Comparative ESR and catalytic studies of ethylene dimerization on Pd(II)-exchanged clinoptilolite, mordenite, ferrierite, and SUZ-4

Catalytic activity for ethylene dimerization is studied in four palladium-e xchanged zeolites, PdH-clinoptilolite, PdH-mordenite, PdH-ferrierite, and P dHK-SUZ-4, with different channel systems. The formation and adsorbate inte ractions of NJ) are also investigated for a better understanding of the rol e of Pd(I) on the catalytic reaction in these zeolites using electron spin resonance (ESR) spectroscopy. Thermal and hydrogen reduction of Pd(II) in c linoptilolite, mordenite, ferrierite, and SUZ-4 produce isolated Pd(I) with somewhat different ESR parameters. The interaction of Pd(I) with various s izes of adsorbates shows some similarities in PdH-mordenite and PdH-ferrier ite, indicating that Pd(I) is located in the main 12-ring and 10-ring chann els of mordenite and ferrierite, respectively, where all the Pd(I) can coor dinate with methanol and pyridine. However, SUZ-4 shows a noticeable differ ence in the way Pd(I) interacts with several adsorbates such as ammonia, me thanol, and ethylene. These adsorbates reduce Pd(II) to Pd(I) at 298 K, lea ding to the formation of two Pd(I) ions situated at two different sites of SUZ-4. One is the same Pd(I) ion site formed by thermal reduction but it ap pears at higher intensity upon adsorption. The other Pd(I) is an isolated P d(I) produced only after prolonged annealing with adsorbates. Our ESR and c atalytic results show that Pd(I) is active for ethylene dimerization in the se four channel-type zeolites. This activity is dependent on the location a nd accessibility of Pd(I) and the reaction temperature. All catalysts deact ivate due to the further reduction of Pd(I) by ethylene and butene. Analysi s of the composition of butene products indicates that PdH-clinoptilolite a nd PdHK-SUZ-4 reach a thermal equilibrium distribution faster than do PdH-m ordenite and PdH-ferrierite.