Supported liquid-phase catalyst system

Supported Liquid-phase Hydroformylation. - A potentially attractive alternative to chemically anchored hydroformylation catalysts is the use of supported liquid-phase catalyst (SLPC) systems for gas-phase hydroformyl-ations. The homogeneous catalyst is dissolved in a non-volatile solvent and then condensed in the pores of a support, where the strong negative capillary forces effectively immobilize the catalyst, thereby preventing metal loss. In addition one might expect that the environment of the homogeneous system... 

The use of supported liquid-phase catalysts (SLPCs) in organic reactions was mentioned in Chapter 6. The diffusion-reaction problem in such catalysts is considerably more complicated than that for solid catalysts (Rony, 1969 Abed and Rinker, 1973 Livbjerget al., 1974, 1976). In a more recent analysis of SLPC systems, it has been shown (Datta and Rinker, 1985 Datta et al., 1985) that a critical parameter is the ratio of the effective diffusivity of a liquid-loaded pellet to that of a dry pellet ( JcA.l/ ca)- This ratio is a strong function of the liquid loading q and the gas-liquid partition coefficient of reactant A, as shown in Figure 7.8. Thus the effectiveness factor is a function of liquid loading. 

For instance substitution chlorination of organic compounds produces hydrogen chloride which must simultaneously desorb back in the gas phase to prevent supersaturation of the liquid phase. Another industrially important process involves "supported liquid phase catalyst", where the reactants have to be transferred from a bulk gas to a liquid reaction phase while the products are released back into the gas phase. Here the catalyst is in the form of a melt on a solid support and it finds applications in alkylation, carbonylation, hydroformylation and oxidation of inorganic and organic compounds. The subject matter was recently reviewed delicately by Villadsen and Liv-berg (11,12). Other examples of these interesting systems are shown in Table 6. 

In the interesting area of simultaneous absorption and reaction in reacting systems the possibility of supersaturation in the intermediate vicinity of the interface was pointed out and properly analyzed. An industrially fully exploited case of simultaneous absorption and desorption into molten catalysts, i.e. a supported liquid phase catalyst, was discussed in another Advanced Study Institute with a heavy emphasis upon a further development of the theoretical basis (12). 

Supported liquid-phase catalysts More detailed consideration of supported and recyclable systems is beyond the scope of the present review and is well covered in a number of recent reviews [151, 174-178]. 

This review is a survey of the applications and properties of supported liquid phase catalysts (SLP). By a supported liquid phase catalyst is meant the distribution of a catalytically active liquid on an inert porous support and the behaviour of such systems raises many interesting questions on catalyst chemistry, mass transfer in catalysts and reactor design. It is noteworthy thou that such systems have been employed in the chemical industry for many decades - indeed for over a century in the Deacon process for obtaining chlorine from hydrogen chloride - and of almost equally respectable antiquity are the vanadium based catalyst systems used for sulfuric acid manufacture but the recognition of SLP catalysts as possessing features of their own is much more recent. 

Before the 1990s there was little in the literature on multiphasic L-L-S and L-L-L-S systems used for chemical reactions. There is, however, a relatively large volume of work done on other types of multiphasic systems related to the present topic supported liquid-phase catalysis (SL-PC), and gas liquid phase transfer Catalysis (GL-PTC). The common denominator in both cases is the presence of an interfacial liquid layer of a hydrophilic compound between the catalyst and the bulk of the reaction. 

Another feature of fused catalytic compounds can be the generation of a melt during catalytic action. Such supported liquid phase (SLP) catalysts consist of an inert solid support on which a mixture of oxides is precipitated which transform into a homogeneous melt at reaction conditions. These systems provide, in contrast to the case described before, a chemically and structurally homogeneous reaction environment. The standard example for this type of catalyst is the vanadium oxide contact used for oxidation of SO2 to S03. 

The technical catalyst is a supported liquid phase system of vanadium pentoxide in potassium pyro-sulfate [16, 17], Other alkali ions influence the activity [18] at the low-temperature end of the operation range, with Cs exhibiting a particular beneficial effect [13]. 

Catalyst discovery research—metal oxides and supports, shape selective and hetero metal substituted molecular sieves, pillared clays, biomimetic, methan-otropic and other bio systems and combinatorial catalytic screening techniques, liquid phase homogeneous systems. 

A porous matrix is sandwiched between two membranes. The matrix supports a liquid-phase catalyst. For the reaction A -> B, membrane 1 passes A but resists B, and membrane 2 passes both freely the function of membrane 2 is to encapsulate the catalyst solution. Reactant A is fed external to membrane 1 the concentration of A drops across the catalyst as it is consumed by reaction, due to diffusional resistance. The product diffuses to the right, reactant A does not. The benefits of this reactor are the liquid phase is encapsulated, the catalyst is separated from the product stream, the product is separated from the reactant, it provides a higher gas-liquid interfacial area, and a product is removed from an equilibrium-limited reaction. The authors suggested that the system be implemented as a shell-and-tube configuration using two different hollow-fiber membranes. 

Silica gels and controlled-pore glass, which were covered with thin films of polar phases such as water, ethylene glycol or ionic liquids, were used as polar solid supports. These systems are limited to very polar, usually ionic catalysts and non-polar reaction media in order to prevent catalyst leaching. This in turn, can be limiting to the range of substrates. Existing catalytic processes in common liquid-liquid biphasic systems can be easily transferred to supported liquid-phase conditions. At the same time the interfacial area between the... 

One possible way to achieve a uniform surface is by coating the solid support material with a thin liquid film, thereby defining the material properties by the liquid s properties. Such supported liquid phase (SLP) materials date back a 100 years ago till 1914, when BASF introduced a silica-supported V205-alkah/pyrosulfate SO2 oxidation catalyst for sulfuric acid production (see Figure 1.1) [3]. This catalyst, which is stiU the standard system for sulfuric acid production today, can be described as a supported molten salt, as it consists of a mixture of vanadium alkali sulfate/hydrogensulfate/pyrosulfate complexes that are present under reaction conditions (400-600 °C) [4],... 

The concept of supported Hquid catalysis is not restricted to liquid salts. In order to apply the concept of uniform surface properties and efficient catalyst immobilization, several authors investigated the SLP concept during the 1970s and 1980s [5-11]. However, later studies revealed that the evaporation of the loaded liquid cannot be avoided completely during operation. This is especially a problem when using water as the Hquid phase [12-17]. In these supported aqueous phase (SAP) systems, the thin film of water evaporated quickly under reaction conditions, making the concept appHcable only for slurry-phase reactions with hydrophobic reaction mixtures. 

Supported aqueous phase catalysts are well known [29, 30]. In these systems, a thin film of water present on the surface of a polar solid support is used to immobilize metal complexes, which are nonvolatile or insoluble in a mobile gaseous or liquid organic phase, respectively [30]. The concept was used successfully, for example, for the hydroformylation of oleyl alcohol over a supported rhodium complex [29]. Here, it was suggested that the reaction occurred at the interface between the aqueous and organic phase. However, the volatility of water necessitated... 

Advantages of the vapor-phase carbonylation using active carbon support in comparison with the liquid homogeneous catalyst system are as follows. Corrosive attack to reactors and pipes of iodide-acetic acid solution can be avoided. No problem of limited supply, such as of rhodium, is associated with use of nickel. Separation of products from catalyst is facilitated by a gas-solid system. These catalysts can work at higher temperature. 

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