Most solid acid catalysts presented excellent catalytic active in the first run. However, a considerable loss in catalytic activity occurred when the catalyst was recycled for several times [51-53]. Hegner et al. [51] reported that glucose yield from the hydrolysis of cellulose with FeCl3/silica was 11 %, while for the second and third cycles, it reduced to 8 % and 7 %, respectively. Dhepe et al. [52] studied the hydrolysis of sucrose using water-tolerant sulfonated mesoporous silicas. The reactions were tested at 80 °C for 24 h up to three recycles, and no decrease in activity was found. However, the catalysts were not tested for cellulose hydrolysis which needs a higher reaction temperature (~150 °C). Up to now, an amorphous carbon bearing — SO3H, — COOH and — OH groups showed the best recyclability among the reported results [42]. No decrease in activity was observed even after 25 cycles of the catalyst (total reaction time, 150 h). Although such sulfonated carbons are highly efficient for cellulose hydrolysis, there are needs for their improvement in the areas of separation and recovery from un-hydrolyzed cellulose residues.

A modification technology to increase the surface area, mechanical strength, and stability of catalyst supports and improve the acid density, strength, and recyclability of acid sites is required. The modification usually starts with discovering the cause of catalyst deactivation and seeking cheap raw materials to reduce the cost of catalyst preparation. Reusability is a unique character of solid catalysts as distinguished from liquid catalysts, also is the key to reduce the cost of catalytic process. Metal oxides, zeolites, cation-exchange resins, and carbonaceous solid are common supports of solid acids. Herein, modification of catalyst supports is discussed in detail.