Reaction and Catalyst Separation

In Section 4.2.2.1 cases are described in which the catalyst stays in the reactor because of the liquid-liquid two-phase technique (LLTP). In other cases only one liquid phase occurs and attention is turned especially to the removal of the catalyst. This removal 

F g-11 Successive reaction and catalyst separation (via solvent distillation and catalyst filtration). 

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In Figure 9 the simultaneous reaction and catalyst separation by membrane has been described. In one part of the reactor a catalyst is dissolved which cannot pass the membrane which is installed in the reactor. Here the starting chemicals A and B form the products C and D in a homogeneous catalyst solution. The products are able to pass through the membrane, perhaps together with a certain amount of the solvent. In the second unit, the distillation step, this solvent is recycled to the reactor and the products are isolated at the bottom of the distillation column. 

Figure 10 shows a similar arrangement, but reaction and catalyst separation now occur in different units. The reaction is succeeded by the membrane separation step, which is then followed by the distillation unit. The products C and D... 

Heterogeneous catalytic systems offer the advantage that separation of the products from the catalyst is usually not a problem. The reacting fluid passes through a catalyst-filled reactor m the steady state, and the reaction products can be separated by standard methods. A recent innovation called catalytic distillation combines both the catalytic reaction and the separation process in the same vessel. This combination decreases the number of unit operations involved in a chemical process and has been used to make gasoline additives such as MTBE (methyl tertiai-y butyl ether). 

Figure 5.6. Process flowsheets of biphasic reaction A+B - C+D and extraction l, Simultaneous reaction and extraction within the reactor 2, Separate reaction and catalyst extraction...

In the ideal biphasic hydrogenation process, the substrate will be more soluble or partially soluble in the immobilization solvent and the hydrogenation product will be insoluble as this facilitates both reaction and product separation. Mixing problems are sometimes encountered with biphasic processes and much work has been conducted to elucidate exactly where catalysis takes place (see Chapter 2). Clearly, if the substrates are soluble in the catalyst support phase, then mixing is not an issue. The hydrogenation of benzene to cyclohexane in tetrafluoroborate ionic liquids exploits the differing solubilities of the substrate and product. The solubility of benzene and cyclohexane has been measured in... 

Thermoregulated phase-transfer and phase-separable catalysis are attractive catalyst recycUng techniques complementing other approaches of multiphase catalysis. They utilize temperature-dependent solubility or miscibiUty phenomena to switch between homogeneous reaction and heterogeneous separation stages. 

The system can be operated in the parallel mode, discontinuously (batch-wise) with each reactor as an independent unit, semi-continuously or as a reactor cascade. Both homogeneous and heterogeneous reactions as well as product and catalyst separation and catalyst recycling are possible. 

Although not all of the factors that influence homogeneous hydrogenation and hydroboration in sc C02 are fully understood, it is clear that the use of sc C02 can lead to an increase in selectivity for some reactions. Additional work is needed to understand the opportunities for further selectivity enhancements and catalyst separation/recycle strategies. Even sc C02 systems that exhibit similar selectivities to those obtained in organic solvents could offer a practical, environmentally responsible method for the production of many important chiral building blocks. 

Heterogeneous catalysis is preferred over homogeneous catalysis. A critical issue is the catalyst design, which should ensure compatibility between the reaction and the separation. The temperature profile dominated by the vapor-liquid equilibrium, as well as the residence-time distribution controlled by the hydrodynamics of internals must comply with the achievable reaction rate and with the desired selectivity pattern. 

Similar to the inorganic supports, organic polymer matrices serve as carriers for halogenation reagents and catalysts providing modified and controllable microenvironment and unique reaction conditions, which occasionally result in enhanced rates and improved selectivities. In some instances procedures for the reactions and, particularly, separations are notably simplified. 

It makes more sense to separate the two units as shown in Figure 17, the successive reaction and catalyst treatment. First, the reaction is carried out as a homogeneously catalyzed conversion using a nonpolar solvent and a nonpolar catalyst. In the second unit, the nonpolar catalyst is converted into a polar one by adding... 

Various catalysts have been studied for these reaction processes. The major disadvantages of these reaction processes are (i) the need for an energy intensive separation step for catalyst recovery and (ii) limiting solubility of CO and O2 in the liquid reactant medium. One of the objectives of our study is to develop a heterogeneous gas-solid catalytic reaction process for the synthesis of methyl-N-phenylcarbamate involving step 3 and dimethyl carbonate involving step 4 over Cu-based catalyst. This gas-solid process would eliminate the solubility limitation and catalyst separation step, thus enhancing the overall economics of the carbamate synthesis (6-8). 

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