Photosensitized Oxidation of Alkenes Adsorbed on Pentasil Zeolites

Photosensitized oxidation of trans,trans-1,4-diphenyl-1,3-butadiene (DPB), trans-stilbene (TS), and 2,3-dihydro-γ-pyran (DHP) included in the internal framework of Na-ZSM-5 zeolites was investigated. The zeolite samples having adsorbed the alkenes were suspended in isooctane or pentaerythritol trimethyl ether (PTE), and the sensitizers, 9,10-dicyanoanthracene (DCA) and hypocrellin A (HA), were dissolved in the solution. The molecular sizes of both of the sensitizers and the solvents are greater compared with the channels of the zeolites. The isolation of the alkenes within the zeolites from the sensitizers in surrounding solution prevents them from undergoing electron transfer. On the other hand, singlet oxygen was produced in the solution and diffused into the internal framework of the zeolites to react with the alkenes. Thus, only singlet oxygen oxidation products were obtained and no product derived from superoxide radical anion was detected. By addition of water after the alkene adsorption, the photosensitized oxidation product distributions can be dramatically varied depending upon the aluminum content of the zeolite framework and the molecular dimensions of the substrates. Possible interpretations of these effects are discussed in terms of the locations of the reactants.