Co-precipitation or filling supports with an aqueous solution of active precursor is a conventional method for preparing solid acid catalysts. Metal oxides are widely used as catalyst supports because of their thermal and mechanical stability, high specific surface area, and large pore size (15 nm) and pore volume (>0.2mL/g) [25, 26]. Because solid acids function the same as [H+] for cellulose hydrolysis, sulfonated metal oxides, such as SO42-/Al2O3, SO42-/TiO2, SO42-/ZrO2, SO42-/SnO2, and SO42-/V2O5, can supply many acidic species. Such solid acids are usually prepared by impregnating the hydroxides from ammonia precipitation of corresponding metal salt solutions with aqueous sulfuric acids followed by calcination. One limitation of these types of solid catalysts is that the acidic sites are leached under hydrolytic con­ditions. It is difficult to control the catalyst particle size and shape, therefore, novel synthesis technology should integrate with conventional methods to resolve these is­sues. Fang et al. [27] have successfully prepared activated hydrotalcite nanoparticles by co-precipitation of Mg(NO3)2 ■ 6H2O and Al(NO3)3 ■ 9H2O in urea solution and subsequent with microwave-hydrothermal treatment. The particles were activated with Ca(OH)2 and used to hydrolyze cellulose. X-ray diffraction (XRD) pattern in­dicated that it had layered and well-crystallized structures with characteristic and symmetric reflections.

Carbonaceous solid acid catalysts are known to have one of the highest catalytic activities for cellulose hydrolysis. These catalysts were typically prepared from car­bohydrates by carbonizing at 400 °C under N2 and then sulfonating at 150 °C [28]. Glucose, sucrose, cellulose, lignin, and activated-carbon can be used as raw materi­als for their preparation [28-31]. The carbon in the catalysts is in amorphous forms consisting of polycyclic aromatic carbon sheets. All S-atoms in the catalysts are in — SO3H groups, which are the active sites. Carboxylic acid species, -COOH, gener­ally provide more active sites than Nafion NR50 and Amberlyset-15 which could not help to hydrolyze cellulose into glucose. Pang et al. [29] reported that high glucose yield of up to 74.5 % with 94.4 % selectivity was obtained at 150 °C and 24 h. Lignin is the second-most abundant natural organic material after cellulose, and the richest aromatic organic biopolymer. It has high carbon content and should be usable as a precursor for activated carbon. Pua et al. [30] prepared a solid acid catalyst from Kraft lignin by treatment with phosphoric acid, pyrolysis, and sulfuric acid, and subsequently it was successfully used as catalyst to synthesize biodiesel from high acid value Jatropha oil. It is speculated that lignin derived carbonaceous catalyst is more advantageous for cellulose hydrolysis.

Homogeneous catalysis by heteropoly acids (HPAs) is in principle similar to sulfuric acid in that [H+] leaches into solution and interacts with the oxygen atoms in the glycosidic bonds of cellulose. However, recovery of the homogeneous catalysts is problematic. Cellulose hydrolysis using solid HPAs was reported by Tian et al. [32]. Several types of acidic cesium salts, CsxH3-xPW12O40 (X = [1-3]), were prepared. The salt Cs1H2PW12O40 was found to give the highest glucose yield (30 %) at 160 °C for 6h reaction time. CsxH3-xPW12O40 catalysts were prepared by adding dropwise the required amount of aqueous solution of cesium carbonate to aqueous solution of H3PW12O40 with cesium content ranging from 1 to 3 at room temperature with stirring. After the resultant milky suspension was aged at room temperature overnight, the solution was slowly heated at 50 °C to obtain white solid powers. It was found that Cs1H2PW12O40, with strong protonic acid sites, showed the best catalytic performance in terms of the conversion of cellulose and the yield for glucose. Cs2,2H0 8PW12O40 showed the highest selectivity in terms of glucose, which is due to its micro-porous structure.

For industrial applications, low cost catalysts with good performance are required. The catalyst cost can be reduced by selecting cheap materials and simple preparation process. Relatively speaking, carbonaceous solid acid catalysts are considered as the cheapest catalyst, since they were obtained from biomass (such as glucose, cellulose, lignin, wood et al.) by a simple process of carbonization and sulfonation.