Titanium dioxide is the most extensively used heterogeneous catalyst for the photooxidation of toluene and other hydrocarbons, but it has low utility for the synthesis of benzyl alcohol, of which little is produced, or benzaldehyde, due to further oxidation to benzoic acid and cresol, among other oxidation products, and eventually complete mineralization to CO2. Et4N[FeCl4] functions as a photocatalyst through the dissociation of chlorine atoms, which abstract hydrogen from toluene, and the photooxidation of toluene proceeds only as far as benzyl alcohol and benzaldehyde. Unlike TiO2, which requires ultraviolet (UV) irradiation, Et4N[FeCl4] catalyzes the photooxidation of toluene with visible light alone. Even under predominantly UV irradiation, the yield of benzyl alcohol plus benzaldehyde is greater with Et4N[FeCl4] than with TiO2. Et4N[FeCl4] photocatalysis yields benzyl chloride as a side product, but it can be minimized by restricting irradiation to wavelengths above 360 nm and by the use of long irradiation times. The photonic efficiency of oxidation in one experiment was found to be 0.042 mol/einstein at 365 nm. The use of sunlight as the irradiation source was explored.

Tetraethylammonium tetrachloroferrate catalyzes the photooxidation of cyclohexane heterogeneously, exhibiting significant photocatalysis even in the visible portion of the spectrum. The photoproducts, cyclohexanol and cyclohexanone, initially develop at constant rates, implying that the ketone and the alcohol are both primary products. The yield is improved by the inclusion of 1% acetic acid in the cyclohexane. With small amounts of catalyst, the reaction rate increases with the amount of catalyst employed, but then passes through a maximum and decreases, due to increased reflection of the incident light. The reaction rate also passes through a maximum as the percentage of dioxygen above the sample is increased. This behavior is due to quenching by oxygen, which at the same time is a reactant. Under one set of reaction conditions, the photonic efficiency at 365 nm was 0.018 mol/Einstein. Compared to TiO2 as a catalyst, Et4N[FeCl4] generates lower yields at wavelengths below about 380 nm, but higher yields at longer wavelengths. Selectivity for cyclohexanol is considerably greater with Et4N[FeCl4], and oxidation does not proceed past cyclohexanone.

FeCl 4 − heterogenized on a Dowex 2-X8 anion exchange resin catalyzes the photooxidation of ethanol to acetaldehyde under visible and near-UV irradiation (>345 nm). The rate of reaction is proportional to the oxygen partial pressure up to 1 atm. Oxidation is suggested to occur through the formation of 1-hydroxyethylhydroperoxide, initiated by the photodissociation of a chlorine atom. The hydroperoxide re-oxidizes the iron(II) species, both the oxidation and reduction steps producing acetaldehyde. This mechanism is consistent with the increases in yield with ethanol concentration in ethanol–toluene mixtures towards an asymptotic limit. Copyright © 2014 John Wiley & Sons, Ltd.

Unactivated MCM ‐41 mesoporous silica catalyzes the photodecomposition of chloroform to phosgene and hydrogen chloride under near‐ UV ( λ > 360 nm) irradiation. The rate of photodecomposition increases toward an asymptotic limit as the O 2 partial pressure is increased. Deuterochloroform does not decompose under the same experimental conditions. Low concentrations of both cyclohexane and ethanol quench the photodecomposition, whereas water, up to its solubility limit, does not. Dissolved tetraalkylammonium salts suppress photodecomposition. The data are consistent with a mechanism in which light absorption by an SiO 2 defect yields an electron‐deficient oxygen atom, which then abstracts hydrogen from chloroform. The resulting CC l 3 radicals react with oxygen to form a peroxy radical that decomposes, eventually yielding phosgene and hydrogen chloride.

