Maria Milina

Expanding the Horizons of Hierarchical Zeolites: Beyond Laboratory Curiosity towards Industrial Realization

In recent years, we have witnessed remarkable progress in the preparation, characterization, and application of hierarchical (mesoporous) zeolites. [1] This wave of research originated because conventional (purely microporous) zeolites, despite having truly unique properties, underperform in many relevant reactions as a result of access and diffusion constraints. The introduction of auxiliary mesoporosity in zeolite crystals improves micropore accessibility and molecular transport. [2] This enhances the activity, selectivity, and lifetime in a large number of heterogeneously catalyzed reactions. [3] Besides catalytic benefits, mesoporous zeolites exhibit improved performance in adsorption and ion-exchange processes. [4] To date, the preparation of mesoporous zeolites has been confined to the laboratory scale, undertaken in gram quantities under precisely controllable conditions. Many of the routes developed are cost prohibitive and currently unthinkable for large-scale production. Furthermore, characterization and catalytic studies encompass pure zeolite powders. Often, advanced porous materials do not make it to real life processes due to the difficulties encountered in extrapolating encouraging laboratory results to an industrial context. Therefore, to truly ascertain future perspectives for mesoporous zeolites as technical catalysts, it is of urgent necessity to demonstrate 1) their largescale preparation and 2) the preservation of their enhanced properties upon shaping into a workable form. The former need requires the identification of economically viable preparation methods, whereas the latter constitutes the formation of a hierarchical material in the broadest dimension, integrating the micro-, meso-, and macroporosity levels in bounded zeolite bodies. This communication reports the first large-scale preparation of such hierarchically structured zeolite catalysts.

Impact of pore connectivity on the design of long-lived zeolite catalysts.

Without techniques sensitive to complex pore architectures, synthetic efforts to enhance molecular transport in zeolite and other porous materials through hierarchical structuring lack descriptors for their rational design. The power of positron annihilation lifetime spectroscopy (PALS) to characterize the pore connectivity in hierarchical MFI zeolites is demonstrated, establishing a direct link with the enhanced catalyst lifetime in the conversion of methanol to valuable hydrocarbons. The unique ability to capture subtleties of the hierarchical structure originates from the dynamic nature of the ortho-positronium response to the pore network. The findings reveal the strong dependence on the way in which the hierarchical zeolites are manufactured, having direct implications for the practical realization of these advanced catalysts.

Hierarchy Brings Function: Mesoporous Clinoptilolite and L Zeolite Catalysts Synthesized by Tandem Acid-Base Treatments

Hierarchical clinoptilolite and L zeolites are prepared using optimized tandem dealumination-desilication treatments. The main challenge in the post-synthetic modification of these zeolites is the high Al content, requiring a tailored dealumination prior to the desilication step. For natural clinoptilolite sequential acid treatments using aqueous HCl solutions were applied, while for L a controlled dealumination using ammonium hexafluorosilicate is required. Subsequent desilication by NaOH treatment yields mesopore surfaces of up to 4-fold (clinoptilolite: 64 m2 g-1, L: 135 m2 g-1) relative to the parent zeolite (clinoptilolite: 15 m2 g-1, L: 45 m2 g-1). A thorough characterization sheds light on the composition, crystallinity, porosity, morphology, coordination, and acidity of the modified clinoptilolite and L zeolites. It is elaborated that, besides the degree of dealumination, the resulting Al distribution is a critical precondition for the following mesopore formation by desilication. Adsorption experiments of Cu2+ and methylene blue from aqueous solutions and the catalytic evaluation in alkylations and Knoevenagel condensation evidence the superiority of the hierarchical zeolites, compared to their purely microporous counterparts. Finally, the post-synthetic routes for clinoptilolite and L are generalized with other recently reported modification strategies, and presented in a comprehensive overview.

Towards more efficient monodimensional zeolite catalysts: n-alkane hydro-isomerisation on hierarchical ZSM-22

A hierarchical (mesoporous) ZSM-22 zeolite displays a greatly enhanced sorption capacity for n-octane, compared to its purely microporous parent. In n-octane hydro-isomerisation, the mesoporous bi-functional Pt/ZSM-22 catalyst clearly outperforms its microporous parent, judged by the higher monobranched isomer yield. This is attributed to an increased number of accessible micropore mouths in the mesoporous zeolite.

Aluminum Redistribution during the Preparation of Hierarchical Zeolites by Desilication

A literature survey reveals a prominent reduction in the concentration of Brønsted acid sites in hierarchically organized zeolites with increasing mesoporous or external surface area independent of the framework type or synthesis route; this suggests a common fundamental explanation. To determine the cause, nature, and impact of the underlying changes in aluminum speciation, this study combines a multitechnique analysis that integrates basic characterization, a detailed synchrotron XRD and multiple-quantum NMR spectroscopy assessment, and catalytic tests to correlate evolution of the properties with performance during successive steps in the preparation of hierarchical MFI-type zeolites by desilication. The findings, subsequently generalized to FAU- and BEA-type materials, identify the crucial impact of calcination on the protonic form, which is an integral step in the synthesis and regeneration of zeolite catalysts; on aluminum coordination; and the associated acidity trends.