Study On The Performance And Kinetics Of GaN-catalyzed CO₂ Oxidized Propane Dehydrogenation Reaction

By comparison with propane dehydrogenation(PDH)and oxidative propane dehydrogenation with O2(ODHP),oxidative propane dehydrogenation with CO2(CO2-OPDH),in general,is conducted under lower reaction temperature and associated with higher propylene selectivity.In addition,CO2 as a mild oxidant in CO2-OPDH process,has the coupling with reverse water gas shift reaction(RWGS)and removing the carbon deposit of catalyst via reverse Boudouard reaction,which is helpful to improve the dehydrogenation reaction activity and catalyst stability.At the present,noble metal and transition metal oxides have been studied in CO2-OPDH reaction,however,low yield of propylene and catalyst deactivation are still big issues.Recently,it is reported that nitrides(IIIA,BN and GaN)are considered as a promising catalytic material because of potential ability for activating C-H bond of alkane.Whereas,a harsh condition is needed for preparation of nitrides,and CO2-OPDH mechanism over nitrides is poorly understood.In this thesis,catalytic performance and kinetics of gallium nitride prepared via a mild one-step strategy in CO2-OPDH were investigated.The details are follows:(1)GaN nanoparticles are prepared by means of in-situ ammoniation under inert atomosphere at high temperture,where the mixed precursors are composed of melamine and gallium nitrate via solid-phase grinding.Moreover,the optimal synthesis conditions were obtained.The structure,texture and morphology were characterized by XRD,N2-physical adsorption and desorption,SEM and TEM.It is determined that GaN is characteristic of hexagonal wurtzite crystalline and having stacking pore.The chemsorption of probe moelcules(H2-TPR,CO2-TPO,C3H8-TPD and CO2-TPD)shows that GaN is stable under reducible atomosphere,and is favorable to adsorption and activation of propane and carbon dioxide.Beyond this,FT-IR spectra demonstrate that the presence of Ga-H and N-H groups is indictive of potential active sites for propane.(2)The as-synthesized GaN catalysts were evaluated in PDH and CO2-OPDH,respectively.By comparison with blank test(without catalyst),it is found that GaN shows higher reactivity for CO2-OPDH at 500-600 ?.Moreover,the propane conversion of 10.6%with propylene selectivity of 95.0%was achieved in CO2-OPDH reaction when the reaction temperature is 550 ?,which is better than that for PDH.Diffusion study reveals that intra-granular and extra-granular diffusion limitation was eliminated when the catalyst granular size is below 60-80 mesh and gas-hourly space velocity(GHSV)is in the range of 3000-27000 mL·g-1·h-1,respectively.Kinetic study shows that rates equations of PDH and CO2-OPDH over GaN catalyst follow(?).In addition,Arrhenius plots determine that apparent activation energy of GaN-catalyzed PDH and CO2-OPDH is 43.1 kJ·mol-1 and 48.3 kJ· mol-1,respectively.