Knowledge Management System Of Institute of process engineering,CAS
|关键词||负载金催化剂 氧化酯化反应 载体效应 铈锆固溶体 反应机理|
酯是重要的化工和有机合成中间体，传统的合成方法是由醛或醇首先氧化制备酸或酸的衍生物，然后酸或酸的衍生物进行酯化反应制备，但制备工艺复杂，而且过程中产生许多有毒副产物。由醛或醇一步氧化酯化制备醛的方法，由于制备工艺简单，越来越受到人们的关注。近年发现,纳米金催化剂比传统Pd-Pb具有更高的反应活性并且无毒,具有非常大的应用前景.本文的研究中,采用不同方法制备了纳米金催化剂,考察催化剂在醛氧化酯化反应的催化性能,研究了载体效应和金与载体相互作用的影响。主要研究内容如下:（1）采用沉积沉淀法制备了Al2O3、TiO2、ZnO、CeO2和ZrO2负载的金催化剂，考察了考察制备温度、老化时间、初始浓度、还原步骤等制备条件的影响，得到了最佳制备条件。将这些催化剂用于异丁醛的氧化酯化反应，其中自制氧化铈作为载体具有最高的活性。（2）通过水热法分别以硝酸铈和硝酸铈铵为铈源制备了铈锆复合氧化物，前者各自保持氧化铈和氧化锆的形貌，后者形成了固溶体结构。在同铈锆含量下，固溶体具有更高的比表面积，更强的可还原性和更多的酸碱中心。将两个系列的金催化剂应用于异丁醛的氧化酯化，发现固溶体系列具有更高的反应活性，进一步推测了反应机理，并且考察了不同底物的影响和催化剂的循环性能。（3）通过浸渍法成功在铈锆固溶体中添加了三种助剂，并通过沉积沉淀法制备了金催化剂，所有催化剂的金颗粒大小和电子状态相似。三种助剂的都增加了相应金催化剂在醛氧化酯化反应上的活性，其中添加助剂氧化镁的催化剂具有最高的反应活性。结合反应活性和表征数据的相关性，推测氧化镁增强了金和载体的相互作用，增加了活性氧量，提高了反应活性。（4）通过水热法制备了球状、立方体和棒状氧化铈，发现棒状氧化铈负载的金催化剂具有最高的氧化酯化活性。通过调节水热中氢氧化钠的浓度，制备了大小均匀分布的棒状Ce-M（Zr、La和Fe）固溶体，并制备了相应的金催化剂，用于醛氧化酯化反应。发现加入镧元素后，催化剂的反应活性下降，加入锆元素催化剂活性略有提高，而加入铁元素后，催化剂的活性明显提高，通过H2-TPR和CO2-TPD表征，推测可能是铁的加入，增强了载体的可还原性和碱性，由此提高催化剂的活性。（5）采用溶胶沉积法以PVA为保护剂制备了Au/ZnO催化剂，研究了三种除去PVA的方法：热处理、热水洗和热水回流，其中300 ℃煅烧具有最好的除去PVA效果，其催化剂具有最高的MAL氧化酯化性能。发现溶胶溶液pH值对于金颗粒的大小分布和负载量有很大影响。将均匀分布的金颗粒负载到了不同载体上，其中以氧化锌负载的金催化剂具有最高的反应活性。这可能是由于Au/ZnO具有较多的碱性中心和金颗粒和氧化锌之间的强相互作用造成的。（6）通过浸渍还原（WIR）法和溶胶沉积（SI）法分别制备了不同Au-Pd比例的双金属催化剂，WIR法制备的二氧化钛负载的单金、单钯和金钯双金属颗粒分布不均匀，部分颗粒发生了团聚。SI法制备的的催化剂，单金属Au、Pd或是Au-Pd颗粒大小分布均匀，并且通过UV-vis和EDS线扫描表征发现Au-Pd形成了合金结构。 将催化剂用于醛的氧化酯化反应，发现加入Pd后，提高了MAL的转化率，但是MMA的选择性没有明显提升。
Ester is a pivotal chemical and organic intermediate. Traditionally, it is synthesized by the complicated two-step procedure: first the carboxylic acids or carboxylic acid derivatives are prepared by oxidation of aldehydes or alcohols and sequentially the ester are prepared by esterification. The direct oxidative esterification of aldehydes with alcohols caused much attention due to its short process flow and environmental benefits. Recently, the supported Au-based catalysts have shown a wide application prospect because of its higher performance than that of traditional Pd-based catalysts.In this dissertation, the Au catalysts prepared by different methods were applied for the oxidative esterification of aldehydes. The effects of the supports and the interaction between Au and support were investigated. The main works are shown as follows:(1) The Al2O3, TiO2, ZnO, CeO2 and ZrO2 supported Au catalysts were prepared by deposition precipitation method. The effects of preparation temperature, aging time, initial concentration and reduction step were investigated and the optimum conditions were obtained. These Au catalysts were applied for the oxidative esterification of isobutyraldehyde and the one loaded on self-made CeO2 presented the best performance.