Supercritical fluid catalytic reactions

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. 

Supercritical Fluid Technology Reactions Table 3 Heterogeneous catalytic reactions in supercritical solvents 2921... 

Several classes of chemical reactions are possible in microemulsions formed in supercritical fluids. Catalytic hydrogenation or oxidation reactions using molecular hydrogen and oxygen as reactants are particularly well suited for these studies as both are very soluble in supercritical fluid solvents. A potentially useful role for these oxidation reactions is the destruction of hazardous chemical wastes or contaminated materials. 

Heterogeneous catalytic reactions in supercritical solvents Obviously, a solid catalyzed reaction takes place only on the active sites of the porous catalyst with the implication of some mass and heat transport steps prior to and after the reaction. The first step is the diffusion of the reactants through the film surrounding the catalyst particle to the external surface of the catalyst, followed by diffusion of the reactants into the catalyst pore to the active site in the pores. These steps are limited by the dif-fusivity and viscosity of the reactants. In the case of a supercritical fluid phase reaction, the diffusivity is higher than the liquid diffusivity, viscosity is less than the liquid viscosity and therefore, the rate of transfer to the active site will be higher. After the adsorption, reaction and desorption steps, the products have to diffuse out of the pore, and again... 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

In recent years, supercritical fluids such as scC02 were considered to be modern green solvents they were non-toxic, readily available, inexpensive, and environmentally benign. They are studied as a reaction medium for catalytic applications because of their interest in product separation and catalyst recovery, and... 

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. 

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... 

In high pressure work, slurry reactors are used when a solid catalyst is suspended in a liquid or supercritical fluid (either reactant or inert) and the second reactant is a high pressure gas or also a supercritical fluid. The slurry catalytic reactor will be used in the laboratory to try different catalyst batches or alternatives. Or to measure the reaction rate under high rotational speeds for assessing intrinsic kinetics. Or even it can be used at different catalyst loadings to assess mass transfer resistances. It can also be used in the laboratory to check the deactivating behaviour. 

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.

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. 

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

It is well known that the properties of supercritical fluids are sensitive to pressure, and thus pressure may drastically influence the catalytic activity or the product selectivity when a reaction takes place in supercritical conditions. The favorable pressure for the Wacker oxidation of styrene is around 16 MPa of total pressure including 3 MPa 02, at which the selectivity toward acetophenone reaches 92 %, while under a total pressure of 9 MPa, the selectivity for acetophenone is lower (86 %). However, C02 with a higher pressure of over 20 MPa might retard the interaction between the substrate and the catalyst, and might cause a low concentration of substrate in the vicinity of the catalyst, thus resulting in a relatively low yield [38]. 

This investigation consisted of essentially two parts. The first involved a feasibility study of catalytic hydrotreating in the presence of supercritical fluid. The second part of our investigation involved the parametric studies to see how reaction parameters affect supercritical hydrotreating. 

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