【作者】 马薇；

【导师】 李保山；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2011， 博士

【摘要】 杂多化合物（POM）具有较强的酸性和氧化还原性，可作为双功能催化剂，是近年来无机化学及催化剂领域研究的热点之一。杂多化合物存在比表面积较小(<10m²·g^-1)、热稳定性差等缺点，这大大限制了其在工业上的应用。因此，将杂多化合物负载于适当的载体上，以提高其比表面积和热稳定性，对于提高其催化活性及重复使用性具有重要的意义。随着介孔分子筛的问世及研究的不断深入，人们开始探索将杂多化合物与这类比表面大、热稳定性高的介孔材料相复合，制备负载型杂多化合物分子筛催化剂。分子筛与杂多化合物复合，一方面可以利用杂多化合物的酸性及分子筛固有的酸性，提高催化剂的活性。另一方面可以利用分子筛特有的孔结构，提高催化剂的择形性，这些研究工作已取得了一定的成绩。目前报道的复合方法主要是浸渍-蒸发法，但这种方法通常会导致分子筛孔道的堵塞，降低催化剂的比表面积及孔容，而且杂多化合物与分子筛之间的作用力较弱，在反应体系中容易脱落，催化剂的活性及重复使用性较差。因此，设计新的复合方法，将杂多化合物组装复合到分子筛结构中，使其牢固的附着或结合于分子筛的孔腔内壁或骨架结构上，有效避免杂多化合物在催化反应过程中从分子筛的溶脱，可以有效提升催化剂的性能，将具有重要的学术价值及实际应用前景。本文旨在从分子筛的形成机理入手，设计新的合成路线，一步法将杂多化合物组装于分子筛的孔道结构中，制备一系列杂多化合物-分子筛复合杂化材料。通过X射线衍射（XRD）、X射线荧光（XRF）、扫描电子显微镜（SEM）、高倍透射电子显微镜（HRTEM）、N2吸附-脱附（N2-sorption）等温线、红外吸收光谱（FT-IR）、拉曼光谱（Raman）、热重-差热分析（TG-DTA）等分析测试手段对合成的样品进行结构表征，研究不同合成条件对杂多化合物-分子筛杂化材料结构的影响，考察样品材料的催化性能。论文的主要研究工作有：1.用直接合成法，在初始pH为10的条件下室温晶化为24h下，成功合成了系列单缺位磷钨酸-MCM-41复合杂化材料。当MCM-41中单缺位磷钨酸（LPOM）含量不高于35wt%时，MCM-41结构和形貌基本不变。复合材料具有规则的六方介孔孔道结构，其比表面积及孔径远远大于浸渍负载法所得样品，例如25%LPOM/MCM-41的比表面积为743m²·g^-1，孔径为2.9nm左右，远远高于浸渍负载型25%LPOM/MCM-41-IM的比表面积536m²·g^-1和孔径2.3nm。另外，在分子筛结构中LPOM的热稳定性也得以提高，其最终分解温度由574oC提高到了596oC。该类杂化材料对酯化反应及模拟油品的氧化脱硫反应均具有很好的催化活性及重复使用性。如25%LPOM/MCM-41样品在实验优化的条件下，可使乙酸与正丁醇的酯化反应中正丁醇的转化率达到89.6%，对乙酸正丁酯的选择性为100%。在60oC下、80min内可使模拟油中的硫含量从500ppm降低到6.85ppm，相应的脱硫率为98.63%。催化剂重复使用4次后，催化活性降低不明显。2.用直接合成法，成功合成了系列过渡金属取代磷钨酸-MCM-41杂化材料。当MCM-41中过渡金属取代磷钨酸（M₁-POM）含量高达25wt%时，MCM-41结构和形貌基本不变。复合材料具有规则的六方介孔孔道结构，其比表面积及孔径远远大于浸渍负载法所得样品，例如15%Ni-POM/MCM-41的比表面积为863m²·g^-1，孔径为2.6nm左右，远远高于浸渍负载型15%Ni-POM/MCM-41-IM的比表面积674m²·g^-1和孔径2.2nm。该类杂化材料对酯化反应具有很好的催化活性及重复使用性。如25%Ni-POM/MCM-41样品在实验优化的条件下，可使乙酸与正丁醇的酯化反应中正丁醇的转化率达到90.2%，对乙酸正丁酯的选择性为100%。然而，25%Cu-POM/MCM-41样品对模拟油的氧化脱硫反应催化活性不高，在60oC下、80min内仅使模拟油中的硫含量从500ppm降低到45.16ppm，相应的脱硫率仅为90.91%。3.用直接合成法，在晶化温度为室温，晶化时间为24h下，成功合成了系列磷钨酸（HPW）-HMS复合杂化材料。复合材料具有有序的指纹状介孔孔道结构，其比表面积在1011-746m²·g^-1范围内，孔径在2.2nm左右，远远大于浸渍负载法所得样品30%HPW-HMS-IM的比表面积(546m²·g^-1)和孔径（1.9nm）。该类杂化材料对酯化反应及模拟油品的氧化脱硫反应均具有很好的催化活性及重复使用性。如30%HPW-HMS-DS样品在实验优化的条件下，可使乙酸与正丁醇的酯化反应中正丁醇的转化率达到86.9%，对乙酸正丁酯的选择性为100%。20%HPW-HMS-DS样品在60oC、60min内可使脱硫率达到98.51%。4.用直接合成法，在晶化时间为36h，晶化温度为40℃下，成功合成了系列单缺位磷钨酸-HMS复合杂化材料。该材料具有有序的介孔孔道和较大的空腔结构，其比表面积在997-626m²·g^-1范围内，孔径在3.8nm左右，该类杂化材料对酯化反应及模拟油品的氧化脱硫反应均具有很好的催化活性及重复使用性。如20%LPOM-HMS-DS样品在实验优化的条件下，可使乙酸与正丁醇的酯化反应中正丁醇的转化率达到88.3%，对乙酸正丁酯的选择性为100%。催化剂重复使用3次后，催化活性没有明显下降。20%LPOM-HMS-DS样品，在60oC下、80min内可使脱硫率达到98.67%。5.本文分别探讨了催化剂对酯化反应及油品氧化脱硫反应的催化机理。认为酯化反应发生在MCM-41分子筛中单缺位磷钨酸表面的B酸位上。正丁醇首先吸附在LPOM的表面，形成了一个碳正离子。然后，乙酸进攻碳正离子形成一个不稳定中间体，不稳定中间体通过脱质子而生成乙酸丁酯和水。6.探讨了杂化复合材料的形成机理，提出了直接法（或一步法）合成杂多化合物-分子筛杂化材料形成的静电作用“夹层机理”。其核心是通过表面活性剂阳离子和杂多化合物阴离子之间的静电作用将杂多化合物引入到二氧化硅层与模板剂胶束之间的“夹层”界面中，最终将杂多化合物组装到分子筛的孔道内。更多还原

【Abstract】 Polyoxometalate （POM） is one of the most promising catalysts used asthe dual function catalyst due to its strong acidity and appropriate redoxproperty. However, POM suffers from some shortcomings, such as the smallsurface area （1-10m2·g-1）, low thermal stability and the difficulties inseparation and recovery. These drawbacks greatly limit their applications incatalytic processes. In order to improve its catalytic performance, the POMwas dispersed into the carriers with high surface area. Some groups have beendevoting into synthesizing the POM catalysts combined with the mesoporousmaterials