【作者】 王玮；

【导师】 马新宾；

【作者基本信息】 天津大学 ， 化学工艺， 2011， 博士

【摘要】 “绿色化学品”碳酸二甲酯（DMC）可用于替代光气,是一种重要的化工中间体,特别是近年来发现DMC可以作为汽油、柴油的含氧添加剂取代甲基叔丁基醚的使用,使其具有更加广泛的应用前景。二氧化碳的大量排放带来了严重的全球性环境问题,对其进行资源化利用一直都是研究者关注的焦点。从CO₂出发合成DMC工艺的研究,不仅可得到高附加值的化工产品,而且将CO₂作为一种可循环利用的碳资源,对改善生态环境也具有重要意义。本文对氧化铈催化剂催化CO₂和甲醇直接反应合成DMC进行了系统的研究。结合不同焙烧温度制备的CeO₂催化剂的NH₃-TPD、CO₂-TPD测试结果和对应的催化反应活性可知,NH₃-TPD脱附峰在340℃处中强酸位的大量出现有利于反应活性的提高,是反应的主要活性酸位;260℃处出现的中强酸位不利于反应活性的提高;强酸位的出现对反应没有直接影响。与碱性位相比,CeO₂催化剂的酸性位是影响反应活性的主要因素。CeO₂晶体具有立方萤石结构,其主要的特征晶面为（111）、（110）和（100）面。由于晶面的晶格排列方式不同,导致晶面与反应物之间具有不同的相互作用,因此CeO₂的各个特征晶面表现出不同的反应活性。HRTEM及SEAD表征表明,利用水热合成反应,通过调控催化剂制备过程的碱溶液浓度和反应温度,可制备出不同晶面占主体的具有不同形貌的CeO₂催化剂。通过对其反应活性的考察,发现纳米棒具有最高的反应活性,（110）面是反应的主要活性晶面;纳米立方体活性非常低,（100）面对该反应体系基本无活性;纳米粒子所具有的活性位由（111）面提供,但其活性要低于（110）面。由三种晶面活性的对比可知（110）面是DMC合成反应的主要活性晶面。直接焙烧法和水热合成法制备的CeO₂催化剂在循环利用后反应活性均下降,这一方面是由于反应后NH₃-TPD脱附峰340℃处具有活性的中强酸位没有出现,另一方面是由于在反应过程中CeO₂的形貌发生了变化,活性（110）面大量减少暴露,从而导致催化剂的反应活性下降。借助Material Studio分析软件,采用密度泛函理论（DFT）对CeO₂催化CO₂和甲醇直接反应合成DMC的反应历程进行量子化学研究,计算结果表明,甲醇与碳酸甲氧基生成产物DMC是反应的速率控制步骤;与CeO₂（111）面相比,CeO₂（110）面在该步的反应能垒较低,产物DMC更容易生成,进一步确定了（110）面是反应的主要活性晶面。更多还原

【Abstract】 Dimethyl carbonate （DMC） has been widely used as an environmentally benign intermediate in chemical industry for the replacement of phosgene for polycarbonate production and other processes. Over the past years, DMC has been considered as a ideal candidate for replacing methyl tert-butyl ether （MTBE） as the additive to gasoline and the utilization of DMC exhibits excellent prospects. Carbon dioxide, which contributes to the increase in global temperatures and the climate change due to the greenhouse effect, has attracted considerable attention for the chemical utilization. The process for direct synthesis of DMC from CO₂ and methanol not only produces economic chemicals with high value, but also benefits for the environment by using CO₂ as a recycled carbon resource. Direct synthesis of DMC from CO₂ and methanol over cerium oxide was studied systematically in this dissertation.Combining the NH₃/CO₂-TPD results with catalytic performance of CeO₂ calcined at different temperatures, it is suggested that the moderate acid sites with desorption peak of NH₃-TPD at 340℃exhibit an excellent catalytic formation of DMC and are the main active acid sites. The moderate acid sites with desorption peak at 260℃are unfavorable for the synthesis of DMC and the strong acid sites have no direct impacts on the catalytic activity. Compared to the base sites, the acid sites on the surface of CeO₂ are the main factors which affect the catalytic performance.The crystal of CeO₂ has the structure of cubic fluorite with the primary characteristic surfaces of （111）, （110） and （100）. Due to the differences in the packing of atoms in bulk structure and certainly at surface, the interaction forces between the surfaces and the reactants would be different. By controlling the concentration of alkali solution and the reaction temperature, CeO₂ with different morphologies was synthesized through the hydrothermal method. Combining with HRTEM and SEAD, the surfaces of CeO₂ with different morphologies were confirmed. Connected with the catalytic performance, it is suggested that nanorods exhibit the highest reactivity for the synthesis of DMC and （110） is the main active surface. Nanocubes show the lowest activity for the reaction indicating that （100） surfaces have little activity for the synthesis of DMC. The active sites of nanoparticles are provided by （111） surfaces with lower catalytic activity than （110）. Comparing the catalytic reactivity of the type of surfaces, it is showed that （110） is the main active surface for the synthesis of DMC.The catalyst lifetime of CeO₂ prepared by calcination and hydrothermal method was quite short as after the first cycle the activity decreased. It could be due to the disappearance of the active middle strong sites with desorption peak of NH₃-TPD at 340℃. On the other hand, the morphology of CeO₂ was changed during the reaction and the active （110） surfaces have a significant reduction in exposure, leading to the decrease of the catalytic performance.Quantum chemical study of the reaction mechanism for direct synthesis of DMC from CO₂ and methanol over CeO₂ catalyst was first explored using density functional theory （DFT） by Material Studio software. It is demonstrated that the formation of DMC from methanol and methoxycarbonyl is the rate-determining step during the reaction. The reaction energy barrier of the rate-determining step for （110） is lower than that for （111） suggesting that the formation of DMC is more readily compared to the （110） surface. The calculation results support experimental observation that （110） of CeO₂ is the main active surface for direct synthesis of DMC from CO₂ and methanol.更多还原