Substrate size-selective catalysis with zeolite-encapsulated gold nanoparticles.

Over the years, many strategies have been developed to address the problem of sintering of nanoparticle catalysts, including encapsulating metal nanoparticles in protective shells, and trapping nanoparticles in the cavities of certain zeolites in post-synthesis steps. In general, materials that contain metal nanoparticles that are only accessible via zeolite micropores are intriguing, specifically, but not exclusively, for catalytic applications. The encapsulation of carbon nanoparticles during zeolite crystallization is a well-known approach for making carbon–zeolite composites that afford mesoporous zeolites after combustion. Herein, we show that metal nanoparticles can also be encapsulated during zeolite crystallization, as exemplified by silicalite-1 crystals that are embedded with circa 1–2 nm-sized gold nanoparticles that remain stable and catalytically active after calcination in air at 550 8C. Moreover, we show that the encapsulated gold nanoparticles are only are accessible through the micropores of the zeolite, which makes this material a substrate-size selective oxidation catalyst. Currently, more than 175 different zeolite structures have been reported, and these can be tuned according to the desired acidity and/or redox properties. Expanding the scope from pure zeolites to hybrid materials, by combining the properties of zeolites with other components, significantly widens the field of zeolite materials design. Aside from posttreatment methods, two types of approaches have been pursued for preparing hybrid zeolite–nanoparticle materials. The first type of approach involves crystallization of the zeolite from a gel that contains metal ions that are immobilized in the zeolite during crystallization. With this kind of approach, it is very difficult to control the properties of the non-zeolite component in terms of, for example, particle size. The other type of approach is to first synthesize the nonzeolite component and subsequently encapsulate this in the individual zeolite crystals during crystallization. Indeed, this strategy is also well-known and an entire family of materials, known as mesoporous or hierarchical zeolite crystals, are based on the embedding of carbon nanoparticles, nanofibers, nanotubes, or other nanostructures during zeolite crystallization (and subsequent combustion) in a process known as carbon templating. Concerning the embedding of metal nanoparticles in zeolites, Hashimoto et al. reported a top down approach that features downsizing gold flakes to approximately 40 nm particles by laser ablation, and subsequent encapsulation of these particles during crystallization. A reduction in particle size by one order of magnitude is necessary for an efficient use of costly noble metals in catalytic applications. However, a reduction of the particle size enhances the tendency for sintering, owing to the increase in surface free energy. To mitigate this problem, we report herein a bottom-up approach for the preparation of hybrid zeolite-nanoparticle materials that contain small metal nanoparticles, dispersed throughout the zeolite crystals. This synthetic approach comprises three steps (Figure 1): First, a metal nanoparticle colloid is prepared with suitable anchor points for the generation of a silica shell. Second, the particles are encapsulated in an amorphous silica matrix. Third, the silica nanoparticle precursor is subjected to hydrothermal conditions in order for zeolite crystallization to take place. Using this approach, we successfully prepared a material that consisted predominantly of circa 1–2 nm sized gold particles that were embedded in silicalite-1 crystals. X-ray diffraction revealed that the material contained exclusively gold as well as MFI-structured material (generalized silicalite-1 crystal structure type). Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid material that consists of gold nanoparticles embedded in silicalite-1 crystals. The SEM image reveals that the material is mainly composed of circa 1–2 mm long coffinshaped crystals, with a minor fraction of intergrown coffin[*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen, S. Helveg, Prof. C. H. Christensen, K. Egeblad Haldor Topsøe A/S Nymøllevej 55, 2800 Kgs. Lyngby (Denmark) E-mail: [email protected] [email protected]