Hydrogen pressure effect

The hydrogen pressure effect has been investigated in the following range 300 torr < PH2 < 700 torr at the standard temperature of 350 K. 

Pt-CEO, at the expense of aromatization while the selectivities for producing MCP and isomers were similar to Pt-B. The residual activity of both supported Pt was much higher than that of Pt-B. The hexene selectivity on EPT increased less dramatically than on Pt-B. No hexene was produced on Pt-CEO. The S/So values for benzene were almost the same for all three catalysts. Isomer selectivity decreased to a smaller extent and MCP selectivity even increased after deactivation. Details of temperature and hydrogen pressure effects will be reported elsewhere. 

Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

A corresponding pressure effect on the flow stress was observed on hydrostatic extrusion of conical specimens of the same alloy through a die of 5 mm dia at 220°C. The basic alloy was extruded at P = 12 kbar up to the area ratio equal to 4.1 while the alloy hydrogenated to a = 0.20 extruded to the area ratio of 7.6 even at a lower pressure, P = 11 kbax . Similar pressure/hydrogen effects on the flow stress were also observed on hydrostatic extrusion of ZrH,c, VH,c and Nb,c alloys with x = 0 and 0.1-0.2 wt.%. 

T emary alloys Ti-Al based materials mechanical properties of Titanium alloys hydrogenated stram effects pressure effects Tight-binding LMTO CPA... 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

Many other authors studied the catalytic activity of palladium in more complicated hydrogenation reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical temperature for coexistence of the (a + /3)-phases in the other case the hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

The variation of enantioselectivities with temperature and pressure was investigated. The effects of these two factors are very substrate dependent and difficult to generalize even in a single substrate serie. However, it seems that enantioselectivities are shghly better at 25-40 °C than at lower temperatures (0 °C or less). The stereoselectivity can be inverted for specific alkenes (formation of the S or R enantiomer preferentially). For several substrates, the reactions tend to proceed to completion with optimal ee s when performed at lower hydrogen pressure (2 bar) instead of 50 bar (Fig. 13). Pronoimced variation of enantioselectivities with hydrogen concentration in solution may indicate the presence of two (or even more) different mechanisms which happen to give opposite enantiomers for some substrates. 

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