Hydrogen on ruthenium

Gallezot, P., Nicolaus, N., Fleche, G., Fuertes, P., and Perrard, A. (1998) Glucose hydrogenation on ruthenium catalysts in a trickle-bed reactor. J. 

P. Gallezot, N. Nicolaus, G. Fleche, P. Fuertes and A. Perrard, Glucose Hydrogenation on Ruthenium Catalysts in a Trickle-Bed Reactor, Journal of Catalysis 180 (1998) 51. 

Polymeric carbon refers to chains of carbon monomers (surface carbide) that are connected by covalent bonds. It has been shown recently47 that the barrier for C-C coupling on flat surfaces (1.22 eV) is half that for a step site (2.43 eV), and may indicate that the growth of these polymeric species is favored on terraces. Polymeric carbon may also refer to carbon chains that contain hydrogen. In the case of CO hydrogenation on ruthenium catalysts, polymeric carbon has been identified as a less reactive carbon that forms from polymerization of CHX and has an alkyl group structure.48... 

There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

Figure 3.1 Typical adsorption isotherms at room temperature for hydrogen on ruthenium-copper aggregates containing 5 at.% copper (3). (Reprinted with permission from Academic Press, Inc.)...

The hydrogenation of ring A aromatic steroids over ruthenium occurs, almost invariably, from the a side and all substituents on the original aromatic ring are cis in the resulting cyclohexane. Estrone (62) is hydrogenated over ruthenium to 5a,10a-estrane-3/3,17j6-diol (63) in 85-90% yield. 

Role of adsorbed hydrogen species on ruthenium and molybdenum sulfides. Characterization by inelastic neutron scattering, thermoanalysis methods and model reactions. 

In a somewhat different approach, supported-aqueous-phase-catalysts (SAPC, see Chapter 5, Section 5.2.5 of this book) have been combined with supercritical CO2 in catalytic hydrogenation [55], Ruthenium was supported on silica and combined with the ligand TPPTS in water, after which a scC02/H2 phase was applied together with the substrate. Better levels of conversion were obtained using scC02 than the equivalent system with toluene for the hydrogenation of cinnamaldehyde. 

There is much current excitement and activity in the field of homogeneous hydrogenation using ruthenium catalysts. This is reflected in the recent, explosive increase in the number of research publications in this area, now rivaling those for rhodium catalysts (Fig. 3.1). Meanwhile, the price of rhodium metal has risen dramatically, becoming about ten times that of ruthenium, on a molar basis. The number of reports on the use of osmium catalysts has remained low, partly because of the higher price of osmium compounds - about ten times that of ruthenium - and partly because the activity of osmium catalysts is often lower. 

Scheme 3.3 Mechanism for the hydrogenation of 1-alkenes catalyzed by RuHCI(PPh3)3. [Ru] represents the RuCI(PPh3)n fragment. The box represents an empty coordination site on ruthenium (II).

Scheme 3.7 Generation of the active hydride catalyst by hydrogen transfer from formic acid or iso-propanol via /5-hydride elimination from formate or alkoxide intermediates. The square represents a vacant site on ruthenium.

Enantioselective catalytic hydrogenation. The ruthenium(II) complexes of (R)- and (S)-l, bearing a chiral BINAP ligand, catalyze asymmetric hydrogenation of N-acyl-l-alkylidenetetrahydroisoquinolines to give (1R)- or (lS)-tetrahydroiso-quinolines in 95-100% ee.1 Thus the (Z)-enamide (2), prepared by acylation of 3,4-dihydropapaverine, is hydrogenated in the presence of (R)-l to (1R)-tetrahydroisoquinolines (3). The enantiomeric (lS)-3 is obtained on use of (S)-l as catalyst. 

BINAP was introduced by Noyori [18], It has been particularly explored for reduction with ruthenium catalysts. While the first generation rhodium catalysts exhibited excellent performance with dehydroamino acids (or esters), the second generation of hydrogenation catalysts, those based on ruthenium /BINAP complexes, are also highly enantioselective for other prochiral alkenes. An impressive list of rather complex organic molecules has been hydrogenated with high e.e. s. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

In the methanol synthesis, a fast hydrogenation of the adsorbed formaldehyde would overcome arguments invoking any thermodynamic limitation to its formation. Formaldehyde adsorbed through both the oxygen and the carbon ends has been characterized in homogeneous catalysis (43), on oxide surfaces (44) and more recently on ruthenium metal (45). 

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