【作者】 计扬；

【导师】 肖文德；

【作者基本信息】 华东理工大学 ， 化学工程， 2010， 博士

【摘要】 本文主要研究了CO催化偶联合成草酸二甲酯(DMO)过程中的CO偶联反应机理、CO偶联反应本征动力学、蛋壳型Pd/α-Al2O3催化剂和亚硝酸甲酯(MN)再生反应过程。CO催化偶联制备DMO是合成气经DMO制乙二醇(EG)过程的第一步。该过程主要包括两个部分：一、CO与MN反应生成DMO与NO；二、NO与甲醇、O2反应实现MN的再生。NO和MN在两个反应过程之间循环。通过研究本论文主要有如下结论：第一、分别研究了反应温度、原料气配比CO/MN和空速等操作条件对CO偶联反应的影响。随着反应温度的提高CO偶联反应速率增加明显。当反应操作条件为CO/MN=2,空速为3000h-1时,反应温度由100℃增加到140℃MN的转化率由60%增加到95%。但过高的反应温度会导致MN热分解为甲醇、甲醛和NO。为抑制MN的分解反应,反应温度应该低于其分解温度135℃。当其他工艺条件一定的情况下,原料气配比CO/MN存在最优值,此时DMO的时空收率最大。CO/MN大于最优值将导致CO过度占据活性中心,使DMO生成反应速率下降。但CO/MN的最优值随工艺条件的改变而改变；特别是空速对其影响最大。最佳CO/MN值随空速的增加而减小。最优CO/MN的值一般在1-2之间,CO/MN低于1时大量MN在催化剂表面分解为甲醇和甲酸甲酯(MF)。空速对CO偶联反应有很大影响。在不同的空速条件下,CO偶联反应过程中发生的副反应、产物选择性和最佳CO/MN值也不同。当CO偶联反应在空速较低或受外扩散影响较大的情况下,除发生生成DMO和DMC的反应外,还会发生MN催化分解生成MF和甲醇的反应。此时副产物MF和甲醇的选择性均较高。而当反应处于高空速或受外扩散影响较小的情况时,反应产物只有DMO和副产物DMC而几乎没有MF和甲醇。从提高DMO选择性的角度看,适当提高空速利于增加DMO的时空收率。第二、论文探讨了CO偶联反应制DMO的反应机理。通过对原料气CO和MN进入FTIR原位反应池的顺序进行调变,进行了吸附和反应的非定态响应的研究。实验结果证明,在CO偶联反应中主要存在三个过渡态中间体。第一个过渡态中间体是MN在Pd上吸附形成的CH3O-Pd-NO;第二个过渡态中间体是CH3O-Pd-NO与桥式吸附态的CO反应生成的CH3OCO-Pd-NO;第三个过渡态中间体是两分子的CH3OCO-Pd-NO发生偶联生成的双MF钯络合中间体(Pd(CH3OCO)2);最后由Pd(CH3OCO)2得到DMO。主要副产物DMC的生成来自于CH3O-Pd-NO与CH3OCO-Pd-NO之间的反应。在进一步的研究中还发现：只有桥式吸附态的CO参与CO偶联反应；而线式吸附态的CO虽不参与反应但会占据催化剂表面活性中心。原料气中CO分压的上升将导致催化剂表面线式吸附CO增加；而大量线式吸附CO占据催化剂表面活性中心最终引起反应速率的下降。由此,本文提出的CO偶联反应制DMO的反应机理如下图所示：第三、论文研究表明,原料气中的H2和NO对CO偶联反应有较大负面影响。实验表明,原料气中H2或NO含量的上升均会导致催化剂的活性迅速下降,这是由于H2或NO在催化剂表面与MN和CO发生竞争吸附占据活性中心引起的。但这是一个可逆的过程。在不同的空速条件下H2对CO偶联反应的影响程度也不相同。当反应在空速较低或受外扩散影响较大的情况下,反应产物主要是DMO、DMC和甲醇。而当反应发生在空速较高或受外扩散影响较小的情况时,反应的主要产物是DMO、DMC和MF。其原因在于,空速的增加即意味着气体线速度的增加,从而改变了气体组分的气相传质系数。当空速较小时H2由气相主体向活性中心表面的扩散速率大于CO的扩散速率；此时催化剂表面H2主要与MN的吸附物种CH3O-Pd-NO反应生成甲醇。随空速的增加CO由气相主体向活性中心表面的扩散速率也逐渐增加,催化剂表面吸附态CO增加,CH3O-Pd-NO首先与吸附态CO反应生成CH3OCO-Pd-NO,随后与H2反应生成副产物MF。第四、通过论文研究还表明,载体孔结构、Pd分散度和活性组分在球形催化剂中的分布是影响蛋壳型Pd/α-Al2O3催化剂活性的主要因素。通过研究发现：1、相比较比表面积和孔容而言,载体孔结构是催化剂活性的主要因素。提高α-Al2O3载体中1～10nm的孔的比例,有利于催化剂活性的提高。而具备双峰孔结构分布的载体最适合于CO偶联反应体系。2、CO偶联反应是一个结构敏感型反应,CO偶联反应速率随Pd分散度的下降而上升。3、对蛋壳型Pd/α-Al2O3催化剂,减小“蛋壳”即活性层厚度利于催化剂活性提高。最优“蛋壳”厚度为50μm。4、通过比较上述三个因素对催化剂活性的影响,发现活性组分在蛋壳型催化剂内的分布,即所谓的“蛋壳”厚度,是影响催化剂活性的首要关键因素。第五、根据提出的CO偶联反应机理,论文还推导出了CO偶联反应本征动力学模型。动力学实验发现CO偶联反应的控制步骤为催化剂表面的表面反应,本征动力学方程如下：其中：第六、本文还研究了MN再生过程。该过程分为两个部分,首先NO与O2反应生成摩尔比为1：1的NO与NO2的混合物(即N203)；其次N2O3与甲醇反应生成MN。研究发现N2O3与甲醇反应生成MN属于气相反应。动力学实验得到了N203与甲醇气体反应的反应动力学方程,由于N2O3可以认为是摩尔比为1：1的NO与N02的混合物,因此在动力学方程中用NO代替N2O3,提出的动力学方程如下。更多还原

【Abstract】 The process of CO coupling reaction to form dimethyl oxalate (DMO) is the first step of the process of producting ethylene glycol (EG) by using syngas. The process of CO coupling reaction to form DMO is composed of two processes, the catalytic coupling reaction between CO and methyl nitrite (MN) to form DMO and NO, and the reaction between NO,O2, and methanol to regenerate MN. In this dissertation, it was invetstgated the mechanism of CO coupling reaction to form DMO, the intrinsic