【作者】 卢胜波；

【导师】 王修慧；

【作者基本信息】 大连交通大学 ， 材料学， 2010， 硕士

【摘要】 镁铝尖晶石具有很高的热稳定性,不易烧结,高的机械强度和硬度,良好的耐蚀性、耐磨性,低的热膨胀系数,同时具有碱性和酸性两种活性中心这一独特的表面性质,在化学反应中广泛地用作催化剂和催化剂载体。目前制备的以镁铝尖晶石为载体的催化剂,在工业应用过程中,存在稳定性较差,易失活等缺点,主要因为制备载体的骨架结构晶态不完整,载体粒子尺寸较大或焙烧不完全,导致催化剂比表面积和孔容变小。本文旨在采用醇盐水解结合水热法制备具有完整晶态骨架,一定形貌和比表面积的催化剂载体材料,有利于镁铝尖晶石催化剂载体性能的提高及工业化应用。本文采用镁片和铝片合成镁铝双金属醇盐,水解制备均匀溶胶,然后采用水热法制得了特殊形貌和孔结构的镁铝尖晶石催化剂载体。用TG-DTA、SEM、TEM、XRD、N2吸附-脱附对制备样品进行了表征,考察合成过程中水热反应温度、水热反应时间、加水量、表面活性剂CTAB及加量、矿化剂、焙烧温度等条件对合成产物的骨架结构,组织形貌以及比表面积孔结构等性能的影响。实验结果表明:镁铝双金属醇盐室温水解产物是镁铝羟基化合物,主要是MgAl2(OH)8组成,经过水热反应后得到的主要是γ-AlOOH和镁铝羟基复合物,随着水热反应温度和反应时间的改变,制备的前驱物的相组成没有改变,只是衍射峰强度发生稍微改变;制备载体颗粒表面的片状结构不断长大,比表面积不断地减少。随着加水量的增加,制备的镁铝尖晶石由片状不断地长成块状结构,且比表面积大幅度下降。经过水热反应后的镁铝尖晶石载体颗粒表面具有片状结构,且孔道结构呈蠕虫状,具有较大的比表面积,焙烧后,颗粒形貌并没有改变。通过添加表面活性剂CTAB及改变其含量,得到了颗粒表面片层结构排列更规则,孔道结构更规整、更紧密,比表面积增大,适量的表面活性剂能够明显改善镁铝尖晶石载体材料的比表面积和孔结构特征,而过量的表面活性剂对制备的镁铝尖晶石有不利影响。焙烧温度对镁铝尖晶石骨架结构的影响最为明显,醇盐水解产物水热反应后在450℃就有尖晶石相的生成,600℃就得到了完整晶态骨架结构,随着温度的提高,样品的衍射峰强度不断增加,晶粒尺寸长大,比表面积不断降低。更多还原

【Abstract】 Magnesium aluminate spinel (MgAl2O4) is one of the end-members of the spinel group minerals, which shows a unique combination of properties like high thermal stablity, high mechanical strength and hardness, high chemical inertness against both acidic and basic slag, abrasion resistance, low thermal expansion coefficient. Meanwhile, owing to its good catalytic activity and two active centers as acid and alkaline, MgAl2O4 spinel has been widely used as catalysts or catalyst support in chemical reactions. Nowadays MgAl2O4 spinel catalyst supports present the bad stability and easily lose its activity and so on in industrial application process. The main reason is that skeleton crystalline state of as-synthesized MgAl2O4 spinel is imperfect, particle size of supports is larger or imperfect calcination, which leads to diminishing specific surface area and pore volume of catalyst support. The paper’s purpose is that hydration of alkoxide combines hydrothermal methed to prepare the MgAl2O4 catalyst support of perfect skeleton crystalline state, special surface topography and larger specific surface area, with a view to improve the performance of MgAl2O4 catalyst support and in favor of industrial application.The stoichiometric magnesium sheet and aluminum sheet were used as reagents to synthesize the Mgnesium-Aluminum double metal alkoxide. The synthesized alkoxide was hydrolyzed with deionized water under stirring, until generating the homogeneous sol. The hydrolyzate was added into high-pressure autoclave with magnetic stirrer for hydrothermal synthesis, dried and calcined. At last the products of special topography and pore structure were gained. The powders as-sythesized were characterized by TGA-DTA、SEM、TEM、XRD、and N2 adsorption-desorption isotherms to study the effect of different synthesis conditions on the properties of MgAl2O4 spinel or precursor, including hydrothermal temperature、hydrothermal duration、additive water content、surfactant CTAB and additive content、mineralizer and calination temperature, and so on.The results indicated that hydrolysate of Mgnesium-Aluminum double metal alkoxide was Mgnesium-Aluminum hydroxide compounds, mainly including MgAl2(OH)8, and the precursor was mainly composed of AlOOH and Mgnesium-Aluminum hydroxide compounds by hydrothermal reaction. With the increment of hydrothermal temperature and extension of hydrothermal duration, the phase composition of the precursor as-synthesized were not changed, just only intensity of diffraction peak had a little bit of increment, and relative content of both changed a little; the sheet of powder surface grew larger, and its specific surface area diminished gradually. MgAl2O4 spinel grew into blocky structure from sheet structure gradually and had lower specific surface area with the increment of adding water. The results indicated that MgAl2O4 spinel powders particle surface by hydrothermal method was made up of sheet structure. And it had wormlike pore structures and large specific surface area in a structure, which is a relic of the morphology even after calcination. By adding right amount of surfactant CTAB, the sheet structure of particle surface grew regularly, its pore structure was more regular and compacter, and had larger specific surface area, while the excess of surfactant CTAB had adverse effect.The effect of calcination temperature on MgAl2O4 spinel skeleton structure was the most significant. The spinel phase began to be formed at 450℃by hydrothermal reaction with alkoxide hydrolysate. And the perfect crystalline state skeleton structure was formed at 600℃. With the increment of calcination temperature, the diffraction peak intensity of samples increased gradually, the crystalline grain grew bigger and its specific surface area diminished gradually.更多还原