Nitration solid acid catalyzed

FIGURE 2.2 Basic features of solid acid catalyzed toliiene nitration. 

The solid superacidic Nafion-Hhas also been found to catalyze effectively nitration reactions with various reagents.503 The nitrating agents employed were n-butyl nitrate, acetone cyanohydrin nitrate, and fuming nitric acid. In nitric acid nitrations, sulfuric acid can be substituted by Nafion-H and the water formed is azeotropically removed during the reaction (azeotropic nitration).503... 

These solid-acid catalysts are, in principle, applicable to a plethora of acid-catalyzed processes in organic synthesis [18]. These include various electrophilic aromatic substitutions, e.g. nitrations, halogenations, and Fiiedel-Crafts alkylations and acylations, and numerous rearrangement reactions such as the Beckmann and Fries rearrangements. Other examples include a variety of cyclization reactions such as Diels-Alder reactions and the synthesis of pyridines and other heterocycles. 

Nitration with nitric acid in the presence of strong protic acids such as H2SO4, FSO3H, and CF3SO3H or Lewis acids such as boron trifluotide requires subsequent separation of spent acid (due to water formed in the reaction) and neutralization of acid left in the product. One is generally left with a large amount of dilute acid for disposal, which is neutralized in the case of sulfuric-acid-catalyzed nitrations to a mixture of ammonium nitrate and ammonium sulfate. By using a solid acid catalyst most of these environmental problems can be eliminated. The solid acid catalyst is simply separated and recycled for subsequent use. 

Because of the advantages of using solid superacidic catalysts in electrophilic aromatic nitration and in acid-catalyzed reactions in general, Olah et al. have examined the mercury (Il)-promoted azeotropic nitration of aromatics using Nafion-H solid superacidic catalyst [142]. Azeotropic removal of water accelerates the rate of reaction by mitigating the dilution of nitric acid in a static reaction system. The yield of nitroaromatics varies from 48-77% (Table XXVIII). 

Many acid-catalyzed reactions can be advantageously carried out using solid superacids instead of conventional acid systems. The reactions can be carried out in either the gaseous or the liquid phase. Using the example Nafion-H (a perfluoroalkane resin sulfonic acid, developed by DuPont) solid acid, several simple procedures were reported to carry out alkylation, transbromination, nitration, acetalization, hydration, and so on. 

Some nitric acid is used for the manufacture of explosives and chemicals, but much is converted on-site to the potentially explosive high nitrogen fertilizer ammonium nitrate (Section 2.11). Ammonia gas from the Haber plant is absorbed in aqueous HN03, and the NH4N03 solution is evaporated to a liquid melt (< 8% H20) for crystallization, but care must be taken to keep the pH of the solution above about 4.5 and to exclude any material (chlorides, organic compounds, metals) that might catalyze the explosive decomposition of NH4N03. It is also wise to keep the melt mass low and to vent it to avoid pressure buildup. The solid product should be stored well away from the main plant. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

A distinction must be made between chemical and physical stability. While physical stability is important, particularly in the evaluation of solid propellants, the chemical stability is of prime importance in the estimation of the course of decomposition of nitrate esters. The nitrate esters which are processed for use as propellants - unlike nitro compounds, which are relatively stable under these conditions - undergo a steady decomposition, which is due to imperfect purification of the starting materials and to the effect of other parameters such as temperature and air humidity. The rate of this decomposition is auto-catalyzed by the acidic decomposition products and may in certain cases produce spontaneous ignition. In order to reduce the decomposition rate as much as possible, suitable stabilizers are added to the powders, which are capable of accepting the acid cleavage products with formation of the corresponding nitro compounds (- Stabilizers). The stability is controlled by means of several tests (- Hot Storage Tests). 

The nitration of toluene with nitric acid and acetic anhydride with zeolite catalysts was studied by means of multi-nuclear solid-state NMR spectroscopy in order to explain the enhanced / ara-selectivity observed with zeolite beta. The reversible transformation of framework aluminum from a tetrahedral into an octahedral environment was revealed by A1 NMR upon interaction of the zeolite with the different components of the nitrating system. The flexibility of the lattice seems to play an important role in the regio-selectivity of nitration catalyzed by zeolites. 

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