Of all the solid acid supports, carbonaceous support seems to be the most effec­tive. It was reported that — SO3H groups are linked to amorphous structure in the form of C-O-SO3H [31]. As mentioned in Sect. 15.3.1, a carbon material can in­corporate large amounts of hydrophilic molecules, which provide good access to P-1,4glucans by — SO3H groups, giving high catalytic performance for hydrolysis [42]. The incorporation results in the decease of activation energy for the hydrolysis of cellulose.

Suganuma et al. [42] reported that carbonaceous solid acids obtained at mild carbonization temperatures (ca. 450 °C) exhibited high catalytic activity, because — SO3H groups were bonded to carbon sheets with poor cross-linking, which made it easy for access to reactants by the — SO3H groups. However, when carbonized at higher temperature (ca. 550 °C), most of the — SO3H groups bonded to the carbon sheets were not located on surface, which resulted in a poor catalytic performance. Onda et al. [31] used sulfonated activated-carbon catalyst to selectively hydrolyze cellulose at 150 °C for 24 h, and obtained 40.5 % glucose yield with 90 % glu­cose selectivity. Only few amount of SO42- was leached (<0.03 mmol/L) after the reaction.

Carbonaceous solid acid catalysts ground to nanosize (10-100 nm) could achieve high catalytic activity. Vyver et al. [79] proposed an effective conversion tech­nique for cellulose hydrolysis by using a sulfonated silica/carbon nano-composite as catalyst. Glucose formation was faster on the nano-composite as compared with ion-exchange resins (glucose yield of 50 % vs. 29 %) under reaction conditions of 150 °C for 24 h with 0.05 g of ball-milling cellulose, 0.05 g of catalysts, and 5 mL of water. The catalyst was superior in the formation rate of glucose (4.6 ^mol h-1), and turnover frequency (0.37 h-1) as compared with typical sugar catalysts (formation rate of glucose, 0.93 h-1; turnover frequency, 0.06 h-1). However, separation and re­covery of it from un-hydrolyzed cellulose residues are needed for further study. Lai et al. [41] developed a process for recovering carbonaceous solid catalysts by using a paramagnetic solid acid (Fe3O4-SBA-SO3H). When microcrystalline cellulose was pretreated with [BMIM][Cl], glucose yield reached 52 % in 3h. The incorporation of paramagnetic nanoparticles into the carbonaceous carriers not only provides good access of reactants to the — SO3H groups, but also has functional characteristics that allow it to be separated and regenerated.

Carbonaceous solid acid catalysts are considered as the most promising catalyst for cellulose hydrolysis, since they provide good access of reactants to the acidic sites of — SO3H groups. High glucose yields of up to 75 % with 80 % selectivity have been achieved at 150 °C for 24 h with carbonaceous solid acid catalysts. However, sep­aration of carbonaceous solid acid catalysts from un-hydrolyzed cellulose residues after hydrolysis needs further research since these catalysts have similar physical and chemical properties to the residues. Use of functionalized carbonaceous solid acid catalysts that contain paramagnetic groups is one method to improve carbonaceous solid acid catalysts’ separation and reuse.