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Vanadium is said to be the main element needed in the activity of propane oxidation to acrylic acid due to its oxidation number ability to activate propane. Three vanadium compositions are varied to see the development of phases in the samples. It can be clearly seen that vanadium with composition of 0.35 gives the best results for this catalyst sample. A single phase of Te2M20O57 is obtained which agreed with the TPR profile that show a peak of oxygen removed with the surface area of 5.89 m2/g.
From this role functioning of each elements presence in the MoVTeNbO, it is said to be that the main elements help for propane oxidation is the vanadium. In this study, we varied the vanadium composition in MoVTeNbO in order to investigate its effect on the physicochemical properties of the mixed metal oxide.
The composition of Mo:V:Te:Nb used in this study were 1:X:0.16:0.12 in which X is the vanadium composition which was varied to be 0.25, 0.30 and 0.35. These MoVTeNbOx samples was synthesized using ammoniumheptamolybdate tetrahydrate (10.7g) and telluric acid (2.205g) which had been dissolved in hot deionized water forming solution A. Vanadium
oxide sulphate (4.55g) solution was introduced into the solution A followed by ammonium niobium oxalate hydrate (6.39g) solution. The mixed solution was reflux for 72h with continuous stirring and the precipitate formed was separated from the solution by centrifugation and dried in oven at 353 K for overnight. A purplish black precursor was formed and immediately sent for XRD analysis before being calcined at 873 K under N2 flow for 2h. The synthesis process would be repeated with different vanadium composition (0.25 and 0.35) whereas the other elements molar ratio was kept constant.
The calcined samples were characterized by powder X-ray diffraction for phase determination and functional group presence in the compound were analysed using FTIR. Total surface area of the MoVTeNbOx samples determined by BET surface area and the morphology of the structure were viewed through scanning electron microscopy (SEM). The natures of reducing species in the samples were determined using temperature programmed reduction (TPR).
The catalytic reaction of propane oxidation was done at atmospheric pressure in a pyrex tubular reactor (id:4mm,length: mm) heating from room temperature up to the desired temperature. The feed composition of C3H8: O2: N2 was 8:10:82 (mol%) and 500mg of sample also 3g of quartz sand were mixed into tube. The total flow rate was 50ml/min. The reactant and products of the reaction were analyzed by online gas chromatography analytical system. Three separation columns were used which Gaskuropak 54 column to separate the hydrocarbon and CO2, a molecular sieve 13X column to separate N2, O2 and CO and Porapak QS column for separating the oxygenates product.
Results and discussion
The preparation of this catalyst sample was using totally different method from the literature [6-10] where sample were only reflux at the boiling point of water under the atmospheric pressure. Three MoVTeNbOx precursors with different vanadium composition of 0.25, 0.30 and 0.35 were synthesized and labelled as Pre-A, Pre-B and Pre-C, respectively. Figure. 1 shows the XRD pattern for precursor MoVTeNbOx samples. It is clearly shown that all samples exhibit a pseudo-amorphous or semi-crystalline phases which are observed by a broad peak at 2θ=22.4o and 27.5o. This is an agreement with reported data by Botella et al. (2001) and Popova et al. (2009) that showed the appearance of this peaks at 2θ=22.1o and 27.5o[6, 11]. These peaks are matched with (V0.07Mo0.93)5O14 phase (JCPDS File: 31-1437). Some reported that these two peaks is belong to the heteropolyanion Anderson-type crystalline phase (NH4)6TeMo6O24.7H2O  and/or (NH4)7TeMo5VO24.8H2O . However, various molar ratio for this samples showed no direct effect to its phase formation on the precursor samples.
After that the samples were calcined in N2 at 600oC for 2h and labelled as A, B and C. The calcined samples were characterized by XRD and the diffractograms are shown in Fig. 2. All of the samples show high intensities and sharp peaks, confirming the high crystallinity of the samples. Although all the sample have already been calcined, sample A showed the apperance of one sharp peak followed by one broad peak almost similar as the XRD profile for its precursor. However, this pattern showed more crystalline compared to the precursor and the peak is assigned to Mo5-xVxO14 (or Nb) and TeMo5O16 phases. Samples B and C showed a presence of three reflection peaks at 2θ below 10o which are 6.6o, 7.7o and 8.9o indicated that the formation of orthorhombic structure in the samples. The other peaks could be assigned to Te2M20O57 (M=Mo, V, Nb) phase as shown by 2θ at 7.7o, 8.9o, 22.1o, 26.2o, 27.1o, 45.1o in both samples. This strongly suggests that the samples formed contained an orthorhombic (M1) structure. There also have a few reflection peak occured at 2θ = 23.5o, 24.9o, 31.5o, 28.1o and 53.0o that could be assigned to TeMo5O16 and Mo5-xVxO14 (or Nb) phases respectively. From this observation, it could be said that calcination step succesfully removed the organic compund that exist in the precursor of the samples and lead to formation of those phases stated. In addition, increasing vanadium molar ratio in the samples
The morphology of the samples analyzed by scanning electron microscopy (SEM) showed a rod shape particles which agglomerates together. The BET surface area of the samples have a value area of 3.99 m2g-1, 3.82 m2g-1 and 5.89 m2g-1 for A, B and C respectively. Temperature programmed reduction (TPR) profiles shown in Fig. 3 displayed by a single peak evolution in sample C which is due to only single phase presence in this sample, as evidence in XRD diffractogram (Fig. 2). Whereas the TPR profiles for samples A and B show a broad peaks which could be due to the multiphasic property of these samples.
Based on the data obtained, it can be concluded that the composition of the vanadium has a strong effect on the physicochemical properties of MoVTeNbOx. In this work, it is discovered
that vanadium composition of 0.35 is crucial for the formation of single phase orthorhombic MoVTeNb mixed oxide catalyst.