Incorporation of RuO 2 nanoparticles into MFI-type zeolites

Abstract

Ruthenium oxide particles were introduced into the medium-pore zeolite ZSM-5 (MFI) by ion exchange in aqueous suspension and by the addition of ruthenium chloride to the synthesis gel in hydrothermal zeolite syntheses. The latter approach was systematically studied in the presence and absence of tetrapropyl ammonium ions as organic structure-directing agents. The materials were characterized by chemical analysis, XRD, SEM, XANES, EXAFS, HRTEM, hydrogen chemisorption and the competitive hydrogenation of 1-hexene and 2,4,4-trimethyl-1-pentene. This catalytic test reaction was applied to discriminate between ruthenium species in the zeolite pores and on the outer crystal surface. The addition of ruthenium chloride during the zeolite synthesis strongly affects the rate of crystallization. In the case of the template-free synthesis, the presence of ruthenium chloride in the synthesis gel also changes the size and the morphology of the zeolite crystals. Independent of the preparation conditions, the ruthenium species are oxidized during the hydrothermal synthesis or during the post-synthesis modification to ruthenium oxide with ruthenium in the oxidation state +IV. However, by ion exchange in aqueous suspension or by adding ruthenium chloride to the gel of a tetrapropyl ammonium-templated hydrothermal synthesis with subsequent calcination, large anhydrous ruthenium oxide particles are formed. These are predominantly located on the outer surface of the zeolite crystals. Upon adding ruthenium chloride to a template-free gel of a hydrothermal ZSM-5 synthesis, hydrous ruthenium oxide nanoparticles can be prepared. The majority of these particles are between 0.5 nm and 0.9 nm in diameter, located inside the pores of the MFI structure, and are, thus, available for shape-selective catalytic conversions.