Conductivity in supercritical

Enzymatic reactions have also been conducted in supercritical media. 

The N-phenylimine of acetophenone was hydrogenated using an Ir cationic complex with a phosphinodihydrooxazole (S)-la in CH2CI2 under 100 atm of H2 to give the R amine in 87% ee (Table of Scheme 1) [6]. The reaction was completed with a substrate to catalyst molar ratio (S/C) of 667. The reactivity was further increased when the reaction was conducted in supercritical CO2 in-... 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

Russell and coworkers [23] conducted an investigation into the condensation reaction between 1,4-butanediol and bis(2,2,2-trichloroethyl) adipate. The authors showed that low-dispersity polymers could be achieved in supercritical fluoroform at 50°C and 372 bar using PCL. Typically, the polymers had a molecular weight less than 1500 Da. This was the first record of biocatalytic polymerization in a supercritical fluid and opened the way for biocatalytic polymerization in these solvents. Almost all polymerization reactions in supercritical fluids to follow were conducted in supercritical C02. 

Numerous other enzymatic polymerizations have been conducted in supercritical C02 and we have included most of these in the following sections. 

A biocatalytic carbon dioxide fixation reaction was conducted in supercritical carbon dioxide for the first time. It will play a significant role both in identifying enzyme species suitable for biocatalysis in supercritical carbon dioxide and in developing synthetic methods using carbon dioxide. 

Mukaiyama aldol reaction. Reasonably good enantioselection is observed when the catalyzed reaction of ketene silyl acetals derived from thioesters with aldehydes is conducted in supercritical fluoroform. ... 

The reason for the low alkylate yield in the reaction in 2-methyl-butane was most likely the high reaction temperature. High reaction temperatures favored side reactions, reducing the selectivity of alkylate. Indeed, C5-C7 hydrocarbon products formed in high selectivity in the high temperature reaction conducted in supercritical 2-methyl-butane. It is possible that hydrocracking occurred as a side reaction at the same site where the alkylation reaction proceeded. The temperature of the reaction with propane was low and the side reactions were effectively suppressed. The deactivation of this reaction is probably due to the poor extraction capacity of the propane medium, especially at the low reaction temperature used here. The low solubilities of catalyst poisons in supercritical propane at these reaction conditions deactivated the catalyst. 

The observation of electric transport in dense supercritical ionic phases stimulated the investigation of conductance in supercritical metals above 1500 °C and to pressures of about 2000 bar. The conductance of mercury was investigated and continuous metal-nonmetal transition was observed [25]. An impressive amount of further aspects of such metal-nonmetal transitions has been studied later in alkali metals and other fluids (see F. Hensel and coworkers) [25a]. A metal-nonmetal transition was also observed in cesium hydride and cesium in the liquid state at high hydrogen pressures [26] which corresponds to the earlier non-pressure work of Max Bredig [26a] with liquid metal-metal halide systems. 

A potential solution of this problem has been recently developed at Max-Planck bistitut fur Kohlenforschung (MuUieim an der Ruhr, Germany) in close cooperation with industrial partners." Ionic liquids (IL) were used as an immobilizing matrix for catalyst 6 and the metathesis reaction was conducted in supercritical carbon dioxide. This technology can now be operated under continuous-flow conditions and used for the macrocyclization of some pharmaceutical precursors. ... 

In a classical heterogeneous dispersion polymerization, the continuous phase is organic in nature, althou in some instances water has been used as a component of the continuous phase to increase polarity. Research investigations have focused on the composition of the dispersion m um, reaction kinetics, the structure and influence of the stabilizer polymer, particle size, molecular weight and molecular weight distribution (4, S). Dispersions of poly(methyl methacrylate) (8) and poly(styrene) (9) are widely studied and among Ae best characterized systems. Recently, dispersion polymerizations conducted in supercritical carbon dioxide have also been reported (9-12). 

The water-in-C02 microemulsion mentioned previously in this section may provide an effective medium for generating electrical conductivity in supercritical CO2. In 2000, Ohde et al. first reported the results of voltammetric measurements for the redox reactions of ferrocene (FC) and A,fV,iV fV tetramethyl-jc-phenylenediamine (TMPD) in supercritical CO2 in the presence of a water-in-CO2 microemulsion (14). The design of their high-pressure electrochemical cell is shown in Figure 16. The same AOT/PFPE-PO4 water-in-C02 microemulsion described in Section IV.A was used in their voltammetric experiments. Well-defined voltammetric waves were obtained for FC and for TMPD in the microemulsion system as shown in Figure 17. An obvious diffusion current for the redox reaction of FC or TMPD was observed. An electrolysis experiment was also performed with TMPD. After the electrolysis at +0.3 V, the UV-Vis absorption spectrum of the sample collected in hexane was measured. The absorption peak wavelength and the shape of the peak were identical to that for TMPD + in water. The result suggests that TMPD " produced at the electrode surface was in the water core of the water-in-C02 microemulsion, as shown in Fq. (12) ... 

The reaction of a carboxylic acid with N,Af -carbonyldiimidazolellH33 (abbreviated as CDI), forming an imidazolide as the first step followed by alcoholysis or phenolysis of the imidazolide (second step), constitutes a synthesis of esters that differs from most other methods by virtue of its particularly mild reaction conditions.t41,[5] It may be conducted in two separate steps with isolation of the carboxylic acid imidazolide, but more frequently the synthesis is carried out as a one-pot reaction without isolation of the intermediate. Equimolar amounts of carboxylic acid, alcohol, and CDI are allowed to react in anhydrous tetrahydrofuran, benzene, trichloromethane, dichloromethane, dimethylformamide, or nitromethane to give the ester in high yield. The solvents should be anhydrous because of the moisture sensitivity of CDI (see Chapter 2). Even such unusual solvent as supercritical carbon dioxide at a pressure of 3000 psi and a temperature of 36-68 °C has been used for esterification with azolides.[6]... 

