A new route for the synthesis of uniform nanozeolites with hydrophobic external surface in organic solvent medium.

Nanosized zeolite crystals (nanozeolites) with narrow particlesize distributions and sizes less than 100 nm have received much attention because of their great potential applications in catalysis and adsorption. The decrease in the crystal sizes results in high external surface areas, reduced diffusion path lengths, and more exposed active sites. For example, small Y-type zeolite crystals have been reported to increase catalytic activity and improve the selectivity of intermediate cracked products such as gasoline and light gas oil in catalytic cracking of heavy gas oil. This catalyst exhibited lower deactivation rates since the coke formation was suppressed.1 Moreover, nanozeolites can be used as “building units” for constructing hierarchical materials. Zeolite nanoclusters have been employed for assembling mesoporous alumosilicates.2 Materials with semicrystalline zeolitic mesopore walls3 and nanozeolite coated mesoporous aluminosilicates4 were reported by our group. Recently, zeolites with intracrystal mesopores and strong acidity were also synthesized.5 The resulting materials are considered of potential for application in catalysis and separation, owing to easier transport of guest molecules through the mesopores and shorter diffusion pathways in the zeolitic walls. Syntheses of nanozeolites are often carried out in the aqueous phase. During the crystallization, once the nanozeolite precursors are formed, the aqueous phase acts as an effective environment for the incorporation of soluble aluminosilicate species and the aggregation of zeolite crystals. This could lead to the formation of large crystals and aggregates.6 Direct synthesis using a clear gel solution of aluminosilicates can also produce nanozeolites by careful control of the gel composition and crystallizing conditions.7 Another method, which is called confined space synthesis, has been developed for the preparation of nanosized zeolite crystals. The synthesis is conducted within an inert matrix such as porous carbon matrices,8 thermoreversible polymer hydrogels, or microemulsions9 which provides a steric hindered space for zeolite crystal growth. Several synthetic routes have been reported for the preparation of nanocrystalline zeolites. However, none of these attempts has produced an easy means of controlling the small size. Furthermore, the external surface of nanocrystalline zeolites is hydrophilic and thereby has mostly silanol groups that limit catalytic reactivity to the internal pore surface.6 Serrano et al.10 have recently reported the use of organosilane as growth inhibitor. In this study, MFI and â zeolites were synthesized in the aqueous medium, using phenylaminopropyltrimethoxysilane (PHAPTMS). The synthesis is based on reducing the growth of zeolite crystals by silanization of the zeolitic seeds to hinder their further aggregation. However, as investigated by TEM analysis, the obtained MFI sample consisted of particles of about 300-400 nm which were formed by the aggregation of ultrasmall units of 10 nm. Having that large size, the sample was hardly considered as true nanozeolite. Herein, we demonstrate a new route for the synthesis of controlled uniform size nanozeolites with the hydrophobic external surface. An organic solvent is used as the medium for crystallization instead of water. The zeolite precursors are functionalized with organic silane groups. They thus become hydrophobic and highly dispersed in the organic solvent. Because the crystallization occurs in the organic phase and the zeolite precursors are protected by functional groups, catastrophic aggregation can be prevented hence, resulting in small and uniform nanozeolites with hydrophobic external surface. The MFI and faujasite zeolites were selected to illustrate our approach, because they are widely recognized for their unique properties as catalysts. As seen in Scheme 1, this approach is simple and was found to be reproducible when using hexadecyltrimethoxysilane as organosilane agent and the mixture of toluene and n-butanol as organic medium. The XRD patterns of the as-made nanozeolite samples prepared from silylated seeds are shown in Figure 1. Samples prepared by the conventional method in aqueous medium from the same clear zeolite gel solution without organosilane were used as references. The XRD pattern of the nanosilicalite-1 sample is identical to that of the reference, indicating the MFI structure of this nanosilicalite-1 sample. However, there is a clear broadening of the reflections, which is attributed to small crystals. In addition, no significant peak at 2θ ) 20-30° which is characteristic of amorphous phase was observed, indicating a relatively high crystallinity of this sample. A similar trend was also observed for the nanofaujasite sample (Figure 1B). Furthermore, the FTIR spectra of both as-made nanosilicalite-1 and nanofaujasite match well with the typical FTIR peaks assigned to silicalite-1 and zeolite Y, respectively (not shown).11 Representative TEM micrographs of the as-made silylated zeolite samples are shown in Figure 2 and exhibit very uniform nanocrystal sizes mostly with spherical and cubic shaped particles for silicalite-1 and zeolite Y, respectively. The particle size is about 21 nm for silicalite-1 and 27 nm for faujasite. The standard deviation established from the analysis of more than 1500 particles in representative TEM pictures of each sample showed a standard Scheme 1. Schematic Representation of the Single-Phase Synthesis Method Published on Web 03/10/2007