due to their large surface areas and high thermal stabilities. It wasfound that incorporation of POM into molecular sieves not only improved thesurface acidity of molecular sieves, but also greatly increased the selectivity ofmolecular sieves in catalytic reactions.At present, the incorporation of POM into the mesoporous materials wasprimarily through the impregnation methods followed by evaporation of thesolvent. However, the POM in these samples can significantly reduce thesurface area of the mesoporous materials through blocking the pores, and thePOM can also diffuse into the polar solvents, resulting in the great decrease in catalytic activity. Therefore, it is necessary to find a new method which canstrengthen the interaction of mesoporous molecular sieves andpolyoxometalates, reduce the loss of polyoxometalates in the catalytic reaction,and improve the catalytic performance.In this paper, we synthesized the mesoporous molecular sievesincorporated with polyoxometalates via a novel direct method. All of thesamples were characterized by the XRD, XRF, SEM, HRTEM, N2-sorption,FT-IR, Raman and TG-DTA techniques, followed by the catalytic tests. Thedetails are described as followings:1. The ordered mesoporous MCM-41materials incorporated withlacunary polyoxometalate （LPOM） were prepared via an original directsynthesis method at room temperature with24h and under the initial pH valueof10. When the LPOM content is less than35wt%in LPOM/MCM-41samples, the structure and morphology of MCM-41do not changed. Inaddition, the materials contain ordered hexagonal mesoporous structure. Andthe LPOM/MCM-41catalysts possess larger surface areas, larger porediameter and larger pore volume than the impregnation samples. For example,the25%LPOM/MCM-41sample exhibits a specific surface area of743m2/gand a pore size of2.90nm, is much bigger than that of the25%LPOM/MCM-41-IM sample （536m2/g,2.32nm）. In addition, the thermalstability of LPOM in the molecular sieve is also improved, and its finaldecomposition temperature increased from574oC to596oC. The hybridmaterials show good catalytic activities and reusability in both esterification and oxidation desulfurization of simulated oil. In particular, the25%LPOM-MCM-41sample exhibits an excellent catalytic performance.Under the optimized conditions, the conversion of n-butanol is89.7%, and theselectivity of butyl acetate is nearly100%. For the15%LPOM-MCM-41, thesulfur content can be efficiently decreased from500to7.53ppm at60oCwithin80min corresponding to98.51%sulfur removal. Moreover, sulfurremoval is slightly decreased after4runs.2. Well-ordered mesoporous MCM-41with transition metal-modifiedpolyoxometalate M₁-POM （M₁-POM=[PW511O39M₁], M₁=Ni or Cu） hasbeen synthesized by the directed synthesis method. After incorporating of highM₁-POM content, the pore structure and spherical morphology of MCM-41are unchanged. The M₁-POM/MCM-41catalysts possess larger surface areas,larger pore diameter and larger pore volume than the impregnation samples.For example, the15%Ni-POM/MCM-41sample exhibits a specific surfacearea of863m2/g and a pore size of2.6nm, is much bigger than that of the25%LPOM/MCM-41-IM sample （546m2/g,2.2nm）. The materials showgood catalytic activities and