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