Zeolites
Zeolite catalysts have been used in depolymerization of cellulose in cellulose/ionic liquid solutions, as well as cellulose suspensions in water under hydrothermal conditions. The use of zeolite in ionic liquid media by Zhang and Zhao is discussed in Section 7.4.2 of this book. Hydrolysis under hydrothermal conditions with zeolites generally gives poor glucose yields in comparison to ionic liquids. This is due to the fact that natural cellulosic materials have high crystallinity and unless powderized, they have large particle sizes, and thus they are difficult to disperse into the inner pore of a zeolite. For example, only 12% glucose yield is obtained using HZSM-5 catalyst under hydrothermal conditions [72]. This poor performance of the zeolite catalysts was due to their small pore diameters that limited accessibility and their weak acid sites. To use H-zeolite catalysts effectively for cellulose hydrolysis, cellulosic materials must be either dissolved in a solvent or the catalysts must be processed to have pore sizes that allow substrates access to their catalytic sites. Although H-form zeolites have a large surface area and strong acidity, they have been difficult to use in practice due to mass transfer
|
Table 7.3 Selected examples of use of solid acid catalysts for hydrolysis cellulose of corn cobs to sugars in water [2].
hydrotalcite (HTeOHCa); Sulfonated activated-carbon (ACeSO3H); Sulfonic group functionalized magnetic SBA-15 catalyst (Fe3O4eSBAeSO3H); Ru-mesoporous carbon materials (Ru-CMKs); Cellulase immobilized mesoporous silica nanocatalysts (Cellulase-MSNs); Porous coordination polymers decorated with sulfonic acid functional groups (PCPseSO3H); Heteropoly acid H3PW12O40 (HPA). a Total yield of reducing sugars |
limitations. The active species of protons in such catalysts are not freely accessible to the P-1,4-glucans in cellulose.