Investigation of silver based catalysts for ethylene epoxidation: High throughput studies and characterization

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, however. Recently our group has shown that a rational catalyst design methodology can be applied to this chemistry to predict improved catalyst formulations based on microkinetic modeling and density functional theory. A Cu-Ag bimetallic catalyst was predicted and later validated by Linic, Jankowiak and Barteau to improve the selectivity toward EO. Several other bimetallic combinations have been modeled but due to the length of the reactor experiments, they remained without validation. The purpose of this work has been to facilitate further studies, as well as to allow for new material discovery and mechanistic investigations of ethylene epoxidation, by employing a high-throughput catalytic reactor system, which was fabricated for the testing of monolith catalysts. This system, which employs independent temperature control of reactor tubes and Fourier transform infrared spectroscopic imaging for analysis of the effluent, has been used to study the effect of preparation conditions and promoters on Ag catalysts. In particular, high throughput reactor studies of the effect of Cu, Pt, Au, Pd, Cd, and Re on Ag catalysts will be discussed.;Using the high throughput reactor system, several Ag catalysts were initially tested to investigate the effects of calcination temperature, calcination time, reduction temperature, and reduction time on ethylene conversion and EO selectivity. The reduction temperature and time were not found to significantly affect the catalyst performance in the range of 523-623 K and 6-18 hours, respectively. However, calcination temperature and time do play a significant role. Catalysts calcined at temperatures less than 573 K showed no activity. This was attributed to lack of conversion of AgNO3 to Ag, which is reported to occur at 623 K [5]. Catalyst calcination at 673 K resulted in the optimum yield to EO. Calcination time was shown to create dramatic changes in the ethylene conversion with little change to the EO selectivity. In particular Ag catalysts that were calcined for 3 hours presented a maximum in ethylene conversion. This surprising result indicated that calcination longer than 3 hours sintered the Ag catalysts, decreasing the active surface area and therefore decreasing the ethylene conversion.;Promotion of Ag catalysts calcined for both 12 hours and 2 hours was undertaken for comparison with the microkinetic modeling results of Mhadeshwar and Barteau. The reactor studies indicated that the model adequately predicts the behavior of Cu-Ag, Pd-Ag and possibly Au-Ag catalysts. The addition of Pd improved the EO selectivity, similar to the Cu-Ag catalysts, while Au was found to diminish it. Cd and Pt differ significantly from the model however. Model predictions indicated that Cd-promoted catalysts would be completely unselective toward EO. Instead, 58% EO selectivity was achieved, which was almost twice that of an unpromoted Ag catalyst at 31%. Pt-promoted catalysts on the other hand were predicted to leave the EO selectivity unchanged compared to Ag, however 0% EO selectivity was measured. In order to understand the differences between the modeled and experimentally tested catalysts, characterization using scanning electron microscopy (SEM) with energy dispersive spectrometry was undertaken. The micrographs seen for these catalysts were both surprising and interesting when analyzed in combination with the reactor experiments. In particular, the formation of bimetallics for the Au-Ag and Pt-Ag catalysts was not found. Instead Au and Pt particles on the Ag particles led to deactivation of the catalyst due to a reduction in surface area. The Cd-promoted catalysts show a redistribution of the Ag to a trimodal distribution of 50 mum, 1 mum, and 100 nm particles. These results indicated that characterization may play a crucial role in validating modeling results and allowing for accurate catalyst predictions, particularly in future studies to identify improvements needed in the model to predict the observed Cd-Ag performance.;Opposite of the procedure for Cu, Pd, Pt, Au, and Cd promotion, analysis of Re-promoted catalysts was undertaken before models were created. Rhenium has previously been shown to promote the catalyst performance by Lauritzen of Shell Oil Company. 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. 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%. The sequentially impregnated catalysts showed further improvements with the addition of small quantities of an organic chloride co-fed with ethylene as well. However, sequentially impregnated Re-Ag catalysts were significantly less active than unpromoted Ag. Characterization of these catalysts with SEM showed significant variations in catalyst morphology. Catalysts co-impregnated with Re and further promoted with an organic chloride co-feed were found to have dramatic increases in selectivity to >70%. This increase was accompanied by an increase in conversion from 0.5% to ~3.0% ethylene conversion as well. These catalysts also presented dramatically different Ag particle structure than the non-chlorinated versions, with changes in the surface area accounting for the change in activity.