【作者】 郭晓明；

【导师】 卢冠忠；

【作者基本信息】 华东理工大学 ， 工业催化， 2011， 博士

【摘要】 随着大气中C02浓度的增加,温室效应日益严重。在减少C02排放的同时,C02的回收利用也是各国政府和科学研究人员关注的焦点。将CO2转化为有用的化学品是CO2回收利用的有效途径。甲醇是一大宗的化工原料,同时也是化石燃料的潜在替代品。因此,C02加氢合成甲醇在环保、能源和化工等多个领域均具有重要意义。本文分析了CO2加氢合成甲醇用铜基催化剂的研究现状,有针对性地从催化剂的制备方法、催化剂的组成和催化反应机理三个方面开展了研究,取得的主要结果如下：一、铜基催化剂制备方法的研究采用燃烧法制备了CuO-ZnO-ZrO2催化剂,研究了燃料用量、燃料种类及引燃方式等制备条件对催化剂性能的影响,研究了催化剂的组成-结构-性能的构效关系。结果表明,燃料用量和燃料种类是影响催化剂性能的主要因素。燃料用量不同,燃烧焓、燃烧反应持续时间及燃烧反应释放的气体量也不同,从而导致燃烧反应温度不同,并最终影响催化剂的物化性能和催化性能。燃料种类不同,催化剂性能随燃料量变化的规律也明显不同。相对于甘氨酸和尿素的燃烧反应,柠檬酸作燃料的燃烧反应更趋温和,这与燃料本身的组成和结构有关。采用尿素、甘氨酸和柠檬酸作燃料制备的CuO-ZnO-ZrO2催化剂,在温度为240℃、压力为3.0 Mpa、空速为3600 h-1的反应条件下,甲醇收率分别可达9.6%、9.9%和8.1%。燃烧法制备的CuO-ZnO-ZrO2催化剂具有比共沉淀法更高的催化活性,原因是燃烧过程中的短暂高温过程有效促进了各组分之间的相互作用。研究表明,催化剂中Cu分散度的提高有利于催化剂活性的提高,ZrO2的相态影响甲醇的选择性。此外,催化剂的性能与催化剂各组分之间的相互作用密切相关。燃烧法是一种简单、快速且有效的制备CuO-ZnO-ZrO2催化剂的方法,可推广到其它复合氧化物的制备。采用固相合成法制备了CuO-ZnO-ZrO2催化剂,考察了焙烧温度和配位剂用量对催化剂性能的影响,并对固相反应机理进行了探讨。结果表明,结晶水的存在降低了金属盐的晶格能,常温下金属盐即可与配位剂发生固相配位反应,生成的金属配合物在焙烧条件下分解得到金属氧化物。随着焙烧温度的升高,催化剂中Cu的分散度下降,ZrO2发生相转变,CO2的转化率下降,甲醇的选择性升高。固相合成法具有无需溶剂、符合绿色化学理念的优点,可用于CuO-ZnO-ZrO2等复合氧化物粉体的制备。二、铜基催化剂组成和助催化剂的研究以CuO-ZrO2催化剂为基础,考察了La2O3掺杂和碱土金属氧化物掺杂对其性能的影响,研究了ZnO在CuO-ZnO-ZrO2催化剂中的作用。结果表明,La2O3对CuO-ZrO2催化剂的掺杂可改变催化剂中Cu的分散度和催化剂表面的碱性。随着La含量的增加,Cu的比表面积呈火山型变化,催化剂的表面碱中心数和密度持续增加。CO2的转化率随金属Cu比表面积的增加而线性增加,甲醇选择性则与催化剂表面的碱中心分布有关。当La2O3的掺入量为Cu2+、Zr4+(?)总量的5%时,甲醇收率最高。碱土金属的掺杂能明显提高CuO-ZrO2催化剂中金属Cu的比表面积,但同时削弱CuO和ZrO2间的相互作用,使CuO的还原难度增加,削弱作用按Mg<Ca<Sr<Ba的顺序递增。适量Mg的掺杂在保持较高CO2转化率的同时提高了甲醇的选择性。CuO-ZrO2催化剂中ZnO的引入在促进CuO晶化的同时又能抑制CuO晶粒的生长和颗粒的团聚。ZnO的加入大大增加了催化剂表面碱中心的数量,从而明显提高催化剂的催化活性。当Zn/Zr比为2：3时,催化剂的活性最高。三、铜基催化剂中载体的作用和CO2加氢反应机理的探讨研究了铜基催化剂中载体的作用,对CO2加氢反应进行了原位XRD测试和原位红外光谱测试,对铜基催化剂上CO2加氢的催化反应路径进行了探讨。结果表明,载体有三个方面的作用：对CuO起到分散作用；与CuO之间发生电荷作用,改变CuO的还原行为；作为活性中心吸附C02和反应中间体。原位XRD结果表明,催化剂在还原过程和反应过程中均以CuO出现,未检到Cu+的存在。原位DRIFT检测到碳酸氢盐、甲酸盐、表面键合甲醛及甲氧基等反应中间体,这些中间体逐步加氢生成甲醇。对于铜基催化剂上CO2加氢合成甲醇反应而言,其催化反应机理为双活性位机理。更多还原

