Lowering of the selectivity

This lowering of the selectivity may be attributed to competitive binding between the hydroxyl and amide groups to iridium [37]. In entries 6, 7 and 8, the directivity of the hydroxyl group at the bishomoallylic position effectively overrides the effect of the carbamate unit [38]. In the hydrogenation of methylenebi-... 

This effect is complete in the case of porphyrins 6-8,13, which behave in the same way, independent of metal present in the inner core of the macrocycle. In conclusion, the introduction of these alkyl chains from one side gives a higher porosity to the molecular film and, as a consequence, both a speed up of the response and an increase of the sensor response. On the other hand there is a significant decrease of the importance of the selective interactions in the sensing mechanism of the organic material and, as a consequence, a lowering of the selectivity of the sensor. 

Scheme 16 summarizes the results obtained by enantioselective radical reduction of a-bromoester by chiral binaphthyl-derived tin hydride. The reactions were generally performed at - 78 °C. An increase in the temperature resulted in the lowering of the selectivity. All reactions mediated by (S)-configured chiral tin hydride showed an (R)-selective preference in the product. The use of the opposite enantiomer of the chiral stannane resulted in a quantitative reversal of the selectivity (not shown). The selectivity remained modest on addition of magnesium Lewis acids. These reductions were also feasible when a catalytic amount of chiral tin hydride (1 mol %) was employed in combination with an excess of achiral hydride NaCNBH3, providing similar results. 

Above a temperature of about 450°C the catalyst deactivates due to the formation of heavier products which remain on the zeolite, explaining the lowering of the selectivity above this temperature, but at lower temperatures benzaldehyde selectively transforms to benzoic acid. Phenol and benzene are the main by-products of benzaldehyde oxidation with selectivities up to about 10% each. The other by-products observed in toluene oxidation, including phenylbenzoate, were also detected in benzaldehyde oxidation. 

A is enhanced greatly as the solution concentration is decreased. Increasing the concentration results in a lowering of the selectivity of the divalent ion over the univalent ion. 

With the series of promoted platinum catalysts it was seen that those promoted by compounds containing non-transition elements (Sn and Ge) appeared to be promising, the selectivities to furfuryl alcohol were 40.0% and 16.8%, respectively. Promoters like V and Ti compounds did not enhance the selectivity above that of pure platinum and the sodium promoter even caused a lowering of the selectivity to furfuryl alcohol (Figure 2 and Table 2). The values plotted in Figure 2 for the promoted platinum catalysts were obtained in the non-isothermic way. In the non-isothermic experiments the temperature was stepwisely varied in the range of 170° to 210°C. 

Not only does the decreasing catalyst activity lead to a loss of productivity, it is also often accompanied by a lowering of the selectivity. Therefore, in industrial processes great efforts are made to avoid catalyst deactivation or to regenerate deactivated catalyst. Catalyst regeneration can be carried out batchwise or preferably continuously while the process is running. 

A second important point evidenced in [30], and already pointed out earlier in this chapter, is that alcohol dehydrogenation was clearly found to be the main cause for the lowering of the selectivity due to aniline formation. This point is... 

The attempt to design active catalysts is justified by the fact that if they were active at lower temperature, the thermodynamics of the process would be improved and the equilibrium conversion yield increased to more usable values. Therefore, modification of the oxides with phosphoric acid has been carried out in an attempt to modify the acid/basic ratio of active sites. Very often, it has been observed that an increase of catalyst activity is coupled with a lowering of the selectivity the conversion yield is improved but the lower selectivity raises the problem of the cost of separation of the products. Examples of such unfortunate coincidences are given for the following catalysts. Whereas Zr02 is used at 440 K with a low yield (<1 %) and selectivity close to 100 %, H3PWi204o/Zr02 is active at 373 K, but dimethylether-DME and CO are formed as by-products [49, 50, 52-54],... 

In addition to hydride, other groups may be abstracted from neutral ti -cyclohexadiene complexes. The reaction between the homochiral complex 37 and the triphenylcarbenium ion resulted in removal of a methoxide anion and formation of a 5 1 mixture of the two possible isomers 43 and 44. Trifluoro-acetic acid (TFA) may also be used. For 37 this resulted in a lowering of the selectivity, but there are examples where regioisomers do not arise and TFA is the reagent of choice for the removal of methoxide. ... 

The more selectively adsorbed components are stripped from the adsorbent bed during the countercurrent purge step. By purging into the product end of the bed, a lower residual loading of the selectively adsorbed species can be achieved in the portion of the adsorber that determines product quaUty. 

Use the Nikolskii-Eisenman equation to explain why lowering of the detection limits requires careful attention to the selectivity of the resulting ISE. 

The selectivity to citraconic anhydride decreases and that to acetic acid increases as the temperature is raised. The results indicate that the activation energy for the formation of citraconic anhydride is much lower than that for the formation of acetic acid. The selectivity to acetic acid decreases steadily with a lowering of the temperature. However, the highest selectivity to citraconic anhydride is obtained at 200°C. Possibly vaporization of pyruvic acid may become difficult at temperatures below 200°C. The yield of citraconic anhydride reached 71 mol% and that of acetic acid was 7 mol% at the pyruvic acid conversion of 98% the loss was about 20 mol%. 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

The catalyst composition has a role in the control of selectivity. The rutile-type V/Sb/(Nb) mixed oxide activates the hydrocarbon and ammonia. However, most of the ammonia is burnt to N2, rather than being inserted on the hydrocarbons this likely occurs because the catalyst is not veiy efficient in the generation of the selective Me=NH species when reaction temperatures lower than 400°C are used (11). In fact, with all catalysts the selectivity to A -containing compounds increased when the reaction temperature was increased, and the selectivity to N2 correspondingly decreased (Figure 40.6). The dilution of the active phase with tin... 

Guidelines suggest that if an operation exceeds two half-lives of the selected antimicrobial, then another dose should be administered.1 Repeat dosing has been shown to lower rates of SSI. For example, cefazolin has a half-life of about 2 hours, thus... 

Vreactor=70 ml VCh=Vrcn=10 ml (0.045 mole) 111 1 =0.3 g PH2=80 bar (at RT) was not maintained during reaction NH3/RCN=0.25 without ammonia, selectivity of SB is higher at lower temperatures. The selectivity to RNH2 decreased with reaction time for the experiment performed without NH3. The apparent activation energy of the hydrogenation of RCN on RNi-L catalyst was 30.5 kJ/mol, which is close to the value 46 kJ/mol measured in the liquid phase hydrogenation of acetonitrile on CoB amorphous alloy catalyt [7], RNi-C is more active than RNi-L catalyst (compare Table 1 No 4 and 6 and Table 2 No 7 and 8). 

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