【作者】 郭晓俊；

【导师】 陈标华；

【作者基本信息】 北京化工大学 ， 化学工艺， 2008， 博士

【摘要】 由异丁烯合成甲基丙烯酸甲酯（MMA）须经两步氧化和一步酯化过程,工艺路线长、管线易堵塞;将第二步氧化（甲基丙烯醛（MAL）氧化成甲基丙烯酸（MAA））和酯化（MAA与甲醇酯化为MMA）进行耦合,一步氧化酯化得到MMA是对C₄工艺合成MMA的改进。目前研究的一步氧化酯化工艺多采用贵金属Pd作为催化剂的活性组分,存在催化剂活性低、价格高以及反应时间长、设备投资大、甲醇回收费用高等问题。鉴于杂多化合物（HPCs）能兼作氧化和酯化催化剂,并能通过改变组成对其氧化活性和酸性进行调节,本研究通过改变HPCs制备方法及其组成,试将其用于一步氧化酯化制MMA的反应。通过将该耦合反应拆分为氧化、酯化两步和最终耦合一起的方法,对催化剂进行评价和筛选,优选出较好的一步氧化酯化制MMA催化剂,并对一步氧化酯化的反应机理进行了探讨。首先,合成了3种纯杂多酸和4种杂多酸盐,表征了其结构特征和酸性,考察了其催化氧化MAL制MAA的反应性能。结果表明,H₄PMo₁₁VO₄₀具有较高的分解温度和酸性,表现出较好的氧化催化性能,是合成较好的氧化催化剂的基础底物。考察了喹啉沉淀法和共沉淀法制备HPCs对其结构和催化氧化MAL活性的影响。在HPCs催化剂化学组成相同的情况下,与共沉淀法相比,经过喹啉沉淀法制备的催化剂比表面显著增加,具有较高的MAL转化率。反荷离子对HPCs的催化性能存在重要影响:Cs⁺的存在明显增大了HPCs的比表面,Cu²⁺促进了晶格氧与游离氧的置换,提高了HPCs催化剂的催化氧化活性;而Cu²⁺和Cs⁺的协同作用显著地提高了HPCs的结构稳定性。碱金属反荷离子HPCs,其比表面和结构稳定性随碱金属离子半径的增加,催化活性也随之逐渐增强。镧系金属离子HPCs,其结构稳定性和比表面较小,而酸量较高,其性质类似于纯杂多酸。总的来看,以H₄PMo₁₁VO₄₀为母体酸,Cs⁺为主要反荷离子,含有少量Cu²⁺和As³⁺,并采用喹啉沉淀法制备的HPCs具有较高的结构稳定性和较好的催化性能。在与氧化反应相似的条件下,考察了碱金属离子和镧系金属离子HPCs对MAL氧化、MAA酯化,以及MAL一步氧化酯化制MMA反应的催化性能。其中,以Cs⁺为主要反荷离子的HPCs表现出较优的催化性能。Cs含量对HPCs结构特征和一步氧化酯化催化性能也有很大的影响。随Cs含量的增加,HPCs的比表面而增加,而还原能力和酸性位逐渐下降,结构稳定性增强,MAL一步氧化酯化中MAL转化率先增加后降低,MMA和MAA的总选择性也呈现出同样的规律。总的来看,影响MAL催化活性的主要因素是比表面,酯化反应的主要影响因素是酸量;酸量大有利于提高一步氧化酯化反应中MMA的选择性。为了研究酸性对反应机理的影响,通过离子交换法（Ⅰ）和机械混合法（Ⅱ）在CsCuPMo₁₁VO₄₀中加入一定量的H₃PW₁₂O₄₀。两种制备方法所制得具有较强酸性的复合HPCs结构特征和催化性能没有明显的差别,复合催化剂具有较高的MMA选择性,但MAL转化率较CsCuPMo₁₁VO₄₀低,MMA和MAA的总选择性更低。此外,复合催化剂的寿命很短。说明单纯提高酸性并不能改善反应收率。出现这种现象的原因是:酸性增强,一方面有利于吸附更多的甲醇,利于酯化反应的进行;另一方面,MAL在催化剂上的吸附作用力过强,一段时间后,在催化剂表面可能结焦。MAL的吸附作用力比甲醇、MAA和MMA强;一步氧化酯化反应的实现,说明MAL与甲醇的吸附可能存在两种活性位。然而,MAL与甲醇的吸附顺序对反应存在影响,以此推断也存在一些活性位,对MAL和甲醇均有吸附作用,两者存在一定程度的竞争吸附。考察了CsCuPMo₁₁VO₄₀催化剂上MAL氧化和酯化反应条件对催化性能的影响,进而考察了MAL一步氧化酯化适宜的反应条件。O₂/MAL和水蒸气分压对MAL一步氧化酯化反应的影响的考察结果显示,随O₂的增加,MAL的转化率增加的同时,引起了深度氧化的增加;而MAL增加提高了MMA和MAA的总选择性。水蒸气的存在引起了MAA选择性的增加,同时抑制了CO和CO₂的生成。然而,过量的水蒸气造成了MMA选择性的下降。总的来看,CsCuPMo₁₁VO₄₀催化剂具有较好的稳定性。CsCuPMo₁₁VO₄₀催化剂的失活为结构性失活。失活后比表面和孔体积均减小。分解产物中可以确认的结构有V₂O₅和α-MoO₃等氧化物种。最后,选择了H_2.25Cs_0.7Cu_0.25As_0.1PMo₁₁VO₄₀(PMo₁₁VO₄₀)作为活性物质进行了固载化研究。结果表明,在SiO₂表面的临界负载量为40%左右。与SiO₂载体相比,（C₆H₆N₆O₆）₆载体上具有较好的催化性能:氧化反应的MAL转化率达到了83.5%,MAA选择性达到了92.7%;一步氧化酯化反应得到94.1%的MAL转化率,MMA选择性4.1%,MAA的选择性79.2%的较好结果。更多还原

【Abstract】 Methylmethacrylate （MMA） has been prepared from isobutene by two-stage oxidation and one-stage esterification, which was a complex process and easy to be jammed by high melting point by-products. The modified process that prepared MAL from isobutene, and then produced MMA from MAL by one-step oxidation-esterification reaction have been developed. However, the new processes that needed a rather long operation time used Pd as active material of catalysts. Furthermore, its operating cost was also kept relatively expensive for recycling the excessive methanol, which was fed into reactor to promote the conversion of MAL. Heteropolycompounds （HPCs） are known to be very effective