【作者】 朱海波；

【导师】 谢在库；

【作者基本信息】 上海交通大学 ， 应用化学， 2009， 博士

【摘要】 近年来,具有介孔孔道的沸石材料引起了人们的广泛关注,它是在结晶微孔沸石中引入一定数目的介孔通道。这种复合孔的沸石材料既有微孔沸石良好的热稳定性和强的催化活性,又具有介孔材料通畅的传质孔道。介孔沸石具有潜在的应用价值,可望在催化、环保以及新材料等领域得到广泛的应用。本论文在综述前人工作的基础上,针对该领域的一些基本问题进行了深入研究,并取得了一系列原创性的成果。采用纳米碳酸钙和聚乙烯醇缩丁醛为硬介孔模板合成出了介孔沸石,同时研究了沸石在聚乙烯醇溶液中的晶化过程,最后对合成的介孔沸石的催化性能进行了考察,具体研究方法和结果如下:1.以纳米CaCO₃为硬模板合成出了具有二次介孔的silicalite-1和ZSM-5沸石。在MFI结构的沸石晶化过程中,纳米CaCO₃被共生到了沸石晶体结构中。然后通过酸洗涤,复合到沸石晶体中的纳米CaCO₃被去除从而在沸石中留下介孔。XRD、TEM、SEM和N₂吸附表征手段证明了合成的沸石中含有二次孔,并且这些孔的大小和形状与纳米CaCO₃的形貌一致。研究发现,只有亲水性的纳米碳酸钙才能在沸石的合成中起到介孔模板的作用。当其表面被脂肪酸修饰钝化后,纳米碳酸钙就不能在沸石合成中造孔了,证明了纳米碳酸钙表面羟基对其模板效应有重要的影响。2.研究沸石分子筛在聚乙烯醇体系中的晶化过程及聚乙烯醇在分子筛合成中的软介孔模板作用。聚乙烯醇的存在对沸石分子筛晶化过程影响很小,在聚乙烯醇的溶液中能制备出ZSM-5和ZSM-11沸石。聚乙烯醇体系中合成出来的ZSM-5和ZSM-11沸石的结构中都存在一定量的介孔,并且介孔的体积可以通过加入的聚乙烯醇量进行调节。3.使用聚乙烯醇缩丁醛为硬介孔模板合成出了介孔ZSM-5、ZSM-11和Beta沸石。首先,通过溶胶-凝胶技术制备出了二氧化硅和聚乙烯醇缩丁醛的复合物,并将该复合物用作合成沸石的前驱体。在水热晶化条件下,无定型的二氧化硅转化成了沸石晶体,而聚乙烯醇缩丁醛则被共生到了沸石晶体中。焙烧水热晶化后的产物能够使沸石中的微孔和介孔孔道开放。表征的结果证明沸石中的微孔是有序的,而介孔则无规则的分布在沸石晶体中。聚乙烯醇缩丁醛为硬模板合成介孔沸石的总孔体积和介孔体积可以通过PVB/SiO₂的值进行有效的调节。4.表征了常规沸石与介孔沸石中Al和Si元素的核磁性质。介孔沸石中的铝原子都进入了沸石的骨架并以四配位的形式存在,同时介孔沸石中硅的化学环境也与常规沸石的相同。NH₃-TPD表征进一步证实了介孔沸石结构中的骨架铝产生了相应的酸性质,是催化活性中心的来源。由此可见:模板法引入介孔后对沸石的化学性质没有影响,介孔沸石完全具有与常规沸石相同的化学性质。介孔ZSM-5沸石和ZSM-11沸石在三甲苯（1,3,5-三甲苯和1,2,4-三甲苯）的裂解的探针反应中表现出了比常规沸石高的裂解能力,在产物分布上介孔沸石中更容易生成大分子。在Beta沸石裂解三甲苯的反应中,介孔Beta沸石的失活过程被抑制了。这是由于导致结焦的大分子更容易从介孔沸石晶粒中扩散出去,减小了进一步聚合反应而产生结焦的可能。探针反应证实了介孔沸石的引入对沸石催化性质的改善是有显著效果的。在具有现实意义的甲苯歧化与重芳烃烷基转移反应中,介孔ZSM-5和ZSM-11沸石显示了对重芳烃高的转化性能,这是与其特殊的孔道结构密不可分的。当介孔被引入到沸石中后,这些介孔为C₉A和C₁₀A芳烃提供了一条传质的通道,有利于它们在反应当中的扩散,因而在这个反应中对重芳烃的处理能力提高了。具有双孔分布的介孔沸石将是化学工业中一种有应用前景的新型催化材料。更多还原

