Design of sintering-stable heterogeneous catalysts

One of the major issues in the use of metal nanoparticles in heterogeneous catalysis is sintering. Sintering occurs at elevated temperatures because of increased mobility of nanoparticles, leading to their agglomeration and, as a consequence, to the deactivation of the catalyst. It is an emerging problem especially for the noble metals-based catalysis.

These metals being expensive and scarce, it is worth developing catalyst systems which preserve their activity over time. Encapsulation of nanoparticles inside zeolites is one of the ways to prevent sintering. Entrapment of nanoparticles inside the crystalline framework of a zeolite creates a steric hindrance against agglomeration into larger clusters.

In the present study, experimental protocols for encapsulation of metal nanoparticles inside zeolites were developed. Two different methodologies were proposed to encapsulate gold, palladium and platinum nanoparticles inside silicalite-1 - the pressure assisted impregnation and reduction method (PAIR), and in situ incorporation method.

PAIR is based on modified incipient wetness impregnation technique in which the impregnation of a zeolite with a solution of metal precursor and its reduction are performed under elevated pressure. In situ incorporation is a one-pot procedure based on simultaneous growth of a zeolite and entrapment of metal precursor in a form of ethylenediamine complex inside the forming crystalline network, leading to encapsulated metal nanoparticles after reduction in hydrogen.

The PAIR procedure was used to successfully synthesize gold nanoparticles, 2-3 nm in size inside silicalite-1 and ZSM-5. Silicalite-1 with 2 nm palladium nanoparticles uniformly distributed on the external surface of the crystal was synthesized as well using the PAIR method. The in situ incorporation method was used to produce single metal palladium and platinum nanoparticles, 2-3 nm in size, and bi-metallic palladium/platinum nanoparticles inside silicalite-1.

Materials were primarily characterized using XRD, nitrogen physisorption, TEM, and XRF techniques. STEM and XPS were also used in selected cases. Synthesized catalysts were able to decompose formic acid with ~ 85% selectivity towards hydrogen at temperatures around 100 °C. Palladium/slicalite-1 catalysts were shown to be very active in Suzuki cross-coupling reaction of bromobenzene and 4-methoxyphenylboronic acid in methanol at 70 °C reaching yields of ~ 85% after 45 min.

Oxidation of allyl alcohol to its methyl esters at ambient conditions showed a very low activity of gold/silicalite-1 catalyst due to limited diffusion and possible adsorption of products inside the pores of zeolite. It was shown that the PAIR method and in situ incorporation method are feasible and easy protocols for synthesis of metal nanoparticles encapsulated inside a zeolite matrix.

Small size, stability towards sintering and high activity of nanoparticles obtained using the PAIR method and in situ incorporation method makes these two protocols promising for further research.