Effects of Brønsted and Lewis acidities on activity and selectivity of heteropolyacid-based catalysts for hydrolysis of cellobiose and cellulose

Heteropolyacids (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀) and salts of metal cations (Mⁿ⁺) and PW₁₂O₄₀³⁻ (M_3/nPW₁₂O₄₀) act as effective homogeneous catalysts for selective hydrolysis of cellobiose and cellulose to glucose and total reducing sugars (TRS), respectively, in an aqueous phase. For Brønsted acid catalysts, including mineral acid and heteropolyacids, the activity for both reactions increases with a decrease in the deprotonation enthalpies (DPE), indicating that stronger Brønsted acidity is more favorable. For M_3/nPW₁₂O₄₀ of 11 kinds of metal ions (Ag⁺, Ca²⁺, Co²⁺, Y³⁺, Sn⁴⁺, Sc³⁺, Ru³⁺, Fe³⁺, Hf⁴⁺, Ga³⁺ and Al³⁺), the rate of cellulose hydrolysis increases with Lewis acidity of the cation, while the TRS selectivity was highest for cations with moderate Lewis acidity, such as Sn⁴⁺ and Ru³⁺. For the hydrolysis of cellobiose, cellulose and lignocellulose, H₃PW₁₂O₄₀ and Sn_0.75PW₁₂O₄₀ showed higher TRS yield than H₂SO₄.