アイテム詳細

要旨

The present work concentrates on the systematic kinetic study of the one-step propane oxidation to acrylic acid over a well defined, phase-pure M1 MoVTeNbOx catalyst. The bulk structural stability of the catalyst is a key issue for kinetic studies. The stability of the phase-pure M1 MoVTeNbOx catalyst under various conditions (steam-containing, steam-free, net reducing, stoichiometric and net oxidizing feed compositions) was evidenced by an in-situ XRD experiment which suggested that the bulk structure is homogeneous and constant under reaction conditions. Thereby, the heterogeneously catalyzed reactivity is exclusively determined by the surface properties, which in turn, are controlled by the chemical potential of the gas phase.
A kinetic study on the reaction variables (temperature, steam content and redox potential) was carried out. Stable catalytic performance was observed for all the conditions. Cycling experiments showed the reversibility of the conversion and selectivity decrease upon exposing the catalyst to dry and reducing feed, respectively. Further catalytic experiments revealed that the reactivity spans over 5 orders of magnitude in the order of acrolein oxidation>>propylene oxidation>propane oxidation>>carbon monoxide oxidation~water gas shift reaction. The negligible CO oxidation activity suggested that the CO and CO2 are formed via two independent pathways in propane oxidation over M1. The stage-wise addition of oxygen lead to an improvement of the catalytic performance by 5% compared to the conventional single-tube reactor. Further experiments in the two-stage reactor revealed that the phase-pure M1 is not reoxidized by N2O. The addition of propylene in the two-stage reactor revealed a slight competitive adsorption on the active sites with propane, which observation was supported by the results of microcalorimetric experiments. On the other hand, the addition of CO and CO2 in the two-stage reactor showed that these products do not adsorb competitively with the educt or intermediates.
In the literature much of the kinetic data was reported for ill-defined catalyst surfaces. In contrast to that, the present work reports the kinetic study of propane selective oxidation to acrylic acid on a well defined phase-pure and structurally stable M1 MoVTeNbOx catalyst. This study may contribute to the better kinetic and mechanistic understanding of the propane selective oxidation reaction.