Photocatalytic reactions dehydrogenation

Catalytic dehydrogenation of alcohol is an important process for the production of aldehyde and ketone (1). The majority of these dehydrogenation processes occur at the hquid-metal interface. The liquid phase catalytic reaction presents a challenge for identifying reaction intermediates and reaction pathways due to the strong overlapping infrared absorption of the solvent molecules. The objective of this study is to explore the feasibility of photocatalytic alcohol dehydrogenation. 

When platinum loading is varied on 2 orders of magnitude (0.1 < Pt% <10%), but with a constant particle size 2 nm), there exists an optimum content equal to v 0.5 wt Pt% at which the rate of CDIE is maximum (Fig.3). This optimum is characteristic of the Pt/Ti02 system since it has been found for other photocatalytic reactions, as well in the liquid or aqueous phase (dehydrogenation of (C,- C ) - alcohols (22) or of unsaturated ones (34)(curves (1) and (2 in Fig (3) as in the gas phase (photocatalytic isotopic exchange of oxygen (35)). Such an optimum is general and can be observed for other metals. It was found equal to 5 wt% for nickel (36) and seems to depend upon the nature and especially on the texture (particle size and dispersion) of the metal. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The chief advantages to using dense CO2 as the reaction medium were claimed to be its inertness, its general ability to dissolve the reactants, and its easy separation from the reaction mixture. No mechanistic work was reported, but the same mechanism as previously proposed in alkane solvents was suggested, that is, the key step was oxidative addition to the excited state of the metal complex. The C02/Rh complex system was also reported to be an effective photocatalytic system for the dehydrogenation of cyclooctane. 

Summary The kinetic study of the solid-liquid reaction between photocatalytic titanium dioxide (photo-Ti02) and H- siloxane was investigated. The results showed that the solid-liquid reaction was inhibited in the presence of water, alcohol, ether, or other polar molecules, and supported its characterization as a dehydrogenation condensation reaction. The synthesized silicone-modified photo-TiOa by the solid-liquid reaction was initially hydrophobic, but became super-hydrophilic after irradiation by BLB light. Both ESR and Si-NMR studies suggested that this effect was caused by the photocatalytic oxidation of the silicone present on the photo-Ti02. 

For comparison, AQE for photocatalytic H2 formation from 2-propanol (200 pmol) in an aqueous suspension of platinized Ti02 was also examined and determined to be 4.1 % under the same irradiation conditions. This reaction has often been used as a model reaction to evaluate the activity of a photocatalyst for H2 evolution. The value of AQE in the present reaction larger than that of 2-propanol dehydrogenation shows that oxalic acid efficiently works as hole scavenger for photocatalytic reduction of NS to AS. 

Since reactions without the use of a solvent are more favorable from the point of view of green chemistry, photocatalytic solvent-free dehydrogenation of BnOH was also examined. The reaction was very simple, i.e., only Pt-Ti02 particles were suspended in 5 cm of BnOH and the mixture was photoirradiated under argon. Results are shown in Fig. 9.19. The amount of PhCHO increased linearly along with photoirradiation, and the PhCHO yield reached 1220 pmol after 24-h photoirradiation. Under the solvent-free condition, AQE was calculated to be 31 % after 4-h irradiation of Fig. 9.19. 

In the solvent-free system, the amount of H2 evolved was not determined because H2 was continuously purged from the reactor (test tube) to prevent a large increase in internal pressure. These results include two important points the first point is that photocatalytic dehydrogenation of BnOH occurred without loss of activity in the BnOH solvent as well as the acetonitrile solvent, and the second point is that high efficiency (reaction rate and AQE) was obtained compared with condition of a low concentration of BnOH. 

Even with Ti02 samples having the optimal content of Pt (as was determined in the case of photocatalytic dehydrogenation reactions), a positive effect on photocatalytic oxidations that do not involve H2 evolution has not always been observed. This suggests that the influence of Pt deposits on charge separation was not really fundamental and/or that the overall reduction of O2 was not really improved contrary to the aforementioned expectancies. The role of Pt to combine H atoms is therefore essential [91] as was recently confirmed in a study which also concluded that H" reduction occurs first on Ti02 [92]. 

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