The effects of the nature of TiO2 supports on the catalytic performance of Rh–Mn/TiO2 catalysts in the synthesis of C2 oxygenates from syngas

Abstract

The effects of TiO₂ phase composition on the catalytic and physico-chemical properties of supported Rh catalysts in the synthesis of C₂ oxygenates from syngas were studied. The catalysts supported on anatase containing TiO₂ were more active than those supported on rutile containing TiO₂. Meanwhile, the catalyst supported on pure rutile TiO₂ exhibited a greatly higher C₂ oxygenate selectivity than that supported on anatase TiO₂, and the TiO₂ with mixed phase composition could further improve the C₂ oxygenate selectivity compared with the pure phase TiO₂. The results showed that the higher Rh dispersion and CO adsorption capacity at the reaction temperature were obtained on the anatase/anatase-rich TiO₂ supported Rh catalysts, resulting in their higher CO conversion compared with catalysts supported on rutile/rutile-rich TiO₂. The chemical states of Rh sites can be affected by the different electron-donor properties of TiO₂ with different crystalline phases. The electropositivity of Rh⁺ sites was weakened when using the rutile containing TiO₂ supports, which is conducive to transformation of Rh⁺(CO)₂ into Rh–CO species and weakens the Rh–CO bond strength, finally resulting in the increased CO insertion reaction, and accordingly the increase of C₂ oxygenate selectivity over the rutile containing catalysts. Moreover, the remarkably higher TOF of RM/TiO₂-Ar can be attributed to the higher CO adsorption capacity and fast transformation of Rh⁺(CO)₂ into Rh–CO species.