Sustainable hydrothermal self-assembly of hafnium-lignosulfonate nanohybrids for highly efficient reductive upgrading of 5-hydroxymethylfurfural

The research for highly selective catalytic transfer hydrogenation (CTH) of 5-hydroxymethylfurfural (5-HMF) into 2,5-bis(hydroxymethyl) furan (BHMF) is an extremely important pathway for biomass valorization. Herein, we use lignosulfonate, a waste by-product from the paper industry, as a building block to coordinate with different metal ions (Hf4+, Zr4+, Fe3+, Al3+, Zn2+) and thus a series of inorganic-biopolymer hybrids (M-LigS) were prepared by a hydrothermal self-assembly method. The resulting Hf-LigS hybrid with strong Lewis acid-base couple sites and moderate Bronsted acidic sites from the inherent sulfonic groups in LigS exhibited the best catalytic activity for CTH of 5-HMF with 2-propanol (2-PrOH) in high yields (90%) under mild reaction conditions (100 degrees C in 2 h). This robust bifunctional acid-base Hf-LigS is also demonstrated to be effective in one-step reductive etherification of 5-HMF to 5-[(1-methylethoxy) methyl]-2-furanmethanol (MEFA), a potential biomass-derived fuel additive, with 95% yield. Kinetic studies revealed that the activation energy for CTH of 5-HMF was 62.25 kJ mol(-1), accounting for the high reaction rate. Isotopic labelling experiments demonstrated that intermolecular hydrogen transfer from the alpha-C of 2-PrOH to the alpha-C of 5-HMF was the dominant reaction pathway and the direct hydride transfer on acid-base sites was the rate-determining step. Due to the strong interactions between Hf4+ and phenolic hydroxyl groups, Hf-LigS was highly stable and could be reused without a significant decline in activity.