Carbon under supercritical conditions

Kawanami H, Ikushima Y (2000) Chemical fixation of carbon dioxide to styrene carbonate under supercritical conditions with DMF in the absence of any additional catalysts. Chem Commun 21 2089-2090... 

At beginning, sihca-supported IL-phase catalyst, which was prepared by treating the silica gel in the acetone solution of IL [BMIm](BF4], was used in the synthesis of cyclic carbonate under supercritical conditions [101]. Epoxides with different structures could be efHciently transferred into cycHc carbonates with up 100% yields. The catalyst could be easily separated by filtration and reused for several runs without obvious deactivation. In order to simplify the separation of catalyst from the reaction mixture, sihca gel-modified magnetite was used as the support for covalent immobilization of IL, and similar results were obtained [102]. 

Carbon dioxide and water are the most commonly used SCFs because they are cheap, nontoxic, nonflammable and environmentally benign. Carbon dioxide has a more accessible critical point (Table 6.13) than water and therefore requires less complex technical apparatus. Water is also a suitable solvent at temperatures below its critical temperature (superheated water). Other fluids used frequently under supercritical conditions are propane, ethane and ethylene. 

J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai, and L.-N. He, Synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts under supercritical conditions, /. Mol. Cat. A Chem., 249 (2006) 143-148. 

Oxidation of unfunctionalized alkanes is notoriously difficult to perform selectively, because breaking of a C-H bond is required. Although oxidation is thermodynamically favourable, there are limited kinetic pathways for reaction to occur. For most alkanes, the hydrogens are not labile, and, as the carbon atom cannot expand its valence electron shell beyond eight electrons, there is no mechanism for electrophilic or nucleophilic substitution short of using extreme (superacid or superbase) conditions. Alkane oxidations are therefore normally radical processes, and thus difficult to control in terms of selectivity. Nonetheless, some oxidations of alkanes have been performed under supercritical conditions, although it is probable that these actually proceed via radical mechanisms. 

The subject of chemical reactions under supercritical conditions is well outside the scope of matters of major concern to combustion related considerations. However, a trend to increase the compression ratio of some turbojet engines has raised concerns that the fuel injection line to the combustion chamber could place the fuel in a supercritical state that is the pyrolysis of the fuel in the line could increase the possibility of carbon formations such as soot. The... 

Figure 6.4—Schematic of a supercriticalfluid chromatograph. Carbon dioxide reaches a supercritical state between the pump and the injector. A pressure regulator (restrictor) is located after the column and either before or after the detector, depending on its type. It allows the mobile phase to be kept under supercritical conditions until its exit from the column.

Recently, Ikarya has reported the use of imidazolin-2-ylidenes with N-alkyl and N-aryl substituents and their C02 adducts as catalyst of the carboxylative cydiza-tion of internal and terminal propargylic alcohols [215], The reaction of internal propargyl alcohols with C02 has been carried out also under supercritical conditions. Ikariya et al. have developed a synthetic process to afford Z-alkylidene cyclic carbonates promoted by P( -C4H9)3 with high efficiency [216],... 

An intriguing observation reported by these workers was that the reduction of 9-iodoanthracene produced anthracene (71%) and 9-anthracenecar-boxylic acid (10%), the latter of which may arise from addition of the 9-anthryl radical to one of the C=0 bonds of C02 (eq. 4.9). If this hypothesis is correct, this would be the first example of a carbon-centered radical adding to C02 under supercritical conditions. 

Cui H, Wang T, Wang F et al (2003) One-pot synthesis of dimethyl carbonate using ethylene oxide, methanol, and carbon dioxide under supercritical conditions. Ind Eng Chem Res... 

At the end of considerations dealing with oxirane/carbon dioxide copolymerisation, copolymerisation run under supercritical conditions in carbon dioxide as the reaction medium should be mentioned [239]. 

Ross et al. [10,88] conducted an extensive study on the conversion of several model compounds (e.g., parachlorophenol, dichlorobenzene, hexa-chlorobenzene, and tetrachlorobiphenyl) to simulate the waste streams containing PCBs, under supercritical conditions at 400°C and 3700 psi with sodium carbonate added as a promoter. In their study, no formation of dibenzofurans or dibenzo-p-dioxins was noted during the decomposition of the starting material, even at conversions as low as 50%. These results were confirmed by Mitsubishi Heavy Industries (MHI) in their laboratory-scale testing. 

