Organic amine catalytic organosolv pretreatment of corn stover for enzymatic saccharification and high-quality lignin
Graphical abstract
Introduction
Due to the depleting fossil fuel reserves and increasing greenhouse gas emission, the exploration of feasible pathways for the utilization of abundant and renewable biomass is highly desirable. In this sense, lignocellulosic biomass is a promising candidate because it is an abundant and carbon-neutral energy resource (Somerville et al., 2010). Sugars derived from lignocellulose are the key platform chemicals, which can be further converted to biofuels and some other high value-added chemicals via biological and/or chemical routes (Wettstein et al., 2012, Sheldon, 2014). Enzymatic hydrolysis is considered as a promising method to obtain fermentable sugars from lignocellulose due to its mild processing condition and high selectivity (Chaturvedi and Verma, 2013). However, the recalcitrant structure of lignocellulose resists the enzymatic hydrolysis process (Himmel et al., 2007). Therefore, an efficient and economical pretreatment technology is a prerequisite to enhance the bio-digestibility of enzymes.
Pretreatment techniques can overcome the recalcitrant structure by disrupting cell wall physical barriers as well as cellulose crystallinity and removing the lignin so that hydrolytic enzymes can access the biomass macrostructure. A large number of pretreatment approaches such as alkaline pretreatment, dilute acid pretreatment, hydrothermal pretreatment, steam explosion, and organosolv pretreatment have been investigated on a wide variety of feedstock types and there are several recent review articles which provide a general overview of this field (Ravindran and Jaiswal, 2016, Zhang et al., 2016). Among these methods, organosolv pretreatment provides an efficient fractionation of the lignocellulose into its main constituents, thus allowing the valorization of all fractions. Furthermore, the presence of organic solvent reduces the viscosity of the pretreatment medium, improves penetration into the lignocellulosic matrix, and facilitates a more efficient removal of lignin (Oliet et al., 2002). Various organic solvents especially for the low boiling point polar solvents such as ethanol, methanol, tetrahydrofuran, or acetone were used for organosolv pretreatment due to their easy recovery and low cost (Zhang et al., 2016).
Compared to the pure organosolv pretreatment technology, catalytic organosolv pretreatment (COP), which usually use acid or alkali as a catalyst combined with organic solvent have proven to be an efficient method for the lignocellulosic biomass pretreatment due to its positive effect to enhance the delignification process and enzymatic digestibilities of cellulose (Zhang et al., 2016). In view of the issues derived from acid catalytic organosolv pretreatment (ACOP) such as generating inhibitors, equipment corrosion problems, base catalytic organosolv pretreatment (BCOP) have recently drawn considerable attention (Kim et al., 2016). BCOP could suppress the dissolution of cellulose and hemicellulose, which are often observed in the acid or hydrothermal pretreatment processes (Alvira et al., 2010). Furthermore, it usually causes the swelling of the biomass, thus increasing the accessibility of enzyme to the cellulose and improve the saccharification. Numerous inorganic base catalysts such as NaOH (Li et al., 2013), KOH (Sun et al., 2011) have been used for the BCOP process. However, these processes usually generate significant waste water streams that must be neutralized, which leads to additional disposal cost. Using solid base catalyst such as CaO, MgO still suffers from the separation problem between the catalyst and pulp. From an economic and environmental perspective, the development of a new BCOP method which can simplify the process, promote clean manufacturing, and decreased the disposal cost is highly desirable.
In continuation of our efforts to develop new BCOP method for the lignocellulosic biomass pretreatment (Tang et al., 2017), herein, a novel and efficient organic amine catalytic organosolv pretreatment system (OACOP) was reported. In this study, the catalytic effects of organic amine on the pretreatment results were investigated, and analysis techniques were employed to characterize the structure and property changes of stock before and after pretreatment to evaluate the effect of organic amine catalyst. A mechanism of OACOP was proposed. Furthermore, the recyclability of organic amine along with organic solvent was also studied.
Section snippets
Materials and reagents
Corn stover, provided by local farmer in Lianyungang, Jiangsu, China, was knife milled and screened to 30–50 mesh and dried to constant weight. The chemical composition of the raw corn stover (on a dry weight basis) was 38.7% cellulose, 20.0% hemicellulose, 18.1% lignin, 3.9% ash and 19.3% unknown components. Organic amine (99.5%) and ethanol (99.5%) were purchased from Aladdin (China).
Cellulase (245 FPU/mL), was obtained from Tianguan Co. (Nanyang, China).
Pretreatment of corn stover by OACOP
The OACOP reactions were carried out in
The influence of catalyst type on OACOP
Initial experiments were first carried out using various organic amine (diethylamine, triethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, ethylenediamine) as an alkali catalyst at a dosage of 10 mmol/g dry corn stover, corn stover was fractionated into two primary components: holocellulose (cellulose and hemicellulose) and lignin under the present pretreatment process. Pretreatment without catalyst was used as a blank control. It is important to note that the present
Conclusions
In conclusion, a mild and efficient organic amine catalytic organosolv pretreatment of lignocellulose is presented, which can overcome the recalcitrance of lignocellulose to produce high yield of fermentable sugar and high-quality salt-free lignin. A delignification of 81.7% and total sugar yield of 83.2% was achieved. Compared to the traditional inorganic base or solid base pretreatment system, the catalyst along with the solvent in the present system can be easily recycled and reused.
Acknowledgments
This work was supported by the Program for The National Natural Science Foundation of China (Grant No.: 21406110) and Jiangsu Province Natural Science Foundation for Youths (Grant No.: BK20140938); Changjiang Scholars and Innovative Research Team in University (Grant No.: IRT_14R28); The Major Research Plan of the National Natural Science Foundation of China (Grant No.: 21390204); The State Key Laboratory of Motor Vehicle Biofuel Technology (Grant No.: KFKT2013001); and Jiangsu National
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