Understanding Alkene Interaction with Metal-Modified Zeolites: Thermodynamics and Mechanism of Bonding in the π-Complex.

Zeolites modified with metal cations are perspective catalysts for converting light alkenes to valuable chemicals. A crucial step of the transformation is an alkene interaction with zeolite to afford π-complex with metal cations. The mechanism of alkene bonding with cations is still unclear. To address this problem, propene adsorption on H+ (BroÌ·nsted acid site), Na+, Ca2+, Zn2+, Co2+, Cu2+, Cu+, and Ag+ cationic sites in ZSM-5 zeolite has been studied by quantum chemical calculations in terms of adsorption enthalpy, νC═C frequency, and natural bond orbital (NBO) analysis together with natural energy decomposition analysis (NEDA). It is revealed that the conventional concept of σ- and π-bonding is only partially applicable to alkene interaction with metal cations in zeolites. The orbital interaction between an alkene molecule and a metal site is more complex. Several different bonding mechanisms have been identified depending on the nature and electron configuration of the metal cation. This finding explains the complex correlations observed for propene π-complex stability and νC═C frequency shift or charge transfer from the alkene molecule. The results provide the basis for further understanding the interactions between alkenes and inorganic solid BroÌ·nsted and Lewis acids.

Isobutane Transformation to Aromatics on Zn-Modified Zeolites: Intermediates and the Effect of Zn2+ and ZnO Species on the Reaction Occurrence Revealed by 13 C MAS NMR.

To clarify the effects of different Zn species, zeolite topology and acidity (quantity of Brønsted acid sites, BAS) on alkane aromatization, isobutane transformation on Zn2+ /H-ZSM-5, Zn2+ /H-BEA, and ZnO/H-BEA zeolites has been monitored with 13 C MAS NMR. The alkane transformation has been established to occur by aromatization and hydrogenolysis pathways. Zn2+ species is more efficient for the aromatization reaction because aromatic products are formed at lower temperatures on Zn2+ /H-BEA and Zn2+ /H-ZSM-5 than on ZnO/H-BEA. The larger quantity of BAS in ZnO/H-BEA seems to provide a higher degree of the hydrogenolysis pathway on this catalyst. The mechanism of the alkane aromatization is similar for the zeolites of different topology and containing different Zn species, with the main reaction steps being the following: (i) isobutane dehydrogenation to isobutene via isobutylzinc; (ii) isobutene stabilization as a π-complex on Zn sites; (iii) isobutene oligomerization via the alkene insertion into Zn-C bond of methyl-σ-allylzinc formed from the π-complex; (iv) oligomer dehydrogenation with intermediate formation of polyene carbanionic structures; (v) aromatics formation via further polyene dehydrogenation, protonation, cyclization, deprotonation steps with BAS involvement.