**2. Results and Discussion**

### *2.1. Characterization of the Catalyst*

The Fourier transform infrared (FT-IR) spectrum of bamboo-derived carbonaceous magnetic solid acid prepared by different pretreatment methods is shown in Figure 1. The peak of 1560 cm<sup>−</sup><sup>1</sup> that appeared in all the samples corresponds to the C=C stretching vibration in aromatic carbons [22]. The typical band that appeared at 585 cm<sup>−</sup><sup>1</sup> represents the Fe–O vibration in Fe3O4, indicating that the solid acid was successfully magnetized during magnetization [23]. Moreover, strong bands such as 1041 cm<sup>−</sup><sup>1</sup> and 1380 cm<sup>−</sup><sup>1</sup> (O=S=O asymmetric) indicate that the –SO3H group was successfully introduced into the bamboo-derived magnetic solid materials [24]. However, the strength of 1137 cm<sup>−</sup><sup>1</sup> (O=S=O symmetric stretching vibrations) was varied between solid acids prepared by different pretreatment methods. The order of the strength of the O=S=O peak at 1137 cm<sup>−</sup><sup>1</sup> was: dilute acid pretreated > dilute alkali pretreated > deionized (DI) water pretreated. The activated carbon prepared by dilute acid pretreatment was better combined with sulfuric acid, which was consistent with the subsequent catalytic hydrolysis results. In addition, a band at 3423 cm<sup>−</sup><sup>1</sup> corresponds to the O–H stretching vibration in –COOH, suggesting that phenolic –OH groups were also successfully generated to the prepared solid acid during the carbonization and sulfonation processes [25].

**Figure 1.** Fourier transform infrared (FT-IR) spectrum of bamboo-derived carbonaceous magnetic solid acid.

In order to understand the H+ concentration on the surface of the prepared materials, an acid/base titration test was performed, and the results are shown in Figure 2. H+ concentration on the surface of bamboo-derived carbonaceous magnetic solid acid prepared by different pretreatment methods was in the range of 0.6487 mmol/g to 2.3204 mmol/g. The acidity of the catalyst increased with the increment of acid/alkaline concentration, which may be related to the better interaction between the substrate and chemicals [18]. Compared with dilute alkali pretreatment, the acid density of the solid acids prepared by dilute acid pretreatment is higher, except at high concentration (2%). This phenomenon was consistent with the above FT-IR results. However, the acid density of the sample prepared by 2% KOH was much higher than that of the sample prepared by 2% H2SO4. Furthermore, 2.3204 mmol/g of H+ concentration could be obtained from the DI-pretreated samples. These results indicated that the surface of a bamboo-derived carbonaceous magnetic solid had been successfully acidified.

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**Figure 2.** H+ concentration of the bamboo-derived magnetic solid acid prepared by different pretreatment conditions.

During the carbonization and sulfonation processes, changes occurred in the crystallinity of the material. The XRD spectras of bamboo-derived magnetic solid acid prepared at different pretreatment conditions are shown in Figure 3. The observed sharp peaks at 2*θ* values of 35.61◦, 57◦, and 62◦ were assigned to the (220), (311), and (440) lattice planes of Fe3O4 (JCPDS19-629) [22,24], which indicated that the carbon coated by Fe3O4 existed in the form of polycyclic aromatic hydrocarbons, and their structures were kept stable during the sulfonation process. Moreover, material prepared by 0.5% H2SO4 pretreated bamboo showed the strongest intensity at 35.61◦, which may lead to the better catalytic performance for the reducing production of sugars. This speculation was also verified by the following catalysis experiments. Compared with the solid acid prepared by dilute alkali pretreated under the same concentration, the peak intensity of the dilute acid pretreated sample was higher, which was consistent with the results of the subsequent catalytic output. In addition, both patterns exhibited a broad and weak diffraction peak at 2*θ* = 20–30◦, which was due to the presence of amorphous carbon, which was composed of aromatic carbon sheets oriented in a random fashion [26].

**Figure 3.** XRD spectra of bamboo-derived carbonaceous magnetic solid acid prepared with different pretreatment conditions (A: 0.5% H2SO4 pretreated sample; B: 2% H2SO4 pretreated sample; C: 0.5% KOH pretreated sample; D: 2% KOH pretreated sample; E: Deionized (DI) water pretreated sample).

The thermal behavior of the bamboo-derived magnetic solid acid (pretreated by 0.25% H2SO4) was shown in Figure 4. The thermal decomposition of the prepared catalyst can be divided into three stages. The weight loss which occurred before 150 ◦C was mainly because of the evaporation of water adsorbed on the surface of the material [27]. The weight loss stage from 180 ◦C to 420 ◦C corresponded to the decomposition of –SO3H groups [3]. The last stage that occurred above 400 ◦C was attributed to the further condensation of amorphous carbons [25]. About an 11% weight loss of the Fe precursor may be occurred between 180–500 ◦C, which may be ascribed to the decomposition and conversion reaction of the Fe species of the catalyst at high temperatures [24].

