纳米含Mg复合金属氧化物制备、表征及催化性能研究

【作者】 马宁；

【导师】 高滋；

【作者基本信息】 复旦大学 ， 物理化学， 2003， 博士

【摘要】 纳米粒子由于其独特的反应活性和优良的选择性而被看作是一类有前途的新型催化材料。复合金属氧化物在催化领域已得到广泛应用。由不同金属氧化物可组成多种结构形式的复合金属氧化物，包括晶形的和无定形的。对复合金属氧化物，可通过改变化学组成、织构性质或调变氧化物之间的相互作用等多种途径来提高催化剂性能。根据已有的关于纳米粒子和金属氧化物的知识，可以预见，纳米复合金属氧化物将具有更广泛的应用前景，纳米复合金属氧化物的制备、表征及催化性能研究是一个值得深入的领域。MgO是一种具有中强碱性的金属氧化物。在催化研究中，通过MgO与其它氧化物之间的相互作用对催化剂进行改性，从而使催化剂性能得到提高的例子不胜枚举。本论文围绕苯乙烯液相选择氧化和乙苯CO₂氧化脱氢这两个具有工业应用价值的催化反应，研究了具有尖晶石相的纳米MgxFe₃-xO₄复合金属氧化物以及由水滑石前驱体获得的含Mg、Fe纳米复合金属氧化物的制备、表征和催化性能，论文主要内容如下：1. 采用共沉淀方法和柠檬酸溶胶-凝胶方法制备纳米MgFe₂O₄尖晶石。在共沉淀方法中，采用NH₄OH和NH₄HCO₃作沉淀剂，研究了改变NH₄OH/NH₄HCO₃比例和焙烧温度对样品性能的影响。在柠檬酸溶胶-凝胶方法中，研究了改变柠檬酸用量和焙烧温度对样品性能的影响。XRD和TEM测试结果证明这两种方法都可以制得纳米MgFe₂O₄尖晶石，但粒子形貌存在较大的差异。共沉淀法样品的粒径在80 nm左右，而柠檬酸法样品的粒径不仅小一倍，约30 40 nm，并且样品分散性更好。TG/DTG/DTA结果显示共沉淀方法样品的尖晶石成相温度约在600oC，而柠檬酸溶胶-凝胶方法样品的尖晶石成相温度约在400oC。不同方法制得的尖晶石粒子形貌的不同可能是由焙烧温度的差异造成的。通过比较说明，采用柠檬酸溶胶-凝胶方法更适合于制备分散性好和粒径小的纯相纳米MgFe₂O₄粒子。2. 研究了以过氧化氢为氧化剂时纳米MgxFe₃-xO₄尖晶石复合金属氧化物对苯乙烯选择氧化反应的催化性能。发现采用柠檬酸溶胶-凝胶方法制得的催化剂具有更高的催化活性，而且非化学计量比的MgxFe₃-xO₄催化剂的活性优于纯相MgFe₂O₄尖晶石。催化剂上苯乙烯首先发生C=C双键断裂，再发生氧化反应，反应产物主要是苯甲醛（60 70%），和少量的苯乙醛和苯乙烯环氧化物（ 10%）。以Mg_0.4Fe_2.6O₄催化剂为代表，考察了溶剂、反应温度、反应时间和苯乙烯/过氧化氢（摩尔比）对催化性能的影响。结果显示，极性溶剂和较低的反应温度有利于苯甲醛选择性的增加。在最佳反应条件下，苯乙烯转化率达到32.0%，苯甲醛产率达到20.2%。将催化剂表征结果与反应数据相关联，认为非化学计量比的催化剂表面具有的较多的缺<WP=6>陷结构以及在尖晶石结构中分散着的一定量的α-Fe₂O₃微晶有可能是促进催化剂活性提高的主要原因。3. 在上述工作基础上，考察了在非化学计量比的纳米Mg-Fe尖晶石复合氧化物中掺杂第三组分对催化性能的影响。发现加入适量的Al有助于进一步提高催化剂活性和苯甲醛选择性。以Mg_0.4Fe_2.3Al_0.3O₄催化剂为代表，研究了反应温度、反应时间和苯乙烯/过氧化氢（摩尔比）对催化性能的影响。在最佳反应条件下，苯乙烯转化率达到38.5%，苯甲醛产率达到30.0%。TPR数据显示，Al的加入促使Mg-Fe复合氧化物催化剂中产生更多高活性的α-Fe₂O₃微晶，从而导致催化剂活性增加。4. 以含Mg水滑石为前驱体，制备了二元，三元和四元纳米复合金属氧化物催化剂，考察了该类型催化剂对乙苯CO₂脱氢反应的催化性能。纳米Mg/Fe复合氧化物催化剂对该反应具有较高的催化活性和苯乙烯选择性，其乙苯转化率为35.0%，苯乙烯选择性为99.8%。在Mg/Fe水滑石中加入适量的Al，可有效提高催化剂的比表面和乙苯转化率，而苯乙烯选择性仍保持99.5%。在Mg/Al/Fe水滑石中加入Ni，Co和Zn等元素，可在不影响苯乙烯选择性的前提下，进一步提高乙苯转化率。其中Mg/Zn/Al/Fe（3/3/1/2）催化剂活性最高，500oC时乙苯转化率达到52.7%，苯乙烯选择性为99.6%。通过将催化剂的表征结果与反应数据相关联，以含Mg水滑石为前驱体制备催化剂，不仅可提高催化剂的比表面和氧化铁的分散度，从而增加催化活性；同时，Mg、Al和Zn等元素的加入还可以促进Fe3+还原成Fe²⁺，抑制Fe²⁺还原成Fe0，增加参与氧化还原循环的活性位数目；另外，加入Mg、Al和Zn等元素还可以调节催化剂的酸碱性，Mg/Zn/Al/Fe（3/3/1/2）催化剂的高活性可能与其表面具有较多的中强酸位和中强碱位有关。考察了反应条件对Mg/Zn/Al/Fe（3/3/1/2）催化剂性能的影响。在N2中预活化催化剂，可提高催化剂活性。升高反应温度可提高乙苯转化率，但苯乙烯选择性降低，失活速率加快。Mg/Zn/Al/Fe（3/3/1/2）催化剂的乙苯转化率随反应时间增加而逐步下降，通空气可以使催化剂活性恢复，表明催化剂的失活为暂时性的结焦失活。5. 通过比较采用GC和HPLC分析苯酚羟基化反应获得的数据，观察到采用GC分析产物，由于溶液中残余的过氧化氢会与苯酚及其羟基化产物在GC高温系统中继续发生氧化反应，使得苯酚转化率数据偏高，邻苯二酚/对苯二酚比值更多还原

