Evidence for Migration of Palladium Species during Ethylene Dimerization on Palladium-Exchanged Na-Y and Ca-Y Zeolites Studied by Gas Chromatography Correlated with Electron Spin Resonance and Electron Spin-Echo Modulation Spectroscopies

Article abstract of DOI:10.1021/j100346a074

Various palladium-exchanged Na-Y and Ca-Y zeolites (0-14 Pd²⁺/unit cell) have been studied for ethylene dimerization.The reaction was found to occur on all O2-pretreated Pd-exchanged Y zeolites at 25 degC after an induction period that is dependent on the palladium content, type of location used (Na⁺ or Ca²⁺), and reaction temperature.This induction period is apparently due in part to formation of active Pd⁺ species and their migration toward the zeolite supercage.The generation of Pd⁺ species during catalyst pretreatment in H2 ether eliminates or greatly shortens the induction period.This together with electron spin resonance (ESR) and electron spin-echo modulation (ESEM) results confirms that monovalent palladium cations are active for dimerization reaction.ESR spectra of NaPd-Y and CaPd-Y zeolites recorded at various reaction times show that various Pd⁺ species (g_<*>=2.88-2.33, g_<*>=2.10) are coordinated to ethylene during the reaction.These can be detected by ESEM methods and confirm that Pd⁺-ethylene complex is formed during the dimerization reaction.The results are discussed in terms of a mechanism, for ethylene dimerization on zeolites.The results are also compared to previously obtained data on palladium-exchanged Na-X and Ca-X zeolites to examine the effect of the Si/Al ratio in the generation and migration of palladium species in the zeolites.