Some supported solid acid catalysts (e. g., sulfonated carbon based solid acid, sul- fonated metal oxides and sulfonated activated-carbon) showed good activity in cellulose hydrolysis [24, 33, 34]. Supported solid acid catalysts are promising for the depolymerization of cellulose in water, since they have substantial surface acidic species (e. g., 1.5mmol/g; sulfonated activated-carbon) as compared with zeolites (e. g., 0.3 mmol/g; HSM-5B) and transition-metal oxides (e. g., 0.3 mmol/g; Nb3W7 oxide) [35, 36], and specific functional groups. The active species of protons [H+] in such catalysts are more accessible to the P-1,4-glucans in cellulose than L acid sites [37].

Solid acid catalysts, especially strong acids such as sulfated metal oxides (e. g., SO42-/Al2O3, SO42-/TiO2, SO42-/ZrO2, SO42-/SnO2 and SO42-/V2O5), are being studied extensively in an effort to replace liquid acid catalysts. Sulfated metal oxides are solid acid catalysts with the combination of B and L acids in a certain form. For­mation of proton acid centers is related to the adsorption of hydroxyl groups or H2O on SO42-. However, most acid sites are mainly formed by coordination adsorption of SO42- on the surface of metal oxides. The coordination adsorption makes strong migration of the electron cloud in metal-oxygen bond, leading to strengthening the L acid center. Many studies have proposed that water, present in biomass or produced as a reaction product, converts L acid sites to B acid sites. Therefore, in the hydrolysis of cellulose into glucose, B acid sites play an important role.

The relative activity of B acid sites depends on many factors, such as the structure of supports, the nature of reactions and the polarity of reaction media. Supported acid catalysts are the most extensively studied ones for organic synthesis [38]. As the same kind of catalysts, the number of strong B acid sites is correlated with the catalytic activity. Zhang et al. [39] studied the skeletal isomerization of я-butane and found that the catalytic activity of H4SiW12O40/SiO2 reached a maximum when the loading amount of H4SiW12O40 was 50 wt%. The density of strong B acid sites increased with loading of H4SiW12O40 on SiO2 (10-50 wt%). It was concluded that the direct interaction of H4SiW12O40 with the surface of SiO2 caused strong B sites to form in the second-layer of H4SiW12O40 exposed on the surface. However, in the previous study about the effect of support identity on B acid site density [40], it was found that the reactivity of B acid sites was not affected by the identity of supports. The turnover rate increased with the rising of density of acid sites on all supports.

The influence of supports on activity of B acid sites in cellulose hydrolysis still remains undiscovered. Sulfonic group functionalized magnetic SBA-15 cata­lyst (Fe3O4-SBA-SO3H) [41], which gave high glucose yield (98 %) in cellobiose conversion, can be recovered for reuse by an external magnetic field. Fe3O4-SBA — SO3H not only provides good access of reactants to the — SO3H groups, but also has functional characteristics that allow it to be separated and regenerated. Sulfated ZrO2 [31] has similar active acid sites (sulfonic acid group: 1.2 vs. 1.09 mmol/g) but lower glucose yield (14 % vs. 98 %) as compared with Fe3O4-SBA-SO3H. It still could not prove whether their activities were influenced by effect of supports on activity of B acid sites or by access of reactants to -SO3H groups. Suganuma et al. [42] reported that carbon materials can incorporate large amounts of hydrophilic molecules into the carbon bulk, due to the high density of the hydrophilic functional groups bound to the flexible carbon sheets. Compared with niobic acid, H-mordenite, Nafion, and Amberlyst-15 resins, the carbon catalyst had the highest catalytic activity because it can adsorb |5-1,4-glucans, which are not adsorbed by the other four solid acids.

The activity of solid acid catalysts for cellulose hydrolysis was not only related to the density of B acid sites even for the same support. Onda et al. [31] studied the hydrolysis of cellulose into glucose using sulfonated activated-carbon as catalyst (acid site density of 0.58 mmol/g), and a glucose yield of 41.4 % was achieved at 150 °C for 24 h. They reported a similar glucose yield of 40 % under the same reac­tion conditions using the same sulfonated activated-carbon with 1.25 mmol/g acid site density [43]. The high catalytic activity of the sulfonated carbonaceous material was attributed to (1) its ability to absorb the |5-1,4-glucans, (2) its large effective surface area in water, and (3) the presence of — SO3H groups that are tolerant to hydrolysis [44]. Vigier and Jerome [33] found that an increase in loading of sulfonic sites (i. e., the proton concentration) on poly(tetrafluoroethylene-co-perfluorovinyl ether)-gra/l-polystyrenesulfonic acid (PFA-g-PSSA) membrane surface from 28 % to 63 % enhanced the catalytic activity from 7.5 x 10-3 to 27.5 x 10-3 min-1. It was suggested that this increase in activity might be ascribed to the rise of the amount of accessible sulfonic sites and the catalyst hydrophilicity, thus improving the polymer chains mobility and therefore accessibility of the catalytic sites [24]. Similar phenomenon was obtained by Zhang and Zhao [45] who used different

H-form zeolite catalysts (i. e., HZSM-5a, HZSM-5b and H-beta) for cellulose hydrolysis with microwave-heating at 240 W. HZSM zeolites had a higher glucose yield (35 %) than that of H-beta zeolite (30 %) because of their higher acidity and more reactive sites. Therefore, disregarding the accessibility of catalytic sites to P-1,4-glucans in cellulose, hydrolysis yield is positively related to acid site density.