Danny Verboekend

Design of hierarchical zeolite catalysts by desilication

Hierarchical (or mesoporous) zeolites have received an ever-increasing attention due to their improved performance in catalysed reactions with respect to conventional (purely microporous) zeolites. Desilication in alkaline media has become a widely applied preparation method to tailor these modified zeolites, due to an optimal combination of efficiency and simplicity. This review presents recent developments that have expanded its general understanding and turned this top-down treatment highly versatile, controllable, and scalable. Design aspects of mesoporous zeolites for catalytic applications are emphasised, encircling the establishment of synthesis–property–function relationships. Alkaline treatment is a key step in strategic combinations with other post-synthesis modifications towards superior zeolite catalysts. The outlook of the field, pinpointing present needs and short-term priorities, is discussed.

Desilication Mechanism Revisited: Highly Mesoporous All-Silica Zeolites Enabled Through Pore-Directing Agents

The role of pore-directing agents (PDAs) in the introduction of hierarchical porosity in silicalite-1 in alkaline medium was investigated. By incorporation of various PDAs in aqueous NaOH, homogenously distributed mesopores were introduced in 2.5 μm silicalite-1 crystals. It was proven for the first time that framework aluminum is not a prerequisite for the introduction of intracrystalline mesoporosity by desilication. The pore-directing role is not directly exerted by framework trivalent cations metals, but by species on the external surface of the zeolite. The inclusion of metal complexes (Al(OH)(4)(-), Ga(OH)(4)(-)) and tetraalkyl ammonium cations (tetramethyl ammonium (TMA(+)), tetrapropyl ammonium (TPA(+))) in the alkaline solution led to distinct mesopore surface areas (up to 286 m(2) g(-1)) and pore sizes centered in the range of 5-20 nm. In the case alkaline treatment was performed in the presence of Al(OH)(4)(-), all aluminum partially integrated in the zeolite giving rise to both Lewis and Brønsted acidity. Apart from the concentration and location, the affinity of the PDA to the zeolite surface plays a crucial role in the pore formation process. If the PDA is attracted too strongly (e.g., TMA(+)), the dissolution is reduced dramatically. When the pore-directing agent is not attracted to the zeolites external surface, excessive dissolution occurs (standard alkaline treatment). TPA(+) proved to be the most effective PDA as its presence led to high mesopore surface areas (>200 m(2) g(-1)) over a broad range of PDA concentrations (0.003-0.1 M). Importantly, our results enable to extend the suitability of desilication for controlled mesopore formation to all-silica zeolites.

Mesoporous ZSM-22 zeolite obtained by desilication: peculiarities associated with crystal morphology and aluminium distribution

Mesoporous ZSM-22 zeolite (TON structure) is prepared by controlled silicon extraction in aqueous NaOH under different conditions of concentration, temperature, and time. The first challenge is to synthesize a pure ZSM-22 sample, since ZSM-5 and cristobalite impurities are hard to avoid. For this purpose, hydrothermal syntheses in autoclaves of 30–1000 cm3 using tumbling or magnetic mixing were conducted. The parent (calcined) and alkaline-treated samples are characterised by XRD, AAS, N2 and Ar adsorption, TEM, 27Al MAS NMR, and FTIR. The introduction of mesoporosity in ZSM-22 crystals is not straightforward due to their peculiar characteristics: the rod-like morphology of their small crystallites, the one-dimensionality of the ellipsoidal micropore system, and an uneven Al distribution. Compared to other zeolite frameworks such as MFI, the generation of up to 95 m2 g−1 of (both inter- and intracrystalline) mesopore surface area by NaOH treatment leads to a sizeable drop of the micropore volume (down to 0.006 cm3 g−1), attributed to blocking by re-deposited Al species. A subsequent mild acid treatment in aqueous HCl restores ca. 90% of the original micropore volume and increases the mesopore surface area to 114 m2 g−1. However, due to the particular Al distribution in the parent ZSM-22 crystals, only 37% of the original Bronsted acidity is recovered. A new descriptor ‘desilication efficiency’ is introduced to relate the mesopore area generated to the mass of zeolite dissolved. In the case of ZSM-22 nanorods and ferrierite platelets, the desilication efficiency is relatively low compared to ZSM-5, most likely due to the crystal morphology of the former two zeolites. The auxiliary mesoporosity developed in ZSM-22 brings new prospects to catalytic applications of the zeolite due to the extensive creation of pore mouths in the hierarchical sample.