Supercritical fluid extraction can be used to remove carbonaceous material from spent catalysts. De Filippi and Robey (2) used supercritical carbon dioxide extraction to regenerate adsorbents. Abel (3) tried supercritical carbon dioxide extraction to regenerate a certain catalyst. Tiltscher et al. (4,5) studied the isomerization of 1-hexene on an alumina catalyst and showed that under supercritical conditions, 1-hexene was able to remove the oligomeric compounds (C -C q) from t ie catalyst surface and prevent coking. 

Cyclic voltammetry in supercritical water-0.2 M NaHS04 [88] and ammonia-0.14 M CF3SO3K [88,332] of some organic compounds shows that this electroanalytical technique was applicable under these conditions. The behavior of phenazine in NH3 at —40°C and under supercritical conditions, for example, was analogous two reversible reductions were found in both cases [88]. Dimethyl carbonate has been prepared from CO and MeOH on anodic oxidation in a supercritical mixture of CO2 and MeOH [89]. 

Under supercritical conditions, a gas, such as carbon dioxide, possesses liquid-like density and solubility, and gas-like diffusivity and viscosity, along with zero surface tension. Thus, supercritical fluids work extremely well as a processing media for a wide variety of chemical, biological, and polymer extractions. This solvent power of supercritical fluid has been known since 1879. Nevertheless, its application was not considered until recently when the sharp increases in energy cost, environmental regulations, and performance demands on materials have caused industry to consider alternative... 

For example, sulfur-cured SBR samples that contained no filler, carbon black, silica, or a combination of carbon black and silica were heated with the 2-buta-nol under supercritical conditions. The SBR had an original weight average molecular weight of about 400,000. The weight average molecular weights of the devulcanized SBR samples recovered are reported in Table 2. 

Butane, pentane, and propane are also used as the reaction medium in polymer synthesis because carbon dioxide is not a strong solvent for most polymers. Furthermore, some polymerization reactions (such as polyethylene synthesis) are carried out under supercritical conditions of the monomer. 

Because RuCl2(PMe3)4 is soluble in supercritical carbon dioxide, it catalyzes the formation of formate or formamide from carbon dioxide, hydrogen, and alcohol or amine under supercritical conditions with a maximum 370 000 ton- These reactions may lead to a breakthrough in CO activation [140]. 

Furthermore, one of the advantages of carbon dioxide-based SFE is that normally extractions are carried out at low temperatures. The resolution of this dilemma can be as simple as adding a short sub-step in the beginning to remove the ethyl alcohol in the extraction thimble. Recall that the capsaicin extraction is carried out at 40 C and a density of 0.70 g/mL. The ethyl alcohol can be readily removed at a density of 0.25 g/mL at 40 C in a short three-minute dynamic extraction step. This is well below the "threshold-density" at which the capsaicin would extract or be soluble in supercritical carbon dioxide. The mechanism of using the carbon dioxide to remove ethyl alcohol from the extraction thimble under supercritical conditions is quite different than the evaporation mechanism that would be taking place in the solid trap after expansion of the carbon dioxide plus ethyl alcohol to a mixture of gas plus condensing liquid. Note that the fact that ethanol can be pre-extracted ahead of the analyte of interest demonstrates that the role of the modifier in this application is mainly matrix modification had a more polar solvent (like that afforded by a mixture of ethanol in carbon dioxide) been necessary to solvate the capsaicin, the pre-extraction step to remove the ethanol would have been detrimental to the extraction of the capsaicin. 

Oxazolidinones are useful heterocyclic compounds in organic synthesis. They have a wide range of applications in asymmetric syntheses as chiral reagents and, since they have good antibacterial properties, in medicinal chemistry [53]. Oxazolidinones can be synthesized in traditional solvents such as acetonitrile [54] or DMF [5 5], but it is more environmentally friendly to use scC02 [56]. In the reaction an internal propargyl alcohol, carbon dioxide, and a primary amine participate in a cycloaddition reaction under supercritical conditions to give 4-alkylene-l,3-oxazoli-din-2-ones (Equation 4.30). 

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