Liquid-phase oxidation of benzene to phenol by copper catalysts in aqueous solvent with a high acetic acid concentration

Abstract

The liquid-phase catalytic oxidation of benzene was carried out under mild reaction conditions over Cuimpregnated zeolite catalysts (Cu/NaX, Cu/HX, Cu/NaY, Cu/HY, Cu/NaZSM-5, Cu/HZSM-5, Cu/Namordenite, Cu/Hmordenite, and Cu/Hbeta) using both molecular oxygen and ascorbic acid as the oxidant and the reducing reagents, respectively. Phenol was exclusively produced as the oxidation product and no other product was detected. Among the catalysts tested in this study, the Cu/HY catalyst had the highest activity for phenol formation. H-type zeolites were more effective supports for phenol formation than the corresponding Na-type ones. The yield of phenol was found to increase with the simultaneous increase in the amounts of both the supported Cu and the added ascorbic acid. The Cu species supported on HY began to elute as Cu 2+ species in the solvent from the HY support with the initiation of the oxidation of benzene. The dissolved Cu 2+ species were reduced to form precipitated Cu 1+ species during the benzene oxidation. The homogeneous Cu species dissolved from the Cu/HY catalyst were thus suggested as mainly contributing to the phenol formation. The precipitated Cu 1+ species that had no activity for the benzene oxidation were found to be regenerated in their oxidation activity toward phenol formation by calcination of the Cu 1+ species in flowing air, although parts of the Cu species were found to be converted to aggregated CuO during the calcination.Résumé : On a effectué l'oxydation catalytique du benzène en phase liquide, dans des conditions douces, sur des catalyseurs de zéolite imprégnée de cuivre (Cu/NaX, C/HX, Cu/NaY, Cu/HY, Cu/NaZSM-5, Cu/HZSM-5, Cu/Namordenite, Cu/Hmordenite et Cu/Hbeta), en utilisant de l'oxygène moléculaire et de l'acide ascorbique respectivement comme agents oxydant et réducteur. Le seul produit d'oxydation est le phénol et aucun autre produit n'a été détecté. Parmi les catalyseurs évalués dans cette étude, le catalyseur Cu/HY présente l'activité la plus élevée pour la formation de phénol. Les zéolites du type H sont des supports plus efficaces que les supports correspondants de type Na pour la formation de phénol. On a observé que le rendement en phénol augmente avec une augmentation simultanée dans les quantités à la fois du Cu supporté et de l'acide ascorbique ajouté. Les entités de Cu supportées sur HY commencent à s'éluer du support HY vers le solvant, sous la forme de cations Cu 2+ , dès le début de l'oxydation de benzène. Au cours de l'oxydation du benzène, les cations Cu 2+ sont réduits en espèces Cu 1+ qui précipitent. On suggère donc que les espèces Cu homogènes, dissoutes du catalyseur Cu/HY, sont les principales à contribuer à la formation du phénol. On a observé que, si on soumet les espèces Cu 1+ qui ont précipité à une calcination avec circulation d'air, elles peuvent être partiellement régénérées dans leur activité oxydante conduisant à la formation de phénol, même si une certaine partie de ces entités est transformée en CuO agrégé au cours de cette calcination.

Cu precipitate formed during the benzene oxidation catalyzed by supported Cu in the presence of ascorbic acid and O2

“…However, a drawback in the supported Cu-ascorbic acid-O 2 catalytic system is that the catalytic activity for phenol formation stops during the initial stage of the oxidation [6,7]. Part of the Cu species on the supported Cu catalyst was observed to leach into the reaction solution during the initial stage of the phenol formation [8]. The eluted Cu species once again rapidly precipitated followed by deactivation of the phenol formation [8].…”

“…Part of the Cu species on the supported Cu catalyst was observed to leach into the reaction solution during the initial stage of the phenol formation [8]. The eluted Cu species once again rapidly precipitated followed by deactivation of the phenol formation [8]. The amount of Cu precipitate has been reported [9] to be correlated to the yield of phenol, thus the precursor of the Cu precipitate is thought to be an active Cu species for the benzene oxidation to phenol.…”

Aerobic one-step oxidation of benzene to phenol on copper exchanged HZSM5 zeolites: A mechanistic study

“…Kitamura et al investigated the benzene oxidation to phenol in aqueous solutions on several copper impregnated zeolites [1]. In this study the HY-zeolite showed the highest catalytic activity for the benzene conversion with a reported selectivity towards phenol of 100% but with a phenol yield of only 2.5%.…”

“…Furthermore, these observations indicate that phenol formation in the gas phase reaction can occur by a different reaction mechanism than in aqueous solution [1,9].…”

Liquid phase hydroxylation of benzene to phenol over Cu/ZnO catalysts

“…Gas-phase catalytic oxidation of benzene to phenol over copperimpregnated HZSM-5 catalysts [3] and the liquid phase oxidation of benzene over impregnated zeolite catalysts (Cu/NaX, Cu/HX, Cu/HY, Cu/NaZSM-5, Cu/Namordenite, Cu/Hmordenite and Cu/Hbeta) were some of the recent reports [4]. The oxidation of benzene to phenol using transition metal substituted polyoxometastable compounds in the presence of acetone and sulfonate [5] and V/Al 2 O 3 -SiO 2 catalysts [6] containing around 80% acetic acid and CuO/Al 2 O 3 catalysts [7] prepared by the co-precipitation method was also reported.…”