Catalytic Oxidative Dehydrogenation Of Propane By B₂O₃/SBA-15

The technology for oxidative dehydrogenation of propane not only provides an important way for the utilization of propane resource,but also explores a new direction for the industrial production of propylene.Oxidative dehydrogenation process has the advantages of not being restricted by thermodynamic equilibrium,no carbon deposition in the reaction process,and oxidizing agent belongs to clean energy,but the problem of deep oxidation has always been the key factor restricting the industrialization of oxidative dehydrogenation of propane.For a long time,precious metal catalyst and transition metal oxide catalysts have been the main catalytic systems for the direct dehydrogenation of propane.However,the high production cost of precious metal catalyst and the environmental pollution caused by transition metal oxide have become more obvious.Therefore,it is of great significance to design catalysts with high activity,high selectivity and more environmental greener by using oxidative dehydrogenation of propane as a model reaction.In this thesis,a series of B2O3/SBA-15 catalysts were prepared by using the mesoporous silica(SBA-15)as the support and B2O3 as the active component for the oxidative dehydrogenation of propane.Under the reaction temperature of 500? and the total flow rate of feed gas of 48 mL/min(C3H8:O2:N2=1:1.5:3.5),the catalytic performance of the catalyst for oxidative dehydrogenation of propane was systematically investigated,and the optimal catalyst with high catalytic performance was obtained.At the same time,the X ray diffraction(XRD),N2 physical adsorption,transmission electron microscope(TEM),and oxygen temperature programmed(O2-TPD)were used to explore the structure of the catalyst,and it was correlated with the reaction results of oxidizing dehydrogenation.The relationship between catalyst loading,B2O3 grain size,and reaction activity was revealed.The activation mechanism of active oxygen species for propane was also discusses and the possible reaction path was proposed.The main contents and results are as follows:(1)With boric acid as a precursor,the B2O3/SBA-15 catalysts under different loadings(1?30 wt.%)were prepared by constant volume impregnation method.Combined with the structural characterization results of the catalyst and the catalytic activity of oxidative dehydrogenation of propane,the catalytic activity of the B2O3/SBA-15 catalyst increased first and then decreased.With the increase of the catalyst loading during the process of the catalyst loading gradually increasing.The optimal catalytic performance with a propane conversion of 25.6%and a propylene selectivity of 77.7%was achieved over 10 wt.%B2O3/SBA-15 catalyst Based on the reaction results of TOS at 4 h,combined with the product distribution results,it can be seen that in addition to propylene in the products,more of the by-products are ethylene with industrial value,while the proportion of COx is very low.(2)B2O3/SBA-15 catalysts with different B2O3 grain sizes were prepared by adjusting the cooling rate of muffle furnace during the calcination process.The TEM results were associated with the results of the catalytic oxidation of C3H8 shows that the grain size of active component B2O3 significantly affects the catalytic activity of the catalyst.When the crystalline grain size of B2O3 is 4 nm,the TOF of propane consumption is 0.31 s-1.With increase of the grain size of B2O3,the TOF of propane consumed by the catalyst gradually decreased.It may be caused by the decrease of active sites on the catalyst surface during the growth of B2O3 grains.(3)The characterization results of O2-TPD,C3H8-TPD,C3H6-TPD were associated with the results of the catalytic oxidation of C3H8,the absorption and desorption between the catalyst and C3H8/C3H6 was investigated,along with that the dehydrogenation mechanism was presented,and the deactivation of catalyst was demonstrated.It was observed that firstly O2 is adsorbed and activated into reactive oxygen species on the active component B2O3 of the catalyst surface,and then the C-H bond in C3H8 is activated with the synergy of the reactive oxygen species.The activated C3H8 dehydrogenates into propylene product,which is released directly from the surface of the catalyst,thus avoids its deep oxidation to produce COx.At the same time,the adsorbed activated C3H8 dissociates into hydrogen,which reacts with oxygen to form water.In view that the water generated during the reaction process may dissolve part of the active component B2O3 under the high temperature,resulting in the loss of the active component of the catalyst,which eventually leads to the deactivation of the catalyst during the dehydrogenation process.