Energy Fuels 2010, 24, 464–468 Published on Web 10/22/2009
: DOI:10.1021/ef900802y
Structural Characterization of Fuels Obtained by Olefin Oligomerization Rosalı´ a Rodrı´ guez, Juan J. Espada, and Baudilio Coto* Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan Carlos, C/Tulip an s/n, 28933 M ostoles, Madrid, Spain Received July 28, 2009. Revised Manuscript Received October 5, 2009
Environmental legislation concerning the aromatic and sulfur content of gasoline and diesel fuels is becoming more and more restrictive. For that reason, the study of processes obtaining diesel with a low content of sulfur is of great interest. Among them, the acid catalyzed oligomerization of light olefins appears as a promising way for diesel manufacture. In this work, products obtained by oligomerization of feedstocks with different amounts of sulfur and nitrogen compounds and a FCC naphta were analyzed and compared with a reference diesel fuel. Distillation curves (SDA) were determined and showed similar boiling point ranges. Molecular weights, measured from gel permeation chromatography (GPC), showed average values around C11. The n-paraffin content was very low as it was qualitative determined by differential scanning calorimetry (DSC) and by gas chromatography-mass spectrometry (GC-MS). Moreover, GC-MS results confirm the average molecular weight (around C11) determined by GPC analysis. The cetane indexes were calculated from the CH2/CH3 ratio obtained by 1H nuclear magnetic resonance spectroscopy (1H NMR) analyses, and values for the oligomerization products were slightly lower than that for the reference diesel. According to the characterization results, some of the studied products could be used as diesel fuel. Furthermore, these products present a good behavior at low temperatures since their n-paraffin content is negligible.
esting procedure to produce higher molecular weight hydrocarbon mixtures which can fit required diesel fuel specifications. This process has been deeply studied in literature with good results.8,9 An industrial application for the oligomerization reaction is the use of light naphta obtained by fluid catalytic cracking process (FCC naphta, mainly composed of olefins ranging between 5 and 7 carbon atoms10,11) as feedstock. The main drawback of this process is that sulfur (tiophene, mercaptans, etc.) removal is required to meet specifications.12,13 In this work, the mixtures obtained by oligomerization of synthetic feedstock (1-hexene with different concentrations of sulfur and nitrogen compounds) and of FCC naphta feedstock were analyzed by different techniques to check their properties as diesel fuel. Simulated distillation analysis (SDA carried out following the ASTM-D2887 method), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and proton nuclear magnetic resonance (1H NMR) were used to characterize the oligomerization products. The boiling point, the amount of n-paraffin that can precipitate at low temperature causing problems in the engine, the molecular weight, the structure (CH2/CH3 ratio), and the cetane index were determined for the studied mixtures. Obtained values were compared to those for a reference diesel fuel
Introduction The demand along with the quality of the gasoline and diesel fuel is continually growing.1,2 Environmental legislation concerning the aromatic and sulfur content in gasoline and diesel fuels are becoming more and more restrictive.3,4 Thus, the formulation of fuels with a maximum sulfur content of 10 ppm in EU by 20095 is the forthcoming established aim. Along the sulfur restriction, a reduction of aromatic compounds is required which represents a change in the hydrocarbon composition type toward highly paraffinic fuels.6 However, the presence of n-paraffinic compounds can significantly affect the behavior of the diesel fuel since such compounds can crystallize at low temperatures and wax crystals can block filters and lead to engine failure.7 This is one of the main problems that the oil industry should face. Traditionally, additives (pour point depressants, paraffin inhibitors, or wax crystal modifiers) have been used to improve the low temperature behavior of the fuels, thus avoiding damage in equipment. For all mentioned above, the process of oligomerization of light olefins (C3=-C6=) over acid catalysts can be an inter*To whom correspondence should be addressed. Telephone: þ34 91 488 70 89. Fax: þ34 91 488 70 68. E-mail:
[email protected]. (1) Catani, R.; Madreoli, M.; Rossini, R.; Vaccari, A. Catal. Today 2002, 75, 125–131. (2) Marcilly, C. J. Catal. 2003, 216, 47–62. (3) Dupain, X.; Makkee, M.; Moulijn, J. A. Appl. Catal., A 2006, 29, 198–219. (4) Ishihara, A.; Dumeignil, F.; Lee, J.; Mitsuhashi, K.; Qian, E. W.; Kabe, T. Appl. Catal., A 2005, 289, 163–173. (5) Directive 2003/17/EC of the European Parliament and of the Council on the Quality of Petrol and Diesel Fuels. (6) Coutinho, J. A. P.; Dauphin, C.; Daridon, J. L. Fuel 2000, 79, 607– 616. (7) Zhang, J.; Wu, C.; Li, W.; Wang, Y.; Han, Z. Fuel 2003, 82, 1419– 1426. r 2009 American Chemical Society
(8) Chiche, B.; Sauvage, E.; Di Renzo, F.; Ivanova, I. I.; Fajula, F. J. Mol. Catal. A 1998, 134, 145–157. (9) Hulea, V.; Fajula, F. J. Catal. 2004, 225, 213–222. (10) Van Grieken, R.; Escola, J. M.; Moreno, J; Rodrı´ guez, R. Appl. Catal., A 2006, 305, 176–188. (11) Van Grieken, R.; Escola, J. M.; Moreno, J.; Rodrı´ guez, R. Appl. Catal., A 2008, 337, 173–183. (12) Lomas, D. A. Process for Upgrading FCC Product with Additional Reactor. U.S. 20040140246 A1, July 22, 2004. (13) Dı´ az, L.; Herbert, J.; Cortez, M. T.; Zarate, R.; Santes, V.; Hernandez, G. Petrol. Sci. Technol. 2004, 22, 141–155.
