【作者】 王彦昌；

【导师】 段雪；

【作者基本信息】 北京化工大学 ， 应用化学， 2008， 博士

【摘要】 随着材料科学与技术的发展,科学家正在努力探索运用化学或物理的方法来构建具有特定形貌和孔结构的材料,从而可能实现对某些材料的设计,如新型催化剂载体、大分子聚合物分离膜、生物医药材料和药物载体等,最终调控其应用性能。复合金属氢氧化物（LDHs）是一类可人工合成的、具有二维结构的阴离子型层状功能材料,它们由带有正电荷的主体层板、平衡正电荷的层间阴离子客体和部分水分子组成。其层板元素组成和层间阴离子种类可调的特点能够衍生出很多具有不同物理化学特性的功能性组装体,因此有可能作为高性能催化材料、吸附材料、分离材料、功能性助剂材料、生物材料和医药材料等获得应用。目前,在该研究领域的工作中,LDHs类材料通常是以粉体形式使用,这样就会在实际应用过程中（比如作为催化材料、吸附和分离材料等）产生诸如压力降高、传质/传热效率低以及分离困难等问题。因此,关于LDHs类材料的形貌可控制备具有明显的科学意义和实际应用价值。本论文以LDHs类材料的形貌可控制备为目标,重点研究了球形LDHs类材料的制备工艺和球形MgO/MgAl₂O₄多孔复合材料可控制备及其催化性能。论文首先在无模板存在条件下,采用喷雾干燥工艺制备了球形MgAl-CO₃-LDHs材料。SEM、激光粒度和显微图像分析结果表明,产物为实心的球形颗粒,颗粒直径约为10-50μm;压汞法进一步分析表明球形颗粒为多孔材料,比表面积为43 m²/g,总孔容达1.3 cc/g,最可几孔径为87.9 nm。该球形材料在500℃的条件下焙烧8 h和经过焙烧/再水合处理后依然保持着良好球形形貌,表明该球形材料具有较好的结构稳定性。在上述可控制备的基础上,采用喷雾干燥工艺进一步制备出其他组成如NiAl-CO₃-LDHs、ZnAl-CO₃-LDHs和CuZnAl-CO₃-LDHs多孔微球,研究结果表明该方法对制备球形、实心、多孔性LDHs材料具有普适性。最后,以γ-Al₂O₃为硬模板,在其孔道内原位控制制备得到MgAl-CO₃-LDHs,然后以其为前驱体制备出球形MgO/MgAl₂O₄多孔复合材料。表征了该材料的结构和织构特性,并研究了其催化大豆油与甲醇的酯交换反应的催化性能。研究工作中首先在球形γ-Al₂O₃（0.5～1.0 mm）孔道内原位控制制备得到MgAl-CO₃-LDHs前驱体,然后对其进行焙烧、溶蚀和再焙烧等处理,最终制备出球形MgO/MgAl₂O₄多孔复合材料。XRD、SEM、TEM和HRTEM表征结果显示,球体的直径为0.5～1.0 mm,该球形复合材料主要由MgO和MgAl₂O₄组成的纳米棒构成;TEM和低温氮气吸脱附证实球形产物为多孔材料,BET法计算复合材料的比表面积为92.5 m²/g,总孔容为0.65 cc/g,对低温氮气吸脱附等温线的脱附支采用BJH法进行孔径计算表明其最可几孔径为17.4nm;采用CO₂-TPD技术进一步研究了MgO/MgAl₂O₄多孔复合材料的碱性质,并与用传统浸渍法制备的球形MgO/MgAl₂O₄/γ-Al₂O₃材料相比较,研究结果表明MgO/MgAl₂O₄多孔复合材料具有较高的碱性;进一步以大豆油与甲醇的酯交换反应为探针,研究了MgO/MgAl₂O₄多孔复合材料的催化活性,结果表明MgO/MgAl₂O₄多孔复合材料催化大豆油酯交换反应10 h后的生物柴油产率为57%,比使用传统浸渍法制备的催化剂提高了20%。更多还原