reusability in esterification of acetic acid andn-butanol. For the25%Ni-POM/MCM-41, the conversion of n-butanol is90.2%, and the selectivity of butyl acetate is nearly100%, under the optimizedconditions. However, the catalytic activity of M₁-POM/MCM-41in oxidativedesulfurization of model oil is not high. For the15%Co-POM/MCM-41,sulfur removal is only86.97%at60oC within80min.3. Well-ordered hexagonal mesoporous silicate （HMS） materials with various contents of phosphotungstic acid （HPW） were synthesized via anoriginal direct synthesis method. The crystallization temperature is under theroom temperature and the crystallization time is24h. The samples showhomogeneous dispersion of the HPW molecules and have regularfingerprint-like mesopores and spherical morphology. Compared with theconventional wet impregnation method, the samples obtained by our methodhave better HPW dispersions，larger specific surface area and larger porevolume. The specific surface area of HPW-HMS-DS samples is in the range of1011-746m²·g^-1, and the pore diameter is about2.2nm. And the directsynthesized samples exhibit better catalytic activities than the impregnatedsamples in both esterification and oxidation desulfurization. Under theoptimized conditions, for the30%HPW-HMS-DS, the conversion ofn-butanol is86.9%, and the selectivity of butyl acetate is nearly100%. For the20%HPW-HMS-DS, the sulfur content can be efficiently decreased from500to7.53ppm at60oC within60min corresponding to98.51%sulfur removal.4. The LPOM were successfully incorporated into the hexagonalmesoporous HMS matrix via the directed synthesis method. Thecrystallization temperature is40oC and the crystallization time is236h. TheLPOM-HMS-DS samples possess Compared with the worm-like porestructure of HMS reported previously. The samples prepared in the acidmedium show an order pore structure with many uniform size of cavitystructure （about50nm）. In addition, the material has a high specific surfacearea and big pore diameter. The specific surface area of the samples is in the range of997-626m²·g^-1, and the pore diameter is about3.8nm. The materialsshow good catalytic activities in the esterification and oxidationdesulfurization. For the20%LPOM-HMS-DS sample, the conversion ofn-butanol is88.3%, the selectivity of butyl acetate is nearly100%, and thesulfur removal is98.67%at60oC within80min. In addition, the catalystshave the excellent reusability.5. We investigated the catalyst mechanism for the esterification. Firstly,the alcohol is chemisorbed on the Br nsted acid sites to form the carboniumions. The surface reaction is the rate determining step. Then the acetic acidattacks these carbocations to form an unstable intermediate which generatesbutyl acetate and water by deprotonation.6. We proposed an electrostatic effect “mechanism of sandwich” used inthe direct synthesis （or one step synthesis） of the hybrid materials ofpolyoxometalates and mesoporous molecular sieves. Its core is to introducepolyoxometalate into the sandwich interface between the silicon dioxide layerand the template micelles through the electrostatic interaction of the surfactantcation and polyoxometalate anion, and eventually assemble thepolyoxometalates into the pore canal of molecular sieves.更多还原