*3.2.2. Effect of reaction temperature*

and 1000 K when the contact between NiO and alumina is intimate [33–35]. The patterns of reduction depend on the nature of the metal-support interactions, which can be modified by the calcination temperature employed during the preparation of the monometallic nickel catalysts [34]. Besides, a broad peak is also present in this profile with a maximum at 1000 K, which is attributed to the reduction of nickel aluminates, NiAl2O4, showing a strong metalsupport interaction [35, 36]. According to the calculated degree of reduction, determined by TPR, the bimetallic catalyst has a low percentage of reduced Ni (7%) and Pd (74%). This suggests the presence of strong Pd-Ni intermetallic interaction in the catalyst; however, the

When palladium-supported catalysts are used during the alkyne hydrogenations, the β-phase hydride acts as a hydrogen source that promote over hydrogenation to obtain mainly the alkane, decreasing the selectivity to the alkene. The disappearance of the β-PdH phase is very important because it could impact directly on the activity and selectivity [37]. These authors state that the disappearance of the β-PdH phase considerably decreases alkynes hydrogenation rate to alkanes, thus increasing the selectivity to alkenes formation. For the prepared monoand bimetallic catalysts, the palladium β-phase Pd hydride is not present as it is proved by the

According to XPS and TPR characterizations, it can be concluded that after pretreatment Pd° is present in the low-loaded monometallic catalysts, while Pdδ+OxCly species are formed in the high-loaded Pd catalysts. On the other hand, on the bimetallic catalyst, two kinds of palladium species (Pdδ+, with δ close to 0, and Pdδ−) and Nin+ (with *n* close to 2) are present on the surface.

**Figure 3** presents 1-heptyne total conversion and selectivity to 1-heptene as a function of time for Pd(5%)/Al catalyst pretreated for 1 h in a hydrogen flow at 373 and 573 K, the reaction temperature was 303 K. In the figure, it can be seen that the total conversion increases as the reduction temperature is increased, while the selectivity is slightly lower at the higher reduction temperature (≥90%). From the TOF values displayed in **Table 2**, it may be concluded that the catalyst reduced at higher temperature is nearly twice more active than when it is reduced at 373 K. The activity results can be explained taking into account the electronic state of Pd in each catalyst: the more electron-deficient palladium species the less active is the catalyst for the hydrogenation of 1-heptyne. It is probably that the Pdδ+ species inhibit the interaction between the metal and 1-heptene due to an electronic effect, decreasing its electrondonor character. Therefore, it can say that the presence of electron-deficient Pdδ+OxCly species improves the selectivity to the desired product. Given that the chlorine is not completely removed from the Pd(5%)/Al catalyst after the heat treatments, it seems likely that the role of the remaining chlorine on the catalyst surface could be to stabilize the positively charged palladium structures, resulting in less active but more selective catalysts (electronic and steric factors). Therefore, our results suggest a correlation between the reduction temperature, electron deficiency of palladium species and chloride content, with total conversion and

interaction of Pd and Ni with the support cannot be neglected.

22 New Advances in Hydrogenation Processes - Fundamentals and Applications

TPR profiles at the pretreatment reduction temperature adopted.

**3.2. Catalytic evaluations**

*3.2.1. Effect of the reduction temperature*

Another important factor to evaluate is the reaction temperature. Alkyne hydrogenation reactions must be carefully controlled, especially at large scale when large amount of catalyst are used as in these kinds of exothermic reactions. It is well known that above 313 K, a complete hydrogenation of the alkyne compounds occurs [5]. In order to evaluate this effect, the reaction tests were performed at 280 and 303 K using the Pd(5%)/Al catalyst reduced for 1 h at 573 K (optimal reduction temperature). In **Figure 4**, the total conversion and selectivity to 1-heptene as a function of time at the mentioned temperatures are presented.

**Figure 4.** *Effect of reaction temperature*: Total conversion of 1-heptyne and selectivity to 1-heptene as a function of time for Pd(5%)/Al catalyst: 303 K (filled symbols) and 280 K (opened symbols). Reduction temperature: 573 K.

The obtained results shown in **Figure 4** indicate a marked difference in the total conversion of 1-heptyne when the reaction temperature is increased. The total conversion is markedly higher when the reaction is carried out at 303 K, while similar selectivity to 1-heptene values is obtained at both temperatures. Other authors have found similar results while studying the partial hydrogenation of several alkynes [41, 42].

As hydrogenation reactions are extremely exothermic, an increase in the reaction temperature does not favour the reaction thermodynamically, but it improves the kinetic of the reaction. Experimental results show that the optimum reaction temperature is 303 K because higher total conversions of 1-heptyne are obtained with high selectivity to 1-heptene (≥90%).
