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Preparation, Physicochemical Evaluations and Kinetic Modeling of Vanadium Oxide Nanocatalysts over Carbon Nanostructures for Oxidative Dehydrogenation of Propane (ODHP) Reaction

Fattahi, Moslem | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45251 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Kazemeini, Mohammad; Khorasheh, Farhad; Rashidi, Ali Morad
  7. Abstract:
  8. A series of V2O5 catalysts supported on multiwall carbon nanotube (MWCNT), single wall carbon nanotube (SWCNT) and graphene were synthesized by hydrothermal and reflux methods for oxidative dehydrogenation of propane (ODHP) to propylene. The catalysts were characterized by techniques including the BET surface area measurements, XRD, FTIR, H2-TPR, NH3-TPD, FESEM and UV-vis diffuse reflectance. The performance of the catalysts and the supports were subsequently examined in a fixed bed reactor. The vanadium catalyst supported on graphene with C/V molar ratio of 1:1 synthesized through the hydrothermal method had the best performance under the reactor test conditions of 450°C, feed C3H8/Air molar ratio of 0.6 and the total feed flow rate of 90ml/min resulted in mean values of 53.60 selectivity for the propylene at 50.73% propane conversion. This catalyst was further employed in a series of experiments to study the effects of operating parameters including the; reaction temperature, propane to air ratio and the total feed flow rate on conversions and product selectivity using an experimental design method utilizing the response surface methodology (RSM) with central composite design at three levels. Optimum conditions for maximizing propane conversion and propylene selectivity as well as minimizing the COx selectivity were determined at the temperature of 500°C, C3H8/Air molar ratio of 0.28 and total flow rate of 60ml/min. Ultimately, experiments under optimum conditions revealed satisfactory agreement between the experimental and predicted data.
    Furthermore, the application of design of experiment (DOE) coupled with the artificial neural networks (ANN) in kinetic study of oxidative dehydrogenation of propane (ODHP) over a vanadium-graphene catalyst at 400-500oC and a method of data collection/fitting for the experiments were presented. The reaction network composed of consecutive and simultaneous reactions with kinetics expressed by simple power law equations involving a total of 20 unknown parameters determined through non-linear regression analysis. RSM and ANN techniques were constructed based upon the DOE’s points and were utilized for generating simulated data. These were subsequently used to fit power law kinetic rate expressions for the main ODHP and side reactions. The results of kinetic modeling with simulated data sets from the ANN and RSM models compared with collected experimental data. Both methods were able to satisfactorily fit the experimental data.
    Moreover, the newly synthesized vanadia nanocatalysts were investigated physicochemically. Several vanadium pentoxide (V2O5) nanostructures including; rod or belt-like, tube-like, needle-like and flower-like were successfully synthesized via the reflux and hydrothermal processes utilizing different templates such as monoamine, diamine, aromatic and alcoholic amines. In the synthesis process, morphologies of the products found to be sensitive to the type of amines. The diameters of the V2O5 nanotubes also varied between 60-120nm with a length upto 5μm. The physical characterizations and reactor performance of these nanostructures indicated that V2O5 nanotube synthesized through the dodecylamine had the best reactor performance. In this venue, the as-synthesized vanadium nanostructures with amines doped over graphene with 1:1 molar ratio of V:C prepared both through the reflux and by hydrothermal methods. The structure of the prepared samples was then characterized. Amongst, these new materials, bulk catalyst of V2O5 prepared with dodecylamine and V2O5 synthesized by aniline over graphene were the most active catalysts for the ODHP reaction. The reactor test conditions of 450°C, feed of C3H8/Air molar ratio of 0.5 and the total feed flow rate of 60ml/min over vanadium on Graphene resulted in 53.93% selectivity for the propylene at 47.02% propane conversion after 360min, at the end of reaction. It was revealed that, under these conditions the cracking of the propane or propylene along with the COx formation as the side reaction occurred. Next, the effects of different reaction temperatures and propane to air ratio on propane conversion and product selectivities on two selected catalysts namely V-DDA and V-A-G were investigated.

  9. Keywords:
  10. Nanostructure ; Carbon Nanotubes ; Graphene ; Kinetics Modeling ; Artificial Neural Network ; Propane Dehydrogenation ; Oxidative Dehydrogenation

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