(2) The Ce-Zr solid solutions were synthesized using (NH4)2Ce(NO3)6 as precursor, while the mixed oxides were obtained by Ce(NO3)3 precursor. The solid solutions exhibited higher BET surface area, and larger amount of H2 consumption, stronger acidity and basicity than the mixed oxides at the same Ce/Zr mole ratio. Moreover, the Au catalysts supported on the solid solutions exhibited higher catalytic performance than that of catalysts loaded on mixed oxides. Plausible reaction mechanism for oxidative esterification of aldehydes on Ce-Zr solid solution supported Au nanoparticles was proposed. The effects of different substrates and recycling times were also investigated. (3) The Ce0.6Zr0.4O2 solid solution was modified by MgO, (NH4)2SO4 or MoO3 respectively through impregnation method and then the corresponding Au catalysts were prepared by deposition precipitation method. The Au particles of these catalysts presented similar size distribution and valent state. All the modified Au/Ce0.6Zr0.4O2 catalysts demonstrated obvious enhancement in the catalytic activities and the MgO promoted catalyst presented the best performance. According to the correlation of catalytic activity and characteristic data, we speculated that the MgO increased the amount of active oxygen species caused by the enhancement of the interaction between Au and support leading to the improvement of catalytic activity.(4) Three ceria nanoshapes (particles, cubes and rods) were synthesized by hydrothermal method and the CeO2 rods supported Au catalyst presented the best activity for oxidative esterification of MAL (methacrolein). Further, the Ce-M (Zr, La and Fe) solid solution nanorods were prepared by adjusting the concentration of NaOH during the hydrothermal process and their Au catalysts were applied to oxidative esterification of MAL. It is found that the introduction of La decreased the activity and the addition of Zr improved the activity slightly, while the catalyst modified by Fe promoted the performance obviously. The introduction of Fe maybe enhanced the reducibility and basicity of support leading to the improvement of activity according to the H2-TPR and CO2-TPD analysis.(5)The Au/ZnO catalyst was synthesized by colloid deposition method using poly vinyl alcohol (PVA) as protective agent. Three different pre-treatment: heat treatment, hot water washing and water reflux were compared to remove the PVA stabilizer. The catalyst heat treated at 300 oC presented the best activity due to the minimum content of PVA. It is found that the distribution and loading content of Au particles was closely related to the pH of the colloid solution. Further, the catalysts with the similar size of Au particles loaded on different supports were successfully prepared. The Au/ZnO presented the best performance, which may be due to the strong basic surface sites and the strong interaction between Au and ZnO.(6) Two series of Au-Pd bimetal catalysts were synthesized by impregnation-reduction method (WIR) and sol-immobilization (SI) method. Au, Pd and Au-Pd particles prepared by WIR method presented big size due to the agglomeration, while the particles prepared by SI method showed uniform distribution and presented the alloy form confirmed by UV-vis and EDS line scan. The addition of Pd improved the conversion of oxidative esterification of MAL, while the selectivity of MMA retained unchanged.
|李玉超. 负载金催化剂的制备及其催化醛氧化酯化反应的研究[D]. 北京. 中国科学院研究生院,2016.|
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