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: DOI:10.1021/ef900802y
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showing the possibility of using the studied mixtures as a motor fuel.
Gas Chromatography-Mass Spectrometry. This technique provides detailed information about composition of different kind of mixtures. The equipment was a Varian 3800 series gas chromatograph (GC) equipped with a factorFour VF-1 ms capillary column 30 m long and 0.25 mm internal diameter in which the stationary phase consists of a 1%-diphenil-95%-dimethylpolysiloxane copolymer with a film thickness of 0.25 μm The detection was carried out by a Varian 1200 series mass detector using electronic impact at 70 eV. The injection port temperature was set at 310 °C while the GC oven temperature profile started at 40 °C, is increased at 5 °C/min up to 325 °C, and finally held for 33 min. The helium carrier gas flow was 1 mL/min.18
Experimental Section The samples used in this work were an ASTM D2887 reference gas oil and three products obtained by oligomerization of olefins (P1, P2, and P3). These reactions were carried out in a fixed bed reactor (Microactivity-Pro Reactor from ICP Engineering Process Control Group) at 200 °C and 5 MPa during 4 h using an acid catalyst (Al-MTS) and a feedstock reported elsewhere.11 P1 and P2 were obtained from synthetic feedstock of a light olefin (1-hexene) and different amounts of sulfur and nitrogen compounds. P1 was 1-hexene þ 700 ppm of tiophene þ 25 ppm of butylamine, and P2 was 1-hexene þ 7000 ppm of tiophene þ 25 ppm of butylamine. P3 was obtained by oligomerization of a light FCC naphta in which the sulfur and nitrogen content was similar to that of the P1 mixture. Distillation Curve. The simulated distillation curves were obtained according to the ASTM D-2887 method.14 The studied mixture was dissolved in CS2 (5 wt %) and analyzed. The equipment was a 3900 Varian GC with an automatic injector, a cryogenic system, a flame ionization detector (FID), and on-column injection system. The column was a 10 m length 0.53 mm internal diameter with a 0.17 mm width silicone stationary phase. Differential Scanning Calorimetry. Samples were analyzed with a DSC Mettler-Toledo DSC822e. The sample was heated from 25 to 80 °C at 3 °C/min and then cooled from 80 to -120 °C at 3 °C/min. The heat transfer and the temperature were recorded, and phase change determined thus allowing the experimental quantification of wax precipitation in petroleum mixtures15,16 and the wax appearance temperature (WAT) to be carried out. Gel Permeation Chromatography. The average molecular weight of the different mixtures was determined by gel permeation chromatography. A Waters Alliance GPCV 2000 equipped with refractive index and viscosimeter detectors and three different columns (two PLgel 10 μm MIXEDB, 300 7.5 mm and a PLgel 10 μm 10E6 A˚, 300 7.5 mm) was used. The mobile phase was 1,2,4-trichlorobenzene, the flow rate was set at 1 mL/min and the temperature at 145 °C. Samples were dissolved in 1,2,4-trichlorobenzene, and the concentration was around 1.2-1.5 mg/mL. Universal calibration obtained from pure n-paraffins was used in this work.17 Nuclear Magnetic Resonance Spectroscopy (1H NMR). A Varian Mercury Plus NMR spectrometer (C/H dual 5 mm probe, frequency 400 MHz) was used to quantify the different types of hydrogen atoms. Samples were prepared by solving 15-20 mg of the mixture in 0.45 mL of CDCl3 using 5 mm samples tubes. The number of scans was 64, with a 30° pulse and a 1 s delay time between scans. This technique allows the aromatic content and degree of branching of the samples to be determined following a procedure previously reported.18
Results and Discussion Distillation Curves. Distillation curves for all the mixtures are shown in Figure 1. All the oligomerization products (using 1-hexene or FCC naphta as feedstock) present similar results regarding the boiling temperature range (150650 °C). However, slight differences in the shape of SDA curves can be found. Thus, curves for P1 and P2 mixtures (obtained by oligomerization of 1-hexene with different amounts of nitrogen and sulfur compounds) present three clearly defined steps, whereas P3 mixture (obtained from the FCC naphtha) shows a continuous and more homogeneous SDA curve. Such shape in both P1 and P2 curves can be related to a noncontinuous distribution of products. The appearance of the three groups of products (dimers, trimers, and heavy oligomers) in the