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.

Molecular traffic control for catalytic oxidation reaction in TS-1 zeolite

A 2H NMR study has been conducted on the sorption of benzene and p-xylene in an MFI framework with a high silica content. A wide variety of sorbate loadings have been examined and the results are compared with adsorption in an isostructural ZSM-5 framework of lower silicon-to-aluminium ratio. The results from the silicalite study provide direct evidence of adsorption site heterogeneity for both the benzene and p-xylene systems. Single-component studies of benzene at low loadings suggest the presence of adsorbed species demonstrating only restricted motions in mid-channel sites in both the straight and sinusoidal channels. An increase in sorbate loading causes adsorption sites associated with greater sorbate mobility to be accessed; adsorption within the channel intersections is consistent with these observations. Results for p-xylene adsorption at low loadings reveal that the adsorbed molecules remain in the straight channels. An increase in the concentration of p-xylene causes the molecules to access a new site in the sinusoidal channels. The deuterium study thus supports the concept of molecular traffic control within the silicalite framework. In particular, at loadings in excess of one molecule per quarter unit cell (or channel intersection), benzene is restricted to 1D (one-dimensional) pathways whereas p-xylene can access the full 3D pore structure. The presence of aluminium in the MFI framework was found to have a marked affect on the nature of the adsorption sites and the mobility of the sorbed species.

The dynamics of a linear alkane, n-octane, adsorbed in zeolite ZSM-5 was studied using deuterium solid-state NMR (2H NMR) and quasi-elastic neutron scattering (QENS). It has been found that at the loading of 1.8 molecules per unit cell, adsorbed n-octane molecules are essentially located in the straight channels and diffuse along the direction of the straight channels with a diffusion coefficient D = 12.0 × 10-11 m2/s at 300 K. In the course of translational movement along the straight channels, some coupled rotational motions of all CHn− (n = 2, 3) groups of the hydrocarbon skeleton of the molecule take place. They are reflected in the 2H NMR spectrum of deuterated n-octane-d18, in the temperature range 253−373 K, as fast rotations of the separate methylene and methyl groups simultaneously around two and three C−C bonds of the molecule with a characteristic time τC ≈ 10-11 s and an activation energy ER ≈ 10−12 kJ/mol. These internal motions may correspond to fast interconversion between trans and gauche conformations in the adsorbed alkane molecule while the molecule moves along the straight channels. Upon heating at 373 K for 1 h, n-octane molecules, formerly located in the straight channels, become redistributed over straight and zigzag channels. Subsequent translational motion of n-octane consists of two independent modes of motion. One of them represents the movement along the tortuous zigzag channels. The other one represents the movement along the straight channels, disturbed by collisions with the other molecules at the channel intersections. For a loading of 3.5 molecules per unit cell, a liquidlike line shape appears at 253 K in the 2H NMR spectrum. This line shape corresponds to isotropically reorienting n-octane molecules, changing the direction of their translational motion (from straight to zigzag channels) under collision with the other molecules at the channel intersections.

The whereabouts of benzene molecules adsorbed at two loadings in zeolite ZSM-5 at room temperature have\nbeen studied with powder neutron and synchrotron X-ray diffraction. The structures have been refined by the\nRietveld method in space group <i>Pnma</i> by fitting simultaneously to the X-ray and neutron data. With\napproximately 3.4 molecules per unit cell, the best agreement is obtained with a model which has two molecules\nin mutually exclusive positions in the intersection of the straight and sinusoidal channels. The two positions\nhave occupancies of 2.10(5) and 1.32(5) molecules per unit cell, respectively. At the higher benzene coverage\nof around 7.6 molecules per unit cell, molecules are found at three locations: in the intersection, in the\nsinusoidal channel, and in the straight channel. These have occupancies of 4.0, 2.45(4), and 1.15(4) molecules\nper unit cell, respectively. The intersection is essentially full. There appear to be favorable interactions between\nthe molecules in the intersection and those in the sinusoidal and straight channels that help stabilize the\narrangement.

