Bulk reactions pressure effect

The importance of steric bulk in the transition structure in order to obtain a large AAV can also be seen in the cycloaddition of 78 and 61a to give the cycloadducts 79 and 80 described by Boger et al. [47]. In this reaction the pressure effect on the stereoselectivity seems to be negligible since the endo/exo ratio of 5.7 1.0 was observed to be the same at 0.62 and 1.3 GPa. This is in good agreement with the lower steric demand of the oxabutadiene (78) compared to 60a. 

Brennecke and coworkers (90,320) investigated the uncatalyzed esterification of phthalic anhydride with methanol in SCCO2 as a probe reaction to show that augmented local densities and cosolvent compositions in the near-critical region represent enhanced reactant concentrations that result in increased reaction rates. The authors report kinetic data for the esterification reaction at both 40 C and 50 C and pressures ranging from 97.5 to 166.5 bar. Based on bulk fluid compositions, a dramatic pressure effect was exhibited for the measured bimolecular rate constants. For example, at 50°C the value of the rate constant decreased 25-fold from 0.0348 L/mol-min at 97.5 bar to 0.00138 L/mol-min at 166.5 bar, which represents one of the largest pressure effects ever reported for a reaction in an SCF. Based on calculations of the thermodynamic pressure effect on the rate constant from transition state theory and estimates of the local concentrations from literature data, the authors conclude that the dramatic pressure... 

Sonochemistry is strongly affected by a variety of external variables, including acoustic frequency, acoustic intensity, bulk temperature, static pressure, ambient gas, and solvent (47). These are the important parameters which need consideration in the effective appHcation of ultrasound to chemical reactions. The origin of these influences is easily understood in terms of the hot-spot mechanism of sonochemistry. 

In the aqueous biphasic hydroformylation reaction, the site of the reaction has been much discussed (and contested) and is dependent on reaction conditions (temperature, partial pressure of gas, stirring, use of additives) and reaction partners (type of alkene) [35, 36]. It has been suggested that the positive effects of cosolvents indicate that the bulk of the aqueous liquid phase is the reaction site. By contrast, the addition of surfactants or other surface- or micelle-active compounds accelerates the reaction, which apparently indicates that the reaction occurs at the interfacial layer. 

Bulk or forced flow of the Hagan-Poiseuille type does not in general contribute significantly to the mass transport process in porous catalysts. For fast reactions where there is a change in the number of moles on reaction, significant pressure differentials can arise between the interior and the exterior of the catalyst pellets. This phenomenon occurs because there is insufficient driving force for effective mass transfer by forced flow. Molecular diffusion occurs much more rapidly than forced flow in most porous catalysts. 

The effect of the bulk solution temperature lies primarily in its influence on the bubble content before collapse. With increasing temperature, in general, sonochemical reaction rates are slower. This reflects the dramatic influence which solvent vapor pressure has on the cavitation event the greater the solvent vapor pressure found within a bubble prior to collapse, the less effective the collapse. In fact, one can quantitate this relationship rather well (89). From simple hydrodynamic models of the cavitation process, Neppiras, for example, derives (26) the peak temperature generated during collapse of a gas-filled cavity as... 

The oxidation of chloride at the anode results in an anode boundary layer that contains less chloride than the bulk anolyte (Fig. 6.2). The membrane is pressed against the anode by differential pressure, and thus is integrated into the anode boundary layer. Good internal mixing is necessary to minimise the layer effect and to maintain a steady supply of chloride to the anode for reaction. The ionic concentration and the thickness of this layer will have an effect on the water content, the... 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

The first reported study of a reaction of wood with an epoxide appears to be that of McMillan (1963). This involved the use of gaseous ethylene oxide (Figure 4.9, R=H) at a temperature of 93 °C and a pressure of 3 atmospheres (0.3 MPa). In some cases, the wood was diffusion pre-treated with trimethylamine vapour as the catalyst. A 65 % ASE at 20 % WPG was obtained, attributed to a bulking effect due to in situ polymerization of the epoxide. There was no effect on the static bending strength of samples, and the modified wood became distinctly brown at higher levels of treatment. 

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