New Perspectives and Insights Into Direct Epoxidation of C3H6 Using O2 and Ag Based Catalysts and Measurement of Active Ag Site Concentration of Promoted Ag Catalyst for C2H4 Epoxidation by H2 Pulse Titration Over Oxygen Pre-covered Surface
Date of Award
Open Access Dissertation
John R. Monnier
John R. Regalbuto
Direct epoxidation of C3H6 using molecular O2 and Ag catalysts to produce propylene oxide (PO) selectively is a long-sought goal in the field of heterogeneous catalysis. In the early 1990’s Scientists at ARCO reported promising results of 58 – 59% selectivity to PO at 3.2% conversion of C3H6 over an unusual composition of 54.0 wt% Ag, 2.0 wt% K+, 0.5 wt% Mo and balance CaCO3. Feed additives of ethyl chloride (EtCl), nitric oxide (NO) and CO2 were stated necessary to achieve the claimed high PO selectivities. The roles of relatively high levels of NO and EtCl (200 ppmv each) and 2.0 wt% K+ , and 0.5 wt% Mo were not explained, nor was the choice of CaCO3, rather than the typically used α-alumina support. Ag supported on ZrO2 promoted/modified with Mo and W has been reported to achieve high selectivity to PO 53% to 67% at moderated 2.0% to 12.0% conversion of C3H6 at a significantly high (400°C – 460°C) temperature compared to the temperature (240ºC – 280ºC) reported by ARCO for propylene epoxidation. For ethylene epoxidation to ethylene oxide and 1,3 butadiene epoxidation to 3,4-epoxy-1-butene (EpB) reported reaction temperature was 200ºC – 270ºC. Therefore, these results for propylene epoxidation to propylene oxide (PO) using Mo and W promoted 20 wt% Ag/ZrO2 catalyst at high temperature (400 – 460°C) need to be confirmed with further investigation.
In this dissertation, a series of Ag catalysts supported on low surface area α-Al2O3, CaCO3 and ZrO2 was prepared and evaluated for direct epoxidation of C3H6 using molecular O2. Active surface Ag site concentration was measured by H2 pulse titration over oxygen pre-covered surface. XRD analysis, SEM imaging with EDXS analysis, BET surface area and BJH pore size distribution analysis was performed to characterize all the catalysts. Isomerization of PO was studied over α-Al2O3, CaCO3 and ZrO2 supports to examine PO stability at a temperature range from 220ºC to 300ºC. Effects of promoters (K+ , Cs+ , MoO3) and gas feed additives (EtCl, NO) were studied rationally for fundamental understanding. EtCl showed enhancement in PO selectivity by inhibiting the total oxidation of PO. However, over chlorination of the catalyst surface showed irreversible formation of AgCl and caused the loss in catalyst activity. A critical amount of NO (50 ppmv) was found to be required to achieve improvement in both activity and PO selectivity. Further increase in NO concentration at the feed > 50 ppmv showed only improvement in activity. K+ showed resistance to AgCl formation when 50 ppmv EtCl was present at the feed. In the presence of EtCl, NO and promoted with K+ both 12 wt% Ag/α-Al2O3 and 56 wt% Ag/CaCO3 showed > 35.0% selectivity at a 4.0% – 6.0 % conversion of C3H6. The 20 wt% Ag – 4 wt% Mo/ZrO2 catalyst at 460ºC showed the formation of acrolein as the selective product instead of PO as claimed in the literature. PO stability study at 360ºC showed PO was not stable at this high temperature. Catalytic evaluation for partial oxidation of C3H6 over a 4 wt% Ag/ZrO2 catalyst at 460ºC showed Ag was not required for the formation of acrolein.
Rahman, M.(2023). New Perspectives and Insights Into Direct Epoxidation of C3H6 Using O2 and Ag Based Catalysts and Measurement of Active Ag Site Concentration of Promoted Ag Catalyst for C2H4 Epoxidation by H2 Pulse Titration Over Oxygen Pre-covered Surface. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/7309