Wilhelm Schwieger

Zeolitic Materials with Hierarchical Porous Structures

During the past several years, different kinds of hierarchical structured zeolitic materials have been synthesized due to their highly attractive properties, such as superior mass/heat transfer characteristics, lower restriction of the diffusion of reactants in the mesopores, and low pressure drop. Our contribution provides general information regarding types and preparation methods of hierarchical zeolitic materials and their relative advantages and disadvantages. Thereafter, recent advances in the preparation and characterization of hierarchical zeolitic structures within the crystallites by post-synthetic treatment methods, such as dealumination or desilication; and structured devices by in situ and ex situ zeolite coatings on open-cellular ceramic foams as (non-reactive as well as reactive) supports are highlighted. Specific advantages of using hierarchical zeolitic catalysts/structures in selected catalytic reactions, such as benzene to phenol (BTOP) and methanol to olefins (MTO) are presented.

Porous glasses in the 21st century––a short review

This article reviews recent studies concerning the preparation, modification, characterization, modeling and application of porous glasses on the basis of phase-separated alkali borosilicate glasses.

Assemblies of Mesoporous FAU-Type Zeolite Nanosheets†

Hierarchical porous materials are of great scientific as well as technological interest because the presence of porosity on different scales has the potential to affect the transport characteristics of molecules within the pore system. The targeted design of the pore hierarchy in such materials will result in improved performance in transport-based applications, such as adsorption, catalysis, and separation. 2] Hierarchical materials containing zeolites, combine characteristics of systems with pore sizes on at least two different length scales. Compared to the allover microporous channels in conventional zeolite crystals, such hierarchical pore systems, with wide transport pores intersecting the micropore network like motorways linking a narrow road system, are already proven to reduce diffusion limitations for molecules within zeolitic catalysts. While for high-silica zeolites, such as ZSM-5, several methods for incorporating additional transport pores were developed during the past few years, 4, 11] such a substantial collection of approaches is not available for low-silica zeolites. Herein, we present the synthesis and characterization results of Faujasite (FAU)type zeolite X (Si/Al< 1.5) grown as house-of-cards-like nanosheet assemblies with intracrystalline mesoporosity. The resulting pore system covers all three pore size levels (micro-meso-macro) in a hierarchical interconnection. The reduction of diffusion limitations in microporous zeolites can be achieved by the creation of intracrystalline transport pores (in the meso or macro pore range) or by the reduction of the zeolite crystal dimensions itself. Zeolite crystals with additional meso (or in few cases macro) pores can be produced by different methods, for example, desilication by an alkaline post-treatment 15] or by the use of hard 17] and soft 18–20] templates during zeolite synthesis. However, all the methods are usually limited to a certain group of zeolites or just to a special zeolite type. For example, for zeolites with a Si/Al molar ratio below about 15, desilication is not an appropriate method for the introduction of additional transport pores. Thus, it is very challenging to introduce transport pores into low-silica (high aluminum content) zeolites. To date, only the low-silica zeolites LTA and SOD could be synthesized with intracrystalline mesoporosity by soft-templating with organosilane surfactants such as 3(trimethoxysilyl)propyl hexadecyl dimethyl ammonium chloride (TPHAC). 20] There are a few reports on the synthesis of mesoporous FAU-type zeolite Y, either by using carbon aerogel as a hard template, by steaming, or by a combined acid–base post-treatment. With the acid–base technique a threefold hierarchical pore system, combining the zeolitic micropores and two ranges of mesopores, was obtained with a resulting Si/Al molar ratio of around 20. But owing to the underlying extraction mechanism, this combined acid–base post-treatment is (as well as the single alkaline post-treatment) not suitable for introducing mesopores into low-silica zeolites with Si/Al molar ratios close to 1. Furthermore, all the examples reported for hierarchical zeolites involve either a micro/meso 16, 17,19–22] or a micro/ macro pore size combination, but to our knowledge there is no zeolitic material which combines pores of all three (micromeso-macro) size levels hierarchically interconnected within one particle. In fact, for Faujasite-type zeolite X, a highly hydrophilic large-pore zeolite with a pore diameter of about 0.74 nm and with a low Si/Al molar ratio of 1.0–1.5, a successful method for the creation of additional transport porosity has not been reported up to now. This limitation is a drawback, especially from the viewpoint of using renewable feedstocks for the production of so-called new platform chemicals. Such processes usually involve the transformation of larger molecules, such as fatty acids or sugars. In this area especially basic catalysts such as zeolite X are of high importance, for example for transesterification reactions. 25] Thus it seems to be an essential task to design the properties of such catalysts according to the future demands. Consequently, our major aim was to find a facile synthesis pathway for the implementation of additional transport porosity in Faujasite-type zeolite X. For the synthesis of the mesoporous zeolite X nanosheets the organosilane template TPHAC was used. The as-synthesized material was designated NaX-T, and after template removal by calcination as NaX-T-cal. For comparison purpose a conventional microporous zeolite X (NaX-R, NaX-R-cal) was synthesized using the same synthesis conditions and composition but without TPHAC. As can be seen from the powder X-ray diffraction (XRD) patterns in Figure 1, the reflections of both synthesis products could be attributed to the Faujasite structure and were indexed accordingly. Competing crystalline phases such as zeolite LTA and zeolite P (GIS structure) were not observed. Furthermore, the absence of a halo between 2q = 25–308 in the XRD patterns as well as the absence of sponge-like material in the SEM pictures in Figure 2 indicate the absence of amorphous material. Elemental analysis gave a Si/Al molar ratio of 1.2 [*] A. Inayat, Prof. W. Schwieger Chair of Chemical Reaction Engineering, Department of Chemical and Bioengineering, University of Erlangen-Nuremberg Egerlandstrasse 3, 91052 Erlangen (Germany) E-mail: wilhelm.schwieger@crt.cbi.uni-erlangen.de

