【作者】 杨海贤；

【导师】 贾立山；

【作者基本信息】 厦门大学 ， 工业催化， 2008， 硕士

【摘要】 二氧化碳催化加氢直接合成二甲醚既可以利用二氧化碳制得有用的化学品,又可以解决二氧化碳的环境污染问题,对于保证经济的高速发展和实现长期可持续发展战略均具有重要意义。CO₂加氢合成二甲醚的双功能催化剂由甲醇合成和甲醇脱水组分复合组成,只有当两种活性组分“协同催化”时,方能充分发挥催化剂的整体功能。本文以铜锰基甲醇合成催化剂和HZSM-5分子筛甲醇脱水催化剂所构成的双功能复合催化剂作为主要研究对象,利用XRD、BET、H₂-TPR、SEM、H₂-TPD、NH₃-TPD、XPS和加压固定床活性评价等多种研究方法系统地考察了下列诸因素对复合催化剂结构和性能的影响:甲醇合成催化剂制备过程中的沉淀温度、PH值、铜/锰比例;甲醇合成催化剂和HZSM-5的复合方法及焙烧温度;对甲醇合成催化剂的助剂改性处理。通过研究主要得出以下结果:在机械法制备的Cu-Mn/HZSM-5催化剂基础上,考察了共沉淀温度、PH值、不同铜锰比和焙烧温度对催化剂催化加氢性能的影响。结果表明:在2.0MPa、250℃、空速2100h^-1、n（H₂）/n（CO₂）=3.2的反应条件下,n（Cu）/n（Mn）=4/3、焙烧温度为350℃,共沉淀温度为70℃和pH值为7-8时,CO₂的加氢活性及二甲醚的选择性均最好。其CO₂转化率为21.83%,二甲醚的收率可达6.68%。助剂SiO₂的加入能显著提高Cu-Mn/HZSM-5催化剂的性能,在2.0MPa、250℃、空速2100h^-1;V（H₂）/V（CO₂）=3.2下,当催化剂中W（SiO₂）=3.49%时,二氧化碳的转化率和二甲醚的收率分别为23.86%和9.33%。XRD和H₂-TPR表明,适量SiO₂的加入,使CuO晶相峰明显减弱,促进表面Cu的分散;铜锰复合氧化物进一步向非晶态转化,阻止Cu的深度还原,从而提高了催化剂的活性。XPS结果表明,活性组分铜以Cu⁺形态存在,支持Cu⁺物种是甲醇合成活性中心的观点。在共沉积法制备的Cu-Mn-Si/HZSM-5上,对温度、压力、空速和氢碳摩尔比等反应操作条件的考察结果显示,当温度260℃、压力2.0MPa、空速2100h^-1,氢碳摩尔比3.2时,催化剂具有较好的活性和选择性,CO₂转化率26.03%,DME的选择性和收率分别为42.42%和11.04%。研究还表明,提高反应温度有利于提高CO₂转化率,但使二甲醚的选择性降低;增大压力和氢碳比有利于提高CO₂转化率和二甲醚的选择性;增大空速会使CO₂转化率和二甲醚选择性均呈现下降趋势。焙烧温度对共沉积法制备的Cu-Mn-Si/HZSM-5催化剂的催化性能影响研究表明:焙烧温度过高、过低均不利于CO₂加氢。当焙烧温度在400℃时,在反应过程会有更多Cu⁺存在。而焙烧温度超过400℃时,峰形发生了质的变化,晶相CuO颗粒变大,催化剂的活性明显下降。CO₂-TPD结果表明,中强CO₂吸附中心与CO₂加氢合成二甲醚有关。在考察的范围内,最适宜的焙烧温度为400℃。为了进一步研究催化剂的表面活性中心物种,在共沉积法基础上采用超声波处理,来考察超声波处理对Cu-Mn-Si/HZSM-5催化剂的催化加氢活性的影响,并与以上两种制备方法相比较。结果表明:经过超声处理后,更有利于Cu⁺和Mn³⁺生成和稳定,促进Cu⁺与Mn³⁺之间相互协同作用,在一定程度上增强了中强的CO₂吸附中心,提高催化剂表面的加氢能力,使催化剂表面酸强度和强酸位中心数的增加,有利于甲醇脱水生成二甲醚反应的进行,从而促进了催化剂整体催化活性的提高。本文认为Cu⁺和Mn³⁺共同构成了Cu-Mn-Si/HZSM-5催化剂的活性中心。更多还原

