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甲基丙烯醛（Methacrolein，MAL）选择性氧化制甲基丙烯酸（Methacrylic acid，MAA）是甲基丙烯酸甲酯（Methyl Methacrylate，MMA）清洁生产工艺的关键步骤，其核心技术是开发优异性能的以磷钼酸和磷钼钒酸为基础的Keggin型杂多酸催化剂。铵根基团（NH4+）是磷钼钒酸类杂多酸催化剂的一个较为特殊的抗衡阳离子，对于催化剂的形貌、结构和性能均有重要的影响，能有效提高杂多酸催化剂在MAL氧化为MAA反应中的催化活性。但是有关杂多酸中的铵根基团的引入方式、对杂多酸物理化学性质及催化活性的作用机制尚没有相对系统的研究。由此本文分别从引入无机铵、含氮有机分子和离子液体等三个方面探讨了不同前体引入的含氮化合物继而转化的铵根基团对杂多酸催化剂的晶相结构、表面形貌、酸性、氧化还原性及其催化活性的作用机制。具体研究内容如下：（1）制备了系列催化剂 Cs(NH4)xH3-xPMo11VO40（x = 0.5、1.0、1.5、2.0、2.5和3.0），考察了铵根离子的含量（x）对催化剂的结构和性能的影响规律，确定了性能最好的催化剂为Cs(NH4)1.5H1.5PAV，通过连续评价近100 h 后催化剂仍然保持较高的活性（MAL转化率和MAA选择性分别达到80%和90%）。研究揭示了一定量的NH4+基团对催化剂的改性作用主要体现在：调控了催化剂的晶相结构、形貌和比表面积；改变了催化剂表面的酸量；提升了催化剂表面的氧化还原活性位。（2）分别以十六烷基三甲基溴化铵（CTAB）和甲壳素（chitin）为改性前体，制备了CTAB-CsH3PMo11VO40（CTA-CsPAV）和Chitin-CsH3PMo11VO40（Ch-CsPAV）杂化催化剂，获得了形貌均匀、高选择性的纳米微球颗粒催化剂。通过连续评价实验Ch-CsPAV表现出良好的稳定性（MAL转化率和MAA选择性连续反应100 h时分别达到80%和90%以上），结合原位红外技术阐明了MAL气相氧化生成MAA的反应机制。研究揭示了含氮有机物前体对于杂多酸催化剂的改性机制主要表现在：含氮有机物整齐有序排列的分子结构能有效调控杂化催化剂的晶相结构和表面形貌；季铵盐类的CTAB与具有一定碱性的甲壳素分子均能有效调控杂化催化剂的酸性；焙烧过程形成的铵根基团能有效调控杂化催化剂的氧化还原性。（3）初步探究了咪唑类离子液体对磷钼钒酸催化剂改性机制，考察了离子液体中不同阴离子（NO3-、AC-、BF4-）对磷钼钒酸催化剂的形貌、晶相结构、氧化还原性和催化活性的影响，获得了具有高选择性（98%）的杂化催化剂EMIMAC-HPAV。揭示了EMIMAC-HPAV表现高选择性的原因是由于[AC]-类离子液体体系的强的氢键碱性所致。提出了[AC]-类咪唑离子液体在杂化催化剂制备中的作用机制：咪唑阳离子与磷钼钒酸阴离子之间的氢键诱导自组装作用能有效调控杂化催化剂的微观表面形貌；焙烧过程中离子液体阳离子与磷钼钒酸阴离子之间的电荷转换，是铵根基团的形成原因；形成的铵根基团和[EMIM][AC]离子液体中强的氢键碱性增强了杂化催化剂的氧化还原能力。;Selective oxidation of methacrolein (MAL) to methacrylic acid (MAA) is one of the key steps leading to methyl methacrylate (MMA) in industry. Various catalyst systems have been developed, wherein the Keggin-type heteropoly catalysts composed of P, Mo, and V behave relatively well. It has been demonstrated that the type of counter-ions has an important impact on the morphology, structure, and properties of catalysts, thereby enhancing the reaction activity. In this regard, introducing NH4+ into the Keggin-type heteropolyacid catalyst has a positive effect on the catalytic oxidation efficiency. However, systematic studies on the NH4+-introduction modes and function mechanism of heteropolyacid in the catalytic performance of heteropolyacid are highly demanded. Our study was, therefore, devoted to find the relationship between the introduction modes of ammonium ion and the structure of heteropoly materials, such as crystal structure，surface morphology, acidity and redox property, thereby providing efficient strategies of modification. The catalytic behavior of the newly prepared materials in the oxidative conversion of MAL to MAA is also investigated. The contents could be categorized into three sections as the following:(1) A series of Cs(NH4)xH3-xPMo11VO40 with different x value were synthesized. The influence of loading amount of ammonium (x) on the structure of heteropoly materials was investigated. In addition, their catalytic performance in the selective oxidation of MAL to MAA was evaluated. Mechanistic studies suggest it helps to adjust not only the crystal structure, surface area and morphology of the heteropoly materials but also the amount of acid sites and redox properties on its surface by varying the x value. As a result, Cs(NH4)1.5H1.5PMo11VO40 (x = 1.5) provided the optimum reaction efficiency. A high conversion of MAL (80%) and selectivity to MAA (90%) can still be reached even after reaction for 100 h.(2) Two hybrid heteropoly catalysts, CTAB-CsH3PMo11VO40(CTA-CsPAV) and Chitin-CsH3PMo11VO40 (Ch-CsPAV), were prepared using cetyltimethyl ammonium bromide (CTAB) and chitin functioning as the modified precursor, respectively. Each of them has an uniform and well-defined nanoparticle structure and was successfully employed as the catalyst in the oxidative transformation of MAL to MAA. Specifically, Ch-CsPAV shows good stability during the long-term stability test, wherein a 80% conversion of MAL and a 90% selectivity to MAA was maintained for100 h. Mechanistic studies demonstrated that the formation of ammonium group upon calcination is promising no matter what kind of organic nitrogen precursor was introduced, which can thereby modify the redox property of heteropolyacid hybrids. The improved crystal structure, surficial morphology and catalytic performance were attributed to the linear sequential nitrogen atom in the organic molecules launched. In this regard, both CTAB and chitin can provide the optimized surface acidity. (3) A series of heteropolyacid hybrids of [C8H14N2O2(EMIM)-H4PMo11VO40] were synthesized using imidazole-bsed ionic liquids bearing different anion (NO3-、AC-、BF4-) as the modifying agent. The influence of ionic liquid’s structure on the morphology, crystal structure, redox property and catalytic performance of the resulted hybrids was investigated. The optimized structure of EMIMAC-HPAV was obatined, which provided a high catalytic selectivity (98%) in the aerobic oxidation of MAL to MAA. As compared to [EMIM][NO3] and [EMIM][BF4], [EMIM][AC] shows relatively strong H-bond basicity, which is considered as the main reason for the high activity upon hybridization with HPAV. The positive effects of [EMIM][AC] on the HPAV hybrids were categized as the following: (I) the H-bonding interaction between EMIM+ and PAV- provided a self-assembling structure of IL-HPAV precursor, which, thereby, leading to an uniform morphology of hybrids upon calcination; (II) the formation of ammonium group during the calcination was attributed to the potential charge transfer between EMIM+ and PAV-; (III) The enhanced redox property of EMIMAC-HPAV hybrids was rationized by the formation of NH4+ group and the high H-bond basicity of [EMIM][AC].
|曹云丽. 甲基丙烯醛氧化为甲基丙烯酸的杂多酸催化剂的铵基改性研究[D]. 中国科学院大学,2018.|
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