Cat-CrNP has been tested and characterized towards its physicochemical propeties by multi analytical methods such as XRD, elemental analysis, IR, MALDI-TOF-MS, potentiometric conductometric titrations, thermal analysis and scanning electron microscopy. Cat-CrNP is new nitrilotriacetate complex compound of chromium(III) with 1,10-phenanthroline (Fig. 1).
The title compound has an interesting structure. Cat-CrNP was obtained as violet-gold crystals. In the crystal of title compound, molecules of nitrilotriacetate complex compound of chromium(III) with 1,10-phenanthroline are linked via C–H···O hydrogen bonds and π-π interactions to produce blocks along b-axis, whereas water molecules interact via O(water)–H···O(water) interactions to form a centrosymmetric, alternately arranged (H2O)4 (cyclic-planar, tetrameric 0-4-A structure) and (H2O)8 (cyclic-chair, octameric 0-8-I structure) water clusters, which produce tapes extending along a-axis [21,22,23]. In the crystal packing, the adjacent blocks and tapes are connected through C–H···O(water) and O(water)–H···O hydrogen bonds to form a 3D framework structure (Fig. 2).
Elemental analysis of as-prepared Cat-CrNP revealed that it is composed of C (44.18 %), H ( 4.51 %) and N (8.54 %). Anal. Calcd.: C, 43.91 %, H, 4.51 %, N, 8.53 %. Thus, experimentally determined composition of the chromium(III) complex compound perfectly fits theoretical findings.
The IR analysis showed that medium intensity stretch band from carboxylic acid occurs at 3440.39 cm-1. In the range 2969.65-2938.75 cm-1 O - H stretching vibrations of crystallization water are observed. The range 1684.67-1664.16 cm-1 confirms the presence of C = O stretching of the carbonyl group. The peak at 444.76 cm-1 corresponds to deformation vibrations Cr - N.
MALDI-TOF-MS revealed that Cat-CrNP fragments into M - COO and M - COOH. The potentiometric titration has been used to investigate the stability of the title complex compound by Hyperquad program. The value of the logβ for Cat-CrNP equal to 8.97 ± 0.05. The conductometric analysis allow to confirm the composition of the chromium(III) complex compound. The decrease in the conductivity of the solution is maintained to nNaOH:nH3NTA = 4, then nNaOH:nH3NTA > 4 there is an increase in conductivity. At nNaOH:nH3NTA equal to 3 the conductivity of the solution is lowered due to the formation of the complex compound.
Cat-CrNP was subjected to thermal analysis using the thermogravimetric method. The analysis of the thermal decomposition of the coordination compound was carried out in the temperature range from 0 °C to 1000 °C. Cat-CrNP undergoes thermal decomposition in 7 stages. The first stage of decomposition reached the temperature of 80 °C, where the weight loss was 8.23%. In the first stage of thermal analysis, a peak is observed at 62.2 °C, where there was a weight loss of 2.68%. The second stage of thermal decomposition occurred in the temperature range of 80 °C to 140 °C, where a second peak is observed, weight loss (1.23%) at 90.8 °C. The third stage of thermal analysis of Cat-CrNP occurs in the temperature range of 140 °C - 380 °C. The weight of the sample during the analysis in this temperature range decreased by 24.65%. In further analysis of TG up to 440 °C a mass loss equal to 10.67% and a peak at 437.7 °C is observed, where 2.38% of the sample mass has been decomposed. Then the sample decomposed in 10.67% to the temperature of 460 °C and in the next stage by 35.94% to 660 °C. In the last stage of decomposition of the sample, the weight loss was 1.53%. Finally, SEM analysis was used for catalysts morphology imaging. As it was shown in Fig. 3, the chromium complex exhibited a plate-like structure with regular edges and smooth surfaces.
Cat-CrNP has been used for the oligomerization of 2-chloro-2-propen-1-ol and ethylene. The oligomers obtained in the oligomerization of 2-chloro-2-propen-1-ol have been characterized by IR. Medium intensity stretch band of the O-H group in the oligomer chain participating in hydrogen bonding occurs at 3474.9 cm-1. CH2 bending vibration in the oligomer chain is confirmed by peaks in the range 1456.84 - 1401.64 cm-1. C-O stretching vibration occurs at 1177.54 cm-1. C-Cl bending vibrations in the oligomer chain have been confirmed by the presence of the peak at 575.02 cm-1. MALDI-TOF-MS analysis showed that the oligomer sample contains from 4 to 7 mers in the chain. The oligomerization product of 2-chloro-2-propenol was tested for thermal stability. The sample was decomposed in 5 stages. The first stage of decomposition of the sample resulted in a weight loss of 10.21% and it was recorded up to 140 °C. A peak was recorded in the above-mentioned temperature range at a temperature of 101.1 °C, where the weight loss was 1.18%. The second stage of thermal decomposition of the tested sample resulted in a weight loss of 5.91% in the temperature range of 140 ° C - 200 °C. In the third stage 24.62% of the sample decomposed in in the temperature range of 200 °C - 440 °C. In this interval, a peak was recorded at 236.0 °C (weight loss 1.49%). In the penultimate stage, the sample decomposed in 12.01% (440 °C - 750 °C), while in the last stage it was 2.37% (750 °C - 1000 °C).
