In our study, along with oil fractions rich in aromatic hydrocarbons (RPF and PPF) the alkylation of α-olefins (hexene-1, octene-1, decene-1) with individual aromatic hydrocarbons (mainly xylenes) was carried out in the presence of ILCS and the results were compared. The regularities of the reactions carried out with individual hydrocarbons were also observed in the reactions carried out with RPF and PPF. Moreover, reactions to two-phase catalysis were carried out in this direction. It is known that the reactions occur on the surface of the IL. In this case, a diffusion equilibrium is formed and the reaction continues.[4, 10] In (oligo) alkylation processes, the reaction follows a similar situation. This confirms itself in both cases—when taking both individual hydrocarbons and RPF. The yields of AP are comperable. However, considering the corrosive ability of AlCl3 and because of economical considerations, it has been decided to reduce the amount of AlCl3 and replace a certain ratio of this with ZnCl2. On the other hand, “harder” acid-centered catalytic systems are being replaced by “softer” active centers. The changing of the nature of the active center allows to regulate the molecular parameters of AP. (Table 2) Furthermore, NMPC has been taken as a modifier in the composition of ILCS. In this case, although the yield of AP decreases, it is possible to regulate molecular parameters. The use of NMPC taken as a modifier improves the dispersibility of ILCS according to our investigations. This shows a great impact on two-phased catalytic systems. Moreover, the selectivity of the target product increases. The studies on the IL demonstrate that the investigations carried out in this field are of current interest and have a wide scientific and practical significance. At the same time, the reduction of the used amount of IL and the absence or a strong limitation of IL can contribute to significant consequences from an economical point of view. It was also possible to recycle the catalytic system by recovering the IL phase and reusing it in the next cycles with moderate results. The yields of the products and the conditions of alkylation of a liquid fraction rich in aromatic hydrocarbons and individual aromatic hydrocarbons obtained from the reforming process (RPF) and the pyrolysis process (PPF) with α-olefins based on IL were illustrated in Table 1.
In this table alkylation processes carried out in the presence of pure IL and NMPC has been presented and results of experiments carried out with individual aromatic hydrocarbons and RPF and PPF is compared. It is obvious that individual aromatic hydrocarbons are much more reactive than oil fractions rich in aromatic hydrocarbons (RPF and PPF). This originates from olefins and substances containing iso-structural substituted aromatic hydrocarbons in the oil fractions. So this influences the rate of a chemical reaction and the yield of AP. Likewise, all these opinions were proved by our investigations and the results were presented in Table 1. On the other hand, it is shown from the Table 1 that using RPF without separation into narrow fractions (RFB, RFT, and RFX) increases the yield of AP (> 70% weight) significantly in the concentration of 1–3% AlCl3 and the presence of [(C2H5)3NH]+[Al2Cl7]−, compared to using the narrow fractions of RPF and PPF. After the third re-use of AP 14, the yield of which is 94%, as shown in Table 1, the yield falls to 75%.
According to the results of the analysis, the RPF (80–145 °C) fraction contains up to 60% of aromatic hydrocarbons. It is known that due to the presence of this oil fraction, it contains not only aromatic hydrocarbons but also non-reactive aliphatic hydrocarbons. Of course, when RPF is divided into narrower fractions (RFB, RFT, RFX), they also contain other non-reactive hydrocarbons. Therefore, compared to pure individual hydrocarbons, the yield is reduced in this case. However, when RPF is used directly, it has a synergistic effect due to the presence of various aromatic hydrocarbons, and the yield of AP increases. The yield of AP increases up to 80%. Therefore, economically and ecologically, it is more acceptable to use RPF directly. Our main goal is to use RPF directly, too. Furthermore, because of less viscosity and flowing ability, the use of the TEAHX IL leads to an increase in the yield of AP impressively compared to the PHX IL. But high viscosity of PHX influences to the diffusion factor in alkylation processes.
On the other hand, because of the existence of olefins the use of PPF is not recommended for alkylation processes, therefore a dramatic decrease in the yield of AP (< 35% weight) was observed. During this process mainly oil polymer resin (OPR) (> 70% weight) was obtained through the use of PPX. It has been shown from researches that PPF and their narrow fractions can be recommended in producing OPR.
