用于苯甲醇氧化反应中的高效金基双金属催化剂的合成及其性能研究

【作者】 王英；

【导师】 戴维林；

【作者基本信息】 复旦大学 ， 物理化学， 2011， 硕士

【摘要】 苯甲酸及其钠盐是非常重要的精细化工产品,主要应用于医药、食品、染料、香料等方面。工业上制备苯甲酸及其钠盐的方法主要是通过用以乙酸、苯甲酸的钴盐或锰盐以及溴化物为催化剂,高温高压下催化氧化甲苯制得。目前的生产工艺存在很多问题,比如产品和均相催化剂分离困难,大量有毒废物的产生以及设备的腐蚀等等。本论文以负载型金基双金属材料作为催化剂,以常压空气为氧化剂,在较为温和的条件下无溶剂一锅法合成苯甲酸钠,为苯甲酸及其钠盐的合成提供了一条绿色、清洁并且高效的途径。负载在氧化物载体上的纳米金催化剂具有独特的催化活性,近年来引起了催化领域的广泛关注。在单金属催化剂中添加第二种金属,形成双金属催化剂,在保持良好催化活性的同时,提高了催化剂的稳定性。不同金属间的相互作用不仅可以改变两种金属的电子和几何性能,产生新的协同效应,而且能改变催化剂的配位效应和整体效应。双金属并不是各组分的简单加和,其性质会因加入别的金属形成合金而具有独特性能。研究表明,它们对化学吸附的强度、催化活性和选择性等都有影响,这使得双金属催化剂成为值得单独研究的一类催化剂。本论文合成的金基双金属催化剂在催化苯甲醇氧化反应中表现出很高的催化活性。通过对负载型金基双金属催化剂合成条件、载体、预处理方法和双金属组分的优化,得到了具有高催化活性和高稳定性的负载型金基双金属催化剂。本论文的主要工作及结果如下：1.对苯甲醇氧化制备苯甲酸的反应条件如催化剂用量、反应温度和反应时间进行了优化,得到的最佳反应条件为：苯甲醇1.08 g,NaOH 0.5g,催化剂与底物摩尔比0.004,反应温度200℃,反应时间10h。采用先负载Ag后负载金的方法和Au-Ag同时沉淀的方法制得了两种双金属催化剂,活性测试结果表明先负载Ag后负载金的方法相对Au-Ag同时沉淀的方法可以得到活性更高的催化剂。通过改变双金属催化剂预处理条件如干燥、还原和焙烧温度,最后选定催化剂的预处理条件为浸渍法负载Ag后在500℃焙烧,沉淀金之后在300℃焙烧来制备Au-Ag双金属催化剂。改变双金属催化剂中的Au和Ag的含量,发现双金属催化剂相比单金属的Au和Ag催化剂活性更高,且在金银比为1：3时活性达到最佳值,金银之间存在一定的协同作用。2.在AuPd/CeO2双金属催化剂应用于苯甲醇氧化制备苯甲酸及其钠盐的反应中,载体对Au-Pd双金属催化剂活性有显著影响,其中过渡金属氧化物载体相比于主族金属氧化物可以得到活性更高的Au-Pd双金属催化剂。在过渡金属氧化物中以CeO2为载体的AuPd/CeO2催化活性最高。Au-Pd的负载顺序对催化剂的活性也有影响。催化活性的高低顺序为AuPd/CeO2> Au-Pd/CeO2> PdAu/CeO2。在AuPd/CeO2双金属催化剂中,Au/Pd摩尔比会影响催化剂上活性物种的粒径大小和尺寸分布,改变表面金属物种组成,通过Au-Pd之间的电子效应和协同效应,从而影响其催化活性。相比于Ag/TiO2, AuPd/CeO2在催化剂金与底物的摩尔比例减少一半,降低温度和减少反应时间的条件下,产物得率仍可媲美。当Au/Pd摩尔比为1/3时,催化活性最高,得率达92%。同时,催化剂具有良好的稳定性,3Au1Pd/CeO2套用7次后仍然具有较高的催化活性,苯甲酸产物得率>79%。更多还原

【Abstract】 Benzoic acid and sodium benzoate are important commodity chemicals, with wide use in food additive, preservative, spicery, plasticizer and mordant. The commercial production of benzoic acid and sodium benzoate via the catalytic oxidation of toluene is achieved by heating mixture of the substrate, cobalt acetate and bromide promoter in acetic acid to 250℃at high pressure. The process gives rise to problems such as difficult separation of products and homogeneous catalyst, large amount of toxic waste and equipment corrosion. In addition, requirement of expensive and specialized equipments also makes the cost of production very high, along with large amount of energy consumed by operation under extreme conditions. In this dissertation, the gold-based catalysts were used to catalyze the benzyl alcohol under one-pot solvent-free condition to find new and environmental-friendly routes, which is by performing the reaction under mild conditions using a variety of solid catalysts, ionic liquids or supercritical fluids.Nobel metal nanostructures have recently received much attention because of their unique optical, catalytic, and electrochemical properties which make them suitable materials for potential applications in various field. Supported bimetallic nanoclusters have been at the focus of interest for decades due to their unique catalytic properties. Gold-based bimetallic NPs exhibit interesting electronic, optical, chemical, and biological properties due to their new bifunctional or synergistic effects. The introduction of second metal into Au NPs has been expected to enhance the catalytic activity and selectivity.Their reactivity is affected by chemical compositions, for the addition of a second metallic element often modifies the stability and catalytic activity of the pure metal clusters by affecting both geometry and electronic properties.A novel efficient gold-based bimetallic catalysts were developed with high stability for one-pot solvent-free synthesis of sodium benzoate and benzoic acid from the green oxidation of benzyl alcohol using air as the oxidant under ambient pressure without any wastes emission. The preparation conditions of the catalysts, support crystal phase and the mole ratio of the two metal were optimized, and finally we prepared the gold-based bimetallic catalysts with high activity and stability were obtained. The conclusions are as follows:1. By optimizing the reaction temperature, reaction time and catalyst amount, the best reaction condition is as follows:1.08 g benzyl alcohol,0.5 g NaOH, bimetal: benzyl alcohol molar ratio 0.004,200℃,10 h. Precipitation order of An and Ag also has influence on the activity of gold catalysts. Activity results reveal that loading Ag before the precipitation of Au gives better catalytic activity than the catalyst obtained by precipitation of Au and Ag simutaneously. By altering the pretreatment temperature Au-Ag catalyst, e.g. drying, calcination and reduction, the AuAg/P25(500)-C catalyst which is calcined at 500℃after impregnation of Ag, followed by precipitation of Au and calcining at 300℃shows the highest activity. Au-Ag catalysts with different Au/Ag molar ratio are also prepared. It is found that the activity of Au-Ag bimetallic catalysts is better than that of monometallic ones. Meanwhile, catalyst with Au/Ag molar ratio of 1:3 shows the best activity. It is concluded that there is synergistic effect between Au and Ag.2. The AuPd/CeO2 bimetallic catalysts were prepared for the application in the oxidation of benzyl alcohol. In particular, the Pd promotional effects are studied through the systematic investigation set of AuPd/CeO2 bimetallic catalysts with a range of Au/Pd ratios. The addition, Pd nano-species show great influence on the gold catalysts, including changes in particle size, electronic structure and etc. The reaction activity increases with the increase of the Pd content. The optimum performance for the synthesis of benzoic acid from benzyl alcohol is observed over a catalyst with the Au/Pd of 3:1 molar ratio.更多还原