Solid acids are widely used for the study of biomass hydrolysis that will be introduced in Sect. 15.3 in detail. The hydrolysis mechanistic route of a water-soluble polysac­charide consists of the following steps: (1) The soluble polysaccharide diffuses onto the surface acidic sites of a solid acid; (2) в-1,4 glycosidic bonds access to the acidic sites; and (3) These bonds cleave randomly, and the polysaccharide is hydrolyzed to mono-sugars. Therefore, ideally, cellulosic materials need to be dissolved in a solvent and converted into short-sugar chains to make full use of the acid sites.

Liquid acid has some important limitations including the corrosion of reactor, recycle, and neutralization of the acid for microbial fermentation. Formation of degraded products and release of fermentation inhibitors are other characteristics of liquid acid pretreatment.

Unlike hydrolysis, in the pretreatment process, few mono-sugars are formed. Therefore, it performs under milder conditions, such as lower temperature, short processing time, and weaker acidity that can be selectively controlled. After pretreat­ment, biomass can be further hydrolyzed by either enzymatic or chemical method. Solid acid catalysts mediated pretreatment is superior in the destruction of biomass structure as compared with traditional liquid acid pretreatment because it achieves low reaction rate, and fewer side-reactions occur. After pre-hydrolysis of cellulose, the formed oligosaccharides can extend into the vicinity of Brpnsted or Lewis acid sites where catalytic hydrolysis occurs. Pre-converting cellulose into oligomers is important to the subsequent hydrolysis with solid acid catalysts.

Solid acid pretreatment is a novel method for biomass pretreatment. Although little work is reported, it can be considered as the initial stage or lesser degree of hydrolysis, and is easily controlled by changing the parameters of solid acid hydrolysis conditions. With the development and application of solid acids in biomass hydrolysis, they will find applications in pretreatment. So, hydrolysis is focused on the following sections.