New chromium(III)-based catalysts for ethylene oligomerization

The report focuses on the new precatalysts for ethylene oligomerization. The five chromium(III) complex compounds containing the following ligands: dipicolinate anion, oxalate anion, 5-aminopyridine-2-carboxylate anion, 2,2′-bipyridine and 4,4′-dimethoxy-2,2′-bipyridine have been examined towards catalytic activity for ethylene oligomerization. The chromium(III) complexes have been activated by modified methylaluminoxane. The obtained oligomers have been investigated by MALDI-TOF–MS, thermal analysis and infrared spectroscopy. The results revealed that the examined chromium(III) complexes are highly active catalysts for ethylene oligomerization. The values of catalytic activities of the examined complexes are in the range 1860 – 3798 g∙mmol-1∙h-1∙bar-1.


Results and discussion
The catalytic activity of the ethylene oligomerization reaction using chromium(III) complex compounds ( Fig. 1)  O during the analysis, DHB matrix-2,5-dihydroxybenzoic acid was used. In the registered mass spectrum we see a peak at m/z = 177.013, which comes from the matrix used during the analysis. Peaks at m/z = 273.022, m/z = 354.974, m/z = 449.338 contain 10, 13 and 16 mers respectively in the oligomer chain. The molecular peak at a m/z = 332.984 contains 11 mers in the oligomer chain. The main product contains 24 carbon atoms in the oligomeric chain. The molecular peak has a signal intensity of 9955 [a.u.].
The mass spectrum in Fig. 2   The CAA matrix was used in the case of the analysis of the ethylene oligomer obtained with (5-aminopyridine-2-carboxylate) 3 chromium(III) complex. Peaks at m/z = 379.069 and 839.326 are signals from the used matrix. The molecular peak was characterized by 260.936 m/z. It can be concluded, that it contained 9 mers in the oligomer chain which is built from 18 carbon atoms. Its signal intensity was 27,146 [a.u.]. The peak at m/z = 294.054 contained 10 mers in the chain.
The oligomers obtained during these studies were tested by thermal analysis in order to determine the temperature gradient distribution characteristics of the tested samples.  20.31% in the temperature range 280 °C -720 °C , that in the last stage up to 990 °C, the sample decomposes by 4.52%. Sample 2 also decomposed in 4 stages. The third stage is a weight loss of 19.31% in the temperature range 280 °C-640 °C, and then slightly decomposing the sample to 3.01% weight loss-decomposition carried out to 860 °C. In the case of sample 3, thermal decomposition took place in three stages. Stage 2 is a weight loss of 9.52% in the temperature range of 320 °C-680 °C. The last stage is sample distribution in 3.63% to 910 °C. Sample 4 was broken down in three stages. The third stage was 6.34% weight loss in the temperature range from 400 °C to 955 °C. The fifth sample in the temperature range 380 °C -520 °C decomposed by 4.54%, so that in the last stage to 905 °C the sample mass decreased by 6.80%. A different number of stages of thermal decomposition of the studied products of the ethylene oligomerization reaction may result from different branching and structure of oligomeric chains. Oligomers formed as a result of the ethylene oligomerization reaction catalysed by complex compounds were thermally decomposed with the probable separation of the following products: carbon monoxide(IV), carbon monoxide(II) and water.
The ethylene oligomers have been studied by IR. The obtained results confirm the composition of the oligomer chain (Tables 2, 3 O) complex has been synthesized as follows. 5-Aminopyridine-2-carboxylic acid (4 mmol, 0.55 g) was dissolved in ethanol and added to the aqueous solution containing CrCl 3 • 6 H 2 O (2 mmol, 0.54 g). All mixed up. Then, the mixture was heated at 50° C and mixed for 15 min. In the next step, the mixture was cooled and trichloromethane and methanol in the volume ratio 1 : 2 was added. After 2 months the (5-aminopyridine-2-carboxylate) 3 chromium(III) complex precipitated.

the oligomerization process
A solution of the complex as a catalyst was prepared by dissolving 1 μmol of the complex in 1 mL of toluene in a glass tube located above the magnetic stirrer. The entire polymerization process was carried out at atmospheric pressure, at room temperature and nitrogen atmosphere. In the next reaction stage, 1 mL of MMAO-12 solution was introduced. MMAO-12 was purchased from Sigma-Aldrich. It was 7 wt% aluminum in toluene.[(CH 3 ) 0.95 (n-C 8 )H 17 ] 0.05 AlO] n is linear formula of MMAO-12. The molar ratio of [Al]/[Cr] was equal to 1000:1. The solution was allowed to mix for a while, then ethylene was added all the time until the process was complete. The reaction was terminated according to the procedure described in the literature 21 . The pressure ethylene was released and the catalytic system was quenched with water.
MALDi-tof-MS. MALDI-TOF-MS analysis was performed for ethylene oligomerization reaction products using chromium complexes as reaction catalysts. Sample analysis was performed on DHB (2,5-dihydroxybenzoic acid) and CAA (α-cyano-4-hydroxycinnamic acid) matrices 22 . thermal analysis. Thermal analysis (TG) was performed on NETZSCH TG 209 instrument. Measurements were made in the temperature range from 0 °C to 1000 °C. The mass of samples subjected to thermal analysis was about 5 mg. Thermal analysis was carried out in an argon atmosphere. infrared spectroscopy. IR analysis was carried out using the KBr pastil. Measurements were carried out from 4000 cm -1 to 600 cm -1 .