Hydrogenation in supercritical fluids

Asymmetric hydrogenations have also been carried out in scCOi. a-Enamides have been hydrogenated by using a cationic rhodium complex [19]. Enantiomeric 

Processes in which catalysts immobilized in ionic liquids have been combined with product extraction using SCCO2 show considerable promise [20], The ionic liquids are poorly soluble in the SCCO2 which allows products to be extracted without contamination from the ionic liquid or the catalyst. 

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Enantioselective hydrogenation in supercritical fluids. Limitations of the use of supercritical C02... 

Ramirez, E. Zgarni, S. Larrayoz, M. A. Recasens, F. Short compilation of published reaction rate data for catalytic hydrogenations in supercritical fluids. Eng. Life Sci. 2002, 2 (9), 257-264. 

Minder B, Mallat T, Baiker A. Enantioselective hydrogenation in supercritical fluids. Limitations of the use of supercritical CO2. In von Rohr PR, Trepp C, eds. High Pressure Chemical Engineering Proceedings of the 3rd International Symposium on High Pressure Chemical Engineering, Zurich, Switzerland, 7-9 October, 1996. Amsterdam Elsevier, 1996 139-144. 

Hydrogenations in supercritical fluids are close to industrial-scale application. In the process developed by Harrod and colleagues [15], the hydrogenation of fatty acid methyl esters is carried out in propane, because the solubilities of the esters in supercritical CO2 are too low. In propane, the reaction can be carried out with only one supercritical phase contacting the solid catalyst, and with enough hydrogen to ensure an extremely fast and selective reaction. 

T Tacke, C Rehren, S Wieland, P Panster, SK Ross, J Toler, MG Hitzler, F Small, M Poliakoff. Continuous hydrogenation in supercritical fluids. In FE Herkes, ed. Catalysis of Organic Reactions. New York Marcel Dekker, 1998, pp 345-356. 

P. G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, Homogeneous Catalysis in Supercritical Fluids Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides ,J. Am Chem Soc 1996,118, 344-355. 

There are an increasing number of applications of high pressure NMR in supercritical fluids to homogeneous catalysis [266]. Using their toroidal pressure probe, Rathke and coworkers [249, 267-269] have extensively studied the Co2(CO)g-cata-lyzed hydroformylation of olefins in scCOj (Eq. (14)). The hydrogenation of Co2(CO)g (Eq. (15)) is a key step in this reaction. 

Various equations of state have been developed to treat association in supercritical fluids. Two of the most often used are the statistical association fluid theory (SAFT) (60,61) and the lattice fluid hydrogen bonding model (LFHB) (62). These models include parameters that describe the enthalpy and entropy of association. The most detailed description of association in supercritical water has been obtained using molecular dynamics and Monte Carlo computer simulations (63), but this requires much larger amounts of computer time (64—66). 

Hitzler, M. G., Smail, F. R., Ross, S. K., Poliakoff, M. Selective Catalytic Hydrogenation of Organic Compounds in Supercritical Fluids as a Continuous Process. Organic Process Research Development. 1998, 2, 137 - 146. 

Minder, B., Mallat, T., Pickel, K.H., Steiner, K., Baiker, A. Enantioselective Hydrogenation of Pyruvate in Supercritical Fluids. Catalysis Letters. 1995, 34, 1-9. 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

Selective and Complete Hydrogenation of Vegetable Oils and Free Fatty Acids in Supercritical Fluids... 

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 ... 

Gupta RB, Panayiotou CG, Sanchez IC, Johnston KP. Theory of hydrogen bonding in supercritical fluids. AIChE J 1992 38 1243-1253. 

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. 

We have recently reviewed the use of vibrational spectroscopy in supercritical fluids [2] and the theme common to most of our projects is the use of spectroscopy for real-time optimisation of processes in supercritical solution. Such optimisation is considerably more important in supercritical fluids than in conventional solvents because the tunability of the fluids results in a greater number of parameters which can affect the outcome of a reaction. Thus, the chances of hitting the optimal conditions purely by trial and error are much less in supercritical solution than in conventional reactions. Below, we give three examples of our approach, synthesis of polymers, transition metal hydrogen compounds, and the use of flow-reactors. 

Tacke, T. Wieland, S. Panster, P. Selective and complete hydrogenation of vegetable oils and free fatty acids in supercritical fluids. In Green Chemistry Using Liquid and Supercritical Carbon Dioxide, Desimone, J.M., William, T., Eds. Oxford University Press Inc. New York, 2003 228-240. 

Jessop, P.G. Hsiao, Y. Ikariya, T. Noyori, R. Homogeneous catalysis in supercritical fluids hydrogenation of supercritical carbon dioxide to formic acid, alkyl formates, and formamides. J. Am. Chem. Soc. 1996, 118 (2), 344-55. 

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