Gallium Nitride Preparation And Oxidative Dehydrogenation Of Propane M Particles

Propylene is a very important raw material in chemical industry.Propane dehydrogenation is one of the most important strategies to the massive production of propylene.The direct dehydrogenation of propane is restricted by the reaction thermodynamic equilibrium,resulting in low propane conversion.Oxidative dehydrogenation of propane in the presence of O2 is an exotherm reaction,which can reduce the energy consumption and eliminate the deposition of coke.While the introduction of O2 leads to the severly deep oxidation of propane.CO2 is believed to be a weak oxidant which can not only reduce the thermodynamic requirement,but also reduce occurrence of cracking reactions.Currently,the active components of the catalysts for the dehydrogenation of propane to propylene in the presence of CO2 are mainly focused on the precious metal(Pt),?A nitride(BN),and oxides(Ga2O3,In2O3).However,those catalysts generally have problems of rapid deactivation and environmental pollution.Gallium nitride(GaN)may act as a new dehydrogenation material for that it can effectively activate C-H bond.The structural characteristics of GaN make it possible to exhibit excellent activity and stability in the oxidative dehydrogenation of propane to propylene.It is of great significance to systematically study and apply GaN in the oxidative dehygrogenation(ODH)of propane.This thesis focuses on the preparation of GaN nanoparticles,and the application of the GaN nanoparticles to the ODH of propane.The main contents are summarized as follows.(1)Preparation of GaN nanoparticles.A novel "one-step" method to prepare GaN nanoparticles was presented.Melamine and gallium nitrate hydrate were mixed together and then grinded in crucible.After that,the grinded powder was calcined in N2.The targeted GaN nanoparticles can be successfully produced with the average size of 13-25 nm.The operational conditions are as follows:the mass ratio of gallium nitrate hydrate to melamine is 1:0.5-1:4,calcination temperature is 800-900 ?,calcination time is 1 h.The obtained GaN nanoparticles were characterized by XRD,SEM-EDS and HRTEM,and a hexagonal wurtzite was confirmed.The results of TGA and elemental analysis showed that there was a small amount of the residue of coke in the prepared GaN.Thus heating treatment in air is prerequisite,via which the residue of coke can be eliminated and the purity of GaN was improved.(2)Catalytic oxidative dehydrogenation of propane in the presence of CO2 over GaN.The reaction performance over the obtained GaN nanoparticles was investigated under atmospheric pressure,550 ?.The molar ratio of C3H8,CO2 and N2 is 1:2:7,and GHSV is 3000 L·kg-1·h-1.The initial propylene selectivity and the initial propane conversion over GaN were 92.5%and 12.6%.After 8 h,the selectivity of propylene was 90.9%.However,the conversion of propane sharply decreased to less than 1%.XRD,N2-physical adsorption desorption,XPS,elemental analysis,TGA and Visible Raman spectrum were employed to characterize the structure of GaN before and after the reaction.It was indicated that the main reason for the rapid deactivation of GaN may be the deposition of coke.(3)Catalytic ODH of propane in the presence of CO2 over supported GaN catalyst.In order to eliminate the deposition of coke and improve the activity and stability of GaN,GaN nanoparticles were uniformly dispersed on different supports,such as SBA-15,?-Al2O3 and SiO2(Q-x),to prepare GaN-supported catalysts.Among which,15 wt%GaN/SiO2Q-3 showed an excellent catalytic performance with the initial propylene selectivity and the initial propane conversion was 97.8%and 12.6%.After 60 h,the propylene selectivity was 97.5%,while the propane conversion dropped to 7.5%.XRD,N2-physical adsorption desorption,elemental analysis and TGA were employed to characterize these catalysts and found that GaN/SiO2(Q-3)has the largest specific surface area and the smallest pore volume as compared to other catalysts.It is indicated that the space restriction of supports may play an important role in dispersing the active GaN,limiting the growth of the deposition of coke,and thus improving the stability of GaN and the conversion of propane.