Quantum mechanical study of propylene epoxidation on various metal oxide surfaces

Tezsevin, İlker
Silver is a very successful catalyst for the ethylene oxide synthesis, however, its performance for propylene oxide (PO) production is very limited. This thesis work aims to investigate the reasons of the low PO selectivity on the silver catalyst and to study the possible enhancements to make PO formation feasible. In order to investigate the reason of the low PO performance, the partial oxidation mechanism of propylene on silver catalyst is examined by using density functional theory (DFT) calculations. Ag2O (001) surface is used as the active oxide phase of the catalyst through the study. The mechanistic study on an ideal surface revealed that even though the direct propylene oxide formation on the silver oxide catalyst is the most probable pathway, the high desorption barrier of propylene oxide leads to allyl radical formation causing further oxidation reactions. After finding the reason behind the problem, possible enhancements were studied to make PO desorption easier and to block undesired allyl formation pathways. For this aim, effects of gold and chlorine doping on the Ag2O (001) surface were investigated both individually and together in seven different scenarios. It is observed that gold doping alters the binding properties of the oxygen atom responsible for the PO and PO isomer formation and decreases the desorption barrier of the PO from the surface. On the other hand, only gold doping cannot inhibit allyl formation pathways, which are still easier than the PO desorption. Chlorine promotion shows two different effects. First, Cl addition slightly decreases the PO desorption barrier. Second, Cl blocks the oxygen site responsible for allyl formation. It is found that, in order to block the allyl formation pathway completely, two neighboring O atoms of the central oxygen should be replaced with Cl atoms. Therefore, silver oxide unit cell modified with 2 Cl gives better results and favors PO production without allyl formation problem. Studies for combined effects of gold and chlorine doping show that one gold-silver and two oxygen-chlorine replacements gives the best performance among the Au & Cl doped scenarios by both reducing the PO desorption barrier and blocking the allyl formation paths. As a result, this study shows that silver can be an effective catalyst for the propylene oxide production with appropriate modifications. This study also encourages experimental researchers for further effort on the modified silver (oxide) catalyst for the direct propylene epoxidation.


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Citation Formats
İ. Tezsevin, “Quantum mechanical study of propylene epoxidation on various metal oxide surfaces,” Ph.D. - Doctoral Program, Middle East Technical University, 2017.