The microstructural parameters and acid density of the carbonaceous materials are summarized in Tab.2. The raw material GP and C600 had low surface areas of 4.32 and 9.83 m
2/g, with small pore volumes of 0.02 and 0.05 cm
3/g, respectively. Such values could be ascribed to the release of volatile substances (such as HCl, H
2O, CO, and CO
2) from the dehydration and decomposition of the carbon matrix and FeCl
3, which increased the surface area and pore volume of C600. However, the surface area and pore volume of C600-S significantly increased to 175.1 m
2/g and 0.16 cm
3/g, respectively. The specific surface area of the magnetic carbonaceous carrier increased significantly after sulfonation, possibly due to the corrosion of embedded Fe
3O
4 and carbon surface by concentrated sulfuric acid. Zhang et al. [
27] reported a similar result with the synthesized Jatropha hulls-based magnetic acid catalyst. Moreover, Fig.3 showed the N
2 adsorption-desorption isotherms and pore size distribution of the C600-S catalyst. As Fig.3(a) illustrates, the C600-S catalyst exhibited a Type-IV isotherm profile, with a notable H
4 type hysteresis loop, characteristic of mesoporous materials. Furthermore, Fig.3(b) illustrates that the pore size distribution of C600-S was centered at 3.87 nm. Based on the relatively large surface area and mesoporosity, reactants could also access the active sites on the internal pore walls [
28], significantly reducing diffusion difficulty and ensuring improved catalysis. In addition, Tab.2 indicates that the total acid (including -OH, -COOH and -SO
3H groups) and -SO
3H densities of C600-S were 3.38 and 0.74 mmol/g, respectively. The acidic property of C600-S was similar to those of other sulfonated biomass-based catalysts but with a superior surface area (Tab.2).