摘要
首次较系统地对尿素与甲醇合成碳酸二甲酯反应体系进行了热力学分析。结果表明：主反应分两步进行，即尿素先与甲醇反应生成氨基甲酸甲酯，后者再与甲醇反应生成碳酸二甲酯。其中，第二步反应在337K以上才可能实现，而且平衡常数较小。主要副反应是氨基甲酸甲酯的甲基化反应。提高反应温度对主、副反应都有利。反应过程中及时移出产物碳酸二甲酯既可以促进氨基甲酸甲酯向碳酸二甲酯转化，又可以有效地抑制副反应发生。
采用沉淀法和溶胶—凝胶法，并结合超临界干燥技术制备出ZnO、PbO、La_2O_3和ZnO-PbO、ZnO-La_2O_3纳米金属氧化物催化剂，借助XRD、TEM、BET等手段对催化剂的晶相结构、表面形貌和比表面积进行表征，在间歇反应装置上评价了各催化剂的催化活性。研究发现，沉淀法和溶胶—凝胶法均制备出纳米ZnO粒子；溶胶—凝胶法制备出一维纳米尺度的PbO和La_2O_3粒子；沉淀法和溶胶—凝胶法都成功得到纳米ZnO-PbO粒子。研究还发现，PbO会因晶相变化和还原反应而失活，而ZnO中添加PbO或La_2O_3后，催化活性都有所提高。
首次开发出用于尿素与甲醇反应制备碳酸二甲酯用Zn和Pb金属催化剂，并对其负载化进行了深入研究。采用TPR技术确定出制备负载型金属催化剂的还原温度，研究了载体种类、负载量、还原温度等对催化剂活性的影响，从而确定了负载型金属催化剂的最佳制备方法；对负载型Zn-Pb双金属催化剂的催化效果进行初步探索；最后利用XRD和XPS对催化剂样品进行了表征，对载体与活性组分之间的相互作用进行了理论分析。
自行设计并建立了高压连续反应装置，对其操作性能进行了初步考核。以ZnO为催化剂优化了连续反应条件，并对不同因素对反应过程的影响进行了深入分析。相同反应条件下评价了不同催化剂的催化效果。结果表明，采用连续操作方式，碳酸二甲酯收率大幅提高，最高达到46.5％。
The reaction of urea and methanol catalyzed heterogeneously for synthesis of dimethyl carbonate (DMC) has been systematically studied in this thesis.
The reaction system of synthesizing DMC from urea and metlianol has been analyzed thermodynamically. The conclusion is as follows: firstly, the main reaction proceeds in two steps: urea reacts with methanol to give methyl carbomate (MC), and then MC is converted to DMC by reaction with methanol. The second step can not happen until the reaction temperature rises above 337K, and the equilibrium constant of this step is very small. Secondly, the most important side reaction is methylation of MC with DMC to N-methyl methyl carbomate. Thirdly, elevating reaction temperature is in favor of both the main reaction and the side reaction. On the other hand, removing DMC out of the reaction zone in time can not only promote the reaction of MC to DMC, but also suppress the side reaction effectively.
ZnO, PbO and LaiOs as well as ZnO-PbO, ZnO-LaiOa nano-meter metal oxide catalysts have been prepared by precipitation and sol-gel process in combination with supercritical drying technique. The crystal structure, surface morphology and specific surface area of the catalysts have been characterized by means of XRD, TEM and BET, and the catalytic activities have been evaluated in batch reactor system. It can be found that nm-ZnO particles can be prepared through both precipitation and sol-gel process. PbO particles can be made in one-dimensional nano-meter range through sol-gel process. Also, nm-I^Os particles can be obtained using sol-gel process in combination with supercritical drying technique. Nm-ZnO-PbO binary metal oxide catalysts are available from both methods. As far as PbO catalyst is concerned, it can be deactivated during the reaction process due to transition of crystal structure and reduction reaction. The catalytic activity of ZnO can be improved after PbO or I^Os is added.
For the first time, Zn and Pb have been developed as catalysts in the reaction for DMC synthesis from urea and methanol, and their supporting methods have been studied emphatically. After the reduction temperature has been determined by TPR technique, the effects of support and amount of supported metal atoms on the activities of catalysts have been evaluated. And the catalytic properties of Zn-Pb binary metal catalyst have also been investigated. Finally, on the basis of characterization by XRD arid XPS, the interaction of support with the active component has been analyzed theoretically.
In order to improve DMC yield, a set of apparatus for continuously producing DMC has been built, and its operational suitability test has also been performed. On this basis, the operation conditions over ZnO catalyst have been optimized, and each influential factor has been discussed in the mean time. Under the same conditions, the catalytic properties of different
ii
catalysts have been evaluated. The results show that DMC yield can be greatly improved under continuous operation conditions; the highest DMC yield reaches 46.5%.
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