3.2.1. Catalyst Characterization

The Fourier transform infrared (FT-IR) spectrum of the prepared bamboo-derived magnetic solid acid was recorded on a spectrophotometer (Tensor 27, Bruker Optics, Karlsruhe, Germany). An acid-base titration test was performed to verify the free hydrogen ion (H+) concentration in the bamboo-derived magnetic solid acid. In briefly, 0.1 g of catalyst was added into 20 mL of NaCl solution (20 mmol/L), and then stirred for 24 h (100 rpm) at room temperature. Subsequently, the mixture was filtered, and a liquid fraction (5 mL) was titrated with 50 mmol/L of NaOH solution. Phenolphthalein was used as the indicator. Experiments were done in triplicate, and the average value was obtained. The X-ray diffraction (XRD) patterns of the prepared catalysts were detected in the 2*θ* range of 10◦ to 90◦ on a Bruker D8 ADVANCE X-ray diffractometer (Karlsruhe, Germany) with Cu Kα radiation. The

thermostability of the samples was tested with thermogravimetric analysis (TA Q200, New Castle, DE, USA).

### 3.2.2. Two-Step Preparation of Bamboo-Derived Magnetic Carbonaceous Solid Acid

The hydrothermal pretreatment of bamboo was carried out in a high-pressure reactor (SLM-100, Shenlang Co. Ltd., Beijing, China). In this study, a mixture of 10 g of feedstock and 100 mL of liquid (KOH/H2SO4/water) was added into the reactor. The concentrations of the KOH and H2SO4 solution were 0.25%, 0.5%, 0.75%, 1%, and 2% respectively. A control group was performed using DI water to compare with dilute acid and alkali pretreatments. The pretreatment time, temperature, and the agitation rate were 120 ◦C, 30 min, and 300 rpm, respectively. Once the reaction finished, flowing water was used to cool the reactor quickly. After that, the mixture was separated by filtration. Solid residue was washed with DI water several times and dried at 60 ◦C overnight for the following preparation of bamboo-derived magnetic solid acid. The liquid products were determined by high-performance liquid chromatography (HPLC, Waters 2414, America) coupled with a refractive index detector (RID) and a Bio-rad Aminex HPX-87H (300 × 7.8 mm) column. Five mM of H2SO4 was employed as the eluent with a flow rate of 0.5 mL/min at 60 ◦C.

Five g of pretreated residue was dispersed in 500 mL of the FeCl3 •6H2O solution (10 mmol/L), and then stirred at room temperature (150 rpm) for 5 h. After impregnation, bamboo was carbonized at 500 ◦C for 1 h under nitrogen atmosphere to produce the bamboo carbon. The bamboo-derived carbon was mixed with a concentrated sulfuric acid at a ratio of 1:10 (g/mL) and sonicated for 15 min. Subsequently, the mixture was heated under vigorous at 90 ◦C for 10 h to introduce the sulfo-group (–SO3H) to the surface of the bamboo-derived magnetic solid precursor. After the reaction, the mixture was diluted with DI water and dried in a vacuum oven at 80 ◦C for 12 h.

### 3.2.3. Hydrolysis of Corncob by Bamboo-Derived Magnetic Solid Acid

One g of ball-milled corncob, 0.2 g of bamboo-derived magnetic solid acid, and 20 mL of DI water were mixed first and ultrasonicated for 30 min, and then added into the reactor. The reaction time, temperature, and the agitation was 120 ◦C, 30 min, and 300 rpm, respectively. At the same time, a catalyst-free reaction including 1 g of ball-milled corncob and 20 mL of DI water was also conducted at the same reaction conditions. After the reaction, the product was filtered with a 0.22-μm syringe. Liquid fractions were stored in the fridge prior to HPLC analysis, as mentioned above. Solid residues were washed with DI water several times and oven-dried at 60 ◦C to a constant weight.
