Effect of temperature on selectivity

Although temperature has been proposed as a variable in altering selectivity, it has not been widely used, because the majority of analytes show very similar changes on changing temperature (especially over the limited conventional temperature range). Significant differences may be observed if temperature can cause ionization changes or if analytes with very different functional 

A few application and studies have examined temperature effects, such as the selectivity dependence of the carotenoids [21] on different columns from 25 5°C. Studies of the prediction of the influence of temperature and solvent strength on the separation of 47 basic acidic and neutral drugs compounds were reported by Zhu et al. [22] in an interlaboratory collaborative study. More recently the influence on temperature on selectivity has been reviewed by Dolan [23, 24]. 

Berthod et al. [28] examined the effect of temperature on chiral separations between 5°C and 45°C using four macrocyclic glycopeptides phases and although the efficiencies increased with temperature, in 83% of cases the chiral selectivity decreased. 

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An example of the effect of temperature on selectivity (yield) for the case of two reactions where A goes to product P by a first-order reaction, and P goes to impurity X by a second-order reaction is shown in Figure 3.9. Say that the undesired reaction is highly exothermic. If the product P is removed as soon as it is formed, the second (undesired) reaction will not occur. It is evident that the overall reaction would be more hazardous and the yield of product P less if an incorrect reactor type is selected. From Figure 3.9, it can be seen that the higher the temperature, the greater the decrease in selectivity. At low... 

Li, J. Effect of temperature on selectivity in reversed-phase liquid chromatography a thermodynamic analysis. Anal. Chim. Acta 1998, 369, 21-37. 

WATEQ2 consists of a main program and 12 subroutines and is patterned similarly to WATEQF ( ). WATEQ2 (the main program) uses input data to set the bounds of all major arrays and calls most of the other procedures. INTABLE reads the thermodynamic data base and prints the thermodynamic data and other pertinent information, such as analytical expressions for effect of temperature on selected equilibrium constants. PREP reads the analytical data, converts concentrations to the required units, calculates temperature-dependent coefficients for the Debye-HKckel equation, and tests for charge balance of the input data. SET initializes values of individual species for the iterative mass action-mass balance calculations, and calculates the equilibrium constants as a function of the input temperature. MAJ EL calculates the activity coefficients and, on the first iteration only, does a partial speciation of the major anions, and performs mass action-mass balance calculations on Li, Cs, Rb, Ba, Sr and the major cations. TR EL performs these calculations on the minor cations, Mn, Cu, Zn, Cd, Pb, Ni, Ag, and As. SUMS performs the anion mass... 

Walter et al. [84] discussed several common experimental methods to estimate the influence of internal mass transfer resistances on the observed rates of heterogeneous catalytic reactions. For example, when the reaction temperature is varied, because the intrinsic reaction rate increases more strongly with temperature than the rate of diffusion, the influence of mass transfer becomes more important and the observed apparent activation energy decreases as the temperature increases effects of temperature on selectivity may be more complex. 

Regarding the effect of temperature on selectivity, reliance on equations such as... 

An important point which is often misinterpreted in high pressure chemistry, is the effect of temperature on selectivity. Thus, in many cases the observed improvement of selectivity under high pressure conditions is not due to a favorable AAV, but results from carrying out the reaction at a lower temperature. For transformations where a large difference in reaction enthalpy (AAH ) exists leading to the different isomers, lowering the temperature has a strong effect on selectivity. The pressure and enthalpy effect of the reaction can be cooperative but it can also be opposed. An accurate differentiation of these two influences can only be achieved by a separate determination of the AV and AH for the different reaction channels. 

Figure 9.5. Hydrogenation of ethyne over Al203-supported metals effect of temperature on selectivity (Pe = 50 Torr Ph = 200 Torr). ...

Figure 6. Effect of temperature on selectivity of C2+ products at lowest oxygen content.

In selecting the boiling temperature, consideration must be given to the effect of temperature on heat-transfer characteristics of the type of evaporator to be used. Some evaporators show a marked drop in coefficient at low temperature—more than enough to offset any gain in available temperature difference. The condenser cooling-water temperature and cost must also be considered. 

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

Figure 8.62, Effect of temperature on the catalytic rates of C02, N2 and N20 formation and on the corresponding N2 selectivity, for open (unpromoted) and closed (NEMCA) circuit conditions on Rh/YSZ during NO reduction by C3H6.67,68 Reprinted from ref. 68 with permission from Elsevier Science.

Although the effect of temperature on each of the steps in an overall electrode process is readily predictable, it is surprising to find in the literature very few systematic studies of this variable or attempts to use it to change the rate, products or selectivity of an organic electrosynthetic process. A recent paper has, however, discussed equipment and suitable solvents for low-temperature electrochemistry (Van Dyne and Reilley, 1972a). 

Fig. 1 Effect of temperature on NH3 oxidation. Closed symbols electric finnace, open symbols microwave heating,, O NH3 conversion, A, A N2 selectivity,, NO selectivity, BO N2O selectivity.

Fig. 2. Ihe effects of temperature on the conversions of CO2 and CH4 and the selectivity overNi-YSZ-Ce02 catalyst.

