CO₂和甲醇直接合成碳酸二甲酯的研究

【作者】 张智芳；

【导师】 刘昭铁；

【作者基本信息】 陕西师范大学 ， 应用化学， 2009， 博士

【摘要】 碳酸二甲酯（DMC）作为20世纪以来发展最快的绿色化学品之一,近年来受到了广泛的关注。其主要用途有:可作为无毒的羰基化试剂和甲基化试剂用于有机合成工业,是集清洁性与安全性于一身的绿色溶剂,还是继甲基叔丁基醚（MTBE）之后提高汽油辛烷值的新型添加剂。主要合成路线有光气法和非光气法。20世纪80年代以来,国内外对DMC的非光气合成进行了大量的研究,在越来越注重环境和关注节约型社会的今天,合成DMC的路线正朝着简单化、无毒化和无污染化的方向发展,CO₂和甲醇直接合成DMC的路线越来越受到了关注。CO₂既是潜在的C₁资源又是一种工业排放的废气,同时还是引起温室效应的主要气体之一。因此,CO₂的减排及其资源化利用已引起人们的广泛关注。由CO₂和甲醇直接合成DMC,无论是从能源的角度还是环境的角度,对社会的可持续发展都具有重要意义。该合成路线的关键问题是CO₂的活化。研究表明,铈锆固溶体Ce_xZr_1-xO₂具有较高的催化活性。为了进一步探索铈锆固溶体的制备方法、结构及其催化直接合成DMC性能之间的关系,本课题主要从以下几个方面进行了研究:1.通过热力学估算,从热力学角度分析了压力和温度的变化对CO₂和甲醇直接合成DMC的平衡转化率的影响,并且探讨在该体系中添加2,2-二甲氧基丙烷（DMP）、二甲氧基甲烷（DMM）、三甲氧基甲烷（TMM）以及三甲氧基乙烷（TME）等脱水剂对反应过程的影响,力求从热力学的角度为选择化学脱水剂和反应条件提供指导。2.分别以水杨酸、酒石酸、己二酸和柠檬酸等为配位剂,铈和锆的硝酸盐为前躯体,采用配位.分解法制备Ce_0.5Zr_0.5O₂固溶体。通过XRD、Raman、SEM、液氮温度下的氮气吸脱附和化学吸附等方法对固溶体的晶型、孔型结构、比表面积以及氧化还原性能和CO₂的化学吸附性能等进行考察,并结合其催化直接合成DMC的活性,将固溶体的催化性能与其结构的关系进行了关联和解析。结果表明,配位酸的种类和焙烧温度是影响Ce_0.5Zr_0.5O₂的晶形和是否发生晶相分离的关键因素;铈锆固溶体的晶形及其是否晶相分离是影响其催化合成DMC活性的关键因素。当以水杨酸为配位酸并且焙烧到1273 K时,制备的铈锆固溶体为四方晶形,基本上没有相分离,其催化活性最好;当以酒石酸为配位酸且焙烧到1273 K时,铈锆固溶体发生相分离,没有检测到DMC的生成,几乎没有催化直接合成DMC的活性。3.采用配位.