Reactive homogeneously catalyzed

The mechanism of a homogeneously catalyzed reaction, again in the sense of that part of the chemistry occurring in solution which converts substrate to product, involves a series of highly reactive metal complexes any one of which has as much right as any other to be called the catalyst, and it is therefore more correct to speak of a catalytic cycle or catalyst system than of a catalyst. Since the term catalyst will doubtless remain in use for homogeneous systems, the above should be borne in mind when dealing with the literature. 

The first single-site metal catalyst which was shown to homogeneously catalyze the coupling of epoxides and C02 was (tpp)AlCl (tpp = tetraphenylporphyrin) in the presence of a quaternary organic salt or triphenylphosphine [20]. Although this catalytic system was extremely slow at ambient temperature, copolymers from ethylene oxide, PO, and CHO and C02 were obtained that possessed very narrow molecular weight distributions (polydispersity = 1.06-1.14). The low reactivity of... 

Usually, the effect of chemical reactions in reactive absorption processes is advantageous only in the region of low gas-phase concentrations due to limitations by the reaction stoichiometry or equilibrium [5]. Further difficulties of reactive absorption applications may be caused by the reaction heat through exothermic reactions and by relatively difficult solvent regeneration [6, 7]. Most of the reactive absorption processes are steady-state operations, either homogeneously catalyzed or auto-catalyzed. Recently, an application of a reactive absorption process based on using secondary amine groups on solid supports as immobilized activators has been reported [8]. 

Reactive distillation was soon extended to other types of reactions, as nitrations, sulfonations, and sap-onifications [3], All of these reactions were homogeneously catalyzed, i.e. the catalyst was in the liquid phase, flowing down the column, and had to be recovered from the residue. 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

Heterogeneously catalyzed reactive distillation offers several advantages when compared to the homogeneously catalyzed process alternative. The size and location of the reactive section can be chosen independently of thermodynamic constraints, and an additional process step for separating the catalyst from the product can be avoided. In order to fix the catalyst in the reactive section of the column, heterogeneously catalyzed reactive distillation processes require special internals, which will be discussed in Section 3.2.4. 

Whereas homogeneously catalyzed reactive distillation can be carried out in conventional tray columns (sometimes modified to ensure sufficient residence time of the reactants), a heterogeneous catalyst has to be fixed in the reactive section with the help of special internals. These internals have to combine good wetting characteristics to achieve a good contact between the Hquid and vapor phases with a large amount of catalyst that is readily accessible by the liquid in order to avoid macro-kinetic influences. 

As noted above one source of chemical reactivity is the observation of phosphine oxidation which lowers the availability of ligating functionality. Another mode of deactivation is actual rupture of the phosphorus-carbon bond. In order to understand the chemical instability of transition metal phosphine complexes on resins, we must first examine some recent data on the chemical stability of tertiary phosphines during homogeneously catalyzed... 

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