ADSORBATE INTERACTIONS OF PARAMAGNETIC PALLADIUM(I) SPECIES IN PD(II)-EXCHANGED NA-MCM-22 ZEOLITE

Adsorbate interactions of paramagnetic palladium(I) species in Pd(II)- exchanged Na-MCM-22 zeolite are studied by electron spin resonance (ES R) and electron spin-echo modulation (ESEM) spectroscopies. Oxidation and subsequent evacuation at 825 K of Na-MCM-22 zeolite exchanged with Pd(NH3)(4)Cl-2 results in the formation of a Pd(I) species. Monovalen t palladium can be produced in PdNa-MCM-22 material by room temperatur e hydrogen reduction after dehydration and O-2 treatment at high tempe rature. Addition of hydrogen to PdNa-MCM-22 after dehydration, O-2 tre atment, and evacuation (activation) is found to increase the concentra tion of Pd(I) at room temperature and at higher temperature is found t o reduce Pd(I) to Pd(0)(n) (palladium clusters). Adsorption of O-2 and H2O to an activated sample containing Pd(I) generates a Pd(II)-O-2(-) species with g(perpendicular to) > g(parallel to). Adsorption of CO o n an activated sample results in a Pd(I)-(CO)(2) complex which partial ly transforms to Pd(I)-CO on evacuation. Adsorption of ethylene on an activated sample produces Pd(I)-C2H4 and Pd(I)-C4H8 complexes, suggest ing that Pd(I) catalyzes dimerization of ethylene. Methanol and ethano l behave differently when adsorbed on an activated sample. While metha nol reduces Pd(I) to Pd(0)(n) at room temperature, ethanol forms a pal ladium complex suggested to be Pd(I)-CH3CH2OD. Adsorption of ammonia o n activated PdNa-MCM-22 leads to two Pd(I)-(ND3)(1) complexes, probabl y in different environments. On the other hand, adsorption of pyridine results only in a single species, perhaps due to steric effects. The kinetic size of the adsorbates plays a major role in forming complexes with Pd(I) in MCM-22 zeolite. A significant amount of Pd(I) formed du ring activation is located in sites inaccessible to relatively large m olecules while it reacts immediately with smaller adsorbates.