Simultaneous reactions selectivity

The Diels Alder reactions of maleic anhydride with 1,3-cyclohexadiene, as well the parallel reaction network in which maleic anhydride competes to react simultaneously with isoprene and 1,3-cyclohexadiene [84], were also investigated in subcritical propane under the above reaction conditions (80 °C and 90-152 bar). The reaction selectivities of the parallel Diels-Alder reaction network diverged from those of the independent reactions as the reaction pressure decreased. In contrast, the same selectivities were obtained in both parallel and independent reactions carried out in conventional solvents (hexane, ethyl acetate, chloroform) [84]. 

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

When the effectiveness factors for both reactions approach unity, the selectivity for two independent simultaneous reactions is the ratio of the two intrinsic reaction-rate constants. However, at low values of both effectiveness factors, the selectivity of a porous catalyst may be greater than or less than that for a plane-catalyst surface. For a porous spherical catalyst at large values of the Thiele modulus s, the effectiveness factor becomes inversely proportional to (j>S9 as indicated by equation 12.3.68. In this situation, equation 12.3.133 becomes... 

It was shown in the preceding text that even in the simplest systems many different chemisorbed particles originate on the surface during the catalytic reaction. In principle most of them can interact with each other and probably with gaseous reaction components as well. As a consequence, any catalytic reaction represents a system of simultaneous reactions, and the problem is how to influence the course of a particular reaction—in other words, it is essentially the selectivity problem. Thus in catalysis by metals, probably the modification of the surface properties (by forming the alloys, stable surface complexes, or by the addition of promotors, etc.) seems to be the most promising direction of the further fundamental research. 

The full advantage of the chromatographic reactor (simultaneous reaction and separation) is fully realized only for selected types of reactions, which are briefly summarized below [132]. 

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

Imposing oscillations in the feed concentrations for a continuous back-mixed reactor can also result in beneficial changes of reaction selectivity [58]. Such changes are likely to be more significant with intermediates in consecutive reactions than with products from simultaneous reactions in parallel [59]. 

Obviously, reaction selectivity will depend on temperature. For isothermal operation, the temperature can be chosen to maximise the yield of desirable product. For example, consider the simultaneous parallel reactions (12). If the rate coefficients conform to the Arrhenius expression, e.g. 

Since all the sink terms, initial distribution and one-reactant densities, n, are of the same form for all reactants, the double summation can be replaced by N N — 1)12. To preserve the independence of the reactants, the sink term. )k must be strongly localised to minimise the chance of simultaneous reaction paths being available and the form 5( r - — th = R) is suitable. Consequently, selecting the reactants 1 and 2 as representative of the reaction, the double integral is... 

Influence on Selectivity for Simultaneous Reactions Suppose the two competing reactions are... 

Fig. 7. Influence of segregation on selectivity for simultaneous reactions along solid lines, no influence +, segregation favors reaction 1 —, segregation favors reaction 2.

The term selectivity S is used to describe the relative rates of two or more competing reactions on a catalyst. Such competition includes cases of different reactants undergoing simultaneous reactions or of a single reactant taking part in two or more reactions. For the latter case, S may be defined in two ways. The first of these defines a fractional selectivity SF for each... 

Since is proportional to VF we may conclude that for competing simultaneous reactions strongly influenced by diffusion effects (where is large and tanh — 1) the selectivity depends on Vs (where S = kjk2), the square root of the ratio of the respective rate constants. The corollary is that, for such reactions, maximum selectivity is displayed by small sized particles and, in the limit, if the particle size is sufficiently small (small so that tanh - )the selectivity is the same as for a non-porous particle, i.e. S itself. 

Reactive extraction processes involve simultaneous reaction and liquid-liquid phase separation and can be effectively utilized to obtain significant improvements in yields of desired products and selectivities to desired products in multireaction systems, thereby reducing recycle flows and waste formation. The combination of... 

In general, the temperature of the reactor is established in a number of different ways that depend very strongly on the chemistry and the kinetics. For the simple irreversible reaction studied in this section, in which the only issue is to achieve the desired conversion, it would appear that the reactor temperature should be made as high as possible. This would give the largest specific reaction rate and therefore the smallest reactor size, thus minimizing capital investment. However, as we show below, there are dynamic controllability considerations that must be factored in when selecting reactor temperature. For the more complex reactions considered in later sections (such as reversible, consecutive, or simultaneous reactions), in which issues of both conversion and yield are important, the selection of reactor temperature must consider the production of undesirable products as well as reactant conversion. 

Selectivity is a concept that applies to processes with multiple simultaneous reactions. It is used to quantify the relative rates of the individual reactions. However, any discussion about multiple reactions and the analysis of these is beyond the scope of this chapter. Refer to the further reading material identified at the end of this chapter for more information. 

The formation probability of products in a system of coupled reactions can be calculated from the simultaneous equilibria 3. 36]. For complex systems like the Fischer-Tropsch synthesis, however, a simplillcation has to be made by the assumption that the reactions selected are independent of each other. 

Type I selectivity is that occurring when two simultaneous reactions are taking place.33 An example of this type is the hydrogenation of the C4 acetylenes... 