oligomerization process is reported previously in the literature.19-21 The reference diesel fit in such boiling temperature range and can be considered a good reference to compare with. No remarkable differences were found when comparing the temperature range of distillation curves for P1, P2, and P3 with that obtained for the diesel used as reference. Consequently, in all cases, the obtained products can be considered as fuel mixtures regarding their boiling temperature range. A mean molecular weight of each mixture was estimated from the T50% value (boiling temperature at which 50% of the sample volume is distillated and taken directly from the SDA curves in this work) following the procedure of Riazi et al.22 Values for molecular weight thus obtained were 234, 260, 234, and 249 for the P1, P2, P3, and reference diesel mixtures, respectively. These values correspond to the range C17-C18 in terms of carbon atoms, but as it will be discussed later, such values are higher than expected and are considered not very reliable. GPC Analysis. Figure 2 shows the molecular weight distribution obtained for all mixtures by means of GPC. The average molecular weight for the mixtures can be calculated from these distributions. In this work, the average in mass (Mw) was considered. Obtained values were 157, 153, and 152 for P1, P2, and P3 mixtures, respectively, which correspond to C11 in terms of carbon atoms. These results are lower than those obtained by SDA, but they are considered more reliable because the main oligomerization products should be those obtained in the dimerization reaction that
(14) ASTM. ASTM Standard D-2887-99; 2003; Vol. 5. (15) Coutinho, J. A. P.; Ruffier-Meray, V. Ind. Eng. Chem. Res. 1997, 36, 4977–4983. (16) Coutinho, J. A. P.; Ruffier-Meray, V. Fluid Phase Equilib. 1998, 148, 147–160. (17) Espada, J. J.; Coto, B.; Pe~ na, J. L. Energy Fuels 2009, 23, 888– 893. (18) Martos, M. C.; Coto, B.; Espada, J. J.; Robustillo, M. D.; G omez, S.; Pe~ na, J. L. Energy Fuels 2008, 22, 708–714.
(19) Pater, J. P. G.; Jacobs, P. A.; Martens, J. A. J. Catal. 1998, 179, 477–482. (20) Pater, J. P. G.; Jacobs, P. A.; Martens, J. A. J. Catal. 1999, 184, 262–267. (21) De Klerk, A. Ind. Eng. Chem. Res. 2005, 44, 3887–3893. (22) Riazi, M. R.; Nasimi, N.; Roomi, Y. A. Ind. Eng. Chem. Res. 1999, 38, 4507–4512.
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Energy Fuels 2010, 24, 464–468
: DOI:10.1021/ef900802y
Rodríguez et al.
Figure 1. SDA curves for oligomerization mixtures and reference diesel.
Figure 2. GPC molecular weight distribution for oligomerization mixtures and reference diesel. Figure 3. DSC thermograms for oligomerization mixtures and reference diesel.
occurs at the greatest extent, and consequently the range of carbon atoms C11-C12 is expected for these mixtures. A value of 199 was obtained for the reference diesel, which is equivalent to C14. As can be seen in Figure 2, different distribution functions are obtained for the studied samples. The curve obtained for the reference diesel is slightly shifted to higher molecular weight values, as expected from the higher average values obtained. The curves obtained for the P1 and P2 samples show slight humps in the low molecular weight region. These humps are placed around molecular weight of 80, i.e., C6 in carbon atom, and can be related to steps shown in the SDA curves. DSC Analysis. Paraffinic compounds present in fuels can precipitate when temperature decreases, and their content is an important parameter to be determined as representation
of the behavior of the fuel at low temperatures. In this work, DSC analysis was performed to determine the presence of paraffins in the studied mixtures. Figure 3 shows the obtained thermograms. As can be seen, a pronounced signal was obtained for the diesel reference mixture and the obtained WAT was about 10 °C, thus revealing the presence of n-paraffins. However, thermograms for the P1, P2, and P3 mixtures did not show such signal, thus revealing an extremely low content of n-paraffin compounds. This indicates that P1, P2, and P3 mixtures should present better properties at low temperatures than the diesel used as reference, despite the similar boiling range obtained for all the mixtures. 1 H NMR Spectroscopy. This experimental technique was used to quantify different kinds of hydrogen atoms in the studied samples. The integration procedure for the 1H NMR 466
Energy Fuels 2010, 24, 464–468
: DOI:10.1021/ef900802y
Rodríguez et al.