The ability of five substances – Amberlite IRA-900 anion exchange resin in the Cl- form, CuCl4 2- supported on Dowex 2-X8 anion exchange resin (CuCl4 2-/Dowex), FeCl4- supported on Dowex 2-X8 (FeCl4-/Dowex), FeCl3 supported on silica gel (FeCl3/SiO2), and unmodified MCM-41 mesoporous silica – to catalyze the photodecomposition of chloroform to COCl2 and HCl in a suspension irradiated under oxygen at wavelengths above 345 nm was compared, choosing amounts of each catalyst to optimize the decomposition rate. Amberlite, FeCl4-/Dowex, and MCM-41 were the most active. The same comparison was made for chloroform irradiated under air by sunlight, for which Amberlite, CuCl4 2- /Dowex, and MCM-41 were the most active. After irradiation, all samples had a COCl2 to HCl ratio greater than 10:1. A photodecomposition mechanism different from that observed in the gas phase is proposed that includes the participation of trichloromethylhydroperoxide. Keywords: Amberlite IRA-900, chloroform, FeCl3, MCM-41, photodecomposition, tetrachlorocuprate(II), tetrachloroferrate( III), trichloromethylhydroperoxide.

Iron(III) chloride adsorbed on silica gel catalyzes the photodecomposition of dichloromethane under near-UV irradiation, functioning through photodissociation to produce chlorine atoms that initiate a radical chain. While the photocatalytic activity under these conditions is similar to that of FeCl4¯ heterogenized on an ion exchange resin, under solar irradiation the FeCl3/silica system is considerably more effective.

Heterogenized on a polystyrene anion exchange resin and in the presence of oxygen, CuCl4(2-) catalyzes the photodecomposition of chloroform at wavelengths above 345 nm with greater efficiency than an equivalent amount in homogeneous solution. The reaction is proposed to proceed in two stages, the first stage yielding CCl4 and HO2(-) as products, the second consisting of a chain reaction resulting from the CuCl4(2-)-catalyzed photodissociation of CCl4, yielding phosgene with CCl3 radicals as chain carriers. Photodecomposition is retarded by added Cl(-), CH3CN, C6H12 or C2H5OH, which is ascribed to the displacement of CHCl3 molecules from the vicinity of the copper by attraction to the polystyrene matrix or to the alkylammonium cation sites.

The FeCl_4^− ion, heterogenized on a Dowex ion exchange resin, catalyzes the aerobic photodecomposition of neat CH_2Cl_2. Phosgene production was used to characterize the extent of decomposition, although it appears to be a secondary product from the decomposition of chloroform, which is suggested to arise from the reaction of dichloromethanol with hydrogen chloride. The yield of CHCl_3 increases when the production of phosgene is suppressed by water or acetonitrile. CuCl_4^2−, likewise heterogenized on Dowex, is photocatalytically inactive.

When the yield of a heterogeneously photocatalyzed reaction is seen to pass through maximum as the mass of catalyst is increased in a suspension, this behavior may be attributable to a regime in which back-reflection most often occurs after a single photon-particle encounter, while absorption requires multiple internal reflections. This was modeled with a double exponential equation, which predicts that the yield will pass through a maximum with increasing catalyst mass when the catalyst has low absorptivity, but approach an asymptotic limit when the absorptivity is high. The rate of photodecomposition of chloroform, catalyzed by the chloride form of a polystyrene anion exchange resin (low absorptivity), was found to pass through a maximum as the amount of resin was increased, while catalysis by the CuCl4 2- form of the resin (higher absorptivity) caused an increase in the rate of decomposition with catalyst mass to an asymptotic value. Keywords: Heterogeneous photocatalysis, suspensions, chloroform photodegradation, reflectance, catalyst mass

Irradiation at wavelengths above 360 nm of a suspension of Dowex 2-X8 anion exchange resin (Cl¯ form) in chloroform causes the decomposition of chloroform with formation of COCl2 and HCl. The presence of CCl4 as the most important secondary product, along with the lack of any photodecomposition in CDCl3, the absence of photodecomposition when O2 is excluded, and the lack of any secondary products related to CHCl2 radicals rule out a mechanism based on sensitized C–Cl dissociation, as occurs in the direct UV photolysis of chloroform. A mechanism is proposed in which an initial photoreaction produces COCl2 and HCl at a slow rate, along with CCl4. A second photoreaction causes Cl dissociation from CCl4, greatly increasing the net rate of COCl2 and HCl formation after a sufficient concentration of CCl4 is attained. Keywords: Chloroform, photocatalysis, photodecomposition, Dowex, anion exchange resin, phosgene.

