Synthesis Of Nano - Gold Catalysts Supported On Different Manganese Oxide And Their Catalytic Application In The Synthesis Of Lactones By Diol Oxidation

The nano-sized gold is a catalyst with high performance in the low temperature CO oxidation, oxidation of alcohols, water-gas shift reaction and hydrogenation of α,β-unsaturated aldehydes or ketones. Because of their low temperature activity and high selectivity, the gold catalysts are becoming more and more important. Generally, metallic oxide with reductive property is considered as a desired support for gold-based catalyst. Manganese oxides are economical, environmental friendly, stable and reductive, so they can be used in electrode material and catalytic reaction. There are various manganese oxides, which can transform to each other and afford a mass of surface oxygen species that is beneficial to the oxidation process. There is little report on the manganese oxide as the support of gold catalyst in alcohol oxidation reaction. Especially, manganese oxide octahedral molecular sieve (OMS-2) has been rarely studied. Thus, we tried to synthesize the manganese oxide supported gold catalyst, and the particle size, electronic state and support effect were studied in this dissertation to investigate the catalytic properties of gold catalyst.The lactones and their derivatives are widely distributed in nature. The lactone ring exists in the molecules of many bioactive substances and metabolic intermediates. Due to their high boiling point, solubility, conductivity and stability, the lactones are widely used as solvent, extraction agent and also can be used for the synthesis of a variety of polymers. In this dissertation, the nano-sized gold catalysts were used to catalyze the aerobic oxidative dehydrogenation of diols to lactones. The reaction was carried out at low temperature with air as oxidant, which is a clean route in accordance with the demand of green chemistry and sustainable development.Manganese oxides supported gold catalysts prepared in this dissertation showed high acticity in the oxidative dehydrogenation of a series of diols, such as1,4-butanediol,1,5-pentanediol,1,6-hexanediol and1,2-benzenedimethanol to the corresponding lactones. The preparation conditions of the catalysts, reaction conditions, support compositions, support morphology and support crystal phase were controlled and optimized, and the catalysts catalyzed the reaction of diols to lactones with high activity, selectivity and stability. The main results are as follows:1. Influence of the crystal phase of manganese oxides on the catalytic performance of Au/MnOx catalysts in the oxidative cyclization of diols to lactones MnO, MnO2and Mn2O3were utilized as support, and the gold catalysts were prepared by the homogeneous deposition-precipitation using urea as precipitation agent. In the reaction of1,4-butanediol to y-butyrolactone, the order of catalytic activity is:Au/Mn2O3> Au/MnO2> Au/MnO. The Au/Mn2O3had the best catalytic performance with98%of conversion and100%of selectivity after8hours’reaction. The average gold particle size of Au/Mn2O3is the smallest (2.6nm), and there are appropriate amount of Au0and Auδ+in Au/Mn2O3. Furthermore, Au/Mn2O3could be reused and be preserved for a long time with the same catalytic activity, and it could catalyze the oxidation reaction of other diols to lactones.2. Influence of catalyst preparation and reaction conditions on the catalytic performance of Au/Mn2O3catalysts in the oxidative cyclization of1,4-butanediol to γ-butyrolactoneThe calcination temperature and gold loading had effect on the size of gold particle and the interaction between gold and support. The best catalytic performance was obtained in Au/Mn2O3catalyst with5%gold loading and calcined at573K. By optimizing the preparation temperature, preparation time, precipitant species and precipitant amount, the best preparation condition is:urea as the precipitant,200/1of urea/Au molar ratio,363K,4h. Moreover, the optimal reaction condition is:1.25MPa air,200/1of diol/Au molar ratio,393K,8h. Besides, the participation of water and illumination of sunlight had no effect on the catalytic performance of catalysts during the preparation and reaction process.3. Influence of manganese oxide octahedral molecular sieve on the catalytic performance of Au/MnOx catalysts in the oxidative cyclization of diols to lactonesThe variety of acids led to the diverse manganese oxide octahedral molecular sieve (OMS-2) with the same crystal phase, but different particle size. All of the OMS-2supported gold catalysts could catalyze the oxidation reaction of1,4-butanediol to y-butyrolactone. Gold particles deposited on the surface of OMS-2which was prepared by formic acid had the highest activity:59%of conversion,100%of selectivity. The smallest average size of gold particle and the most abundant gold were obtained in this catalyst, and it could also catalyze the reaction of other diols to the corresponding lactones with high selectivity.