分解法制备Ce_xZr_1-xO₂铈锆固溶体。通过XRD、Raman、SEM、液氮温度下的氮气吸脱附和化学吸附等方法对该固溶体的晶型、孔型结构、BET比表面积以及氧化还原性能和CO₂的化学吸附性能等进行考察,并结合其催化直接合成DMC的活性,对催化剂的性能与其结构的关系进行了关联和分析。结果表明,铈锆的物质的量之比和焙烧温度对其晶形影响很大:催化剂的晶形对其催化活性起着关键性的作用。当铈锆的物质的量之比接近1,且焙烧温度为1273 K时,铈锆固溶体为Ce_0.5Zr_0.5O₂四方晶相,其催化性能最好。为了促使反应平衡向右移动,采用在反应体系中添加化学脱水剂的方法来消耗所生成的水。结果表明,在同样的反应条件下,甲醇的转化率由未加TMM的1.8%提高到7.9%;当TMM为50 wt%,在20 MPa、373 K条件下反应34 h,甲醇的转化率达到了10.4%,甲醇的转化率得到显著提高;四方相的铈锆固溶体催化剂以及水的消耗是直接合成DMC的有效途径,并对该反应机理进行解析。4.为了提高催化剂的活性,对铈锆固溶体催化剂进行了改性,并对其催化合成DMC的性能进行探讨。（1）采用在铈锆固溶体中添加钇的方法对铈锆固溶体改性。配位-分解法制备铈锆钇固溶体(Ce_xZr_1-x-0.1Y_0.1O₂)催化剂,通过对其结构的表征,并结合其催化直接合成DMC的活性,对催化剂的性能与其结构的关系进行了关联和分析。结果表明,各金属的物质的量之比和焙烧温度对其孔型、比表面积、孔容以及氧化还原等的影响很大;而影响铈锆钇固溶体催化性能的主要因素是其比表面积和孔容。其中,焙烧1073 K的铈锆钇Ce_0.5Zr_0.4Y_0.1O₂固溶体具有大的比表面积和孔容,其活性较好。然而,其催化活性并没有Ce_0.5Zr_0.5O₂（焙烧1273 K）的催化活性高。（2）采用负载离子液体的方法对铈锆固溶体进行改性。浸渍法制备一系列负载离子液体催化剂[emim]Br/Ce_0.5Zr_0.5O₂,并用于催化直接合成DMC。结果表明,负载[emim]Br的量对催化剂的活性起着关键性的作用。当[emim]Br:Ce_0.5Zr_0.5O₂=1:2时,[emim]Br/Ce_0.5Zr_0.5O₂的活性较高,远远高于[emim]Br或Ce_0.5Zr_0.5O₂。这可能是离子液体和铈锆固溶体的协同作用,促进了催化剂的活性提高。采用负载离子液体的方法改性铈锆固溶体,当[emim]Br:Ce_0.5Zr_0.5O₂=1:2时,其催化性能远远好于Ce_0.5Zr_0.5O₂（焙烧到1273 K时的铈锆固溶体）的催化性能。更多还原