【Abstract】 The zeolite-based hierarchical porous materials with two or more levels of porosity have drawn much attention in the field of porous materials, because of their potential applications in the industrial fields. These materials have been proven to be the promising catalysts that combine the advantage of shape selectivity with the efficient mass transport. The presence of mesopores in zeolite has improved catalytic activity and selectivity significantly in comparison with conventional zeolite catalysts. Most importantly, some zeolites with mesoporosity have been used in a number of industrial processes including the cracking of heavy oil fraction over zeolite Y, the production of cumene, hydro-isomerization of alkanes over mordenite, and the synthesis of fine chemicals over Y, ZSM-5, and beta. The enhanced catalytic activities are due to the reduction of diffusion path and easier access to the active site as the result of the introduction of mesopores into the zeolite framework. In this dissertation we used nano-sized CaCO₃ and polyvinyl butyral gel as the hard mesopore directing agent for the synthesis of mesoporous zeolite. Meanwhile, we investigated the zeolite crystallization in the polyvinyl alcohol system, and it was found that the polyvinyl alcohol in zeolite synthesis could produce mesopores within the zeolite. Lastly, the catalytic performance of mesoporous zeolite was studied in the tri-methyl benzene cracking reaction as well as toluene disproportionation and transalkylation with C₉ and C₁₀ aromatics. The main results in this dissertation are as follows:1. The silicalite-1 and ZSM-5 crystal with secondary pores in the range of 50-100 nm was synthesized by using the nanosized CaCO₃ as a hard template. The nanosized CaCO₃ can be trapped into the MFI zeolite crystals during the crystallization process. By means of acid dissolution, the encapsulated nanoparticles were removed, giving rise to the intracrystal pores within the zeolite crystal. Characterization techniques including XRD, TEM, SEM, and N2 adsorption provided the detailed information on this hierarchical pore structure. The hydroxyl groups on the surface of CaCO₃ are essential to taking the hard template effect. The secondary pores within zeolite correspond well to the morphology of the nanosized CaCO₃, which confirms the template effect of nanosized CaCO₃. These results suggest that using CaCO₃ as a hard template may be a useful approach for the synthesis of hierarchical porous materials.2. The zeolite crystallized in the presence of polyvinyl alcohol solution was systematically studied. The existence of polyvinyl alcohol in zeolite synthesis did not exert negative effect on the crystallization of zeolite, meanwhile ZSM-5 and ZSM-11 could be synthesized in the polyvinyl alcohol system. The characterization technique indicated that the zeolites obtained from polyvinyl alcohol not only possessed micropore but also had additional mesopore. The mesopore volume can be controlled by the amount of polyvinyl alcohol in the synthesis.3. The mesoporous zeolites with BEA, MFI, and MEL topology were synthesized by using polyvinyl butyral gel as the mesopore directing agent. The PVB/silica composite was readily prepared by sol-gel technique and then used as precursor for the synthesis of mesoporous zeolites. Upon the direct hydrothermal crystallization of PVB/silica composite, the silica was transformed into zeolite, whereas the PVB gel was incorporated in the zeolite crystals. Calcination of the obtained PVB/zeolite composite yielded the opening of micropores and mesopores in zeolites, respectively. The characterization techniques confirmed that the obtained porous materials consisted of well-defined microporosity and irregular mesoporosity. Raising the ratio of PVB/SiO2 in the synthesis leaded to the increase in mesopore volume of mesoporos zeolite, indicating the mesopore volume in mesoporous zeolite is controllable. 4. The NMR analyses on the mesoporous zeolites suggested that the mesoporosity in zeolite had little influence on the Si and Al species in the framework. Furthermore, the characterization data from the temperature-programmed desorption of ammonia （NH₃-TPD） on the H-form conventional zeolites and mesoporous zeolites confirmed that the mesoporous zeolites shown the same acidity as that of conventional zeolites. The mesoporous zeolites exhibited better catalytic activity and enhanced resistance toward deactivation in the cracking of 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene, as compared with conventional zeolites. Also, the mesoporous zeolites show improved catalytic activity in the toluene disproportionation and transalkylation with C9 and C10 aromatics. The enhanced catalytic performance is due to the presence of mesopores for increasing the diffusion rate of the reactant. Bimodal porosity as well as strong acidity will enable these materials to be a promising catalyst in a wide range of chemical industry.更多还原