Catalytic roles of the acid sites in different pore channels of H-ZSM-5 zeolite for methanol-to-olefins conversion

Fe-modified ZSM-5 and β zeolites were prepared by adopting liquid ion-exchange method and their catalytic performance was studied in the N2O decomposition reaction. The state of Fe loaded on Fe-zeolites was investigated by means of UV-vis diffuse spectra, infrared spectroscopy, EPR and H2-TPR. The results of IR of hydroxyl stretching and UV-vis investigationSubscript texts indicated that part of the iron-ions was introduced into zeolites at the charge-balancing sites. The results of EPR and H2-TPR investigations showed that the same iron species were loaded on ZSM-5 and β zeolites. However, the results of IR of the perturbed anti-symmetric T-O-T vibrations of iron-ions indicated that different types of ZSM-5 and β zeolites resulted in different distributions of charge-balancing iron cations. The iron-ions could replace Brönsted acid protons at the straight channel wall (α sites), intersection of straight and sinusoidal channels (β sites), and sinusoidal channel wall (γ sites) within the ZSM-5 zeolite. In the case of Fe-β zeolites, iron-ions mainly located in the straight channels. We observed that the catalytic activity of the iron ions located on the α sites of ZSM-5 zeolites was better than those of iron ions located on β and γ sites in N2O direct decomposition, since the former was the most easily reduced from Fe3+to Fe2+in H2. Furthermore, it was found that Fe-β zeolite showed higher catalytic activity than Fe-ZSM-5 zeolite. This difference was attributed to the active sites located almost exclusively in the straight zeolite channels.

Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites

Single-crystal structure of a pyridine sorption complex of zeolite HZSM-5 (H-MFI)

The entrance and diffusion pathway of CO2 and dimethyl ether (DME) in MFI-type zeolite channels were investigated by a selective sealing method using large silicalite-1 crystals. The MFI-type zeolite has two kinds of orthogonal channels: straight channels and sinusoidal channels. The mouths of the straight channels are on (010) crystal faces, while those of the sinusoidal channels are on (100) faces. The channel mouths are directly sealed by silicone resin on the (100) and (010) faces so as to restrict the entrance and diffusion pathways to straight and sinusoidal channel pathways, respectively. The locations and loadings of the guest CO2 and DME molecules are determined by single-crystal X-ray diffraction structural analysis. The loadings show the difference of the adsorption rates between the pathways. The straight channel pathway is 4.2 times faster than the sinusoidal channel pathway for the CO2, and the sinusoidal channel pathway is 5.1 times faster than the straight channel pathway for the DME. It reveals their dominant pathways and the anisotropy of adsorption. The dominant pathway correlates to the stability of the channel as adsorption sites.

Correlation of Brønsted acid sites and Al distribution in ZSM-5 zeolites and their effects on butenes conversion

DIFFUSION OF BENZENE, P-XYLENE AND THEIR MIXTURE IN SILICALITE-1 USING A FREQUENCY RESPONSE TECHNIQUE

Differentiating the diffusivity in straight channel and sinusoidal channel of ZSM‐5 zeolite is a prerequisite for catalyst optimization for the required product distribution with specific orientation. Herein, the intracrystalline diffusivities of n‐butane and iso‐butane in power ZSM‐5 zeolite were investigated by pulse field gradient (PFG) NMR, respectively. Different diffusion behaviors of the selected isomers were revealed. The effective self‐diffusion coefficient of n‐butane is about three orders of magnitude higher than that of iso‐butane. For the selected isomers, the opposite loading‐dependence were presented in self‐diffusion coefficients. Moreover, an equation containing diffusion tensors in different directions in orthogonal coordinate was successfully applied to fit the diffusion coefficients of n‐butane in straight and sinusoidal channel of ZSM‐5, respectively. The diffusivity of n‐butane in straight channel is nearly one order of magnitude faster than that in sinusoidal channel, predicting its dominant diffusion path.

Guest–host interactions of arenes in H-ZSM-5 and their impact on methanol-to-hydrocarbons deactivation processes

Coke formation in high-silica zeolites