State-of-the-art propylene epoxidation Ag/CaCO
3 catalysts achieve 60% selectivity only in presence of potassium (~0.02 g K (g
cat)
-1) as well as gaseous co-feeds—2-10 vol% carbon dioxide, 5-20 ppmv allyl chloride, and 10-1000 ppmv nitric oxide. Co-fed alkyl chlorides are known to decompose and deposit Cl on Ag surfaces as evidenced by post-reaction Cl titration. Propylene epoxidation selectivity increases from 5% to 55% as allyl chloride concentration increases from 0-20 ppm [1-3]. This promotion occurs concomitant with deposition of ~0.15 mol Cl (mol Ag
surf)
-1 and corresponds to an increase in the epoxidation-to-combustion ratio ~20× without significantly altering rate-determining O
2 activation. Co-promoting nitric oxide, carbon dioxide, and potassium are required in addition to Cl for selective propylene epoxidation. We show that alkali co-promotion substantially alters the magnitude and nature of active surfaces and the promoting influence of all gaseous co-promoters on Ag/CaCO
3 catalysts. On catalysts without K promotion, effects of CO
2, NO, and allyl chloride are muted, and the catalyst remains non-selective at all conditions. At a fixed level of K promotion, co-fed NO increases selectivity at low concentrations and decreases selectivity at high concentrations. Increasing K loading (~0.024 to 0.1 mol K (mol Ag
bulk)
-1) decreases NO concentrations required for selective oxidation (~100 to 10 ppm NO), revealing cooperative promotional effects between K and NO. On Ag/CaCO
3 with K promotion, co-fed CO
2 is invariably observed to inhibit oxidation rates while co-promoting effects of CO
2 on selectivity are observed to be inverse to the effects of NO but depend sensitively on levels of K promotion. These co-promoter interactions in direct oxidation of propylene by molecular oxygen and their mechanistic underpinnings will be discussed.
- Esposito, J. and Bhan, A. ACS Catal. 13, 7578 (2023).
- Esposito, J. and Bhan, A. Catal. 431, 115356 (2024).
- Esposito, J. and Bhan, A. ACS Catal. 14, 14339 (2024).
