*3.2.2. Cis/trans isomerization*

where H2c is the hydrogen consumption for decay in iodine value; *t* is the reaction time; *m*oil is mass of oil; and Nilcc is the nickel loading in the catalyst charge for hydrogenation run (in

The results of SBO hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in the slurry

**rH2c e (μmol H2**

 **min−1 goil −1)**

pressure); 750 rpm (stirring rate).

**A (μmol H2**

 **min−1 gNi**

**−1 goil −1)**

**H2c d (mol)**

Ni-Mg/D 0.9972 89.3 75 8.018 21.3 21.4 Ni-Mg-Ag0.16/D 0.8985 90.6 81 7.762 19.2 21.3 Ni-Mg-Ag1.55/D 0.8790 90.0 155 7.880 10.2 11.6 Ni-Mg-Ag5.88/D 0.8377 90.0 255 7.880 6.2 7.4

The obtained results showed clearly the influence of the silver addition on the catalyst activity. Under the same process conditions, Ni-Mg/D and silver modified catalysts exhibited different activities toward SBO hydrogenation (see **Table 8**). By comparing the results of catalytic test runs over Ni-Mg/D and Ni-Mg-Ag0.16/D catalysts, it can be observed that the activity of the sample without silver is slightly higher. The modification by silver inhibits hydrogenation activity, this effect being more obvious as the Ag loading is higher. From these results, the hydrogenation activity for the studied catalyst (**Table 8**) increases in the following order:

**Table 8.** Soybean oil hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in slurry pilot plant reactor system—

Ni loading in catalyst charge for hydrogenation run (catalyst concentration: 0.05 wt%, i.e., 2.5 g catalyst/5000 g oil).

The observed differences in the activity of the studied catalysts could be attributed to nickel dispersion and different textural properties of the catalysts. From the chemisorption results, the silver-modified Ni catalyst sample with high loading (Ni-Mg-Ag5.88/D) demonstrated 6.0% nickel dispersion and an average nickel crystallite size of ≈ 11 nm. A higher nickel dispersion and a smaller nickel crystallite size were obtained for the Ni-Mg/D catalyst sample (**Table 6**). Besides, among the studied catalysts Ni-Mg catalyst sample had the highest SBET surface area (**Table 4**). This indicates that the role of catalyst texture and dispersion of the active phase is critical in assessing the catalytic efficiency. In considering an explanation for the diminished hydrogenation activity of silver modified nickel catalyst it can be also assumed a physical blocking of nickel active sites or even changes in the morphology of nickel metal particles by the silver modifier. It is difficult to discriminate between these different possibilities. Apart from the effect of co-metal blocking of the surface nickel atoms, it should also be noted that an

pilot-plant reactor system (**Figure 4**) are presented in **Table 8**.

**IVfinal <sup>c</sup> t (min)**

160 New Advances in Hydrogenation Processes - Fundamentals and Applications

dHydrogen consumption for decrease of iodine value by *ca* 40–41 units.

Process parameters: 160°C (temperature); 0.16 MPa (H<sup>2</sup>

Rate of hydrogen consumption per unit mass of oil.

Ni-Mg-Ag5.88/D < Ni-Mg-Ag1.55/D < Ni-Mg-Ag0.16/D < Ni-Mg/D.

grams).

a

b

c

e

**Sample code Activity resultsa**

**Nilcc b (g)**

Iodine value determined by Wijs method.

catalytic activity test runs results.

The factors influencing the *cis*/*trans* composition are catalyst activity and loading, as well as process conditions that include the hydrogen pressure, temperature and stirring rate. These parameters determine the hydrogen concentration on the catalyst surface, which is crucial not only for hydrogenation of double bonds but for *cis*/*trans* isomerization as well.

The hydrogenation of SBO is a complex network of chemical reactions involving consecutive saturation of C18:3*c* to C18:2*c*, C18:2*c* to C18:1*c* and subsequent saturation of C18:1*c* to C18:0 as well as parallel reversible isomerization of C18:2*c* to C18:2*t* and C18:1*c* to C18:1*t*. The overall hydrogenation reaction scheme may also involve a simple step hydrogenation of C18:1*t* to C18:0 and C18:2*t* to C18:1*t*.

