*5.2.2. Alkyne hydrogenation*

The partial hydrogenation of alkynes to obtain olefinic compounds is a reaction of industrial and academic interest. It is of big importance for the petrochemical, pharmaceutical, and agrochemical industries. Most of the times the alkyne compounds to be hydrogenated are not raw materials for synthesizing new products but impurities to be removed from a certain process feedstock. The conversion of these alkynes, when the feedstock has a high concentration of alkenes, demands specialized catalysts that can react with the triple bond while keeping intact the double bonds of the alkenes. Moreover, the conversion of alkynes to valuable alkenes is usually desired [36–40].

**Figure 13** shows the scheme of reaction of the selective hydrogenation of 1-heptyne, a terminal alkyne, to 1-heptene.The objective of this reaction is to hydrogenate 1-heptyne with a maximum selectivity to the intermediate product 1-heptene. The product of deep hydrogenation, n-heptane, should be completely disfavored.

**Figure 13.** Scheme of the reaction for the selective hydrogenation of 1-heptyne to 1-heptene.

**Figure 14** shows the values of conversion of 1-heptyne and selectivities to 1-heptene and nheptane as a function of time for the catalysts 0.3PdAl, 0.3PdCNR, Lindlar, and 0.3PdUTAl. It can be seen that the egg-shell Pd supported catalyst base on the UTAl support is the most active for hydrogenation. At 3 h,95% conversion of heptyne is seen while the other tested pellet catalysts did not reach 30%.The Lindlar catalysts (0.75 g) with a Pd content of 5% had at 180 min values of conversion similar to 0.3PdUTAl but with selectivities toheptene close to 85%, lower than the selectivity of 0.3PdUTAl. When compared to 0.3PdUTAl, the Lindlar catalyst is disadvantaged as it is supplied in powder form. Once the reaction is finished it must be filtered from the reaction medium with a process that is lengthy and costly due to the high efficiency required for recovering the high priced, high Pd content (5%) Lindlar catalyst. 0.3PdUTAl has a much lower Pd content (0.22%), and is cheaper and has no filtering problems.

**Figure 15** shows the scheme of reaction of selective hydrogenation of 3-hexyne, a nonterminal alkyne, to 3-hexene. In this reaction the desired compound is the product of mild hydrogenation, the intermediate 3-hexene, while the product of deep hydrogenation, n-hexane, is undesired.

New Strategies for Obtaining Inorganic-Organic Composite Catalysts for Selective Hydrogenation http://dx.doi.org/10.5772/65959 197

**Figure 14.** 1-heptyne hydrogenation. (a) 0.3PdAl, (b) 0.3PdCNR, (c) Lindlar, and (d) 0.3PdUTAl.Reaction conditions:toluene solvent, *C*<sup>0</sup> 1-heptyne = 0.350 M, 0.15 MPa H2, 303 K, *W*Cat = 0.75 g.

**Figure 15.** Scheme of the reaction of selective hydrogenation of 3-hexyne to 3-hexene.

It can be seen that for all catalysts the pattern of conversion as a function of time are quite similar, though the catalytic activity is slightly better for the composite 0.3PdUTAl and

The partial hydrogenation of alkynes to obtain olefinic compounds is a reaction of industrial and academic interest. It is of big importance for the petrochemical, pharmaceutical, and agrochemical industries. Most of the times the alkyne compounds to be hydrogenated are not raw materials for synthesizing new products but impurities to be removed from a certain process feedstock. The conversion of these alkynes, when the feedstock has a high concentration of alkenes, demands specialized catalysts that can react with the triple bond while keeping intact the double bonds of the alkenes. Moreover, the conversion of alkynes to valuable alkenes

**Figure 13** shows the scheme of reaction of the selective hydrogenation of 1-heptyne, a terminal alkyne, to 1-heptene.The objective of this reaction is to hydrogenate 1-heptyne with a maximum selectivity to the intermediate product 1-heptene. The product of deep hydrogenation,

