Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design.

The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

Visualizing the crystal structure and locating the catalytic activity of micro- and mesoporous ZSM-5 zeolite crystals by using in situ optical and fluorescence microscopy.

A combination of optical and fluorescence microscopy was used to study the morphology of micro- and mesoporous H-ZSM-5 zeolite crystals (17 x 4 x 4 microm) and to evaluate, in a spatially resolved manner, the effect of mesoporosity, introduced via desilication, on catalytic performance. For this purpose, the oligomerization of various styrene molecules was used as a model reaction, in which the carbocation intermediates formed in the zeolite pores act as reporter molecules. In situ confocal fluorescence measurements after the template removal process showed that the crystals generally consist of three different subunits that have pyramidal boundaries with each other. Examination of these crystals during styrene oligomerization revealed differences in the catalytic activity between the purely microporous and the combined micro- and mesoporous crystals. The introduction of intracrystalline mesoporosity limits the formation to dimeric carbocation intermediates and facilitates the transport of styrene molecules inside the zeolite volume. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, the oligomerization of various styrene compounds, which differ in their reactivity, provides a good way of estimating the Brønsted acid strength in a spatially resolved manner, showing a nonhomogeneously distributed Brønsted acidity over the volume of the crystals. More detailed information on the structure of the ZSM-5 crystals was revealed for mesoporous crystals during the oligomerization of 4-methoxystyrene. This reaction induced an "explosion" of the crystal leading to the formation of a complex system with at least eight different subunits. Finally, polarized-light microscopy was used to unravel the pore geometry in these individual building blocks. The observed differences in catalytic behavior between micro- and mesoporous ZSM-5 crystals are strengthened by the microspectroscopic techniques employed, which show that upon desilication the crystal morphology is affected, the product distribution is changed towards less conjugated carbocation intermediates, and that a gradient in Brønsted acid strength appears to be present.

Single-template synthesis of zeolite ZSM-5 composites with tunable mesoporosity.

Hierarchically structured composites (TUD-C) with ZSM-5 crystals embedded in a well-connected mesoporous matrix were synthesized by using only one organic templating/scaffolding molecule (TPAOH).

In situ monitoring of desilication of MFI-type zeolites in alkaline medium.

In situ pH and Attenuated Total Reflection (ATR) infrared techniques have been successfully applied in order to gain insights into the dissolution process connected to mesopore formation occurring upon alkaline treatment of ZSM-5 zeolites. Online pH measurements reveal a similar consumption of OH(-) ions in the initial stage of the reaction independent of the Si/Al ratio of the zeolite. In view of the greatly different mesoporosity development, the extraction of polymeric silica entities is anticipated, its structure depending on the framework Si/Al ratio. In agreement, ATR-IR experiments have confirmed dissolution of polymeric silicon-containing species that in the course of the alkaline treatment disintegrate into smaller entities. A direct relation between the type of porosity developed and the process of silicon extraction as measured in the liquid phase cannot be drawn.

Direct demonstration of enhanced diffusion in mesoporous ZSM-5 zeolite obtained via controlled desilication.

A 2 orders of magnitude gas transport improvement in a medium pore ZSM-5 zeolite has been achieved upon introduction of intracrystalline mesoporosity in gradient-free crystals by desilication post-treatment in alkaline medium.

Alkaline treatment of iron-containing MFI zeolites. Influence on mesoporosity development and iron speciation.

The effects of alkaline treatment on the mesoporosity development and iron speciation in Fe-MFI zeolites have been investigated. To this end, a variety of samples derived from different synthetic routes and having distinct Si/Al ratios and Fe content were treated in NaOH solutions and characterized by N2 adsorption, SEM, TEM, UV/vis spectroscopy, and EPR. The alkaline treatment induces a significant intracrystalline mesoporosity development by framework silicon extraction and promotes disintegration of oligomeric iron species. Iron in framework positions has shown to provoke mesopore formation, whereas nonframework iron species suppresses silicon leaching and lowers the extent of extra porosity.

Creation of hollow zeolite architectures by controlled desilication of Al-zoned ZSM-5 crystals.

Hollow zeolite architectures on different length scales have been obtained upon controlled desilication of Al-zoned ZSM-5 zeolites in alkaline medium. The hollow ZSM-5 crystals possess a functional Al-rich exterior and a tunable internal porosity and accessibility.

