Secondary reactions, minimization

Reducing waste from multiple reactions producing waste byproducts. In addition to the losses described above for single reactions, multiple reaction systems lead to further waste through the formation of waste byproducts in secondary reactions. Let us briefly review from Chap. 2 what can be done to minimize byproduct formation. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Conditions must be controlled so as to minimize loss of the product by the secondary reaction ... 

In liquid phase reactions the base catalysed reaction may only proceed as far as its self-condensation to DAA with minimal dehydration to MO, however when operating in the gas phase at elevated temperatures, secondary reactions can take place (4). 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

The isomer distribution in the hydroxylation of phenol, anisole and toluene or other aromatics on TS-1 is influenced by the reaction conditions, but is characterized by a tendency towards p-selectivity [105-106]. Furthermore secondary reactions leading to polynuclear aromatic byproducts are minimized. Both phenomena are ascribed to the pore structure of the catalyst, which is isostructural to ZSM-5 [96]. The selectivity for hydroxylation as well as the H2O2 efficiency decrease with increasing conversions as is shown in Figure 14 for the hydroxylation of phenol [106]. 

With the FTIR spectroscopic method, these free radical reactions cannot be studied individually under completely isolated conditions since competing side reactions and also secondary reactions involving the molecular products must be taken into account. These mechanistic complications can be greatly reduced by appropriate selection of the method of free radical generation, as described in Section II.B. In general, to minimize the occurrence of secondary reactions, the conversion of the molecular reactants, and consequently the product yields, have to be kept as small as is permissible in order to obtain accurate concentration measurements. Also, the reaction time required for such chemical analysis must be kept as short as possible to minimize photochemical and heterogeneous losses of labile products. 

Reactions of atomic carbon, produced by nuclear reactions, with a number of hydrocarbons have been studied by Wolfgang and his collaborators (69). To minimize radiation induced secondary reactions which occur when use is made of C14, a technique has been developed using short-lived C11 produced by a neutron exchange reaction between a platinum foil and a C12 ion beam from a heavy ion accelerator. Part of the scattered Cu atoms has been allowed to penetrate through the thin brass foil wall of a brass vessel and come in contact with the compound wrhose reaction is studied. Products have been analyzed by gas chromatography using a technique of simultaneous mass and radioactivity determination. 

The mass spectrometric method has the advantage of permitting direct detection of initially formed radicals, since under the experimental conditions, secondary reactions are minimized. However, precise quantitative data are difficult to obtain, owing to the difficulty in controlling experimental parameters in this particular reaction system. 

The role of the secondary and bimolecular reactions is ambiguous. Obviously, the rate at which the network grows, depends on the number of surface chlorine groups after the trichlorosilylation. This number could be increased by minimizing the secondary species. In this point of view, secondary reactions are undesirable, since they have a restricting effect on the rate of the nitrogen increase. 

The thermal decomposition experiments performed by Solomon and co-workers (5,12-15) were done in a thin bed under vacuum. Under these conditions, the tar molecules may be removed quickly from the reacting bed and undergo minimal secondary reactions. Therefore, many of the coal structural elements are preserved in tar and careful analysis of these products can supply clues to the original structure. For example, the average molecular weight of the PSOC 170 tar was determined to be about 370 by VPO and 490 and 385 by GPC (16). 

During a typical batch electrolysis, the minimal COD value can be estimated (CODfe min = 4.25mmol dm-3 or 136ppm) by assuming a typical value of minimal hydroxyl production current density (iu,mm = 5.0mA cm-2), and a characteristics value of mass-transfer coefficient (km = 3 x 10 s m s ). The obtained minimal COD value is higher than the final treatment value that is usually required (CODf). Consequently, it can be stated that the electrochemical treatment loses a part of electric charge supplied in secondary reactions in this final step... 

In these experiments, the role of KX1 produced in the secondary reaction of X with K2 was minimized by raising the temperature in order to reduce the concentration of K2. 

For example, AMP, the product of the primary reaction, reaction (1), may undergo secondary reactions to form adenosine and phosphate or IMP and ammonia. Other secondary reactions (e.g., the degradation of ATP to ADP) could involve ATP. These secondary reactions are summarized in Figure 4.1 in the step marked Incubation. While secondary reactions can be eliminated or their significance minimized, they should not be overlooked in the analysis and design of the assay system. 

By adjusting reaction conditions to minimize activity of secondary reactions. 

Thus, good dispersion or mass transfer favors olefin isomerization (to isobutene), isobutene dimerization, and maximizes hydrogen transfer and primary alkylation reactions, i.e., yielding the greatest amount of high-octane-number trimethylpentanes, and minimizing low-octane-number byproducts from secondary reactions such os excess polymerization. 

It is reasonable to assume that the presence of a catalyst may accelerate the scission of given bonds, and thereby reduce the temperature requirement for degradation to the point at which secondary reactions leading to extensive cracking and polymerization are minimized. 

---