Effect on product selectivity

Along with catalyst activity, product selectivity is a key issue in cobalt-based FTS.1 For GTL processes the preferred product is long-chain waxy hydrocarbons. It is well known that FT reaction conditions have an important effect on product selectivities. High temperatures and H2/CO ratios are associated with higher methane selectivity, lower probability of hydrocarbon chain growth, and lower olefinicity in the products.105... 

The indicated first-order rate dependence on [HCo(CO)4] is observed in 1,4-dioxane solvent, as shown by the constant value of k over a range of HCo(CO)4 concentrations. A slightly higher concentration dependence may be observed in 2,2,2-trifluoroethanol (e.g., Expt. 9). It is proposed that such behavior, if significant, could be the result of greater ionic dissociation of HCo(CO)4 at the lower concentrations in this more polar solvent. A catalyst concentration effect on product selectivity was reported by Keim et al. (59), but rate effects are not reported and possible secondary reactions are not taken into account. 

There are clearly some interesting observations reported in the above papers1217 that seem to pervade stereoselective reactions. First of all, the nature of the solvent in homogeneous stereoselective reactions is critical. Secondly, unique steric requirements for reactants on solid surfaces appear to lead to enhanced stereoselectivity. Thirdly, modifiers and promoters in both solution phase and solid phase catalysis appear to have marked effects on product selectivity. 

It is convenient to examine structure-sensitivity as revealed only by rates, since much of the available information concerns ethane there are however major and important effects on product selectivities in the reaction of n-butane especially. 

Substituent effects are complicated and can perturb all the key steps in the mechanism. Steric effects might enhance or reduce electronic effects. A substantial shortening of the kinetic chain length (as observed for 6 in Figure 7.4) makes the anticipation of substituent effects on product selectivities even more difficult because they are no longer determined by the radical chain propagation steps only. The strength of computational studies is the abUity to probe individual reactions. 

Figure 8.47 shows the effect of the dimensionless potential THFUwr/RT, on product selectivity, S, under constant feed conditions. The selectivity to h2co can be varied deliberately between 0.35 and 0.60 by varying the catalyst potential. 

Figure 8.52. Effect of dimensionless catalyst potential n(=FUwn/RT) on product selectivity to H2CO, CO and CH4 during CH3OH dehydrogenation and decomposition on Ag. Conditions and symbols as in Fig. 8.51.56 Reprinted with permission from Academic Press.

In our previous studies on chlorination of toluene we had found that solvent had an important effect on the selectivity. In particular, the use of diethyl ether as a cosolvent was advantageous for the production of a high proportion of the para-isomer (ref. 9). An experiment in which the amount of ether in a tetrachloromethane/diethyl ether solvent mixture was varied under otherwise identical reaction conditions (Ih reaction at 18°C with 1.04 molar equivalent of tert-butyl hypobromite) demonstrated that diethyl ether also had a marked influence on the selectivity of the bromination reaction (Fig. 6). There was also an effect on the yield of the reaction as performed under these standard conditions. As the... 

Product selectivities fiom the different CuCl catalysts are shown in Table 1. There is no visible detrimental effect on the selectivities due to the introduction of the dispersants during CuCl preparation. The best selectivity was obtained with the CuCl prepared with the addition of SDBS. Therefore, SDBS is recommended as a dispersing agent that can be used in practical production. 

Since it is reported in the literature that small amounts of water may improve the catalyst life with time-on-stream, reducing the formation of tar [26], a small percentage of water was added in the feed (Table 39.5), producing a very negative effect on the selectivity in MDB. Furthermore the formation of a new by-product, 2-methoxyphenol (2-MP), was observed. On the contrary, no increase was found in the C-balance values. 

Fe/4.6Si/1.44K/2.0Cu and 100Fe/4.6Si/5.0K/2.0Cu catalysts were found to be 0.92 and 0.94, respectively. Since the amount of Cu was essentially identical in both catalysts and the steady CO conversion rate of the 100Fe/4.6Si/5.0K/2.0Cu catalyst was similar to that of the 100Fe/5.1Si/5.0K catalyst, we postulate that the difference in CO conversion level between the two Cu-promoted catalysts is due primarily to a difference in the K (and perhaps Si) content. In general, selectivities in FTS are compared at a similar CO conversion level for each catalyst. However, wide variations in the CO conversion level in the current study make it difficult to evaluate the effects of different promoters on product selectivity. In spite of the differences in CO conversion rates, an effort was made to qualitatively compare the promotional effect of Cu and K on product selectivity at similar, or nearly similar, CO conversion levels. 

These results are in agreement with the literature results, especially with data concerning temperature effects on CO conversion.121619 20 In case of temperature effect on product distribution, there are many studies that, in apparent disagreement with what is presented here, report an increase of selectivity to the lighter products with increasing temperature. These data, however, are compatible with our results if one considers the narrow temperature interval (220-235°C) investigated in this study. 

Judicious selection of excipients for parenteral dosage forms is critical due to their systemic administration.106107 Excipients of a parenteral dosage form may have a significant effect on product safety including injection site irritation or pain. Permissible excipients for parenteral dosage forms are far less than those for oral dosage forms. 

Effect of nitrating agent and reaction conditions on product selectivity... 

Effect of Steam Treatment of ZSM-5 Catalyst on Product Selectivity During n-hexadecane Cracking... 

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