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HZSM-5负载氧化锰催化剂的制备及其对VOCs催化氧化反应性能研究
黄河
学位类型博士 ; 学位论文 ; 博士 ; 学位论文
导师李学兵
2016-05
学位授予单位中国科学院大学
学位授予地点北京
学位专业化学反应工程
关键词挥发性有机化合物 催化氧化 酸性 负载型氧化锰催化剂 尺寸效应
摘要挥发性有机化合物(Volatile organic compounds,简称VOCs)是大气污染的主要来源之一,严重危害生态环境和人类健康。在诸多的VOCs处理技术中,催化氧化由于净化效率高、能耗低,且反应过程中将有机污染物直接转化为二氧化碳和水,被认为是最具应用前景的净化技术。目前,催化氧化技术的关键是获得具有优异催化性能且价格低廉的催化剂。尽管负载型贵金属催化剂表现出较高的低温催化活性,但是昂贵的价格限制了其实际应用。过渡金属氧化物对VOCs具有优良的高温催化活性和抗中毒性能,得到了研究人员的广泛关注。本论文以甲苯的催化氧化为模型反应,制备了一系列MnOx/HZSM-5催化剂并考察其催化氧化性能。采用X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、氮气物理吸附(N2 physisorption)、氨气程序升温脱附(NH3-TPD)和氢气程序升温还原(H2-TPR)等表征技术,研究了催化剂的物化性质和催化性能间的相互关系。论文主要取得了以下研究结论: (1)采用等体积浸渍法,合成了具有不同负载量的y%MnOx/HZSM-5(y=2.5、5.0、7.5、10、15)催化剂。催化反应测试结果发现,随着MnOx负载量的增加,催化剂的催化活性逐渐提高;当负载量高于10%时,催化活性并没有得到提升反而下降。催化剂的催化活性由高到低依次为10%MnOx/HZSM-5>7.5%MnOx/HZSM-5≈15%MnOx/HZSM-5>5.0%MnOx/HZSM-5>2.5%MnOx/HZSM-5,其中10%MnOx/HZSM-5催化剂表现出了最优的催化活性(T50=261 ℃,T90=267 ℃)、良好的催化稳定性和抗水性能。HZSM-5分子筛表面的MnOx颗粒可以明显的抑制反应过程中积碳的生成并促进高温下积碳的分解,低温可还原性能是影响催化剂反应活性的关键因素。以商品级SiO2和Al2O3为载体制备的10%MnOx/SiO2和10%MnOx/Al2O3催化剂的催化活性均远低于10%MnOx/HZSM-5的催化活性。主要归因于HZSM-5分子筛中丰富的Brønsted酸中心,与MnOx颗粒氧化还原活性位间的协同作用显著地促进了甲苯催化氧化反应的进行。 (2)采用离子交换法合成了一系列具有不同Na/Al值的NaZSM-5分子筛,并以此为载体制备了相同负载量的10%MnOx/NaZSM-5-y(y=0.16、0.31、0.51)催化剂。各催化剂的总酸量和酸强度随着Na/Al值的增大而逐渐减小,而酸性的改变并未对其低温可还原性能造成明显的影响。在甲苯的催化氧化反应中,10%MnOx/HZSM-5催化剂仍表现出最好的催化活性和最低的表观活化能,且甲苯在单位锰活性位上的反应速率最高;然而随着催化剂酸量的减少,其对甲苯的催化活性也逐渐下降,单位锰活性位上的甲苯转化速率也逐渐降低;特别对于10%MnOx/NaZSM-5催化剂,其T50和T90相比10%MnOx/HZSM-5催化剂均高20 ℃以上。通过对物化表征结果与催化氧化活性综合分析发现,催化剂的催化活性随着总酸量的增加而提高,呈线性相关;催化剂较强的酸性有利于降低反应的表观活化能,对甲苯转化具有明显的促进作用。 (3)合成了具有不同硅铝比的HZSM-5分子筛,作为载体制备了相同负载量的10%MnOx/HZSM-5-y(y=25、50、75)。随着载体硅铝比的增加,催化剂的酸强度和总酸量逐渐降低。在甲苯的催化氧化反应中,10%MnOx/HZSM-5-25催化剂表现出最佳的催化活性和最低的表观活化能,其T50和T90分别为253 ℃和261 ℃。随着载体硅铝比的增加,其催化反应活性逐渐降低,再次证明了载体的酸性是影响催化剂整体催化性能的关键因素,更多的酸性位对甲苯的催化氧化反应具有显著的促进作用。 (4)合成了具有不同晶粒尺寸和内部结构的HZSM-5分子筛,考察了载体的尺寸效应对于MnOx负载型催化剂催化活性的影响。相比于微米尺寸的Micro-10%MnOx/HZSM-5催化剂,具有纳米结构的Nano-10%MnOx/HZSM-5催化剂具有更大的比表面积,有利于活性组分在其表面的分散,从而具有更好的可还原性能。对于Hollow-10%MnOx/HZSM-5催化剂,分子筛载体的中空结构能够进一步提高活性组分的分散性,使得催化剂的氢气还原温度大幅降低为277 ℃和375 ℃,表现出优异的氧化还原性能。甲苯催化氧化的反应结果发现,Hollow-10%MnOx/HZSM-5催化剂具有最佳的催化活性,其T50和T90分别为245 ℃和255 ℃,均远低于另外两个催化剂的甲苯转化温度。其最低的表观活化能(Ea=101.3 kJ mol-1)表明在较低的反应温度下更易于发生甲苯的转化。中空结构的存在,使催化剂的外表面积和介孔体积都有显著的增加,这不仅有利于活性组分MnOx颗粒在载体表面的分散,提高催化剂的还原性能并进一步提高其甲苯催化氧化活性,而且还缩短了反应过程中各物质的传递距离,促进了催化氧化反应的进行。
其他摘要Volatile Organic Compounds (VOCs), one of the main sources of air pollution, pollute the environment and are harmful to human health. Various applicable technologies have been developed in order to efficiently remove VOCs pollutants. Among all, catalytic oxidation has been proven to be an effective and energy-saving process where organic pollutants could be completely decomposed into those harmless products such as carbon dioxide and water at relatively low temperature. The main concern in the field of VOCs catalytic oxidation is to search a proper catalyst with high catalytic activity and stability but low cost. In spite that supported noble metals are highly active in catalyzing VOCs oxidation at low temperature, high costs and easy deactivation are the main defects limiting their wide application. It has been accepted that transition metal oxides are another branch of catalysts for VOCs oxidation because of their good catalytic activities at high temperature and resistant ability to poisoning. In this thesis, catalytic performances of MnOx/HZSM-5 catalysts were evaluated for the catalytic oxidation of toluene, which was typically selected as a model reaction for ordinary VOCs abatement. Numerous characterization techniques including XRD, N2 physisorption, SEM, TEM, H2-TPR and NH3-TPD were conducted to investigate the correlation between the physicochemical properties