Bio-based polymer from heterogeneous catalytic polymerization of vegetable oils

2.1 Materials

Chemical materials employed in the fabrication of samples, including nickel nitrate (Ni (NO₃)₂.6H₂O, Merck, Darmstadt, Germany), copper nitrate (Cu (NO₃)₂.3H₂O, Merck, Darmstadt, Germany), and ethylenediamine (C₂H₈N₂, Sigma-Aldrich, St. Louis, MO, USA) were used without further purification.

2.2 Synthesis of nickel and copper complexes

For tris(ethylenediamine)nickel(II) nitrate synthesis, at first, an aqueous solution of Ni (NO₃)₂.6H₂O (1 mmol, 0.29 g) in 20 ml of deionized water was prepared and heated in a reaction flask up to 100°C. At next step, C₂H₈N₂ (3 mmol, 0.8 g) was added to the solution dropwise with stirring, and then, the reaction proceeded at 100°C for 3 h under reflux conditions. The precipitant was aged for 24 h at ambient temperature under stirring and then dried at 80°C for 20 h. The bis(ethylenediamine)copper(II) nitrate was fabricated using 1 mmol Cu (NO₃)₂.3H₂O in 10 ml of deionized water and 2 mmol C₂H₈N₂ using the same procedure as discussed for the nickel complex synthesis except for the drying temperature (100°C) and the duration (30 h).

2.3 Synthesis of NiO and CuO catalysts

For both NiO and CuO catalyst fabrication, nickel- and copper-synthesized complexes were calcined at a rate of 10°C/min in open atmosphere up to 400°C and maintained at the same temperature for 3 h.

2.4 Sample characterization

For the synthesized complexes, electronic spectra were obtained using a Perkin-Elmer, Lambda 25 spectrophotometer. Infrared spectra were measured on a FT-IR PHILIPS, PU9624 spectrophotometer with KBr pellets. The crystalline structures of the NiO and CuO catalysts were studied by XRD using a EQUINOX 3000 diffractometer via Cu Kα (λ=1.5418 Å) as incident radiation. Crystalline phases were identified through tabulated powder diffraction files of the International Center of Diffraction Data (ICDD). In addition, the sample morphology was investigated using a microscope FESEM, model ALS2100.

2.5 Polymerization reaction system

Polymerization reaction occurred in a batch reactor system including a stainless steel reactor with a volume of 1 l equipped with a thermowell. The heating power was supplied through an electrical heating jacket mounted around the reactor, and the reactor temperature was controlled using a PID controller. For each run, 400 ml of soybean oil was charged to the reactor, and after heating up to 300°C, a weighed amount of catalyst sample was added to the reactor content. The reaction was performed under agitation, and the samples were taken within specific time intervals. Kinematic viscosities of the samples taken were measured by a Ubbelohde viscometer at 20°C according to ASTM D445. It is noteworthy to mention that the viscometer was calibrated using various concentrations of sucrose solutions at 20°C. Figure 1 depicts a scheme for the polymerization reactor system.

Figure 1:

Scheme of the reactor system.

3.1 Nickel and copper complex characterization

The electronic spectra obtained from [Ni(C₂H₈N₂)₃](NO₃)₂ and [Cu(C₂H₈N₂)₂](NO₃)₂ aqueous solutions are presented in Figure 2. As can be seen in Figure 2A, the electronic spectra for the nickel complex indicate four electronic transitions, an intensive peak observed at 280 nm, which can be assigned to the transition of nitrate ion. Three other characteristic peaks at 300, 360, and 530 nm are assigned to the d-d transitions of Ni²⁺ ion, including ³A_2g→³T_2g, ³A_2g→³T_1g(F), and ³A_2g→³T_1g(P). Also, in Figure 2B, the peak appearing in 280 nm corresponds to the nitrate ion, and the peak at 560 nm is assigned to the d-d transition of Cu²⁺ (³E_g→³T_g).

Figure 2:

Electronic spectra for (A) tris(ethylenediamine)nickel(II) nitrate and (B) bis(ethylenediamine)copper(II) nitrate.

The IR spectra for [Cu(C₂H₈N₂)₂](NO₃)₂ (Figure 3A) show all absorption bands assigned to the ethylenediamine ligand, as absorption modes at 3000–3500, 2964, 1606, 1275, 1043, and 977 cm⁻¹ can be attributed to υ(N-H), υ(C-H), N-H deformation, δ(NH), υ(C-N), and υ(C-C), respectively [19]. Furthermore, absorption bands at about 525 and 1382 cm⁻¹ are attributed to Cu-N stretching vibration and nitrate ion, respectively [20]. Also, in the IR spectrum of [Ni(C₂H₈N₂)₃](NO₃)₂ (Figure 3B), all absorption bands corresponding to the ethylenediamine ligand, are observable, and absorption bands at about 500 and 1235 cm⁻¹ can be attributed to the Ni-N stretching vibration and the nitrate ion existence, respectively [21].

Figure 3:

FT-IR spectra of (A) bis(ethylenediamine)copper(II) nitrate and (B) tris(ethylenediamine)nickel(II) nitrate.

