分子筛晶内介孔的原位构筑及其对催化性能的影响

Zeolites are widely used in industrial processes such as adsorption, separation, catalysis, etc. The relatively small micropores are beneficial to shape-selective catalytic reactions, but can significantly influence the mass transport of the reactants to and from the active sites located within them, severely hindering its application in industrial processes. Synthesis of zeolites with hierarchical porosity is an effective way to reduce diffusion limitation during the reactions. Dual templating method, post-treatment method and nanozeolite assembly method were used to construct mesopores in zeolites. In order to avoid the use of the expensive meso-scale template and environmental pollution during the combustion of template, to simplify the complex post-treatment procedures and reduce the waste discharges, to balance the catalytic performance and the shape-selectivity, herein, we propose, in this proposal, to construct intracrytalline mesoporosity in zeolite during synthesis. Using zeolites or zeolite precursors which are relatively unstable in alkaline systems as seeds to synthesize zeolites, we propose to in-situ dissolve the unstable core of the zeolite crystals formed from the seeds and thus to construct mesoporosity in zeolite by elaborate control of the stability of the seeds and the rate of the zeolite crystallization. The influence of the topology structure, the introduction of heteroatoms, the stability and the size of seeds, alkalinity and the concentration of the synthesis systems and the use of auxiliary organic structuring agents on the morphologies of the crystals and formation of the intracrystalline mesoporosity will be investigated. Alkylation of toluene and naphthalene with alcohols such as methanol and isopropyl alcohol will be used as probe reactions to evaluate the influence of intracrystalline mesoporosity on the catalytic performances and the shape-selectivity.

分子筛的微孔孔道在催化反应过程中提供了优异的择形选择性，但其较小的微孔孔道意味着扩散限制，分子筛的活性中心利用率低，影响其的催化性能，合成具有多级孔道结构分子筛是减少反应过程中扩散限制的有效手段。目前，多级孔道结构分子筛制备方法包括双模板结构导向法、后处理产生介孔法以及无介孔模板剂的单一模板直接合成法等，为避免双模板剂法中昂贵的模板剂的使用及其焙烧除去过程中带来的环境问题；避免后处理法繁琐步骤、减少制备过程中的废弃物排放；协调催化性能与择性选择性的关系，本项目提出利用在碱性条件下相对不稳定的分子筛或其前驱体作为晶种，在常规分子筛合成体系中通过对晶种相对稳定性和分子筛晶化速率调控，原位溶解不稳定的晶核，构筑分子筛晶内介孔。研究分子筛拓扑结构、杂原子引入、晶种稳定性及晶粒尺寸、合成体系的碱度及浓度、辅助模板剂的使用等对晶体形貌和晶内介孔形成的影响。考察晶内介孔引入对分子筛催化活性及择性选择性的影响。

本项目属于无机材料及多相催化剂制备的研究范畴，针对微孔沸石分子筛应用过程中的扩散限制以及分子筛的活性中心利用效率低等问题，采用晶种导向法辅助以表面活性剂以及双结构导向剂来构建具有多级孔结构、核壳结构的沸石分子筛催化材料，开展了小晶粒TS-1分子筛的制备、系列多级孔道结构ZSM-5和FER分子筛的构筑以及具有核壳结构的“Y型分子筛@棒状γ-Al2O3”的设计等工作。以胶态纯硅silicalite-1为晶种，以氨水做碱源，辅助以少量TPABr做结构导向剂制备了小晶粒TS-1分子筛的制备，具有良好的催化正己烯环氧化和环己酮氨肟化性能；以1, 6-己二胺做结构导向剂，十六烷基三甲基溴化铵做介孔模板制备了多级孔道结构ZSM-5分子筛，考察了其晶化机理；以少量晶种导向制备得到了具有多级孔道纳米ZSM-5聚集体，所得沸石无需焙烧即可进行离子交换，方法绿色、廉价，具有广泛的应用前景；开发了一步脱硅磷活化制备高水热稳定性多级孔ZSM-5的方法，以四乙基氢氧化磷为磷源直接处理微孔ZSM-5沸石得到含磷多级孔ZSM-5沸石，增加了磷物种与铝物种之间的相互作用，磷物种优先交换分子筛骨架上铝原子附近用来平衡电荷的钠离子，从而提高了稳化骨架的效果；以哌啶和四甲基氢氧化铵做双结构导向剂，利用四甲基氢氧化铵的有利于形成FER笼的性质，促进FER笼的形成并形成大量晶核，以哌啶原位组装FER笼形成具有多级孔道结构的FER分子筛聚集体，次级孔的引入使得FER分子筛活性位的可接触性显著提高，催化大分子反应的能力明显改善。利用单分散棒状拟薄水铝石自身的正电荷与Y型沸石表面负电荷的相互作用制备了具有核壳结构的Y型分子筛@棒状γ-Al2O3材料，材料具有优化的结构性质和酸性位接触性，在三异丙苯裂解中呈现出优异的性能，做为FCC助剂催化重油裂解时表现出突出的汽油选择性。在项目资助以及全体成员的共同努力下，发表SCI检索论文6篇，申请国家发明专利2项；项目已经按照计划书的承诺圆满完成了预期计划任务和指标。