Metal Oxide Supported Solid Acid Catalysts
Sulfonated metal oxides are another class of solid acid catalysts that can be used in water at high temperatures. Metal oxides are widely used as catalyst supports because of their thermal and mechanical stability, high specific surface area, large pore size (>15 nm) and pore volume (>0.2 mL/g) [83, 84]. The most common examples of using sulfonated metal oxides are SO42-/Al2O3, SO42-/TiO2, SO42-/ZrO2, SO42-/SnO2 and SO42-/V2O5. These solid acids are usually prepared by impregnating the hydroxides from ammonia precipitation of corresponding metal salt solutions with aqueous sulfuric acid followed by calcination. Additionally, metal oxide supported Pt or Ru for conversion of cellulose to sugar alcohols have also being reported [85]. These catalysts are active in heterogeneous liquid-solid systems and are recoverable and reusable. However, one major limitation of these types of solid catalysts is that the acidic sites may leach from silica surfaces under hydrolytic conditions. Thus, the thermal stability under hydrolytic conditions of the supported solid acid catalysts is an important factor in considering their application since this affects both product separation and catalyst recyclability.