Mechanism of hierarchical porosity development in MFI zeolites by desilication: the role of aluminium as a pore-directing agent.

The role of the concentration and the nature of aluminium in the creation of hierarchical porosity in both commercial and synthesized MFI zeolites have been investigated through controlled mesoporosity development by desilication in alkaline medium. Framework aluminium controls the process of framework silicon extraction and makes desilication selective towards intracrystalline mesopore formation. An optimal molar Si/Al ratio in the range 25-50 has been identified; this leads to an optimal mesoporosity centred around 10 nm and mesopore surface areas of up to 235 m(2) g(-1) while preserving the intrinsic crystalline and acidic properties. At lower framework Si/Al ratios the relatively high Al content inhibits Si extraction and hardly any mesopores are created, while in highly siliceous ZSM-5 unselective extraction of framework Si induces formation of large pores. The existence of framework Al sites in different T positions that are more or less susceptible to the alkaline treatment, and the occurrence of re-alumination, are tentative explanations for the remarkable behaviour of Al in the desilication process. The presence of substantial extra framework Al, obtained by steam treatment, inhibits Si extraction and related mesopore formation; this is attributed to re-alumination of the extraframework Al species during the alkaline treatment. Removal of extraframework Al species by mild oxalic acid treatment restores susceptibility to desilication, which is accompanied by formation of larger mesopores due to the enhanced Si/Al ratio in the acid-treated zeolite.

Aldol condensations over reconstructed Mg-Al hydrotalcites: structure-activity relationships related to the rehydration method.

Two different rehydration procedures in the liquid or gas phase have been applied to reconstruct mixed oxides derived from calcined hydrotalcite-like materials to be used as catalysts for aldol condensation reactions. The as-synthesized hydrotalcite, its decomposition product, as well as the reconstructed solids upon rehydration were characterized by XRD, N(2) adsorption, He pycnometry, FTIR, SEM, TEM, (27)Al MAS-NMR and CO(2)-TPD (TPD=temperature-programmed desorption). Compared to the Mg-Al mixed oxide rehydrated in the gas phase (HT-rg), that rehydrated in the liquid phase (HT-rl) exhibits a superior catalytic performance with respect to the aldol condensation of citral with ketones to yield pseudoionones and in the self-aldolization of acetone. The textural properties of HT-rl and HT-rg differ strongly and determine the catalytic behavior. A memory effect led to a higher degree of reconstruction of the lamellar structure when the mixed oxide was rehydrated in the gas phase rather than in the liquid phase, although liquid-phase rehydration under fast stirring produced a surface area that was 26 times greater. This contrasts to typical statements in the literature claiming a higher degree of reconstruction in the presence of large amounts of water in the medium. CO(2)-TPD shows that the number of OH(-) groups and their nature are very similar in HT-rg and HT-rl, and cannot explain the markedly different catalytic behavior. Accordingly, only a small fraction of the available basic sites in the rehydrated samples is active in liquid-phase aldol condensations. Our results support the model in which only basic sites near the edges of the hydrotalcite platelets are partaking in aldol reactions. Based on this, reconstructed materials with small crystallites (produced by exfoliation during mechanical stirring), that is, possessing a high external surface area, are beneficial in the reactions compared to larger crystals with a high degree of intraplatelet porosity.

Adsorption of butane isomers and SF6 on Kureha activated carbon: 1. Equilibrium.

Adsorption equilibria of butane isomers and SF6 on Kureha activated carbon were investigated using the volumetric method and the tapered element oscillating microbalance (TEOM) technique. The isotherm data of the butane isomers measured by the TEOM technique are in good agreement with those determined by the volumetric method. Single-component adsorption isotherms are reported at temperatures in the range from 298 to 393 K and at pressures up to 120 kPa. SF6 molecules are mainly adsorbed in the larger micropores, resulting in a lower adsorption capacity. The amount adsorbed for n-butane is slightly higher than that for isobutane in the whole range investigated. This is attributed to the fact that the linear n-butane molecule can adsorb in the smaller micropores. The T6th model appropriately describes the equilibrium data of the butane isomers, while the isotherm data of SF6 can be fitted by the Langmuir model. The isosteric heats associated with adsorption for these three adsorptives show different loading dependences. The present study indicates that the activated carbon can be well characterized by the probe molecules having different molecular sizes.