The definition of AP via spectroscopic methods.
The IR – spectroscopy of samples obtained as a result of AP has observed the following absorption bands: 691 cm−1 and 725 cm−1 deformation pendulum vibration of C – H bond which belong to aromatic hydrocarbons; 1033 cm−1 and 1079 cm−1 flat deformation pendular vibration of C-H bond; 1803 cm−1, 1860 cm−1 and 1943 cm−1 common deformation pendular vibration of C-H bond; deformation (1494 cm−1) and valent (3026 cm−1) pendular vibration of C-H bond of the aromatic ring; 1603 cm−1 valent pendular vibration of C=C bond of the aromatic ring; deformation (1380 cm−1, 1456 cm−1) and valent (2870 cm−1, 2920 cm−1) pendular vibration of C-H bond of methyl groups. IR spectrums of a heavy faction of AP (Tboil. > 250 °C) are similar and appropriate to their substituted aromatic hydrocarbons. It should be noted that, substituted groups in the aromatic chain are also isostructural and absorption bands of these groups prove this opinion by IR – spectroscopy (1304 cm−1). Some IR–spectroscopies of RPF and AP obtained based on RPF results were illustrated in Fig. 1.
RPF, and based on it, the obtained AP were analyzed comparatively by UV–spectroscopy. It has been shown that high intensity and 2 characteristic absorption bands of mononuclear aromatic hydrocarbons (basically benzene, toluene and xylene) were observed in 198 nm and 200 nm regions in UV spectra of RPF. Likewise, intensive peaks were observed in 201 nm and 204 nm regions in UV spectra of AP. Thus, hypsochromatic shifts were observed because of RPF.
The results of the definition of the structural parameters of products by 1H NMR spectroscopy method, which were obtained from the alkylation of RPF with C6–C12 α-olefins are consistent with the data of IR spectral analysis. In 1H NMR spectra of all synthesized AP the resonant absorption band of the protons of methyl (0.98 ppm), methylene (1.28–1.3 ppm) and methine (1.4–1.7 ppm), corresponding to the chemical shift of the protons in the alkyl group, appears. In the 1H NMR spectra weak bands of protons of the methylene groups (5.0—5.5 ppm) with CH=CH2, and vinylene (4.5—5.7 ppm) CH=CH groups appear, indicating a small number of double bonds in the composition of the products obtained. At the same time, multiple signals corresponding to the substituted aromatic rings were observed in 6.8–7.9 ppm region. The triplet bands corresponding to the aromatic rings were observed in 1H NMR spectra of AP in 7.39 ppm, 7.29 ppm and 7.16 ppm regions. It has been determined that the state of bands and integral dependence belong to a different alternative phenyl group, so 7.39 ppm corresponded to ortho-substituted, 7.29 ppm corresponded to meta-substituted and 7.16 ppm corresponded to para-substituted. It is shown from Fig. 2 that the nature of multiplet signals belonging to aromatic hydrocarbons of RPF and AP have been changed.
As mentioned above, according to the 1H NMR images of AP 18 (Fig. 2b) and AP 28 (Fig. 2c) aromatic hydrocarbon signals are present at 7.39, 7.29 and 7.16 ppm. They differ from each other because of the nature of multiplet signals. This means that the addition of NMPC to the catalytic system has some effect on the direction of the reaction, and the structure of the aromatic product changes to some extent during alkylation process.
Thus, the results of IR-, UV- and NMR- spectra indicate the presence of alkyl aromatic compounds in the heavy fraction of AP (Tboil > 250 °C).
