Interfacial nitration

At present, HpNC is significantly easier to make than ONC, which is an expensive explosive and also difficult to make. Research is now focused on finding an economical synthetic route and to make it directly by tetramerization of dinitro-acetylene (a compound not yet known). By exploiting the property of TNC (of highly acidic nature) and use of interfacial nitration, TNC is converted to PNC [261]. Acetylene (parent hydrocarbon of dinitroacetylene and a cheap starting material available in abundance) is acidic in nature and therefore, it is speculated that acetylene may be converted to dinitroacetylene by following the approach of conversion of TNC to PNC, followed by its tetramerization resulting in the formation of ONC. 

Many industrial processes involve a chemical reaction between two Hquid phases, for example nitration (qv), sulfonation (see Sulfonation and sulfation), alkylation (qv), and saponification. These processes are not always considered to be extractions because the main objective is a new chemical product, rather than separation (30). However these processes have many features in common with extraction, for example the need to maintain a high interfacial area with the aid of agitation and the importance of efficient phase separation after the reaction is completed. 

Most ionic nitrations are performed at 0—120°C. For nitrations of most aromatics, there are two Hquid phases an organic and an acid phase. Sufficient pressure, usually slightly above atmospheric, is provided to maintain the Hquid phases. A large interfacial area between the two phases is needed to expedite transfer of the reactants to the interface and of the products from the interface. The site of the main reactions is often at or close to the interface (2). To provide large interfacial areas, a mechanical agitator is frequently used. 

Rates of nitration determined over a range of temperatures in two-phase dispersions have been used to calculate energies of activation from 59—75 kj/mol (14—18 kcal/mol). Such energies of activation must be considered as only apparent, since the tme kinetic rate constants, NO2 concentrations, and interfacial area all change as temperature is increased. 

Increased agitation of a given acid—hydrocarbon dispersion results in an increase in interfacial areas owing to a decrease in the average diameter of the dispersed droplets. In addition, the diameters of the droplets also decrease to relatively low and nearly constant values as the volume % acid in the dispersions approaches either 0 or 100%. As the droplets decrease in si2e, the ease of separation of the two phases, following completion of nitration, also decreases. 

Reactions 8 and 9 are important steps for the Hquid-phase nitration of paraffins. The nitric oxide which is produced is oxidized with nitric acid to reform nitrogen dioxide, which continues the reaction. The process is compHcated by the presence of two Hquid phases consequentiy, the nitrogen oxides must transfer from one phase to another. A large interfacial area is needed between the two phases. 

Because the highest possible interfacial area is desired for the heterogeneous reaction mixture, advances have also been made in the techniques used for mixing the two reaction phases. Several jet impingement reactors have been developed that are especially suited for nitration reactions (14). The process boosts reaction rates and yields. It also reduces the formation of by-products such as mono-, di-, and trinitrophenol by 50%. First Chemical (Pascagoula, Mississippi) uses this process at its plant. Another technique is to atomize the reactant layers by pressure injection through an orifice nozzle into a reaction chamber (15). The technique uses pressures of typically 0.21—0.93 MPa (30—135 psi) and consistendy produces droplets less than 1 p.m in size. The process is economical to build and operate, is safe, and leads to a substantially pure product. 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

The vast majority of chemical reactions are sufficiently slow not to observe a dramatic influence of mixing on yields and selectivities. Exceptions are polymerizations, interfacial polycondensations, precipitations, and some fast reactions - usually performed in semibatch mode - such as autocatalytic reactions, neutralizations, nitrations, diazo couplings, brominations, iodinations, and alkaline hydrolysis, which are often encountered in the manufacture of fine chemicals. 

They recorded such a polarization curve for zinc, for copper in the presence of gelatin and for silver in nitrate solution. Under this mechanism, a negative fluctuation in concentration drives the current density up, resulting in further reduction in interfacial concentration. For this instability to be expressed, the surface concentration must be free to respond to variations in current. As a result, the instability is seen only far from the limiting current, where the interfacial concentration is pinned at zero. At high Peclet numbers, the concentration disturbance is propagated downstream by convection, and the striations follow the streamlines. 

