Recent developments in the selective oxidation of propane to acrylic and acetic acids

Abstract

The oxidation of propane to acrylic and acetic acids has been studied using a Mo 1Nb 0.08Sb 0.25V 0.3 mixed oxide catalyst, calcined and activated before reaction under different conditions ( T=500 or 600 °C in atmospheres of N 2, static air, and flowing air (dry or with water or ammonia addition)). Catalytic testing was performed at 400 °C in a plug flow microreactor and the characterisation of the catalysts was carried out by XRD, XPS, BET, FT-IR and ammonia TPD. The best conditions for acrylic acid (AA) formation are calcination at 500 °C under N 2, followed by activation at 500 °C prior to reaction under helium. The selectivity to acetic acid was found to be rather high, which was assigned to the presence of Sb instead of Te, which is used most often. The presence of all four elements, Mo, Nb, Sb and V, was found necessary to achieve high selectivities in acrylic and acetic acids. The presence of several crystalline phases, such as Sb 4M 10O 31/Sb 2M 10O 31 with M: Mo (major), Nb and/or V (minor), SbVO 4, sub-oxides of MoO 3, such as Mo 8O 23 and (M x M y Mo 1− x− y ) 5O 14 with x and y referring to low amounts of V and Nb, and some amorphous phases, such as Sb 2O 4, was shown to be important in order to orientate the reaction toward the production of acrylic and/or acetic acids. The presence of MoO 3 in all air-calcined samples was found to be detrimental to the selective oxidation of propane. This work exemplifies further the general concept of multicomponent oxide catalysis for heterogeneous selective oxidation, which involves low acidity, a synergistic effect between several solid phases and the role of shear/defective and even XRD amorphous, structures facilitating the redox mechanism of the reaction.