Copper dispersed natural kaolinite as high-performance catalysts for the hydrolysis of cellulose in water

Kaolinite is a layered silicate material with a Janus character and has been directly used as the solid catalyst for the hydrolysis of cellulose in our previous work. Reducing sugar (RS) is one of the most important intermediates for the utilization of cellulose. In the present study, metal ions dispersed on kaolinite was easily prepared and further evaluated for the hydrolysis of cellulose to RS in water. The metal ions dispersed kaolinite was characterized by XRD, BET, SEM(EDS), and XPS. The catalytic results showed that copper was the best co-catalyst among the tested metal ions and the kaolinite treated with 4(wt)% copper nitrate by calcination at 623 K showed the highest total reducing sugar (TRS) yield of 51% under 478 K with the mass ratio of catalyst to cellulose of 0.2 and weight ratio of water to cellulose of 7 for 3 h. Moreover, the catalyst could be easily regenerated by calcination and the yield of TRS on the regenerated catalyst decreased from 51 to 43.2% after four times reuse. The characterization results revealed that copper was highly dispersed on the outer SiO₄ tetrahedral layer. The surface cuprous species were the active sites and could make the C-O bond cleavage easily by attacking the positively charged carbon atoms of the β-(1,4)-glycosidic linkage in the cellulose. The leaching of the cuprous species was responsible for the deactivation of the catalyst. The copper dispersed kaolinite is the efficient catalyst for the hydrolysis cellulose to reducing sugars and worth further development.