Bifunctional catalysts for the transformation of cellobiose into sorbitol

Details

Supervisors

Hermans, Sophie ; Debecker, Damien

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries

Abstract

Due to the environmental crisis and an ever-increasing demand for energy, materials and chemicals, new and green technologies are needed to step aside from the fossil fuel heavy utilization. A sustainable feedstock is lignocellulosic biomass. It is comprised of several biopolymers of which the most abundant is cellulose. Cellulose is made of glucose monomers and can be processed in a biorenery (factory converting biomass into energy and chemicals) in order to produce several platform chemicals among which is sorbitol. Its production from cellulose requires a bifunctional system to both hydrolyze the bond between the monomers and hydrogenate glucose into sorbitol. The most studied systems contain both acid species and ruthenium metallic nanoparticles (Ru NPs). However, the glycosidic bond can also be hydrolyzed with alkaline species. We designed this study in order to set the question of the basic hydrolysis. We chose to focus on Ru NPs supported on alkali functionalized carbon. Nitrogen doping of the supports was identied as the most interesting method to introduce basic functions on the surface of the materials. Nitrogen function type can be tuned with a heat treatment under inert atmosphere to incorporate the atoms within the carbon lattice, enabling a control to favor the most basic groups. Besides, N atoms in the C network can enhance the activity of metallic sites thanks to electronic effects. As cellulose is a challenging molecule to work with (highly crystalline and insoluble in water due to inter and intra molecule H-bonds), we used cellobiose, a dimer of glucose, as a model compound. Two pathways were chosen to obtain N functions: ethylene diamine (EDA) reaction and urea deposition. Both were followed by a heat treatment to give N-doped supports. To explore the potential of various carbon supports, four commercial materials were chosen: two activated carbon (SX+ & CB) and two nanofibers. XPS and Boehm titration were used routinely to characterize the N incorporation and the basicity of the supports, respectively. Only the EDA grafting on SX+ led to a major change in the properties of the support, especially for the heat-treated material. The alkaline behavior of this sample was confirmed with TPD-CO2. Then, Ru NPs were deposited on the surface of the material with a good dispersion. This was confirmed by both ICP and TEM microscopy. A textural analysis was conducted and led us to the conclusion that the micro and mesoporous, high surface area of SX+ was retained through the entire manipulation to yield the catalysts. Two catalytic tests were conducted: one focusing on the sole hydrolysis of cellobiose into glucose and the other one both hydrolysis and hydrogenation. The former gave out negative results as every alkali-functionalized support performed lower than the blank. We link this inactivity to the acidity of the solvent (mQ water) that is thought to neutralize the basic functions. However, the tests on the two steps reaction let us report an increased activity of the Ru NPs supported on basic materials, which we associate to the doping of the support. Also, the role of surface O and N moieties is discussed as the formation of H-bonds with the reactants and the increased hydrophilicity can lead to a better adsorption of the reactants on the surface of the supports. Finally, a kinetic study was carried out that shed light on the reaction pathways and let us conclude that a high sorbitol yield can be obtained with our system.