*3.2.7. Commercial Lindlar catalyst*

In **Figure 7**, it can be observed that both catalysts present very similar behaviour on 1-heptyne total conversions and on selectivity to 1-heptene. As shown in Section 3.1, the characterization techniques indicated the presence of reduced palladium after the pretreatment with hydrogen flow at 573 K, and also there is neither chlorine nor nitrogen species on the surface of both catalysts (absence of electronic and steric factor). As different dispersions were obtained with chemisorption analysis, differences in TOF (shown in **Table 2**) indicate that PdN(0.4%)/Al is twice active than Pd(0.4%)/Al. So, geometric factors are responsible of the higher activity of

As the low-loaded palladium catalyst prepared with the nitrate precursor salt was more active than that prepared with chloride salt, the addition of Ni to the monometallic PdN(0.4%)/Al catalyst was evaluated in order to improve 1-heptyne total conversion. In **Figure 8**, 1-heptyne total conversion and selectivity to 1-heptene as a function of time obtained during the hydrogenation test for the monometallic Pd(0.4%)/Al and bimetallic Pd-Ni/Al catalysts are plotted. The bimetallic catalyst was reduced for 1 h at 673 K, and the temperature of the catalytic evaluations

**Figure 8.** *Effect of the addition of Ni*: Total conversion of 1-heptyne and selectivity to 1-heptene as a function of time for PdN(0.4%)/Al (filled symbols) and Pd-Ni/Al (opened symbols). Reduction and reaction temperatures: 573

According to **Figure 8**, the bimetallic and monometallic catalysts present similar and very high selectivities to the desired product (97% c.a.). Therefore, it can be said that the addition of Ni as a second metal to the alumina-supported palladium catalysts has no influence on the

*3.2.6. Effect of the addition of a second metal: bimetallic catalyst*

28 New Advances in Hydrogenation Processes - Fundamentals and Applications

PdN(0.4%)/Al.

was 303 K.

and 303 K, respectively.

The Lindlar catalyst is very often used as a reference for the selective hydrogenation of alkynes, so in this work the performance of the commercial catalyst at 303 K was obtained. **Figure 9** presents the total conversion of 1-heptyne and selectivity to 1-heptene versus reaction time for this catalyst.

**Figure 9.** *Commercial Lindlar catalyst*: Total conversion to 1-heptyne and selectivity to 1-heptene as a function of time for Lindlar catalyst. Reaction temperature: 303 K.

In **Figure 9**, it can be observed that the Lindlar catalyst allows obtaining high values of 1 heptyne total conversion with high selectivity to 1-heptene, between 82% and 86%. Comparing the results obtained with the prepared high-loaded catalysts (5 wt% of Pd, similar to that of Lindlar), a slightly better yield to 1-heptene is obtained with the Pd(5%)/Al catalyst than that obtained with Lindlar catalyst. Besides, it must be remarked that the prepared catalyst has the advantage of being a pelletized material. On the other hand, the lowest yield to 1-heptene is obtained with Pd(5%)/AC. Differences in total conversion can be associated to electronic effects, as the surface palladium species on the Lindlar catalyst are more electron-deficient than those present on Pd(5%)/Al or Pd(5%)/AC.