Dissolved hexachlororuthenate(IV) effectively catalyzes the photodecomposition of chloroform to hydrogen chloride and phosgene under near‐UV (λ > 345 nm) irradiation, whereby RuCl62− is not itself photocatalytically active, but is photochemically transformed into a species that is active, possibly RuCl5(CHCl3)−. Conversion to a photoactive species during irradiation is consistent with the acceleration of the decomposition rate during the early stages and with the apparent inverse dependence of the decomposition rate on the initial concentration of RuCl62−. The displacement of Cl− by CHCl3 in the coordination sphere to create the photoactive species is consistent with the retardation of photodecomposition by both Cl− and H2O. The much smaller photodecomposition rate in CDCl3 suggests that C–H bond dissociation occurs during the primary photochemical event, which is also consistent with the presence of a CHCl3 molecule in the first coordination sphere.

The protonation of tetraphenylporphyrin followed by spectrophotometric detection of the resulting dihydrogen tetraphenylporphyrin dication has been evaluated as a means to determine micromolar concentrations of strong acids in chloroform. The equilibrium constants for the process are large (> 10(10) for benzenesulfonic and methanesulfonic acids), but not large enough to ensure stoichiometric conversion to the dication unless only order of magnitude precision is required. The incorporation of a correction term in the stoichiometric approximation, based on a dilution of the analytical sample, yields an acceptable result for these two acids.

Unlike other chlorometallate complexes that catalyze the photodecomposition of haloalkanes through photodissociation of a chlorine atom, both PdCl42- and Pd2Cl62- catalyze chloroform decomposition through a process that appears to involve C-H bond breakage from an excited state association complex with chloroform. This would account for the greatly retarded rate of decomposition in CDCl3 and for the generation of CCl4 as a side product. In chloroform, Pd2Cl62- and PdCl42- are in slow equilibrium with each other. The rate for the conversion of Pd2Cl62--PdCl42- in chloroform at 23 degrees C obeys the expression (0.03 M (1) s (1)) [PdCl42-][Cl]. The equilibrium constant, K = [Pd2Cl62-][Cl ](2)/[PdCl42-](2), was estimated to be 3 x 10 (3) M in CHCl3. (C) 2010 Elsevier B.V. All rights reserved.

The chloride form of the polystyrene-divinylbenzene anion exchange resin Amberlite IRA-900 was found to catalyze the photodecomposition of carbon tetrachloride in ethanol at wavelengths above 350 nm. With sulfate, bromide, and perchlorate as counterions, the resin was inactive. The major products are acetaldehyde, phosgene, chloroform, and hydrogen chloride. The photoreaction is much slower under 1.0 atm O(2) than under air, while in deoxygenated solutions it is also much slower and produces no phosgene. Much of the observed behavior can be explained by a model in which the poly(styrene-co-divinylbenzene) matrix absorbs light and transfers energy to CCl(4), which undergoes photodissociation, assisted by a chloride ion to stabilize the chlorine atom as Cl(2)(-). Two major reaction channels for the trichloromethyl radicals produced by photodissociation are proposed, one in which CCl(3) abstracts hydrogen from ethanol and the other involving addition of O(2) to form trichloromethylperoxy radicals. (C) 2010 Elsevier Inc. All rights reserved.

Broadband (lambda > 320 nm) irradiation of solutions of (Bu4N)(3)RhCl6 in CHCl3 exposed to air causes chloroform decomposition, with the production of HCl and substances, including phosgene and peroxides, capable of oxidizing iodide ion, and lesser amounts of CCl4 and C2Cl6. There is a short induction period as RhCl63- is replaced by an unknown rhodium species, the spectrum of which is stable after 15 or 20 min. The rate of photodecomposition is reduced by the addition of chloride ion, and it ceases nearly completely in deoxygenated solutions or in CDCl3. Mechanistic possibilities in keeping with these observations include hydrogen atom transfer from chloroform during the primary photochemical step and a prior equilibrium involving chloride ion dissociation. A rhodium hydroperoxide offers one possible explanation for the required participation of O-2. (C) 2010 Elsevier B.V. All rights reserved.

