Simultaneous and Efficient Production of Furfural and Subsequent Glucose in MTHF/H2O Biphasic System via Parameter Regulation.

Efficient production of furfural from cornstalk in 2-Methyltetrahydrofuran/aqueous (MTHF/H2O) biphasic system via parameter regulation (e.g., VMTHF/VH2O, temperature, time, and H2SO4 concentration) was proposed. The resulting solid residues achieved from the different MTHF/H2O system conditions for furfural production were also to prepare glucose by adding cellulases to increase the high-value applications of cornstalk. A maximum furfural yield (68.1%) was obtained based on reaction condition (VMTHF:VH2O = 1:1, 170 °C, 60 min, 0.05 M H2SO4). Among these parameters, the concentration of H2SO4 had the most obvious effect on the furfural production. The glucose yields of the residues acquired from different MTHF/H2O processes were enhanced and then a maximum value of 78.9% based on the maximum furfural production conditions was observed. Single factor may not be sufficient to detail the difference in glucose production, and several factors affected the hydrolysis efficiency of the residues. Overall, the MTHF/H2O system effectively converted cornstalk into furfural and glucose via a simple and environment-friendly process, thus was an ideal manner for the food industries.


Introduction
Furfural is a high value platform compound that has potential to produce bio-based chemical in the many industries and interests amongst scientists that surpasses [1][2][3]. Generally, agricultural and forestry biomasses are some of the main raw materials for furfural production because they contain rich hemicellulosic polymer constituents [4]. Cornstalk is often used for mulching. In fact, cornstalk is an ideal source for furfural production because it is xylan-rich polymers. Unfortunately, cornstalk is intractable for disintegrating hemicelluloses owing to the rigid and dense cell wall structure [5]. Therefore, a pretreatment process is usually required to destroy the inherent structure of cornstalk, thereby improving the furfural production and then glucose via enzymatic hydrolysis [6].

Furfural Yields by the MTHF/H 2 O System
The MTHF/H 2 O system process was proposed to reduce recalcitrance of cornstalk and enhance furfural yield. Figure 1 shows the effects of the volume ratios of MTHF/H 2 O, temperatures, times, and H 2 SO 4 concentrations on the furfural yields from cornstalk. The MTHF/H 2 O process conditions greatly influenced the furfural yields. It can be seen from Figure 1a that the furfural yields were closely related to the volume ratios of MTHF/H 2 O. Under the reactions at 160 • C for 30 min with 0.1 M H 2 SO 4 conditions, the furfural yields appeared gradual and was a slight enhancement from 25.7% to 31.7% with the volume ratios of MTHF/H 2 O reduced from 3:1 to 1:1. A possible reason was that the strengthening of xylan hemicelluloses depolymerization and D-xylose dehydration [19]. However, as the volume ratios of MTHF/H 2 O further reduced to 1:2, the furfural yields quickly reduced from 31.7% to 19.8%, which was mainly the furfural degradation during the MTHF/H 2 O process. This was also in keeping with the previous facts based on a MIBK/H 2 O biphasic system process [19]. In other words, the furfural constituent was labile and degraded quickly at low volume ratios of MTHF and H 2 O under the conditions given. Based on the results of Figure 1a, the volume ratios of MTHF and H 2 O of 1:1 were selected as a maximum production volume ratio of MTHF/H 2 O for furfural preparation from cornstalk. MIBK/H2O process, the maximal furfural yield was 65.9% and the recovery of residue was 46.9% under an optimal reaction condition [19]. In the current study, a maximum production conditions with yield of 68.1% for furfural from cornstalk by MTHF/H2O process was achieved at 170 °C for 60 min with 0.05 M H2SO4 when the volume ratio of MTHF/H2O was 1:1. The furfural yield with MTHF/H2O process was higher than that of the MIBK/H2O process; thus, the MTHF/H2O process was a potential method to prepare furfural from cornstalk.

