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

轻质烷烃(C2、−、C4)存在于石油和天然气中，可以在沸石催化剂上以芳香烃(甲苯、二甲苯等)的混合物进行加工，这些芳烃在石化工业中被广泛用于合成更有价值的物质或改善汽车燃料的质量。用金属(如锌)对沸石进行改性可提高轻(小)烷烃加工成芳香烃的效率。在沸石孔道中，除布朗斯特酸中心(BAS)外，通常还含有不同的锌物种，包括大小不同的锌离子或氧化锌物种。不同的锌物种在烷烃一次活化和随后的芳构化中的作用尚不清楚。该项目旨在进行两个ZSM-5或BEA类型的沸石样品的定向合成，这些样品在孔中只包含一种类型的锌物种，要么是锌离子，要么是尺寸在1 nm数量级的小氧化锌(ZnO)。这将有助于建立C2−C4烷烃在含锌和锌阳离子物种的沸石上的活化和芳构化机理的差异，并有助于我们找出哪些特定的锌物种能够有效地将小分子烷烃转化为芳香族分子。该项目的实施包括以下几个主要阶段：1.确定BAS和锌物种在烷烃C-H键活化中的作用。2.测定了BAS与锌物种在小分子烷烃C-H键活化中的协同作用。烷烃与锌物种相互作用形成的初级中间体的鉴定。测定锌和氧化锌沸石中间体结构的异同。用~(13)C MAS核磁共振原位分析烷烃与含锌沸石相互作用的中间体的性质和结构。BAS和各种锌物种在烷烃CH键活化中作用的合理性将建立在分析BAS与不同CHn(n=1−3)链段之间H/D氢交换的区域选择性和速率的基础上，这种交换是通过原位1HMAS核磁共振方法监测这种交换动力学得到的。通过~1H-MAS和~(67)Zn-MAS核磁共振对BAS和ZN中心的研究完善了应用方法，所获得的基础信息将有助于阐明不同的锌中心和BAS在分子筛催化剂上烷烃特定C-H键的活化及其芳构化过程中的作用。这为锌改性沸石多相芳构化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.