No abstract

No abstract

The photoassisted monooxygenation of C6H12 to C6H10O and C6H11OH by molecular oxygen has been studied on TiO2 powder catalyst dispersed in neat C6H12 and in C6H12/CH2Cl2 mixtures. The composition of the mixed solvent has a strong influence on the selectivity of the process: an increase in the content of CH2Cl2 brings about both an enhancement in the rate of formation of mono-oxygenated products and a decrease in the production of CO2. At the same time, the alcohol to ketone ratio increases in the mixed solvent. An explanation of this behavior is proposed which is based on the observed decrease in the adsorption strength of intermediates (C6H11OH and radicals) as the solvent composition is varied from pure C6H12 to mixtures of it with increasing amounts of CH2Cl2. The results of experiments with different O2 partial pressures are reported. The process is unaffected for O2 partial pressures > 200 Torr. For lower values the formation of dicyclohexyl becomes significant and reaches a maximum at a pO2 of 60 Torr. In O2-free media containing C(NO2)4 as the electron scavenger, the formation of C6H10O decreases markedly while that of C6H11OH is essentially the same as that in oxygenated media. In the mechanism proposed, the reaction of cyclohexyl radicals with O2 and/or activated oxygen species is the main route leading to the ketone.

No abstract

No abstract

The photochemical and photocatalytic properties of iron meso-tetraarylporphyrins bearing an OH(-) axial ligand and different substituents in the beta-positions of the porphyrin ring are reported. Irradiation (lambda = 365 nm) in the absence of dioxygen leads to the reduction of Fe(III) to Fe(II) with the formation of OH(*) radicals. Substituents at the pyrrole beta-positions are found to markedly affect the photoreduction quantum yields. Under aerobic conditions, this photoreaction can induce the subsequent oxidation of cyclohexane to cyclohexanone and cyclohexanol by O(2) itself. The process occurs under mild conditions (22 degrees C; 760 Torr of O(2)) and without the consumption of a reducing agent. The polarity of the solvent and the nature of the porphyrin ring have a remarkable effect on the selectivity of the photooxidation process, likely controlling the cleavage of O-O bonds of possible iron peroxoalkyl intermediates. In particular, in pure cyclohexane, oxidation occurs with the selective formation of cyclohexanone; in contrast, in dichloromethane/cyclohexane mixed solvent, the main oxidation product is cyclohexanol. Phenyl-tert-butylnitrone (pbn) has been found to quench the radical chain autooxidation of the substrate thus increasing the yield of cyclohexanol. This becomes the only oxidation product when iron 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin hydroxide (Fe(III)(TDCPP)(OH)) is used as photocatalyst.

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibilities. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the -NO2 group, affording 4-aminobenzaldehyde, takes place through surface states at potentials positive of the flatband potential (E(fb)). Dark currents caused by reduction of the aldehyde group are observed only at potentials more negative than E(fb), and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k=0.059 s(-1)), whereas that of the aldehyde group is slower (k=0.001 s(-1)) and requires electron photoaccumulation. Control of the photogenerated charge is a prospective means for achieving chemoselective reductions.

Proper reaction conditions have been found for the conversion of geraniol, citronellol, trans-2-penten-1-ol and 1-pentanol to the corresponding aldehydes with good chemo-selectivity (>70%) by photochemical excitation of suspensions of P25-TiO(2). It is demonstrated that adsorption of the alcohol on the surface as an alcoholate is necessary for its oxidation. ESR-spin trapping experiments point out that oxidation of alcohols starts with the formation of alkoxide radicals. Water content in the dispersing medium strongly inhibits alcohol adsorption and subsequent oxidation. In fact, water increases the polarity of the dispersing medium favouring the affinity between the polar alcohol and the CH(3)CN-H(2)O mixture itself; moreover, water competitive adsorption with the alcohol causes the removal of the latter from the photocatalytic surface with consequent difficult oxidation, as evidenced by ESR-spin trapping investigation. The reactivity of the alcohol on the surface of photoexcited P25-TiO(2) is also affected by the nature of its hydrophobic aliphatic chain: geraniol and citronellol are more susceptible to the water content than their short analogues trans-2-penten-1-ol and 1-pentanol. Moreover, in anhydrous CH(3)CN, specific interaction between the surface and the OH group enhances the reactivity of the primary aliphatic alcohols towards their partial oxidation to aldehyde, which can be accumulated in the reaction environment.