Catalytic activity of germanosilicate zeolites in the aerobic oxidation of cyclohexene: Effect of Ge ‘open’ sites

In this work we disclose for the first time the catalytic activity of Ge in germanosilicate zeolites for the aerobic oxidation of cyclohexene. Our results show that Ge in the -ITV framework displays a higher catalytic activity than Ge in the BEC framework, despite having a similar Ge content. This must be related to the different configuration of Ge in both frameworks, since Ge in -ITV locates preferably in interrupted ≡T-OH Q3 positions, similar to the more active ‘open’ sites of Ti(IV) and Sn(IV) in zeolites, while in the BEC framework Ge is in Q4 ‘closed’ configuration. Interestingly, pre-treatment of both calcined germanosilicate materials with 1-butanol results in a notable improvement of the catalytic activity, especially in terms of a reduction of the induction period. A detailed spectroscopic study reveals a special interaction of cyclohexene with ≡Ge-OH interrupted sites in the -ITV framework, displaying two types of interactions, through H-bonding and through a direct interaction with Ge. Moreover, cyclohexene is shown to form some type of complex with Ge sites in both BEC and -ITV frameworks, but they show a different nature as revealed by their distinct UV absorption bands, suggesting that Ge is able to pre-activate in some way cyclohexene for the oxidation reaction.