The extraction of uranyl nitrate from 1 M aqueous solution into 30% tributylphosphate in oil is accompanied by an initial interfacial turbulence (41), with more transfer than calculated, even though re-solvation of each uranyl ion at the interface must be a relatively complex process. If the turbulence is suppressed with sorbitan mono-oleate, transfer proceeds at a rate in excellent agreement with theory. 

In liquid-liquid extraction using wetted-wall columns, analysis is possible only by dimensionless groups (75) for the core fluid, flowing up inside the tube, k varies as approximately D and for the fluid falling down the inner walls, varies as Systems studied include phenol-kerosene-water, acetic acid-methylisobutylketone-water, and uranyl nitrate between water and organic solvents (7S, 80-82) interfacial resistances of the order 100 sec.cm." are observed in the last system. These resistances are interpreted as being caused by a rather slow third-order interfacial exchange of of solvent molecules (S) coordinated about each UOa" ion ... 

Bancroft cites cases in which disintegration is effected more readily in mixed solvents than by either solvent alone such as cellulose nitrate in ether alcohol mixtures. The interfacial surface tensions of such mixtures do not appear to have been measured. 

Kinetics of Aromatic Nitrations. The kinetics of aromatic nitrations are functions of temperature, which affects the kinetic rate constant, and of the compositions of both the acid and hydrocaibon phase. In addition, a larger interifacial area between the two phases increases the rates of nitration since the main reactions occur at or near the interface. Larger interfacial areas are oblaincd by increased agitation and by ihc proper choice of the volumetric % acid in the liquid-liquid dispersion. The viscosities and densities of the two phases and the interfacial tension between the phases are important physical properties affecting the interfacial area. 

Enzyme micro-encapsulation is another alternative for sensor development, although in most cases preparation of the microcapsules may require extremely well-controlled conditions. Two procedures have usually been applied to microcapsule preparation, namely interfacial polymerization and liquid drying [80]. Polyamide, collodion (cellulose nitrate), ethylcellulose, cellulose acetate butyrate or silicone polymers have been employed for preparation of permanent micro capsules. One advantage of this method is the double specificity attributed to the presence of both the enzyme and the semipermeable membrane. It also allows the simultaneous immobilization of many enzymes in a single step, and the contact area between the substrate and the catalyst is large. However, the need for high protein concentration and the restriction to low molecular weight substrates are the important limitations to this approach. 

The extension of the interfacial area by emulsification explains Miyagawa s [95] observation that the nitration rate can be considerably increased by the action of ultrasonics on a reacting system. For example, nitration of m- xylene to trinitro-m- xylene, which generally takes 2 hr, takes only 30 min when ultrasonics are used. There is no evidence as yet whether and how ultrasonic waves effect group orientations. 

Insufficient mixing may easily result in a low nitration rate owing to the small interfacial area. It can also lead to a non-uniform nitration process. Owing to inadequate construction of the stirrer, too low a speed of rotation, or an interruption in stirring, so-called dead spaces may easily be formed in which non nitrated or not fully nitrated substances accumulate. If a rather large quantity of the mixture is stirred suddenly, rapid extension of the interfacial area takes place, followed by the generation of large amounts of heat and a rise in temperature. This may cause a spontaneous decomposition of the reaction mass in the nitrator, and then an explosion. 

This involves the use of tertiary amine extraction of the An ions from acidic 11 M LiCl solutions. Spectroscopic studies have indicated that, in the cases of Am and Nd at least, the octahedral trianionic hexachloro complexes are extracted from 11 M LiCl. Stability constant data for the chloride complexing of Am , and Cfin media of ionic strength 1,0 have been reported. Tertiary amines also extract Pu and a study of extraction from nitrate media by trilaurylamine (TLA) in xylene has been reported. " This showed that the mass transfer rate was controlled by the reactions between Pu from the bulk phase and interfacially adsorbed TLA-HNOs. The separation of individual transplutonium elements from the Tramex actinide product may be achieved using ion exchange or precipitation techniques." ... 

In the case of COa absorption, often used to determine interfacial parameters in gas-liquid reactors, Ratcliff and Holdcroft (R2) have found that the diffusivities in aqueous solutions of chlorides, nitrates, and sulfates of sodium and magnesium vary roughly as (rather than as in the Wilke and Chang formula). Nijsing et al. (N11) and Danckwerts and Alper (D7) found the diffusivity of COa in sulfates of sodium and magnesium, and in NaaCOa, to vary as However, all these results... 

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