How do copper sites in Cu/H-ZSM-5 zeolites interact with adsorbed alkenes?

Abstract

Copper-modified zeolites can be used for light alkene transformation to valuable chemicals, such as higher alkenes, simple aromatic hydrocarbons, and oxygenates (aldehydes, carboxylic acids, etc.). With the aim of clarifying the properties of Cu2+ and Cu+ sites in zeolites for alkene C=C bond activation, C2-C4 alkene (ethene, propene, isobutene) interaction with copper-modified ZSM-5 zeolites at 296 K has been monitored by FTIR and EPR spectroscopy, as well as by XANES. Alkene π-complexes with Cu2+ and Cu+ sites of Cu2+/H-ZSM-5 and Cu+/H-ZSM-5 samples have been characterized by their specific absorption bands. FTIR, EPR, and XANES data demonstrate partial reduction of Cu2+ to the Cu+ state upon the reaction with alkenes, with the alkene reactivity changing in the order: isobutene > propene > ethene. The formation of hydrocarbon paramagnetic species from alkenes on Cu2+/H-ZSM-5 has been detected by EPR spectroscopy. Possible mechanisms of Cu2+ site reduction are discussed. It is suggested that the electron transfer from an alkene π-bond to Cu2+ can lead to Cu2+ becoming effectively Cu+ and alkene transforming to a radical cation. The reduction of Cu2+ ions by allylic species is found to be improbable according to the experimental data obtained.