【Abstract】 Nanoparticles are regarded as new active catalytic materials because of their high reactivity and good selectivity. On the other hand, complex metal oxides have already been used widely in the catalysis. Complex metal oxides with different structures and compositions can be prepared from various kinds of metal oxides. The catalytic performance of complex metal oxides are often improved through fine adjustment in chemical composition, textural structure and the interaction among active components. Based on the former knowledge about the nanoparticles and complex metal oxides, nanosized complex metal oxides undoubtedly possess great potentials for application in catalysis，and the preparation, characterization and catalysis of these materials deserve intensive investigation. MgO is a kind of metal oxide with medium-strength basicity. In catalysis, it is common to improve the performance of catalysts through introduction of MgO. This dissertation is focused on two catalytic reactions of practical interest, i.e., selective oxidation of styrene and the ethylbenzene dehydrogenation in the presence of CO₂. In the work, the preparation, characterization and catalytic properties of MgxFe₃-xO₄ complex oxide catalysts and Mg-Fe complex metal oxide prepared from hydrotalcite-like precursors are studied. The main contents are as follows:1. Nanosized spinel MgFe₂O₄ were prepared by coprecipitation and citrate gel methods. In coprecipitation, NH₄OH and NH₄HCO₃ were used as precipitants. The influence of molar ratio of NH₄OH to NH₄HCO₃ and calcination temperature on the preparation was studied. In citrate gel method, the influence of citric acid/Mg2+ ratio and calcination temperature on the preparation was studied. XRD and TEM results reveal that both methods are successful for getting MgFe₂O₄ spinel nanoparticles. However, the particle size and morphology of the samples are different. The mean size of MgFe₂O₄ prepared by coprecipitation is about 80 nm, whereas the particles prepared by citrate gel method are smaller with a mean size of 30-40 nm and more dispersive. TG/DTG/DTA measurements show that the crystallization temperature of MgFe₂O₄ spinel prepared by citrate gel method is about 200oC lower than that by coprecipitation method. Such a temperature difference leads to the differences in the properties of the spinel products. The citrate gel method is more suitable for the preparation of samples with higher dispersity and smaller particle size.2. Selective oxidation of styrene with H₂O₂ as oxidant on MgxFe₃-xO₄ spinel complex<WP=9>oxide catalysts was studied. MgxFe₃-xO₄ catalysts obtained by citrate gel method are more active due to their higher dispersity and smaller particle size. The oxidation reaction is favored on nonstoichiometric MgxFe₃-xO₄ catalysts rather than on stoichiometric MgFe₂O₄. Styrene undergoes a C=C bond cleavage preferentially over the spinel-type catalysts to give benzaldehyde （ 65 - 70 mol%）, phenylacetaldehyde and styrene oxide are formed as minor reaction products （ 10 mol%）. The effects of solvent, reaction temperature, reaction time and styrene/H₂O₂ （molar ratio） on the performance of Mg_0.4Fe_2.6O₄ catalyst were studied. Aprotic solvent and lower reaction temperature are favorable for increasing the selectivity for benzaldehyde. Under the optimum reaction conditions, the styrene conversion is 32.0%, and the selectivity for benzaldehyde is 20.2%. The catalyst characterization and reaction results reveal that being rich in defect structure, and the existence of nanosized α-Fe₂O₃ crystallites in the non-stoichiometric Mg-Fe spinel complex oxide catalysts are responsible for the enhanced catalytic activity.3. The effect of metal promoters on the catalytic properties of Mg_0.4Fe_2.6O₄ was studied. The results show that adding an appropriate amount of Al enhances the activity of the catalyst and the selectivity for benzaldehyde. The effects of reaction temperature, reaction time and styrene/H₂O₂ （molar ratio） on the performance of Mg_0.4Fe更多还原