catalysts for both acid-catalyzed and redox reactions. Moreover, the properties of oxidation and acid catalysis of HPCs can be modified by changing their preparation methods or compositions. In this dissertation, HPCs have been modified by choosing proper preparation methods or compositions, so that some HPCs can be used as catalysts in one-step synthesis of MMA from MAL. The project was performed by both-step reactions, MAL oxidation and MAA esterification. HPCs were evaluated and chosen by MAL oxidation, then some of them were used in one-step oxidation-esterification. And the reaction mechanism of one-step oxidation-esterification had been investigated.Firstly, Three kinds of pure heteropoly acid and four kinds of pure heteropoly salts containing P, Mo and V were prepared. Their structure and acid properties were studied by different characterizations methods. Catalytic performances of the HPCs were investigated in MAL oxidation reaction. Among the HPCs, H₄PMo₁₁VO₄₀ showed a higher decomposition temperature and better catalytic activity and it became a preferential substrate of HPCs catalysts. Effects of preparation method on the structural characters properties and catalytic performances of HPCs were investigated. Compared to coprecipitation method, HPCs catalysts with the same chemical composition prepare by quinline sedimentation method had more surface area and stronger catalytic activities of MAL oxidation. Moreover, counter-ions affected heavily catalytic performances: Cu²⁺ increased the redox of the HPCs; Cs⁺ improved the surface area; and synergetic effect of Cs⁺ and Cu²⁺ increases the structural stability. Acid acid properties and structural stabilities of HPCs with Cs⁺, Rb⁺ and K⁺ as counter-ions increased with the increasing of their ion radius. And HPCs with La³⁺ and Ce⁴⁺ as counter ions had lower structural stability and less surface area. Their properties were similar to pure heteropoly acid. Anyhow, H₄PMo₁₁VO₄₀ as the parent acid, HPCs with Cs⁺ as main counter-ions, containing less Cu²⁺ and As³⁺, prepared by the quinline sedimentation method, showed better catalysis.Under the similar reactive conditions of MAL oxidation, catalytic behaviors of HPCs with alkali metals （Cs⁺, Rb⁺ and K⁺） and lanthanide (La³⁺ and Ce⁴⁺) were investigated in esterification and MAL oxidation-esterification. HPCs with Cs⁺ showed good catalytic performances. Further, effects of cesium content on the properties and catalytic performances of HPCs were studied. With the increasing of Cs content, surface area and structural stabilities of the HPCs increased, while the acidity and reducibility decreased. On the other hand, MAL conversions increased in the beginning and then decreased, and the total selectivity of MMA and MAA showed a similar