Half-hydrogenation times

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L"1 d"1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d"1 with an ee of >99%. The catalyst half-life times were... 

The tin hydride method suffers from one major disadvantage, namely the efficiency of the reagent as a hydrogen atom donor. For successful synthesis, alkenes have to be reactive enough, otherwise direct reduction of the starting precursor becomes a considerable side reaction. In practice, the yields are increased by slow addition of a solution of tin hydride and a radical initiator into the reaction mixture containing an excess of alkene. However, a delicate balance must be maintained. If a large excess of olefin is used, polymerization can compete. 2,2-Azobisisobutyronitrile is the most commonly employed initiator, with a half-life time for unimolecular scission of 1 h at 80°C. 

In crystals, hydrogen-bond structures are sometimes found where two half-hydrogen atoms are located approximately along the lines of centers, (X, A) - - -H -i-H - (A, X), with yH - H separations of about 1.0 A. This distance is considerably shorter than the 2.4 A expected for a van der Waals H H separation, and requires the two H atoms to be in mutually incompatible positions, corresponding to a time- or space-averaged equilibrium, and that X and A be the same atom type... 

From the values of TOF, the increasing order of activity for the fresh catalysts in the hydrogenation of ethylbenzene is Pd < Ni < Pt < Rh < Ru, Concerning the deactivation process, the intrinsic order of sulfur resistance appears to be Ru Rh Ni > Pd > Pt. On the other hand, the half deactivation time and the catalyst life decrease in the order Rh > Ru Pd Ni > Pt. This difference is due to the fact that the lifetime of a particular catalyst is an extensive property, which depends both on the deactivation rate constant (k l and the initial number of exposed metal atoms (N). Finally, we want to point out the small differences in the activity found for the fresh catalysts (CRu/CPd - 1.6), as compared with the greater values of their sulfur resistance (CRu/CPd = 6.5 or CRu/CPt = 12.5). 

Figure 1 shows the effect of reaction time on tlie productions of gasses from pulp. Decomposition ratio was slightly increased with increasing reaction time. The influence of reaction time on the decomposition of pulp is low compared with Naphthalene [16]. In the case of Naphthalene, half reaction time led to approximately half decomposition ratio. Pulp seems to be easy to decomposition. The volume of produced hydrogen was virtually the same as one among three difference reaction time parameters. 

In fig. 8.8, we show the dependence of typical profiles on the value of the shape parameter C. There are two parts to the figure which correspond to changing the sign of B so that the zero in the cross section occurs either just above or just below a hydrogenic point. As expected, all the curves meet three times in a cycle, and once at the half-hydrogenic point. 

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