3.2 Nickel and copper oxide characterization

The XRD pattern for the NiO catalyst (Figure 4) implies the presence of nickel oxide (JCPDS No. 78-0423) and nickel (JCPDS No. 3-1043) phases in the product. Also, all diffraction lines in the XRD pattern of the CuO (Figure 5) catalyst are assigned to copper oxide (JCPDS No. 80-1916). Also, the SEM image of the Ni/NiO and CuO catalysts are depicted in Figure 6, from which it can be seen that the spherical shape for the metal oxide particles has a rather high degree of agglomeration.

Figure 4:

XRD pattern of synthesized nickel oxide.

Figure 5:

XRD pattern of synthesized copper oxide.

Figure 6:

SEM images for (A) synthesized nickel oxide and (B) synthesized copper oxide.

3.3 Soybean oil polymerization

It is well known that the polymerization reaction take place at temperatures above 260°C when the dienes obtained from the conjugation of double bonds react via Diels-Alder reaction to form a cross-link between chains or through the combination of free radicals. Scheme 1 depicts the possible polymerization reaction of vegetable oils.

Scheme 1:

The polymerization of vegetable oil.

On the other hand, the occurrence of the reaction at temperatures above 350°C leads to thermal cracking of triglycerides that results in the production of such compounds as hydrocarbons, carboxylic acids, and other oxygenated materials [18]. Therefore, the reaction temperature was set at 300°C, which is within the two above-mentioned limitations.

Soybean oil polymerization occurred at 300°C with various catalyst samples, including nickel and copper complexes and oxides, and the reaction progress was plotted as viscosity (cSt) versus time (h) in Figures 7–9. As can be seen in Figure 7, by thermal polymerization, without using any catalyst, after 3 h, the viscosity of the oil increased 4 times. By adding 0.1 wt% tris(ethylenediamine)nickel(II) nitrate to the reaction, the viscosity of the reaction product was enhanced about 75% in comparison with the non-catalyzed thermal polymerization. By increasing the nickel complex loading to 0.25 and 0.5 wt%, the product viscosity increases 2.3 and 2.7 times compared with the non-catalyzed thermal polymerization, respectively, suggesting trivial enhancement in viscosity by increasing the catalyst loading from 0.25 to 0.5 wt%. The kinematic viscosity obtained using a 0.1 wt% nickel complex catalyst is close or slightly higher than the reported results with the same amount of catalyst [3].

Figure 7:

Soybean oil polymerization by Ni complex catalyst at 300°C.

Figure 8:

Soybean oil polymerization by Cu complex catalyst at 300°C.

Figure 9:

Soybean oil polymerization by Ni/NiO and CuO catalysts at 300°C.

An acceleration in viscosity increase is observed for almost all catalytic runs after passing 2 h from the beginning of the reaction, which can be ascribed to the formation of molecules with longer chains as a result of the combination of molecules with longer chains and also the formation of cross-link bonds [22]. Figure 8 presents the enhancements of 43%, 65%, and 130% in product viscosity compared with thermal polymerization for 0.1, 0.25, and 0.5 wt% loadings of bis(ethylenediamine)copper(II) nitrate, respectively.

As shown in Figure 9, compared to the complex catalysts, the presence of nickel and copper oxide catalysts does not have significant influence on the reaction progress and only leads to a slight increase in the viscosity for the nickel oxide catalyst. Better activity of the complex catalysts compared with the metal oxides can be ascribed to the complex chemical structure and their intrinsic reactivity. It can be postulated that, first, the ethylenediamine ligand separates from the metal ions, and in the next step, the reaction was catalyzed by the metal ion with oxidation states [23], whereas the metal oxide catalysts, because of their stability, are needed to be reduced before catalyzing the reaction, demanding more energy that can be supplied at temperatures above 300°C.

Also, comparing the results shown in Figures 7 and 8 reveals that the nickel complex catalyst exhibited a better performance in the soybean oil polymerization reaction. This observation can be justified by the fact that, despite more oxidative potential of Cu²⁺ compared with that for Ni²⁺, Cu²⁺ is not able to act as an effective oxidative agent due to instability and oxidative sintering at temperatures above 200°C [24].

Figure 10 presents the polymerization progress of soybean oil in the presence of nickel and copper complexes after 2.5 h from the beginning of the reaction using FTIR analysis.

Figure 10:

FT-IR spectra of non-polymerized and polymerized soybean oil by Ni and Cu complex catalysts.

As can be seen in the IR spectrum, the C=C and C-H alkenes bands appeared at 1650 and 3015 cm⁻¹, respectively [25]. For both samples, the strength of the mentioned bands decreases, indicating breaking double bands and progressing the polymerization reaction. Also, weaker C=C and C-H alkenes bands in the IR spectrum of the sample polymerized by the nickel complex reveals a greater amount of broken double bonds in the polymerization process compared with the process catalyzed by the copper complex, indicating a higher degree of polymerization when using the nickel complex catalyst. This observation is in accordance with higher magnitudes of kinematic viscosity resulting from the polymerization process catalyzed by the nickel complex catalyst (Figures 7 and 8).