Energy & Fuels 2007, 21, 2473-2474
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Communications Polymerization of n-Heptane under Mild Conditions Hongzhu Ma, Xue Cui, Bo Wang,* and Hongwei Chen Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, Institute of Energy Chemistry, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, People’s Republic of China ReceiVed March 27, 2007. ReVised Manuscript ReceiVed May 16, 2007 Introduction It is well-known that solid superacids SO42-/MxOy (Ho < -11.93) have been used in isomerization, alkylation, acylation, polymerization, oligomerization, esterification, oxidation, etc., for their high strength of acidity, nontoxicity, and high activity at low temperatures.1,2 As one of the strongest solid superacids, the acid strength of sulfated tin oxide is lower than -16 on the Hammett function scale and it has been proven to be a useful catalyst in isomerization reactions, owing to its basic properties. Metal-promoted superacids, which were highly active for many reactions, were investigated to improve its acid sites activity, selectivity, and stability. The cetane number (CN), dependent upon the chemical composition of diesel fuel, is a measure of the ignition quality of diesel fuel. Generally, normal paraffins have high CNs that increase with molecular weight, and isoparaffins have lower CNs than n-paraffins at the same carbon number. Isomers with several short side chains have lower CNs than those with one long side chain. The high cetane standard (primary reference fuel) is n-hexadecane (trivial name cetane), with an assigned CN ) 100. One approach for improving CN is to use compounds known as “cetane boosters”. These compounds, frequently organic nitrates, improve the cold-start performance and may reduce particulate matter (PM) emissions. However, at the same time, cetane boosters, increasing the flammability of the fuel, are potentially more hazardous because of ultrafine particle emissions and degrade the storage stability of the fuel. As another option to reach a higher CN, while simultaneously reducing aromatics, is further upgrading of the petroleum stream by deep hydrotreating. The resulting CN greatly depends upon the nature of the crude as well as the refinery blending strategies. Even after deep hydrogenation, the expected high-CN regulations may not be met. Some researchers have suggested that a chain polymerization process is a potential solution for significantly improving CN.3-5 In recent years, chain polymerizations, especially free-radical polymerizations, have evoked a lot of * To whom correspondence should be addressed. Telephone: +86-2985308442. Fax: +86-29-85307774. E-mail:
[email protected]. (1) Hinoa, M.; Aratab, K. Appl. Catal., A 1998, 173, 121-124. (2) Matsuhashi, H.; Miyazaki, H.; Kawamura, Y.; Nakamura, H.; Arata, K. Chem. Mater. 2001, 13, 3038-3042. (3) Santana, R. C.; Do, P. T.; Santikunaporn, M.; Alvarez, W. E.; Taylor, J. D.; Sughrue, E. L.; Resasco, D. E. Fuel 2006, 85, 643-656. (4) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C.; Padmakumari, K. Energy 2006, 31, 2524-2533. (5) Lu, X. C.; Yang, J. G.; Zhang, W. G.; Huang, Z. Energy Fuels 2005, 19, 1879-1888.
interests. The aim of this work is to propose an easy methodology to carry out chain polymerizations at room-temperature catalyzed by solid superacid. To exemplify the approach, n-heptane polymerization at room-temperature catalyzed by SO42-/SnO2-Co2O3 solid superacid has been selected. The distributions of the products were investigated by gas chromatography mass spectrometry (GCMS), and the results show that the main products are the C number of n-alkane from C14 to C22, which can all be used as diesel fuel components that have higher cetane numbers. Experimental Section SnO2-Co2O3 binary oxides (the concentration of Co was 2.0 wt %) were prepared by a chemical coprecipitation method.6 The catalyst was pretreated at 573 K for 1 h in a nitrogen atmosphere before each term. Typically, 5 g of catalyst and 100 mL of n-heptane were used for each experiment, unless stated otherwise. The finely ground catalyst particles were placed on quartz wool. The experiments were carried out in a catalyst column of 250 mL at atmospheric pressure and room temperature. Products were analyzed with an on-line GC-MS.