【Abstract】 In recent years,much attention has been paid to dimethyl carbonate（DMC）,because DMC is one of the fastest growing environmentally benign chemical products.DMC has many potential applications:first,its versatility as a reagent and a solvent.It can be used as a chemical intermediate, such as a non-toxic carbonylaring and methylating agent.Second,it is non-toxicity for human health and environment.It can also be used as a potential gasoline fuel additive which has about three times higher in oxygen content than that of methyl tert-butyl ether（MTBE）.Several reaction routes have been reported for DMC synthesis so far.Among the investigated methods,the direct synthesis of DMC from CO₂ and CH₃OH is the more preferred route as it matches the view of the so-called "Sustainable Society" and "Green Chemistry".Moreover,conversion of carbon dioxide to useful industrial compounds has attracted much attention because of its environmentally benign nature （nontoxic,noncorrosive,and nonflammable） and also a greenhouse gase and a major emission product.The direct synthesis of DMC from CO₂ and CH₃OH has been attempted to match the view of the so-called "Sustainable Society" and "Green Chemistry".The activation of CO₂ is the key problem in its conversion process.Some effective catalysts,such as organometallic compounds, carbonate salts and oxides have been reported for the rifled reaction.The Ce_xZr_1-xO₂ solid solutions showed potentially higher catalytic performance.In order to further explore the relationships between the structure and the catalytic activities of Ce_xZr_1-xO₂ solid solutions as catalysts for synthesis of dimethyl carbonate direct from carbon dioxide and methanol,the fellowing work has been done in this dissertation.1.A thermodynamic analysis of the synthesis processes is carried out with the emphasis on the calculations of the effect of the addition of the 1,1,1-trimethoxy methane（TMM）,trimethoxy ethane（TME）,2,2-dimethoxy propane（DMP） and 2,2-dimethoxy methane（DMM） at different temperatures and pressures.Optimum reaction conditions for DMC synthesis were investigated in this work,which will provide the thermodynamic basis for designing DMC synthesis process and developing catalysts for this process.2.The Ce_0.5Zr_0.5O₂ solid solutions were prepared by a complex-decomposition method using salicylic acid,tartaric acid,adipic acid,and citric acid as complexants and cerium and zirconium nitrides as precursors,respectively.These solid solutions can be used in dimethyl carbonate（DMC） synthesis from methanol and CO₂.Combined with the characterization results by means of X-ray diffraction（XRD）,Raman,SEM,N₂ adsorption-desorption isotherms,H₂-TPR and CO₂ pulse chemisorption,we discussed the relationships between the catalytic activities and catalytic properties. Results indicated that crystal structure of Ce_0.5Zr_0.5O₂ was the key factor in determining the catalytic activity for DMC synthesis.The solid solution prepared by salicylic acid as complexants and calcined at 1273 K showed a good catalytic activity for the selective DMC synthesis from CH₃OH and CO₂.Moreover,the catalytic activity of the Ce_0.5Zr_0.5O₂ was significantly decreased when phase separation as a result of the enrichment either of Ce or Zr was occurred.3.The solid solutions of Ce_xZr_1-xO₂ with various x values were synthesized by complex-decomposition and used as catalysts for the titled reaction.Combined with the characterization results by means of X-ray diffraction（XRD）,Raman,SEM,mercury porosimetry, N₂ adsorption-desorption isotherms,H₂-TPR and CO₂ pulse chemisorption,we discussed the relationships between the catalytic activities and catalytic properties.Results indicated that Ce_0.5Zr_0.5O₂ with tetragonal phase exhibited very effective in methanol conversion.In order to increase the methanol conversion and the selectivity to DMC,the method of removing H₂O was used with an additive of TMM.Results indicated that,the addition of TMM was very effective for the increase of the methanol conversion.The methanol conversion increased from 1.8%to 7.9%at the same reaction conditions over Ce_0.5Zr_0.5O₂.The methanol conversion reached about 10.4%using TMM as dehydrating reagent at 423 K and 12 MPa after 34 h reaction.The combination of this selective bimodal porous catalyst with an effective H₂O removal system can provide a novel process of selective DMC production starting from CO₂ with a dramatic efficiency. The elucidation of the mechanism of the activity enhancement enables catalyst design for much higher performance.4.In order to enhance the catalytic activity,the methods of modifying the solid solutions of Ce_xZr_1-xO₂ were used.These solid solutions modified can be used in dimethyl carbonate（DMC） synthesis from methanol and CO₂.（1） The method of doping Y on Ce_xZr_1-xO₂ was used.The solid solutions of Ce_xZr_1-x-0.1Y_0.1O₂ with various x values were prepared by complex-decomposition method,and applied for the direct synthesis of dimethyl carbonate in a batch reactor under 20 MPa.The performance of Ce_xZr_1-x-0.1Y_0.1O₂ for the titled reaction strongly depends on x values and calcination temperatures, and the Ce_0.5Zr_0.4Y_0.1O₂ calcined at 1073 K showed the optimal performance.However,the catalytic activity of Ce_0.5Zr_0.4Y_0.1O₂ calcined at 1073 K was lower than the Ce_0.5Zr_0.5O₂ solid solution calcined at 1273 K. （2） The method of loading ionic liquid（ILs） on the solid solutions was used.The [emim]Br/Ce_0.5Zr_0.5O₂ catalysts were prepared by impregnation method and applied for the direct synthesis of dimethyl carbonate.The results indicated that the[emim]Br contents of [emim]Br/Ce_0.5Zr_0.5O₂ catalysts influenced the DMC formation.Moreover,the catalytic performance was higher for the solid solution loaded ILs with the[emim]Br:Ce_0.5Zr_0.5O₂=1:2 as catalyst.It may be that the synergistic effect of ILs and the solid solution improves the catalytic performance for the titled reaction.It is very interesting that the catalytic activity of the ionic liquid loaded solid solution catalyst is higher than the Ce_0.5Zr_0.5O₂ solid solution calcined at 1273 K.更多还原