Chemical catalytic systems with liquid acids have the difficulty in separating the homogeneous catalysts from product solutions. Solid acid catalysts have many ad­vantages over liquid catalysts. They are widely studied as direct replacements for liquid acids to reduce pollutants and operating costs. A solid acid catalyst is defined as solid that can donate protons (Brpnsted, B acid) or accept electrons during re­actions (Lewis, L acid). The catalytic function for a solid acid catalyst is derived from its acidic centers, existing mainly on its surface. Accordingly, solid acids with B acid sites can catalyze biomass hydrolysis. Because L acid catalysis of cellulose conversion was reviewed [24], this section focuses on B acid catalysis.

The mechanistic route of cellulose hydrolysis by solid acid catalysts consists of the following steps: (1) crystalline cellulose dissolves in some chemical agents, such as ILs; (2) the soluble polysaccharide diffuses onto the surface of the solid acid catalysts; (3) the polysaccharide undergoes hydrolysis over the acid sites; (4) some hydrolysis oligosaccharides diffuse into the internal pores of solid acid catalysts; (5) the poly-oligosaccharides undergo hydrolysis over the surface and internal acid sites; (6) the hydrolysis products, mainly glucose, diffuse into the reaction medium. Therefore, the properties of solid B acid catalysts, such as acid site density, acid strength, structure of supports, acid site distribution, and tolerance to water, have great influence on their activities and selectivities. Under ordinary circumstances, acid strengths and catalytic activities of solid acid catalysts decrease in the presence of water. Moreover, most solid acids do not function effectively for cellulose hydrolysis because the surfaces of these solids do not have strong acid sites or cannot allow their close contact to |5-1,4-glucans. Therefore, it is a challenge to develop hydrothermal catalytic hydrolysis processes with solid acid catalysts. It is important to acquaint with the properties of solid B acid catalysts considerably.