EPR investigation of PdI species in palladium-exchanged ZSM-5 and beta zeolites

Abstract

Paramagnetic Pd^I cations have been formed and characterised by EPR spectroscopy in high silica ZSM-5 and beta zeolites. In the case of Pd-ZSM-5, two dominant sites for Pd^I stabilisation were identified in the open channels of the structure, while a third minor species was also found in inaccessible positions, likely inside the five-membered rings. By comparison, only a single Pd^I cation site is visible in Pd-beta; this site is also located in the open-channel structure. A single molecule of pyridine and acetonitrile was shown to interact with the Pd^I centres in ZSM-5 at 100 K. The Pd^I-beta sample was also found to form a Pd^I–H₂ adduct reversibly at low temperatures (ca. ∽100 K). Carbon monoxide readily interacts with the cations in both Pd-ZSM-5 and Pd-beta, but the resulting distribution of adsorbed Pd–[CO]_n complexes is quite different depending on the zeolite framework.