Efficient and stable dehydrogenation of ethylbenzene over vanadia supported on ultra-fine Al2O3 nanofibers with excellent physicochemical properties

Abstract

The design and development of a highly efficient and stable catalyst for styrene (ST) obtained from ethylbenzene (EB) dehydrogenation reaction is urgently required because a commercial Fe–K-based catalyst often undergoes severe deactivation resulted from the active phase decomposition into inactive ferrite compound at high temperatures (>600 °C). Herein, ultra-fine Al₂O₃ nanofibers which are further self-assembled to form uniform rod-like morphology (R–Al₂O₃) were fabricated via a facile sol-gel strategy and further utilized as support to dispersing vanadium species (V/R–Al₂O₃). The physicochemical properties of the samples were systematically characterized by XRD, Raman, SEM, TEM, H₂-TPR, CO₂-TPD, XPS, and ²⁷Al MAS NMR techniques. It is found that except that R–Al₂O₃ support has the merits of a high specific surface area, lager pore volume, and uniform mesopore structure, the presence of the unsaturated pentacoordinate Al³⁺ ions plays a pivotal role in better dispersing V active species, with the existence of the isolate and/or polymeric forms. As compared to V/C–Al₂O₃ (commercial Al₂O₃ as support), the V/R–Al₂O₃ catalyst endows outperformed catalytic performances for oxidative dehydrogenation of ethylbenzene in the presence of CO₂ (OEBDH-CO₂). In which, the V/R–Al₂O₃ catalyst gives a high EB catalytic activity and good stability without significant deactivation phenomenon being detected even after the third regeneration test. Such a result can be principally ascribed to that R–Al₂O₃ support with excellent textural properties can better disperse V species so as to expose more active sites, which is beneficial for improving the conversion efficiency of EB. More importantly, keeping/regaining high valence state of V species in the V/R–Al₂O₃ catalyst under CO₂ as a soft oxidant effectively promotes Mars-Van Krevelen redox cycle, further suppressing the catalyst deactivation. This work provides a rational design strategy for a high efficiency and stable dehydrogenation catalyst by optimizing the textural properties of Al₂O₃ support and introducing the unsaturated pentacoordinate Al³⁺ ions.