Yields and Constituent Analysis of the Resulting Residues
During the MTHF/H2O process, xylan hemicellulosic constituents were mainly converted into furfural via continuous depolymerization and dehydration processes [23]. Thus, yields and constituent analysis of the resulting residues changed significantly under the biphasic system conditions given. Table 1 shows the yields and chemical constituents of the resulting residues obtained by different MTHF/H2O processes. As compared to the residue yield (59.0%) at 170 °C for 60 min without adding H2SO4, the residue yields were continuously reduced (48.9-57.9%) with H2SO4 concentrations increased under the same temperature and time conditions. This was mainly  . It was found that the process temperature affected the furfural yield, releasing furfural from MTHF/H 2 O system with the highest yield (46.7%) acquired at 170 • C. In addition, the effect of the process times (15,30,60,90, and 120 min) on furfural preparation was determined and the data are displayed in Figure 1c. The results demonstrated that the highest furfural yield (52.8%) was gained at 170 • C for 60 min with 0.1 M H 2 SO 4 . According to the report, whether in a single-phase aqueous solution or in an organic/aqueous biphasic system, the H 2 SO 4 concentration significantly influenced the conversion of xylose to furfural [2,29]. Thus, under the conditions above (V MTHF :V H2O = 1:1, 170 • C, 60 min), various H 2 SO 4 concentrations (0, 0.02, 0.03, 0.05, 0.1, 0.3, and 0.5 M) were also added to the MTHF/H 2 O biphasic system for evaluating its impacted on the furfural production ( Figure 1d). The furfural yield was obviously increased to 68.1% with 0.05 M H 2 SO 4 concentration as compared to without adding H 2 SO 4 (10.4%). However, when the H 2 SO 4 concentrations further increased to 0.1-0.5 M, the furfural yields began to go down gradually, which was probably related to the further degradation of furfural under high concentration acid conditions. Therefore, it is not necessary to add high H 2 SO 4 concentrations for furfural production during the MTHF/H 2 O system process. Based on the above analysis, it can be seen that the H 2 SO 4 concentration was the most important parameter for furfural production because it had the most obvious influence on the furfural yield. In addition, during the MIBK/H 2 O process, the maximal furfural yield was 65.9% and the recovery of residue was 46.9% under an optimal reaction condition [19]. In the current study, a maximum production conditions with yield of 68.1% for furfural from cornstalk by MTHF/H 2 O process was achieved at 170 • C for 60 min with 0.05 M H 2 SO 4 when the volume ratio of MTHF/H 2 O was 1:1. The furfural yield with MTHF/H 2 O process was higher than that of the MIBK/H 2 O process; thus, the MTHF/H 2 O process was a potential method to prepare furfural from cornstalk.

Yields and Constituent Analysis of the Resulting Residues
During the MTHF/H 2 O process, xylan hemicellulosic constituents were mainly converted into furfural via continuous depolymerization and dehydration processes [23]. Thus, yields and constituent analysis of the resulting residues changed significantly under the biphasic system conditions given. Table 1 shows the yields and chemical constituents of the resulting residues obtained by different MTHF/H 2 O processes. As compared to the residue yield (59.0%) at 170 • C for 60 min without adding H 2 SO 4 , the residue yields were continuously reduced (48.9-57.9%) with H 2 SO 4 concentrations increased under the same temperature and time conditions. This was mainly due to the degradation of polysaccharides in cornstalk under the given conditions. In addition, as the MTHF/H 2 O process temperatures and times increased, the residue yields were also reduced. As shown in Table 1, only a small amount of hemicellulosic polysaccharide (7.6%) was detected in R170-60-0, suggesting that the hemicellulosic constituents were prominently destroyed and degraded during the MTHF/H 2 O processes under the conditions given. The cellulose content of the residues first increase from 54.7% (in the experiment condition at 170 • C for 60 min without adding H 2 SO 4 ) to 64.3% (in the experiment condition at 170 • C for 60 min with 0.05 M H 2 SO 4 ), but then decreased to 51.1% at 170 • C for 60 min with 0.1 M H 2 SO 4 , suggesting that the degradation of partial cellulose occurred when the H 2 SO 4 concentrations was higher than 0.05 M. Meanwhile, the difference in cellulose content of the residues presented a similar result with the MTHF/H 2 O process temperature elevated. In short, the MTHF/H 2 O process was effective to produce furfural and collect the cellulose-rich residues for the preparation of glucose by the hydrolysis experiment.

Enzymatic Hydrolysis of the RM and Resulting Residues
The MTHF/H 2 O system not only enabled efficient preparation of furfural, but also simultaneously obtained cellulose-rich residue to prepare glucose by the hydrolysis experiment. In the current study, the enzymatic hydrolysis time of the substrate was set to 60 h. The glucose yields of the RM and resulting residues are emerged in Figure 2. As can be seen, the RM showed a low glucose yield (34.2%) because the dense structure hindered the accessibility of cellulases [19]. However, glucose yields of the resulting residues displayed various variation tendencies under different MTHF/H 2 O biphasic process conditions. Among these substrates, glucose yields of the R 170-60-0 (33.6%) were similar to that of RM, implying that the MTHF/H 2 O process without adding H 2 SO 4 did not have an effect on the reducing cornstalk materials recalcitrance. However, under the same MTHF/H 2 O temperatures and times (170 • C, 60 min) conditions, with the addition of H 2 SO 4 from 0 to 0.05 M, the glucose yields appeared to notably increase from 33.6 to 78.9%. The increasing fact may be attributed to the enhancement of degradation and removal of hemicelluloses in RM during the MTHF/H 2 O system process under the current treatment conditions [30]. Nevertheless, as the H 2 SO 4 concentrations further increased to 0.1 M, the glucose yields decreased to 65.0%, which was as a result of the degradation and removal of partial cellulose during the process. Under the same process times and H 2 SO 4 concentrations (60 min,