【Abstract】 With the development of materials science and technologies, scientists are exploring some methods involving chemistry or physics to construct materials with specific morphology and pore architecture and realizing the design and adjust and control of some functional materials such as new types of catalyst supports,membranes for the separation of large polymers,biomedical materials with macroporosity and drug carriers.Layered double hydroxides（LDHs）are a large family of synthetic anion type of layered materials with two-dimensional nanostructures consisting of positively charged layer with charge balancing anions and water moleculars between the layers.As a result,a large calss of functional isostructural materials with widely varied physicochemical properties can be obtained by changing the nature and molar ratios of the metal elements as well as the type of intercalated anions.Thus,LDHs are promising functional materials for a large number of applications in catalysis,adsorption,separation,functional additives,pharmaceutics and biomaterials.So far,LDHs involved in the above fields such as catalysis,adsorption and separation are usually in powder form,which inevitablly give rise to problems such as high pressure drops,poor mass/heat transfer,poor contact efficiency and difficulties in separation.Thus,it is essential both in science and practice to undertake the morphology control study related to LDHs materials.In the present thesis,we take the morphology control of synthetic LDHs as target and put emphasis onto the preparation of microspherical LDHs with porous architecture and macrospherical composites consisting of magnesia and magnesium aluminate（MgO/MgAl₂O₄）,which was used to catalyze the transesterification of methanol and soybean oil.First,we investigated the preparation of microspherical MgAl-CO₃-LDHs by using spray drying technique without the help of templates.Scanning electron microscopy（SEM）,low-angle laser light scattering and microscopic particle size analysis system（MPSAS） techniques were adopted to characterize the products.The resulting products are composed of nanosized LDH particles aggregated into solid microspheres with an diameter of 10-50μm,a sphericity of 0.84. Mercury porosimetry was further consolidated that the microspheres were porous with modal pore access diameter of 87.9 nm,surface area of 43 m²/g and total pore volume of 1.29 cc/g.When calcined for 8 h at 500℃in N₂ atmosphere or calcined and rehydrated in decarbonized dioxide water,the spherical morphology was well restained,which demonstrated that these microspheres have a good structural stability.Besides,in order to demonstrate the common utility of the spray drying method,a variety of LDHs with different compositions such as NiAl-LDHs,ZnAl-LDHs and CuZnAl-LDHs with interlayer carbonate anions were successfully prepared and fabricated into microspheres using the same procedure as for MgAl-CO₃-LDHs microspheres.This confirms that the spray drying method is a very simple yet effective common process for the preparation of solid LDHs microspheres with macroporous.At last,macrospherical composites（MgO/MgAl₂O₄）consisting of magnesia and magnesium aluminate were prepared by using hard template method.We thoroughly characterize the composition, morphology,structure and architecture and choose the methanolysis of soybean oil as probe reaction to investigate its catalysis performance. The process involved in situ growth of magnesium-aluminum layered double hydroxides into the channels of theγ-Al₂O₃ （MgAl-CO₃-LDHs/γ-Al₂O₃）macrospheres（0.5-1.0 mm in diameter）by using urea hydrolysis method,followed by calcination,tuning of the base strength through etching of excess aluminium with aqueous alkali and a final calcination step.X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）, high-resolution transmission electron microscopy（HRTEM）,elemental analysis and low temperature N₂ adsorption-desorption studies demonstrate that the composite materials（MgO/MgAl₂O₄）are composed of nanosized rod-like particles which aggregated into a spherical framework with specific surface area of 92.5 m²/g,total pore volume of 0.65 cc/g and the most optimization pore radius of 17.4 nm.At the same time,temperature programmed desorption with carbon dioxide as probe molecular（CO₂-TPD）was applied to characterize the base strength for MgO/MgAl₂O₄ prepared by a conventional impregnation method and MgO/MgAl₂O₄/γ-Al₂O₃.The general shift to higher desorption temperatures for the MgO/MgAl₂O₄ framework catalysts compared with those of MgO/MgAl₂O₄/γ-Al₂O₃ can be attributed to the increase of base strength resulting from the leaching of acidic Al³⁺ions from the materials. Catalytic reactivity was investigated by using methanolysis of soybean oil as probe reaction.The MgO/MgAl₂O₄ composite showed a biodiesel yield 57%,which was increased by 20%compared to MgO/MgAl₂O₄/γ-Al₂O₃ with the same loading of magnesium.The enhanced catalytic activity of MgO/MgAl₂O₄ can be ascribed to its higher base strength,specific surface area,pore volume and pore size.更多还原