kinetics of the CO coupling reaction, the egg-shell Pd/α-Al2O3 catalysts applied in CO coupling reaction, and the process of MN regeneration.The main conclusions of this dissertation are as follows:1. The influence of operating conditions on CO coupling reaction was investigated. It was discovered that the reaction temperature had large influence on the reaction. When the reaction temperature was increased from 100℃to 140℃, the conversion of MN increased from 60% to 95% under the operating conditions of CO/MN=2,3000h-1.But the raising of reaction temperature also resulted in MN decomposition to form methanol and formaldehyde. So, the CO coupling reaction should be carried out below the decomposition temperatue of MN,135℃.There was an optimal value of CO/MN with the aim of the maximum value of DMO yield. The optimal value of CO/MN could vary with changing space velocity. Generally, the optimal value of CO/MN is between 1 and 2 generally. The MN catalytic decomposition to form methanol and methyl formate (MF) would be raised if the value of CO/MN was below 1; and if the value of CO/MN was above the optimal value, the rate of DMO formation would be slow, because excess CO could occupy active centers over catalysts.Space velocity has great influence on the side reaction, the selectivties of products, and the optimal value of CO/MN. When the CO coupling reaction occurred under the low space velocity, by-prodcuts of MF and methanol, which were resulted from MN catalytic decomposition, could be detected. But when the reaction occurred under the high space velocity, few MF and methanol can be detected. Moverover, it was discovered that with the increasing of space velocity, the optimal value of CO/MN could decrease to 1.2. The mechanism of CO coupling reaction to form DMO was investigated by using in situ FTIR. It was identified three intermediates, CH3O-Pd-NO, CH3OCO-Pd-NO, and Pd(COOCH3)2 during the reaction. And the third intermediate could generate DMO. Moverover, the by-product DMC is generated from the reaction between CH3O-Pd-NO and CH3OCO-Pd-NO.Futhermover, the role of adsorbed CO in the reaction was investigated in this dissertation. It was discovered that only bridge bonded CO rather than linearly bonded CO participated in the reaction. With the increasing of CO partial pressure, the ratio of linearly bonded CO to bridge bonded CO increased over catalysts. It means the more linearly bonded CO the more active centers were occupied, which resulted in the slow reaction rate. The detailed mechanism is decribed as follow figure.3. H2 and NO could resulte in inactivation of Pd/α-Al2O3 catalysts. However, the inactivation resulted from H2 and NO is reversible; the catalysts could be reactivated by outgasing with N2.Furethermore, H2 could give rise to different influence on CO coupling