Heteropolyacids (H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀) and salts of metal cations (Mⁿ⁺) and PW₁₂O₄₀ ³⁻ (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.

The development of heterogeneous catalysts for green chemical synthesis is a growing area in catalysis and green-sustainable chemistry. This review summarizes recent examples of hydrogen transfer-type reactions using supported transition metal catalysts with special emphasis on the direct synthesis of chemicals by borrowing hydrogen methodology. The highlighted reactions are: (1) transfer hydrogenation of carbonyl compounds, (2) transfer dehydrogenation of alcohols, (3) N-alkylation of amines or NH₃ with alcohols, (4) alkylation of acidic CH₂ or CH₃ group with alcohols, (5) self- and cross-alkylation of alcohols, (6) N- or C3-alkylation of indoles with alcohols, (7) synthesis of quinolines from nitroarenes and alcohols, (8) synthesis of thioethers from thiols and alcohols, and (9) cross-alkylation of amines with different amines. These catalytic reactions have a common mechanistic feature: a domino dehydrogenation–condensation–hydrogenation sequence of alcohols (or amines) and nucleophiles. A key concept in the catalyst design in some of the systems is the multifunctionality of metal-loaded acidic and/or basic metal oxides, in which the acid and/or base sites selectively catalyze the condensation reactions, and the metal site catalyzes the transfer dehydrogenation of alcohol (or amines) and transfer hydrogenation of the condensation products as intermediates. In comparison with the recent examples of organometallic catalysis with borrowing hydrogen methodology, the advantages of heterogeneous catalysis are discussed in terms of catalytic efficiency and sustainability.