Heterometallic PdII–Cl–CuI Catalyst for Efficient Hydrolysis of β-1,4-Glycosidic Bonds in 1-Butyl-3-methylimidazolium Chloride

Efficient hydrolysis of β-1,4-glycosidic bonds of cellulose is a pivotal step in biorefineries of cellulosic biomass. In this work, we discovered that paired PdCl2–CuCl exhibited excellent synergetic catalytic activity over the single PdCl2 or CuCl in the hydrolysis of β-1,4-glycosidic bonds in 1-butyl-3-methylimidazolium chloride ([BMim]Cl) and further investigated the catalytic species in this system. The catalytic activity increased and reached a plateau at a CuCl/PdCl2 ratio of 1 with increasing molar ratio of CuCl/PdCl2 while keeping the Pd concentration constant. A combination of experimental evidence based on in situ far-infrared absorption, X-ray absorption, X-ray photoelectron, and electron paramagnetic resonance spectroscopies and DFT calculations demonstrate the coupling reaction between [PdIICl4]2– and [CuICl3]2– via the Cl bridge to form the heterometallic complex anion [Cl2PdII–Cl–CuICl2]2–. The heterometallic species is stable during the reaction and maintains its catalytic performance in the presence of various oxygenate additives. Using paired PdCl2–CuCl as a catalyst, the activation energy of hydrolytic cleavage of β-1,4-glycosidic bonds of cellobiose is determined as low as 95.8 kJ/mol. PdCl2–CuCl/[BMim]Cl combined with CrCl3 can be reused five times in cellobiose conversion to 5-hydroxymethylfurfural (5-HMF) without an apparent decrease of catalytic activity. The performance of this catalytic system is further verified for efficient cellulose conversion. More than 60% product yield is obtained at 100 °C from the cellulose, affording monosaccharides for efficient production of high value-added chemicals, such as 5-HMF, when CrCl3 is present to catalyze the conversion of glucose to 5-HMF.