【Abstract】 With an increase in carbon dioxide (CO2) concentration in the atmosphere, global warming problems are becoming more and more serious in recent years. Conversion of CO2 to useful chemicals is one of the most promising ways to utilize CO2. Methanol synthesis by CO2 hydrogenation is of great significance from the viewpoint of environmental protection, energy sources and chemical industry, because methanol is a common chemical feedstock for several important chemicals and a potential alternative energy to fossil fuels.In this work, the research progress of Cu-based catalysts for methanol synthesis from CO2 has been reviewed, and then the preparation method and composition of Cu-based catalyst and the mechanism of catalytic reaction were investigated. The results obtained are mainly as follows.1. Studies on the preparation methods for Cu-based catalystsCuO-ZnO-ZrO2 catalysts were prepared by the combustion method. The effects of fuel amount, fuel type and ignition manner on the properties of catalysts have been investigated. Many efforts have been focused on elucidating the relationship of the composition structure—property of the catalyst. The results show that the physicochemical and catalytic properties of CuO-ZnO-ZrO2 are affected strongly by fuel content and fuel type. Changing the fuel amount would lead to the variation of the combustion enthalpy, the duration of combustion and the amount of the gases evolved in the combustion process, finally resulting in a change of the combustion temperature, which is related closely to the properties of catalysts. The variation regularities of the properties of CuO-ZnO-ZrO2 with the change of fuel content are different for different types of fuel. The combustion intensity of citric acid-nitrate is markedly weak in comparison with glycine-nitrate and urea-nitrate. The reason can be ascribed to the difference in composition and structure of fuel.The maximum methanol yields of 9.6%,9.9% and 8.1% under the reaction conditions of T=220℃, P=3.0 MPa and GHSV=3600 h-1 were obtained over the CuO-ZnO-ZrO2 catalysts prepared by combustion method using urea, glycine and citric acid as fuel, respectively. In comparison with the catalysts prepared by co-precipitation method, the catalyst prepared by combustion method exhibit a higher activity for methanol synthesis. This is due to the high temperature produced during combustion process favoring an interaction between the components of catalyst in a short time. The results show that the catalytic properties of catalysts are related to the dispersion of Cu, the phase state of ZrO2 and an interaction between the compositions in catalysts. The combustion method is a simple, fast and effective method for the preparation of CuO-ZnO-ZrO2 catalysts, and it can be used to prepare other complex oxide powder.The CuO-ZnO-ZrO2 catalysts were synthesized by a route of solid-state reaction, and the effects of calcination temperature and the complexant amount on the properties of catalysts have been investigated. Furthermore, a mechanism of solid-state reaction was proposed. The results show that, the complexes of metal-citrate at ambient temperature can be formed by the solid-state reaction, because the crystallization water in hydrated metal salts decreases the lattice energy. The CuO-ZnO-ZrO2 catalyst can be obtained by the decomposition of the Cu-Zn-Zr citrate precursors. With an increase in calcination temperature, the dispersion of copper species decreases, resulting in a low catalytic activity for methanol synthesis from CO2 hydrogenation. When the transformation of t- ZrO2 to m- ZrO2 occurred in this catalyst, the CO2 conversion is reduced and the methanol selectivity is increased. The route of solid-state reaction is a solvent-free method that meets the requirement of green chemistry, and can be used to prepare the complex oxide catalysts.2. Studies on the composition of catalyst and the promoterThe effects of La2O3 and alkaline-earth oxide doping on the properties of CuO-ZrO2 catalysts were investigated and the role of ZnO in CuO-ZnO-ZrO2 was studied. The results show that, the presence of La2O3 affects the dispersion of Cu and the property of basic sites in the catalysts. With an increase in La loading, the Cu surface area takes on a volcano variation trend, and the amount and density of basic sites over CuO-ZrO2 catalysts increase continually. The conversion of CO2 depends on the surface area of metallic Cu, and there is a linear relationship between them. The methanol selectivity is related to the distribution of basic sites on the surface of catalyst. A suitable amount of La doing is beneficial for the catalytic activity of CuO-ZrO2 and a maximum methanol yield is obtained as the La loading is 5% of the total amount of Cu2+ and Zr4+. With the doping of alkaline-earth metal oxide, the surface area of Cu in catalysts is increased obviously, but the interaction between CuO and ZxO2 became weaker, leading to the rise in the reduction temperature of CuO, in which the effect extent is in the sequence of Mg<Ca<Sr<Ba. The doping of a suitable amount of Mg results in an increase in CH3OH selectivity and further an increase in CH3OH yield. The presence of ZnO in CuO-ZnO-ZrO2 catalysts can promote the crystallization of CuO, and depress the growth of CuO crystal and its conglomeration. While the amount of basic sites on the catalyst surface increases markedly by an introduction of ZnO, resulting in a remarkable increase of the catalytic activity. The highest activity can be obtained when the ratio of Zn/Zr is 2/3 in the CuO-ZnO-ZrO2 catalyst.3. Roles of carrier in the Cu-based catalyst and the reaction mechanisms for methanol synthesis from CO2 hydrogenationThe roles of the carrier in Cu-based catalysts were studied, and the valence of Cu and the intermediate species in the catalytic reaction were investigated by in-situ XRD and in-situ DRIFT techniques. The results show that there are three roles of the carrier in the Cu-based catalysts for methanol synthesis from CO2 hydrogenation:(ⅰ) the carrier can disperse the CuO component; (ⅱ) there is an electronic effect between the carrier and the CuO, which affects the reduction performance of CuO; (ⅲ) it is very important that the carrier participates the catalytic reaction directly as an active sites of adsorbing CO2 and reaction intermediate species. The results of in-situ XRD show that, the Cu species exists in the form of Cu0 during the catalytic process, and no Cu+ can be detected. With the in-situ DRIFT technique, the reaction intermediate species including bicarbonate, formate, surface-bound formaldehyde and methoxide can be detected, which were hydrogenated step by step to form methanol. The dual-site mechanism is reasonable for the methanol synthesis by CO2 hydrogenation over Cu-based catalysts.更多还原