Theoretical Study On The Oxidation Of Propane By Carbon Dioxide Over Metal Modified Boron Nitride Catalyst

The propane oxidation reaction is an inevitable and important step for its efficient utilization,and the catalyst plays an important role in the reaction process.Metal oxides such as vanadium-based and chromium-based are effective catalysts for propane oxidation,but there are problems such as product propylene over-oxidation and low selectivity.Replacing carbon dioxide with a weak oxidant can solve the problem of over-oxidation of propylene,but this type of catalyst is not sensitive to the activation of carbon dioxide.BN cages and related boron-containing materials are very promising catalysts for carbon dioxide reduction,but their degree of activation for light alkanes needs to be improved.While doping metal atoms into fullerenes can adjust the electronic properties,the electron-deficient nature of boron gives it a unique advantage in metal atom substitution doping,which can provide a degree of freedom for the development of new catalysts for metal doping.In this paper,the oxidation mechanism of carbon dioxide and propane on the surface of metal-modified BN cages was systematically studied based on density functional theory using VASP software combined with aperiodic model.The purpose is to explore efficient and stable catalysts for the preparation of olefins from light alkanes,and to provide a theoretical basis for the study of the mechanism of carbon dioxide oxidation of propane.Since the C-H bond cleavage of primary and secondary carbons in propane is selective on different catalyst surfaces,the following studies were done:（1）Density functional theory（DFT）was used to study the adsorption and the reaction mechanism of the rate control steps of propane and carbon dioxide on the surface of B₂₈N₂₈,V₄-B₂₄N₂₈,Cr₄-B₂₄N₂₈,Ni₄-B₂₄N₂₈,Ni₃Cu-B₂₄N₂₈,Ni₂Cu₂-B₂₄N₂₈ and Ni Cu₃-B₂₄N₂₈.The adsorption energies of propane,carbon dioxide and corresponding intermediates on different catalyst surfaces as well as the reaction heats and activation energies under six possible pathways were calculated.The calculation results showed that propane and carbon dioxide are physically adsorbed on the surface of the above catalysts,and the preferential cleavage of the C-H bond of the propane methylene group is the most favorable path,and the order of activation energy on different catalyst surfaces is V₄-B₂₄N₂₈（3.52 e V）,B₂₈N₂₈（3.09 e V）,Cr₄-B₂₄N₂₈（2.86e V）,Ni₄-B₂₄N₂₈（1.75 e V）,Ni₃Cu-B₂₄N₂₈（1.62 e V）,Ni₂Cu₂-B₂₄N₂₈（1.57 e V）,Ni Cu₃-B₂₄N₂₈（1.42 e V）.It can be seen that in the system of metal-doped BN fullerene catalyzing the propane oxidation by carbon dioxide,the content of doped metal Cu directly affects the catalytic activity of its rate-controlling step,that is,Ni Cu₃-B₂₄N₂₈ has certain advantages in catalyzing the preliminary oxidation of propane.（2）The mechanism of the conversion of propane and carbon dioxide to propylene and formic acid on the surface of Cu₄-B₂₄N₂₈ was studied.On the surface of Cu₄-B₂₄N₂₈,there are two ways from propane to propylene:one is the dehydrogenation of the primary carbon of propane;the other is the dehydrogenation of the secondary carbon of propane.There are three ways from carbon dioxide to formic acid:one is O-H bond;the second is C-H bond;the third is another O-H bond with different activity.The calculation results showed that the dehydrogenation of the secondary carbon of propane combines with the carbon atoms of carbon dioxide to form the HCO₂ intermediate,and then the secondary carbon continues to dehydrogenate and combines with the HCO₂ intermediate to generate the C₃H₆ intermediate and formic acid,and then the hydrogen atoms of the primary carbon of the C₃H₆ intermediate are rearranged to the secondary carbon to generate the propylene is the optimal path（CH₃CH₂CH₃+CO₂→CH₃CHCH₃+OCHO→CH₃CCH₃+HCOOH→CH₂CHCH₃）,the rate-controlling step of the reaction process is the rearrangement of the hydrogen atom from the C₃H₆ intermediate,the reaction activation energy is 2.25 e V,and then formic acid and propylene is desorbed with desorption energy of-0.23 e V and-0.90 e V,respectively,and can be further recycled.However,although Cu₄-B₂₄N₂₈ has good propane activation ability,due to the large difference between the lowest unoccupied orbital and the highest occupied orbital（LUMO-HOMO）of its rate-controlled reaction,the electronic transition is difficult,resulting in a high reaction activation energy.（3）The mechanism of the conversion of propane and carbon dioxide to propylene and formic acid on the surface of Cu₅-B₂₄N₂₈ was studied,that is,one Cu atom was adsorbed on the outer surface of the Cu₄-B₂₄N₂₈ catalyst model.The calculation results showed that the main reaction path of propane oxidation by carbon dioxide on the surface of Cu₅-B₂₄N₂₈ is CH₃CH₂CH₃+CO₂→CH₂CH₂CH₃+OCHO→CHCH₂CH₃+HCOOH→CH₂CHCH₃,which is different from that of Cu₄-B₂₄N₂₈.The primary carbon of propane is easier to dehydrogenate,but the rate-controlled reaction is still the rearrangement of the hydrogen atom from the C₃H₆intermediate,the reaction activation energy is 1.71 e V,and the products propylene and formic acid are also easily desorbed.In addition,the activation energy of each elementary reaction is slightly lower than that of Cu₄-B₂₄N₂₈,so Cu₅-B₂₄N₂₈ has better catalytic effect than Cu₄-B₂₄N₂₈.