Supercritical ammonia synthesis

George et al. studied supercritical ammonia synthesis on four alkaline, silver, Sb sulfide catalysts including MAg2SbS4, M2AgSbS4 (M K, Rb), and characterized their structures. 

Kolis et al. reported the synthesis of some metal sulfide salts of homolep-tic lanthanide ammine complexes using supercritical ammonia as a reaction medium (Scheme 12) [49]. They proposed that these reactions proceed via a... 

Scheme 12 Synthesis of metal polysulfido complexes using supercritical ammonia as a solvent...

SCFs will find applications in high cost areas such as fine chemical production. Having said that, marketing can also be an issue. For example, whilst decaffeina-tion of coffee with dichloromethane is possible, the use of scCC>2 can be said to be natural Industrial applications of SCFs have been around for a long time. Decaffeination of coffee is perhaps the use that is best known [16], but of course the Born-Haber process for ammonia synthesis operates under supercritical conditions as does low density polyethylene (LDPE) synthesis which is carried out in supercritical ethene [17]. 

Potentially, supercritical carbon dioxide (SCCO2) is the ideal green solvent. It is non-toxic for both humans and the environment. It is chemically inert under most conditions, whether they be non-flammable or non-protic, and it is inert to radical and oxidizing conditions. This gas can be obtained in large quantities as a by-product of fermentation, combustion, and ammonia synthesis and it is relatively cheap, particularly compared with conventional solvents. Supercritical carbon dioxide presents other practical advantages as well, such as the possibility of achieving product isolation to total dryness by simple evaporation. 

It should be noted that on an industrial scale, reactions or other processes in SCF media are not new. Many industrial reactions developed in the early part of the twentieth century are actually conducted under supercritical conditions of either their product or reagent including ammonia synthesis (BASF, 1913), methanol synthesis (BASF, 1923) and ethylene polymerization (ICI, 1937). 

Other supercritical fluids can be used for chemical reactions, such as supercritical ammonia in the synthesis of labeled guanidines. ... 

The initial alcohol/amine ratio can determine the product distribution. In the synthesis of primary amines a rather high ammonia/alcohol molar ratio (up to 10-25), and usually high pressure, are required to compensate for the low reactivity of ammonia and suppress the formation of secondary amines. Selectivity for primary diamines could be improved in the amination of 1,3-dihydroxy compounds when using supercritical ammonia as solvent and reactant in a continuous fixed-bed reactor [12]. The remarkable changes in selectivity in the near-critical region (100-110 bar) are attributed to the increased concentration of ammonia on the metal surface as a result of elimination of mass-transport limitations in the two-phase system, and to suppression of hydrogenolysis and water elimination reactions which lead to monofunctional by-products. An example is shown in Figure 1. 

There are few reports of successful one-step synthesis of primary diamines, and the examples are limited to amines with a special structure. Amination of 1,4-cy-clohexanediol in supercritical ammonia (135 bar) over a Co-Fe catalyst alforded 67 % 1,4-diaminocyclohexane [21]. Excess ammonia, as both supercritical solvent and reactant, and short contact time in the continuous fixed-bed reactor favored the desired reactions. In the best example the cumulative selectivity for the diamine and the intermediate amino alcohol was 97 % at 76 % conversion. Recycling of the unreacted diol and amino alcohol can provide an alternative to the eurrent process, the hydrogenation of pnra-phenylenediamine. The high seleetivity was because of the rigid structure and the relative positions of OH functionality in the substrate. For comparison, amination of 1,4-butanediol under similar conditions yielded pyiTolidine as the major product 1,4-diaminobutane was barely detectable. When 1,3-cyclohexanediol was aminated with the same catalyst in the continuous system, the yield of 1,3-diaminoeyclohexane dropped below 5%, mainly because elimination of water led to undesired monofunctional products via a,/9-unsaturated alcohol, ketone, and/or amine intermediates [22]. 

In the following sections some aspects of (potential) applications of sc-fluids in the fine chemical industry with respect to product separation/purification and catalytic reactions are discussed. Earlier industrial applications of supercritical fluid reactions, for example the Haber-Bosch process for the synthesis of ammonia, synthesis of methanol from hydrogen and carbon monoxide, or the polymerization of ethene will not be discussed. An extensive overview on the use of sc-fluids in the synthesis of bulk chemicals is given in the book edited by fessop and Leitner [12],... 

