Reactions in SCFs

Poliakoff and co-workers have been very active in the field of organometallic chemistry in SCFs. Much of the chemistry has involved photochemical synthesis and IR detection, where the transparency of SC Xe makes it an ideal solvent [39,55]. The extremely high concentrations of gaseous reagents possible 

In collaboration with PoliakofiTs group we repeated their original synthesis 

Supercritical fluids have been studied for over a century, but it is only in recent years that their employment as a replacement for conventional solvents has 

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. 

For an excellent discussion on properties and uses of supercritical fluids see M. A. McHugh and V. J. Krukonis, Supercritical Fluid Extraction Butterworth, Stoneham, MA, 1986. 

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Note that, compared to conventional liquid solvents, SCFs are not always a panacea. They have both merits and disadvantages. Many chemical reactions are better performed in ordinary fluid solutions. However, chemistry of the reaction in SCFs still is a young and fully unexplored scientific field. We need deeper understanding of the microscopic and macroscopic properties of SCFs. The industrial... 

In particular, catalytic and analogous reactions in SCFs will become increasingly attractive because of the environmental legislation. To implement future processes based on the use of SCCO2 as medium for catalysis, representative data need to be provided, which describe the states present under reaction and separation conditions. Such data are crucial to prevent losses of catalysts and/or expensive ligands and to achieve the desired product purities [24]. 

The wide diversity and the large number of apparatuses used for investigations and processing of reactions in SCFs make a systematization difficult. They have been developed during the time from the first digester by Denys Papin in 1680 to Ipatiev s high-pressure bomb [198] and up to the first... 

Besides being of considerable commercial interest, Diels-Alder reactions are clean, well-characterized reactions that generally proceed in a single step through a pseudoaromatic transition state. There have been studies on the pressure effect on ionic reactions in SCFs by Zhang et al. (1996) who measured the rates of aryhnethyl cation ion-neutral reactivity in SCF. 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

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. 

Heterogeneously catalyzed reactions in SCFs must, by definition, involved at least two separate phases, the reaction mixture and the solid catalyst, and frequently may contain more. There has been considerable argument as to whether a singlephase reaction mixture is needed to exploit the advantages of a supercritical fluid, but these arguments really lie out of the scope of this chapter [25]. Suffice it to say... 

There are many more reports and patents of reactions in SCFs than the selected examples mentioned here, especially in the period 1910-1945. Since 1945, the literature has become so extensive that it is not possible to review... 

Supercritical or.near-critical water has found technical applications for hydrothermal syntheses, as discussed in detail in Chapter 4.1. Another more recent industrid application of chemical reactions in SCFs is the oxidative destruction of chemical wastes in SCH2O (SCWO, supercritical water oxidation). A detailed coverage of the large and prolific field of SCWO is outside the scope of this book on chemical synthesis. The extensive pilot plant activity, primarily by MODAR (now General Atomics) and Eco Waste Technologies, has recently been sununarized by Schmieder [171]. The first commercial plant was opened by Huntsman Chemical in collaboration with Eco Waste. 

There are already several excellent reviews and multiauthored books that describe various designs for high pressure spectroscopic vessels [10,12-16]. This chapter demonstrates the use of vibrational spectroscopy for in situ monitoring of chemical reactions in SCFs. 

Figure 3.1-1 Cross-sectional view of a microscale spectroscopic vessel for monitoring reactions in SCFs C = CaF2 window P = port S = stainless steel window holder and T = Teflon O-ring (PTFE).

Monitoring of Fast Reactions in SCFs using Time-resolved Vibrational Spectroscopy... 

The most widely used vibrational spectroscopic technique is time-resolved resonance Raman spectroscopy (TR ) [65]. This has been used successfully to obtain structural information about organic excited states in SCCO2. McGar-vey and co-workers probed the excited triplet state of anthracene in SCCO2 [66]. However, TR experiments involve data collection over many laser pulses, with all of the problems associated with secondary photolysis. These problems have prevented TR being used effectively to follow chemical reactions apart from highly photoreversible processes. To our knowledge, TR has not yet been used to follow chemical reactions in SCFs. Recently, however. 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

Figure 3.1-10 illustrates how step-scan FTIR can be used to monitor more complicated photochemical reactions in SCFs. Visible photolysis of trans-[CpMo(CO)3l2 in scCOa generates [CpMo(CO)3] radicals which recombine at a diffusion controlled rate to form the stable trans and unstable gauche forms of [CpMo(CO)3]2. Gauche-[CvMo(CO)-i 2 slowly isomerizes to trans-[CpMo(CO)3l2 (Scheme 3.1-2). 

In general, the large variety of high pressure NMR equipment ensures that NMR spectroscopy will continue to help explore the principles of chemical reactions in SCFs. 

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