Solvent-Catalyst High-Pressure Synthesis

Solvent-Catalyst Reaction. The solvent-catalyst process was developed by General Electric Eind others. It establishes a reaction path with lower activation energy than that of the direct transformation. This permits a faster transformation under more benign conditions. As a result, solvent-catalyst synthesis is readily accomplished and is now a viable and successful industrial process. 

Not all carbon materials are suitable for the solvent-catalyst transformation. For instance, while graphitized pitch cokes form diamond readily, no transformation is observed with turbostratic carbon.i °i 

The solvent-catEdysts are the transition metals such as iron, cobalt, chromium, nickel, platinum, and palladium. These metal-solvents dissolve carbon extensively, break the bonds between groups of carbon atoms and between individual atoms, and transport the carbon to the growing diamond surface. 

The solvent action of nickel is shown in Fig. 12.2. When a nickel-graphite mixture is held at the temperature and pressure found in the cross-hatched area, the transformation graphite-diamond will occur. The calculated nickel-carbon phase diagram at 65 kbar is shown in Fig. 12.3. Other elemental solvents are iron and cobalt.i i However, the most common 

The hydraulic process is currently producing commercial diamonds up to 6 mm, weighing 2 carats (0.4 g) in hydraulic presses such as the one shown in Fig. 12.5. Micron-size crystals are produced in a few minutes producing a two-carat crystal may take several weeks. Typical crystals are shown in Fig. 12.6. Even larger crystals, up to 17 mm, have recently been announced by de Beers in South Africa and others. Research in high-pressure synthesis is continuing unabated in an effort to lower production costs and produce still-larger crystals. 

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The synthesis of lineatin, a pheromone of the insect Trypodendron lineatum, involves the enantioselective addition of methane to the double bond of alkene A in the presence of a chiral rhenium catalyst on silica (Re /silica see Figure 2.19). Michael performed eight separate experiments for this reaction in a 500 mL high-pressure stirred autoclave reactor using liquid methane as a solvent and 0.05 mmol catalyst. The results are shown in Table 2.2. 

Subsequently, Posner published the completely regioselective and highly stereoselective cyclo additions of racemic 3-(p-tolylsulfinyl)-2-pyrone (141) (Scheme 70) with 1,1-dimethoxyethylene [133],vinylether,and vinylthioethers [134]. With the first dienophile, the best diastereoselectivity (an 88 12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h). A 10 1 endo/exo mixture of cycloadducts was obtained with vinyl-ether in the presence of ZnBr2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures. The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives. Although enantio-merically pure samples of compound 141 could be obtained [134] it was not used as a starting material for asymmetric Diels-Alder reactions (the low yield of (S)-141 precluded this). 

Limited supply, increasing demand, and high cost have led to an intense search for an alternative, dependable source of diamond. This search led to the high pressure (ca 5 GPa (0.5 x 106 psi)), high temperature (ca 1500°C) (HP—HT) synthesis of diamond from graphite in the mid-1950s (153—155) in the presence of a catalyst—solvent material, eg, Ni or Fe, and the subsequent development of polycrystalline sintered diamond tools in the late 1960s (156). 

Among the most attractive tasks for a plasma chemist is the possibility of preparation of diamond, a high temperature and high pressure modification of carbon. The conventional synthesis of diamond is performed in its stability region at a pressure of several ten kilobars and several thousand degrees Kelvin, using suitable solvent catalysts in order to overcome the kinetic barrier for the transition from sp to sp hybridization of C-atoms. Without a catalyst the graphite diamond transition occurs only at pressures of several hundred kilobars... 

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