The fatty acid compositions of hydrogenated SBO at conversion of 30.8 ± 0.5% are shown in **Table 9**. The experimental data of the fatty acid compositions in hydrogenated SBO summarized in **Table 9** showed that in all the cases, there was an increase in the concentration of stearic acid (C18:0). On the contrary, a decrease in linoleic acid (C18:2*c*) was also observed in all the cases. It should be noted that linolenic acid (C18:3*c*) conversion after 30 min of reaction was 100% when the hydrogenation was performed over any of the studied catalysts. Taking into account a known fact that silver modified catalysts show high saturation selectivity toward linoleic acid (C18:2*c*) [102], disappearance of linolenic acid in the early stage of the hydrogenation was an expected result. As the present work is focused on the control of the TFAs, the geometric isomers are presented in **Table 9** without taking into account the position of double bond in the fatty acid chain, only distinguishing between *cis* fatty acids (CFAs) and TFAs.

From these results, it is evident that the Ni-Mg-Ag5.88/D catalyst formed the least TFAs of all catalysts (23%) at the same conversion level. On the contrary, the Ni-Mg/D catalyst was demonstrated to have the highest content of TFAs (62.1%), which could be associated with its activity manifested in SBO hydrogenation and a higher total surface area compared to the catalyst with a silver modifier. It is well known that a large surface area encourages the isomerization reactions, due to the greater accessibility to the nickel active sites [103]. A small increase of stearic acid in the order: Ni-Mg-Ag5.88/D < Ni-Mg-Ag1.55/D < Ni-Mg-Ag0.16/D < Ni-Mg/D could be explained by the differences observed in their textural properties (**Tables 4** and **5**). According to Balakos and Hernandez [13], small pores favor fatty acid saturation, since the successive hydrogenation is made easier by the mobility difficulties of the bulky molecule. The catalytic test results clearly show that adding silver to the Ni-Mg/D system have a considerable effect on the distribution of CFAs and TFAs in hydrogenated oil. A higher ratio of unsaturated *cis* fatty acids to *trans* fatty acids over the Ni-Mg-Ag5.88/D catalyst can be attributed to the smaller formation of TFAs, suggesting a more selective hydrogenation reaction. The modification with silver is beneficial in limiting the C18:1 *cis*/*trans* isomerization during the SBO hydrogenation. On the other hand, the addition of silver did not have significant effect on the formation and distribution of C18:2*t* isomers (C18:2*c,t* + C18:2*t,c* + C18:2*t,t*), indicating that the main reason for the difference in the specific isomerization selectivity (S*<sup>i</sup>* ) was favored isomerization of elaidic fatty acid (C18;1*t*) over the Ni-Mg/D catalyst sample. Concerning the isomerization selectivity (S*<sup>i</sup>* ), the addition of silver provoked the reduction of TFAs (R*trans*) in the range of 4–57% during SBO hydrogenation with respect to the Ni-Mg/D catalyst (see **Table 9**).


<sup>a</sup>Conversion (%) = [(IV₀–IVf)/IV₀] × 100 = 30.8 ± 0.5.

bSum of *ct, tc* and *tt* fatty acid isomers.

<sup>c</sup>Sum of C14:0, C16:0, C16:1, C20:0, C20:1 and C22:0 fatty acids.

<sup>d</sup>Selectivity basically means that the reaction is guided toward a particular unsaturated bond in preference to others; d1Si specific isomerization selectivity, defined as quotient of TFAs (%) in hydrogenated oil product and change in iodine value between the starting oil and hydrogenated product; d2S1—selectivity 1, defined as the amount of monounsaturated fatty acids (C18:1) formed with respect to the amount of diunsaturated (C18:2) converted: S1 = (C18:1–C18:1(0))/(C18:2(0)–C18:2). eR*trans*—reduction TFAs, defined as: R*trans* = {1 − [Δ(C18:1*t*+C18:2*t*)catalyst2/Δ(C18:1*t*+C18:2*t*)catalyst1]} × 100, Δ(C18:1*t*+C18:2*t*)catalyst1 > Δ(C18:1*t*+C18:2*t*) catalyst2,C18:2*t*—sum of *ct, tc* and *tt* fatty acid isomers.

