Ethane, supercritical

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

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]. 

Figure 5 Free energies for attachment to solutes MeSt— methylst5rene. Sty—styrene, Bph— biphenyl, CO2, Pyr—pyrimidine, Tph— triphenylene, Dfb—p-difluorobenzene, Tol— toluene. But— 1,3-butadiene, Pyz—pyrazine in TMS, 2,2,4-trimethylpentane, and -hexane at 298 K and in supercritical ethane at 310 K. (From Refs. 90-99.)...

Figure 9 Rate constants for electron attachment to CO2 vs. the free energy of reaction in different fluids 0—2,2,4-trimethylpentane [126], —2,2-dimethylbutane [139], O— TMS [126], —supercritical ethane [99],...

High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

The equipment depicted in Fig. 17 also allows monitoring of species adsorbed on a solid catalyst. For this application, the ZnSe IRE is coated with a layer of the catalyst before assembly of the cell and the start of the reaction. This approach was chosen for investigation, for example, of the interaction of the reactant with the catalyst during the asymmetric hydrogenation of ethyl pyruvate catalyzed by cinc-honidine (CD)-modified Pt/Al2O3 in the presence of supercritical ethane (79). 

Figure 2. Fraction (%) of cross-coupling photoproduct AB obtained by photolysis of l-(4-methylphenyl)-3-phenyl-2-propanone in supercritical ethane as a function of pressure. T=35°C, 39mM reactant, 2.5 min exposure.

Solubilities of Five Solid n-Alkanes in Supercritical Ethane... 

The effect of solid structure on the solubilities of n-alkanes in supercritical ethane has been investigated at a temperature just above the critical point of ethane. Solubilities of n-alkanes containing 28 to 33 carbon atoms in ethane at 308.15K and pressures up to 20 MPa are reported in this work. The enhancement factor is shown to exhibit a regular trend with the number of carbon atoms in the n-alkane, although different trends are exhibited by the odd and even members of the series. 

The n-alkanes are an interesting homologous series because they display great regularity in their behavior. Many of their fluid phase properties, for example, can be correlated with the number of carbon atoms in the molecules ( 1, 2 ). In order to develop general relations for supercritical extraction, therefore, we have stud.ied the solubilities of solid n-alkanes containing 28 to 33 carbon atoms in supercritical ethane. 

Figure 3. Experimental and calculated solubilities of even-numbered n-alkanes in supercritical ethane at 308.IK. experimental data for n-C28H58 (k.j - - 0405) A-experimental data forCn-CggH ) (kj. = -0306 o-experimental data for - 32 66 ij = 0383). Calculations using the Patel-...

Figure 4. Experimental and calculated solubilities of odd-numbered n-alkanes In supercritical ethane 308.1k.

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

In this chapter, we describe the density- and temperature-dependent behavior of the vibrational lifetime (TO of the asymmetric CO stretching mode of W(CO)6( 2000 cm-1) in supercritical ethane, fluoroform, and carbon dioxide (C02). The studies are performed from low density (well below the critical density) to high density (well above the critical density) at two temperatures one close to the critical temperature and one significantly above the critical temperature (68-70). In addition, experimental results on the temperature dependence of Ti at fixed density are presented. Ti is measured using infrared (IR) pump-probe experiments. The vibrational absorption line positions as a function of density are also reported in the three solvents (68,70) at the two temperatures. 

Using supercritical fluids as solvents in the study of vibrational relaxation permits the independent control of temperature and density. Figure 2 shows a typical pump-probe scan of the asymmetric CO stretching mode of W(CO)6 in a supercritical fluid. This decay was taken in supercritical ethane... 

Figure 2 Pump-probe data for the asymmetric stretching CO mode of W(CO)6 ( 1990 cm-1) in supercritical ethane at the critical density (6.88 mol/L) and 343 K. The heavy line is a fit to a single exponential. The lifetime, Ti, equals 278 ps. Data scans in other solvents, temperatures, and densities were of similar quality.

Our last specific system involves the solute W(CO)6 in supercritical ethane. In the experiments (8-11) a particular vibrational mode of the solute is excited to n = 1, and the population of this level is subsequently probed as a function of time. VER in this system in principle embodies all the... 

Figure 3 Solvent-induced VER rate for the asymmetrical CO stretch mode of W(CO)6 in supercritical ethane at 307.15 K (critical temperature = 305.33 K) as a function of density. The solid diamonds are the experimental points, and the theory is given by the open circles.

The experiments by Fayer and coworkers measure VER rates of the asymmetrical (Tlu) CO stretching mode of W(CO)6 over a wide range of densities and temperatures in supercritical ethane (8-11) (as well as in other solvents that we do not consider herein). We use the following interaction parameters for this system (12) es/k = 233 K, rrs = 4.24 A,... 

One aspect of the last set of experiments on W(CO)6 in supercritical ethane that we have not yet discussed involves the possible role of local density enhancements in VER and other experimental observables for near-critical mixtures. The term local density enhancement refers to the anomalously high solvent coordination number around a solute in attractive (where the solute-solvent attraction is stronger than that for the solvent with itself) near-critical mixtures (24,25). Although Fayer and coworkers can fit their data with a theory that does not contain these local density enhancements (10,11) (since in their theory the solute-solvent interaction has no attraction), based on our theory, which is quite sensitive to short-range solute-solvent structure and which does properly include local density enhancements if present, we conclude that local density enhancements do play an important play in VER and other spectroscopic observables (26) in near-critical attractive mixtures. 

The Pt-catalyzed enantioselective hydrogenation of ethyl pyruvate to (/ )-ethyl lactate was considerably faster (by a factor of 3-3.5) in supercritical ethane than in the conventional apolar solvent toluene, whereas the enantioselectivity was unaffected. Complete catalyst deactivation was observed in C02, which was shown by FTIR to be due to the reduction of C02 to CO via reverse water gas shift reaction. The catalyst could be regenerated by exposing it to ambient air, while hydrogen treatment was less efficient. This is the first evidence to the limitation of catalytic hydrogenations over Pt metals in supercritical C02. 

Solubilities of heavy hydrocarbons in supercritical fluids depend on the type of solvent (6). Moradinia and Teja (7) showed that the solubilities of solid n-alkanes (n-C2g, n-C Q, n-C ) are about ten times higher in supercritical ethane than in carbon dioxide. Therefore, it is reasonable to search and find an appropriate solvent which can disintegrate and dissolve the carbonaceous deposits from hydrotreating catalysts, resulting in their decoking and regeneration. 

In the studies described here, we examine in more detail the properties of these surfactant aggregates solubilized in supercritical ethane and propane. We present the results of solubility measurements of AOT in pure ethane and propane and of conductance and density measurements of supercritical fluid reverse micelle solutions. The effect of temperature and pressure on phase behavior of ternary mixtures consisting of AOT/water/supercritical ethane or propane are also examined. We report that the phase behavior of these systems is dependent on fluid pressure in contrast to liquid systems where similar changes in pressure have little or no effect. We have focused our attention on the reverse micelle region where mixtures containing 80 to 100% by weight alkane were examined. The new evidence supports and extends our initial findings related to reverse micelle structures in supercritical fluids. We report properties of these systems which may be important in the field of enhanced oil recovery. 

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