Selectivity consecutive reactions

Equation 1 is referred to as the selective reaction, equation 2 is called the nonselective reaction, and equation 3 is termed the consecutive reaction and is considered to proceed via isomerization of ethylene oxide to acetaldehyde, which undergoes rapid total combustion under the conditions present in the reactor. Only silver has been found to effect the selective partial oxidation of ethylene to ethylene oxide. The maximum selectivity for this reaction is considered to be 85.7%, based on mechanistic considerations. The best catalysts used in ethylene oxide production achieve 80—84% selectivity at commercially useful ethylene—oxygen conversion levels (68,69). 

Selectivity A significant respect in which CSTRs may differ from batch (or PFR) reaclors is in the product distribution of complex reactions. However, each particular set of reactions must be treated individually to find the superiority. For the consecutive reactions A B C, Fig. 7-5b shows that a higher peak value of B is reached in batch reactors than in CSTRs as the number of stages increases the batch performance is approached. 

Selectivity of parallel and consecutive reactions and of reac tions in a porous catalyst... 

Also from the examples shown in Fig. 5 (the transient case where no step is clearly rate determining) it is evident that the selectivity of the consecutive reaction A —> B —> C, as estimated from the curves, will be in... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

Saddle point. 170 Salt effects. 206-214 Scavenging (see Reactions, trapping) Second-order kinetics. 18-22, 24 in one component, 18-19 in two components (mixed), 19-22 Selectivity. 112 Sensitivity analysis. 118 Sensitivity factor, 239-240 Sequential reactions (see Consecutive reactions)... 

Select a short residence time reactor to avoid undesired consecutive reactions taking place with time. 

The selectivity of the gas phase amination is highly dependent on the type of zeolite as shown in Figure 11 for five zeolites all containing 3 wt % of Cu. When the zeolite becomes more spacious (L, Beta) the consecutive reaction increases which is understandable. Over Cu-Y reduction predominates. 

The benefits refer to the ability to achieve defined thin, highly porous coatings in micro reactors. In combination with the small length scales of the channels, diffusion to the active sites is facilitated. The residence time can be controlled, accurately minimizing consecutive reactions which may reduce selectivity. 

A selectivity-conversion plot for the product, the ring-substituted by-product and other, so far unidentified, by-products is given in [6] (Figure 5.22). While the content of the ring-substituted product decreased with increasing conversion (12-5%), the other by-products were formed in much larger amount (8-29%). This also indicates that the other by-products are formed by consecutive reactions at longer residence times. 

The motivation of an industrial development was to increase selectivity for monochlorination of acetic acid to give chloroacetic acid [57]. This product is amenable under suitable reaction conditions by further chlorination to give dichloroacetic acid by consecutive reaction. The removal of this impurity is not simple, but rather demands laborious and costly separation. Either crystallization has to be performed with high technical expenditure or an expensive hydrogen reduction at a Pd catalyst is needed. 

From product distribution analysis it could be concluded that larger particles present higher selectivity to glycerate due to the reduction of consecutive reaction, i.e. oxidation of glycerate to tartronate, remaining glycolate amount being almost stable. 

In an analoguous case, two-phase telomerization of butadiene with ammonia to give octadienylamine has been reported where higher selectivity is realized in a two-phase system of water-toluene. Here, octadienylamine is more reactive than ammonia and consecutive reaction leads to sec and ten amines. By adopting a two-phase strategy, a primary amine selectivity as high as 91 % has been realized (Drieben-Hoscher and Keim, 1998). 

Hydrogenations involving consecutive reactions are common in the organic process industry and even in the hydrogenation of fats. In the fine chemicals industry we have examples of acetylenic (triple) bonds to be selectively converted to olefinic (double) bonds. Lange et al. (1998) have shown, for the comversion of the model substance 2-hexyne into cis-2-hexene, how catalytically active microporous thin-film membranes can accomplish 100% selectivity. This unusual selectivity is attributed to avoidance of backmixing. 

Consecutive reactions, Ei = E2. The selectivity is temperature-independent. The highest allowable temperature should be applied to maximize reactor productivity. 

