Catalysts And Reaction Mechanism For Hydrogenolysis Of Glycerol

Recently, much attention has been paid to the utilisation of renewable energy. Among them, biodiesel, which could replace petro-diesel, has been reported popularly all over the world for its safety and renewability. Biodiesel is produced from vegetable oils or animal fats via the transesterification process. Glycerol, as a major byproduct, is largely produced in this process. In recent years, with the expanding demand for biodiesel, glycerol is oversupplied. It is of great importance to convert glycerol into value-added chemicals. Hydrogenolysis of glycerol to propanediols (PDOs) has been regarded as one of the most promising processes.Cu-based catalyst has been investigated popularly for its lower price and high selectivity to1,2-propanediol (1,2-PDO) owing to its lower activity for C-C bond cleavage. The aim of this dissertation is to synthesize highly effective catalysts via layered double hydroxide (LDH) precursors under controlled drying, calcinations, and reduction. The main achievements of this work are:Highly effective bimetallic noble metal-Cu/solid base catalysts have been prepared from the LDH precursors by introducing a second active metal (Pd, Rh, Ru, Re). Characterizations disclosed that Pd, Rh, Ru, and Re promoted the reduction of Cu2+, enhanced its ability of adsorbing hydrogen, and lowered the reaction temperature, pressure and time significantly. The activity of surface Cu atoms in Pdo.o4Cuo.4/Mg5.56Al2O8.56reached9.7mol-gly/mol-Cu/h at180℃.A series of Cu/Zn-Mg-Al catalysts with different Zn/Mg molar ratios were prepared. The morphology, structure, acid-base properties, the dispersion of copper and hydrogen spillover were characterized. It was found that the acidity (and basicity) of these catalysts could be manipulated by adjusting the molar ratio of Zn/Mg. Cuo.4/Zn0.6Mg5.0Al2O8.6exhibited the best performance for hydrogenolysis of glycerol in aqueous solution at180℃. The calculated activity of surface Cu atom reached9.7mol-gly/mol-Cu/h, which was comparable to that of bimetallic Pd-Cu/solid base catalyst. It was concluded that the conversion of glycerol over Cu/Zn-Mg-Al catalyst depended strongly on both the basicity and the dispersion of Cu. At the same time, hydrogen spillover from ZnO to Cu also enhanced its performance. The catalyst could be recycled while maintaining a good catalytic activity. During the controlled thermal decomposition of LDH, the decomposition of interlayer anions and dehydroxylation of brucite-like sheets lead to the disordered stacking of lamellae. Certain places of the lamella could not be used efficiently owing to the overlap of lamellae. In order to solve this problem, multiwall carbon nanotube (MWCNT) was selected as the support, and a series of MWCNT-pillared layered Cu0.4/Mg5.6Al2O8.6materials with stable structure, doublet meso-pore channels and high surface area were fabricated and used for glycerol hydrogenolysis. Characterizations disclosed that the close contact of layered Cuo.4/Mg5.6Al2O8.6with MWCNT enhanced the reducibility of Cu2+. And the hydrogen spillover from MWCNT to Cu contributed to the conversion of glycerol. This material showed enhanced catalytic performance in glycerol hydrogenolysis reactions. The highest activity of surface Cu atom reached as high as17.2mol-gly/mol-Cu/h at180℃.Core-shell structured magnetic Fe2O3@CuMgAl LDH catalysts were synthesized for selective hydrogenolysis of glycerol. Characterizations of XRD, N2-adsorption, H2-TPR, SEM, TEM, CO2-TPD and XPS disclosed that the thermal stability of the LDH framework, the dispersion of Cu and its activity were enhanced simultaneously in the presence of Fe2O3. These magnetic catalysts could be easily separated by an external magnetic field and showed high efficiency in reuse.At last, synthesis of1,2-PDO with the required hydrogen produced from hydrogen donor molecule instead of H2was carried out. The reactions were performed over Cu-Mg-Al catalysts with different (Cu+Mg)/Al molar ratios. Different alcohols were selected as the hydrogen donors. It was found that the performance of Cu-Mg-Al catalysts for this reaction depended mainly on their basicity, and Cu was indispensable for this reaction because alcohol dehydrogenation was performed on Cu. With ethanol as the hydrogen donor, the conversion of glycerol over Cu0.4/Mg6.28Al1.32O8.26reached95.1%at210℃, and the selectivity of1,2-PDO is higher than90%in most experiments.