Supercritical conditions synthesis under

H. Jiang, J. Zhao, A. Wang, An efficient and eco-friendly process for the conversion of carbon dioxide into oxazolones and oxazolidinones under supercritical conditions. Synthesis (2008) 763-769. 

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. 

SCFs will find applications in high cost areas such as fine chemical production. Having said that, marketing can also be an issue. For example, whilst decaffeina-tion of coffee with dichloromethane is possible, the use of scCC>2 can be said to be natural Industrial applications of SCFs have been around for a long time. Decaffeination of coffee is perhaps the use that is best known [16], but of course the Born-Haber process for ammonia synthesis operates under supercritical conditions as does low density polyethylene (LDPE) synthesis which is carried out in supercritical ethene [17]. 

Another way to produce biphenyl derivates using flow was described by Leeke et al. [34] where they performed a Pd catalyzed Suzuki-Miyaura synthesis in the presence of a base. First experiments were carried out in toluene/methanol solvent. A reaction mixture was passed through the encapsulated Pd filled column bed length 14.5 cm (some cases 10 cm) x 25.4 mm id. 45 g of PdEnCat. Base concentration, temperature and flow rate were optimized and at optimum parameters (0.05 M base concentration, 100°C and 9.9 mL/min) the conversion was 74%. Then the reaction was performed under supercritical conditions using supercritical CO2 at high pressure and temperature. After optimizing the concentration of base, flow rate, pressure and temperature, the highest conversion rate (81%) was observed at 166 bar and 100°C where the reactant mixture was monophasic in the supercritical state. This system is able to produce 0.06 g/min of the desired product. 

Figure 9.2-3. Design of continuous experimental apparatus for synthesis under supercritical conditions S, substrates 1, molecular sieves 2, saturation column 3, packed-bed enzymatic reactor 4,5, separators P, high-pressure pump PI, pressure indicator TI, temperature indicator H, heat exchanger [17].

A mixed reaction medium, composed of scC02 and ILs, has been defined as a new biphasic system by Advanced Industrial Science and Technology (AIST), and used for selective and efficient CC synthesis. For example, 1-alkyl-3-methylimidazolium salts represent a suitable system when used under supercritical conditions for the synthesis of CCs [156] from epoxides and C02. Kanawami et al. [159] have reported that the use of 1-octyl-3-methylimidazolium tetrafluoroborate under supercritical conditions resulted in a 100% conversion into PC, with 100% selectivity, within only a few minutes (Equation 7.15). 

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... 

Figure 2. Design of continuously operating experimental apparatus for the synthesis of oleyl oleate under supercritical conditions 1-substrates, 2-saturation column, 3-enzymatic reactor, 4, 5-separators, P-high pressure pump, Pi-presssure indicator, T-temperature indicator,...

Solvothermal process is now becoming a powerful technique for preparing nanomaterials. It is analogous to hydrothermal synthesis, except that non-aqueous solvents replace water as reaction medium. From the chemical reaction point of view, solvents in supercritical conditions play a significant role in reaction and crystallization. New materials, especially those having metastable phases and special nanostructures, can be obtained under mild conditions. By sealing the reaction system in an autoclave, the reactants and products prevent effectively from oxidation, hydrolysis and volatilization, and the reaction and crystallization can be realized synchronously. 

It should be noted that on an industrial scale, reactions or other processes in SCF media are not new. Many industrial reactions developed in the early part of the twentieth century are actually conducted under supercritical conditions of either their product or reagent including ammonia synthesis (BASF, 1913), methanol synthesis (BASF, 1923) and ethylene polymerization (ICI, 1937). 

Scheme 6 2-pyrone-synthesis from 3-hexyne and CO2 under supercritical conditions. 

The use of supercritical conditions for the homogeneous hydrogenation of CO2 with following thermal esterification leads to high yields of methyl formate under mild conditions. Therefore, it might be possible to develop an industrial procedure for the synthesis of methyl formate with CO2 as Ci-building block when catalysts and reaction conditions are optimized. 

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. 

In addition to the synthesis of specific ligands and the resulting catalysts new ideas and concepts begin to combine with transition metal chemistry to open up new areas in homogeneous catalysis, like catalysis under supercritical conditions, colloidal catalysis , organometallic electrocatalysis and multi-metallic catalysis " ( cooperative catalysis ). 

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). 

Some other industrial processes are or have been carried out under supercritical conditions even though the fact may not have been generally recognized. For example, McKee and Parker pointed out in 1928 that some oil cracking processes performed industrially at the time occurred above the critical temperature of the reaction mixtures [151]. These processes have since been replaced by lower temperature catalytic cracking methods. Other historical industrial processes which probably involved supercritical phases include the synthesis of melamine [152] from dicyanodiamide, N2 and NH3, and the alkylation of aniline by methanol [152]. 

Scheme 4J-4 Synthesis of oxalate from CO2 and CO under supercritical conditions.

Scheme 4.7-1 Synthesis of formic acid and its derivatives by hydrogenation of CO2 under supercritical conditions. Species 1-4 are some ruthenium complexes that have been used as catalysts or catalyst precursors.

Hybrid catalysts derived from cocondensation of Group 8 metal-chloro complexes with Si(OEt)4 via a sol-gel process were highly active for the synthesis of A/,iV-dimethyIformamide from CO2, H2 and dimethylamine under supercritical conditions, affording turnover numbers up to 100800 at 100% selectivity [61]. The activity of the catalysts, containing methylphosphine ligands, decreased in the order Ru>Ir>Pt,Pd>Rh. It seemed that the high activity of silica matrix stabilized ruthenium complexes was due to the formation of an active hydride intermediate by hydrogenolysis of the Ru-Cl bond. 

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