Table 1. 1H NMR Results for the Studied Mixtures sample
aromatic H (wt %)
HR (wt %)
Hβ (wt %)
Hγ (wt %)
Hβ/Hγ
cetane index
reference diesel P1 P2 P3
2.5 0.8 1.3 1.8
11.3 11.8 13.6 12.0
62.1 51.0 54.9 49.1
24.1 36.5 30.2 37.1
2.6 1.4 1.8 1.3
73 49 59 47
Figure 4. GC-MS spectra for oligomerization mixtures and reference diesel.
spectra was reported by Martos et al.18 Table 1 shows the obtained results for each sample in weight percent. The CH2/ CH3 ratio (Hβ/Hγ) for all mixtures is also listed. The aromatic content obtained in all cases is lower than 2.5% (wt), and all the oligomerization mixtures present even lower aromatic content than that for the reference diesel, with values lower than 2.0% (wt). As shown in Table 1, the reference diesel mixture presents higher content of Hβ and lower content of Hγ, thus indicating the highest content n-paraffins (in good agreement with the results obtained by DSC analysis). The cetane index (CI) is a parameter of great interest in diesel fuels. It is related to the presence of n-paraffins. O’Connor et al.23 analyzed different alkane mixtures, founding the relation between the cetane index and the CH2/CH3 ratio (Hβ/Hγ) determined by 1H NMR. In this work, a nonlinear relation between these parameters, reported by those authors, was used. Table 1 shows the cetane index values for the considered mixtures calculated using this relation. As expected, the reference diesel mixture presents a higher value of CI than the rest of the considered mixtures. This can be due to the formation of different carbocationic species from light olefins which lead to the formation of branched alkanes.
P1 and P3 mixtures show lower CI than that for P2, which reveals lower content of linear paraffins. The difference of CI values found between P1, P3, and P2 can be related to the amount of sulfur and nitrogen compounds in the feedstock mixture. Thus, a higher content of sulfur and nitrogen (P2) would reduce the reaction extent, leading to less branched alkanes. GC-MS Analysis. In order to qualitatively check the content of n-paraffin in each mixture, GC-MS analysis was performed. Obtained spectra, shown in Figure 4, indicate higher content of n-paraffins for diesel reference and P2 mixtures in good agreement with the results obtained by 1 H NMR analysis. As shown in Figure 4, different groups of peaks for P1, P2, and P3 can be observed in good agreement with the SDA and GPC results. The first group of peaks (retention time around 18 min) corresponds to light compounds. The peaks centered at 30 and 38 min are associated with dimers and trimers formed in the oligomerization process using 1-hexene or naphta as feedstock. The obtained results confirm the presence or different component groups, in good agreement with SDA results commented on above. Moreover, as can be seen in Figure 4, dimers are the main products from the oligomerization of 1-hexene, which confirms the average molecular weight (around C11-C12) estimated by GPC analysis. Furthermore, the low content of n-paraffins in
(23) O’Connor, C. T.; Forrester, R. D.; Scurrell, M. S. Fuel 1992, 71, 1323–1327.
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Energy Fuels 2010, 24, 464–468
: DOI:10.1021/ef900802y
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P1, P2, and P3 mixtures obtained by GC-MS confirm DSC results.
for the diesel mixture, which indicates good behavior at low temperatures. Cetane index values were calculated for the oligomerization products from the CH2/CH3 ratio determined by 1H NMR. Obtained results were all within the range of fuel mixtures. According to the obtained results, the products yielded by oligomerization are promising as diesel mixtures even improving some properties as their behavior at low temperatures due to their negligible n-paraffin content and their low sulfur content.
Conclusions Products obtained by oligomerization of feedstock containing 1-hexene with different amounts of sulfur and nitrogen compounds or of a FCC naphta show similar boiling range temperature to a diesel petroleum mixture. However, molecular weight by GPC is lower (around C11) than that for the reference diesel, which is confirmed by GC-MS results. DSC results show an extremely low content of n-paraffin compounds for the studied mixtures in contrast to that obtained
Acknowledgment. We thank to REPSOL for providing the FCC naphta sample used in this research.
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