Recyclable Cu Salt-Derived Brønsted Acids for Hydrolytic Hydrogenation of Cellulose on Ru Catalyst

Brønsted acid-catalyzed hydrolysis of cellulose, the most abundant source of biomass in nature, is the critical step for its depolymerization and subsequent conversion to sugar alcohols and other important chemicals. Mineral acids are highly active; however, the difficulties in recycling and disposal limit their practical applications. Herein, we have developed a novel and facile approach to efficiently hydrolyze cellulose using recyclable H₃PO₄and H₂SO₄generated in situ in the cellulose reaction via the reduction of the insoluble Cu salts Cu₂(OH)PO₄and Cu₃(OH)₄SO₄by H₂. After the reaction, such in situ formed H₃PO₄and H₂SO₄were readily reconverted to insoluble Cu₂(OH)PO₄and Cu₃(OH)₄SO₄salts, which were then separated by simple filtration from the product solutions to completely recycle the insoluble Cu salts and mineral acids. The in situ formed H₃PO₄and H₂SO₄produced pH values as low as 1.5 in the reactionmedia that effectively catalyzed the conversion of microcrystalline cellulose to C₆alcohols, including hexitols, sorbitan, and isosorbide, in combination with hydrogenation catalyst Ru/C, affording a high yield of 93.7% at 473 K. This novel approach provides a rationale for designing recyclable mineral acids with wide applications that are important for developing green chemical processes.