Metal oxides are usually used to synthesize solid superacids as introduced in Sect. 15.3.1. Active sites supported on single metal oxides performed high cat­alytic activity in many organic reactions [54-56]. Sulfonated metal oxides, such as SO42-/Al2O3, SO42-/TiO2, SO42-/ZrO2, and SO42-/V2O5 [57, 58], can supply much acid species, which function the same as [H+] in sulfuric acid for cellulose hydrolysis. Such acid solid catalysts are widely used in biodiesel synthesis, but it is generally very difficult to retain a strong B acid site as sulfonic acid in the framework. Jitputti et al. [57] studied the transesterification of crude oil by different solid acids, such as SO42-/SnO2, SO42-/ZrO2, and resulted in the production of over 90 % fatty acid methyl esters (FAMEs; biodiesel). It was found that the spent SO42-/ZrO2 could not be directly reused for the transesterification. Such solid acid catalysts cannot be used for cellulose hydrolysis at the current structure and composition. Kulkarni and Muggli [59], reported that an apparent increase in B acidity was found upon treat­ing SO42-/TiO2 with H2O, which proved that H2O displaced isopropylamine from approximately one-third to one-half of the B acid sites.

In order to improve the catalytic efficiency and stability of sulfonated metal oxides, some modifications are usually proposed as below: (1) Introducing other metals or metal oxides. Metals that promote the catalytic activity include aluminum, iron, and manganese, and platinum can also increase the stability of solid catalysts [60, 61]. (2) Introducing lanthanum. Lanthanun was widely used to improve the stability of active sites [62]. (3) Synthesizing nanoparticles. Nano-catalysts have the advantage of supplying larger surface area, good stability, and high effect active, which will be introduced in detail in Sect. 15.4.