Ronald L. Bruening

Comparison of Bulk, Emulsion, Thin Sheet Supported, and Hollo Fiber Supported Liquid Membranes in Macrocycle-Mediated Cation Separations

Abstract The advantages, disadvantages, and possible applications of macrocycle-mediated bulk, emulsion, supported and hollow fiber liquid membranes have been investi-gated. The relative transport rates of the alkali metal cations and of Zn(II), Cd(II) and Hg(II) in sin-gle and competitive cation experiments are studied and compared in the different membrane types. The four membrane types demonstrate similar selectivities but significantly different cation fluxes under comparable conditions using analogous macrocyclic carriers. The degree of distribution of the macrocycle to the organic membrane which is necessary for significant transport varies dramatically among the membrane types, each of which requires unique solvent characteristics. In the experiments, either l8-crown-6, dicyclohexano-l8-crown-6, or 4,1′(5)bis(1-hydroxyheptylcyclohexano)-l8-crown-6 were incorporated into bulk (chloroform, raethylene chloride), emulsion (toluene), supported (phenylhexane) and hollow fiber (phenylhexane or 1-octanol) ...

Challenges to achievement of metal sustainability in our high-tech society

Achievement of sustainability in metal life cycles from mining of virgin ore to consumer and industrial devices to end-of-life products requires greatly increased recycling rates and improved processing of metals using conventional and green chemistry technologies. Electronic and other high-tech products containing precious, toxic, and specialty metals usually have short lifetimes and low recycling rates. Products containing these metals generally are incinerated, discarded as waste in landfills, or dismantled in informal recycling using crude and environmentally irresponsible procedures. Low recycling rates of metals coupled with increasing demand for high-tech products containing them necessitate increased mining with attendant environmental, health, energy, water, and carbon-footprint consequences. In this tutorial review, challenges to achieving metal sustainability, including projected use of urban mining, in present high-tech society are presented; health, environmental, and economic incentives for various government, industry, and public stakeholders to improve metal sustainability are discussed; a case for technical improvements, including use of molecular recognition, in selective metal separation technology, especially for metal recovery from dilute feed stocks is given; and global consequences of continuing on the present path are examined.

Accomplishment of difficult chemical separations using solid phase extraction

A solid phase extraction (SPE) system is described which is capable of highly selective removal of trace amounts of metal ions from solutions containing matrices which normally make separations difficult. Highly acidic or basic solutions and concentrated salt mixtures are included in the matrices. Significant volume reductions can be achieved with SPE. Separations using SPE have been commercialized in both industrial and analytical applications.

Characterization of a supported liquid membrane for macrocycle-mediated selective cation transport

Macrocycle-mediated cation transport has been studied using aqueous phases separated by a liquid membrane supported on a microporous polypropylene film. A macrocycle-containing organic phase was incorporated into the pores of the film. In this system, phenylhexane was found to be superior to chloroform, toluene and 1,2-dichlorobenzene as a membrane solvent in maintaining membrane integrity due to its lower water solubility and higher boiling point. Transport experiments using a bis (1-hydroxyheptyl) derivative of dicyclohexano-18-crown-6 (DC18C6) as carrier gave the selectivity sequecnes K+ > Rb+ > Cs+ >Na+ > Li+ and Sr2+ > Ba2+ > Ca2+. Furthermore, the addition of t-butyl or hydroxyheptyl side-chains to DC18C6 improved cation transport over that observed with DC18C6 due to the decreased distribution of the ligand from the membrane to the adjacent water phases. Cation flux was found to be independent of the rate of stirring of the water phases and to be a direct function of carrier concentration in the membrane.

Macrocycle-mediated cation transport using hollow fiber supported liquid membranes

Abstract Macrocycle-mediated separation and transport of cations using a stable hollow-fiber supported liquid membrane system have been investigated. Mathematical models describing membrane diffusion-limited, macrocycle-mediated transport in cylindrical coordinates have been derived and found to model experimental cation fluxes within a few percent. The effects of flow rate and macrocycle concentration on facilitated transport through hollow fibers have proven analogous to those of stirring speed and macrocycle concentration, respectively, in thin-sheet supported liquid membrane studies. The selectivities observed in the transport of K+ over Na+, K+ over Li+, and Sr2+ over Ba2+ as the NO3 - salts and of Cd (SCN)2 over Zn (SCN2 are identical to those found using thin-sheet supported liquid membrane systems. Similar selectivities were found using emulsion and bulk liquid membrane systems in which the only variation in the chemistry was that macrocycle substituent groups were less hydrophobic and the membrane solvent was different.

