Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity

Abstract

A simple donor-acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr þ -Mes) was incorporated into nanosized mesoporous silica-alumina to form a composite, which in acetonitrile is highly dispersed. In this medium, upon visible light irradiation, the formation of an extremely long-lived electron-transfer state (Acr • -Mes •þ ) was confirmed by EPR and laser flash photolysis spectroscopic methods. The composite of Acr þ -Mes-incorporated mesoporous silica-alumina with an added copper complex [ðtmpaÞCu II ]ðClO 4 − Þ 2 (tmpa ¼ trisð2-pyridylmethylÞamine) acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to produce p-tolualdehyde and hydrogen peroxide. Thus, incorporation of Acr þ -Mes into nanosized mesoporous silica-alumina combined with an O 2 -reduction catalyst (½ðtmpaÞCu II 2þ ) provides a promising method in the development of efficient and robust organic photocatalysts for substrate oxygenation by dioxygen, the ultimate environmentally benign oxidant.copper complex catalyst | donor-acceptor dyad | nanosized silica-alumina | p-xylene | photoinduced electron transfer

Improvement of durability of an organic photocatalyst in p-xylene oxygenation by addition of a Cu(ii) complex

“…16 p -Methylbenzyl alcohol is further oxidized by the same catalytic cycle, even more efficiently, to produce p -tolualdehyde as the final oxidized product of p -xylene. 14,18 On the other hand, the Acr ˙ moiety can react with O 2 as evidenced by nanosecond laser flash photolysis (Fig. 5a).…”

“…8–14 For example, oxidation of benzene to phenol where the oxygen atom derives from water as its source has been made possible without producing further oxygenated compounds. 15,16 Bromination of various aromatic compounds has also been achieved without using toxic bromine.…”

“…18 The electron-transfer state (Acr ˙ –Mes ˙+ ), produced upon photoexcitation of Acr + –Mes, has both strong oxidizing and reducing ability, and this enables it to not only oxidize alkyl aromatic compounds but also to reduce O 2 . 10,14 However, the accompanying formation of reactive oxygen species (ROS) may be problematic, because they may irreversibly oxidize the organic photocatalyst leading to its deactivation.…”

Selective photocatalytic reactions with organic photocatalysts

“…99,100 Such an improvement is not observed by the addition of Cu(ClO 4 ) 2 in the absence of organic ligands. The addition of [(tmpa)Cu] 2+ in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu II ] 2+ .…”

Away from TiO2: A critical minireview on the developing of new photocatalysts for degradation of contaminants in water

“…Examples include 2Fe 2 S complexes [79], the report on water stable Zr-benzenedicarboxylate [80], and a donor-acceptor linked dyad comprising of 9-mesityl-10-methylacridinium ion incorporated into nanosized mesoporous silica-alumina [81]. Likewise, LFP was used to study the direct action of organic sensitizers leading to the degradation of water contaminants.…”