Catalytic Performances And Mechanism Of Propane Dehydrogenation Over Supported V-Based Catalysts

Propylene is one of the most important building blocks for the chemical industry,and the demand for propylene as well as its derivatives（polypropylene,propylene oxide,acrylonitrile,phenol,etc.）is growing rapidly.Propylene can be commercially produced as a byproduct by steam cracking and fluid catalytic cracking（FCC）of naphtha,light diesel,and other oil byproducts.Recently,growing production of shale gas drives the price of propane down,which makes the propane dehydrogenation（PDH）process commercially attractive for propylene production.Pt and CrO_x are generally employed as catalytically active materials in industrial PDH processes and they have been extensively investigated.However,the high cost of Pt and the hypertoxicity of CrO_x somehow restrict the development of PDH industry.Supported VO_x catalysts have been extensively employed in oxidative dehydrogenation of light alkane,and they also showed considerable activity and propylene selectivity in non-oxidative PDH process.This thesis describes our mechanistic studies of the PDH reaction on supported VO_x catalysts and provides methods to improve catalystic performace of the supported VOx catalysts.Firstly,this thesis presents a practicable method to promote the stablility of VO_x/Al₂O₃ catalysts for PDH reaction.Although the VO_x/Al₂O₃ catalysts show great activity and propylene selectivity,the rapid deactivation caused by severe coke formation seriesly confined the application of VO_x catalysts in PDH.In this study,a small amount of Mg was used to modify the VO_x/Al₂O₃ catalysts,which was found quite effective to suppress coke formation and increase the stablility.By using a number of characterization techniques including Raman spectroscopy,Ultraviolet-visible（UV-Vis）spectroscopy and Transmission electron microscope（TEM）elemental mapping,we further explored the mechanism of this promotion effect.Rather than changing the surface acid-base properties of the supported VO_x catalyst,the addition of a small amount of Mg（i.e.,1 wt%）helps dispersing the 3D V₂O₅ nanoparticles.Additionally,in situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）was used to study the coke formation process on the 12V/Al₂O₃ catalyst with and without Mg modification and the results further confirm our conclusion.This thesis also investigates the surface structure of a VO_x/Al₂O₃ catalyst under the circumstance of non-oxidative dehydrogenation and describes the direct connection between the surface hydroxyl groups and the PDH performace of the supported VO_x catalysts.Great efforts have been made to study the surface structure of supported vanadium.The surface models for different V loading samples had been proposed and widely accepted.However,very few study concerns about the detailed structure of the surface VO_x species under reaction conditions.With the help of in situ DRIFTS experiments,we show that hydroxyl groups on the VO_x/Al₂O₃ catalyst（V-OH）are produced under H₂ pre-reduction,and the catalytic performance for PDH are closely connected to the concentration of superficial V-OH species on the catalyst.The hydroxyl groups are found to promote the catalyst,which leads to better stability by suppressing the coke deposition.