrates and cumulative product distribution as a function of treatment severity. Bioresour. Technol. 1991, 36, 121−131. (9) van Walsum, G. P.; Allen, S. G.; Spencer, M. J.; Laser, M. S.; Antal, M. J.; Lynd, L. R. Conversion of lignocellulosics pretreated with liquid hot water to ethanol. Appl. Biochem. Biotechnol. 1996, 57−58, 2−15. (10) Weil, J.; Sarikaya, A.; Rau, S. L.; Goetz, J.; Ladisch, C. M.; Brewer, M.; Hendrickson, R.; Ladisch, M. R. Pretreatment of yellow poplar sawdust by pressure cooking in water. Appl. Biochem. Biotechnol. 1997, 68, 21−40. (11) Yang, B.; Wyman, C. E. Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of corn stover cellulose. Biotechnol. Bioeng. 2004, 86, 88−95. (12) Yang, B.; Wyman, C. E. Characterization of the degree of polymerization of xylooligomers produced by flowthrough hydrolysis of pure xylan and corn stover with water. Bioresour. Technol. 2008, 99, 5756−5762. (13) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673−686. (14) Nitsos, C. K.; Matis, K. A.; Triantafyllidis, K. S. Optimization of hydrothermal pretreatment of lignocellulosic biomass in the bioethanol production process. ChemSusChem 2013, 6, 110−122. (15) Laskar, D. D.; Zeng, J.; Yan, L.; Chen, S.; Yang, B. Characterization of lignin derived from water-only flowthrough pretreatment of Miscanthus. Ind. Crops Prod. 2013, 50, 391−399. (16) Mok, W. S.; Antal, M. J. Productive and Parasitic Pathways in Dilute Acid-Catalyzed Hydrolysis of Cellulose. Ind. Eng. Chem. Res. 1992, 31, 94−100. (17) Bobleter, O. Hydrothermal degradation and fractionation of saccharides and Polysaccharides. In Polysaccharides: Structural Diversity and Functional; Marcel Dekker, Inc.: New York, 2005. (18) Phaiboonsilpa, N.; Yamauchi, K.; Lu, X.; Saka, S. Two-step hydrolysis of Japanese cedar as treated by semi-flow hot-compressed water. J. Wood Sci. 2010, 56, 331−338. (19) Yan, L.; Zhang, L.; Yang, B. Enhancement of total sugar and lignin yields through dissolution of poplar wood by hot water and dilute acid flowthrough pretreatment. Biotechnol. Biofuels 2014, 7, 76. (20) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.;Crocker, D. Determination of Structural Carbohydrates
CONCLUSIONS Our results indicate a temperature-dependent cellulose decrystallization mechanism in both water-only and dilute acid flowthrough pretreatment processes. The significant disruption of crystalline cellulose occurred when the temperature reached 270 °C for water-only operation and 220 °C for dilute acid (0.05 wt % H2SO4) pretreatment. Under such conditions, lignin with guaiacyl units was the dominant solid fraction in pretreated whole slurries nearly without crystalline cellulose, whereas hemicellulose was degraded to xylose and xylooligomers with the DP predominantly ranging from 2 to 6. In addition, the preferential degradation of lignin syringyl units over guaiacyl units during both water-only and dilute acid pretreatment was demonstrated. Furthermore, results of enzymatic hydrolysis of pretreated whole slurries revealed significant enhancement of cellulose reactivity by flowthrough pretreatment at temperature of 270 °C for water-only and 220 °C for 0.05 wt % H2SO4. Thus, flowthrough pretreatment provides a promising platform for dissolution of whole lignocellulosic biomass to generate high yields of reactive intermediates (i.e., sugars and lignin) for biofuels productions.
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HPLC, High-performance liquid chromatography IC, Ion chromatograph NMR, Nuclear magnetic resonance XRD, X-ray diffraction
AUTHOR INFORMATION
Corresponding Author
*B. Yang. Fax: +1 509 372 7690. Tel: +1 509 372 7640. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the DARPA Young Faculty Award No. N66001-11-1-414, The Sun Grant-DOT Award No. T0013GA-Task 8, the National Science Foundation Award No. 1258504, and U.S. DOE-EERE No. DE-EE0006112 for funding this research. Part of this work was conducted at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility located at the Pacific Northwest National Laboratory (PNNL) and sponsored by the Department of Energy’s Office of Biological and Environmental Research (BER). We also thank Dr. Hongfei Wang and Ms. Marie S. Swita for insightful discussions.
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ABBREVIATIONS DP, Degree of ploymerization FTIR, Fourier transform infrared spectrometer H
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ACS Sustainable Chemistry & Engineering and Lignin in Biomass; Technical Report NREL/TP-510-42618; National Renewable Energy Laboratory: Golden, CO, 2008. (21) Yan, L.; Laskar, D. D.; Lee, S.-J.; Yang, B. Aqueous phase catalytic conversion of agarose to 5-hydroxymethylfurfural by metal chlorides. RSC Adv. 2013, 3, 24090−24098. (22) Li, X.; Converse, A. O.; Wyman, C. E. Characterization of molecular weight distribution of oligomers from auto-catalyzed batch hydrolysis of xylan. Appl. Biochem. Biotechnol. 2003, 105−108, 515− 522. (23) Ghose, T. K. Measurement of cellulase activities. Pure Appl. Chem. 1987, 59, 257−268. (24) He, Y.; Pang, Y.; Liu, Y.; Li, X.; Wang, K. Physicochemical Characterization of Rice Straw Pretreated with Sodium Hydroxide in the Solid State for Enhancing Biogas Production. Energy Fuels 2008, 22, 2775−2781. (25) Kumar, R.; Mago, G.; Balan, V.; Wyman, C. E. Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies. Bioresour. Technol. 2009, 100, 3948−3962. (26) Oh, S. Y.; Yoo, D. I.; Shin, Y.; Kim, H. C.; Kim, H. Y.; Chung, Y. S.; Park, W. H.; Youk, J. H. Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy. Carbohydr. Res. 2005, 340, 2376− 2391. (27) Pandey, K.; Pitman, A. FTIR studies of the changes in wood chemistry following decay by brown-rot and white-rot fungi. Int. Biodeterior. Biodegrad. 2003, 52, 151−160. (28) ElHage, R.; Brosse, N.; Sannigrahi, P.; Ragauskas, A. Effects of process severity on the chemical structure of Miscanthus ethanol organosolv lignin. Polym. Degrad. Stab. 2010, 95, 997−1003. (29) Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal Structure and Hydrogen Bonding System in Cellulose Ir from Synchrotron X-ray and Neutron Fiber Diffraction. J. Am. Chem. Soc. 2003, 125, 14300− 14306. (30) Wada, M.; Ike, M.; Tokuyasu, K. Enzymatic hydrolysis of cellulose I is greatly accelerated via its conversion to the cellulose II hydrate form. Polym. Degrad. Stab. 2010, 95, 543−548. (31) French, A. D.; Miller, D. P.; Aabloo, A. Miniature crystal models of cellulose polymorphs and other carbohydrates. Int. J. Biol. Macromol. 1993, 15, 30−36.
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