Catalytic performance of [Ti,Al]-beta in the alkene epoxidation controlled by the postsynthetic ion exchange

Novel methods for enhancing the epoxide selectivity in the alkene epoxidation over [Ti,Al]-Beta have been developed. When as-synthesized [Ti,Al]-Beta was treated with aqueous ammonium nitrate solution and successively calcined at low temperature, a dramatic enhancement of epoxide selectivity was attained in the liquid-phase epoxidation of cyclohexene using H2O2 as an oxidant in protic solvent methanol. The optimum thermal treatment temperature to achieve the maximum epoxide yield was 473 K, where the postsynthetic [Ti,Al]-Beta exhibited a catalytic activity comparable to the sample directly calcined at 793 K; nevertheless, the epoxide selectivity was as high as 63% for the former in contrast to 0% for the latter. The ion-exchange treatments with quaternary ammonium salts over calcined [Ti,Al]-Beta showed similar effects, although the treatments with alkali and alkaline earth metal ions were detrimental to the catalytic activity. It is suggested that the quaternary ammonium cations selectively blocked the acid sites deriving from the framework Al, which resulted in preventing the ring-opening hydrolysis of the epoxide, whereas the inorganic cations poisoned not only the acid sites but also the Ti active sites contributing to the catalytic epoxidation. The ion-exchanged catalyst was regenerated readily by repeated calcination and following ion exchange and thus turned out to be an active, selective, and reusable epoxidation catalyst.