Supercritical catalytic reactions

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

In the past, the majority of high-pressure homogeneous catalytic reactions were conducted in batch systems, which may cause problems in scale-up for SCFs because of the higher pressures needed for achieving the supercritical state. Therefore, continuous processing has also been investigated in the last years. It would be preferable for industrial-scale SCF reactions, because it involves smaller and, hence, safer equipment [144-150]. In addition, capital costs are likely to be lower than in batch systems. 

Supercritical fluids (SCFs) offer several advantages as reaction media for catalytic reactions. These advantages include the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. Benefits derived from the SCF phase Fischer-Tropsch synthesis (SCF-FTS) involve the gas-like diffusivities and liquid-like solubilities, which together combine the desirable features of the gas- and liquid-phase FT synthesis routes. 

Hyde, J.R. and Licence, P. and Carter, D. and Poliakoff, M. (2001). Continuous catalytic reactions in supercritical fluids. Applied Catalysis A General. 222. 119-131. 

HP IR spectroscopy has been coupled with each of these approaches, leading to a number of recent advances in both cell-design and mechanistic understanding. Another important area of current interest is the use of supercritical fluids as reaction media for catalytic reactions. By their very nature, supercritical fluids require high pressures, and HP IR has also played a significant role in this field. 

The last ATR cell described here in detail was designed for the study of catalytic reactions at high pressures and in particular in supercritical fluids. A schematic representation of the design is shown in Fig. 17 (76). An important issue in this type of reaction is the phase behavior of the system, which can have a large influence on the catalytic reaction 77,IS). The cell consists of a horizontal stainless-steel cylinder. It is designed to allow monitoring of the phase behavior via a video camera. For this purpose, one end of the cylinder is sealed with a sapphire window, behind... 

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

The cyclic substrate 32 and other disubstituted olefins such as 35a were oxidized in sc C02 to give the corresponding epoxides with reasonable rates (>95% conversion in less than 18 h) and excellent selectivities (>98%) under otherwise similar reaction conditions (Loeker and Leitner, 2000). It is important to note, however, that no addition of a metal catalyst was required in the supercritical reaction medium. Detailed control experiments revealed that the stainless steel of the reactor walls served as efficient initiator for the epoxidation under these conditions. Terminal olefins 35b,c were oxidized with somewhat reduced rates and either epoxidation or vinylic oxidation occurred as the major reaction pathway depending on the substrate (eq. 5.11). Apart from providing the first examples for efficient and highly selective oxidation with 02 in sc C02 (earlier attempts Birnbaum et al., 1999 Loeker et al., 1998 Wu et ah, 1997), this study points to the possible importance of wall effects during catalytic reactions in this medium (see also Christian et ah, 1999 Suppes et ah, 1989). 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

Figure 14.2. Laboratory-scale apparatus for catalytic reactions in supercritical fluids in combination with countercurrent extraction.

At the present time, catalytic reactions in supercritical carbon dioxide for the manufacture of fine chemicals appear to be more commercially feasible (McCoy, 1999). A large range of reactions have been investigated in cooperation with the University of Nottingham and Thomas Swan Co., Ltd. (Consett, Co. Durham, U.K.) (Meehan et al., 1999). Very high space-time... 

Pereda, S., Bottini, S.B. and Brignole, E.A. (2005) Supercritical fluids and phase behavior in heterogeneous gas-liquid catalytic reactions. Appl. Catal. A Gen., 281, 129. 

Non-catalytic reaction pathways and rates of reaction of diethyl ether in supercritical water have been determined in a quartz capillary by observing the liquid- and gas-phase XH and 13C NMR spectra.37 At 400 °C, diethyl ether undergoes, competitively, proton-transferred fragmentation and hydrolysis as primary steps. The former path generates acetaldehyde and ethane and is dominant over the wide water density range up to... 

Arai M, Fujita S, Shirai M (2009) Multiphase catalytic reactions in/under dense phase C02. J Supercrit Fluids 47(3) 351-356... 

A considerable amount of effort has already been devoted to producing dimethyl carbonate (DMC) from methanol and CO2, and some of the reactions have been catalyzed by organotin alkoxides. However, the catalytic activities so far obtained have been very low due to the decomposition of the catalysts by water generated during the reaction. The supercritical C02 reaction with trimethyl orthoacetate leads to the desired reaction and gives DMC and methyl acetate. Although di- -butyltin dimethoxide is less effective, the addition of tetrabutylphosphonium iodide substantially enhances the catalytic activity of the system (Equation (96)).261 262... 

Flow reactors offer considerable advantages over sealed autoclaves for supercritical reactions. Not only do flow-reactors require a much lower volume than a batch reactor for a given throughput of material (with obvious safety advantages) but also it is much easier to optimise reaction conditions in a flow reactor. We have already reported [4,5] the use of a miniature flow-reactor for the photochemical preparation of unstable metal complexes. We are now extending these techniques to the study of thermal and catalytic reactions. As an initial stage we... 

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