A DFT Study On The Mechanism Of BN Catalyzed Propane Oxidative Dehydrogenation

BN has been the focus of propane oxidative dehydrogenation because of the excellent thermal stability and selectivity to propene.Although some of these experimental and theoretical works shed light on the propane ODH reaction on hexagonal boron nitride catalysts,there still remain unresolved questions,for example,the identification of active sites,the complete reaction pathway and the reason of excellent selectivity to propene.In here,oxidative dehydrogenation reaction mechanism for propane on oxygen terminated BN edges active sites have been studied by Gaussian09 D.01 package using the B3LYP-D3 functional with the 6-31G(d,p)basis set.Four kinds of oxygen terminated BN edges were used as active sites: Armchair-a,Armchair-b,Zigzag-B and Zigzag-N.They are classified into three groups:(1)the peroxide oxygen molecule terminated BN edges(Armchair-a);(2)the dimer oxygen molecule terminated BN edges(Zigzag-B);(3)the oxygen atom terminated BN edges(Armchair-b and Zigzag-N).The stability order is ZigzagB>Armchair-a>Armchair-b>Zigzag-N.On oxygen atom terminated Zigzag-N and Armchair-b active sites,two hydrogen atoms of propane can be abstracted by two adjacent oxygen atoms in a concerted way.The corresponding activation energy was calculated to be 62.25 and 114.73 k J/mol.There is no intermediate formation,the concerted way can explain the excellent selectivity of propane to propene.On Armchair-a,Armchair-b and Zigzag-B active sites,propane dehydrogenation through two-step procedure,activation energy of rate-determining step was 168.37,174.73 and 241.54 k J/mol.In either ways,the propene and OH-terminated BN edges can be obtained,and the stretching vibration frequency of O-H bond is most obvious in 3200 cm-1.The oxidant oxygen molecule facilitates the removal of the hydroxyls to regenerate the active sites and form H2O2.Large activation energy(>325.31 k J/mol)for C-C bond cleavage indicates that propyl intermediate kinetically favorably proceeds a second hydrogen atom abstraction(the activation energy <240.90 k J/mol)to form propene not C-C bond cleavage to form ethene.It can explicate the excellent selectivity of propane to propene on BN catalyst.