ZEOLITE-SUPPORTED METAL-CARBONYLS - SENSITIVE PROBES FOR INFRARED SPECTROSCOPIC CHARACTERIZATION OF THE ZEOLITE SURFACE

The metal carbonyls [M(CO)(6)] (M = Mo or W) were found to be molecula rly adsorbed on the surface of H-ZSM-5 zeolite crystals. Three types o f adsorbed species were detected by IR spectroscopy: weakly physisorbe d metal carbonyls and [M(CO)(6)] molecules interacting (via a CO ligan d) with OH groups or with Lewis-acid sites (co-ordinatively unsaturate d Al3+ ions). For physisorbed species a single C-O stretching mode (T- 1u) was observed, around 1990 cm(-1). The (OC)(5)MCO ... HOSi species gave a band at 1965 cm(-1) (anchored CO ligand) and the corresponding E + 2A(1) modes at higher frequency. O-Bonding between the metal hexac arbonyls and Lewis-acid centres gives rise to a characteristic IR abso rption band in the range 1750-1850 cm(-1), and corresponding E + 2A(1) modes at <(v)over tilde (CO)> > 2000 cm(-1). Close inspection of thes e spectroscopic features, and comparison with IR spectra of adsorbed C O, enables characterization of the zeolite surface. For medium-pore ze olites an important feature of this method is that [M(CO)(6)] molecule s cannot penetrate inside the zeolite channels, therefore discriminati on between internal and external surface sites can be accomplished. Th us, it was found that both Bronsted-acid sites [bridged Si(OH)Al group s] and silanols are located mainly inside the zeolite channels.