Properties and functions of hierarchical ferrierite zeolites obtained by sequential post-synthesis treatments

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.