The analysis of samples obtained as a result of ethylene oligomerization confirmed the structure of the product obtained. IR spectrum allow to concluded that peaks at 2924.11 cm-1 confirms strong asymmetric CH2 stretching vibration in the chain of the oligomer. The peak at 2853.22 cm-1 correspond with strong symmetrical CH2 stretching vibration. The presence of strong deformations of CH2 in the oligomer chain is confirmed by the peak at 1513.59 cm-1. Peaks in the range 1467.69 - 1426.22 cm-1 allow to conclude that weak symmetrical deformation of the terminal CH3 occurs in the oligomer chain. The peak at 856.96 cm-1 confirms the presence of CH2 rocking vibration. MALDI-TOF-MS showed that the product of ethylene oligomerization contains from 12 to 24 mers. In order to understand thermal stability of the obtained oligomers, the sample of oligomers was subjected to the thermal analysis. The sample decomposes in 5 steps. In the first stage of 5.17% undergoes decomposition to 100 °C. Additionally a peak at 83.6 °C was recorded, where the weight loss was 1.23%. The second step of thermal analysis (100 °C - 320 °C) resulted in 27.83% weight loss of the sample. The peak at 149.5 °C was recorded, where the sample weight decreased by 2.15%. In the next stage of thermal analysis (320 °C - 520 °C) the sample decomposed by 9.38%. The penultimate stage of the sample decomposition was recorded in the temperature range of 520 °C - 740 °C (weight loss 6.68%). In the last stage of sample decomposition, the sample decreased by 7.08%.
Glass transition temperatures were determined for the products of oligomerization of 2-chloro-2-propen-1-ol and ethylene by DSC method. In the case of a sample of the 2-chloro-2-propen-1-ol oligomerization product, the cooling curve records the thermal change (Tg) at -113 °C, and in the case of the heating curve at T = -95 °C. In the case of the ethylene oligomerization product sample, the cooling curve records the thermal change (Tg) at -109 °C, and in the case of the heating curve at T = -91 °C.
The morphology of the 2-chloro 2-propen 1-ol and ethylene oligomers were investigated by SEM, and the results are shown in Fig. 4. SEM images show that oligomers presented small, highly coagulated, and nonuniform particles. According to the literature,[24,25] such a one-dimensional structure might physically provide a better dispersion in the monomers and better exposure of active sites to the reactants thus achieved high catalytic activity [24,25].
The yield of catalytic oligomerization over Cat-CrNP equaled to 213.92 g ∙ mmol-1 ∙ h-1∙ bar-1 and 3232 g ∙ mmol-1 ∙ h-1 ∙ bar-1 for the 2-chloro-2-propen-1-ol and ethylene, respectively. It means that Cat-CrNP is very highly active catalyst for the ethylene oligomerization and it is highly active catalyst for the 2-chloro-2-propen-1-ol oligomerization (Figures 5 and 6). Analyzing other chromium(III) complexes with catalytic properties in ethylene oligomerization (Figure 5), it can be concluded that the Cat-CrNP - new catalytic material shows catalytic activity similar to the best precatalysts. Catalytic oligomerization of ethylene over Cat-CrNPs is only a slightly lower than that previously reported for [Cr(dipic)2]Hdmbipy·2.5 H2O (3232 and 3798 g·mmol-1·h-1·bar-1, respectively), nonetheless, synthesis of material reported in this work does not require the usage of thrichlormethane and methanol, and precipitation time was shortened from 2 months to 14 days. It should also be noted that the investigated oligomerization processes with the participation of the new material Cat-CrNP proceed in very mild conditions. Although other catalysts with higher catalytic activity values than the catalyst described in this work are known in the literature, but synthesis of Cat-CrNP is very simple, cheap and it crystallizes very quickly, comparing to synthesis procedure of commercially available catalysts. In addition, the synthesis uses mainly an environmentally friendly solvent - water.