Selectivity hydrogen

In hydrogenation, it is often desirable to hydrogenate selectively, leaving some unsaturated bonds intact. A review of hydrogenation reactions is available (89). 

The A -double bond is readily hydrogenated selectively in the presence of a A -doiible bond. With other double bonds selectivity is not so easily achieved. ... 

Ergosterol (a triene) rapidly takes up 1 mole of hydrogen selectively to give 5a,6-dihydroergosterol, recrystallized from methanol-chloroform, mp 173-174°. 

GP 8[ [R 7] Rhodium catalysts generally show no pronoimced activation phase as given for other catalysts in other reactions [3]. In the first 4 h of operation, methane conversion and hydrogen selectivity increases by only a few percent. After this short and non-pronounced formation phase, no significant changes in activity were determined in the experimental runs for more than 200 h. 

Other poisons (modifiers) used to create such selective Pd catalysts may be metals 23 Zn, Cd, Zr, Ru, Au, Cu, Fe, Hg, Ag, Pb, Sb, and Sn or solvents (organic modifiers) 24 pyridine, quinoline, piperidine, aniline, diethylamine, other amines, chlorobenzene, and sulfur compounds. Hydroxides have also been used to increase the half-hydrogenation selectivity of Pd. 

The C=C double bond in enamides was hydrogenated selectively without the cleavage of the C-N bond. A mixture of the enamide, sodium carbonate, and 10% Pd/C in EtOH was stirred under 1-3 atm hydrogen pressure for 16— 48 hours (Scheme 4.86).335... 

When the membrane pore size is reduced further to molecular dimensions, gas species separation can occur by molecular sieving. To separate hydrogen selectively from the other syngas components (CO, C02, CH4, and H20), porous membranes need to be able to discriminate molecules in the 0.3-0.4 nm size with 0.1 nm or less in size difference. 

Prabhu, A.K. and S.T. Oyama, Highly hydrogen selective ceramic membranes Application to the transformation of greenhouse gases,. Membr. Sci, 176, 233-248, 2000. 

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 presence of [RhH(TPPMS)3] causes substantial changes in the mechanism of hydrogenation, that most probably follows a conventional monohydride mechanism as shown in Scheme 1.1. This is also reflected in the rates and the hydrogenation selectivities [27]. 

In general, the rate of alkene hydrogenation is typically ordered as follows terminal>di-substituted>tri-substituted >tetra-substituted. In fact, this allows terminal or di-substituted olefins to be hydrogenated selectively in the presence of tri- or tetra-substituted ones. Additionally, the rate of hydrogenation of al-kynes is much slower than that of alkenes, although the cis-alkene intermediate... 

Hydrogenation of butadiene with K3Co(CN)5, which is known to hydrogenate selectively conjugated dienes [75], was possible with 100% conversion and selectivity and a TOF up to 72 h-1 in the ionic liquid [BMIM][BF4], but the catalyst was deactivated after the first run and the inactive complex (BMIM)3Co(CN)5 was formed [69]. 

Addition of some sodium borate to increase the fructose —> mannitol hydrogenation selectivity due to fructose-borate complex formation [21, 22]. 

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