Butanal Condensation Chemistry Catalyzed by Brønsted Acid Sites on Polyoxometalate Clusters

Abstract

Abstract The connection of active site structures and their catalytic chemistry during butanal deoxygenation on polyoxometalate clusters with varying H + site densities and identity of central atoms [H x Na 4− x SiW 12 O 40 ( x =0–4) and H y Na 3− y PW 12 O 40 ( y =0–3)] was established with rate assessment, IR spectroscopic, and chemical titration methods. Butanal adsorbs on the H + or Na + ions on polyoxometalate clusters and forms RC=O⋅⋅⋅H + or RC=O⋅⋅⋅Na + complexes at 348 K. A portion of the adsorbed butanals on the H + sites converts to surface acetates through their reactions with vicinal framework oxygen atoms, as confirmed from the detection of ν as (OCO) band at approximately 1580 cm −1 , and remains as the spectator species. Bimolecular reactions of butanals on the remaining H + sites lead to 2‐ethyl‐2‐hexenal as the predominant products, within which a small fraction undergoes sequential cyclization–dehydration to produce aromatics. A trace amount of butanal converts through minor, competitive pathways that form light olefins and dienes. These findings on the connection between active sites and their catalytic chemistry provide mechanistic insights useful for tuning the rates of the various concomitant paths and thus yields towards the different products during deoxygenation reactions.