From fundamental studies to rational catalyst design: A hybrid experimental/theoretical investigation of ethylene epoxidation.

摘要

One of the ultimate goals in catalysis is to design new catalysts from fundamental studies. Rational catalyst design utilizes first principles to quantitatively predict activity and/or selectivity patterns of surface reactions and to provide directions for catalyst design and improvement.; We have demonstrated that surface oxametallacycles are key intermediates in selective olefin epoxidation. By a combination of surface science experiments and Density Functional Theory (DFT) calculations, we have synthesized the first stable surface oxametallacycle on a silver surface and have verified its identity and structure. DFT calculations have been used to investigate the reaction coordinate for ethylene epoxidation on silver, and to develop an ab-initio micro-kinetic model for this process. Selectivity is determined by competing ring closure of the oxametallacycle which leads to ethylene oxide, and isomerization reaction of the oxametallacycle which leads to undesirable products. Understanding the factors that govern selectivity in this process provides a platform, not only for predicting the performance of traditional silver catalysts, but also for exploring the influence of catalyst promoters at the molecular level, and ultimately for rational catalyst design. We have demonstrated that Cs, a promoter used in industrial catalysts, enhances the selectivity to ethylene oxide via long-range dipole-dipole interactions.; The information gained from the atomistic surface science and DFT studies was used as an input in a rapid quantum computational screening of a number of bimetallic alloy catalysts. The aim of this screening was to obtain an alloy catalyst that is more selective than the traditional silver catalyst. This rapid quantum computational screening has demonstrated that a Cu/Ag alloy offers enhanced selectivity for ethylene epoxidation with respect to the traditional Ag catalyst.; We have also carried out experimental studies of Cu/Ag alloy catalyst. We synthesized high surface area Ag and Cu/Ag monolith-supported catalysts. Catalytic studies were performed at several different temperatures and conversions. It was observed that the Cu/Ag alloy catalyst achieves much higher selectivity to ethylene oxide than the Ag catalyst for a wide range of conditions. Typically, the ethylene oxide yield was about one-third greater for the Cu/Ag alloy as compared to the pure Ag catalyst. This is in quantitative and qualitative agreement with the predictions from first principles.