*3.2.3. Effect of support*

It is important to compare different materials employed as supports of the active phases, so two kinds of supports were selected: alumina (inorganic) and activated carbon (a carbonaceous material, called AC). The evaluated catalysts were Pd(5%)/Al and Pd(5%)/AC, reduced for 1 h at 573 K and run at 303 K. The total conversion and selectivity to 1-heptene versus time are shown in **Figure 5**.

Analysing the data presented in **Figure 5** and the TOF values shown in **Table 2**, it can be noted that better performance is achieved when Al2O3 is used as support. Initially, the obtained selectivity values with both supports are higher than 90%, but for total conversion higher than 60% the selectivity to 1-heptene slightly decays.

**Figure 5.** *Effect of support*: Total conversion of 1-heptyne and selectivity to 1-heptene as a function of time for Pd(5%)/Al (filled symbols) and Pd(5%)/AC (opened symbols). Reduction and reaction temperatures: 573 and 303 K, respectively.

**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

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

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

It is important to compare different materials employed as supports of the active phases, so two kinds of supports were selected: alumina (inorganic) and activated carbon (a carbonaceous material, called AC). The evaluated catalysts were Pd(5%)/Al and Pd(5%)/AC, reduced for 1 h at 573 K and run at 303 K. The total conversion and selectivity to 1-heptene versus time are

Analysing the data presented in **Figure 5** and the TOF values shown in **Table 2**, it can be noted that better performance is achieved when Al2O3 is used as support. Initially, the obtained selectivity values with both supports are higher than 90%, but for total conversion higher than

total conversions of 1-heptyne are obtained with high selectivity to 1-heptene (≥90%).

partial hydrogenation of several alkynes [41, 42].

24 New Advances in Hydrogenation Processes - Fundamentals and Applications

60% the selectivity to 1-heptene slightly decays.

K.

*3.2.3. Effect of support*

shown in **Figure 5**.

As both catalysts present quite similar dispersion values as well as Pdδ+ electron-deficient species and Cl/Pd atomic ratios, the observed differences in activity must be assigned to the characteristic of the support. The influence of the support on the physicochemical properties and, therefore, on the catalytic behaviour of metals is well established in the literature [43].

During the preparation step of the catalysts, the specific support properties of the carbons (such as chemical nature, texture, pore structure, surface state, etc.) can modify the morphology and/ or localization of the metal particles, electronic structure of the surface metal atoms, adsorption-desorption equilibrium of reactants, and so on. This can generate differences in the conversion and selectivity values. Thus, as our results suggest, the activity and selectivity of palladium-supported catalysts is a complex property of the whole catalyst and cannot be related to a single parameter. Considering the above, the slightly higher selectivity to 1-heptene at the highest conversion values found for Pd(5%)/AC catalyst could be associated to shape selectivity induced by the porous support. In this way, this might be due to the 1-heptene molecule has a planar end, unlike the more voluminous end of the fully saturated n-heptane. The increase of the selectivity to the desired product may be associated with the localization of the Pd species in narrow pores (micro- and supermicropores) in the carbon support. If this is the case, it could also be suggested that the lower total conversion of Pd(5%)/AC catalyst is due to the narrower porosity of the activated carbon, as it is probable that fewer 1-heptyne molecules could reach the active sites located in the supermicropores. If a significant fraction of the active species are located in pores of a particular size (larger supermicropores, practically absent in Pd(5%)/Al catalyst, and mesopores), the concentration of 1-heptene in the neighbour-

hood of the Pd species could be enhanced, thus favouring the consecutive hydrogenation of 1-heptene to heptane. Although the surface chemistry of GF-45 support is quite unlike that of alumina, the similar dispersions and electronic states of palladium on Pd(5%)/Al and Pd(5%)/AC reinforce the idea that their different catalytic behaviours are related to the differences in the support porosities.

#### *3.2.4. Effect of the metal loading*

The price of a metal, its toxicity, easy handling and safety are a set of properties to take into account during the preparation of the catalysts. Industrially profitable processes with highly active, selective and cheaper catalysts are continually researched. As the cost of the catalyst is important, a reduction of the metal loading on the final catalyst was considered and its effect on activity and selectivity during the hydrogenation reaction was assessed.

In **Figure 6**, the total conversion and selectivity to 1-heptene for the palladium catalysts supported on alumina with a metal loading of 5 and 0.4 wt% of Pd are presented. The used precursor salt was PdCl2, the catalysts were reduced at 573 K and the catalytic tests were carried out at 303 K.

**Figure 6.** *Effect of metal loading*: Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd(5%)/Al (filled symbols) and Pd(0.4%)/Al (opened symbols). Reduction and reaction temperatures: 573 and 303 K, respectively.

In the figure, it can be seen that decreasing the metal concentration of the catalyst, at identical operational conditions, decreases the total conversion (70–22% at 180 min). On the other hand, considering TOF values of both catalyst shown in **Table 2**, it can be concluded that the lowloaded Pd(0.4%)/Al catalyst is 2.8 times more active than the Pd(5%)/Al. The selectivities to the desired product (1-heptene) were 95% average without large changes when the metal loading is decreased. This fact is very important from an economic and industrial point of view.

