Herein, the four ILs namely [DBU][OTf], [BtTzm][OTf], [DMEA][OAc] and [DEEA][OAc]were used as a dual solvocatalyst in a 20:1 ratio of IL: RS. Structures of the different ILs used here in this study are shown in Fig. 1. Similar type of pretreatment study was already done where the biomass to solvent ratio was maintained to be 1:10 g/g. In such study the solvent was used as a binary mixture of [TEA][HSO4]: water with 4:1 w/w ratio and they have conducted the experiments in triplicate at 120 °C for 24 hrs (Chambon et al. 2018). The acidic character of the ILs served to be useful for its dissolution and its larger amount helped for its behavior as a solvent.
Moreover, scanning electron microscopy (SEM) is an imperative tool for examine the morphological aspects of biomass. The surface of the untreated RS was found to be well organized due to the lignin; which acts as skeleton where we can see a well-organized rib-cage like structure or binding agent to hemicellulose and cellulose (shown in Fig. 2a and 2b). Fig. 2c-g represents the pretreatment of RS with [DBU][OTf]; where, small and long fibers started appearing in the 90 °C and 120 °C ionothermal treatment due to the thermo-mechanical force applied besides the acidic media. The ionothermal treatment at 140 °C RS was observed to the similar of untreated RS, which shows the worst effect on the structural change. This could be mainly attributed due to the degradation of the IL as it turned black instantly. Further, this degradation was confirmed through the NMR of the extracted IL via post-ionothermal treatment, which is entirely different from the original NMR of [DBU][OTf].
Rice straw after pretreatment with [DBU][OTf] at different temperatures are viewed at different sizes are shown in figures (c) – (h) where c) 90 °C at 100 µm; d) 90 °C at 50 µm; e) 120 °C at 100 µm; f) 120 °C at 50 µm; g) 140 °C at 100 µm and h) 140 °C at 50 µm.
Further, for examine the effect of cation having same anion ([BtTzm][OTf]) on pretreatment of RS we have started to solubilize in similar extent. Fig. 3a-d represent the morphology of RS after pretreated with [BtTzm][OTf] at a lower temperature 90ºC. In the presence of triazolium cation only a few fibers were formed in the diameter range of 4.5 to 5.2 µm. Thus, the fibers were not completely released when triazolium cation was used for pretreatment. Here, a lot of fibers were visible in case however those fibers were still bound to each other. Moreover, two more ILs were compared having a common anion as acetate and their cation were varied. However [DMEA][OAc] and [DEEA][OAc] showed eroded surfaces and formation of fewer fibers were observed only in the case of DEEA (Fig. 3e-h). Overall, observed SEM data suggest that complete deorganization of the lignocellulosic material when [DBU][OTf] whereas in case of the other three studied ILs we did not see any proper reorganization. Since DBU-based IL has proved a better result among others studied ILs, we went ahead to characterize other properties of the pretreated product obtained through [DBU][OTf].
Further for validating the above result, we have done the PXRD to determine the crystallinity index (CrI) which is a vital parameter to understand the intensity of crystallinity of any substance. The XRD intensity profile for the original RS (shown in orange, Fig. 3) showed the characteristic crystalline peak (I200) and amorphous peak (I110) or (Iam) at 2θ value of 22° and 18°, respectively. Hence, the crystallinity index (CrI) was calculated using the below formula (equation 1) (Segal et al. 1959).
CrI = ((I200-Iam)/(I200))×100 (1)
Where I200 is the peak intensity at plane (2 0 0) (2θ = 22°) and Iam /I110 is the minimum intensity at the valley between plane (2 0 0) and (1 1 0) (2θ = 18).
The CrI for original RS (shown in orange, Fig. 3) and treated RS with IL at 90 °C (shown in green, Fig. 3) and RS treated with 120 °C (shown in violet, Fig. 3) were 50.77%, 35.27% and 18.16%, respectively. This indicates that ionothermal treatment at 90 °C and 120 °C is capable of decreasing the crystallinity of the polymer and temperature plays a significant role in decreasing the crystallinity. The decrease in crystallinity can be attributed to the fact that dissolution of some components from the polymer occurs in the IL. The SEM results completely agree with the PXRD results which mentions the change in the crystallinity and tearing up of the lignocellulose to fibers occurred when ionothermal treatment at 90 and 120 ºC was provided.
