Platinum–zeolite interactions in alkaline L zeolites. Correlations between catalytic activity and platinum state

Abstract

Alkaline platinum L zeolites have been studied with regard to the state of the platinum and the catalytic activity of the platinum sites in benzene hydrogenation and n-hexane dehydrocyclisation. X-ray diffraction, electron micrography and infrared studies of CO adsorption led to the conclusion that there are four types of Pt particles: large 100–600 Å particles outside the channels, crystals 10–25 Å in diameter and small metallic cylinders inside and outside the channels and very small particles in cavities, the latter giving reversible Pt carbonyl clusters upon CO adsorption. The Pt active sites in benzene hydrogenation and n-hexane dehydrocyclization (10–25 Å crystals and small cylinders) are strongly dependent upon their environment. In the absence of any acidity their activity increases with the zeolite basicity, while no electron deficiency is observed. The infrared band at 2060–2065 cm^–1 of adsorbed CO suggests that Pt particles have an excess of electrons and/or typical faces, corners or edges. It is suggested that the L zeolite structure and field induced these unusual Pt state and catalytic properties.