and the catalytic performances over the prepared catalysts. The main results obtained in the thesis are as follows: (1) Series of y%MnOx/HZSM-5 (y=2.5, 5.0, 7.5, 10, 15) catalysts were successfully prepared by an incipient impregnation method and used for the catalytic oxidation of toluene. Catalytic testing results indicated that the catalytic activities of HZSM-5 supported MnOx catalysts decreased in the order of 10%MnOx/HZSM-5> 7.5%MnOx/HZSM-5≈15%MnOx/HZSM-5> 5.0%MnOx/HZSM-5> 2.5%MnOx/ HZSM-5, where 10%MnOx/HZSM-5 exhibited the optimum catalytic activity, excellent catalytic durability in dry condition as well as good regeneration capability in humid condition. The dispersion of MnOx species on HZSM-5 surface greatly reduced the formation of coke and simultaneously efficiently accelerated the oxidative elimination of surface coke deposition during the catalytic oxidation of toluene which was the factor for toluene oxidation reaction. Based on a comparative analysis with those SiO2 and Al2O3 supported MnOx catalysts, it was predicated that the Brønsted acid sites of HZSM-5 zeolite make a promotional effect on the catalytic oxidation of toluene. The cooperative action between the redox ability of MnOx species and the acidity property of HZSM-5 zeolite resulted in high catalytic reactivity for toluene oxidation. (2) Using the ion exchange method, series of 10%MnOx/NaZSM-5-y (y=0.16, 0.31, 0.51) catalysts were prepared. The total acidity of the catalysts was decreased with the increasing of Na/Al ratio and there was no obvious relationship between the acdity and the reductibility for the catalysts. By comparing to the ion changed samples, 10%MnOx/HZSM-5 catalyst showed the best catalytic performance with the lowest Ea value. After a comparative analysis and discussion based on the obtained characterizations and catalytic testing results, it showed that the acidity of catalysts highly improved the catalytic activities in the toluene oxidation reactions by lowering the activation energy. (3) A range of HZSM-5 zeolite with various Si/Al ratio (25, 50, 75) suppored MnOx catalysts had been successfully prepared via hydrothermal systhesis and impregnation. The Si/Al ratios strongly influenced the acidity of 10%MnOx/ HZSM-5-n (n=25, 50, 75) catalysts. Catalytic tests in toluene oxidation showed that 10%MnOx/HZSM-5-25 exhibits the highest activity (T50=253 ℃ and T90=261 ℃) and the lowest activation energy value among all the catalysts. The catalytic activity was reduced with the increasing of Si/Al ratio, indicating that actidiy of the catalyst is the key factor responsible for the catalytic activity. (4) In order to investigate the influence of size effect on the catalytic activites, series of catalysts with different partical sizes and inner structure were prepared. Campared with micro-sized sample Micro-10%MnOx/HZSM-5, the Nano-10%MnOx/ HZSM-5 catalyst exhibited a better reducing property which was attribute to its higher specific surface area. In the catalytic tests for toluene oxidation, Hollow-10%MnOx/ HZSM-5 catalysts exhibited the best catalytic activity (T50=253 ℃ and T90=261 ℃) and the lowest activation energy value. The special hollow structure significant improved the dispersion of MnOx on the catalyst surface, lowered the reaction activation energy and promoted the toluene oxidation. In addition, the short transfer route was another advantage of the hollow structure.
作者部门多相催化转化团队
学科领域化学工程
公开日期2021-06-30
学位类型博士 ; 学位论文 ; 博士 ; 学位论文
语种中文
文献类型学位论文
条目标识符http://ir.qibebt.ac.cn/handle/337004/9757
专题多相催化转化团队
作者单位中国科学院青岛生物能源与过程研究所
推荐引用方式
GB/T 7714
黄河. HZSM-5负载氧化锰催化剂的制备及其对VOCs催化氧化反应性能研究[D]. 北京. 中国科学院大学,2016.
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