**Figure 4.** Thermal behavior of the bamboo-derived magnetic solid acid (pretreated by 0.25% H2SO4).

### *2.2. Effects of Pretreatment Conditions on the Chemical Composition of Bamboo*

The detection of chemical composition of lignocelluloses can visually verify their structure in different conditions. In order to evaluate the effects of dilute acid and alkali pretreatments on the major sugar compositions of bamboo, dewaxed bamboo was treated by dilute H2SO4 and KOH in various concentrations (0.25%, 0.5%, 0.75%, 1%, and 2%) in a solid ratio of 1:10 (g/mL) at 120 ◦C for 30 min, respectively. The major sugar components of hydrolysates and solid residues are shown in Tables 1 and 2.


**Table 1.** Concentrations of major sugars in bamboo hydrolysates (mg/L).

a ND: Undetectable.

**Table 2.** Major sugar composition of bamboo residues (g/g).


a ND: Undetectable.

In the absence of a catalyst (H2SO4 and KOH), the dissolution of reducing sugars (xylose, arabinose, and glucose) was not observed under the investigated conditions (Table 1, entry 11). However, after dilute H2SO4 was added as the catalyst, the contents of reducing sugars in the hydrolysates enhanced sharply (Table 1, entries 1–5). The productions of xylose, arabinose, and glucose increased from 1995.35 mg/L to 8258.9 mg/L, 510.5 mg/L to 711.0 mg/L, and 166.85 mg/L to 613.3 mg/L with the acid concentration increased from 0.25% to 2.0%, respectively. This result indicated that about 74.74% of hemicellulose and 1.56% of cellulose could be extracted by 2.0% H2SO4 at 120 ◦C for 30 min. Therefore, the major compounds in the acid-treated bamboo residues may be cellulose and lignin. This phenomenon illustrated that a higher acid concentration benefited the dissolution of bamboo hemicellulose, especially for xylose during the pretreatment process. In addition, the hemicellulose structure of bamboo was elucidated as arabinoxylan oligosaccharides with xylose as the backbone and arabinose as the side chain [27]. Compared with xylose (16.61–68.83%), about 25.52–35.55% of arabinose was removed during dilute H2SO4 pretreatment, which suggested that the main chain of bamboo hemicellulose breaks faster than the side chain. However, glucose, xylose, and arabinose could not be detected in the KOH-pretreated hydrolysates (Table 1, entries 6–10), which suggested that dilute alkali pretreatment showed less power for the dissolution of hemicellulose and cellulose from bamboo than dilute acid pretreatment, which was consistent with the previous reports [2]. As shown in Table 1, xylose was the dominating sugar in the liquid fractions, and its content increased gradually as the pretreatment acid concentration increased. Moreover, it was found that arabinose was the secondary sugar constituent in the liquid fractions, and the yield of glucose was much lower than that of the former two. These results indicated that the dilute acid pretreatment mainly promotes the dissolution of hemicellulose rather than cellulose.

The hydrolyzed residues were simultaneously subjected to sugar analysis to verify the effect of pretreatments on the change of bamboo composition (Table 2). The contents of xylose and arabinose decreased gradually as the concentration of dilute acid increased (Table 2, entries 1–5), which was consistent with the results in Table 1. Moreover, a sugar composition analysis of dilute alkali-pretreated bamboo showed that the contents of major monosaccharides were kept stable in different KOH concentrations. This phenomenon suggested that cellulose and hemicellulose did not change much after the dilute alkali pretreatment. No monosaccharides could be detected in the hydrolysates after the dilute alkali and DI water pretreatments, which may be due to their further conversion into formic acid, acetic acid, furfural, etc. [17].

An FT-IR spectra of the bamboo residues obtained from dilute acid and dilute alkali pretreatments was performed to complete the study of all the samples from the experimental design (Figure 5). A band near 1733 cm<sup>−</sup><sup>1</sup> was ascribed to the stretching of C=O in hemicelluloses [28]. However, the absorption of this band decreased with the increment of sulfuric acid concentration, which suggested that the hemicellulose partly dissolved during the pretreatment process. Additionally, strong bonds at 1205 cm<sup>−</sup><sup>1</sup> (OH in-plane bending in cellulose I and cellulose II), 1160 cm<sup>−</sup><sup>1</sup> (C–O–C stretching asymmetric), 1100 cm<sup>−</sup><sup>1</sup> (glucose ring-stretching asymmetric), 898 cm<sup>−</sup><sup>1</sup> (*β*-glycosidic linkages between glucose unites in cellulose) could be observed both in dilute acid and alkali pretreated samples [27]. The strong bands at 1596 cm<sup>−</sup><sup>1</sup> and 1267 cm<sup>−</sup><sup>1</sup> corresponded to the aromatic skeletal vibration breathing with C=O stretching and vibration of the guaiacyl ring of lignin, respectively [16], which suggested that hemicelluloses could be removed effectively during the pretreatment process, resulting in the increment of the relative content of lignin in the bamboo residues. Therefore, the FT-IR results showed that the relative amount of the lignin and cellulose increased during the dilute acid/alkaline pretreatment process, and the change of the chemical composition of bamboo may affect the catalytic ability of the following prepared bamboo-derived carbonaceous magnetic solid acid.

