Alexandra Inayat

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

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.

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.

Probing Mass Transfer in Mesoporous Faujasite-Type Zeolite Nanosheet Assemblies

Pulsed field gradient nuclear magnetic resonance (NMR) diffusion studies are performed by using cyclohexane to probe transport properties in a NaX-type zeolite with a hierarchical pore structure (house-of-cards-like assemblies of mesoporous nanosheets), which is compared with a purely microporous sample. With guest loadings chosen to ensure saturation of the micropores, and the meso- and macropores left essentially unoccupied, guest diffusion is shown to be enhanced by almost one order of magnitude, even at room temperature. Diffusivity enhancement is further increased with increasing temperature, which may, therefore, be unambiguously attributed to the contribution of mass transfer in the meso- and macropores.

Effect of the Structure and Morphology of Natural, Synthetic and Post-processed Graphites on Their Dispersibility and Electronic Properties

Twenty-two different graphite grades of the families natural, synthetic, intercalated, expanded, amorphous, and turbostratic graphite, as well as platelets and activated carbon, were analyzed on their dispersibility in N-methylpyrrolidone, cyclohexylpyrrolidone and water-sodium cholate solution. They were characterized for their properties (grain size, density, purity, surface area, pH in water) and morphology. Thermogravimetric analysis, Raman spectroscopy studies, and electrical conductivity measurements were also used. XRPD followed by a Rietveld refinement provided information on the amounts of the rhombohedral (3R) and hexagonal (2H) phases for the crystalline part of the material and the interlayer distances. The properties of graphites favoring better dispersibility are: small grain size and bulk density, neutral pH in water. The electrical conductivity in inversely proportional to the Raman D-band intensity and is highest for the graphites with shortest interlayer spacing. The D-, G- and 2D-bands in the Raman spectra of graphites are in an exponential relation.

In-situ ultrasound study of the kinetics of formation of zeolites Na–A and Na–X from coal fly ash

The kinetics of synthesis of zeolites Na–A and Na–X from fused South African class F coal fly ash were studied by using an ultrasound device as a real-time, in-situ diagnostic tool. Ex-situ techniques, for example XRD, ICP, and SEM, were used to complement the results of the kinetic study. Reaction rate, reaction order, and activation energy of crystallization processes in clear solution extracted from fused fly ash were calculated on the basis of ultrasound signal data recorded at different crystallization temperatures. Zeolite Na–X and zeolite Na–A crystals were both obtained without ageing. The zeolite Na–X sample showed contaminations of zeolite P and sodalite depending on the synthesis temperature. For zeolite Na–A the impact of ageing on the process of formation was also studied.

Recent advances in the synthesis of hierarchically porous silica materials on the basis of porous glasses

The thermal phase separation and subsequent leaching of sodium borosilicate glasses is a well established route for the preparation of porous glasses exhibiting adjustable pore sizes in the range of 1 nm up to almost 1 μm as well as a very flexible geometric shape. The combination of this route with a large spectrum of synthesis strategies for the implementation of an additional pore system enables the preparation of hierarchically porous glass-based materials. This review covers a wide range of preparative routes for hierarchically porous silica materials starting from the sodium borosilicate glass with a special emphasis on the very recent developments in this versatile field of materials engineering.

Comparative Study between Direct and Pseudomorphic Transformation of Rice Husk Ash into MFI-Type Zeolite

Pre-shaped mesoporous amorphous rice husk ash (RHA) and MCM-41 derived from RHA as a silica source were transformed into MFI-type zeolites using two different structure-directing agents. Tetrapropylammonium hydroxide (TPAOH) was utilized as an alkali source for silica dissolution and structure control during the direct transformation of RHA into zeolite. A monopropylamine (PA)-containing alkaline solution (NaOH) was used for the pseudomorphic transformation of RHA or MCM-41 into zeolite. The hydrothermal conversion of RHA or MCM-41 into MFI-type zeolites was investigated as a function of reaction time at 175 °C. With PA as template, the crystallization took place inside and on the outer surface of RHA or MCM-41 without losing the original shape of the initial silica sources, while TPAOH led to the formation of conventional MFI-type zeolite crystals due to the complete dissolution of RHA. The final products were characterized by X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and optical emission spectroscopy.

Development of Novel Mesoporous Silica-Based Bioactive Glass Scaffolds with Drug Delivery Capabilities

Novel silica-based bioactive glasses were successfully prepared by the sol-gel method. The optimized glass composition for fabrication of the scaffolds was (in mol.%) 60% SiO2 – 30% CaO - 5% Na2O - 5% P2O5 (60S30C5N5P). This composition was confirmed to develop a thick hydroxycarbonate apatite (HCA) layer in Simulated Body Fluid (SBF) after 7 days, as revealed by Fourier Transform Infrared Spectroscopy (FTIR), indicating the bioactive character of the scaffolds. The mesoporous nature of the glass structure allows the load of tetracycline and a sustained release of the drug in PBS during 7 days was measured.

Thermally Induced Growth of ZnO Nanocrystals on Mixed Metal Oxide Surfaces

An in situ method for the growth of ZnO nanocrystals on Zn/Al mixed metal oxide (MMO) surfaces is presented. The key to this method is the thermal treatment of Zn/Al layered double hydroxides (Zn/Al LDHs) in the presence of nitrate anions, which results in partial demixing of the LDH/MMO structure and the subsequent crystallization of ZnO crystals on the surface of the forming MMO layers. In a first experimental series, thermal treatment of Zn/Al LDHs with different fractions of nitrate and carbonate in the interlayer space was examined by thermogravimetry coupled with mass spectrometry (TG-MS) and in situ XRD. In a second experimental series, Zn/Al LDHs with only carbonate in the interlayer space were thermally treated in the presence of different amounts of an external nitrate source (NH4NO3). All obtained Zn/Al MMO samples were analysed by electron microscopy, nitrogen physisorption and powder X-ray diffraction. The gas phase formed during nitrate decomposition turned out to be responsible for the formation of crystalline ZnO nanoparticles. Accordingly, both interlayer nitrate and the presence of ammonium nitrate led to the formation of supported ZnO nanocrystals with mean diameters between 100 and 400 nm, and both methods offer the possibility to tailor the amount and size of the ZnO crystals by means of the amount of nitrate.