Hydrogenation selective partial

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Scheme 89 Total synthesis of sophorolipid lactone 449 by sequential RCAM and -selective partial hydrogenation [186]...

Scheme 90 Fiirstner s total synthesis of epoC (237c) via sequential RCAM of diyne 450 and (Z)-selective partial hydrogenation [187]...

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

Another useful compound is the 1 2 telomer of malonate and butadiene, 137. The first example is the synthesis of pellitorine (138), a naturally occurring pesticide (126). The terminal double bond was hydrogenated selectively with RuCl2(PPh3)3 as a catalyst. Partial hydrolysis afforded the monoester, which was treated with PhSeSePh to displace one of the carboxyl group with phenylselenyl group. Oxidative removal of the phenylselenyl group afforded 2,4-decadienoate (139), which is converted to pellitorine (138) ... 

The selective partial hydrogenation of polyenes is interesting from both preparative and commercial points of view. Success depends on the nature of the polyene as well as on a careful choice of catalyst and conditions. 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

Another interesting reaction of importance is the selective partial hydrogenation of acetylene or of dienes on heterogeneous catalysts. The selective partial hydrogenation reaction will be established by one of the following three mechanisms ... 

Selective partial hydrogenation of acetylene or dienes caused by adsorption induced activation and deactivation of the surface. 

Selective partial hydrogenation of conjugated diene via alkylallyl anion intermediates. 

Industrial processes were developed for the selective partial hydrogenation of 4-vinylcyclohexene with Ni catalysts exhibiting minimized isomerization activity in the presence of additives298,299. For example, supported nickel arsenides prepared by reducing nickel arsenate with NaBFLt display high selectivity in the formation of 4-ethylcyclohexene (96% selectivity at 96% conversion on Ni-As-Al2C>3, 398 K, 25 atm H2, acetone additive). 

Mayer et al. [43] compared their results generated at a micro structured monolith (see Section 2.4.3) with literature data [137]. The degree of conversion and the hydrogen selectivity of the rhodium monolith outperformed both metal-coated foam monoliths, Pt-Rh gauzes and extruded monoliths. This was partially attributed to the higher activity of the rhodium monolith, but also to the lower cross-sectional channel area of the metallic monolith, which reduced mass transfer limitations, and to the improved heat conductivity. 

The solvated ion pair [(C8Hi7)3NMe] [RhCl4]", formed from aqueous rhodium trichloride and Aliquat-336 in a two-phase liquid system, hydrogenates a,p-unsaturated ketones and esters selectively at the C==C double bond. The reduction of benzylideneacetone follows first-order kinetics in substrate below 0.2 M, and approaches second-order in hydrogen at partial pressures below 0.12 atm (1 atm = 101.3 kPa). The catalysis also depends on the nature of the solvent, the phase transfer catalyst and stirring rates. 

The selective hydrogenation of a triple bond to give an alkene without concomitant positional or geometric isomerization is particularly important in synthetic procedures and many industrial processes. In the absence of any isomerization, selective partial hydrogenation of a disubstituted alkyne produces the cis alkene. Small amounts of the trans alkene are sometimes formed in these reactions, but catalytic processes do not lead to the production of the irans olefin as the primary product. The Irons alkenes can be produced as a primary product by metal-ammonia reduction of disubstituted alkynes.2... 

The selective partial hydrogenation of symmetric cyclic diketones was accomplished by interrupting the reaction after the consumption of one equivalent of hydrogen, which indicates a stronger mode of adsorption for the diketone than for the ketol product. 2 The hydrogenation of 1,4-cyclohexanedione (30) to the ketol (Eqn. 18.23) was catalyzed by nickel, copper, palladium, platinum, iridium, and ruthenium. Iridium was the most active and selective of these catalysts. Hydrogenations run over this catalyst in iso-propanol at 20°C and 6 atmospheres... 

Velu, S., Suzuki, K., and Osaki, T. Selective production of hydrogen by partial oxidation of methanol over catalysts derived from CuZnAl-layered double hydroxides. Catalysis Letters, 1999, 62 (2), 159. 

It is helpful to think of a simple membrane process as shown in Figure 8.1. A hydrogen-selective membrane is sealed within a housing (pressure vessel) to make a membrane module. The feed stream enters the membrane module, and hydrogen selectively permeates the membrane. The hydrogen-depleted raffinate stream exits the membrane module as does the permeate stream (enriched in hydrogen). The hydrogen partial pressure in each stream is denoted by PU2 where the subscripts f,... 

In sub-configuration (A) hydrogen will permeate through the hydrogen selective membrane tube under the influence of a pressure difference over the membrane and it will be carried away with an inert sweep gas (steam). The partial pressure of hydrogen in the reaction mixture will decrease cmd the equilibrium will shift to the product side. 

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