Consecutive reactions, E < Ei. The selectivity of the desired product decreases with temperature. However, a low temperature disfavours the reaction rate. A nonuniform temperature-time profile should be applied to maximize reactor productivity (see Fig. 5.4-72). At the start, no desired product is present in the reaction mixture. The temperature should then be as high as possible to keep the rate of P formation high. During the course of reaction, the amount of P in the reaction mixture increases. Therefore, the temperature should be lowered to minimize the rate of formation of the unwanted product from the desired product. 

The selectivity in a system of parallel reactions does not depend much on the catalyst size if effective diffusivities of reactants, intermediates, and products are similar. The same applies to consecutive reactions with the product desired being the final product in the series. In contrast with this, for consecutive reactions in which the intermediate is the desired product, the selectivity much depends on the catalyst size. This was proven by Edvinsson and Cybulski (1994, 1995) for. selective hydrogenations and also by Colen et al. (1988) for the hydrogenation of unsaturated fats. Diffusion limitations can also affect catalyst deactivation. Poisoning by deposition of impurities in the feed is usually slower for larger particles. However, if carbonaceous depositions are formed on the catalyst internal surface, ageing might not depend very much on the catalyst size. 

MIBK is a valuable industrial solvent used primarily in the paint and coating industry and in metallurgical extraction processes. It is also used as a precursor in the production of specialty chemicals such as pesticides, rubber anti-oxidants as well as antibiotics and pharmaceuticals (1). Historically, MIBK has been produced commercially from acetone and hydrogen feedstock in three stages. First, acetone is dimerized to produce diacetone alcohol (DAA). Second, DAA is dehydrated to produce MO and water. Finally, the carbon-carbon double bond of MO is selectively hydrogenated to produce MIBK. These consecutive reactions are outlined in equations (1-3). 

The results of Figure 1 may in part be attributed to a consecutive reaction on 3-MA, the contribution of which becomes relevant at high m-cresol conversion. However, the comparison between the distribution of products obtained when starting either from m-cresol (Figure 1) or from 3-MA (Figure 2) evidences some important differences, in particular the selectivity to polyalkylates. This considerably increases in m-cresol methylation at above 300°C, and can be attributed to the consecutive C-methylation occurring on DMPs and 3-MA. This reaction becomes the dominant one at high temperature. 

Selectivity parameters can be used to compare the catalytic performance of the different catalysts, and to find relationships between catalysts performance and physico-chemical features. Specifically, the following parameters were chosen (a) the O/C-methylation ratio, that is the ratio between the selectivity to 3-MA and that to 2,3-DMP+2,5-DMP+3,4-DMP (b) the ortho/para-C-alkylation ratio, that is the ratio between the selectivity to 2,3-DMP+2,5-DMP and the selectivity to 3,4-DMP (c) the 2,5-DMP/2,3-DMP selectivity ratio. Table 2 compares these parameters for MgO, Mg/Al/O and Mg/Fe/O catalysts. Data were reported at 30% m-cresol conversion, thus under conditions of negligible consecutive reactions. In this way it is possible to compare the ratio of the sole parallel... 

This interpretation is further confirmed by the results obtained in the liquid-phase methylation of m-cresol with Mg/Fe mixed oxides having different Fe contents Mgi.xFexOi+i/2x. In these catalysts the number of Lewis acid sites was proportional to the Fe content [4], Figure 4 plots the selectivity parameters and the number of Lewis acid sites as a function of the Mg/Fe ratio in catalysts (selectivity was calculated at very low m-cresol conversion, thus in the absence of any consecutive reaction). 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

In all cases, the high activity and selectivity of this zeolite as compared to other three-dimensional zeolites, such as beta or dealuminated Faujasite zeolite, is related to the easier and/or faster diffusion of the products and to minimization of undesired consecutive reactions. 

ITQ-21 presents excellent catalytic properties for the production of cumene, being more active and stable towards deactivation and presenting lower selectivity to NPB than a comparable beta zeolite. The benefits of ITQ-21 can be directly related to its open three-dimensional crystalline structure that favors diffusion of the products and minimizes undesired consecutive reactions. 

---