Broadband (λ > 320 nm) irradiation of (Bu4N)2Pd2Br6 in chloroform causes the conversion of to . During the conversion, chloroform is decomposed photocatalytically at a rate that accelerates then decelerates as coordinated bromines are replaced by chlorines. The primary bromine-containing product is CCl3Br. The observations are consistent with a mechanism in which and the intermediate complexes undergo homolytic photodissociation of a bromine or chlorine atom, which terminates with a trichloromethyl radical or abstracts a hydrogen from chloroform, respectively.

Broadband (λ > 320 nm) irradiation of chloroform solutions of either [Ru(bpy)2Cl2] or [Ru(bpy)2Cl2]Cl exposed to air led to a photostationary state, in which [Ru(bpy)2Cl2]+ predominated, and to the continuous decomposition of CHCl3, as evidenced by the accumulation of HCl, hydroperoxides (CCl3OOH and CHCl2OOH), and tetra-, penta-, and hexachloroethane. The addition of Cl− increased the rate of photodecomposition, while the replacement of Cl− by F− greatly decreased the rate. The observations are consistent with a photocatalytic cycle in which [Ru(bpy)2Cl2]+ is photochemically reduced to [Ru(bpy)2Cl2], which is thermally reoxidized by CCl3OO or CCl3OOH. In the absence of air a much slower photodecomposition reaction takes place leading to continuously increasing concentrations of chloroethanes. The data are consistent with a catalytic cycle in which [Ru(bpy)2Cl2]+ is photoreduced, as in aerated solutions, while [Ru(bpy)2Cl2] is photooxidized with chloroform as the substrate.

Near-UV irradiation of solutions of (Bu4N)AuCl4 in aerated ethanol-stabilized chloroform causes the continuous decomposition of chloroform, as evidenced by the production of many equivalents of HCl and peroxides. At the outset of irradiation, most of the AuCl4 − is reduced to AuCl2 −, but the reduction stops and is reversed. The same experiments done in ethanol-free chloroform cause chloroform decomposition only until the irreversible reduction of the gold is complete. In deoxygenated ethanol-free chloroform, irreversible reduction to AuCl2 − is accompanied by the formation of HCl and CCl4, while the main decomposition products in deoxygenated ethanol-stabilized chloroform are HCl and C2Cl6. It is proposed that, in ethanol-free chloroform, photoreduction of AuCl4 − begins with the concerted elimination of HCl from an association complex of CHCl3 with AuCl4 −, and that ethanol suppresses\( \{ {\text{CHCl}}_{3} \cdot {\text{AuCl}}_{4}^{ - } \} \) complex formation, leaving a slower radical process to carry out the photoreduction of AuCl4 − in ethanol-stabilized chloroform. In the presence of oxygen, the radical process causes a build-up of CCl3OOH, which reoxidizes AuCl2 − to AuCl4 − and allows the photodecomposition of CHCl3 to continue indefinitely.

Hexachloroosmate(IV) effectively catalyzes the photodecomposition of chloroform in aerated solutions. The decomposition products are consistent with a mechanism in which excited state OsCl(6)2- reduces chloroform, rather than one involving photodissociation of chlorine atoms. Trace amounts of ethanol or water in the chloroform lead to photosubstitution to form OsCl5(EtOH)- or OsCl5(H2O)-, neither of which is photocatalytically active.

Irradiation (λ > 320 nm) of ferrocene in chloroform causes decomposition of chloroform and the accumulation of HCl, CCl3OOH, and C2Cl6. This appears to occur initially through a cycle in which (a) ferrocene is oxidized to ferrocenium and tetrachloroferrate ions, (b) FeCl4 − undergoes photodissociation, and (c) ferrocenium reoxidizes the chloroferrate(II) species. On extended photolysis, the concentrations of CCl3OOH and FeCl4 − build up and a competing cycle in which FeCl4 − is restored through oxidation of the chloroferrate(II) species by CCl3OOH accelerates the decomposition rate.