The decrease in the crystallinity can be because of two reasons: first removal or conversion of lignin and second removal or conversion of cellulose to some furans such as HMF (via glucose) and gases/volatile components. Complete removal of lignin may not be a possibility as the temperature was lower than 300 °C. The glycosidic bonds linking the glucose units in cellulose are not very strong. There are inter and intramolecular hydrogen bonds associated per glucosyl unit in raw cellulose. The packing of numerous cellulose flatsheets is mainly through van der Waals forces and H-bonding. (Scheme 1). However, the weak glycosidic bonds in cellulose cleave under acid or high-temperature conditions. The latter was proven due to the qualititative analysis of 5-hydroxymethylfurfural (HMF) using TLC sheets in the IL part post-filtration (Scheme 1).
This decrease in crystallinity made it more porous than before and hence further treatments like enzymatic hydrolysis can be carried out more effectively. The effectiveness was tested by pyrolysis method mentioned in the last section. The high temperature would have removed the volatile components and due to these two factors the morphological structures differed before and after treatment as also agreed with SEM images (Fig. 2).
Moreover, Interaction of –OH groups present in cellulose with both cation and anion of ILs make it soluble in those solvents. Oxygen atoms that belong to the –OH groups of cellulose function as electron donors and hydrogen atoms serve as electron acceptors. IL anions also behave as electron donors. Cations having an electron-rich aromatic π system is not interacting much with hydroxyl oxygen atom through either nonbonding electrons or π electrons whereas anions prefer to interact through hydrogen bonding to the hydroxyl proton of cellulose moiety (Feng and Chen 2008).
Such interactions were mainly investigated through the FTIR analysis. Thus it was carried out for the RS as well as ionothermally treated RS at three different temperatures as shown in Fig. 4. The -OH stretching at 2945 from original RS disappeared in the 90 °C and 120 °C indicating cleavage of some H-bonds (mainly O(6)H----O(3)) during the process of conversion of cellulose to HMF via glucose. At 140°C, charring has been occurred so that product was not analyzed. In addition, FTIR was carried out for the original RS as well treated RS at 90, 120 and 140°C. The FTIR at 140°C also confirmed that IL degraded at this temperature and so there was hardly any change in the RS’s composition besides the charring and color of IL turning to black. Treated RS at 140 °C was also used to measure the IR data and no modification was observed in its peak because their chemical constituent looks quite similar to the one that was untreated. The FTIR confirmed the removal of some lignin and cellulose H-bonding when the RS was treated at 120°C. Visible changes were observed in the texture and composition of the RS after the dissolution in IL for 6 h. These conversions for the recycled ILs were obtained by calculation of mass before and after the treatment.
Next, the band at 3383 cm−1 specifies C-H and O-H stretching band of cellulose which clearly broadens on treatment at 90 °C and 120 °C. O-H stretching is obtained more in case where the treatment occurred at 120 °C. The peak at 2945 cm−1 corresponded to O-H and C-H stretching band of the lignocellulosic matrix and this was not present in the RS treated at 90 °C and 120 °C. The peak at 2400 cm−1 corresponded to a methyl group of lignin (between 2915 and 2847 cm−1) which disappeared at 90 °C and 120 °C treated RS. The peak around 1607 cm−1 corresponded to lignin bonds which also disappeared in lower temperature treatments. The peak at 1211 cm−1 corresponds to the syringyl ring and C-O stretching lignin character vibration. The peaks between 1000 and 400 cm−1 corresponded to the presence of silica bonds in the lignocellulose. The treatment at 120 °C showed deformation in the silica bonds as the peaks did not appear. Barriers on the RS towards enzyme accessibility were removed after ionothermal treatment and the cellulose as well as some hemicellulose portion was exposed.
Additionally, the decrement of extent of lignin has been monitored through TGA analysis, where we can see that removal of cellulose and lignin and that was the reason lesser biochar were obtained in these cases. The removal of some lignin components also agreed with the TGA data which clearly showed that the pyrolysis temperature decreased for the pretreated RS.
