Potential use of pure and diluted choline chloride-based deep eutectic solvent in delignification of oil palm fronds
Introduction
In recent years, the exhaustion of global petroleum reserves, combined with unstable geopolitical energy-dependence and associated environmental issues are encouraging the replacement of fuel production by renewable resources such as hydro, solar and biomass as future feedstock for chemical industries in order to provide alternative platforms of biofuels and chemicals (Domínguez de María, 2014; Gunny et al., 2015). Hence, compared to other renewable resources, lignocellulosic biomass is regarded as one of the more favorable alternatives to achieve a sustainable global environment due to its amenability for conversion and abundance in availability (Luo et al., 2014). Presently, Malaysia is the second largest producer of palm oil in the international market with oil palm plantations dominating the largest sector of agriculture, up to 5.7 million hectares of plantation area in 2016 (MPOB, 2017). As a result, approximately 26.2 Mt/y of oil palm fronds (OPF) are generated per million fresh fruit bunches (FFB) processed. The production of OPF has severely outweighed other by-products such as empty fruit bunches (0.23 Mt/y) and oil palm trunks (7.0 Mt/y) (Yunus et al., 2010). Also, the OPF in Malaysia is underutilized because it is typically used for mulching in oil palm plantations. Therefore, the valorization of OPF into other value added-products could improve the agricultural waste management as well as rural economy and eventually lead to the techno-economic advancement in the country.
OPF is composed of lignocellulosic components that consists of lignin and complex carbohydrates such as hemicellulose and cellulose (Tan et al., 2016). These complex carbohydrates can be converted into high-value products such as biofuel, bioethanol as well as fermentable sugar (Solarte-Toro et al., 2018). Nevertheless, due to the recalcitrant structure of lignin in the lignocellulose, the direct conversion of lignocellulosic biomass into products is not energy-favorable (Clark and Deswarte, 2015) because lignin serves as a protective barrier, causing the biomass to be strongly resistant against biological and chemical attacks (Loow et al., 2016a). Hence, the removal of lignin from the lignocellulosic biomass by pretreatment is a necessity to facilitate the conversion into other bio-based products.
Over the past two decades, many pretreatment methods on lignocellulosic biomass have been developed and conducted. By applying an appropriate pretreatment stage, a more energy-efficient and simpler downstream process can be ensured (Loow et al., 2016b). Liu et al. (2009) and Yu et al. (2011) stated that using ionic liquids (ILs) as biomass pretreatment have drawn much attention from the scientific community due to its effectiveness and favorable results in the process of dissolving lignin (delignification) in lignocellulosic biomass valorization. Nonetheless, the “greenness” of ILs is often challenged due to their biocompatibility, biodegradability as well as sustainability (Paiva et al., 2014). In year 2003, a new solvent foundation was discovered, namely, deep eutectic solvents (DESs) (Loow et al., 2017a). This solvent has been recognized as one of the potential alternative green solvents for lignocellulosic biomass pretreatment due to their favorable properties in extracting bioactive phenolic compounds (Bubalo et al., 2016; Dai et al., 2013). Furthermore, DES exhibits similar physicochemical properties to classical ILs but possesses additional advantages such as simpler preparation and purification processes, low cost, biodegradable as well as low toxicity (Dominguez de Maria and Maugeri, 2011; Gunny et al., 2015).
Although the development of using DESs in the field of biomass processing is still in its early stage, many studies done in the recent years have showed the potential of using DES in pretreatment processes of biomass (Loow et al., 2017a). Bubalo et al. (2016) stated that the high viscosity of DES was one of the crucial factors that restricted its effectiveness in lignin extraction. Thus, the integration of water into DES may enable the reduction in viscosity and improve mass transfer during biomass pretreatment. To the best of our knowledge, no studies have been done to investigate the efficiency of pure and diluted DES in removal of lignin from the OPF in conjunction with the physical properties of DES. It was hypothesized that the implementation of water content into DES may further enhance lignin dissolution by improving mass transfer in the OPF. Therefore, the objective of this research was to study the effects of implementing water content (10–50%) in choline chloride:urea (1:2) on the delignification of OPF as well as examining the physical properties of pure and diluted DES.
Section snippets
Preparation of OPF
Fresh OPF were obtained from Universiti Kebangsaan Malaysia (UKM) without its leaflets. Subsequently, the liquid from the stalks of the OPF was removed by a press machine. Then, the pressed stalks were sun-dried for two days. Next, the dried stalks were cut and ground in a pulveriser (8000 rpm). The ground OPF were sieved and only the particles with ≤0.5 mm would be used in this study. Subsequently, the sieved OPF were rinsed for several times with hot distilled water to remove any contaminants
Chemical compositions of raw OPF
From this study, raw OPF by dry weight were comprised of lignin (19.53 ± 0.07%), xylan (19.45 ± 0.33%), arabinan (1.19 ± 0.64%), glucan (44.8 ± 0.27%), ash (0.40 ± 0.12%) as well as water extractives (11.33 ± 0.71%) and ethanol extractives (3.77 ± 0.49%). The result obtained was compared and found to be varying slightly with other literature findings (Goh et al., 2012; Tan et al., 2016) The variation was expected due to the different maturity of OPF in different geographical location (Loow and
Preliminary economic and environmental assessments
One of the challenges in commercializing chemicals and fuels derived from lignocellulosic biomass was to reduce operating costs, especially during the pretreatment stage in order to generate favourable investment returns. In general, the cost of the pretreatment process should be sensible at industrial scale with lower energy consumptions and lower demands of expensive chemicals (Wyman et al., 2005). Besides, the main costs which governed a biorefinery could be categorized into three, namely
Conclusion
The effects of water content in ChCl:urea on delignification of OPF were investigated. From the standpoint of lignin removal, ChCl:urea with 30 vol% distilled water was determined to be 16.31%, which was higher than pure ChCl:urea (11.10%) in delignification of OPF at pretreatment condition of 120 °C and 4 h. The effectiveness of ChCl:urea with 30 vol% distilled water in delignification of OPF was attributed to its lower viscosity that resulted in better mass transfer during pretreatment. The
Acknowledgments
The funding of this research is supported by Ministry of Higher Education under Fundamental Research Grant Scheme (FRGS/1/2016/WAB01/MUSM/02/2) and Long Term Research Grant Scheme (LRGS/2013/UKM-UKM/PT/01). Additionally, the authors would like to thank Monash University Malaysia for providing E.K. New, C.B.T.L. Lee and Y.-L. Loow with postgraduate scholarships. Lastly, the authors would like to thank School of Engineering for providing UROP to L.Y.W. Foo an early opportunity to undergo a
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