(49e) Hydrogenation of Cellulose to Sorbitol Over Sulfated Zirconia Supported Ru Catalysts

Lignocellulosic biomass, composed of cellulose, hemicellulose and lignin, abundantly exists in nature and could be a promising alternative to fossil derived chemicals. Cellulose is a polymer of D-glucose units connected through β-1,4-glycosidic bonds. Dueing to the highly crystalline structure with extensive intra- and inter-molecular hydrogen bonds, it is more resistant to depolymerisation. Up to now, extensive efforts have been devoted to the conversion of cellulose in various reaction media including hot water, mineral acids and ionic liquids. It is of great significance and challenge to achieve direct conversion of cellulose to sugar alchohols, which combining hydrolysis and hydrogenation. Furthermore, previous studies have demonstrated the effectiveness of heterogeneous solid acid catalysts in hydrogenolysis of cellulose to hexitols.

In this work, we prepared a series of sulfated zirconia supported Ru (SO₄^2-/ZrO₂-Ru) catalysts by precipitaion methods followed by impregnation in diluted H₂SO₄, and loading a proper amount of Ru. BET method, powder X-ray diffraction (XRD), and temperature programmed desorption of NH₃ (NH₃-TPD) were used for the catalysts characterization. The results showed that the sulfated zirconia calcined at 773K was completely tetragonal phase which was responsible for the acidity; with the calcination temperature raised, component of monoclinic phase increased, thus the NH₃ adsorption capacity was reduced. The concentration of H₂SO₄ solution and the kind of doping metal affected the BET surface area more than the amount of Ru loading on the sulfated zirconia.

The activity of as-prepared catalysts for hydrogenation of cellulose was investigated in a lab-scaled batch reactor. The main products detected were sorbitol, slight amount of xylitol and C2~C3 polyols. The results showed that acidity strength of catalyst had a remarkable influence on cellulose conversion and the yield of sorbitol. The increase of acidity on catalyst surface promoted hydrolyzing of cellulose, thereby enhanced the conversion of cellulose.While dueing to the side reactions such as dehydration of glucose/hexitols and C-C cleavage of sorbitol, too dominant acid catalysts caused lower selectivity of sorbitol. The strength of acidity can be adjusted by changing of calcination temperature, concentration of H₂SO₄ and the amount of doping metal. With satisfaction, 71.1% conversion of cellulose and 58.1% yield of  sorbitol  were obtained under the mild conditions (at 473K after 10h, 5Mpa H₂ pressure; cellulose,0.25g; catalyst, 0.05g; distilled water 5g) catalyzed by SO₄^2-/ZrO₂-SiO₂-Ru catalyst (Zr/Si = 100:15, 5%Ru). The plausible reaction pathway was proposed that cellulose was firstly hydrolyzed to sugars over acid-treated ZrO₂, and the sugars were immediately hydrogenated to sorbitol over Ru.