Modem plants for ammonia production by the Haber-Bosch process [145] operate at 100-350 bar and 4()0-530°C, well above both Tc and Pc for the reaction mixture. Most notably, the largest plants use the highest pressures because this allows a greater throughput per unit volume of vessel. llie actual ammonia synthesis step may not be Ae only part of the plant that operates at supercritic conditions. The production of H2 in the Shell and Texaco ammo-... 

Example 10.19 Discharge of supercritical ammonia and subsequent dispersion Ammonia is employed in a process for NH3 synthesis at pi = 221 bar and Ti = 573.15 K. A leak of cross sectional area Fl = 490 mm opens, but can be isolated after 10 min. The jet emanating from the leak is supposed to have a length of 30 m in the horizontal direction. At which maximum distance from the point of release do we reach a maximum concentration of 150 ppm (ERPG-2 value, vid. Table 2.35) ... 

In the case of the ammonia synthesis, the enthalpy of ammonia decreases since attractive forces dominate, whereas for both highly supercritical compounds N2 and H2, the forces are mainly of repulsive nature. 

Zeuneretal. [74] applied the ammonothermal technique to synthesizeM2Si5Ng Eu (M = Ca, Sr, Ba) phosphors. In their synthesis, metal amides M(NH2)2 (M = Eu, Ca, Sr, Ba) were first prepared by dissolution of the respective metals in supercritical ammonia at 150 °C and 300 bar. Then the target M2Si5Ng Eu phosphors were synthesized by reacting metal amides with silicon diimide at 1150-1400 °C (Eq. 16.2.12). 

A Fischer, T Mallat, A Baiker. Synthesis of 1,4-diaminocyclohexane in supercritical ammonia. J Catal 182 289-291, 1999. 

The single largest use of ammonia is its direct apphcation as fertdizer, and in the manufacture of ammonium fertilizers that have increased world food production dramatically. Such ammonia-based fertilizers are now the primary source of nitrogen in farm soils. Ammonia also is used in the manufacture of nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammonium salts. Liquid ammonia is used as a solvent for many inorganic reactions in non-aqueous phase. Other apphcations include synthesis of amines and imines as a fluid for supercritical fluid extraction and chromatography and as a reference standard in i N-NMR. 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

The problems of properly characterizing the compounds from ammonia solutions caused a major hiatus in the exploration of the cluster anions. However, in 1970 Kummer and Diehl reported that liquid ammonia may be replaced by more easily handled and considerably more stable polyamines, most conveniently by tetraethy-lenediamine (en). Extraction of sodium-tin alloys in en and subsequent precipitation with THE or monoglyme yields the reasonably stable compound (Na+)4(Sn9 ) 6-8en. Kummer and Diehl pointed out the analogy between Sn9" and the previously characterized cluster cation Bi9 , and a partial structural characterization was reported for (Na+)4(Sn9 ) 7 en. However, in terms of the synthesis of well-defined, stable cluster compounds, the route pioneered by Kummer and Diehl left a lot to be desired. Nevertheless, supercritical amines have recently been found to be good reaction media for the synthesis of extended chal-cogenide structures. 

Only limited work has been reported on microemulsion-mediated synthesis of aluminum hydroxide [44,45]. In the two publications available [44,45], AOT served as the surfactant. It is possible to form reverse micelles in supercritical fluid media [130], and Matson et al. [44] used such a medium and the microemulsion-plus-reactant technique to synthesize A1(0H)3 particles at 110°C. With supercritical propane as the continuous phase, anhydrous ammonia was injected into the reversed micellar solution containing solubilized Al + [as an aqueous A1(N03)3 solution]. Referring to Fig. 1 and Table 2, the resulting precipitation process followed reaction path AP3 the added ammonia reacted with water molecules in the aqueous pseudophase of the microemulsion to generate hydroxide ions ... 

Elucidation of solvation characteristics of supercritical fluids is indispensable to their utilization as media for separation or reaction. One powerful method for elucidating chemical equilibrium and solvation in SCFs is voltammetry. However, voltammetric measurement in pure supercritical CO2 is extremely difficult because CO2 is nonpolar. Electrochemical processes in several polar SCFs including acetonitrile, ammonia, and sulfur dioxide, were investigated by Bard and coworkers in the late 1980s (61-65). Dombro et al. reported the electrochemical synthesis of dimethyl carbonate from carbon monoxide and methanol in super-... 

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