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The design of hierarchical zeolite catalysts is attempted through the maximization of the hierarchy factor (HF); that is, by enhancing the mesopore surface area without severe penalization of the micropore volume. For this purpose, a novel desilication variant involving NaOH treatment of ZSM‐5 in the presence of quaternary ammonium cations is developed. The organic cation (TPA+ or TBA+) acts as a pore‐growth moderator in the crystal by OH−‐assisted silicon extraction, largely protecting the zeolite crystal during the demetallation process. The protective effect is not seen when using cations that are able to enter the micropores, such as TMA+ Engineering the pore structure at the micro‐ and mesolevel is essential to optimize transport properties and catalytic performance, as demonstrated in the benzene alkylation with ethylene, a representative mass‐transfer limited reaction. The hierarchy factor is an appropriate tool to classify hierarchically structured materials. The latter point is of wide interest to the scientific community as it not only embraces mesoporous zeolites obtained by desilication methods but it also enables to quantitatively compare and correlate various materials obtained by different synthetic methodologies.

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).

Partial detemplation of zeolites followed by desilication in alkaline medium is demonstrated as a powerful and elegant approach to design hierarchical zeolites with tailored degree of mesoporosity. This achievement, illustrated for large beta crystals, is based on the fact that the template‐containing zeolite is virtually inert to Si leaching upon treatment in aqueous NaOH solutions. Partial removal of the structure‐directing agent creates regions in the crystal susceptible to mesopore formation by subsequent desilication, while template‐containing regions are protected from silicon extraction. Variation of the calcination temperature in the range 230–550 °C determines the amount of template removed and enables control of the extent of mesopore formation in the zeolite (20–230 m2 g−1) upon alkaline treatment. The functionality of the introduced mesoporosity in the hierarchical beta crystals is demonstrated by the improved performance in the catalytic pyrolysis of low‐density polyethylene. The partial detemplation–desilication treatment enhances the tuning options of this demetallation method.

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A post-synthesis route involving a sequence of up to three treatments in aqueous NaAlO2, HCl, and NaOH solutions, was designed to prepare a variety of complex hierarchical ferrierite zeolites, that is, microporous materials with auxiliary mesoporosity. The precise effect of each step on the composition, structure, morphology, porosity, and acidity of the samples was assessed by means of a multitechnique approach, including inductively coupled plasma-optical emissions spectrometry (ICP-OES), N2 sorption at 77 K, Ar sorption at 87 K, transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), liquid and solid-state nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and temperature-programmed desorption of ammonia (NH3-TPD). NaAlO2 desilicates a portion of the zeolite crystal by alkaline attack, although most of the extracted silicon is retained in the solid forming nanocrystals because of Al(OH)3 precipitation on the zeolite surface. A subsequent HCl washing removes the aluminum-containing deposit from the composite, uncovering a dual network of mesopores associated with the desilicated ferrierite platelets and the Si-rich nanocrystals. The latter species were selectively dissolved by mild NaOH washing. The impact of the relative amount of micro- and mesoporosity in the zeolites on the transport properties was studied by elution of butane isomers, and the pyrolysis of low-density polyethylene was selected as a model reaction to assess differences in catalytic performance. In addition, spray deposition was applied to support palladium nanoparticles onto selected ferrierite samples. The hierarchical zeolite yielded higher metal dispersion and size uniformity of Pd nanoparticles in comparison with the parent zeolite. Our study shows that an appropriate sequence of post-synthesis treatments comprises a powerful approach to tune the properties and functions of hierarchical zeolites.