IR studies and DFT quantum chemical calculations concerning interaction of some organic molecules with Cu~+ sites in zeolites

摘要

It is well-known that Cu~+ cations in zeolites,especially in ZSM-5 activate NO molecule resulting in its decomposition.Quantum chemical calculations evidenced that it is due to Tt-backdonation.The aim of our studies was to answer the question whether Cu~+ in zeolites is also able to activate other re-electron systems according to the same mechanism,i.e.by Tt-backdonation of d electrons of copper to pi~* antibonding orbitals.We studied the adsorption of alkenes (ethene,propene,cis-but-2-ene,trans-but-2-ene),acetylene,benzene,and acetone with Cu~+ sites in CuX,CuY,CuZSM-5.The main experimental method was IR spectroscopy but we also performed DFT calculations.Both evidenced weakening C=C and C ident to C bond.IR red shift reaches 78-115 cm~(-1) for C=C band and 168 cm~(-1) for acetylene.Moreover,in the case of ethene and acetylene the stretching of the C=C and C ident to C bonds which were IR inactive in free molecules became IR active when interacting with Cu+,indicating the loss of symmetry.Contrary,the C=C stretching in trans-but-2-ene was still IR inactive when trans-but-2-ene interacted with Cu~+.At high loading some Cu~+ ions were able to bond two alkene molecules.Although the activation is small,it occurs also for aromatic C-C bond in benzene adsorbed on Cu~+ site;the red band shift was 13 cm~(-1),i.e.much less than in the case of alkenes and acetylene.Similarly,the activation of C=O bond in acetone interacting with Cu~+ results also in the red band shift of 38 cm~(-1).All the systems have been studied by DFT modeling which reveals the function of zeolite as a host for transition metal cationic centers which act as electron transmitters.Flow of electrons and subsequent activation is determined by the electronic structure of the molecules,their electron affinity and symmetry as well as the ability of the site to pi-backdonation.