Ethene/ethane and propene/propane separation via the olefin and paraffin selective metal-organic framework adsorbents CPO-27 and ZIF-8.

Two types of metal-organic frameworks (MOFs) have been synthesized and evaluated in the separation of C2 and C3 olefins and paraffins. Whereas Co2(dhtp) (=Co-CPO-27 = Co-MOF-74) and Mg2(dhtp) show an adsorption selectivity for the olefins ethene and propene over the paraffins ethane and propane, the zeolitic imidazolate framework ZIF-8 behaves in the opposite way and preferentially adsorbs the alkane. Consequently, in breakthrough experiments, the olefins or paraffins, respectively, can be separated.

Minkowski tensor shape analysis of cellular, granular and porous structures

Predicting physical properties of materials with spatially complex structures is one of the most challenging problems in material science. One key to a better understanding of such materials is the geometric characterization of their spatial structure. Minkowski tensors are tensorial shape indices that allow quantitative characterization of the anisotropy of complex materials and are particularly well suited for developing structure-property relationships for tensor-valued or orientation-dependent physical properties. They are fundamental shape indices, in some sense being the simplest generalization of the concepts of volume, surface and integral curvatures to tensor-valued quantities. Minkowski tensors are based on a solid mathematical foundation provided by integral and stochastic geometry, and are endowed with strong robustness and completeness theorems. The versatile definition of Minkowski tensors applies widely to different types of morphologies, including ordered and disordered structures. Fast linear-time algorithms are available for their computation. This article provides a practical overview of the different uses of Minkowski tensors to extract quantitative physically-relevant spatial structure information from experimental and simulated data, both in 2D and 3D. Applications are presented that quantify (a) alignment of co-polymer films by an electric field imaged by surface force microscopy; (b) local cell anisotropy of spherical bead pack models for granular matter and of closed-cell liquid foam models; (c) surface orientation in open-cell solid foams studied by X-ray tomography; and (d) defect densities and locations in molecular dynamics simulations of crystalline copper.