**Table 9.** Soybean oil hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in slurry pilot plant reactor system—fatty acids compositionᵃ and selectivitiesd, d1, d2.

In general, the overall hydrogenation selectivity decreased while the isomerization increased with conversion. The mechanisms of the hydrogenation and *cis*/*trans* isomerization are strongly interrelated. An addition-elimination mechanism according to Horiuti-Polanyi is often assumed to describe the formation TFAs in the hydrogenation processes [58]. The original concept of Horiuti-Polanyi mechanism provides that hydrogenation and isomerization should both be described by a half-hydrogenated state mechanism. Since hydrogenation is accompanied by isomerization, it can be proposed that the electron donor characteristics of the silver modified nickel catalyst would also affect this reaction. If the chemisorbed half-hydrogenated intermediate is removed quickly enough, it may not have time to isomerize to *trans* or in the case of linoleic acid, when hydrogenation of one of the double bonds is complete, the fatty acid molecule is released before hydrogenation of the second double bond can occur. The limited formation of C18:1*t* over silver modified Ni-Mg-Ag/D catalytic system compared to that of the Ni-Mg/D system may be explained by both, the changing of electronic properties and the presence of geometric effect. The promoting effect of adding silver to the Ni-Mg/D system for SBO hydrogenation is manifested for all the catalysts tested in this work when considering the reduced TFA formation.

#### *3.2.3. Kinetic study of SBO hydrogenation*

test results clearly show that adding silver to the Ni-Mg/D system have a considerable effect on the distribution of CFAs and TFAs in hydrogenated oil. A higher ratio of unsaturated *cis* fatty acids to *trans* fatty acids over the Ni-Mg-Ag5.88/D catalyst can be attributed to the smaller formation of TFAs, suggesting a more selective hydrogenation reaction. The modification with silver is beneficial in limiting the C18:1 *cis*/*trans* isomerization during the SBO hydrogenation. On the other hand, the addition of silver did not have significant effect on the formation and distribution of C18:2*t* isomers (C18:2*c,t* + C18:2*t,c* + C18:2*t,t*), indicating that the main reason

elaidic fatty acid (C18;1*t*) over the Ni-Mg/D catalyst sample. Concerning the isomerization

4–57% during SBO hydrogenation with respect to the Ni-Mg/D catalyst (see **Table 9**).

C18:0 8.6 8.1 6.2 5.8 C18:1*c* 15.9 18.7 22.5 22.7 C18:1*t* 53.1 49.1 16.5 10.8 C18:2*cc* 0.9 1.2 22.7 31.7 C18:2*tt* 1.7 1.4 2.5 2.0 C18:2*ct* 2.9 3.9 7.6 6.9 C18:2*tc* 3.5 4.2 8.3 6.6

<sup>b</sup> 8.1 9.5 18.4 15.5 C18:3*c* none none none none Others<sup>c</sup> 13.3 13.6 13.7 13.5 CFAs/TFAs 0.3 0.3 1.3 2.1

d1 1.5 1.5 0.9 0.6 S1d2 1.1 1.1 3.1 1.8 R*trans* (%)<sup>e</sup> – 4.2 43.0 57.0

<sup>d</sup>Selectivity basically means that the reaction is guided toward a particular unsaturated bond in preference to others; d1Si specific isomerization selectivity, defined as quotient of TFAs (%) in hydrogenated oil product and change in iodine value between the starting oil and hydrogenated product; d2S1—selectivity 1, defined as the amount of monounsaturated fatty acids (C18:1) formed with respect to the amount of diunsaturated (C18:2) converted: S1 = (C18:1–C18:1(0))/(C18:2(0)–C18:2). eR*trans*—reduction TFAs, defined as: R*trans* = {1 − [Δ(C18:1*t*+C18:2*t*)catalyst2/Δ(C18:1*t*+C18:2*t*)catalyst1]} × 100, Δ(C18:1*t*+C18:2*t*)catalyst1

**Table 9.** Soybean oil hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalysts in slurry pilot plant reactor system—fatty