The addition of HCN to aldehydes has been a well-known reaction since the 19th century, especially in the context of the Kiliani-Fischer synthesis of sugars. Even older is the Strecker synthesis of amino acids by simultaneous reaction of aldehydes with ammonia and HCN followed by hydrolysis. The challenge in recent years has been to achieve face-selectivity in the addition to chiral aldehydes. These face-selective additions, known as nonchelation-controlled processes, refer to the original formulation of Cram s for the reaction of nucleophiles with acyclic chi carbonyl compounds. The chelation-controlled reactions refer also to a formulaticxi of Cram s, but whose stereochemical consequences sometimes differ. 2... 

The point selectivity, given by Eq. (2-91) for first-order simultaneous reactions, is also equal to kjk. Although point and overall selectivities are identical for this type of first-order system, the two selectivities differ for most complex reactions. 

This means that the yield of B and the overall selectivity will vary with time. This is in contrast to the result for simultaneous reactions, Eq. (2-100). ... 

Using succinate chitosan form very much improved the reaction selectivity the maximum of selectivity to cis-I,4-butenediol was achieved over catalyst N°2. But in this case, the reaction was not terminated at the stage of 1,4-butenediol formation. The rate of further hydrogenation of 1,4-butenediol (second stage) was rather high. Introducing Pb in chitosan completely suppressed the further 1,4-butenediol hydrogenation. The reaction was spontaneously finished over catalyst N°3 after consumption of 1 mole of Ha. Simultaneously, the selectivity with respect to cis-1,4-butenediol was improved by that chitosan modification. 

These are all devices which reduce both the product concentration and residence time in the gas phase and so minimize product losses through subsequent reactions. Methods (1), (2), and (3) are means of controlling selectively the residence time and concentrations of certain specific products in the discharge and a choice can only be made between them in the light of the physical properties of the various constituents of the gas phase. The fourth method, which does not discriminate between the concentrations or residence times of reactants and products, can only be evaluated when the relative rates of the competing reactions are known. Of the various techniques available, simultaneous reaction and absorption in a heterogeneous reactor (Method 2) opens up interesting... 

CDTech uses catalytic distillation to convert isobutene and methanol to MTBE, where the simultaneous reaction and fractionation of MTBE reactants and products takes place [51], A block diagram of this process is shown in Figure 3.31. The C4 feed from catalytic crackers undergoes fractionation to extract deleterious nitrogen compounds. It is then mixed with methanol in a BP reactor where 90% of the equilibrium reaction takes place. The reactor effluent is fed to the catalytic distillation (CD) tower where an overall isobutene conversion of 97% is achieved. The catalyst used is a conventional ion-exchange resin. This process selectively removes MTBE from the product to overcome the chemical equilibrium limitation of the reversible reaction. The MTBE product stream is further fiactionated to remove pentanes, which are sent to gasoline blending, whereas the raffinate from the catalytic distillation tower is washed with water and then fractionated to recover the methanol. 

Benzyl alcohols have been resolved in parallel kinetic resolutions that use two reagents with selectivities for opposite isomers to make carbonates in simultaneous reactions (10.27).55... 

One of the best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider the following two simultaneous reactions. One reaction produces the desired product D... 

It is important to realize, that within a certain lipid class there is still a very large number of individual lipid molecules, which can be formed by a great variety of combinations of two fatty acids differing in their chain length (carbon number), in unsaturation (number of C=C bonds) and in their position on the glycerol backbone (Cl or C2). For simplicity, fatty acids are often denoted by the number of carbon atoms number of C=C bonds, e.g. 18 0 for stearic, 18 1 for oleic/elaidic, 18 2 for linoleic, 18 3 for linolenic acids, etc. Any chemical manipulation on lipids of natural origin usually involves simultaneous reactions of quite a few similar but not identical substrate molecules. On the other hand, individual lipids may have specific biological role which -at least in principle- could be assessed by a selective chemical transformation or removal of a lipid of particular composition. 

The anionic polymerisation of PO is in fact a competition of two simultaneous reactions the propagation reaction (R ) and the transfer reaction (Rtr). An interesting way to obtain it directly from the synthesis of low unsaturated polyether polyols, is to accelerate selectively the propagation reaction while the transfer reaction remains unchanged or lower. It is well known that low unsaturation of polyether polyols represents a low monol content and leads to better physico-mechanical properties in the resulting PU, because in the PU chemistry the monol (a monofunctional compound) is a chain stopper, (i.e., it stops the MW increase). 

Polycyclic arene(tricarbonyl)chromium complexes.h These complexes arc best prepared by treatment of polycyclic arcnes with (NHj),CT(CO)37 and BFj ethcratc. As in complcxation with Cr(CO), the terminal or most aromatic ring is complexed selectively. However, the lower temperatures used in the newer method are advantageous with thermally labile polycyclic arcnes. These complexes are useful for substitution reactions at positions that arc not available by electrophilic substitution of the arenc directly. One such reaction is hydroxylation effected by simultaneous reaction with a base (BuLi or I. DA) and tributoxyborane (excess) followed by H2O2/HOAC workup. Rcgiosclectivc silylation is effected by reaction of the complex with LiTMP and (CHj SiCI with... 

The classical kinetic analytical methods [1-3] are mainly appUed in two versions (1) kinetic catalytic method based on catalytic reactions and (2) kinetic differential method based on the use of systems with simultaneous reactions of a reagent with several mixture components with similar properties. These versions are recommended for enzyme reactions with a view to determining enzymes, inhibitors and substrates. These reactions are highly sensitive and specific their use is without any doubt of particular interest for some systems for the selective and highly sensitive determination of some components of systems [3] to which GC can be applied. 

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