The interaction of butenes with Cu⁺ ions in CuMCM-41 studied by IR spectroscopy

As CuMCM-41 was found to be active in skeletal isomerisation of n-butenes, we followed by IR spectroscopy the interaction of but-1-ene, cis-but-2-ene, and trans-but-2-ene with Cu⁺ sites in CuMCM-41. It has been revealed that Cu⁺ activated strongly CC bond, what resulted in a frequency shift of CC stretching band by about 100 cm^-1. Moreover, the vibration of CH bonds in CH₂ and CH groups neighbouring to the double bonds was also perturbed. We suppose that the activation of CC bond is a result of π-back donation of d electrons of Cu⁺ to π ^* antibonding orbitals of butenes. IR results evidenced also the heterogeneity of Cu⁺ sites in CuMCM-41 and the presence of Cu⁺ sites of various electron donor properties. But-1-ene molecules bonded to Cu⁺ ions are less prone to accept protons from Brønsted acid sites than molecules bonded by hydrogen bonding to non-acidic SiOH groups, even though they are more negative than free or physisorbed molecules. The IR studies of coadsorption of CO and but-1-ene suggest that the interaction of CO with Cu⁺ is much weaker if but-1-ene is present, than without butene.