The concentration and strength of acid sites in solid acid catalysts are usually measured by amine titration, infrared spectroscopy (IR), temperature-programmed desorption (TPD), solid-state nuclear magnetic resonance (NMR) spectroscopy and thermal analysis [46]. The traditional amine titration method can be used for quan­titative analysis of acid amount. However, it could not accurately analyze the types of acid and the distribution of acidic centers on the surface of solid acid catalysts. The analysis of total acid amount and strength of solid acid catalysts using TPD method is very accurate, except for distinguishing acid types. As a supplementary method, IR spectroscopy can be used well to distinguish B — and L acids but not suitable for exact quantitative analysis of acid amounts. Therefore, the combination of TPD method and IR spectroscopy is more accurate to get acidic characteristics of solid acid catalysts. Currently-developed solid-state NMR spectroscopy can realize qualitative and quantitative analysis of acidic center distribution, acid amount, and acid strength, but the analysis is complex and costly.

The acidic strength and acid site density are determined by the preparation process conditions, such as calcination temperature, crystal form of oxides, and sulfonation process. The main functions of calcination are as follows: (1) change amorphous oxides into crystals; and (2) promote solid phase reaction between sulfuric acid and metal oxides, forming acid structure of strong bonding of sulfuric acid with metal oxides. An appropriate calcining temperature is conducive to have good porous structure, many acid sites and high acid strength. Loss of sulfur species, decrease of specific surface and crystal transformation will occur if calcination temperature is too high. On the contrary, the ideal acid structure, crystal and pore structure will not be obtained if calcination temperature is too low. Pang et al. [29] found that the higher the sulfonation temperature, the higher the acid density of the sulfonated carbon. The active carbon sulfonated at 300 °C (AC-SO3H-300) showed an acid density of 2.19mmol/g, which was 15-fold that of the untreated active carbon. However, a higher temperature (>250 °C) caused the decomposition of sulfonate. The highest glucose yield of 74.5 % was achieved by AC-SO3H sulfonated at 250 °C, which possessed the highest density of -SO3H groups.

Mesoporous metal oxides with high catalytic efficiencies have the following spe­cial structure properties: (1) high specific surface area, (2) adjustable pore size, and (3) enhanced thermal stability [47]. However, it has been shown that highly crys­talline pore walls and high mesoporous order cannot always be achieved for the same material. High-temperature heat treatment helps to increase the crystallinity of pore walls, but leads to the collapse of mesoporous structure. Many reports on sulfated metal oxides showed that the amorphous metal oxides are needed for preparation of solid superacids except for y-Al2O3 [47].

During the preparation process of metal oxides, the pH should be controlled by adding precipitation agent to obtain the precipitates of metal hydroxides. Ammonia and urea are the most used precipitation agents. The precipitation process using am­monia is more slowly, thus improving crystal growth efficiency. B acid sites (proton donors) can be generated from highly polarized hydroxyl groups. They can also be formed on oxide-based catalysts via proton balance of a net negative charge intro­duced by substituting cations with a lower valence charge [48]. Sulfide type should be considered as the main factor in acid strength, acid type, and catalytic activ­ity. Commonly-used sulfides include H2S, SO2, SO3, CS2, H2SO4, (NH4)2SO4 and benzenesulfonic acid, but only high-valence sulfides perform acid action. In order to obtain enough strong superacid centers, the optimum calcination temperature should be slightly lower than the decomposition temperature of acid sites. Besides sulfated single metal oxides, manipulation of sulfate content and activation temperature also provides the means for controlling the strength of surface B acid sites in the sulfated mixed metal oxides [49].

Certainly, not only sulfides are used as active sites. Metal oxide supported Pt or Ru showed highly-active for converting cellulose to sugar alcohols with 31 % yield being reported [50]. They were active for catalyzing cellulose conversion and glucose isomerization, simultaneously with 88 % selectivity of glucose being obtained. It should be noted that they presented L acid catalysis.