Mechanisms of small alkane activation and aromatization on zinc-modified zeolite catalysts

轻石蜡（C2−C4）存在于石油和天然气中，可以在沸石催化剂上以芳烃（甲苯、二甲苯等）的混合物进行加工，在石化工业中广泛用于合成更有价值的物质或提高汽车燃料的质量。用金属（如锌）修饰沸石可以提高轻（小）烷烃加工成芳烃的效率。锌改性沸石除了含有Brønsted酸位（BAS）外，通常还含有不同种类的锌，包括不同尺寸和位置的Zn2+阳离子或氧化锌。不同种类的锌在烷烃的初次活化和随后的芳构化中的作用尚不清楚。该项目拟进行ZSM-5型或BEA型两种沸石样品的定向合成，在孔隙中只含有一种锌，要么是Zn2+阳离子，要么是1纳米量级的小氧化锌（ZnO）。这将有助于建立C2−C4烷烃在含有Zn2+或ZnO阳离子的沸石上的活化和芳构化机制的差异，并有助于我们找出哪种特定的锌能确保小烷烃有效转化为芳香分子。项目的实施主要包括以下几个阶段：1.项目的实施。建立Brønsted酸性位点（BAS）和锌在烷烃C-H键活化中的作用。2. 2 . BAS和锌在活化小烷烃C-H键中的协同作用的测定。烷烃与锌相互作用形成的初级中间体的鉴定。3 .测定Zn2+ -和zno -沸石中间体结构的异同。分析了初级中间体向次生中间体的演化，并进一步转化为芳香产物。用13C - MAS NMR原位分析了烷烃与含锌沸石相互作用中间体的性质和结构。BAS和各种锌在烷烃CH键活化中的作用将基于两者的分析，区域选择性和BAS与烷烃中各种CHn （n = 1−3）片段之间的H/D氢交换速率，通过原位监测这种交换动力学，BAS和各种CHn （n = 1−3）片段之间的氢交换速率。BAS和Zn位点的1H MAS NMR和67Zn MAS NMR研究完成了应用方法。所得的基本信息将有助于阐明不同锌中心和BAS在沸石催化剂上活化烷烃特定碳氢键及其芳构化中的作用。这一新发现为锌改性沸石制备小烷烃芳构化多相催化剂的合理设计和操作提供了依据。

Light paraffins (C2−C4) are contained in oil and natural gas and can be processed on zeolite catalysts in a mixture of aromatic hydrocarbons (toluene, xylenes, etc.), which are widely used in the petrochemical industry to synthesize more valuable substances or improve the quality of motor fuels. Zeolite modifications with metals (e.g., zinc) increase the efficiency of light (small) alkane processing into aromatic hydrocarbons. Zinc-modified zeolites contain usually, besides Brønsted acid sites (BAS), different Zn species including Zn2+ cations or zinc oxide species of different dimensions and localizations in the zeolitic pores. The role of different Zn species in the primary activation of the alkane and the subsequent aromatization is still unclear.The project intends to perform a targeted synthesis of two zeolite samples of ZSM-5 or BEA type, containing exclusively one type of zinc species in the pores, either Zn2+cations or small zinc oxide species (ZnO) with a size by order of magnitude of 1 nm. This will help to establish differences in the mechanisms of activation and aromatization of C2−C4 alkanes on zeolites containing cations Zn2+ or ZnO species, and it will help us find out what particular zinc species ensure the efficient conversion of small alkanes into aromatic molecules.The implementation of the project includes the following main stages:1. Establishing the role of Brønsted acidic sites (BAS) and zinc species in the activation of the C-H bonds of alkanes. 2. Determination of synergistic action of BAS and zinc species in the activation of C-H bonds of small alkanes.3. Identification of primary intermediates formed during the interaction of alkanes with zinc species. Determination of the similarity and difference in the structure of these intermediates for Zn2+ - and ZnO-zeolites.4. Analysis of the evolution of primary intermediates into subsequent secondary intermediates, and further transformation in to aromatic products.An analysis of the nature and structure of the intermediates of the interaction of alkanes with zinc-containing zeolites will be performed by 13C MAS NMR in situ. Rationalization of the role of BAS and various zinc species in the activation of CH bonds of alkanes will be based on the analysis of both, the regioselectivity and the rate of H/D hydrogen exchange between BAS and various CHn (n = 1−3) fragments of alkanes obtained by monitoring the kinetics of this exchange by the 1H MAS NMR method in situ. 1H MAS NMR and 67Zn MAS NMR studies of the BAS and Zn sites complete the applied methods.The obtained fundamental information will make it possible to clarify the role of different zinc centers and BAS in the activation of particular C-H bonds of alkane and its aromatization on zeolite catalysts. The new information provides the basis for the rational design and operation of heterogeneous catalysts for aromatization of small alkanes based on zinc modified zeolites.