Unusual Complete Reduction of Cu2+ Species in Cu-ZSM-5 Zeolites under Vacuum Treatment at High Temperature

By electron paramagnetic resonance (EPR), coupled to elemental and thermogravimetric analysis and diffuse reflectance spectroscopy (DRS), we investigated the unusual quasi-complete reduction of Cu 2+ species in Cu-ZSM-5 zeolites (Si/Al = 80 and 25) at different copper loadings induced by high-temperature vacuum treatments. It has been found that at high temperatures (673–773 K) in vacuum a quasi-complete reduction of Cu 2+ unexpectedly occurred in all the “as-prepared Cu-ZSM-5 samples. Evidence is given that such extensive reduction of Cu 2+ species is caused by carbonaceous deposits. In order to completely remove any residual carbonaceous species, the “as-prepared” Cu-ZSM-5 materials must be heated in air (or O 2 ) at high temperature (673–773 K) for prolonged time. Differently, some carbon residue, as in particular originated from the organic template used in the zeolite synthesis, can be left within the channels of the ZSM-5 structure. We show also that such a carbon residue, even when escapes the detection accuracy of elemental analysis, can be checked by a careful EPR characterization.