(415g) Selective Conversion of Cellobiose to Hexitols Over Solid Acid Supported Ru Catalysts Under Mild Conditions

Catalytic conversion of cellulose to hexitols under mild conditions has always been a great challenge because of the strong ß-1,4-glycosidic bond as well as intra- and inter- molecular hydrogen bonds in cellulose. A bifunctional catalyst, which offers balanced activity for hydrogenation and hydrolysis, is ideal to control the selectivity of the reaction. In the present study, we used two strategies to prepare catalysts containing both acid sites for hydrolysis and metallic sites for hydrogenation simultaneously. We then used cellobiose, a glucose dimer linked by the ß-1,4-glycosidic bond, to test the acidity of the supports and examine the overall hydrogenolysis performance of these solid acids supported Ru catalysts.

In the first approach, we synthesized an insoluble ionic liquid (IL)–HPA hybrid and used it as a support for Ru. We characterized the catalyst, Ru/[Bmim]₃PW₁₂O₄₀,  using FT-IR spectra of pyridine adsorption and measured pH changes after treating the catalyst in H₂. The results confirmed the in-situ generation of Brønsted acidic sites as a result of hydrogen spillover from the Ru sites to the O sites of the support. The catalyst, which combines the Ru sites for hydrogenation and both Lewis and Brønsted acidic sites for hydrolysis, exhibited a superior selectivity toward sorbitol over the catalyst resulted from mixing [Bmim]₃PW₁₂O₄₀ and Ru/C. In addition to generate Brønsted sites on the oxygen sites, proton transfer of the hydrogen spilt over onto the oxygen sites acidifies the liquid product. These acidic active sites work synergistically with the supported Ru and contribute to the overall hydrogenolysis activity. The unreduced Ru(III) may also contribute to the observed hydrolysis activity through its Lewis acidity.

In the second approach, we use zirconia modified SBA-15 as a support for Ru. Herein, zirconia was used as a modifier to improve the acidity of the surface of the inner pores of SBA-15. In fact, coating of zirconia onto SBA-15 not only led to generation of acidic sites but also improved dispersion of Ru in the mesopore of Zr-SBA-15. The catalyst enables both hydrolysis and hydrogenation reactions to convert cellobiose into sorbitol. The reaction of cellobiose on Ru/Zr-SBA-15 follows the major reaction path of hydrogenolysis/hydrogenation of the C₁-O-C₅ ether bond in one glucose monomer of the cellobiose molecule and hydrogenates it to 3-β-D-glucopyranosyl-D-glucitol on Ru, hydrolysis of 3-β-D-glucopyranosyl-D-glucitol to sorbitol and glucose on acid sites, and hydrogenation of glucose to hexitols on Ru. Kinetic fittings suggested that hydrolysis of 3-β-D-glucopyranosyl-D-glucitol to sorbitol and glucose on the acid sites is the rating determining step of the overall reaction towards hexitols. We also showed that optimal Ru particles and well preserved mesopore with acid sites could be achieved with a Zr/Si ratio of 0.25, which allows the fast transportation of reactants to the active sites (both Ru and acidic sites). With Ru/Zr-SBA-15(0.25), full conversion of cellobiose and hexitols yield of 72.1% could be achieved in 1 hour at 140 ¢ªC.