**4. Conclusions**

The results in **Table 12** indicate that the values of the rate constants show a significant difference, regardless of the catalyst activity and hence the contribution of the individual reactions

(min−1) **9.6 × 10**<sup>−</sup>**<sup>3</sup> 2.3 × 10**<sup>−</sup>**<sup>3</sup> 2.8 × 10**<sup>−</sup>**<sup>3</sup>**

 (min−1) **1.1 × 10**<sup>−</sup>**<sup>2</sup>** 1.1 × 10−7 **2.2 × 10**<sup>−</sup>**<sup>3</sup>** *k*2*<sup>c</sup>*-0 (min−1) **4.3 × 10**<sup>−</sup>**<sup>3</sup>** 8.0 × 10−9 **2.7 × 10**<sup>−</sup>**<sup>3</sup>**

(min−1) 8.5 × 10−7 1.0 × 10−7 2.4 × 10−7

 (min−1) 2.0 × 10−7 6.7 × 10−7 1.4 × 10−6 *k*2*<sup>t</sup>*-0 (min−1) **1.1 × 10**<sup>−</sup>**<sup>2</sup> 4.7 × 10**<sup>−</sup>**<sup>3</sup> 9.2 × 10**<sup>−</sup>**<sup>3</sup>**

(min−1) **2.6 × 10**<sup>−</sup>**<sup>3</sup> 6.3 × 10**<sup>−</sup>**<sup>4</sup> 8.0 × 10**<sup>−</sup>**<sup>4</sup>**

 (min−1) 1.2 × 10−5 5.3 × 10−7 2.6 × 10−7 *k*1*<sup>c</sup>*-0 (min−1) **8.2 × 10**<sup>−</sup>**<sup>4</sup>** 2.0 × 10−7 9.2 × 10−8 *k*1*<sup>t</sup>*-0 (min−1) **1.8 × 10**<sup>−</sup>**<sup>3</sup> 8.7 × 10**<sup>−</sup>**<sup>4</sup> 3.6 × 10**<sup>−</sup>**<sup>3</sup>**

(min−1) **2.2 × 10**<sup>−</sup>**<sup>3</sup> 1.7 × 10**<sup>−</sup>**<sup>3</sup> 2.0 × 10**<sup>−</sup>**<sup>3</sup>**

(min−1) 2.5 × 10−8 9.3 × 10−10 4.6 × 10−10

**Ni/SiG-A Ni/SiG-B Ni/SiG-C**

In order to simplify the reaction pathway shown in **Figure 13a**, without compromising the accuracy of predicting the concentration of fatty acid change as criteria of importance for some rate constants, we used a value of 1% of the highest rate constant for the particular catalyst (1% *k*max). Rate constants higher than 1% *k*max were declared as significant and marked bold in **Table 12**. On the contrary, rate constants less than 1% *k*max are associated as insignificant. It was shown that the number of significant rate constants increases as the activity of the catalysts grows higher. Significant constants, higher than 1% *k*max, show that Ni/SiG-A has 8, Ni/SiG-C has 7, while the least active catalyst, Ni/SiG-B, has the lowest value 5. Using the criteria of 1% *k*max, we can rewrite a reaction pathway (**Figure 13a**), which will include only significant rate constants for each individual catalyst and are given in

Using the reduced reaction pathways, reaction rates were rewritten, using only significant rate constants and the process of hydrogenation was simulated with only those reaction rates. Values of rate constants then obtained were the same as the ones obtained by the initial reaction mechanism, which indicates that the reduction of the initial reaction pathway changes as

in the reaction mechanism can be recognized.

168 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Rate constantsa Sample**

Values in bold show significant rate constants.

**Table 12.** Kinetic constant values for Ni/SiG catalysts at 160°C and 0.2 MPa.

*k*2*c*-1*<sup>c</sup>*

*k*2*c*-1*<sup>t</sup>*

*k*2*t*-1*<sup>t</sup>*

*k*2*t*-1*<sup>c</sup>*

*k*2*c*-2*<sup>t</sup>*

*k*2*t*-2*<sup>c</sup>*

*k*1*c*-1*<sup>t</sup>*

*k*1*t*-1*<sup>c</sup>*

a

**Figure 14**.

a function of catalyst activity.

The characteristics, structure and catalytic behavior of high loading nickel-based catalysts supported on diatomite, silica gel and perlite have been analyzed. Nickel-based supported catalyst precursors were prepared by the precipitation-deposition method. The results show that the state, reducibility and dispersion of nickel in supported nickel-based catalysts vary depending on the nature of support and the preparation parameters.

Combined nitrogen physisorption and mercury porosimetry studies showed that the studied nickel-based supported systems had a high specific surface area and a well-developed porous structure, containing mesopores stable to thermal reduction treatments.

The results concerning the influence of the preparation stage and nature of the support and the modifier clearly illustrate the features of the supported Ni2+ phase and demonstrate that IR and XRD measurements may offer as an effective tool to identify nickel species and their interaction with support in differently supported and modified nickel-based catalyst precursors. From the results obtained by both IR and XRD instrumental techniques, it could be concluded that during the deposition reaction under alkaline conditions, the silica as the constitutive component of all studied supports reacts with the basic nickel carbonate precipitate and generates the new supported nickel hydrosilicate phase.

The TPR results demonstrate rather well the differences between Ni compounds formed on the surface of supports. The weak metal-support interaction in the Ni-Mg/PF system is probably responsible for the hydrosilicate formation at a low level, which could decrease the difficulty in the system reduction. The Ni-Mg/D and Ni/SiG systems are difficult to reduce and are comparable in reduction characteristics to nickel hydrosilicates. The addition of silver to the Ni-Mg/D system significantly affected reducibility of nickel-based catalysts. Larger nickel crystallites in silver modified nickel catalysts displayed easier nickel reduction than smaller ones in the Ni-Mg/D catalyst.

The hydrogen chemisorption study showed that the size of nickel nanoparticles obtained in the studied catalyst precursor systems depended on the nature of precursor nickel salt from which they are formed, the kind and loading of metal modifier and the type of support used.

The XPS study of Ni/SiG, Ni-Mg/D and Ni-Mg-Ag/D precursor samples confirm the formation surface species with different strength of interaction and different dispersion of the supported nickel species.

The silver modifier inhibits hydrogenation activity, this effect being more obvious as the Ag loading is higher. Modification by silver allowed us to promote the selectivity toward the *cis* isomers, but the catalyst is less active than the non-modified catalyst in the partial hydrogenation of soybean oil.

Among the catalyst samples studied, the highest activity in the sunflower oil hydrogenation was observed over the Ni-Mg/PF-1 catalyst suggesting that the Ni-Mg/PF-1 catalyst is a promising catalyst for SFO hydrogenation. Although Ni/SiG catalysts show a lower overall activity, this system also could be considered as good, since they produced less amount of stearic acid compared to the Ni-Mg/PF system.

The kinetic models include the saturation of double bonds along the fatty acids chains and *cis*/*trans* isomerization. Under studied operating conditions models proved to adequately fit the experimental data for the evolution of product distribution with reaction time. It was shown that the catalysts of different activities had different reaction pathways. The more active catalysts, the reaction pathways multiply and require more complex reaction scheme to describe the results of the catalytic tests.