In 1988, Terry and coworkers attempted to homopolymerize ethylene, 1-octene, and 1-decene in supercritical C02 [87], The purpose of their work was to increase the viscosity of supercritical C02 for enhanced oil recovery applications. They utilized the free radical initiators benzoyl peroxide and fert-butyl-peroctoate and conducted polymerization for 24-48 h at 100-130 bar and 71 °C. In these experiments, the resulting polymers were not well studied, but solubility studies on the products confirmed that they were relatively insoluble in the continuous phase and thus were not effective as viscosity enhancing agents. In addition, a-olefins are known not to yield high polymer using free radical methods due to extensive chain transfer to monomer. 

In 1994, we reported the dispersion polymerization of MM A in supercritical C02 [103]. This work represents the first successful dispersion polymerization of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, we took advantage of the amphiphilic nature of the homopolymer PFOA to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymerization (> 3000) in supercritical C02. These polymerizations were conducted in C02 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN were employed as the initiators. The results from the AIBN initiated polymerizations are shown in Table 3. The spherical polymer particles which resulted from these dispersion polymerizations were isolated by simply venting the C02 from the reaction mixture. Scanning electron microscopy showed that the product consisted of spheres in the pm size range with a narrow particle size distribution (see Fig. 7). In contrast, reactions which were performed in the absence of PFOA resulted in relatively low conversion and molar masses. Moreover, the polymer which resulted from these precipitation... 

The use of a copolymerizable macromonomer constitutes another approach to the dispersion polymerization of MMA. We have recently demonstrated the utility of this approach in C02 by employing a PDMS monomethacrylate as the stabilizer (see Fig. 8) [121], Although several groups have studied the behavior of polysiloxanes in C02 [54,55,57], this work represents the first successful use of PDMS based polymeric stabilizers in C02. The reactions were conducted in either liquid C02 at 30 °C and 75 bar or supercritical C02 at 65 °C and 340 bar. 

In the past, the majority of high-pressure homogeneous catalytic reactions were conducted in batch systems, which may cause problems in scale-up for SCFs because of the higher pressures needed for achieving the supercritical state. Therefore, continuous processing has also been investigated in the last years. It would be preferable for industrial-scale SCF reactions, because it involves smaller and, hence, safer equipment [144-150]. In addition, capital costs are likely to be lower than in batch systems. 

An important innovative technique to replace water as the solvent in dyeing processes is the use of supercritical fluids, for example, supercritical CO2 for dyeing processes. Successful trials have been conducted in various scales with different fibers and full-scale production has been performed in the case of PES dyeing [62,63]. Besides the handling of high pressure equipment, the development of special dyestuff formulations is required. 

Electron attachment to O2 has been investigated in supercritical hydrocarbon fluids at densities up to about 10 molecules/cm using the pulsed electric conductivity technique [110], and the results have been explained in terms of the effect of the change in the electron potential energy and the polarization energy of 2 in the medium fluids. In general, electron attachment to O2 is considered to be a convenient probe to explore electron dynamics in the condensed phase. 

The electron will be solvated in a region where the solvent molecules are appropriately arranged. There must be a cluster of electrons of a size of 4-5 to support the formation of the solvated electron from the results of Gangwer et al., [23], Baxendale [24,25], and Kenney-Wallace and Jonah [16]. This behavior does not depend on the specific alcohol or alkane and even occurs in supercritical solutions, as has been shown in experiments done using mixtures of supercritical ethane-methanol mixtures [19]. Experiments have also shown that the thermodynamically lowest state might not be reached. For example, the experiments of Baxendale that measured the conductivity of the solvated electron in alcohol-alkane mixtures showed that when there was a sufficient concentration of alcohols to form dimers, there was a sharp decrease in the mobility of the electron [24,25]. This result showed that the electron was at least partially solvated. However, the conductivity was not as low as one would expect for the fully solvated electron, and the fully solvated electron was never formed on their time scale (many microseconds), a time scale that was sufficiently long for the electron-alcohol entity to encounter sufficient alcohols to fully solvate the electron. Similarly, the experiments of Weinstein and Firestone, in mixed polar solvents, showed that the electron that was observed depended on the initial mixture and would not relax to form the most fully solvated electron [26]. 

Chemical reactions at supercritical conditions are good examples of solvation effects on rate constants. While the most compelling reason to carry out reactions at (near) supercritical conditions is the abihty to tune the solvation conditions of the medium (chemical potentials) and attenuate transport limitations by adjustment of the system pressure and/or temperature, there has been considerable speculation on explanations for the unusual behavior (occasionally referred to as anomalies) in reaction kinetics at near and supercritical conditions. True near-critical anomalies in reaction equilibrium, if any, will only appear within an extremely small neighborhood of the system s critical point, which is unattainable for all practical purposes. This is because the near-critical anomaly in the equilibrium extent of the reaction has the same near-critical behavior as the internal energy. However, it is not as clear that the kinetics of reactions should be free of anomalies in the near-critical region. Therefore, a more accurate description of solvent effect on the kinetic rate constant of reactions conducted in or near supercritical media is desirable (Chialvo et al., 1998). 

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