Propylene epoxidation on CoO and Li promoted CoO catalysts: a density functional theory study

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2014
Gezer, Miray
Propylene oxide is a significant intermediate chemical which has many derivatives used as raw materials in many industries such as automobile, cosmetic, medicine etc. However, its current production methods, chlorohydrin process and hydroperoxide process, are not preferred since they are economically and environmentally disadvantageous. Considering these negative effects, heterogeneous catalyst for direct propylene epoxidation is still being investigated. With an objective of filling this catalyst gap in the literature, propylene epoxidation mechanism on CuO (001) and Li promoted CuO catalysts are investigated theoretically by means of DFT calculations where VASP code is used. The ultimate goal is to determine the possibilities of the formation of the probable products and to find the energy profiles for both of the catalysts. With this aim, the most probable product formed on CuO catalyst is explored and the effect of Li promoter is observed for the propylene epoxidation mechanism. To begin with, for partial propylene oxidation on CuO (001) surface, there are two possible reaction pathways for propylene. One of the pathway is propylene oxide or acetone formation through oxygen bridging intermediate surface. Differs from the other studies in literature, oxygen bridging surface is discovered in this study which refers to the chemical adsorption of propylene on the catalytic surface. The other path is acrolein formation through allyl radical on CuO surface. For the first pathway, the activation barriers between the oxygen bridging and propylene oxide is found as 2.89 eV. In addition to that, energy barrier between the oxygen bridging and acetone is calculated as 2.47 eV. These high barriers show that it is not possible to obtain both propylene oxide and acetone on CuO surface. Then, analysis of the second pathway is conducted. After optimizing the geometries of allyl radical and acrolein formation, it is detected that there is no activation barrier between these two structures. Thus,these results clearly show that for the first propylene send to the CuO surface, formation of acrolein product has the highest possibility. It is predicted result for this reaction since the propylene tends to form acrolein on the heterogeneous catalysts due to its allylic hydrogen containing group. After desorption of the acrolein, study continues with the investigation of water on this surface. Two alternatives are tried for the water formation mechanism. One of them is using lattice oxygen to obtain water. The other alternative is adsorbing oxygen molecule to the vacancy that arise from the desorption of oxygen from the surface, and used this oxygen molecule for water formation. For both of these options, results remain unchanged that hydrogen atoms do not want to attached to the same oxygen atom. It is concluded that water is not formed directly on the CuO surface, this catalyst has an ability of splitting water. Afterwards, second propylene is send to the lattice oxygen of the surface and propylene oxide is formed directly. During the research in literature, experimental studies about CuO catalyst show that formation of acrolein and combustion products are observed for propylene epoxidation mechanism. With this information in mind, both possible products, acrolein and propylene oxide, are investigated with respect to their tendency to combust. Calculations for combustion indicate that combustion products of acrolein is two carbon dioxide and one carbon monoxide; however, the energy requirement for this combustion path is really high. In addition to this, combustion results of propylene oxide has a lower energy barrier ( 1 eV) with the highly oxidizable products of ethenone and formaldehyde. It seems that combustion of propylene oxide is more possible than the acrolein combustion in the gas phase which is consistent with the information gained from the literature. With the objective of increasing the propylene oxide selectivity on the CuO surface, Li is substituted in the catalyst. After optimizing the Li substituted CuO surface, propylene oxide and acrolein formation on this catalyst are investigated for propylene epoxidation. It is clearly seen that Li promoter block the route for acrolein formation by increasing its activation energy to 1.17 eV. Moreover, promoter of Li increases the possibility of propylene oxide formation by decreasing the activation barrier to 0.49 eV. To conclude, it can be said that compared with the CuO catalyst, Li promoted CuO catalyst is more active for propylene oxide formation.
Citation Formats
M. Gezer, “Propylene epoxidation on CoO and Li promoted CoO catalysts: a density functional theory study,” M.S. - Master of Science, Middle East Technical University, 2014.