Recycling platinum

Just as the hydrosilylation reaction is important in the synthesis of silanes, so it also serves as a useful route to functional polysiloxanes. The same variety of chemistry as previously described for silanes is available and two further examples are given in equations 82 and 83. One of the main advantages of this route is that the silicon hydride prepolymers are well characterized and readily available materials, which in turn leads to well-defined organofunctional products. A disadvantage is the low efficiency in the use of the precious metal catalyst and the difficulty of its recovery. This is particularly true for systems containing a low level of functionality but this can be partially overcome by using recyclable platinum catalysts on solid supports216,217. 

Using an alternative to Speier s catalyst, Chauhan and coworkers reported that recyclable platinum nanoclusters can function as catalysts in PBD hydrosilylation (Fig. 14). These nanoclusters, which were prepared via reduction of Me2Pt(COD) and recovered after the reaction by centrifugation, showed consistent activity up to five cycles of consecutive hydro-silylations. Complete conversion of 1,2-PBD was achieved with a variety of silane structures and yielded the hydrosilylation product via anti-Markovnikov addition at the terminal positions of 1,2-butadiene units. The retention of a narrow molecular weight (M /M =. 4—. 5) in the GPC analysis confirmed that no chain scission or cross-finking occurred in the polymer chains during hydrosilylation. 

Figure 14 Selective anti-Markovnikov hydrosilylation of 1,2-PBD with a variety of silane structures using recyclable platinum nanoclusters.

The catalyst is then transferred back to the first process reactor and is reheated to the reforming process temperature at the reactor inlet using a flow of hydrogen-rich process recycle gas, thereby achieving reduction of the platinum to a catalyticaUy active state. 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

In one patent (31), a filtered, heated mixture of air, methane, and ammonia ia a volume ratio of 5 1 1 was passed over a 90% platinum—10% rhodium gauze catalyst at 200 kPa (2 atm). The unreacted ammonia was absorbed from the off-gas ia a phosphate solution that was subsequently stripped and refined to 90% ammonia—10% water and recycled to the converter. The yield of hydrogen cyanide from ammonia was about 80%. On the basis of these data, the converter off-gas mol % composition can be estimated nitrogen, 49.9% water, 21.7% hydrogen, 13.5% hydrogen cyanide, 8.1% carbon monoxide, 3.7% carbon dioxide, 0.2% methane, 0.6% and ammonia, 2.3%. 

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

Precious metals reclamation Precious metals reclamation is the recycling and recovery of precious metals (i.e., gold, silver, platinum, palladium, iridium, osmium, rhodium, and ruthenium) from hazardous waste. Because U.S. EPA found that these materials will be handled protectively as valuable commodities with significant economic value, generators, transporters, and storers of such recyclable materials are subject to reduced requirements. 

Figure 1.7 Catalyst recycling in the hydrosilylation reaction under inert (N2) conditions with several platinum precursors, [K2PtCI4] (black), [PtCI2(C6Hno)]2 (dark grey), [PtCI2(PPh3)2] (white), and [Pt(PPh3)4]...

Recently, Dupont and coworkers described the use of room-temperature imi-dazolium ionic liquids for the formation and stabilization of transition-metal nanoparticles. The potential interest in the use of ionic liquids is to promote a bi-phasic organic-organic catalytic system for a recycling process. The mixture forms a two-phase system consisting of a lower phase which contains the nanocatalyst in the ionic liquid, and an upper phase which contains the organic products. Rhodium and iridium [105], platinum [73] or ruthenium [74] nanoparticles were prepared from various salts or organometallic precursors in dry 1-bu-tyl-3-methylimidazolium hexafluorophosphate (BMI PF6) ionic liquid under hydrogen pressure (4 bar) at 75 °C. Nanoparticles with a mean diameter of 2-3 nm... 

Although several noble-metal nanoparticles have been investigated for the enantiomeric catalysis of prochiral substrates, platinum colloids remain the most widely studied. PVP-stabilized platinum modified with cinchonidine showed ee-values >95%. Several stabilizers have been also investigated such as surfactants, cinchonidinium salts and solvents, and promising ee-values have been observed. Details of a comparison of various catalytic systems are listed in Table 9.16 in one case, the colloid suspension was reused without any loss in enantioselectiv-ity. Clearly, the development of convenient two-phase liquid-liquid systems for the recycling of chiral colloids remains a future challenge. 

Earle, M. J., McCormac, P. B. Seddon, K. R. Diels-Alder reactions in ionic liquids a safe recyclable green alternative to lithium perchlorate-diethyl ether mixtures. Green Chem., 1999, 1(1), 23-25 Doherty, S. Goodrich, P. Hardacre, C. et al. Marked enantioselectivity enhancements for Diels-Alder reactions in ionic liquids catalysed by platinum diphosphine complexes. Green Chem., 2004, 6(1), 63-67. 

In addition to the presence of these elements in ores, they are also available from recycled feeds, such as catalyst wastes, and as an intermediate bulk palladium platinum product from some refineries. The processes that have been devised to separate these elements rely on two general routes selective extraction with different reagents or coextraction of the elements followed by selective stripping. To understand these alternatives, it is necessary to consider the basic solution chemistry of these elements. The two common oxidation states and stereochemistries are square planar palladium(II) and octahedral platinum(IV). Of these, palladium(II) has the faster substitution kinetics, with platinum(IV) virtually inert. However even for palladium, substitution is much slower than for the base metals so long as contact times are required to achieve extraction equilibrium. 

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