Butynes, hydrogenation

For 1,3-butadiene hydrogenation, the toxicity of sulfur is 3 (Fig. 13). which is lower than the toxicity for olefin hydrogenation. The hydrogenation of 1-butyne has also been studied for various ratios of sulfur over palladium. As was already published (86), the 1-butyne hydrogenation rate increases with time. The same effect has been observed on sulfided palladium. The turnover number is consequently presented for 1-butyne hydrogenation versus the sulfur content for various 1-butyne conversions (see Fig. 14). During the first minutes of reaction (0-25% conversion), the toxicity of sulfur appears close to 1 the rates are proportional to the free surface. However, at higher conversion, the rate becomes independent from the sulfur ratio. The toxicity is zero. 

Initial and Final Sulfur Content of Palladium Catalyst Used in the I-Butyne Hydrogenation... 

The different toxicities found for 1-butene, 1,3-butadiene, and l-butyne hydrogenation can be explained by assuming that the energetic adsorption of unsaturated hydrocarbons destabilizes the metal-sulfur bond producing a real desulfurization with l-butyne. The destabilization exists also with the butadiene, as has been shown on platinum (71). 

In the liquid phase at room temperature, using alcohol as a solvent and palladium supported on barium sulfate as catalyst, the only products observed from 1-butyne hydrogenation were 1-butene (98%) and n-butane (2%) (57). The gas phase reaction using 0.03% palladium on alumina catalyst gave 1-butene (99.1%), cis- and product distributions were maintained until at least 76% removal of the parent hydrocarbon but isomerization and hydrogenation of the 1-butene occurred after complete removal of the alkyne. Thus, l-butjme must displace 1-butene from the surface before its isomerization can occur, and it must prohibit the re-entry of 1-butene into the reacting surface layer. This represents the operation of a powerful thermodynamic factor. 

The rate of l,4-dihydroxy-2-butyne hydrogenation was determined in the trickle phase with a 120-ml tubular reactor using 40 ml of either 3-mm activated tablets or hollow spheres. This hydrogenation was performed at 60 bar hydrogen pressure, 135°C, and the LHSV values of 0.80 and 1.6 h with a 50 wt.% 1,4-dihydroxy-2-butene aqueous solution whose pH was adjusted to 7 with NaHC03. 

Figure 12 Reaction scheme for l,4-dihydroxy-2-butyne hydrogenation.

Catalytic activities for butyne hydrogenation evidence effects due to particle size modifications, but also other effects necessitating a finer study of Mo Pd intaeractions. The modification of alumina by controlled molybdates deposition allows significant improvements in hydrogenation activities. 

As we found in our discussion of alkene stabilities (Section 11-5), heats of hydrogenation also provide convenient measures of the relative stabilities of alkyne isomers. In the presence of catalytic amounts of platinum or palladium on charcoal, the two isomers of butyne hydrogenate by addition of two molar equivalents of H2 to produce butane. Just as we discovered in the case of alkenes, hydrogenation of the internal alkyne isomer releases less energy, allowing us to conclude that 2-butyne is the more stable of the two. Hyperconjugation is the reason for the greater relative stabihty of internal compared with terminal alkynes. 

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