Photochemical separation

We will first consider possible assignments for the fluorescing states in laser-excited PuF6(g) based on available energy level structure and thermodynamic information. We will then consider some of the implications of the long-lived PuF6 fluorescence we have observed in terms of potential photochemical separation processes. 

Research Opportunities. The presence of a long-lived fluorescing state following either 532 nm or 1064 nm excitation of PuF6(g) provides a valuable opportunity to study the extent to which electronic energy in a 5f electron state is available in photochemical and energy transfer reactions. Such gas phase bimolecular reactions would occur in a weak interaction limit governed by van der Waals forces. Seen from the perspective of potential photochemical separations in fluoride volatility... 

Borgarello E, Serpone N, Emo G, Harris R, Pelizzetti E, Minero C (1986) Light-induced reduction of rhodium (III) and palladium (II) on titanium dioxide dispersions and the selective photochemical separation and recovery of gold (III), platinum (IV) and rhodium (III) in chloride media. Inorg Chem 25 4499-4503... 

Pi and P2 are the photochemical reaction center serving also as light-harvesting unit. They can be two kinds of compounds or a single compound (P) such as a metal complex. The photoreaction center must have a strong absorption in the visible region. Tt and T2 are the electron mediators which take out photochemically separated charges rapidly to prevent back reactions. C and C2 are the reduction and oxidation... 

In all the systems considered above the photosensitizer was embedded in the membrane symmetrically, i.e. identical S molecules are located near both the inner and the outer surfaces of the vesicle membranes. Of great interest would also be to create asymmetric membranes providing a specially organized gradient of the redox potential across the lipid bilayer. Asymmetry of a membrane can be realized, e.g. if one locates the molecules with different redox potentials within the membrane near its inner and outer interfaces. An asymmetric membrane containing the components required for photochemical separation of charges at the lipid // water... 

In the case of photolysis of acidic aqueous solutions of Eu2+ ion at 366 nm, the excited state results from 4f —> 5d transitions localized on the metal centre. These excited states have also considerable MLCT character because of strong mixing of metal 5d orbitals with ligand orbitals. In the case of the reaction of Eu3+ with H2 which occurs on photolysis at 254 nm, the photo reaction is due to the formation of an LMCT excited state. This process has been successfully used in the photochemical separation of Eu from other members of the lanthanides because Eu2+ is the only member of the lanthanide series which is at suitably low energy that an LMCT state is accessible [98]. Yb3+ and Sm3+ ions behave in a similar fashion to Eu3+ as far as their photochemical behaviour is concerned. Aqueous solutions of Sm3+ or Yb3+ containing 2-propanol on photolysis at 185 nm give hydrogen and acetone as products probably by a mechanism similar to Eu3+ ion. 

Dodsworth ES, Vlcek AA, Lever ABP (1994) Factorization of hgand-based reduction potentials. Inorg Chem 33 1045-1049 Donard OFX, Weber JH (1988) Volatilization of tin as stannane in anoxic environments. Nature 332 339-341 Donohue T (1977) Photochemical separation of europium from lanthanide mixtures in aqueous solution. J Chem Phys 67 5402-5404... 

All of the basic requirements for photochemical separations were recognized in this early work ... 

The first three requirements relate to the inherent chemical properties of the mixture to be separated. The requirements for the light source depend on the properties of the mixture, but its availability is a technological variable. Lasers, with their narrow wavelength ranges and high intensities, have generated new interest in photochemical separations. Infrared lasers have also made possible the completely new field of infrared-induced chemistry, in which some significant isotope separations have been reported. 

The four principles of photochemical separation apply to the separation of other chemical entities, elements and compounds, as much as they do to the separation of isotopes. Since the spectra of elements and compounds differ to a much greater degree than the spectra of isotopes, the problem is simpler. 

Photochemical separation, with the new capabilities of lasers as light sources, provides many new areas of investigation. Application of photochemical techniques to actinide separations alone has an enormous potential. As lasers become more reliable and economical, photochemical separation should become an attractive technique for many systems. 

Toth, L. M., Friedman, H. A., and Bell, J. T., "Photochemical Separation of Actinides in the Purex Process," Paper presented at the Plutonium Fuel Cycle Mtg., Bal Harbour, FL., 1977. 

Gunning, Pertel, and their coworkers reported the photochemical separation of mercury isotopes [92-95] in a flow reactor which consisted of a microwave-operated discharge lamp [52, 96] cooled by a flowing film of water. A filter cell and a circulation system, to prevent heating of the filter solution and the cell, were placed concentrically and coaxially with the lamp. A similar reactor, for small-scale laboratory photolysis of organic compounds in the solution or gas phase, has been proposed by Den Besten and Tracy [91]. In this arrangement the EDL was placed in a reaction solution and was operated by means of an external microwave field from a radio or microwave-frequency transmitter (Fig. 19.11). The quantum output of the lamp was controlled by changing the output of the trans-... 

Donohue (1977) described the photochemical separation of Eu from other rare earths using a low pressure mercury lamp with no filter at 185nm and with a Vycor filter at 254 nm, and an ArF excimer laser operating at 193 nm. Equimolar mixtures (0.01 M) of binary or ternary lanthanide perchlorates and 0.05 M K2SO4 in 10% isopropanol were used in the experiments. The separation factors for the binary mixture Eu/Ln varied from 1 for Eu/Pr to > 200 for Eu/Tm. 

Separation of Eu from a solution mixture of SmCls, EUCI3, and GdCl3 in a rare earth saturated ethanol—isopropanol system by photoreduction of Eu with a high pressure mercury lamp has been carried out by Qiu et al. (1991). The yield of Eu(II) was 95% and the purity of the precipitated Eu was 92%. EuCh was produced by photoreduction and precipitated from the alcohol mixture. The photochemical separation process is as follows ... 

Bautista begins the volume by reviewing important new solvent extraction procedures as well as emerging alternative separation processes such as photochemical separation, precipitation stripping, and supercritical extraction, in Chapter 139. Scientific and industrial procedures are illustrated. 

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