Surface Morphology of the RM and Resulting Residues
To evaluate the various MTHF/H2O processes on the effect of surface morphology of RM, SEM images of resulting residues were observed at magnifications of 1000 ( Figure 3). The surface of RM and R170-60-0 showed a smooth and dense morphology, resulting in enzymes having difficulty with contacting cellulose components in RM and R170-60-0 [31]. Thus, low glucose yields by the hydrolysis experiment were obtained from RM and R170-60-0. By contrast, the surfaces of the resulting residues obtained from the MTHF/H2O process by adding H2SO4 were broken and some cracks and particle-sized debris to different degrees emerged, which resulted in an increase of glucose yields of the residues [19]. For example, in all of the resulting residues obtained from the MTHF/H2O process under adding H2SO4 conditions, the surface damage degree of R150-30-0.1 was minimal, leading to the lowest glucose yield. As expected, as the H2SO4 concentrations increased from 0 to 0.05 M under the same process temperatures and times, the damage of the residues was aggravated. The fact indicated that the surface damage of the residues was conducive to the production of glucose by enzymatic hydrolysis.

Surface Morphology of the RM and Resulting Residues
To evaluate the various MTHF/H 2 O processes on the effect of surface morphology of RM, SEM images of resulting residues were observed at magnifications of 1000 ( Figure 3). The surface of RM and R 170-60-0 showed a smooth and dense morphology, resulting in enzymes having difficulty with contacting cellulose components in RM and R 170-60-0 [31]. Thus, low glucose yields by the hydrolysis experiment were obtained from RM and R 170-60-0 . By contrast, the surfaces of the resulting residues obtained from the MTHF/H 2 O process by adding H 2 SO 4 were broken and some cracks and particle-sized debris to different degrees emerged, which resulted in an increase of glucose yields of the residues [19]. For example, in all of the resulting residues obtained from the MTHF/H 2 O process under adding H 2 SO 4 conditions, the surface damage degree of R 150-30-0.1 was minimal, leading to the lowest glucose yield. As expected, as the H 2 SO 4 concentrations increased from 0 to 0.05 M under the same process temperatures and times, the damage of the residues was aggravated. The fact indicated that the surface damage of the residues was conducive to the production of glucose by enzymatic hydrolysis.
This phenomenon was mainly due to the increase in the accessibility of cellulase to cellulose by the release of a large number of adsorption sites [32]. When the H 2 SO 4 concentration was further increased to 0.1 M, the surface of R 170-60-0.1 was similar to R 170-60-0.05 . However, the glucose yields of two residues showed different values. This phenomenon suggested that the glucose yield was not only affected by surface morphology of the substrates, but also by several other factors, such as cellulose crystallinity and particle size of the substrates [33,34]. In short, the MTHF/H 2 O process led to surface destruction of RM, forming a large number of adsorption sites of cellulases on the surface of the substrates, thereby improving the efficiency of enzymatic hydrolysis. the residues [19]. For example, in all of the resulting residues obtained from the MTHF/H2O process under adding H2SO4 conditions, the surface damage degree of R150-30-0.1 was minimal, leading to the lowest glucose yield. As expected, as the H2SO4 concentrations increased from 0 to 0.05 M under the same process temperatures and times, the damage of the residues was aggravated. The fact indicated that the surface damage of the residues was conducive to the production of glucose by enzymatic hydrolysis.