Depending on the rise of temperature, change in the speed of heat flux in the DSC curves were observed. It can be seen from the DSC curve of RPF that an endothermic peak with an initial temperature (Ti) – 99.00 °C, maximal temperature (Tm) – 125.61 °C and last temperature (Tl) – 137.16 °C was observed. This process corresponds to the boiling temperature of RPF and its enthalpy (ΔH) is 59.03 J/g. Additionally, an endothermic peak belonging to RPF was observed in the DSC curve of the product obtained in the alkylation of RPF with decene-1. AP is stable about 227.20 °C and Ti = 266 °C, Tm = 290.16 °C, Tl = 325 °C; ΔH = 42,99 J/g. An exothermic peak was observed in the DSC curve of the product obtained in the alkylation of RPF with hexene − 1, AP is stable about 153.24 °C and Ti = 236.5 °C, Tm = 247.50 °C, Tl = 265 °C; ΔH = 15.22 J/g. Additionally, exothermic peak was observed in the DSC curve of AP obtained in the alkylation of RFT with hexene-1, AP is stable about 172.73 °C and Ti = 215.43 °C, Tm = 250.79 °C, Tl = 305.75 °C; ΔH = 76,74 J/g. The DSC researchers of AP (> 250 °C) defined that, obtained products are generally thermostable around 190 °C. The results have been illustrated in Fig. 3.
It is known that molecular characterizations are some of the important data to obtain various products, including AP. For AP according to the size exclusion chromatography, the following parameters were determined Mw = 302–1372; Mn = 260–1125; Mw/Mn = 1.05–1.28. Also an average numeral functionality (fn) was calculated based on molecular data of the obtained reaction products. According to the value of the fn it was determined that alkylate products with di- and tri-(oligo)alkyl aromatic fragments and oligomeric macromolecules without aromatic rings were obtained in the process. Thus it can be reasonably assumed that the following types of macromolecules are formed in the studied reaction: I–Ar- (Ol)n; II–Ar- (Ol)n-Ar; III–(Ol)n. The formation of the Ar-(Ol)n-Ar structure in these processes is presumed based on the analysis of exclusion chromatography. Based on the molecular parameters of (oligo) alkylates, their average numeral functionality (fn) was determined for aromatic fragments. When the average numeral functionality for aromatic groups is higher than one (fn ≥ 1), it can be assumed that the AP also contains fractions with functionality fn = 2, in other words, dimeric and tetrameric oligomeric chains with two aromatic finite fragments type–Ar- (Ol)n-Ar, where n = 2–4. These data change, when is used various ILCS. We assume that when both the alkylation and oligomerization processes take place at the same time, the double bond in the substituent is exposed to re-alkylated and this structure can be formed. Molecular characterizations of obtained AP were illustrated in Table 2. The names of the AP correspond to Table 1.
Regularities are followed for both yields and molecular parameters. The yield was 90% for individual hydrocarbons and 80% for RPF. The data have been illustrated in Table 1 and in the diagram. According to the exclusion chromatography, for individual hydrocarbons Mw–345–780, Mn–286–630, Mw/Mn–1.11–1.24, for oil fraction Mw–355–1372, Mn–281–1125, Mw/Mn–1.05–1.22. The results have been presented in Table 2. The regularity of the reaction refers mainly to these parameters. As individual hydrocarbons, we have taken benzene (80,1 °C), toluene (1106 °C) and xylenes (o–xylene—144 °C, m-xylene–139 °C, p-xylene–138 °C) in our research, and accordingly, as an oil fraction the boiling point of RPF rich in aromatic hydrocarbons, is 80–145 °C. The boiling point of the oil fraction is close to the boiling point of individual aromatic hydrocarbons. Molecular parameters were regulated depending on the reaction conditions and the selection of the optimal composition of ILCS. The results prove that, is possible to obtain important petrochemical alkyl aromatic products with preassigned properties and structures in the presence of ILCS in the future.
Fluorescent Indicator Adsorption (FIA) was used to more accurately study the hydrocarbon content of the obtained AP and to determine the effect of the composition of ILCS on it. FIA analysis is considered informative for more accurate study of the composition of (oligo) alkylates. The results are presented in Table 3.
According to the results of FIA analysis, the addition of NMPC and ZnCl2 orient the process in a more alkylating direction. In this case, the composition of the reaction product changes and becomes more homogeneous. No paraffin was found in these samples.