**Figure 14** shows the values of conversion of 1-heptyne and selectivities to 1-heptene and nheptane as a function of time for the catalysts 0.3PdAl, 0.3PdCNR, Lindlar, and 0.3PdUTAl. It can be seen that the egg-shell Pd supported catalyst base on the UTAl support is the most active for hydrogenation. At 3 h,95% conversion of heptyne is seen while the other tested pellet catalysts did not reach 30%.The Lindlar catalysts (0.75 g) with a Pd content of 5% had at 180 min values of conversion similar to 0.3PdUTAl but with selectivities toheptene close to 85%, lower than the selectivity of 0.3PdUTAl. When compared to 0.3PdUTAl, the Lindlar catalyst is disadvantaged as it is supplied in powder form. Once the reaction is finished it must be filtered from the reaction medium with a process that is lengthy and costly due to the high efficiency required for recovering the high priced, high Pd content (5%) Lindlar catalyst. 0.3PdUTAl has a much lower Pd content (0.22%), and is cheaper and has no filtering problems.

**Figure 15** shows the scheme of reaction of selective hydrogenation of 3-hexyne, a nonterminal alkyne, to 3-hexene. In this reaction the desired compound is the product of mild hydrogenation, the intermediate 3-hexene, while the product of deep hydrogenation, n-hexane, is

**Figure 13.** Scheme of the reaction for the selective hydrogenation of 1-heptyne to 1-heptene.

0.3PdBTAl catalysts.

*5.2.2. Alkyne hydrogenation*

is usually desired [36–40].

undesired.

n-heptane, should be completely disfavored.

196 New Advances in Hydrogenation Processes - Fundamentals and Applications

The 0.3PdUTAl catalyst was also tried in the hydrogenation of 3-hexyne to 3-hexene. The results of total conversion of 3-hexyne (X3HI) and selectivity to 3-hexene (SHE) and n-hexane (SHA) are plotted in **Figure 16**. Values of 99% and 94% selectivity at 180 min reaction time can be seen. Also, no deactivation of the catalyst was detected.

**Figure 16.** 3-hexyne hydrogenation.0.3PdUTAl catalyst. Toluene solvent, *C*<sup>0</sup> 3-hexyne = 0.350 M, 0.15 MPa H2, 303 K, *W*Cat =0.75 g.

#### *5.2.3. 2,3-butanedione hydrogenation*

**Figure 17** shows the scheme of reaction for the selective hydrogenation of 2,3-butanedione to 3-hydroxybutan-2-one. In this reaction one of the carbonyl groups requires to be hydrogenated with an alcohol group.

**Figure 17.** Scheme of reaction for the selective hydrogenation of 2,3-butanedione to 3-hydroxybutan-2-one.

The reaction of hydrogenation of 2,3-butanedione to 3-hydroxybutane-2-one is a reaction of interest for the industry of scents and fragrances [41–43]. **Figure 18** shows the graphs of total conversion of 2,3-butanedione (X2,3BD) as a function of time for the catalysts 0.3PdUTAl, 0.3PdBTAl, 1.3PdUTAl, and 1.3PdBTAl. In all cases the selectivity to 3-hydroxybutane-2-one was higher than 98%.

**Figure 18.** 2,3-butanedione hydrogenation. Reaction conditions: isopropyl alcohol solvent, *C*<sup>0</sup> 2,3-butanodione = 0.057 M, 4.0 MPa H2, 368 K, *W*Cat = 2 g.

All catalysts of Pd supported over composite supports, independent of the kind of polymer used and of the metal content, were active and highly selective for the reaction of interest. Among the composite catalysts series, those based on the BTAl support were more active than those based on the UTAl. This was more evident in the case of the catalyst of low metal concentration.

The catalysts 1PtBTAl and 1PtUTAl (see **Figure 19**) were also tried in this reaction. In these catalysts a decrease is seen in the selectivity to 3-hydroxybutane-2-one due to the appearance of the product of consecutive hydrogenation of 2,3-butanediol. This pattern of lower selectivity of Pt as compared to Pd has already been reported in the literature [44].

**Figure 19.** 2,3-butanedione hydrogenation. Reaction conditions: isopropyl alcohol solvent, *C*<sup>0</sup> 2,3-butanodione = 0.057 M, 4.0 MPa H2, 368 K, *W*Cat = 2 g.