Fig.2. Effect of temperature on the conversion and selectivity in a CSTR at SOOpsig.

Figure 3. Effect of temperature on the selectivity of the SCR reaction over CoZSM-5 (A) and HZSM-5 (B) catalysts. Feed contained 0.28% CH4, 0.21% NO and 2.6% O2 in He at a flow rate of 75 ml/min ( flow rates of CH4 and NO were 9.375 and 7.03//mol/min).

Temperature variation may also be a relevant factor in flowrate stability. Since the viscosity of the solvent is temperature dependent, wide swings in the ambient temperature can directly affect pump performance. The direct effects of temperature on pump performance usually are far smaller, however, than the effects on retention and selectivity therefore, control of column temperature is generally sufficient to obtain high reproducibility. 

Figure 1 Effect of temperature on catalytic performance of Mg/Al/O catalyst in m-cresol methylation m-cresol conversion (u), selectivity to 3-MA (v), 2,3-DMP (X), 2,5-DMP (ct), 3,4-DMP (p), polyalkylates ( ).

The effect of temperature on the catalytic performance of Mg/Fe/O is reported in Figure 3. The behavior was quite different from that of the Mg/Al/O catalyst. The conversion of m-cresol with Mg/Fe/O was always lower than that with Mg/Al/O. The selectivity to 3-MA was almost negligible in the whole range of temperature. The selectivity to polyalkylates and to 3,4-DMP was also much lower than that observed with Mg/Al/O. Therefore, the catalyst was very selective to the products of ortho-C-methylation, 2,3-DMP and in particular 2,5-DMP. This behavior has to be attributed to specific surface features of Mg/Fe/O catalyst, that favor the ortho-C-methylation with respect to O-methylation. A different behavior of Mg/Al/O and Mg/Fe/O catalysts, having Mg/Me atomic ratio equal to 4, has also been recently reported by other authors for the reaction of phenol and o-cresol methylation [5], The effect was attributed to the different basic strength of catalysts. This explanation does not hold in our case, since a similar distribution of basic strength was obtained for Mg/Al/O and Mg/Fe/O catalysts [4],... 

The aldol condensation reaction of acetone was performed over CsOH/Si02 at a range of reaction temperatures between 373 and 673 K (a typical product distribution is shown in Figure 2). Table 1 displays the conversion of acetone along with the selectivities for the products produced once steady state conditions were achieved. Figure 3 presents the effect of temperature on the yield of the products. The activation energy for acetone conversion was calculated to be 24 kJ. mol 1. 

Figure 3. Effect of temperature on cyclohexanone oxime conversion and products selectivity.

Lowering the temperature of the reaction would certainly decrease the rate of acetal hydrolysis and thereby partially remove one of the causes of overoxidation. This would simultaneously decrease the rate of oxidation by periodate. Although no comprehensive study of the effect of temperature on oxidation rates has been made, the number of reactions successfully dealt with in the temperature range of 0 to 6°31 39 78 113 126 130 i33, i64, us, 203,210,266,267 indicates that lowered temperatures do not affect the rates unfavorably. In order to obtain the maximum of selective oxidation and the minimum of overoxidation, periodate oxidations should be performed at as low a temperature as is practicable in relation to the solvent system used and the solubility of the reactants therein. 

Macek, K.J., C. Hutchinson, and O.B. Cope. 1969. The effects of temperature on the susceptibility of bluegills and rainbow trout to selected pesticides. Bull. Environ. Contam. Toxicol. 4 174-183. 

The effects of temperature on enantioselectivities have been examined using a Rh-Et-DuPhos catalyst in both MeOH [56d] and THF [144]. With /5-dehydro-amino acid derivative 73 in MeOH, an increase in temperature was found to have a slight beneficial effect for both ( ) and (Z)-isomers over a 70°C range, with maximum values being observed between 0°C and 25°C. In THF, however, the effect is much more pronounced, especially for the (Z)-isomer which varies in selectivity from 65% ee at 10 °C to 86% ee at 25 °C. Interestingly, when substrate 72 was reduced with a Rh-Et-BPE catalyst in THF, this temperature dependence on enantioselectivity for the (Z)-isomer was most apparent, the se-lectivities varying from 43% ee (10°C) to 90% ee (40°C). Examination of these results also seemed to indicate that the hydrogenation of /9-dehydroamino acid derivatives follows an unsaturated pathway (vide supra) [144]. 

FIGURE 4.11 The effect of temperature on retention and selectivity in packed column SFC and open tubular GC. Conditions 15 cmx250 xm ID capillary packed with 5 xm porous (300 A) silica particles encapsulated with P-CD polymethylsiloxane (10% w/w) and end capped with HMDS, 160 atm, CO2, FID, 10 cmx 12 xm ID restrictor. (B) 25 mx250 xm ID cyano-deactivated capillary cross-linked with P-CD polymethylsiloxane (0.25 xm d ) He FID. (Adapted from Wu, N. et al. 2000. J. Microcol. Sep. 12 454-461. With permission.)... 

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