reaction under different space velocity. When the coupling reaction occurred under the low space velocity, the addition of H2 resulted in the increasing of the selectivity of methanol, and no MF was detected. However, when the CO coupling reaction occurred under the high space velocity, the main byproduct was MF, and no methanol was detected, which is very different from the one under low space velocity. The particular phenomenon can be explained as the difference of diffusion rates from gaseous buck to catalysts surface between H2 and CO. The reaction between CH3O-Pd-NO and H2 to form methanol occurs under low space velocity. With the increasing of space velocity, the diffusion rate of CO was increased, which resulted in the increasing of adsorbed CO over catalysts. With the inceasing of adsorbed CO, CH3O-Pd-NO reacted more easily with adsorbed CO to form CH3OOC-Pd-NO than with H2 to form methanol. The reaction between CH3OOC-Pd-NO and H2 resulted in the formation of MF.4. There are three factors, the pore structure, Pd dispersion over catalysts, and the Pd distribution in egg-shell Pd/α-Al2O3 catalysts, affecting catalytic performance of egg-shell Pd/α-Al2O3 catalysts. It was discovered:(1) The specific surface area and pore volume of the support are not as much important as the pore distribution for the activity of Pd/α-Al2O3, and the double pore structure of the support is necessary for preparing catalysts with high activity. Especially, increasing the proportion of the pore with diameter ranging from 1 nm to 10 nm is positive for high catalytic activity. (2) The CO coupling reaction is a so-called structure sensitive reaction. (3) The shell thickness of the egg-shell Pd/α-Al2O3 catalyst with the best catalytic activity was 50μm. (4) The shell thickness of egg-shell catalysts has the greatest influence on catalytic activity in the three factors above-mentioned.5. The reaction between CO and MN to form DMO was investigated in a continuous flow integral-type fixed-bed reactor with the aim of kinetic modeling studies. Based on the latest work on reaction mechanism, a new kinetic model was proposed. It was discovered that the rate-determining step is the surface reaction over catalysts surface. The kinetic expression is described as follows.6. In present paper, The MN regeneration was composed of two reactions, the NO oxidation to N2O3 and the reaction between N2O3 and methanol to form MN. It was found that the reaction between N2O3 and methanol occurred in gaseouse phase. And the reaction was investigated in a double-stirring reactor with the aim of kinetic modeling studies. A power law rate model was used to represent the formation of MN. The expression is described as follows. PNO is used to represent PN2O3, because the mixture of NO and NO2 (NO:NO2=1:1) is considered as N2O3.更多还原