中孔分子筛的制备及其选择性催化还原氮氧化物的研究(附答辩文稿)

摘 要 汽车尾气中的氮氧化物是主要的大气污染物之一。用选择性催化还原的方法去除氮氧化物对于控制城市空气污染有着重要的意义。本论文在系统的考察了MCM-41合成条件的基础上，对铜离子交换的MCM-41中孔分子筛选择催化还原氮氧化物进行了研究。 首先，论文采用不同的合成方法制备了具有较高晶化度和长程有序度的纯硅MCM-41和Al-MCM-41中孔分子筛，同时考察了它们的高温和水热稳定性。通过XRD表征，确定样品具有典型的MCM-41中孔结构。氮气吸附、透射电镜和扫描电镜测试也表明，材料有蜂窝状中孔结构。而且材料比表面积高达1100m2/g。 其次，在样品的合成过程中，使用三甲苯（TMB）对MCM-41和Al-MCM-41进行了扩孔处理。例如采用TMB/CTAB=8的TMB加入量时，纯硅MCM-41的孔径可以从2nm左右扩增到5nm。此外还对纯硅MCM-41和MCM-48进行了嫁接后处理。特别经过氯化铝溶液嫁接处理后，材料的高温和水热稳定性都得到了明显的提高，而且仍然保持了原有的长程有序结构。 在氮氧化物催化还原的活性评价实验中，考察了铜离子交换的Al-MCM-41对丙烯选择还原一氧化氮的催化活性。实验中发现Cu-Al-MCM-41比传统的Cu-ZSM-5催化剂具有更好的水热稳定性。500℃下5％水蒸汽的通入使得氮氧化物在Cu-ZSM-5上的转化率从72.0％降低到61.7％，而Cu-Al-MCM-41上的转化率只从35.9％降低到35.4％。而且样品经过负载镍的处理后，它在水蒸气条件下400℃以上的催化活性还有一定提高。为了提高Cu-Al-MCM-41的催化活性还采用了提高酸性和担载过渡金属等处理方法。经过以上处理后，氮氧化物在300℃的转化率最高能达到42％。此外，对嫁接处理制备的Cu-Al-MCM-41的催化性能作了比较全面的考察。发现离子交换对催化剂的活性影响较大，经低温水热离子交换的样品会有更宽的活性温度窗口。 最后比较了Cu-Al-MCM-41与Cu-ZSM-5对氮氧化物的吸附能力，为讨论反应机理提供了依据。 关键词：氮氧化物，MCM-41，选择性催化还原，嫁接处理 Abstract Nitric oxides (NOx) emission from automobiles is one of the major air pollutants. Selective catalytic reduction of NOx is one of important routes to control of air pollution in city. After systematically investigating several synthesis parameters of MCM-41, the thesis focused on the selective catalytic reduction of NOx over Cu ion-exchanged MCM-41 mesoporous molecular sieves. Firstly, the crystalline and long-ordered MCM-41 and Al-MCM-41 mesoporous materials were synthesized by hydrothermal synthesis and low temperature assembly synthesis, and the thermal and hydrothermal stabilities were both examined. XRD patterns showed that the samples were typically mesoporous. N2 adsorption, TEM and SEM were also used to characterize these mesoporous samples. The highest surface areas can reach 1100 m2/g, with the pore size at 2.2nm. Secondly, the pore diameters of MCM-41 and Al-MCM-41 were tuned with addition of trimethylbenenze (TMB) during the synthesis process. For example, the pore diameter of MCM-41 was enlarged from 2nm to 5nm after adding TMB with the ratio TMB/CTAB=8. Furthermore the siliceous MCM-41 and MCM-48 were treated by grafting methods. Thermal and hydrothermal stabilities of the materials were both improved significantly after aluminium chloride solution grafting. Moreover these materials still kept well ordered mesoporous structures. Thirdly, selective catalytic reduction of NO with propene over Cu ion-exchanged Al-MCM-41 has been studied. The results show that activity on Cu-Al-MCM-41 is more stable in the presence of water vapor as compared to that on conventional Cu-ZSM-5. The NO conversion on Cu-ZSM-5 decreased about 10% from 72.0% to 61.7% in 5% water vapor at 500oC, while the value decreased just from 35.9% to 35.4% on Cu-Al-MCM-41. And the catalytic activity of the sample doped with Ni was enhanced in water vapor above 400 oC. In order to improve NO conversion on Cu-Al-MCM-41, the post-synthesis were carried out by either doping them with acids and transition metal oxides. The highest NO conversion reached 42% at 300 oC. In addition, the catalytic activities on Cu-Al-MCM-41 treated by alumina-grafting methods were studied. It was found that the ion-exchange methods affected the catalytic activities on these grafted samples significantly. Ion-exchanging at low temperature led to a broad activity temperature window. Finally, the adsorption of NO on Cu-Al-MCM-41 and Cu-ZSM-5 was also compared to explain the mechanism of NOx reduction on these mesoporous materials. Keywords: nitric oxide, MCM-41, SCR, grafting