Reactivity and applications of layered silicates and layered double hydroxides

Layered materials, such as layered sodium silicates and layered double hydroxides (LDHs), are well-known for their remarkable adsorption, intercalation and swelling properties. Their tunable interlayers offer an interesting avenue for the fabrication of pillared nanoporous materials, organic-inorganic hybrid materials and catalysts or catalyst supports. This perspective article provides a summary of the reactivity and applications of layered materials including aluminium-free layered sodium silicates (kanemite, ilerite (RUB-18 or octosilicate) and magadiite) and layered double hydroxides (LDHs). Recent developments in the use of layered sodium silicates as precursors for the preparation of various porous, functional and catalytic materials including zeolites, mesoporous materials, pillared layered silicates, organic-inorganic nanocomposites and synthesis of highly dispersed nanoparticles supported on silica are reviewed in detail. Along this perspective, we have attempted to illustrate the reactivity and transformational potential of LDHs in order to deduce the main differences and similarities between these two types of layered materials.

Transformation of porous glass beads into MFI-type containing beads

Abstract Macroporous glass beads (Na2O–B2O3–SiO2) with a particle diameter of ∼300 μm and an average pore diameter of 92 nm, obtained by the VYCOR-process, were transformed into MFI-type containing beads, that retained the original shape. The conversion was achieved through a hydrothermal treatment with an aqueous mixture of an Al-source (sodium aluminate or aluminum sulfate) and a template (tetrapropylammonium bromide (TPABr) or dipropylamine (DPA)). The resulting products were characterized by X-ray diffraction, 11B MAS NMR, N2-adsoption and SEM. The filtrates were analyzed by ICP-OES. Using two different templates and two kinds of Al-source MFI-type beads with varying properties were obtained. The boron atoms, which are constituents of the glass material, were used as probe atoms for the characterization of the synthesis procedure. With DPA the crystallization took place inside and on the outer surface of the spheres, while TPABr led to a shell like crystallization starting from the outer surface of the glass particles. The type of Al-source influences the pH value of the reaction mixture and varies the Si/B ratio in the obtained products.

Hydrothermal transformation of a layered sodium silicate, kanemite, into zeolite Beta (BEA)

Hydrothermal transformation of a layered sodium silicate, kanemite, in the presence of tetraethylammonium hy- droxide (TEAOH, 30%), into a commercially important zeolite Beta (BEA) has been investigated. During the hy- drothermal transformation of kanemite, zeolite Na-P1 was formed as an intermediate product and subsequently transformed into highly crystalline and phase-pure zeolite Beta as indicated by different physico-chemical characteri- sation techniques. The influence of various parameters such as SiO2/Al2O3 ratio, temperature, and TEAOH/SiO2 ratio were examined. For comparisons, hydrothermal transformation of other layered silicates such as Na-magadiite and Na- ilerite were also studied under similar reaction conditions. The catalytic activity of a zeolite Beta sample obtained by the hydrothermal transformation of kanemite was also demonstrated in the cracking of n-hexane at atmospheric pressure in a fixed bed reactor.

Micro/Macroporous System: MFI‐Type Zeolite Crystals with Embedded Macropores

Zeolite crystals with an embedded and interconnected macropore system are prepared by using mesoporous silica particles as a silica source and as a sacrificial macroporogen. These novel hierarchical zeolite crystals are expected to reduce diffusion limitations in all zeolite-catalyzed reactions, especially in the transformation of larger molecules like in the catalytic cracking of polymers and the conversion of biomass.

MFI-type (ZSM-5) zeolite-filled TiO2 nanotubes for enhanced photocatalytic activity

The present work demonstrates enhanced photocatalytic activity for zeolite-filled TiO2 nanotubes. ZSM-5 zeolite nanocrystals were grown on and into a TiO2 nanotubular skeleton (TiNT/ZSM-5) by multi-step hydrothermal synthesis consisting of in situ seeding and multiple in situ crystallization (MISC). The resulting zeolite nanocrystals were in the range of a few nanometers and they adhere well to the nanotubular inner walls. After crystallization, the photocatalytic activity of this zeolite-filled nanotube catalyst system was compared with neat anatase TiO2 nanotube (TiNT) and with calcined ZSM-5 powder. The results show for TiNT/ZSM-5 a highly enhanced efficiency for the decomposition of acetophenone (used as an aromatic model organic pollutant).