高硅ZSM-5分子筛烷基化活性位识别及其催化性能调控

Toluene alkylation with methanol can produce para-xylene selectively, greatly reducing the cost for separating xylene isomers in conventional production methods. Although the modified ZSM-5 zeolites exhibit a high selectivity towards para-xylene, the activity loss resulted from surface modification process is inevitable. Thereby, how to achieve a high para-xylene selectivity and catalytic activity simultaneously, is one of the key issues in developing the shape-selective alkylation catalyst..In our preliminary study, a high selectivity towards para-xylene has been obtained over the high-silica ZSM-5 zeolite, but the alkylation activity is poor and the nature of active sites is uncertain. To determine the active sites of high-silica ZSM-5 zeolite for alkylation, this work is going to fabricate zeolite samples with various defect degrees by modulating the synthesis and post-treatment processes, and further correlate their defect density and the amount of different silanol groups with the catalytic performance. The corresponding catalytic mechanism is also uncovered through some probe reactions. Subsequently, the micro-mesoporous zeolites will be fabricated through the alkaline treatment (desilication) method. The presence of the micropores guarantees the shape selectivity, while the introduction of mesopores not only increases the density of silicon-defect induced active sites and the reaction activity, but also enhances the molecule diffusion and suppress the coke formation. In general, this project realizes the combination of high activity and selectivity in shape-selective alkylation reaction through the “micro-mesoporous high-silica ZSM-5 zeolite” strategy, and also guides the development of efficient alkylation catalyst theoretically.

甲苯甲醇择形烷基化，可高选择性合成对二甲苯，大大降低传统生产方法中异构体分离的成本。改性ZSM-5分子筛虽具有高对位选择性，但活性损失不可避免。因此，如何兼顾对二甲苯选择性和反应活性，是催化剂开发过程中的关键问题。.前期工作表明，高硅ZSM-5分子筛可获得高对二甲苯选择性，但烷基化活性较低，且活性中心不明确。本课题将通过合成和后处理条件精准调控，制备不同缺陷程度的高硅ZSM-5分子筛，借助分子筛缺陷度和各类硅羟基数量与反应活性的关联，识别烷基化活性位；并利用探针反应，揭示其催化作用机制。随后，采用碱处理脱硅方法，实现高硅分子筛微介孔道复合；微孔的存在保证了反应择形性，而介孔的引入一方面可增加硅缺陷诱导产生的潜在活性位数量，提高反应活性，另一方面又可强化分子扩散，抑制缩合生焦。本项目所提出“高硅分子筛介孔化”策略，不仅兼顾了择形烷基化反应活性与选择性，还从理论上指导了高效烷基化催化剂的开发。

甲苯甲醇择形烷基化作为一类高选择性合成对二甲苯的技术，有效降低了后续异构体分离的成本，日益受到人们的关注。传统改性ZSM-5分子筛催化剂虽具有较高的对位选择性，但表面改性损失了部分催化活性，所以对二甲苯选择性和反应活性的兼顾是高效择形烷基化催化剂开发的关键。本项目初步开发了高硅ZSM-5分子筛烷基化催化剂，分别从催化剂构效关系、活性位识别及性能调控等方面取得了系列研究结果，主要包括以下几个方面：（1）分析了烷基化反应所需酸催化中心的属性及扩散限制的影响，并构建了甲苯甲醇烷基化反应网络；（2）基于分子筛硅羟基类型、数量与反应活性的关联，识别高硅分子筛催化烷基化反应的活性位为硅缺陷诱导产生的硅羟基窝，揭示了其催化作用机制；（3）通过碱处理脱硅制备了微介孔复合分子筛催化剂，实现了择形催化反应活性、选择性及稳定性的兼顾；（4）进一步经水热合成原位引入金属前驱物的方法，制备了金属-分子筛双功能催化剂，将其成功应用于甲醇直接制对二甲苯联产低碳烯烃过程。项目在执行期间公开发表论文15篇，其中SCI收录12篇；培养博士生1名，硕士生7名及本科生6名。本项目不仅在理论上深化了人们对高硅ZSM-5分子筛催化烷基化反应机制的认知，更在实际上指导了高效择形烷基化催化剂的开发。