Density Functional Theory Study of Propylene Epoxidation
Propylene oxide (PO) is one of the major industrial products with production of more than seven million tons annually . PO is an important intermediate in petroleum industry for the production of large variety of consumer products. Present commercial production methods of PO are chlorohydrin and hydroperoxide processes. Chlorohydrin process produces large amounts of hazardous chlorinated side products. Amount of these side products can be as high as 40 times of PO produced. On the other hand, hydroperoxide process produces styrene or t-butyl alcohol as side products with equimolar amount of PO. Because of this reason, economy of hydroperoxide process depends on the market availability of these side products. In recent years some new technologies, hydrogen peroxide based PO (HPPO) and cumene PO are introduced but these methods still have additional feed or some by-products to process. Due to these limitations, propylene oxide production with direct epoxidation of propylene is an important research topic in petrochemical industry [2-5]. No industrial-scale process using molecular oxygen for the direct partial epoxidation of propylene is achieved yet and a catalyst with both high conversion and selectivity toward propylene oxide is not reported either. Additionally, detailed mechanism of the propylene epoxidation process is still unclear and PO from propylene and molecular oxygen is still one of the most challenging goals of catalysis. Major studies in the literature are focused on the photocatalytic processes developments, usage of N2O or H2O2 as an alternative oxidant, in-situ production of hydrogen peroxide by gold catalyst and usage of silver catalyst for direct epoxidation [6, 7]. In the previous literature, mechanism of gas phase ethylene reaction with surface lattice oxygen on silver oxide (Ag2O) (001) surface has been theoretically investigated using DFT calculations . According to these studies Ag2O (001) surface has positive effect for ethylene oxide formation. By using the analogy between ethylene oxide and propylene oxide, silver oxide catalyst may be used for the propylene epoxidation. There is no theoretical study for propylene epoxidation on silver oxide (Ag2O) surface in literature . This study aims to focus on the silver oxide surface for propylene partial oxidation mechanism and possible promotions for propylene oxide formation on the surface. Study uses periodic implementation of density functional theory calculations (DFT) by using Vienna Ab-initio Simulation Package (VASP).
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