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 molecularly adsorbed on the surface of H-ZSM-5 zeolite crystals. Three types of adsorbed species were detected by IR spectroscopy: weakly physisorbed metal carbonyls and [M(CO)(6)] molecules interacting (via a CO ligand) with OH groups or with Lewis-acid sites (co-ordinatively unsaturated 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 hexacarbonyls and Lewis-acid centres gives rise to a characteristic IR absorption 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 these spectroscopic features, and comparison with IR spectra of adsorbed CO, enables characterization of the zeolite surface. For medium-pore zeolites an important feature of this method is that [M(CO)(6)] molecules cannot penetrate inside the zeolite channels, therefore discrimination between internal and external surface sites can be accomplished. Thus, it was found that both Bronsted-acid sites [bridged Si(OH)Al groups] and silanols are located mainly inside the zeolite channels.