Catalytic multi-phase systems

The compatibility of some ILs with water has allowed the development of a highly efficient multi-phase system for the Rh-catalyzed hydrogenation of enamides (turnover numbers of >10,000). In many cases, such IL/water combinations are superior to conventional organic solvents and biphasic ILs/organic co-solvents media with respect to catalytic performance as well as to catalyst separation and recycling. The best results were obtained with Rh-ferrocenyl-diphosphine catalysts (>99% ee) in [G8GiIm]BF4/water. ... 

To better understand the role played by a catalytic membrane reactor in a multi-phase system, it can also be fruitful to start from the concept of the... 

Table 4.5 lists some references on the application of catalytic membrane reactors to liquid-liquid multi-phase systems. 

The complexity of many heterogeneous systems used in multi-phase reactions, the use of a solid support, the difficulty in analyzing highly dispersed active sites and the bifunctional nature of many solid supported metal catalysts, make a detailed and complete study challenging. The simpler homogeneous systems teach many of the principles of catalysis active sites, reaction mechanisms, reaction kinetics and catalytic cycles, which can often be applied elsewhere. 

Abreu, F.R. M.B. Alves C.C.S. Macedo L.F. Zara P.A.Z. Suarez. New multi-phase catalytic systems based on tin compounds active for vegetable oil transesterification reaction. J. Mol. Catal. A Chemical 2005,227, 263—267. 

Multi-phase catalysis performed in ILs can lead to various phase systems where the catalyst should reside in the IL. Prior to the reaction, and in cases where there are no gaseous reactants, two systems can usually be formed a monophase, that is, the substrates are soluble in the IL and biphasic systems where one or all the substrates reside preferentially in an organic phase. If a gas reactant is involved, biphasic and triphasic systems can be formed. At the end of the reaction, three systems can be formed a monophasic system a biphasic system where the residual substrates are soluble in the ionic catalytic solution and the products reside preferentially in the organic phase and triphasic systems, formed, for example, by ionic catalytic solutions, with an organic phase containing the desired product and a third phase containing the byproducts. In most cases, catalysis performed in ILs involves two-phase systems (before and after catalysis). 

Problem of creation of multi-phase reaction systems with developed surface of phase contact is especially actual under polymer synthesis. In particular at the stages of reaction mixture formation under emulsion [1, 80] and suspension [142] copolymerization, halogenation of elastomers [55, 143], decomposition and removal of electrophilic catalysts and Ziegler-Natta catalytic systems out of polymer [1], saturation of solvent by monomers [78, 79], formation of heterogeneous and micro-heterogeneous Ziegler-Natta catalytic systems [144] and so on. 

A more sophisticated (3-cyclodextrin-based catalytic system combining different functions in the same molecule was also conceived. Rhodium complexes of multi-component ligands featuring a chelating diphosphine covalently linked to the upper riin of P-cyclodextrin (Fig. 6) were used as catalysts for the hydrogenation and the hydroformylation of alkenes in water-organic two-phase systems. 

Heterogeneous catalysis are bi- or multi-phased they have dominated the industrial sector even though the fundamental principles involved are largely unknown. Advancements in analytical instrumentation, however, are allowing increased understanding of the catalytic phenomena in these systems. An important aspect of heterogeneous catalysis is the synthesis of active sites via attachment of metal complexes with a given chemical composition to the support surfaces (4). 

In a multi-phase catalytic reactor, the reacting species are dissolved in two different fluid phases (e.g., in a gas-liquid system or in liquid-liquid system) which are separated by a phase interface, and the catalyst is located in one of the fluid phases or in a dissolved form (as for example an homogeneous catalyst) or as a third heterogeneous phase (e.g., a solid phase). When a gas-liquid system is in the presence of a solid phase catalyst, the reactor is referred to as a three-phase reactor. Multi-phase reactors represent one of the most important classes of chemical reactors and they are widely used in many industrial sectors, as for example chemical, petrochemical, biotechnological, pharmaceutical and food processing industries (Barnett, 2006 Biardi and Baldi, 1999 Henkel, 2000 Nauman, 2008). Multi-phase reactors have typical industrial application in ... 

In a multi-phase catalytic reactor, the catalyst is usually a solid phase in contact with the liquid phase. Figure 4.1 shows a typical multi-phase catalytic system, where one fluid phase (gas or liquid) is dispersed in a liquid phase which contains porous catalyst particles. The reactants need to diffuse from their respective phases to the catalytic site where reaction products are formed and then they can diffuse back to one or both fluid phases. The overall reaction rate of the process will be affected by the inter-phase mass transfer rates near the gas-hquid and the liquid-solid interfaces, as well as by the intra-phase mass transfer rate competing with the intrinsic reaction rate inside the catalyst structure. 

The aim of the chapter is to summarize the general features of catalytic membrane reactors apphed to gas-liquid and hquid-liquid systems in order to show the capabilities, advantages and hmitations of this emerging class of multi-phase reactors. 

Typical oxidation reactions investigated by using a multi-phase catalytic membrane reactor are reported in Table 4.4 (gas-liquid systems) and in Table 4.5 (liquid-liquid systems). 

Returning to the general liquid phase catalytic system, assume that you have chosen an appropriate spectroscopy to investigate the system under reaction conditions. The spectroscopy provides spectra, i. e. absorbance A(t), at specific intervals in time. If S denotes the complete set of all species that exist at any time in the physical system, then Sjo s is the subset of all observable species obtained using the in situ spectroscopy. This requires that the pure component spectra aj..as obs are obtainable from the multi-component solution spectra A t) without separation of constituents, and without recourse to spectral libraries or any other type of a priori information. Once reliable spectroscopic information concerning the species present under reaction are available, down to very low concentrations, further issues such as the concentrations of species present, the reactions present, and reaction kinetics can be addressed. In other words, more detailed aspects of mechanistic enquiry can be posed. 

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