Results and Discussion The search of the IR spectra (Figure 1) of modified sulfated tin oxide samples shows absorption bands at 1195, 1128, 1097, and 1050 cm-1 assigned to the bidentate sulfate ion coordinated to metal, such as Sn4+ or Co3+; these results are very similar to those reported by other authors.7-10 Deactivation was less remarkable for H2SO4-treated SnO2-Co2O3. The binding energy and its composition of the catalyst were shown in Figure 2. The spin-orbit doublets, Sn 3d3/2 and Sn 3d5/2, are located at 485.25 and 486.58 eV, respectively, with a separation of 1.33 eV, indicating the formation of SnO2.11 The binding energy of Co 2p peaks at 780.77 and 779.30 eV are attributed to Co 2p1/2 and Co 2p3/2, respectively, indicating the promotion of metal Co in the SnO2-Co2O3 catalyst. In (6) Lu, G. Z. Appl. Catal., A 1995, 133, 11-18. (7) Arata, K.; Nakamura, H.; Shouji, M. Appl. Catal., A 2000, 197, 213219. (8) Salas, P.; Hernhndez, J. G.; Montoya, J. A.; Navarrete, J.; Salmones, J. J. Mol. Catal. A: Chem. 1997, 123, 149-154. (9) Ecormier, M. A.; Wilson, K.; Lee, A. F. J. Catal. 2003, 215, 5765. (10) Sohn, J. R.; Lee, S. H.; Lim, J. S. Catal. Today 2006, 116, 143150. (11) Kwoka, M.; Ottaviano, L.; Passacantando, M.; Czempik, G.; Santucci, S.; Szuber, J. Appl. Surf. Sci. 2006, 252, 7734-7738.
10.1021/ef0701539 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/02/2007
2474 Energy & Fuels, Vol. 21, No. 4, 2007
Figure 1. Infrared spectra of SO42-/SnO2-Co2O3.
Communications
Figure 3. GC-MS chromatogram of the products.
that have high CNs with C numbers increasing, with the abundance of the long-chain compounds indicating the effectiveness of the solid superacid SO42-/SnO2-Co2O3 on the polymerization of n-heptane. Although the conversion of n-heptane is low (only 14.9%), fortunately, polymerization of alkanes in one pot have been successively accomplished and assisted by modified sulfated oxide at room temperature and an air atmosphere. Conclusions
Figure 2. XPS of the catalyst after the reaction.
comparison to the Co3+ that occupied 100% of the total cobalt element before the reaction, it can illuminated that Co3+ was reduced to Co2+ during the polymerization process.12 Alternatively, GC-MS has been known for its superior separation of complex organic compounds, greater sensitivity, and shorter measuring time and, hence, is better suited for the detection and identification of volatile organic compounds.13 The sample of 60 min was analyzed by GC-MS. The GC of the products was shown in Figure 3 (the peak of the solvent was taken off). The difference in the chain length and its distribution between the polymers produced can be found. The conversion ratio of n-heptane is 14.9%, and main products are the C number of long, straight chain n-alkane from C14 to C22 (12) Yea, D. X.; Pimanpang, S.; Jezewski, C.; Tang, F.; Senkevich, J. J.; Wang, G.-C.; Lu, T.-M. Thin Solid Films 2005, 485, 95-100. (13) Giumanini, A. G.; Verardo, G. Ind. Eng. Chem. Res. 2001, 40, 1449-1453.
We have attempted to present the recent works on the synthesis of solid superacid for the transformation of alkanes. A new solid superacid catalyst, SO42-/SnO2-Co2O3, was prepared by doping SnO2 with Co and modifying with sulfate simultaneously. Its catalytic activity on the polymerization of n-heptane was investigated, and high activities for alkane polymerization were observed. In comparison to polymerization by traditional methods, this process became rather easy with modified solid superacid. A simple work-up procedure, milder reaction conditions, and a shorter reaction time are the advantages associated with the SO42-/SnO2-Co2O3 catalysis process. The products gained by this method are long, straight chain hydrocarbons with high CNs. Therefore, this approach has highlighted the importance of using a solid superacid catalyst to realize not only for n-alkane polymerization under mild conditions but also for other processes, which, because of specific requirements, have the need of activating alkane in many cases. Acknowledgment. We are grateful to be supported in part by a graduated innovation fund from Shaanxi Normal University. EF0701539
