Oxidation of benzyl alcohol over Co(II)NaY zeolites

Abstract

Benzyl alcohol oxidation was carried out with a Y-type zeolite catalyst ion-exchanged with cobalt(II) ion (Co(II)NaY) in a flow system operated at atmospheric pressure and temperatures of 320 to 390 °C. The Co(II)NaY catalyst was found to have high selectivity for benzaldehyde, a partial oxidation product, in comparison with that of Cu(II)NaY catalyst. The catalytic activity was proportional to the degree of Co(II) ion exchange up to a level of about 65 to 70%. This dependence of the oxidation activity of Co(II)NaY catalyst upon cobalt(II) ion content emphasizes the importance of this exchanged cobalt(II) ion as the main active site for benzyl alcohol oxidation. The rate of formation of benzaldehyde was second order in benzyl alcohol and 0.5 order in oxygen ( r φCHO = k · [ φCH 2OH ] 2 · [ O 2 ] 1 2 ). An activation energy of 25.6 kcal/mole was observed in the temperature range 320 to 390 °C. The effect of amine addition in the Co(II)NaY catalytic system was investigated by the variation of the amount of amine addition. The addition of pyridine or piperidine increased the oxidation activity, while the addition of ethylenediamine decreased the oxidation activity. Increased concentrations of ethylenediamine resulted in the complete deactivation of Co(II)NaY catalyst for this oxidation. Cobalt(II)-pyridine or -piperidine adduct formation within the large cavities of a Co(II)NaY catalyst was demonstrated by infrared measurements. A mechanism for benzaldehyde formation over Co(II)NaY catalyst, in which dissociative oxygen species participate as a form of [CoO], is proposed on the basis of the kinetic result obtained here. The presence of the dissociative oxygen species adsorbed on cobalt ions was suggested to contribute to the high selectivity for benzaldehyde, a partial oxidation product of benzyl alcohol.