Porous ZSM-5 Research Articles

Hierarchical ZSM‐5 Zeolites in Shape‐Selective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation

The isomerization of o-xylene, a prototypical example of shape-selective catalysis by zeolites, was investigated on hierarchical porous ZSM-5. Extensive intracrystalline mesoporosity in ZSM-5 was introduced by controlled silicon leaching with NaOH. In addition to the development of secondary porosity, the treatment also induced substantial aluminum redistribution, increasing the density of Lewis acid sites located at the external surface of the crystals. However, the strength of the remaining Brønsted sites was not changed. The mesoporous zeolite displayed a higher o-xylene conversion than its parent, owing to the reduced diffusion limitations. However, the selectivity to p-xylene decreased, and fast deactivation due to coking occurred. This is mainly due to the deleterious effect of acidity at the substantially increased external surface and near the pore mouths. A consecutive mild HCl washing of the hierarchical zeolite proved effective to increase the p-xylene selectivity and reduce the deactivation rate. The HCl-washed hierarchical ZSM-5 displayed an approximately twofold increase in p-xylene yield compared to the purely microporous zeolite. The reaction was followed by operando infrared spectroscopy to simultaneously monitor the catalytic performance and the buildup of carbonaceous deposits on the surface. Our results show that the interplay between activity, selectivity, and stability in modified zeolites can be optimized by relatively simple post-synthesis treatments, such as base leaching (introduction of mesoporosity) and acid washing (surface acidity modification).

Shape-controlled synthesis of monolithic ZSM-5 zeolite with hierarchical structure and mechanical stability

Columned ZSM-5 zeolite monoliths with hierarchical structure and excellent mechanical strength were successfully prepared by a hydrothermal transformation method. The pre-formed extrudates of aluminosilicate embedded with β zeolite were subjected to a hydrothermal synthesis system and the hierarchical porous ZSM-5 zeolite monoliths with the same columned shape were obtained after the hydrothermal treatment. The intra-particle hollow structure formed during hydrothermal synthesis was attributed to the digestion of inner β zeolite accompanying with the growth of ZSM-5 zeolite shell. The related data of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) analysis, N 2-sorption experiment and mercury intrusion prosimetry (MIP), showed ZSM-5 zeolite monolith had enriched multi-porosity and excellent mechanical stability, which contribute to its further application in field of catalysis and separation. The obtained hierarchical ZSM-5 zeolite monoliths showed superior catalytic performance in α-pinene isomerization for their proper acidity and good diffusion, compared with the ZSM-5 zeolite sample prepared from the precursor without β zeolite.

The effect of temperature on the pulsed current processing behaviour and structural characteristics of porous ZSM-5 and zeolite Y monoliths

Hierachically porous monoliths of ZSM-5 and Y zeolites have been prepared by pulsed current processing (PCP). The densification behaviour and structural characteristics during rapid thermal treatment of the zeolites have been determined and related to the influence of the Si:Al ratio on the thermal stability of the zeolites. Monoliths of macroscopic shape can be prepared with an insignificant loss of surface area and micropore volume when the PCP-treatment was performed at temperatures below a critical maximum PCP temperature ( T PCP). Full-profile fittings of the powder X-ray diffraction patterns showed that the lattice strain of zeolite Y increases rapidly above the critical T PCP while the ZSM-5 zeolites undergo a phase transition from orthorhombic to monoclinic. The use of a novel ceramic processing route for the production of the zeolite monoliths that do not significantly influence the structural characteristics and surface area of the starting materials has a potential to be of importance in catalysis and gas separation applications.

Quantification of enhanced acid site accessibility in hierarchical zeolites – The accessibility index

An accessibility index (ACI) was derived from infrared spectroscopy of substituted alkylpyridines with different size (pyridine: 0.57 nm, 2,6-lutidine: 0.67 nm, and 2,4,6-collidine: 0.74 nm) over hierarchical porous ZSM-5 crystals. The samples were prepared by selective silicon extraction of a commercial sample in NaOH (desilication), and they contained different degrees of intracrystalline mesoporosity. Our results demonstrate the enhanced accessibility of Brønsted acid sites in the hierarchical zeolites. For example, a relatively bulky molecule such as collidine, which probes practically no acid site of the parent medium-pore MFI structure, can access up to 40% of the acid sites in the mesoporous sample. The ACI is a powerful tool to standardize acid site accessibility in zeolites, and can be used to rank the effectiveness of synthetic strategies to prepare hierarchical zeolites.

Molecular and Meso- and Macroscopic Properties of Hierarchical Nanocrystalline ZSM-5 Zeolite Prepared by Seed Silanization

Surface-passivating silanization of protozeolitic units has been shown to be an effective strategy for the preparation of ZSM-5 nanocrystals, showing a controlled aggregation degree and a hierarchical porosity. ZSM-5 zeolite materials are thus obtained with adjustable and relatively uniform mesoporosities that have a strong influence on resulting macroscopic reaction properties, especially for macromolecular reagents. The mean sizes of the nanounits and, therefore, the textural and accessibility of these materials can be varied by changing the precrystallization conditions and the concentration of the seed-silanization agent. In addition to conventional characterization techniques, solid-state two-dimensional (2D) nuclear magnetic resonance (NMR) spectroscopy measurements and the application of the NLDFT model to the argon adsorption isotherms have allowed both the local and the mesoscopic compositions, as well as the structures of the hierarchically porous ZSM-5 materials, to be established. The resultin...

Visible light photoreduction of Cr(VI) in aqueous solution using iron-containing zeolite tubes

In this study, a novel visible light photocatalyst consisting of iron-loaded zeolite tubes was designed to reduce Cr(VI) to Cr(III) in aqueous solution. Iron was incorporated into hollow, porous ZSM-5 tubes using two different methods. In the first method, the iron-encapsulated ZSM-5 tubes were prepared by introducing iron into the mesoporous silica template prior to the formation of the hollow ZSM-5 tubes resulting in an encapsulated iron species. In the second method, iron was ion-exchanged into the ZSM-5 shell during a post-synthesis treatment. The iron-loaded ZSM-5 structures were extensively characterized by powder X-ray diffraction (XRD), elemental analysis, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), electron microscopy with energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy (XPS). The iron in the iron-exchanged ZSM-5 tubes tended to aggregate compared to the iron in the iron-encapsulated ZSM-5 tubes. Both the iron encapsulated and iron-exchanged ZSM-5 hollow tubes were active for the visible light photoreduction of Cr(VI) to Cr(III) in aqueous solution. The iron-exchanged ZSM-5 tubes were very active initially but suffered from substantial iron leaching after the first use in aqueous Cr(VI) photoreduction relative to the iron-encapsulated ZSM-5 tubes which showed very little leaching of the iron.