| Summary: | Molybdenum based catalysts have been used successfully as catalysts for the oxidative dehydrogenation of propane to propene. Metal phosphate catalysts have also been reported to exhibit high yields of propene in the oxidative dehydrogenation of propane to propene. Therefore, MoO2HPO4.H2O was used as a precursor to prepare different bulk and supported molybdenum phosphate phases based on the methodology used in the preparation of vanadium phosphate (VPO) catalysts which are used commercially for selective butane oxidation. The materials obtained were tested as catalyst for propane oxidative dehydrogenation. MoOPO4 was prepared by a novel procedure. The unsupported molybdenum phosphates were not active for propane oxidative dehydrogenation to propene. However, the activity of the molybdenum phosphates was significantly enhanced after being impregnated on the supports (Al2O3, SiC2, TiO2 and MoO). The enhancement of the activity was attributed to the enhancement of the reducibility of the supported molybdenum phosphates. The more reducible the catalyst the more active it is. Moreover, these supported catalysts exhibited high selectivity at the initial reaction temperature. However, the selectivity decreases as the reaction temperature increases, which was attributed to a consecutive oxidation of formed propene. No crystalline phase has been detected by XRD in all supported molybdenum phosphates, which is attributed to a strong interaction between the impregnated molybdenum phosphate material and the supports. Niobia-supported molybdenum phosphate (heated in nitrogen at 500 C) exhibited the highest alkene selectivity of 51.7% (propene selectivity = 42.5% and ethene selectivity = 9.2%) at a propane conversion as high as 20%. Therefore, the propene and total alkene yield are 8.5 and 10.3%, respectively, which are comparable to that reported in the literature. Therefore, supported molybdenum phosphate catalysts are promising for propane oxidative dehydrogenation to propene. The relative performance of molybdenum phosphate supported on different supports at 500 C (heated in nitrogen to 500 C) is as follows: 20% MoPO/TiO2> 20% MoPO/Al2O3=20% MoPO/SiO2>20% MoPO/Nb205 While the selectivity to propene is as follows: 20% MoPO/Nb2O5> 20% MoPO/SiO2>20% MoPO/Al2O3>20% MoPO/TiO2. This indicates that the higher the activity the lower the selectivity, except for the silica-supported molybdenum phosphate and the alumina-supported molybdenum phosphate. They exhibited the same activity, but the silica-supported molybdenum phosphate was more selective to propene. The in situ XRD confirmed the formation of a stable amorphous phase of molybdenum phosphate when the molybdenum precursor (MoO2HPO4.H2O) was heated in nitrogen up to 400 C. Moreover, heating the amorphous phase formed up to 500 C resulted in the formation of a stable crystalline phase at 500 C. According to the JCPDS data this crystalline phase was molybdenum pyrophosphate ((MoO2)2P2O7).
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