), the addition of silver provoked the reduction of TFAs (R*trans*) in the range of

**Ni-Mg/D Ni-Mg-Ag0.16/D Ni-Mg-Ag1.55/D Ni-Mg-Ag5.88/D**

) was favored isomerization of

for the difference in the specific isomerization selectivity (S*<sup>i</sup>*

162 New Advances in Hydrogenation Processes - Fundamentals and Applications

selectivity (S*<sup>i</sup>*

C18:2*t*

S*i*

**Fatty acid Catalyst sample**

<sup>a</sup>Conversion (%) = [(IV₀–IVf)/IV₀] × 100 = 30.8 ± 0.5.

<sup>c</sup>Sum of C14:0, C16:0, C16:1, C20:0, C20:1 and C22:0 fatty acids.

catalyst2,C18:2*t*—sum of *ct, tc* and *tt* fatty acid isomers.

bSum of *ct, tc* and *tt* fatty acid isomers.

acids compositionᵃ and selectivitiesd, d1, d2.

> Δ(C18:1*t*+C18:2*t*)

Several kinetics models of the hydrogenation of fatty oils containing polyunsaturated fatty acids were devised previously and reaction rate constants were evaluated for the various reactions [104–107]. All of the proposed kinetic models including various reaction pathways were incomplete. From a practical standpoint, it is justified because of the extreme complexity of the complete kinetic model, which would have to include all possible consecutive and isomerization reactions.

A mathematical model has been developed to describe the kinetics of both the hydrogenation and the *cis*/*trans* isomerization of SBO. A simple approach to model the rate constants for SBO hydrogenation over studied catalysts is presented. The rate constant model is constructed assuming the first order rate equations with respect to the compositions of the various *cis* and *trans* fatty acids in the hydrogenated SBO. In this model, the fatty acids are divided into five groups: (i) C18:2*c*—*cis* diunsaturated; (ii) C18:2*t*—*trans* diunsaturated; (iii) C18:1*c*—*cis* monounsaturated; (iv) C18:1*t*—*trans* monounsaturated; (v) C18:0—saturated (stearic acid). Mathematical equations were developed for all groups of fatty acids as a function of reaction time. The *ki* ′s (*i* = 1–8) are the respective first order reaction rate constants. Numerical solutions for the set of ordinary differential equations corresponding to the kinetic model were obtained through the Gear algorithm [108]. The rate constants were computed from the kinetic experiments by minimizing the sums of squares for deviations between the computed and experimental concentrations of studying fatty acids. The minimization was performed by the simplex method [109].

A reaction scheme, time dependence concentration of fatty acids and the estimated rate constants are presented in **Figure 11** and **Table 10**.

**Figure 11.** Hydrogenation of SBO: (a) reaction scheme; (b–d) correlation between experimental data and model predictions for SBO hydrogenation over Ni-Mg/D and Ni-Mg-Ag/D catalytic systems under operating conditions used.


**Table 10.** Hydrogenation of soybean oil over Ni-Mg/D and Ni-Mg-Ag/D catalysts—computed values of the rate constants (T = 160°C).

**Table 10** reveals that the rate constants of isomerization reaction *k*<sup>7</sup> and *k*<sup>8</sup> and constant of saturation reaction *k*<sup>4</sup> , corresponding to the stearic acid formation, are higher in the case of Ni-Mg/D and Ni-Mg-Ag0.16/D catalyst samples. These observations are in agreement with experimental data, shown in **Table 9** related to the formed C18:1*t* and C18:0 during the hydrogenation of SBO.

**Figure 11b**–**d** shows a comparison between experimentally measured and simulated modeling kinetics curves (parity plots for C18:1*c* and C18:1*t*—**Figure 11b**; C18:2*c* and C18:2*t*—**Figure 11c**; C18:0—**Figure 11d**). As it can be seen, generally, the difference between experimental results and model estimation is within 10%, which confirms the accuracy of the results. A model agrees with the general knowledge in hydrogenation and the data were fitted fairly well by the model. The proposed kinetic model could be applicable for the hydrogenation of SBO under studied operating conditions.
