(348e) Investigation of Promoted Catalysts for Ethylene Epoxidation

The direct gas phase epoxidation of ethylene over Ag has been widely studied in an attempt to create more active and selective catalysts. Computational and experimental studies have led to many patents and scientific publications which have shown improvements of ethylene oxide (EO) selectivity with the addition of many different catalyst promoters. Most promoters have been discovered though empirical methods. In contrast, computational chemistry was able to predict more selective bimetallic catalysts for this process, and this prediction was also validated with experiments [1]. To accelerate further studies, as well as to allow for new material discovery and mechanistic investigations of ethylene epoxidation, we fabricated a high-throughput catalytic reactor system for the testing of monolith catalysts [2]. This system, which employs independent temperature control of reactor tubes and FTIR spectroscopic imaging for analysis of the effluent, has been used to study the effect of preparation conditions and promoters on the performance of Ag-based catalysts. In particular, rhenium promoted catalysts will be discussed with and without further promotion from organic chlorides.

Reactor studies of Re-promoted Ag catalysts have shown that Re promotes the catalyst with or without the presence of an organic chloride (Cl) in the feed stream. The catalyst preparation conditions were found to be particularly important in determining performance. Without Cl in the feed, catalysts prepared by sequential impregnation showed an increase in the EO selectivity from ~30% to ~45% at a constant conversion of 1.8% [3]. The sequentially impregnated catalysts showed further improvements with the addition of Cl as well. However, sequentially impregnated Re-Ag catalysts were significantly less active than unpromoted Ag catalysts. Characterization with Scanning Electron Microscopy (SEM) shows distinct changes in the catalyst morphology. Kinetic studies indicate that the change in morphology reduces the number of step sites available for reaction. This leads to increases in selectivity (since terrace sites are more selective to EO[4]) but also decreases conversion upon the addition of Re.

Catalysts co-impregnated with Re and further promoted with an organic chloride co-feed displayed increased EO selectivity up to 70%. This increase was accompanied by an increase in ethylene conversion from 0.5% to ~3.0% . These catalysts also presented dramatically different Ag particle structure than the non-chlorinated versions, with increases in the surface area accounting for the change in catalyst conversion.

1 S. Linc, J. Jankowiak, and M.A. Barteau. Journal of Catalysis 224(2) 2004

2 J.C. Dellamorte, M.A. Barteau and J. Lauterbach. Review of Scientific Instruments 78(7) 2007

3 J.C. Dellamorte, J. Lauterbach and M.A. Barteau. Catalysis Today 120(2) 2007

4 M. Atkins, J. Couves, M. Hague, B.K. Sakakini, and K. Waugh. Journal of Catalysis 235 2005