Electron Diffraction Enables the Mapping of Coke in ZSM-5 Micropores Formed during Methanol-to-Hydrocarbons Conversion

Unveiling the coke formation in zeolites is an essential prerequisite for tackling the deactivation of these catalysts in the transformations of hydrocarbons. Herein, we present the direct mapping of coke in the micropores of ZSM-5 catalysts used in methanol-to-hydrocarbons conversion by single-crystal electron diffraction analysis. The latter technique revealed a polycyclic aromatic structure along the straight channel, wherein the high-quality data permit refinement of its occupancy to about 40 %. These findings were exploited to analyze the evolution of micropore coke during the reaction. Herein, coke-associated signals, which correlate with the activity loss, indicate that the nucleation of coke commences in the intersections of sinusoidal and straight channels, while the formation of coke in the straight pores occurs in the late stages of deactivation. The findings uncover an attractive method for analyzing coke deposition in the micropore domain.

Unravelling Channel Structure-Diffusivity Relationships in Zeolite ZSM-5 at the Single-Molecule Level

The development of improved zeolite materials for applications in separation and catalysis requires understanding of mass transport. Herein, diffusion of single molecules is tracked in the straight and sinusoidal channels of the industrially relevant ZSM-5 zeolites using a combination of single-molecule localization microscopy and uniformly oriented zeolite thin films. Distinct motion behaviors are observed in zeolite channels with the same geometry, suggesting heterogeneous guest-host interactions. Quantification of the diffusion heterogeneities in the sinusoidal and straight channels suggests that the geometry of zeolite channels dictates the mobility and motion behavior of the guest molecules, resulting in diffusion anisotropy. The study of hierarchical zeolites shows that the addition of secondary pore networks primarily enhances the diffusivity of sinusoidal zeolite channels, and thus alleviating the diffusion limitations of microporous zeolites.

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10^-7  mol m^-2  s^-1  Pa^-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

Elucidating Zeolite Channel Geometry-Reaction Intermediate Relationships for the Methanol-to-Hydrocarbon Process

The chemical industry has exploited zeolite shape selectivity for more than 50 years, yet our fundamental understanding remains incomplete. Herein, the zeolite channel geometry-reactive intermediate relationships are studied in detail using anisotropic zeolite ZSM-5 crystals for the methanol-to-hydrocarbon (MTH) process, and advanced magic-angle spinning solid-state NMR (ssNMR) spectroscopy. The utilization of anisotropic ZSM-5 crystals enabled the preferential formation of reaction intermediates in single-orientation zeolite channels, as revealed by molecular dynamics simulations and in situ UV/Vis diffuse-reflectance spectroscopy. The ssNMR results show that the slightly more constrained sinusoidal zeolite channels favor the olefin cycle by promoting the homologation of alkanes, whereas the more extended straight zeolite channels facilitate the aromatic cycle with a higher degree of alkylation of aromatics. Dynamic nuclear polarization experiments further indicate the preferential formation of heavy aromatics at the zeolite surface dominated by the sinusoidal channels, providing further insight into catalyst deactivation.