It is known that along with the process of alkylation of aromatic hydrocarbons with α-olefins, oligomerization of α-olefins also occurs. Because of this issue, the term ‘(oligo)alkylation’ has been used by academician A.H.Azizov in the early 80s [5, 6, 10]. To lead the process more to the direction of alkylation, the amount of aromatic hydrocarbons is taken more than the olefins. In our research, we also have taken the molar ratio of aromatic hydrocarbon to olefins as 2:1. Using ZnCl2 and NMPC in the catalytic system can change the direction of the reaction. To replace a certain ratio of AlCl3 with ZnCl2 causes the reaction to be carried out in a “softer” condition. It is known that Fridel-Crafts alkylation occurs very rapidly in the presence of more reactive AlCl3. IL plays an important role and leads to regulate the direction of this reaction. It is known that reactions occur on the surface of IL. In this case, a diffusion equilibrium is formed and the reaction continues. We have conducted this research in our previous work. More research on oligomerization has also been conducted on the probable mechanism.
It is known that in these processes, along with alkylation in the presence of IL, oligomerization of olefins (dimerization, trimerization, etc.) occurs. In each case, 100% conversion of the olefin is observed. Olefin is used for both alkylation and oligomerization. Obviously, the obtained (oligo) alkylates have a boiling point above 250 °C. According to the result of various analyses, the olefins determined in these samples are clearly higher olefins. The presence of higher α-olefins has been determined by many analytical methods:
The signal intensity of CH2 groups in a long aliphatic chain increases according to 1H NMR.
The signal intensity of CH2 groups in the long aliphatic chain increases according to IR—spectral analysis.
According to the exclusion chromatographic analysis, oligomeric macromolecules without aromatic rings were obtained in the process. It is known that 2 detectors have been used in this method. The first detector measures the refraction of the total product (both oligomeric and alkylaromatic compounds), and the second detector measures ultraviolet signals (obviously, it especially determines alkylaromatic compounds). A comparison of these two indicators shows that along with aromatics, oligomeric products are also obtained in the product.
FIA analysis shows that along with aromatic hydrocarbons, the total product contains higher olefins.
It should be noted that, compared to individual aromatic hydrocarbons, there are specificities of RPF for alkylation processes. Comparative analysis of the alkylating process with RPF rich in aromatic hydrocarbons and individual aromatic hydrocarbons was studied and similar results were achieved (Fig. 4). It has been proven that alkylation process can be carried out in the presence of ionic liquid catalytic systems and modifiers using direct oil fractions. As ILCS, TEAHC IL (IL synthesized from triethylamine hydrochloride and AlCl3), PHC IL (IL synthesized from pyridinium chloride and AlCl3), TEAHC IL+ZnCl2 and PHC IL+ZnCl2 were used. The diagram illustrates the comparative analysis of alkylation process of α-olefins carried out with individual aromatic hydrocarbons and RPF in the presence of different catalytic systems.
As mentioned above, the chief value of alkyl aromatic components lie in their wide range of applications. One of them is the use of AP as an additive for various oils and polymer composites. To find a valuable application field for the obtained AP, some of their properties have been studied. The physical and chemical parameters of the obtained alkylates were also compared. According to some analysis results, the viscosity indices of obtained APs (Tboil. > 250 °C) were between 51–96.3, freezing points between minus 62 °C–minus 45 °C, densities between 0.8681 and 0.8686 and refraction indices between 1.4906 and 1.4964. It has been determined that because of their low freezing temperature, the obtained AP can be used as valuable petrochemical products in refrigeration and lubrication systems.
Furthermore, the obtained AP have been tested as a plasticizer additive in the composition of polyolefin composites. The thermophysical properties of various polyolefins and compositions based on AP were determined. In each case, a drop in the melting point of the initial polyolefin matrix down to 20–30 °C has been determined. It should be noted that in this circumstance the rheological properties of the polymer composite can be improved and easily processed. The composition of composites was determined by IRS, SEM, DSC, TQ, RFA and other methods. It has been shown that different polyolefin-AP composites can be used as different polymeric materials, including phase transition materials in the future . The plasticizing effect of alkylate in the product was the same when we use recovered IL in the next alkylation.