**Figure 5.** *Cont.*

**Figure 5.** FT-IR spectra of dilute acid and alkali pretreated bamboo (( **A**) spectra of dilute acid pretreated bamboo; (**B**) spectra of dilute alkali pretreated bamboo).

### *2.3. Hydrolysis of Corncob with Bamboo-Derived Carbonaceous Magnetic Solid Acid*

It should be noted that the pretreatment conditions had a grea<sup>t</sup> influence on the catalytic performance of the prepared bamboo-derived carbonaceous magnetic solid acid. In this study, experiments were executed for the catalytic hydrolysis of corncob at 120 ◦C for 30 min via the prepared magnetic solid acids. The main components of the hydrolysates and corncob solid residues were shown in Tables 3 and 4. The basic trend of xylose, arabinose, and glucose yields decreased when the pretreatment concentration of H2SO4 rose from 0.25% to 2%, which may be due to the reduction of the carbohydrates contents in the H2SO4-pretreated bamboo residues [29]. A higher carbohydrate content can achieve higher surface areas of specific materials during carbonization, which may be because an incomplete carbonization process is primarily the process of carbohydrates dehydrating and becoming volatile [18]. Moreover, the acidity of solid acids enhanced when prepared by a higher H2SO4 concentration. During the hydrolysis process, the obtained reducing sugar will be further catalyzed to form other products, such as furfural, 5-hydroxymethylfurfural, and so on [30]. However, the opposite phenomenon was observed for bamboo-derived magnetic solid acid prepared by dilute alkali pretreatment. When the KOH concentration was raised from 0.25% to 2%, the yields of xylose, arabinose, and glucose increased from 1856.2 mg/L to 2601.0 mg/L, 27.4 mg/L to 402.7 mg/L, and 672.5 mg/L to 1226.2 mg/L, respectively. This phenomenon may be because lignin was partly removed during the dilute alkali pretreatment, leaving most of the carbohydrates behind [31]. Therefore, higher reducing sugars yields could be obtained from the solid acid prepared by high concentration alkali-pretreated bamboo. In addition, solid acid prepared by authohydrolysis presented the lowest reducing sugars productivity, which may be ascribed to the weak damage strength of the bamboo structure under mild conditions. The highest reducing sugars yield of 37.17% could be obtained from bamboo-derived carbonaceous magnetic solid acid prepared by 0.25% H2SO4-pretreated bamboo. The prepared catalyst possessed a good catalytic performance for the conversion of hemicellulose into xylose and arabinose without further dehydration to form other products, which may be due to its moderate acidity and crystallinity.


**Table 3.** Effect of the pretreatment conditions on the catalytic performance of bamboo-derived magnetic solid acid (mg/L).

Reaction conditions: 1 g of corncob, 0.2 g of bamboo-derived magnetic solid acid, 20.0 mL of DI water, 300 rpm, 120 ◦C and 30 min.

**Entry Sample Xylose Arabinose Glucose** 1 0.25% H2SO4 22.35 0.00 1172.05 2 0.5% H2SO4 46.30 0.00 1288.85 3 0.75% H2SO4 205.85 0.10 874.35 4 1% H2SO4 39.20 0.00 1150.00 5 2% H2SO4 63.55 0.00 1193.85 6 0.25% KOH 42.00 0.00 1038.50 7 0.5% KOH 44.30 0.00 1073.10 8 0.75% KOH 105.30 0.65 959.90 9 1% KOH 197.15 0.50 817.05 10 2% KOH 0.00 0.00 1004.10 11 DI Water 0.00 0.00 999.05

**Table 4.** Analysis of corncob solid residue after catalysis (mg/L).

Determining the recyclability of a catalyst is important to evaluate its efficiency based on economic and environmental factors. The solid acids prepared in this study had the characteristic of being easily separated due to their magnetic properties. In this study, the recyclability of the prepared bamboo-derived magnetic solid acid (pretreated by 0.25% H2SO4) was performed, and the results are shown in Figure 6. After three recycle runs, the prepared catalyst remained active for the conversion of corncob into reducing sugar. In comparison to the fresh catalyst, the yields of xylose, arabinose, and glucose decreased slightly from 4643.4 mg/L to 3902.5 mg/L, 662.8 mg/L to 455.5 mg/L, and 1111.3 mg/L to 805.4 mg/L after three recycles, respectively, which suggested that the prepared catalyst had good reusability. Compared with conventional solid acids, the biomass-based magnetic solid acid used in this study has the advantages of easy recycling, moderate acidity, and widely resourced. Moreover, it possessed good catalytic performance for the conversion of lignocellulosic raw materials into reducing sugars.

**Figure 6.** Recyclability of the bamboo-derived magnetic solid acid (pretreated by 0.25% H2SO4).
