Understanding the Roles of Promoters and Oxygen Species in Ethylene Epoxidation by Ex situ and In situ Spectroscopic Techniques

Abstract

Ethylene oxide (EO) is an important chemical intermediate and a valuable chemical building block to ethylene glycol needed to help make many everyday products including antifreeze, detergents, inks and pharmaceuticals. Industrially, EO is synthesised by direct catalytic oxidation of ethylene with air over supported silver catalysts. The employed Ag/Al2O3 catalysts use promoters, including Cs, Re or both to improve the EO selectivity. Understanding the role of promoters and oxygen species is still debated, but critical for improving the rational design of the catalysts. Here Ag/Al2O3-based catalysts with the addition of Cs and/or Re promoters have been thoroughly investigated with the use of in situ and ex situ spectroscopic and characterisation techniques. There are issues with in situ DRIFTS cells that are currently commercially available. Through the development and optimisation of an in situ DRIFTS cell, these problems have been solved and used to give more information on case studies and provide surface information. Indications of a hybrid atomic molecular oxygen structure of Agx–O2 alongside atomic electrophilic oxygen was found to be present under ethylene epoxidation conditions on Ag/α-Al2O3, where the Ag surface became more oxidised. The addition of 500 ppm of Re caused a decrease in EO selectivity, here Re weakened the Ag-O bond, however, an increase in acidity and hydroxyl groups caused the reduction in EO selectivity. Re was found to be as a +7, whilst 600 ppm of Cs was found to increase the EO selectivity by causing a decrease in Ag crystallite sizes, greater coverage of Oads and a reduction in hydroxyl groups by titrating them. It was found that Cs is likely to be a layer where there is a mixture of peroxide and suboxide species. 600 ppm of Cs and 500 ppm of Re was found to cause a large increase in EO selectivity, due to the individual promotions working in tandem and the interaction between Cs and Re. The Cs and Re were found to weaken the Ag-O bond and increase the amount of Oads whilst titrating the hydroxyl acidic groups on the support. The addition of Cs and Re together appeared to cause a change in the Re-O bonding and caused a significant amount of electron distortion of the Cs. Cs suboxide-like species were found to be present during ethylene epoxidation and the addition of Cs and Re showed an increase in the electrophilicity of oxygen. The use of Cu/Au catalysts is widely reported to give high propylene oxide (PO) selectivity; therefore, these were investigated for the production of EO. The Cu/Au-based catalysts were all found to be relatively active towards ethylene epoxidation. Cu was found in the +2 oxidation state, whilst the catalysts appeared to show metallic Au. The increase in temperature of the pre-treatment to 450 °C was thought to be the reason for the reduction in the acidic hydroxyl groups. This thesis gives new insights into the role of different catalysts, promoters and oxygen species for ethylene epoxidation.