Reactions in Supercritical Fluids

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

T. Ikariya, R. Noyori, Organic Reactions in Supercritical Fluids , In Transition Metal Catalyzed Reactions, (Eds. S.-... 

T. Clifford, K. Bartle, Chemical Reactions in Supercritical Fluids , Chem. Ind. 1996, 449-452. 

Hyde, J.R. and Licence, P. and Carter, D. and Poliakoff, M. (2001). Continuous catalytic reactions in supercritical fluids. Applied Catalysis A General. 222. 119-131. 

A special area of HP NMR in catalysis involves supercritical fluids, which have drawn substantial attention in both industrial applications and basic research [249, 254, 255]. Reactions in supercritical fluids involve only one phase, thereby circumventing the usual liquid/gas mixing problems that can occur in conventional solvents. Further advantages of these media concern their higher diffusivities and lower viscosities [219]. The most commonly used supercritical phase for metal-catalyzed processes is supercritical CO2 (SCCO2), due to its favorable properties [256-260], i. e., nontoxicity, availability, cost, environmental benefits, low critical temperature and moderate critical pressure, as well as facile separation of reactants, catalysts and products after the reaction. 

Nakamura, K. (1990) Biochentical reactions in supercritical fluids. Trends Biotechnol.,... 

Brennecke and Chateauneuf drew several conclusions regarding reactions in supercritical fluids (Brennecke and Chateauneuf, 1999) ... 

Transition State Theory for Reactions in Supercritical Fluids... 

Wu, B.C. Klein, M.T. Sandler, S.I. The Influence of Diffusion on Reactions in Supercritical Fluid Solvents, Paper presented at AIChE National Meeting, Orlando, Fla. March 18-21,1990. 

While studies of reactions in supercritical fluids abound, only a few researchers have addressed the fundamental molecular effects that the supercritical fluid solvent has on the reactants and products that can enhance or depress reaction rates. A few measurements of reaction rate constants as a function of pressure do exist. For instance, Paulaitis and Alexander (1987) studied the Diels Alder cycloaddition reaction between maleic anhydride and isoprene in SCF CO2. They observed bimolecular rate constants that increased with increasing pressure above the critical point and finally at high pressures approached the rates observed in high pressure liquid solutions. Johnston and Haynes (1987) found the same trends in the... 

Figure 8. Schematic representation of our technique for recovering solid products from reactions in supercritical fluids (a) shows the supercritical reactor connected to a computer controlled syringe pump (Lee Scientific Model 501) filled with scC02. (b) shows how the scC02 is used to drive the supercritical reaction mixture (colored) through an expansion valve (Jasco 880/81 Back-pressure Regulator) where the products dissolved in the fluid are precipitated.

Ganapathy, S. Carlier, C. Randolph, T. W. O Brien, J. A. Influence of Local Structural Correlations on Free-Radical Reactions in Supercritical Fluids A Hierarchical Approach, Ind. Eng. Chem. Res. 1996, 35, 19-27. 

Roberts, C. B. Zhang, J. Brennecke, J. F. Chateauneuf, J. E. Laser Flash Photolysis Investigations of Diffusion-Controlled Reactions in Supercritical Fluids. J. Phys. Chem. 1993a, 97, 5618-5623. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

Figure 14.2. Laboratory-scale apparatus for catalytic reactions in supercritical fluids in combination with countercurrent extraction.

Many enzymes are stable and catalyze reactions in supercritical fluids, just as they do in other non- or microaqueous environments (7). Enzyme stability and activity may depend on the enzyme species, supercritical fluid, water content of the enzyme/support/reaction mixture, decompression rates, exposure times, and pressure and temperature of the reaction system. 

Grunwaldt J-D, Baiker A. Time-resolved and operando XAS studies on heterogeneous catalysts - from the gas phase towards reactions in supercritical fluids. In Hedman B, Oia-netta P, editors. AIP Conference Proceedings X-ray Absorption Fine Structure -XAFS13 2007 Vol. CP 882, p. 577-81. 

Supercritical fluids, particularly supercritical C02, scC02, are attractive solvents for cleaner chemical synthesis. However, optimisation of chemical reactions in supercritical fluids is more complicated than in conventional solvents because the high compressibility of the fluids means that solvent density is an additional degree of freedom in the optimisation process. Our overall aim is to combine spectroscopy with chemistry so that processes as varied as analytical separations and chemical reactions can be monitored and optimised in real time. The approach is illustrated by a brief discussion of three examples (i) polymerisation in scC02 (ii) hydrogen and hydrogenation and (iii) miniature flow reactors for synthetic chemistry. 

Jacobs G. P. et al., Utilization of Phoenics in the design of the MODAR SCWO reactor, Presentation in session " Reactions in supercritical fluids", 1992 Annual AICHE Meeting, Miami Beach, FL, nov. 1992... 

In addition to chemicals, biological catalysts such as enzymes can be used to catalyze reactions in SC CO2. Since the first attempt to operate reactions in supercritical fluids published by Randolph et al. [34], various type of enzymes were studied lipase, oxidase, decarboxylase, dehydrogenase, proteinase, etc. [33,35-37]. The effect of different parameters was extensively reported by Ballesteros et al. [35]. Enzyme activity and stability in supercritical conditions as well as the benefits of using supercritical fluids for enzymatic reactions (improved reaction rates, control of selectivity, etc.) have been demonstrated [36]. 

Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

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