Supercritical reactions

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

Water quality standards, 21 581-583 Water reactions, supercritical, 24 16-17 Water-reducers, for cement, 5 485 Water-reduction strategies, in papermaking, 16 126-127 Water removal... 

Supercritical fluids have also been used purely as the solvent for polymerization reactions. Supercritical fluids have many advantages over other solvents for both the synthesis and processing of materials (see Chapter 6), and there are a number of factors that make scCCH a desirable solvent for carrying out polymerization reactions. As well as being cheap, nontoxic and nonflammable, separation of the solvent from the product is achieved simply by depressurization. This eliminates the energy-intensive drying steps that are normally required after the reaction. Carbon dioxide is also chemically relatively inert and hence can be used for a wide variety of reactions. For example, CO2 is inert towards free radicals and this can be important in polymerization reactions since there is then no chain transfer to the solvent. This means that solvent incorporation into the polymer does not take place, giving a purer material. 

Supercritical solvents have a number of advantages which make them excellent reaction media, such as the low cost, non-toxicity, and low viscosity. These advantages have meant that they are increasingly utilized in reactions. Supercritical solvents can be described as fluids with attributes of both liquids and gases. Solubility of the solute in the fluid depends on the vapour pressure of the solute however, addition of different polar/nonpolar compounds can change the solubility. What makes the supercritical solvent to be so imique is that properties, such as solubility, can be tuned by varying the pressure, so the solvent becomes more gas-like or liquid-like [46]. 

Reactions in supercritical media utilize high pressures. Therefore, the effect of pressure on reaction equilibrium as well as reaction rate plays an important role in supercritical phase reactions. Supercritical fluids that exhibit very high negative activation volumes for certain reactions will improve the rate and equilibrium conversion of the reaction. 

Other reactions studied by these authors include the thermal dimerization of methyl acrylate and the reaction of benzene with certain alkynes in a Diels-Alder reaction. Supercritical extraction in coal processing may also be classified under this scheme. This subject is reviewed by Williams (1981) it is not included in this chapter. 

It has been shown in the literature that isobutane (Pc = 36.5 bar, = 408 K)/butene (Pc= 40.2 bar, Tcf= 420 K) alkylation on solid acid catalysts at supercritical temperatures suffers from increased butene oligomerization and cracking reactions at these temperatures, increasing the catalyst deactivation potential [16-18]. Lower temperatures tend to favor the alkylation reaction. Supercritical operation at 95°C can be facilitated by diluting the isopar-affin/olefin feed with suitable amounts of a low inert solvent such as CO2 (Pc = 73.8 bar. Pc = 304 K), and has been shown to give rise to steady alkylation activity on USY and beta zeolites [19]. However, the alkylate yields are very low (< 10%) on these catalysts, attributed to severe pore diffusion limitations on these catalysts. 

Supercritical water as a reaction medium has been investigated sinee the early 1980s, in particular in studies by the research groups of Modell, Antal and Tester. Due to the severe conditions, focus was mainly set on degradation reactions. Supercritical water oxidation of waste compounds can be seen as a further... 

Brownian Dynamics Continuum Solvation Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Monte Carlo Simulations for Complex Fluids Monte Carlo Simulations for Liquids Poisson-Boltzmann Type Equations Numerical Methods Rates of Chemical Reactions Supercritical Water and Aqueous Solutions Molecular Simulation Transition State Theory. 

Organic synthesis Homogeneous catalytic reactions Heterogeneous catalytic reactions Supercritical CO as reactant Hydroformylation Process intensification... 

Ethyl-lactate is a novel ecofriendly solvent with potential applications in supercritical fluid technology, as a co-solvent of earbon dioxide, in high pressiue chemical reactions, supercritical extraction processes and/or anti-solvent precipitation processes. In view of this, knowledge of the phase behavior of (ethyl lactate + CO2) binary is essential for the modeling and design of sueh proeesses. 

Xu, Zhu, Yang [71] Review, different enzymes Stabihty, reactions Supercritical fluids. ... 

DA Hammond, M Karel, AM Klibanov, VJ Krukonis. Enzymatic reactions supercritical gases. Appl Biochem Biotechn 11 393-400, 1985. 

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