A DFT Study On The Mechanism Of Low Carbon Alkane Conversion To Alkene On BN And B₂O₃

The selective catalytic oxidation of low-carbon alkanes to olefins on BN and B₂O₃ were important for the development of natural gas chemical industry.However,there was a great controversy on active sites and reaction mechanism.In this paper,we studied the reaction mechanism of selective oxidation of low-carbon alkanes to alkene on BN and B₂O₃systematically by using Gaussian 09 package,DFT method,B3LYP functional which combined with Grimme-D3 dispersion correction and 6-31 G?d,p?basis set.?1?The mechanism of oxidative dehydrogenation of ethane,propane and n-butane catalyzed by BN could react in a concerted or stepwise way.There were four kinds and eight active sites which includes armchair-a,armchair-b,ziazag-B and ziazag-N after oxygen adsorbed on BNNT and h-BN.The oxidative dehydrogenation can react in a concerted way with an eight-membered ring transition state on armchair-b active site and a nine-membered ring transition state on ziazag-N active site.The reaction activation enregies were between 59and 133 KJ/mol.Due to the low reaction activation energy and without byproduct,the concerted way could explain the high selectivity.The stepwise way could react on armchair-a,armchair-b and ziazag-B active sites.Dehydrogenation can occur in both of concerted and stepwise way on armchair-b active sites.Besides,the regeneration of active sites on BNNT can occur either in a concerted or a stepwise way with the help of oxygen,too.After propane ODH reaction,the desorption energy of propylene was less than the activation energy of C-C bond cleavage which could explain the high selectivity,too.?2?The mechanism of oxidative dehydrogenation of propane catalyzed by boron oxide was studied.The active site on boron oxide and activation energy?177.35 KJ/mol and 234.15KJ/mol?were similar to ziazag-B but higher than that.For selective oxidation of methane on B₂O₃,there were three steps.The first step was the generation of active site,the activation energy was 205.33 KJ/mol.The second step,methane oxidative to formaldehyde could be divided into two steps,the C-H bond cleavage on methane and carbonyl's formation.The activation energy were 271.49 and 191.41 KJ/mol,respectively.Finally,the active site could be regenerated through the activation energy of 136.58 KJ/mol.