Ethyne hydrogenation

Figure 10-5 The proton nmr spectrum and integral of ethynylbenzene at 60 MHz relative to TMS as 0.00. This spectrum also illustrates the use of nmr for detection of small amounts of impurities. The almost imperceptible peaks around 6 ppm are in the correct locations for alkene hydrogens. The integral indicates that the ratio of alkene to ethyne hydrogens is on the order of 1 15. The substance most likely to give rise to the peaks is ethenylbenzene (styrene, C6H5CH=CH2) and, if so, it is present to the extent of about 2%.

The long dashes indicate locations of change in orbital phase. The dotted lines are contour lines of electron amplitude of opposite phase to the solid lines. Notice how the contours of the ethyne hydrogen orbital are distorted toward carbon compared to those of the ethane hydrogen orbital. 

The hydrogens of the —CH2— group of 1,3-cyclopentadiene are acidic. In fact, they are considerably more acidic than the ethyne hydrogens of the 1-alkynes (Section 1 j-8). This means that 1,3-cyclopentadiene is at least 1030 times more acidic than the ordinary alkanes. The reason is that loss of one of the CH2 protons of cyclopentadiene results in formation of an especially stabilized anion ... 

A little work on structure-insensitive reactions has been reported [18]. Both catalysts were very active for ethene hydrogenation, and rapid deactivation occurred even at 176 K. Ethyne and 1,3-butadiene react in a more controlled manner study of ethyne hydrogenation using both l4C-labeled ethyne and ethene showed that ethane formation took place directly from adsorbed ethyne, without the intervention of gas-phase ethene. 

Arrhenius behaviour in ethyne hydrogenation over palladium catalysts. Appl Catal 55 L5... 

Aduriz HR, Bodnariuk P, Dennehy M, Grgola CE (1990) Activity and selectivity of Pd/a-Al Oj for ethyne hydrogenation in a large excess of ethene and hydrogen. Appl Catal 58 227... 

Scheme 9.1. Simplified scheme for the mechanism of ethyne hydrogenation. Note in this and the following Schemes no attempt is made to show all interactions of Jt orbitals with the surface.

As the results are presented in more detail in the following sections, it will be natural to wonder what further development of the mechanistic framework in Scheme 9.1 will be needed to account for them. Such considerations are postponed to Section 9.3.2, but the following points should be borne in mind. The reaction system palladium-ethyne-hydrogen is in fact extremely complex, and there are... 

Ag/Si02 and Ag/Ti02 after activation by oxidation and reduction were active for ethyne hydrogenation at 353-443 K (Table 9.1) both showed 100% selectivity to ethene and no oligomer formation at the lower temperatures, but selectivity fell and more oligomers were made as temperature increased. Rates were slower than for butadiene the activation energy on Ag/Si02 was 39 kJ mol". ... 

TABLE 9.5. Particle-Size Sensitivity of Ethyne Hydrogenation in the Presence of Excess Ethene, and Estimations of... 

CH3) as alternatives to symmetrically-bonded species, either in the predominant route to ethene" and ethane, or just in conditions of low selectivity " (ii) the probable operation of two or three separate types of site during ethyne hydrogenation with excess ethene (Table 9.6) (iii) the likely importance of carbonaceous deposits in determining selectivity or in creating sites at which selective reaction can occur " and (iv) face sensitivity. Other imponderables already noted include the possible formation of carbide and hydride phases in palladium. To add to the misery, we have seen that even the sense of the particle size effect on TOF and selectivity cannot be agreed (Table 9.6), and supports appear to exert an important but poorly understood influence. 

Arrhenius parameters for ethyne hydrogenation on nickel-copper powders are shown as a compensation plot in Figure 9.8. While at 323 K rates decreased progressively as the copper content rose, those at 473 K were maximal at 40 and 60% copper, as indicated by their positions on the compensation plot. As discussed in Section 5.5, this result makes protracted discussion of the relevance of solid-state parameters to catalytic activity of doubtful value, if activity measurements are confined to a single temperature. Results are also available for the nickel-cobalt system. Intermetallic compounds based on cobalt (CoGe, CoGc2, CoAl) were much more selective than cobalt itself. 

Figure 9.8. Compensation plot for ethyne hydrogenation on nickel-copper powders (A in units of rate of pressure fall...

Ethyne - hydrogen fluoride (1/1) (weakly bound complex)... 

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