Co-promoter interactions within silver-catalyzed propylene epoxidation by molecular oxygen

Propylene epoxidation on promoted Ag catalysts requires several promoters including gaseous co-feeds of allyl chloride (5–20 ppm), nitric oxide (100–1000 ppm), and carbon dioxide (2–10 vol%) on K-promoted Ag/CaCO₃. Steady-state rate and selectivity trends with respect to individual promoter concentrations reveal, in addition to direct influence of promoters on oxidation intermediates, co-promoters significantly alter the nature and magnitude of the effect of other co-promoters. On K-promoted Ag/CaCO₃, nitric oxide increases epoxidation selectivity at low concentrations (depending on K loading, up to ∼100 ppm NO) and decreases selectivity at higher concentrations (>200 ppm NO). Co-promoting K concentrations significantly influence the concentrations of NO required for selective oxidation, evincing cooperative co-promotional effects of NO and K. At low levels of K/NO promotion, carbon dioxide (0.2–40 vol%) weakly inhibits O₂ activation rates (∼0.3 order inhibition) and inhibits selectivity while at high levels of K/NO promotion, co-fed CO₂ increases selectivity and reaches maximal order inhibition of rates (at ∼0.5 order inhibition). These results can be rationalized on the basis of selectivity-enhancing, competing alkali-stabilized NO_x and/or CO_x derived from co-fed NO and CO₂. NO renders half-order promotion of steady-state rates on K-promoted catalysts, and transient experiments show NO slowly generates long-lived surface intermediates relevant to catalytic activity for both oxidation and chlorination—consistent either with NO generating an activity-enhancing, co-catalytic NO_x intermediate or removing surface-poisoning, alkali-stabilized CO_x species to promote catalytic activity. K promotion (>0.05 mol K (mol Ag_bulk)⁻¹) is also shown to substantially alter the promoting action of co-fed allyl chloride, with K-bare catalysts featuring significantly weaker selectivity promotion by the organochloride. Insights into co-promoter interactions suggest selective propylene epoxidation occurs only on active sites covered in appropriate concentrations of alkali-stabilized chlorine and oxyanions derived from CO₂ and NO_x.