Lignocellulosic biomass (LCB) is the most abundant and renewable resource on the planet, which can act a low-cost and promising feedstock for production biofuels, biomaterials, and biochemicals, etc., attracting worldwide interests and attentions (Alonso et al. 2017). However, owing to the high biomass recalcitrance, a suitable pretreatment process is required to remove lignin and hemicellulose, depolymerize of cellulose of LCB, and release fermentable sugars for further bioconversion such as bioethanol production (Qi et al. 2018; Tu and Hallett 2019).
To date, several LCB pretreatment techniques have been developed, which are mainly categorized into physical, chemical, physicochemical, biological techniques, and some cocktail of these strategies (Haldar and Purkait 2020). Among them, steam explosion (SE) method is of advantages such as cost-effective and environmentally -friendly supermolecular deconstruction, which is widely applied in LCB especially hardwood and agricultural residual pretreatment (Jacquet et al. 2015). Meanwhile, to improve the efficiency and reduce the severity of SE (e.g., lowering the temperature and pressure, shortening the residence time), some chemical additives, mainly mineral acids or alkalis (such as SO2, H2SO4, H3PO4, and NH4), are introduced into SE process (Zhang et al. 2020).
Dilute phosphoric acid plus steam explosion (DPASE) is a promising alternative LCB pretreatment method (Fockink et al. 2018; Geddes et al. 2011; Oliva et al. 2020), holding the advantages as following: (i) acting as an exogenous weak acid catalyst, dilute H3PO4 results in low sugar loss and yield of toxins simultaneous with SE (Pitarelo et al. 2016; Zeng et al. 2014); (ii) H3PO4 can not only serve as an additional source of nutrients for microbe, but can also be partly recovered and reused as a fertilizer (Geddes et al. 2011); and (iii) H3PO4 is less corrosive to the SE apparatus, which is beneficial for the equipment and efforts at scaling up (Haykiri-Acma and Yaman 2019; Koradiya et al. 2016). However, regarding the distinct components and microstructures of multisourced LCB feedstocks (e.g., dedicated crops, agricultural residues, and short-rotation energy coppices), more researches are needed to expand the application of DPASE.
It has been reported that surfactants, particularly nonionic surfactants (Tween, Triton, and polyethylene glycol, etc.) in the pretreatment process can promote the wettability of LCB, decrease the crystallinity of cellulose, give higher cellulose conversion, improve the delignification rate, maximize the enzymatic convertibility, and enhance the ethanol yields, etc. (Cao and Aita 2013; Jørgensen et al. 2007; Nasirpour et al. 2014; Zheng et al. 2020). A fatty alcohol polyoxyethylene ether based nonionic surfactant agent JFC (R-O(CH2CH2O)5-H, wherein R = C7 − 9), holds the properties of reducing interfacial tension, enhancing the wettability of materials, improving the capillary effect inside fibers, etc., which has been introduced in textile modification (Gao et al. 2017), coal mining (Shi et al. 2019), porous geopolymers preparation (Yan et al. 2021), mineral flotation (Chen et al. 2018), etc. Considering its other advantages such as strong acid, alkali, and high temperature resistance and no bio-toxicity, the surfactant agent JFC holds potential of assisting the LCB steam explosion pretreatment.
In this work, a novel surfactant JFC-assisted dilute phosphoric acid plus steam explosion pretreatment of poplar wood craft was developed. Four crucial factors (phosphoric acid concentration, surfactant concentration, pressure, and residence time) affecting the pretreatment efficiency were optimized using the single factor tests. The morphological and structural characteristics of samples were characterized for analyzing the mechanism of pretreatment and further optimizing of the process.