Solid phase extraction of ions using molecular recognition technology

Solid phase extraction (SPE) materials are described that are capable of highly selective separations of trace levels of cations and anions from aqueous solutions. The extraction kinetics are rapid, either small or large volumes of solution can be treated, and large volume reductions can be. achieved. By varying the SPE system, the separations are effective even in the presence of large concentrations of acid, base, or salt. A review is given of the application of SPE technology to a variety of industrial and analytical separations.

Silica gel-bound aza-crowns for the selective removal and concentration of metal ions

Silica gel-bound macrocycles have been prepared by first hydrosilylating a macrocycle-containing alkene with triethoxysilane followed by coating and heating the resulting macrocycle-containing triethoxysilane on 60-200 mesh silica gel. The macrocycle is attached to silica gel by stable hydrocarbon-ether groups. The aza- crown-containing alkenes were prepared in good overall yields by convenient three- and four-step reactions using N-(2-(2-~hloroethoxy)ethyl)acetamide as a synthon. The silica gel-bound macrocycles were found to be stable at aqueous pH values below 11. In numerous determinations, the log K values for the interaction of the bound- macrocycle with various cations were found to be similar to those for the analogous unbound macrocycle with the same cations in aqueous solution. This result indicates that the bound- and unbound-macrocycles are solvated and form complexes in the same manner. Several separations of heavy metal ions from aqueous mixtures of ions are described.

Industrial applications of molecular recognition technology to separations of platinum group metals and selective removal of metal impurities from process streams

Green chemistry procedures using a novel process based on molecular recognition principles are described for the selective separation and recovery of metals in industrial processes. This process, termed molecular recognition technology (MRT), has the capability to make selective separations at various stages in metal life cycles. Results are given for individual platinum group metal separations, recycling of palladium from end-of-life products, copper purification by control of impurity bismuth concentration levels, and purification of H2SO4 for use in health-related applications by Hg removal to 0.1 mg L−1 concentration levels. In each case, the metals are selectively separated in pure form and can be recovered for reuse or environmentally safe disposal. High metal selectivity is obtained using a pre-designed ligand bonded chemically by a tether to a solid support, such as silica gel. Separations are performed in column mode using feed solutions containing the target metal in a matrix of acid and/or other metals. The target metal is selectively separated by the silica gel-bound ligand, leaving other solution components to go to the raffinate, where individual components can be recovered, if desired. Minimal waste is generated. Elution of the washed column with a small volume of eluent produces a concentrated eluate of pure target metal, which is easily separated in pure form. The MRT process uses innocuous wash and elution chemicals and no solvents. Metal recovery rather than dispersal into the commons is essential from a metal sustainability standpoint. A major benefit of metal recycling is reduction in the amount of virgin ore that must be mined to replace discarded metals. As metal use increases, conservation of this valuable metal resource increases in importance. Metal recycling rates are generally low. From end-of-life high-tech electronic products, they are in the 1–5% range. Separation and recovery results presented here show that green chemistry MRT processes have great promise in increasing metal sustainability in industrial processes.

Effect of co-anion on DC18C6-mediated Tl+ transport through an emulsion liquid membrane

Emulsion membrane systems consisting of an aqueous thallous salt source phase, a toluene membrane containing the macrocyclic ligand dicyclohexano-18-crown-6 (DC 18C6)(0.02M), the surfactant Span 80 (sorbitan monooleate) (3% v/v), and an aqueous 0.01M Li4P2O7 receiving phase were studied with respect to the disappearance of Tl+ from the source phase as a function of time. The salts TlOH, Tl2 CO3, TlAu(CN)2, TlSCN, TlClO4, TlAg(CN)2, Tl4 Fe(CN)6, TlF, Tl2SO4, TlBr, CH2 (COOTl)2, HCOOTl, TlCl, and TlNO3 were investigated singly (salts listed in order of decreasing total percentage of Tl+ transport observed). In competitive transport experiments using equimolar TlAg(CN)2 and TlAu(CN)2, TlAu(CN)2 was found to transport by a factor of six over TlAg(CN)2. Attempts are made to explain the co-anion effect on Tl+ transport by relating transport inversely to the negative free energy of hydration of the co-anion, directly to the amount of ion pairing of the co-anion with a cationic species present, and directly to the amount of deprotonation of the surfactant.

Stable Silica Gel-Bound Crown Ethers. Selective Separation of Metal Ions and a Potential for Separations of Amine Enantiomers*

Silica gel-bound crown ethers and aza macrocycles have been synthesized with the attaching arm connected to the carbon framework of the macrocycles. The interactions of these bound macrocycles with cations are almost identical to those involving the analogous free macrocycles. This has allowed for predictable cation separation, concentration, and removal processes to be performed on a small scale. Quantum mechanical calculations and NMR measurements indicate that similarly bound chiral macrocycles will be capable of use in separating chiral organic amines.