【Abstract】 Direct synthesis of dimethyl ether （DME） from carbon dioxide by catalytic hydrogenation is of great economical and environmental importance, not only to synthesize the useful chemical products, which will utilize carbon dioxide more efficiently, but also to reduce the greenhouse effect resulting from carbon dioxide.The bifunctional catalysts used for this reaction were composed of the methanol synthesis component and the methanol dehydration component, only when the two components catalyzed synergistically, the bifunctional catalysts could exhibit excellent catalytic performance. In this paper, Cu-Mn-based catalyst prepared by co-precipitation method was selected as methanol synthesis catalyst, and HZSM-5 zeolite was selected as methanol dehydration catalyst. The effects of precipitation temperatures, pH values, Cu/Mn ratios, prepared methods, calcination temperatures and modifications of promoter on the physico-chemical and catalytic properties of the composite catalysts were systemically investigated, using the technologies of X-ray diffraction （XRD）, nitrogen adsorption-desorption （BET）, temperature programmed reduction （H₂-TPR）, Scanning Electron Microscope （SEM）, hydrogen temperature programmed desorption （H₂-TPD）, ammonia temperature programmed desorption （NH₃-TPD）, X-ray photoelectron spectra （XPS）,and activity evaluation on a laboratory fixed-bed reactor. The results were obtained as follows:The influences of precipitation temperatures, pH values, and Cu/Mn ratios on the properties of catalysts prepared by co-precipitation method were investigated. The results indicated that the Cu-Mn/HZSM-5 catalysts showed the best catalytic activity when the Cu/Mn ratio was 4/3, the precipitation temperature was 70℃Calcination temperature was 350℃and pH values were 7-8. The conversion of CO₂ reached 21.83% and the yield of DME reached 6.68% under the conditions of 2.0MPa, 250℃, GHSV 2100h^-1, and H₂/ CO₂ volume ratio of 3.2.The addition of SiO₂ could obviously improve the performance of the Cu-Mn/HZSM-5 catalysts. When the amount of SiO₂ was 3.49% of the catalyst mass（calculated as oxides）, the conversion of CO₂ reached 23.86% and the yield of DME reached 9.33% under the conditions of 2.0MPa, 250℃, GHSV 2100h^-1, and H₂/CO₂ volume ratio of 3.2. The XRD and H₂-TPR results suggested that the suitable introduction of SiO₂ can weak the peak intensity of crystal phase CuO, and promote disperse of Cu. The copper-manganese compound oxide translates into amorphous phase from crystal phase, which prevents deep reduce of Cu to promote catalytic activity. The XPS results proved that the active sites of copper species of Cu-Mn/HZSM-5 catalysts might be Cu⁺, which might support the mechanism that Cu⁺ species might compose the active sites for methanol synthesis.The effect of operating conditions （temperature, pressure, space velocity, and mole ratio of H₂/CO₂） on the conversion of CO₂, selectivity to DME, yield of DME and product distribution on the Cu-Mn-Si/HZSM-5 catalysts prepared by co-precipitation method was investigated. CO₂ conversion is up to 26.03% at 260℃and 2.0 MPa, with a selectivity to DME of 42.42% and a yield of DME of 11.04%, for space velocity of 2100h^-1 and a feed made up of H₂/CO₂=3.2. It was also found that increasing reaction temperature could improve the conversion of CO₂, while it might decrease the selectivity of DME. Increasing pressure and H₂/CO₂ molar ratio were contributable to improve the conversion of CO₂ and the selectivity of DME. Furthermore, increasing the space velocity would decrease both the conversion of CO₂ and the selectivity of DME.The effect of calcination temperature on the performance of the Cu-Mn-Si/HZSM-5 prepared by co-precipitation method was also studied. The results showed that moderate temperature could be beneficial to catalytic hydrogenation. When calcination temperature was 400℃, more Cu⁺ existed in the reaction process. When calcination temperature was higher than 400℃, the particles of CuO became bigger resulting in the decrease of catalytic activity. The results of CO₂-TPD showed that the adsorption center to CO₂ was related to the synthesis of dimethyl ether. The moderate temperature was 400℃ in the researched temperature range.To further study the active sites of the catalysts, ultrasonic treatment was carried out to investigate its effect on the catalytic activity of Cu-Mn-Si/HZSM-5. The results showed that ultrasonic treatment could make more Cu⁺ and Mn³⁺ formed, promote the interaction of Cu⁺ and Mn³⁺ which increase the adsorption center to CO₂, and increase surface acidity and the amount of strong acid sites, which is beneficial to the reaction of dimethyl ether, improving the catalytic activity. In this paper, it was thought that the activity sites were both Cu⁺ and Mn³⁺.更多还原