Three VPO catalyst were prepared via direct ultrasound technique using organic, sesquihydrate and dihydrate routes, denoted as VPOuo, VPOus and VPOud respectively. These catalysts were synthesised solely on direct ultrasound technique and calcined in n-butane/air mixture. All catalysts exhibited well-crystallised (VO)2P2O7 phase. VPOus and VPOud showed αII-VOPO4, which led to an increase in average oxidation state of vanadium. All catalysts were showing O1s approx. 530 eV, similar P2p value and V2p3/2 at approx. 517 eV, giving vanadium oxidation state of approx. 4.0 – 4.2. FE-SEM micrographs showed the secondary structure consisting of thin plate-like crystals in different sizes agglomerate to each other due to cavitation effect. HR-TEM demonstrated the existence of polycrystalline phase. The nature of the oxidants was investigated by TPR in H2. VPOus showed highest amount of total removal of oxygen species suggesting that it had highest activity compared to VPOuo and VPOud. The XANES measurement of these catalysts showed the occurrence of vanadium oxide reductions in flowing hydrogen gas, which indicates the presence of V4+ and V5+ species. Catalytic tests demonstrated that the activity and selectivity to maleic anhydride increased with direct ultrasound technique. This study showed that catalyst synthesis time can be reduced to only 2 hours and giving polycrystalline particles compared to conventional method.
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