Homogeneous SCFs reactions

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. 

Srinivas and Mukhopadhyay (246) have investigated the selective thermal oxidation of cyclohexane in SCCO2 to produce cyclohexanone and cyclohexanol as the primary reaction products. Kinetic experiments were conducted at three temperatures (137°C, 150°C, and 160°C) and two pressures (170 and 205 bar), and the presence of a homogeneous SCF was verified experimentally for the initial reactor composition under these conditions (10 mol % cyclohexane, 10% O2, and 80% CO2). Kinetic results were interpreted assuming a free-radical reaction mechanism comparable to that observed in the conventional liquid-phase process, and the reaction was observed to be autocatalytic. Reported conversions are low relative to those in the liquid-phase process, which the authors attribute to... 

A process is described for conducting hydrogenation reactions in a homogeneous SCF phase including hydrogenation of C=C bonds in lipids, of COOR to produce fatty alcohols, and of oxygen to hydrogen peroxide. 

Performance of synthetic reactions is highly sensitive to reaction media However, the general importance of SCFs in homogeneous catalysis was... 

There are two principal reactor types that have been used for reactions in SCF, as seen in Figure 3.8. Batch reactors can be readily equipped with a suitable window to assess homogeneity of the reaction mixture and are widely used in academic research. 

In recent years there has been much interest in the use of supercritical fluids (SCFs) as replacements for conventional liquid solvents, particularly in separation science, but also as reaction media. In addition to their environmental benefits, SCFs have further advantages over conventional liquid solvents, and these are briefly outlined in Section 2. The remainder of the chapter describes the use of SCFs as a medium for NMR spectroscopic studies. First we look briefly at motives for such NMR studies and the techniques employed. We then examine in more detail chemical shifts and nuclear spin relaxation in SCFs. The lower relaxation rates associated with SCFs and consequent sharper lines obtained for quadrupolar nuclei make SCFs excellent solvents. Section 8 describes some NMR studies of organometallic reactions in SCFs. Here the miscibility of supercritical solvents with gaseous reagents proves to be a tremendously useful feature in, for example, homogeneous catalysis. Finally we comment on future possibilities for NMR studies in SCFs. 

A further useful feature of SCFs is their total miscibility with gases. This circumvents the problem of low gas solubility in organic solvents for reactions such as hydrogenation, carbonylation or hydroformylation, and obviates the need for stirring a single homogeneous phase is always present. 

Supercriticial fluids look set to continue to their invasion of the territory of traditional organic solvents in all areas of chemistry. Perhaps the most interesting area for exploration for the organometallic NMR spectroscopist is the use of SCFs for homogeneous catalysis. The presence of a single phase simplifies operation and may lead to enhanced reaction rates, while low viscosity aids observation of quadrupolar nuclei, which includes many of the transition metals. NMR studies in situ could allow observation of reaction intermediates and new insights into the mechanisms of these important reactions. 

In general, the potential benefits of using SCCO2 or other SCFs in homogeneous catalysis can be exploited best under single-phase conditions. For many of the examples discussed in the previous sections, control experiments demonstrate that the presence of a single homogeneous reaction phase is a necessary prerequisite... 

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