Figure 5 suggested the pyrolysis of biomass through TGA analysis, which was significantly affected by the CrI. As displayed in Fig. 5, the decomposition temperature of hemicellulose, cellulose and lignin is observed in the range of 220−315 °C, 314−400 °C and 160 to 900°C respectively, which is the indication of generating a solid residue. The main gases could be CO2, CO, CH4 and some organics, which is mainly responsible for the slow residence times (Yang 2007). Thus, as suggested from Fig. 5, 29%, 21% and 9% biochar was the solid carbon-rich residue left after pyrolysis of pretreated RS biomass for original RS, pretreated at 90 and 120 °C, respectively (Figure 5). Biochar is a low-grade fuel and can be collected in parts from all pyrolysis reactions, which can be further used as a fuel for new pyrolysis reactions. The European Bioenergy Research Institute (EBRI), Aston University runs a pyrolysis unit using biochar. Biochar has also been used as an additive to increase the soil fertility and improves the water holding capacity. All these specify that the RS’s waste after pretreatment can be utilized for various applications. At higher temperatures rapid cleavage of glycosidic bonds occur leading to the formation of gaseous products both condensable and non-condensable and further pyrolysis of the pretreated RS allows the cellulose structure to degrade sharply during the initial stages of fast pyrolysis with the cleavage of more glycosidic bonds thereby leading to lesser char yields.
The TGA profile can be roughly divided into three regions. (i) < 220°C showed weight loss (less than 10%) which may be due to dehydration and removal of volatile components; (ii) between 220 and 360°C showed weight loss upto 40%. Between 200 and 300 the cleavage of intra- and intermolecular hydrogen bonds occur and (iii) at 360°C and higher temperatures. We can see here clearly that the treated RS at 120°C had only 9% remains of the feedstock which clearly suggests that some chemicals were formed and waste is reduced compared to the untreated RS (29% remains) and also RS treated at 90°C where 21% remains. These agree with the FTIR results i.e. removal of some components.
Like petroleum cracking, a large number of reactions take place during pyrolysis of biomass, like condensation, depolymerization, dehydration, isomerization and charring reactions and plenty of pyrolyzed products are released (Wang et al. 2020). Due to all the above mentioned situations the bio-oil or gases when trapped are not pure and have to go through another process of distillation. However, these products were not analysed in this part of our study. Lignin pyrolysis gives mainly aromatic compounds and pyrolysis of cellulose and hemicellulose gives aliphatic components. Due to so many simultaneous reactions going on at the same time it is a difficult task to identify the exact mechanism.
Further, the changes in the CHN composition were also carried out in order to understand the extent of pretreatment. The original RS possessed the elements C, H and N as 9.12%, 0.24% and 0.23%, respectively. On ionothermal treatment at 90 °C and 120 °C the C, H and N percentages changed to 2.65%, 0.05%, 0.38% and 1.81%, 0.38%, 0.08%, respectively (Fig. 6). As the Lewis basic nature of the DBU cation enhances the C6 -O bond breakage, which is mainly due to the tertiary nitrogen in the cation and helps in the dehydration of carbohydrate-kind polymers (Song et al. 2013). Also, old report suggested that anion of ILs were mainly influence the abstraction of the H-atom. Due to this the glycosidic bond weakens and thus the crystallinity reduces. The cellulose finally breaks into glucose which undergoes in presence of the ionothermal condition to give HMF and this was detected using a TLC in the liquid substrate however was not analyzed quantitatively.
Next, the recyclability of IL makes it more sustainable towards the biomass applications. In this regards, the recycling was carried out and the percentage of conversion as recycling process continues is shown in Fig. 7. IL could be recycled at least 3 times for reutilization. In all the cases we can see that the pretreatment at 120 °C was more efficient. ILs was washed with dichloromethane (DCM) as IL is soluble in it also. RS samples were fresh and the IL was used for several cycles after washing. Upto 89% of IL was recycled three times and conversions reduced by 5%, 7%, 10% at 90 °C and 7%, 7%, 10%, at 120 °C. Additionally, water was used as an antisolvent to make the unwanted particles to settle down easily in the vial used for experiments.
So far, owing to multiple issues like high expense behind the production of IL, large scale uptake of ILs is impeded to some extent. Similarly, there are another factors like detailed explanation on micro–macro as well as molecular level of the deconstruction mechanisms which prevents the optimization and modeling, and the requirement of a techno-economic stable assessment on large scale experiments (Halder et al. 2019). Considering the high thermal as well as chemical stabilities of the studied IL, possibilities of reuse do exist and this opens up an economic way of lignocellulose pretreatment in an energy efficient method.