Supercritical fluids , catalysis

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

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

T. Hartmann, E. Schwabe, T. Scheper, Enzyme catalysis in supercritical fluids in R. Patel, Stereoselective Biocatalysis, Marcel Dekker, 2000, 799. 

The development of phase transfer catalysis, of supercritical fluids, of ionic liquids and of course, new reagents, should also have considerable potential in the labeling area. Furthermore there is the possibility of combining these approaches with energy-enhanced conditions - in this way marked improvements can be expected. 

The lesson learned from the catalysis in interphase approach is that the spacer between the insoluble support and the actual catalyst should be sufficiently long and soluble in the solvent of interest to obtain active catalysts. The use of supercritical fluids can also be very beneficial for the activity. Upon using Xantphos immobilised on silica in scC02 for example, the rates are only half of those of the homogeneous catalyst. Expressed as space-time yields the solid catalysts are almost an order of... 

The iridium catalyst was found to be sufficiently soluble for catalysis when in the form of the substrate complex, but precipitated quantitatively once all substrate had been consumed. Supercritical fluid extraction at that stage yielded the solvent- and metal-free product in crystalline form leaving the active and selective catalyst behind for... 

Homogenous catalysis in supercritical fluids has recently been reviewed by Noyori and coworkers [250]. 

P. G. Jessop, T. Ikariya, R. Noyori, Homogeneous Catalysis in Supercritical Fluids , Science 1995,269,1065-1069. 

A. Baiker, Supercritical Fluids in Heterogeneous Catalysis , Chem Rev. 1999, 99, 453-473, and references cited therein. 

The further optimization and development concerning stability and selectivity of the organometallic catalyst in these kinds of media and the application of isolation methodologies similar to CESS (catalysis and extraction using supercritical solutions [43]) together with the physical and chemical advantages of supercritical fluids can lead to high potential catalyst matrices that fulfil the requirements of industrial processes both for bulk and fine chemicals. 

Leitner W (1999) In Knochel P (ed) Reactions in Supercritical Carbon Dioxide (scC02) in Modern Solvent Systems. Top Curr Chem 206 107 Morita DK, David SK (1998) Chem Commun, p 1397 Wegner A, Leitner W (1999) Chem Commun, p 1583 Sellin M, Cole-Hamilton DJ (2000) J Chem Soc, Dalton Trans, p 1681 Solinas M, Pfaltz A, Leitner W (2004) J Am Chem Soc 126 16124 Leitner W, Scurto AM (1998) Imobilization of Organometallic Catalysts using Supercritical Fluids. In Cornils B, Herrmann WA (eds) Aqueous Organometallic Catalysis. WUey, Weinheim, p 664... 

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. 

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. 

In the following, we will present a selected choice of high-pressure cells developed for high-resolution NMR. We have divided the applications of high-pressure NMR into three sections. First, we will describe high pressure equipment used to study liquids under hydrostatic pressure up to 1000 MPa then, we present some special approaches used to study supercritical fluids which, besides moderate pressure, often need high temperatures. Finally, we discuss NMR cells to study solutions under gas pressure, a situation which is quite common in catalysis. 

High-pressure NMR studies for catalysis and with supercritical fluids will lead to a much broader application of sapphire NMR cells and to special applications of toroidal probes. The sapphire tube technique can today be considered as a standard, cheap and easily applicable technique to study samples under medium gas pressures, up to 100 MPa. 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

Yonker, C. R. and Linehan, J. C., A high-pressure NMR investigation of reaction chemistries in a simple salt hydrate,. Supercrit. Fluids, 29, 257 2004. Mehnert, C. R, Supported ionic liquid catalysis, Chem. Eur. ]., 11,50,2005. Giernoth, R. and Bankmann, D., Transition-metal free synthesis of perdeuter-ated imidazolium ionic liquidsby alkylation and H/D exchange, Eur. J. Org. Chem., 2008 (in print). DOT 10.1002/ejoc.200700784. 

THF) under otherwise identical conditions. Supercritical fluids therefore represent a promising medium for homogeneous catalysis (Johnston, 1994). 

Among potentially interesting solvents for enzymatic catalysis, carbon dioxide is the most widely nsed snpercritical fluid. However, there is a growing interest in using other supercritical fluids (e.g., ethylene, fluoroform, ethane, sulfur hexafluoride, and near-critical propane) (Kamat et al., 1995b). 

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