FT-IR of the RM and Resulting Residues
To assess the structural changes of cornstalk after different MTHF/H 2 O processes, the FT-IR spectra of RM and resulting residues are illustrated in Figure 4. As can be seen, two obvious bands (1712 and 1233 cm −1 ) of the acetyl ester in hemicelluloses clearly appeared in the RM and R 170-60-0 [35]. However, the two bands gradually weakened and even disappeared completely in the resulting residues under adding H 2 SO 4 conditions, which was mainly significant removal of xylan hemicelluloses during the MTHF/H 2 O processes, especially at higher process severities. The presence of hemicelluloses inhibited the enzymatic hydrolysis efficiency of cellulose in different biomasses to some extent [36]. Therefore, the removal of hemicelluloses was beneficial to improve the glucose yield, and the results of enzymatic hydrolysis also confirmed this conclusion. The bands about aromatic skeletal vibrations and the C−H deformation were distinctly observed at 1608, 1512, and 1427 cm −1 in all the spectra of the resulting residues [37,38]. This suggested that the enhancement of the process severities during the MTHF/H 2 O system had no evident influence on the structure and content of the lignin. Thus, after the MTHF/H 2 O processes, the improvement of the enzymatic hydrolysis efficiency of the resulting residues was mainly attributed to the removal of hemicellulosic constituents from the RM. about aromatic skeletal vibrations and the C−H deformation were distinctly observed at 1608, 1512, and 1427 cm −1 in all the spectra of the resulting residues [37,38]. This suggested that the enhancement of the process severities during the MTHF/H2O system had no evident influence on the structure and content of the lignin. Thus, after the MTHF/H2O processes, the improvement of the enzymatic hydrolysis efficiency of the resulting residues was mainly attributed to the removal of hemicellulosic constituents from the RM.

Crystallinity of Cellulose in the RM and Resulting Solid Residues
Besides the contents and chemical structures of hemicelluloses and lignin as well as surface morphology of the substrates affecting the glucose yields of resulting residues, the CrI is also considered one of the important factors influencing the production of glucose [39,40]. Thus, the CrIs of cornstalk before and after the various MTHF/H 2 O processes were determined by XRD patterns ( Figure 5). As can be seen, the crystal structure of cellulose in resulting residues collected obtained from the different MTHF/H 2 O processes did not transform, but there was increasing evidence in the CrIs (55.3-65.1%) compared to that of RM (52.1%). Among these residues, the CrI of the R 170-60-0 was only increased by 4.7%, so the glucose yields of the RM and R 170-60-0 showed similar values. As the elevating process severities, the CrIs of the resulting residues first increased and then decreased, implying that the partial cellulose occurred degradation under the higher severities [41]. Combined with the results of glucose yields of the substrates, it was found that there was no linear relation between CrIs and glucose yields of resulting residues since the hydrolysis efficiency of cellulose was not only affected by CrI, but also by several other factors, such as contents and distribution of hemicelluloses or lignins as well as surface morphology of the substrates [42][43][44][45][46]. Cheng et al. examined the influence of the ionic liquid process on the crystal structure of cellulose in different biomasses (microcrystalline cellulose, switchgrass, pine, and eucalyptus), and its effect on hydrolysis kinetics of cellulose. The results indicated that the biphasic process led to a loss of crystalline region for native cellulose. Particularly, there was a significant difference in the transformation process between microcrystalline cellulose and lignocellulosic samples. Microcrystalline cellulose was explained by more thoroughly transforming to cellulose II after the process under the condition given. However, the lignocellulosic samples showed that another other factor, likely lignin-carbohydrate complexes, also impacted the hydrolysis efficiency in addition to CrI [46]. Thus, a single factor may not be sufficient to detail the difference in glucose production, and several factors affected the cellulose hydrolysis of the substrate in the current study [47]. transformation process between microcrystalline cellulose and lignocellulosic samples. Microcrystalline cellulose was explained by more thoroughly transforming to cellulose II after the process under the condition given. However, the lignocellulosic samples showed that another other factor, likely lignin-carbohydrate complexes, also impacted the hydrolysis efficiency in addition to CrI [46]. Thus, a single factor may not be sufficient to detail the difference in glucose production, and several factors affected the cellulose hydrolysis of the substrate in the current study [47].

Conclusions
Simultaneous and efficient production of furfural and subsequent glucose in the MTHF/H2O biphasic system via parameter regulation was a promising approach to convert cornstalk. The results demonstrated that the MTHF/H2O conditions (VMTHF/VH2O, temperature, time, and H2SO4 concentration) had a remarkable influence on the furfural and glucose preparation. The maximum

Conclusions
Simultaneous and efficient production of furfural and subsequent glucose in the MTHF/H 2 O biphasic system via parameter regulation was a promising approach to convert cornstalk. The results demonstrated that the MTHF/H 2 O conditions (V MTHF /V H2O , temperature, time, and H 2 SO 4 concentration) had a remarkable influence on the furfural and glucose preparation. The maximum furfural yield was 68.1% under the reaction conditions (V MTHF :V H2O = 1:1, 170 • C, 60 min, 0.05 M H 2 SO 4 ). The concentration of H 2 SO 4 was the most important parameter for furfural production. The glucose yields of cellulose were improved after different MTHF/H 2 O processes and a maximum value was up to 78.9% under the same MTHF/H 2 O system condition with preparation of furfural. The single factor may not adequately elaborate the differences of glucose yield, and several factors impacted the cellulose hydrolysis of the substrates.