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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01186 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017
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Ball milling for biomass fractionation and
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pretreatment with aqueous hydroxide solutions
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Tianjiao Qu,†,a Ximing Zhang,†,b Xingwei Gu,a Lujia Han,a Guanya Ji,a Xueli Chen,a Weihua
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Xiao*a
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a
College of Engineering, China Agricultural University, Beijing, 100083, P.R. China.
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b
Department of Agricultural and Biological Engineering, Purdue University, West Lafayette, IN
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47907, USA.
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†These authors contributed equally to the work.
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* Corresponding author. Email:
[email protected] 10
Keywords: wheat straw, particle size, alkali treatment, ball milling, enzymatic hydrolysis
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Abstract
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A promising approach in the selective separation and modification of cellulose from raw biomass
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under mild alkali process was proposed. In our study, ball milling was applied to wheat straw
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prior to alkali treatment. With ball milling, ultrafine powder formed an amorphous
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microstructure and displayed higher solubilization in aqueous NaOH compared to general
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ground samples. Alkali-treated ultrafine powder resulted in up to 93.76% removal of
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hemicellulose and 86.14% removal of lignin respectively, whereas cellulose remains largely
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undissolved. High glucose yield (98.48%) was obtained in 72 h enzymatic hydrolysis. XRD and
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solid state 13C CP-MAS NMR analysis revealed an evident of crystalline cellulose I
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transformation to cellulose II in alkali-treated ultrafine wheat straw. Prolonging alkaline
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treatment time can significantly decrease cellulose hydrogen bonding and increase hydrolysis
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yield. The combination of ultrafine ball milling and low-severity alkali treatment played a
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significant role in cellulose supramolecular change, which can be then used for downstream
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biorefinery processes or as a feedstock for biomaterial industry.
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Introduction
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Lignocellulosic biomass is an abundant and renewable resource with potential for fuels and
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chemicals production to displace petroleum-derived products [1, 2]. Extensive research has been
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conducted on the utilization of carbohydrates (hemicellulose and cellulose) in the lignocellulose
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as a feedstock for the biorefining industry [3]. However, crystallinity of cellulose and matrix
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formed by hemicellulose and lignin reduce the efficiency of converting these carbohydrates to
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monomers which can be utilized by biorefineries [4-8]. To address this hurdle, various
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pretreatment methods such as mechanical grinding, alkaline and acidic pretreatment have been
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tested to enhance the reactivity of the carbohydrates region [9, 10].
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Among all the pretreatment methods, alkali pretreatment with concentrated sodium hydroxide
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namely mercerization has been broadly used in the pulp industry for its efficiency in microfibrils
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separation as well as lignin and hemicellulose removal [11-15]. Previous research showed
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sodium hydroxide could productively solubilize lignin and hemicellulose by penetrating the
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inter-fibril region and cleaving α-ether bonds between lignin and hemicellulose, as well as
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swelling cellulose through saponification [16-20]. The benefit of sodium hydroxide pretreatment
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is twofold. First, the reduction of lignin and hemicellulose within lignocellulosic structure
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facilitates the following cellulose utilization in either enzymatic hydrolysis to glucose or catalytic
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conversion to value-added chemicals production [21-23]. Secondly, the alkali aided
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transformation of cellulose chain from parallel (cellulose I) to antiparallel arrangement (cellulose
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II) will decrease and rearrange the hydrogen bonds inner and intra cellulose chains [24-26]. This
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rearrangement enhances cellulose region reactivity and functional properties for applications in
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biomedicine and reinforcing materials [26, 27]. However, delignification of lignocellulose and
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crystalline cellulose transformation both require high sodium hydroxide concentration (15-20%)
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for general ground lignocellulose [11, 24].
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Recent studies indicated mechanical milling (such as general milling and ultrafine grinding)
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could initiate different extent of size reduction and various mechanochemical effects such as
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depolymerization and deacetylation [28]. Ultrafine ground biomass at sub-micrometer particle
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size possess modified physicochemical properties compared to general ground ones [29, 30].
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Hence, ultrafine grinding (e.g. ball milling), allows the possibility to reduce the alkali intensity in
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biomass pretreatment. In our study, wheat straw was subject to mechanical milling and
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sequential alkali treatment. This systematic study focused on pre and post treatment related
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biomass structural changes and enzymatic hydrolysis for the evaluation of pretreatment on
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cellulose utilization.
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Experimental Section
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Materials
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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
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used as received. Other reagents and chemicals (analytical grade) were purchased from the
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Beijing Chemical Plant. CellicTM CTec2 enzyme was donated by Novozymes China R&D Centre
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(Beijing, China). The activity of CTec2 is 167.5 FPU/ml (filter paper unit), which was
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determined according to NREL procedure (TP-510-42628) [31].
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Grinding method
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Air dried wheat straw was manually cut into 4~5 cm in length and then coarsely milled by an
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RT 34 disk mill (Taiwan) through 1.00-mm and 0.50-mm screens respectively, denoted as
