Potential of triphenylphosphine as solid-state NMR probe for studying the noble metal distribution on porous supports

Abstract

Abstract Triphenylphosphine (PPh3) is demonstrated to be a useful molecular probe and tool for quantitatively characterizing the spatial distribution of noble metals on porous supports. The preparation of metal organic complexes by loading the supports with PPh3 was performed by physically mixing the water-free catalysts with solid PPh3 powder in nitrogen atmosphere and heating this mixture at 363 K for 20 h. Based on quantitative 31P MAS NMR spectroscopy, the formation of noble metal (NM) tetrakis(triphenylphosphine) complexes (PPh3)4NM on Pt- and Rh-containing silica and SBA-15 supports was evidenced. The large specific surface areas and mesopore diameters (3.4–4.5 nm) of these supports provide suitable conditions for the quantitative formation of (PPh3)4NM complexes with a molecule size of ca. 2 nm. The micropores and cages of zeolites Y (0.72 and 1.14 nm, respectively), on the other hand, are too small for the formation of these complexes. Consequently, quantitative 31P MAS NMR investigations of noble metal containing zeolites Y indicated that only 7% of the Pt and 6% of the Rh atoms are accessible for the complex formation with PPh3. In other words, these noble metal quantities are located outside of the micropores and cages of the zeolite particles. The non-involved 93% of Pt and 94% of Rh atoms on the zeolites Y under study are located inside the micropores and cages of this support material, making them available for shape-selective heterogeneous catalysis.