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 CrOx 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 CrOx somehow restrict the development of PDH industry.Supported VOx 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 VOx 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 VOx/Al2O3 catalysts for PDH reaction.Although the VOx/Al2O3 catalysts show great activity and propylene selectivity,the rapid deactivation caused by severe coke formation seriesly confined the application of VOx catalysts in PDH.In this study,a small amount of Mg was used to modify the VOx/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 VOx 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 VOx/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 VOx 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 VOx species under reaction conditions.With the help of in situ DRIFTS experiments,we show that hydroxyl groups on the VOx/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. |