Pressure effect on activity

Note that in Fig. 18, KINPTR s prediction of C5- falls below the data points. However, when one considers the large temperature and pressure effect on activity in the C6 system and the fact that these same C6 kinetics are used in KINPTR to make predictions for all reforming feedstocks (full-range naphthas, pure components, etc.), the predictions are certainly acceptable. 

Pressure Effects on Activity and Stability of H3rperthermophilic Enzymes... 

Herbst D, Peper S, Fernandez JF, Ruck W, Niemeyer B. Pressure effects on activity and selectivity of Candida rugosa lipase in organic solvents.J Mol Catal B Enzym 2014 100 104-10. 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

As shown in Table I, at 0.1 mM Ru (C0) 2 concentration, CO pressure has little if any effect on activity. On the other hand, at fixed pressure, the concentration of ruthenium carbonyl has a dramatic effect on activity (see Figure 2). At 0.1 mM Ru CCO), ruthenium carbonyl is very active for the WGSR, small decreases in catalyst concentration lead to substantial increases in activity, and no activity dependenee on CO pressure is observed. At concentrations of 0.5 mM or more, less activity is observed, changes in concentration cause smaller effects in activity and rate dependence on pressure is manifested. Diffusion effects have been shown to be unimportant (26). 

Processes with gaseous reactants are excluded here. Due to the large compressibility of gases an increase of pressure (up to 1 kbar) leads essentially only to an increase of gas concentration, and hence to an acceleration of bimolecular processes in which gases are involved as reactants. The effect of pressure on a chemical reaction in compressed solution is largely determined by the volume of reaction (AV) and the volume of activation (AV ). It is not the purpose of this chapter to provide a complete survey of reactions of dienes and polyenes which have been investigated at elevated pressures. There are many excellent monographs (e.g. References 1-4) and reviews (e.g. References 5-16) on this topic which cover the literature up to early 1990. After a short introduction into the basic concepts necessary to understand pressure effects on chemical processes in compressed solutions, our major objective is to review the literature of the past ten years. 

Laverman and coworkers have reported activation parameters for the aqueous solution reactions of NO with the iron(II) and iron(III) complexes of the water soluble porphyrins TPPS andTMPS (21). These studies involved systematic measurements to determine on and kQ as functions of temperature (298—318 K) and hydrostatic pressure (0.1—250 MPa) to determine values of AH, AS and AV for the on and off reactions of the ferri-heme models and for the on reactions of the ferro-heme models (Table II). Figure 2 illustrates hydrostatic pressure effects on kOTL and kQff for Fem(TPPS). 

Bertole et al.u reported experiments on an unsupported Re-promoted cobalt catalyst. The experiments were done in a SSITKA setup, at 210 °C and pressures in the range 3-16.5 bar, using a 4 mm i.d. fixed bed reactor. The partial pressures of H2, CO and H20 in the feed were varied, and the deactivation, effect on activity, selectivity and intrinsic activity (SSITKA) were studied. The direct observation of the kinetic effect of the water on the activity was difficult due to deactivation. However, the authors discuss kinetic effects of water after correcting for deactivation. The results are summarized in Table 1, the table showing the ratio between the results obtained with added water in the feed divided by the same result in a dry experiment. The column headings refer to the actual experiments compared. It is evident that adding water leads to an increase in the overall rate constant kco. The authors also report the intrinsic pseudo first order rate-coefficient kc, where the overall rate of CO conversion rco = kc 6C and 0C is the coverage of active... 

A study of the temperature and pressure effects on the rate constant k must take Eq. (6) as a point of departure. A plot of In k versus T yields an experimental activation energy (which is not to be interpreted as the internal activation energy) through the relation... 

Jenner [275] has presented a thorough description of several possible contributions to both the intrinsic and the environmental parts of the activation volumes, based on accurate experimental observation of pressure effect on reactions in solutions. The intrinsic contribution to the activation volume essentially derives from the differences in structure between the transition state and the reacting species, so it is directly related to the partial cleavage and formation of chemical bonds in the transition state. In cases where the environmental contribution is negligible, the activation volume variation gives a direct insight in the molecular mechanism [275, 280]. In this case in fact, considering... 

In a liquid-phase system, pressure has no direct effect on activity-the pressure must simply be sufficient to maintain a liquid phase at the appropriate temperature. However, if there is dissolved Hj in the liquid-phase system, then the pressure affects the solubility and can affect the activity or stability of the system. 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

Pressure effect on the product distribution in supercritical media would resolve the problem. If the reaction proceeds via the competitive concerted/ stepwise mechanism, the reaction under a higher pressure is expected to give more exo isomer because the activation volume is considered to be smaller for concerted process than the stepwise one and hence more concerted reaction is expected under a higher pressure. If, on the other hand, bimodal lifetime distribution of trajectories is the origin of the stereoselection, the product ratio is expected to approach to unity under high-pressure conditions, since energy randomization is more effective under a high pressure. 

Equations (6.1) and (6.2) pertain to ideal systems, that is, systems where there are no interactions between the molecules. In a real system the pressure effect on p. in the vapor phase has to be modified by a fugacity coefficient < >, and the effect of mixing on the chemical potential in the liquid phase has to be modified by an activity coefficient 7,. The more general expression for equilibrium (called the 4>-y representation) then becomes... 

The standard state used for the finite concentration data was usually the pure solvent at the temperature and pressure of the mixture. In most cases, the Poynting correction (pressure effect on the liquid) could be neglected. The end result of this approximation is that saturation pressure appears in the expression for the activity coefficient, but not the system pressure. 

Our discussion here explores active connections between the potential distribution theorem (PDT) and the theory of polymer solutions. In Chapter 4 we have already derived the Flory-Huggins model in broad form, and discussed its basis in a van der Waals model of solution thermodynamics. That derivation highlighted the origins of composition, temperature, and pressure effects on the Flory-Huggins interaction parameter. We recall that this theory is based upon a van der Waals treatment of solutions with the additional assumptions of zero volume of mixing and more technical approximations such as Eq. (4.45), p. 81. Considering a system of a polymer (p) of polymerization index M dissolved in a solvent (s), the Rory-Huggins model is... 

Volumes of activation and reaction are themselves also pressure-dependent as shown for the volume of activation in Figure l. There is no theory explaining this pressure dependence which would allow the volume of activation or reaction to be determined over a larger range of pressure. Therefore, several empirical relations are employed to fit the pressure dependencies of rate and equilibrium constants " from which the least-squares fit [hiA (p) = a + b- p, hi= 0) = a, A= -b-R-T orhi f(p) = a - -b p, hiA (p = 0) = a, AV = y RT] is the simplest and in many cases also the most reliable method of computing A and A V, It is only applicable in the low-pressure range (<2000 bar) where the dependencies of hi (p) or In if (p) on pressure p are usually linear. Thus, this method requires a very precise measurement of the rate constants at relatively low-pressures (1-2000 bar) where the pressure effect on the rate constants is relatively... 

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