It is well known that during hydrogenation reactions, metallic centres rich in electrons can cleave the bond in H2 by means of the interaction of a filled *d* metal orbital with the empty sigma antibonding molecular orbital of H2 [44]. The rupture of the hydrogen bond is more easily done on metals with a high amount of available electrons in the external *d* orbital, as it is the case of Pd(0.4%)/Al (with Pd°). This rupture should be less likely on metals with fewer *d* electrons, as in the case of Pd(5%)/Al (with Pdδ+ species). Therefore, the differences in activity between the reported catalysts could be partly attributed to differences in the electronic density of the external *d* orbital of each metal (electronic factor). So, the high activity of Pd(0.4%)/Al could be attributed to different factors: (a) palladium totally reduced Pd°, which favours the dissociative adsorption of hydrogen (electronic factor), (b) high dispersion of the low-loaded catalyst indicating that a high amount of small active sites are present on the surface (geometric factor) and (c) the absence of chlorine, a bulky and electronegativity element, which prevent the adsorption of the alkyne (steric factor).

#### *3.2.5. Effect of precursor salt*

hood of the Pd species could be enhanced, thus favouring the consecutive hydrogenation of 1-heptene to heptane. Although the surface chemistry of GF-45 support is quite unlike that of alumina, the similar dispersions and electronic states of palladium on Pd(5%)/Al and Pd(5%)/AC reinforce the idea that their different catalytic behaviours are related to the differences in the

The price of a metal, its toxicity, easy handling and safety are a set of properties to take into account during the preparation of the catalysts. Industrially profitable processes with highly active, selective and cheaper catalysts are continually researched. As the cost of the catalyst is important, a reduction of the metal loading on the final catalyst was considered and its effect

In **Figure 6**, the total conversion and selectivity to 1-heptene for the palladium catalysts supported on alumina with a metal loading of 5 and 0.4 wt% of Pd are presented. The used precursor salt was PdCl2, the catalysts were reduced at 573 K and the catalytic tests were carried

**Figure 6.** *Effect of metal loading*: Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd(5%)/Al (filled symbols) and Pd(0.4%)/Al (opened symbols). Reduction and reaction temperatures: 573 and 303

In the figure, it can be seen that decreasing the metal concentration of the catalyst, at identical operational conditions, decreases the total conversion (70–22% at 180 min). On the other hand, considering TOF values of both catalyst shown in **Table 2**, it can be concluded that the lowloaded Pd(0.4%)/Al catalyst is 2.8 times more active than the Pd(5%)/Al. The selectivities to the

on activity and selectivity during the hydrogenation reaction was assessed.

26 New Advances in Hydrogenation Processes - Fundamentals and Applications

support porosities.

out at 303 K.

K, respectively.

*3.2.4. Effect of the metal loading*

The effect of precursor salt (palladium chloride and nitrate) on the total conversion and selectivity to 1-heptene during the 1-heptyne partial hydrogenation was studied using Pd(0.4%)/Al and PdN(0.4%)/Al catalysts reduced in a hydrogen flow at 573 K and tested at 303 K. The results of total conversion and selectivity to 1-heptene are shown in **Figure 7**.

**Figure 7.** *Effect of precursor salt*. Total conversion of 1-heptyne )and selectivity to 1-heptene as a function of time for PdN(0.4%)/Al (filled symbols) and Pd(0.4%)/Al (opened symbols). Reduction and reaction temperatures: 573 and 303 K, respectively.

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 PdN(0.4%)/Al.

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

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 was 303 K.

**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 and 303 K, respectively.

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 selectivity to 1-heptene. Therefore, the disappearance of the beta-phase palladium hydride could be the cause, at least in part, of the high selectivities found for mono- and bimetallic palladium catalysts. Besides, the nickel addition to the palladium monometallic catalyst improves 1-heptyne total conversion. TOF values indicate that Pd-Ni/Al is 1.5 times more active than Pd(0.4%)/Al. There is not a simple interpretation to explain the influence of the second metal on the Pd performance. Alkyne partial hydrogenation reactions are more or less sensitive to geometrical and electronic effects, the latter being the most important ones.

As observed by XPS, the nickel addition to the palladium catalyst promotes electron deficiency of Ni (Nin+) and Pd (Pdδ+), and generates electron-rich palladium species (Pdδ−). Since the bimetallic catalyst has a high amount of available electrons in the *4d* Pd orbital than monometallic PdN(0.4%)/Al, the hydrogen cleavage would be favoured on this catalyst. This would explain the higher activity of the bimetallic Pd-Ni catalyst. The modification of the electronic state of Pd could be responsible for the better catalytic behaviour, moreover considering the high electronic density of the triple bond. However, the influence of geometrical effects and/or mixed sites on the activity of the bimetallic catalyst cannot be discarded.