trend. Anyhow, effects on activity of MAL oxidation were surface area. The acidity of HPCs affected MAA selectivity in some degree; the acidity was main factor for esterification of MAA. Furthermore, effects of acid amount on selectivity of MMA were obvious. The more acid amount corresponded to the more MMA selectivity.Certain H₃PW₁₂O₄₀ was added into CsCuPMo₁₁VO₄₀ to improve its acidity by the ion-exchange method or mechanical blend method in order to study effects of acidity on reaction mechanism. Differences were not observed in structural properties and catalysis of the composite HPCs. The more MMA selectivity was observed on the composite HPCs catalysts; however, MAL conversion and the total selectivity of MMA and MAA are rather lower. Especially, life of the composite HPCs catalysts was rather brief. As a result, simplex improvement of acid properties can not promoted the yield of reaction. Reasons may exist that the intense acidity help to adsorb more methanol, so that promote the esterification; on the other hand, excessively intense MAL adsorption caused coking for a while. There may be both active sites corresponding MAL and methanol in the HPCs respectively. Moreover, the competitive adsorption can be concluded in some active sites of the HPCs that adsorbed MAL and methanol.The reaction conditions of MAL oxidation, MAA esterification and MAL oxidation-esterification were studied over CsCuPMo₁₁VO₄₀. And effects of O₂/MAL and steam on MAL oxidation-esterification were evaluated. The results showed MAL conversion increased with increasing of O₂ partial pressure, at the same time, increasing of O₂ partial pressure leaded to improvement of deep oxidation. On the contrary, increasing of MAL improved the total selectivity of MMA and MAA. Steam help to keep a higher MAA selectivity and restrained formation of CO and CO₂. However, the excess steam leaded to decreasing of MMA selectivity. Generally, CsCuPMo₁₁VO₄₀ had better stability than else the HPCs. However, its deactivation was attributed to structural inactivation. Surface area and pore volume of the deactivated HPCs decreased. Furthermore, it can be identified that there were two kinds of structures, i.e., V₂O₅ andα-MoO₃, in the decomposed products.Studies of immobilized H_2.25Cs_0.7Cu_0.25As_0.1PMo₁₁VO₄₀ (PMo₁₁VO₄₀)as active composition were carried out in MAL oxidation and one-step synthesis MMA from MAL. The results showed the critical load amount of was about 40 % on SiO₂ carrier. Results of MAL catalytic oxidation showed MAL conversion was 83.5% and selectivity of MAA was 92.7% on PMo₁₁VO₄₀/（C₆H₆N₄O₂）₆ in certain reactive conditions. On the same catalyst, 4.1% MMA selectivity and 79.2% MAA selectivity with 94.1 MAL conversion can be obtained in one-step synthesis MMA from MAL.更多还原