摘要
碳酸二甲酯(DMC)是一种重要的绿色化工产品，有广泛的应用前景，被誉为21世纪有机合成的“新基块”。目前采用的甲醇氧化羰基化制碳酸二甲酯存在的主要缺陷是：反应过程生成的水引起催化剂的快速失活，同时造成设备的腐蚀。将二甲醚(DME)引入反应体系，利用二甲醚水解及时消耗掉羰基化反应中生成的水，不仅可以缓解水引起的负面效应，同时可以提高DMC的收率。
本论文考察了二甲醚的低温水解性能，开发出低温高活性的DME水解催化剂。并将其与羰基化催化剂进行复合，研究复合型催化剂在共进料体系中的催化性能。结果如下：
1．考察了分子筛的类型、硅铝比及Cu、Pd改性对分子筛催化剂上二甲醚低温水解活性的影响，利用吡啶吸附FT-IR对催化剂的酸性进行表征。结果表明，催化剂的性能主要依赖于分子筛的结构和酸性质。具有超笼结构的MCM-22分子筛显示出较高的DME水解活性。Cu、Pd改性后的MCM-22催化剂，其水解活性得到进一步的提高，150℃时，二甲醚的转化率为14.0％，比改性前提高了3.9％。这有效地解决了水解与羰基化反应的温度匹配问题。
2．采用SEM、XRD、FT-IR、NH_3-TPD和吡啶吸附FT-IR等对自制的分子筛的结构和酸性进行表征。结果表明，所合成的MCM-22分子筛结晶度良好，该分子筛具有两类酸中心，即弱酸中心和强酸中心。
3．将Cu-Pd-TBAB／AC与Cu-Pd／MCM-22进行复合，并将二甲醚引入反应体系中，可以有效提高DMC的时空收率。添加Ce可使催化剂的性能有更进一步的提高，当DME流量为5mL／min时，甲醇的转化率为17.9％，DMC对甲醇的选择性为99.8％，DMC的时空收率为10.18mmol·(g-cat·h)~(-1)。寿命评价结果表明，在氧化羰基化反应体系中引入DME，能够利用二甲醚水解及时消耗掉甲醇氧化羰基化反应中生成的部分水，一定程度上消除甲醇羰基化反应生成的水所导致的催化剂快速失活的问题，提高催化剂的寿命。反应55h后，催化剂的性能才降到最好性能时的一半。
Dimethyl carbonate (DMC), a green chemical intermediate, has considerable potential fororganic synthesis. It is currently produced by the oxidative carbonylation of methanol. Theco-product water in this reaction, however, often induces catalyst deactivation and equipmentcorrosion. In light of the reversible reaction (CH_3OCH_3+H_2O(?)2CH_3OH), the aboveproblems may be diminished somewhat if the dimethyl ether (DME) is introduced as a co-feedgas to the methanol carbonylation reaction. From the thermodynamic point of view, partialconsumption of the produced water via the hydrolysis of DME can also increase the yield ofDMC.
In the present thesis, the hydrolysis performance of dimethyl ether at low temperature wasinvestigated to develop a catalyst for DME hydrolysis with high activity at low temperature.Furthermore, the catalytic performance in the co-feeding system over the bi-functional catalysts,which were prepared by mixing the hydrolysis catalyst with carbonization catalyst, was alsostudied. The results are summarized as follows:
1. The effect of the types of zeolites, silicon aluminum mole ratio, Cu and Pd modificationon the hydrolysis activity of dimethyl ether at low temperature over the zeolite catalysts wasinvestigated. The acidity of the catalysts was characterized by Py-IR. The results show that theperformance of the catalysts was dependent on the structure and acidity of the zeolites. TheMCM-22 zeolite with super-cage structure showed better hydrolysis activity of DME. Thehydrolysis activity of the MCM-22 catalysts modified by Cu and Pd was further enhanced. At150℃, the conversion of dimethyl ether increased by 3.9％to 14.0％when compared with theunmodified MCM-22 catalysts. Based on the above results, the problem of reaction temperaturematching for the coupling DME hydrolysis with methanol oxidative carbonylation was resolvedeffectively.
2. The structure and the acidity of MCM-22 zeolite, which was home-made according to themethod in reference, were characterized by SEM、XRD、FT-IR、NH_3-TPD and Py-IR. The resultsindicated that the MCM-22 zeolite has high crystallization, and there are two kind of acid site, i.e.the strong-acidic site and the weak-acidic site, in this zeolite.
3. The space time yield of DMC over the mixed catalysts of Cu-Pd-TBAB/AC and Cu-Pd/MCM-22 can be improved effectively after the dimethyl ether was introduced to the reactionsystem. The addition of Ce to the catalysts further enhanced the catalytic performance. When theflux of DME is 5mL/min, the conversion of methanol, the selectivity of DMC to methanol andspace time yield of DMC were 17.9％,99.8％and 10.18 mmol·(g-cat·h)~(-1), respectively. Theresults of life-time evaluation indicate that the introduction of DME in the reaction system ofoxidative carbonylation could increase the stability of the catalysts. This is because that thehydrolysis of dimethyl ether can consume some of the water produced in the oxidativecarbonylation of methanol, which, to some extent, resolves the problem of dramatic deactivationof the catalysts due to the water produced in the oxidative carbonylation of methanol. Theconversion of methanol decreased to the half of the highest value after running for 55h.
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