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

轻质链烷烃（C2-C4）包含在石油和天然气中，可以在沸石催化剂上在芳烃（甲苯、二甲苯等）的混合物中进行处理，其广泛用于石化工业以合成更有价值的物质或改善发动机燃料的质量。用金属改性沸石（例如，锌）提高轻质（小）烷烃加工成芳烃的效率。除了布朗斯台德酸位点（BAS）之外，锌改性的沸石通常含有不同的Zn物质，包括在沸石孔中具有不同尺寸和定位的Zn 2+阳离子或氧化锌物质。不同的锌物种在烷烃的初级活化和随后的芳构化中的作用仍然不清楚。该项目打算进行ZSM-5或ZSM-5型的两种沸石样品的有针对性的合成，在孔中仅含有一种类型的锌物种，Zn 2+阳离子或小的氧化锌物种（ZnO），其尺寸为1 nm的数量级。这将有助于确定C2−C4烷烃在含阳离子Zn 2+或ZnO物种的沸石上活化和芳构化机理的差异，并有助于我们发现哪些特定的锌物种确保小烷烃有效转化为芳族分子。该项目的实施包括以下主要阶段：1.建立布朗斯台德酸性位点（BAS）和锌物种在烷烃C-H键活化中的作用。2. BAS和锌物种在小分子烷烃C-H键活化中协同作用的测定.烷烃与锌物种相互作用过程中形成的主要中间体的鉴定。确定了Zn ~（2+）和ZnO分子筛中间体结构的异同。分析初级中间体向次级中间体的演变，以及进一步向芳香族产物的转化。将通过13 C MAS NMR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键活化中的作用的具体化将基于对BAS和烷烃的各种CHn（n = 1−3）片段之间的H/D氢交换的区域选择性和速率的分析，通过1H MAS NMR方法原位监测该交换的动力学而获得。BAS和Zn中心的~ 1HMASNMR和~（67）ZnMASNMR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锌中心和BAS在分子筛催化剂上烷烃特定C-H键活化和芳构化中的作用。这些新的信息为基于锌改性分子筛的小分子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.