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.

Towards a Sustainable Manufacture of Hierarchical Zeolites

Hierarchical zeolites have been established as a superior type of aluminosilicate catalysts compared to their conventional (purely microporous) counterparts. An impressive array of bottom-up and top-down approaches has been developed during the last decade to design and subsequently exploit these exciting materials catalytically. However, the sustainability of the developed synthetic methods has rarely been addressed. This paper highlights important criteria to ensure the ecological and economic viability of the manufacture of hierarchical zeolites. Moreover, by using base leaching as a promising case study, we verify a variety of approaches to increase reactor productivity, recycle waste streams, prevent the combustion of organic compounds, and minimize separation efforts. By reducing their synthetic footprint, hierarchical zeolites are positioned as an integral part of sustainable chemistry.

Hierarchical high-silica zeolites as superior base catalysts

For more than four decades, the design of zeolite base catalysts has relied on the application of aluminium-rich frameworks exchanged with alkali metal cations (preferably Cs+). However, moderate activity associated with access and diffusion limitations, and high manufacturing costs associated with high caesium content (typically over 30%) have hampered their industrial implementation so far. Herein, we have discovered that high-silica USY zeolites outperform their Al-rich counterparts in a variety of base-catalysed reactions of relevance in the fine chemical industry, as well as in the upgrading of biofuels. The benefits of this class of materials are amplified upon the alleviation of diffusion constraints through the introduction of a network of intracrystalline mesopores by post-synthetic modification. For example, the resulting cation-free hierarchical USY provides an up to 30-fold Knoevenagel condensation activity compared to the benchmark Cs–X, and similar observations were made upon application in liquid-phase (nitro)aldol reactions. Moreover, in the gas-phase aldol condensation of propanal, high-silica zeolites provide superior activity, selectivity, and lifetime compared to caesium-containing zeolites and even a strong solid base such as MgO. We decouple the complex interplay between mesoporosity and intrinsic zeolitic properties such as crystallinity, and quantify the increase in catalyst effectiveness upon hierarchical structuring as a function of reactant size. The obtained results are a major step to resolve the drawbacks of zeolites catalysis and thereby revitalise their potential for industrial application.

Hydroisomerization of Emerging Renewable Hydrocarbons using Hierarchical Pt/H‐ZSM‐22 Catalyst

Last site standing: A new generation of hierarchical Pt/H-ZSM-22 zeolites is designed for the efficient processing of upcoming renewable feedstocks. The enhanced accessibility of the active sites is vital for the superior activity and exceptional selectivity in the hydroisomerization of model molecules such as nonadecane and pristane.

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.

Hierarchical zeolites overcome all obstacles: next stop industrial implementation.

This review emphasizes key recent accomplishments towards the industrial exploitation of hierarchically structured zeolites in catalytic processes. A major milestone comprises the demonstration that affordable post-synthetic modifications enable the transformation of any conventional zeolite into hierarchical analogues with tunable porosity and functionality. Through specific examples, belonging to the transformation of fossil fuel and renewable feedstocks, we quantitatively illustrate the spectacular benefits attained upon application of hierarchical zeolite catalysts due to improved accessibility or modification of the type and distribution of active sites. A crucial step for these exciting lab-designed materials to be implemented in industrial processes is to shape them into technical forms. Accordingly, we studied the synthesis, characterization, and catalytic evaluation of millimeter-sized hierarchical zeolite bodies, enriching the fundamental understanding on scale-up and representing an additional solid step towards the commercial application of these materials.