The Effect Of Carrier Morphology On The Active Species Regulation Of VO X /Al 2 O 3 Catalyst And The Performance Of Propane Oxidative Dehydrogenation

Propane dehydrogenation(PDH)is an on-purpose technology for enhancing production of propylene.Compared to non-oxidative dehydrogenation,oxidative dehydrogenation of propane(ODHP)is an exothermic and thermodynamically unrestricted reaction at lower temperatures,resulting in low energy consumption.In addition,the presence of molecular oxygen could minimize the formation of coke deposition and enhance catalyst stability.Unfortunately,the undesirable over-oxidation reactions of reactant(alkane)and product(alkene)give rise to the low yield of propylene.Hence,the key issue for ODHP to propylene is to control reaction process,reduce over-oxidation,improve the selectivity of propylene and realize selective oxidation of propane.In this regard,this thesis aims to acquire a fundamental understanding toward effect of support morphology on chemical state and existence manner of vanadia dispersed on Al2O3,as well as on mass-transfer and diffusion behavior in ODHP reaction process.The knowledge gained offers an insight to prepare the VO,/Al2O3 catalyst with well-defined and isolated active sites.In addition,XRD,SEM,N2 physical adsorption-desorption,CO2/NH3-TPD,UV-Raman,UV-Vis DRS,H2-TPR and MAS NMR characterization techniques were employed to reveal the effect mechanism of support morphology on active species present in catalyst surface.In combination with catalysis and mass-transfer and diffusion,the morphology-performance relationship was elucidated.The detailed contents are as follows:(1)The hydrothermal crystallization and direct roasting methods were employed to synthesize Al2O3 supports with different morphology,including nanoflower-like(F-Al2O3),nanobulk-like(B-Al2O3)and nanosphere-like(S-Al2O3).In the meantime,these as-synthesized supports were compared with commercial Al2O3 with irregular morphology(I-Al2O3).Furthermore,vanadia species with different loading were supported on these Al2O3 supports via incipient wetness impregnation method.A series of characterization techniques were employed to investigate crystalline structure,morphology,pore structure and surface acidic-basic property of Al2O3 supports,as well as surface density,existence manner and local environment of active species present in catalyst.Based on the correlation in characterization,it is found that the surface active species are composed of isolated and polymeric VOx,as well as crystalline V2O5.Upon increasing vanadia loading,the surface density increases and the active species tends to aggregation.It is therefore found that the threshold value of surface density toward the formation of crystalline V2O5 is about 4.8 V/nm2.Among them,B-Al2O3 and I-Al2O3 supports are favorable to disperse VOx species.In addition,the acidic-basic property and A1 coordination environment of Al2O3 support have a profound effect on VOx dispersion and interaction.Specifically,the pentacoordinated-Al is considered as metal centers for maintaining high dispersion of VOx and strong interaction.(2)The effects of vanadia loading and reaction temperature on ODHP reaction performance were performed on a continue-flow fixed-bed stainless steel tubular reactor at atmospheric pressure,GHSV = 8.78 h"1 and the feedstock consisting of a mixture of C3H8/O2/N2 with a molar ratio of 1/2/7.It is found that the isolated and polymeric VOx species display the high catalytic activity for ODHP.The selectivity of propylene decreases with an increase of surface density,and the highest space-time yield can be observed when reaction temperature is at above of 530 ?.Based on diffusion study,it is observed that there is an obvious extragranular diffusion limitation for all catalysts under low flowrate.Compared with 5V-B-Al2O3 and 5V-S-Al2O3 catalysts,the extragranular diffusion limitation is more serious to 5V-F-Al2O3 and 5V-I-Al2O3 catalysts.However,the extragranular diffusion limitation is getting less with increasing flowrate.On the other hand,the intragranular diffusion limitation almost cannot be observed over 5V-B-Al2O3 catalyst,but is most serious for 5V-I-Al2O3 catalyst.In addition,the test in catalytic stability with time on stream(TOS)of 75 h shows that all catalysts could maintain steady-state catalytic activity.