Supercritical fluid catalytic process

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. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

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. 

The use of ecologically harmless SCCO2 as solvent and substrate in chemical reactions is a particularly intriguing prospect. Increased governmental and environmental restrictions on solvent emission make this supercritical fluid more and more attractive as a reaction medium because it can be easily separated from the product and recycled more efficiently than conventional liquid solvents. The special properties (miscibility, transport properties, etc.) of sc CO2 require a development of suitably adjusted catalysts. A simple transformation of catalyst properties from conventional solvents to SCCO2 will mostly fail, and will not lead to higher catalytic efficiency. Supported catalysts could perhaps play a particular role in this field as the possibility of product extraction by depressurization of the supercritical phase and subsequent compression of the CO2 (solvent/substrate) should permit the development of a profitable continuous process. 

There are still many developments in selective hydrogenation, both in terms of new catalysts and process operations. An example of the first is the discovery that Sn-substituted zeolite beta is the most active heterogeneous catalyst for the Meer-wein-Pondorff-Verley reduction of aldehydes and ketones to the corresponding alcohols, with high cis-selectivity (99-100%) in the reduction of 4-alkylcyclohexa-nones [301]. An example of process development is in the heterogeneous catalytic hydrogenation of organic compounds in supercritical fluids (SCFs) [302]. 

Within the last 30 years, micro emulsions have also become increasingly significant in industry. Besides their application in the enhanced oil recovery (see Section 10.2 in Chapter 10), they are used in cosmetics and pharmaceuticals (see Chapter 8), washing processes (see Section 10.3 in Chapter 10), chemical reactions (nano-particle synthesis (see Chapter 6)), polymerisations (see Chapter 7) and catalytic reactions (see Chapter 5). In practical applications, micro emulsions are usually multicomponent mixtures for which formulation rules had to be found (see Chapter 3). Salt solutions and other polar solvents or monomers can be used as hydrophilic component. The hydrophobic component, usually referred to as oil, may be an alkane, a triglyceride, a supercritical fluid, a monomer or a mixture thereof. Industrially used amphiphiles include soaps as well as medium-chained alcohols and amphiphilic polymers, respectively, which serve as co-surfactant. 

MG Hitzler, FR Smail, SK Ross, M Poliakoff. Selective catalytic hydrogenation of organic compounds in supercritical fluids as a continuous process. Org Process Res Dev 2 137-146, 1998. 

In the following sections some aspects of (potential) applications of sc-fluids in the fine chemical industry with respect to product separation/purification and catalytic reactions are discussed. Earlier industrial applications of supercritical fluid reactions, for example the Haber-Bosch process for the synthesis of ammonia, synthesis of methanol from hydrogen and carbon monoxide, or the polymerization of ethene will not be discussed. An extensive overview on the use of sc-fluids in the synthesis of bulk chemicals is given in the book edited by fessop and Leitner [12],... 

Supercritical fluids (SCFs) have proved to be versatile media for a wide range of chemical processes [1] from stereoselective organic chemistry [2] through catalytic hydrogenation [3], polymer synthesis [4] and polymer modification [5] to the preparation of novel inorganic materials [6] and organometallic complexes [7]. IR and Raman spectroscopy have played a significant role [8] in many of these developments. 

Catalytic oxidation in a two-phase water-C02 medium has been investigated for the synthesis of adipic acid from cyclohexene [2b]. The substrate and products were dissolved in the supercritical fluid while the oxidant (e.g. NaI04) resided in the aqueous phase. The catalyst RUO2 was oxidized to RUO4 in the aqueous phase which in turn oxidized the substrate, presumably at the liquid-supercritical interface. This is an example of a Type Ilia process. Unfortunately, catalyst efficiency was fairly low (five catalytic cycles), probably due to deactivation by formation of carbonates in the aqueous phase. 

Different uses of supercritical fluid (SCF) solvents in chemical separation processes have been of considerable research interest since the 1970s. The basic principles of SCF extraction engineering and a number of applications for this technology are described in several review papers [1,2]. As a new field related to SCF technology, the application of supercritical solvents as reaction media attracts increasing attention, especially for catalytic reactions. In such processes, the SCF may either actively participate in the reaction or function solely as the solvent for the reactants, catalysts, and products. 

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