Mobility of Stable π-Complexes of Ethylene with Ag⁺ Cations in Ag/H-ZSM-5 Zeolite: A ²H Solid-State NMR Study

Abstract

Ethylene has been established to form stable π-complexes with Ag + cations in Ag-modified zeolite H-ZSM-5 (Ag/H-ZSM-5), which strongly affect the reactivity of ethylene on this catalyst. With the aim of revealing the catalytic and adsorptive properties of Ag + cations in Ag/H-ZSM-5, a characterization of the mobility of ethylene π-complex, located in the zeolite pores, has been performed with 2 H solidstate NMR spectroscopy. Ethylene molecules were established to remain bound to the Ag + cations at temperatures up to 556 K. In the bound state, ethylene exhibits a 2 H NMR line shape typical for anisotropic motions. It is inferred that being adsorbed on Ag + cation, ethylene molecule is involved in two internal rotations about its symmetry axes: along the C 2 axis aligned with the CC bond and about the second C 2 ′ axis orthogonal to the molecule plane. The first rotation occurs with the rate k 1 of 10−40 kHz and activation barrier E 1 = 2.3 kJ mol −1 , while the second motion occurs with the rate k 2 of 10−400 kHz and E 2 = 4.7 kJ mol −1 . The latter motion is faster than the former one. Both motions occur by four-site jump exchange mechanism. When methane is coadsorbed on the zeolite, the dynamical picture becomes different within two different temperature regions, with T ≤ 392 K and T > 392 K. Below 392 K, the rotation mechanism is similar to that without methane coadsortion. However, the barrier for the rotation about the C 2 ′ axis increases (E 2 II = 7.4 kJ mol −1 ), which is evidence for the methane interaction with the ethylene πcomplex. At temperatures as high as 392 K, the 2 H NMR line shape of adsorbed ethylene becomes typical for isotropic-like motion. The barrier of the motion, E = 25 kJ mol −1 , is accounted for by involvement of ethylene in the presence of methane in some isotropic or translational motions. At T ≥ 491 K, the line shape analysis reveals an H/D exchange between C 2 D 4 and CH 4 .

Supramolecular Organization in Confined Nanospaces

“…Also, the dynamic behavior of the guests can be probed by solid‐state NMR spectroscopy. In this respect, variable‐temperature 2 H NMR experiments are well‐established tools: for instance, they have recently allowed to characterize the dynamics of ethylene bound to Ag + site in Ag/H‐ZSM‐5 zeolite, highlighting two internal rotations of the guest about its symmetry axes …”

2H Solid‐State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL‐100(Al) Metal–Organic Frameworks

“…Despite these studies, only few works approached the characterization of Lewis acid sites in MOFs from the point of the view of mobility of adsorbed probe molecules. In particular when the probe such as pyridine forms the stable complex with CUS site, its structure and stability are greatly influenced by the degree of spatial confinement induced by the organic linkers of porous network . Unlike the case of strongly coordinated molecular probes, pyridine might be mobile in coordinated state and even migrate between neighboring sites.…”

“…Unlike the case of strongly coordinated molecular probes, pyridine might be mobile in coordinated state and even migrate between neighboring sites. In this regard, the information on the dynamics of the molecular probe in such a state can yield valuable information about the effect of network linkers on the potential accessibility of the reactants to catalytically active sites . To our knowledge, such work was performed only for the NO and water .…”

“…In this work we report the first case study of the molecular mobility of a commonly used acidity probe, [D 5 ]pyridine, coordinated to CUS in MIL‐100(Al) MOF. To address the mobility of the selected molecular probe we have used 2 H solid‐state NMR spectroscopy, an established technique to monitor molecular mobility in solids and adsorbed states over a wide range of motional times …”

“…In particular when the probe such as pyridine forms the stable complex with CUS site, its structure and stability are greatly influenced by the degree of spatial confinement inducedb yt he organic linkers of porous network. [14] Unlike the case of strongly coordinated molecular probes, [3c, 15] pyridine might be mobile in coordinated state and even migrateb etween neighboring sites. In this regard, the information on the dynamics of the molecular probe in such as tate can yield valuable information about the effect of network linkersonthe potential accessibility of the reactants to catalytically active sites.…”