Electron Spin Resonance and Electron Spin Echo Spectroscopic Studies of Catalytic Dimerization of Ethylene on Palladium-Exchanged Na-X and Ca-X Zeolites. Proposed Reaction Mechanism for Ethylene Dimerization

Article abstract of DOI:10.1021/ja00232a014

Both NaPd-X and CaPd-X zeolitres are shown to dimerize ethylene selectively to n-butenes after an induction period, which is decreased with an increase of palladium content and is less for CaPd-X than for NaPd-X due to location of more Pd species at sites accessible to ethylene.A shorter induction period is also seen at higher reaction temperature due to migration of Pd species toward sites accessible to ethylene.Paramagnetic Pd⁺ species C (g_<*> = 2.53) and D (g_<*> = 2.33 - 2034), both with g_<*> values of 2.10, are observed to form prior to ethylene dimerization and have been assigned to Pd⁺-C2H4 and Pd⁺-(C2H4)2, respectively.These species are considered to be active intermediates for the reaction.When ethylene is dimerized to butene, paramagnetic species, assigned to Pd⁺ coordinated to butenes, are observed.Ethylene dimerization on the Pd-exchanged zeolites is suggested to occur via coordination of two ethylene molecules to a Pd⁺ paramagnetic species.The initial 1-butene product is isomerized to an equilibrium composition of n-butenes with trans-2-butene predominant.All catalysts are found to deactivate during ethylene dimerization probably due to reduction of Pd species to atomic Pd.