Electron Spin Resonance and Electron Spin Echo Modulation Studies of Catalytic Ethylene Dimerization on Palladium-Exchanged Silicoaluminophosphate Type 5, 8, and 11 Molecular Sieves

Abstract

Oxygen-pretreated, palladium-exchanged silicoaluminophosphate SAPO-11, SAPO-5, and SAPO-8 are catalytically active for ethylene dimerization. The catalytic activity of the silicoaluminophosphates is shown to be due to Pd(I) species and is greatly dependent on the large channel size of the supporting materials which vary from 10-ring (SAPO-11) to 12-ring (SAPO-5) to 14-ring (SAPO-8) sizes. The selectivity for the formation of n-butenes is influenced by the reaction temperature and the channel size, showing that lower temperatures and larger channel diameters favor n-butene formation. Electron spin resonance studies show isolated palladium(I) species A (g∥ = 2.963 and g⊥ = 2.141) and B (g∥ = 2.678 and g⊥ = 2.078) after activation in SAPO-5 and SAPO-11. In SAPO-8 only species A‘ (g∥ = 2.91 and g⊥ = 2.12) can be observed. After ethylene adsorption in all materials, species G (g∥ = 2.245 and g⊥ = 2.003) can be seen, which transforms to species H (g∥ = 2.421 and g⊥ = 2.033) and K (g∥ = 2.641 and g⊥ = 2.076) during the reaction with simultaneous butene formation detected by gas chromatography. Electron spin echo modulation analysis allows the assignment of species G to a Pd(I)−C2D2 complex and species H to a Pd(I)−C4D8 complex. This work shows that in silicoaluminophosphates monovalent palladium cations coordinate ethylene and are catalytically active for ethylene dimerization.