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

• Applicant: Professor Dr. Jürgen Haase
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Solid State and Surface Chemistry, Material Synthesis
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 405567845

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.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Science Foundation
• Cooperation Partner: Professor Dr. Alexander G. Stepanov