Abstract
The formation of an interesting new type of paramagnetic species ( g = 2.00, width 3.5 G) is observed over alumina supported copper oxide catalyst by alkynes and allene at 360 °C. Alkenes and alkanes do not give paramagnetic species (PMS) under experimental conditions. Alumina supported copper oxide contains at least two types of active sites. While one has electron acceptor property (thiodiphenylamine cation radical formation) the other, Cu 2+, has the property to give PMS with alkynes and allene. The affinity of the studied hydrocarbons for the formation of PMS were in the following sequence: acetylene > methylacetylene > allene > ethylacetylene > dimethylacetylene. The intensity of ESR signals dropped drastically as soon as PMS came into contact with air or oxygen. However, on removing the oxygen from O 2 adsorbed PMS either by evacuating or passing CO, CO 2, H 2, D 2, N 2 or acetylene over Cu-alumina at room temperature, the ESR signal reappeared to almost original shape and intensity. A possible mechanism for the formation of these radicals and the effect of oxygen on these is discussed on the basis of charge transfer and an equilibrium shift between PMS and oxygen.
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