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Research Article pubs.acs.org/journal/ascecg
Ball Milling for Biomass Fractionation and Pretreatment with Aqueous Hydroxide Solutions Tianjiao Qu,† Ximing Zhang,‡ Xingwei Gu,† Lujia Han,† Guanya Ji,† Xueli Chen,† and Weihua Xiao*,† †
College of Engineering, China Agricultural University, Beijing 100083, P. R. China Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, Indiana 47907, United States
‡
S Supporting Information *
ABSTRACT: A promising approach in the selective separation and modification of cellulose from raw biomass under a mild alkali process was proposed. In our study, ball milling was applied to wheat straw prior to alkali treatment. With ball milling, ultrafine powder formed an amorphous microstructure and displayed a level of solubilization in aqueous NaOH higher than that of general ground samples. Alkali-treated ultrafine powder resulted in up to 93.76% removal of hemicellulose and 86.14% removal of lignin, whereas cellulose remains largely undissolved. A high glucose yield (98.48%) was obtained via a 72 h enzymatic hydrolysis. X-ray diffraction and solid state 13C cross-polarization magic angle spinning nuclear magnetic resonance analysis revealed evidence of the transformation of crystalline cellulose I to cellulose II in alkali-treated ultrafine wheat straw. Prolonging the alkaline treatment time can significantly decrease the level of cellulose hydrogen bonding and increase the hydrolysis yield. The combination of ultrafine ball milling and lowseverity alkali treatment played a significant role in the cellulose supramolecular change, which can then be used for downstream biorefinery processes or as a feedstock for the biomaterial industry. KEYWORDS: Wheat straw, Particle size, Alkali treatment, Ball milling, Enzymatic hydrolysis
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INTRODUCTION Lignocellulosic biomass is an abundant and renewable resource with potential for the production of fuels and chemicals to displace petroleum-derived products.1,2 Extensive research has been conducted on the utilization of carbohydrates (hemicellulose and cellulose) in lignocellulose as a feedstock for the biorefining industry.3 However, the crystallinity of cellulose and the matrix formed by hemicellulose and lignin reduces the efficiency of converting these carbohydrates to monomers that can be utilized by biorefineries.4−8 To address this hurdle, various pretreatment methods such as mechanical grinding and alkaline and acidic pretreatment have been tested to enhance the reactivity of the carbohydrate region.9,10 Among all the pretreatment methods, alkali pretreatment with concentrated sodium hydroxide, namely mercerization, has been broadly used in the pulp industry for its efficiency in microfibril separation as well as lignin and hemicellulose removal.11−15 Previous research showed sodium hydroxide could productively solubilize lignin and hemicellulose by penetrating the interfibril region and cleaving α-ether bonds between lignin and hemicellulose, as well as swelling cellulose through saponification.16−20 The benefit of sodium hydroxide pretreatment is twofold. First, the reduction of lignin and hemicellulose within the lignocellulosic structure facilitates the following cellulose utilization in either enzymatic hydrolysis to glucose or catalytic conversion to value-added chemical production.21−23 Second, the alkali-aided transformation of © 2017 American Chemical Society
the cellulose chain from a parallel (cellulose I) to antiparallel arrangement (cellulose II) will decrease the number of hydrogen bonds and rearrange the hydrogen bond inner- and intracellulose chains.24−26 This rearrangement enhances the cellulose region reactivity and functional properties for applications in biomedicine and reinforcing materials.26,27 However, delignification of lignocellulose and crystalline cellulose transformation both require a high sodium hydroxide concentration (15−20%) for general ground lignocellulose.11,24 Recent studies indicated mechanical milling (such as general milling and ultrafine grinding) could initiate different extents of size reduction and various mechanochemical effects such as depolymerization and deacetylation.28 Ultrafine ground biomass with a submicrometer particle size possesses physicochemical properties different from those of the general ground form.29,30 Hence, ultrafine grinding (e.g., ball milling) allows for the possibility of decreasing the alkali intensity during biomass pretreatment. In our study, wheat straw was subject to mechanical milling and sequential alkali treatment. This systematic study focused on pre- and post-treatment-related biomass structural changes and enzymatic hydrolysis for the evaluation of pretreatment on cellulose utilization. Received: April 17, 2017 Revised: June 21, 2017 Published: August 10, 2017 7733
DOI: 10.1021/acssuschemeng.7b01186 ACS Sustainable Chem. Eng. 2017, 5, 7733−7742
Research Article
ACS Sustainable Chemistry & Engineering
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Reducing-End Determination. The molar concentration of the reducing end of wheat straw was measured by the BCA method previously described.30 Compositional Analysis. The contents of cellulose, hemicellulose, and lignin of samples were analyzed by following NREL standard laboratory analytical procedure TP-510-42623.34 A Hitachi L7000 high-performance liquid chromatography (HPLC) system equipped with an RID detector and a Bio-Rad Aminex HPX-87P column was used for sugar quantification. Percentages of cellulose, hemicellulose, and lignin of samples were based on dry weight. The solid recovery was calculated as the percentage of total recovery after alkaline treatment. Solubilization of cellulose, hemicellulose, or lignin was defined as a mass reduction percentage of cellulose, hemicellulose, or lignin during alkali treatment.
EXPERIMENTAL SECTION
Materials. Wheat straw obtained from Shangzhuang experiment station of China Agricultural University in 2012 was air-dried. Glucose, xylose, and arabinose were purchased from Sigma-Aldrich and used as received. Other reagents and chemicals (analytical grade) were purchased from the Beijing Chemical Plant. Cellic CTec2 enzyme was donated by Novozymes China R&D Centre (Beijing, China). The activity of CTec2 is 167.5 FPU/mL (filter paper unit), which was determined according to NREL procedure TP-510-42628.31 Grinding Method. Air-dried wheat straw was manually cut into 4−5 cm in length and then coarsely milled by an RT 34 disk mill (Taiwan) through 1.00 and 0.50 mm screens, denoted as
