Selective Positional Isomerization of 2-Butene over Alumina and La-promoted Alumina Catalysts

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1 J. Ind. Eng. Chem., Vol. 13, No. 7, (2007) Selective Positional Isomerization of 2-Butene over Alumina and La-promoted Alumina Catalysts Kyungah Lee, Jong-Ki Jeon, Eui-Hwan Hwang, Young Soo Ko, Young-Kwon Park*, Seong Jun Lee**, and Jae Ho Lee** Department of Chemical Engineering, Kongju National University, Gongju , Korea *Faculty of Environmental Engineering, University of Seoul, Seoul , Korea **Chemicals R&D Center, SK Energy, Yuseong-gu, Daejeon , Korea Received April 20, 2007; Accepted November 5, 2007 Abstract: Positional isomerization of 2-butene to 1-butene over alumina catalysts was investigated. The physical characteristics and acidity of activated alumina and η-alumina catalysts were analyzed by BET, XRD, ammonia-temperature programmed desorption, and infra-red spectroscopy of adsorbed pyridine. The effects of lanthanum addition on the acidity of catalysts and the isomerization were investigated. The yield of 1-butene over η -alumina catalyst was shown to be higher than that of activated alumina catalyst due to the larger BET surface area (287 m 2 /g) as well as the weak acidity of η-alumina. The catalytic activity does not correspond to the amount of Lewis acid sites, explaining the insignificant role of Lewis acid sites in the positional isomerization of 2-butene. It was proposed that the H site might be an active site for the positional isomerization of 2-butene to 1-butene. Keywords: 2-butene, positional isomerization, alumina catalyst, 1-butene Introduction 1) Olefin hydrocarbon is used as a material of fuels, polymers and other chemical products. Considering the position of double bond or the degree of branch, specific olefin isomer can be an important variable for the efficiency of chemical reaction or the properties of products. Accordingly, sometimes it is desirable to isomerize olefin in order to increase the distribution of a desired isomer. Positional isomerization is distinguished from double bond migration isomerization in which the position of double bond changes, cis/trans geometric isomerization in which the spatial arrangement changes, and skeletal isomerization in which the part of bonds forming the molecular structure is broken and rearranged. 1-butene is used as a monomer in the production of linear low-density polyethylene (LLDPE) and polybutene. In general, C 4 fractions produced from petrochemical plants and oil refineries contain high levels of olefin and dio- To whom all correspondence should be addressed. ( jkjeon@kongju.ac.kr) lefin compounds including butadiene, isobutene, 1-butene and 2-butene. After butadiene is separated from C 4 fraction and methyl tertiary buthyl ether (MTBE) is manufactured by reacting isobutene with methanol, C 4 raffinate-ii (1-butene and 2-butene compounds) is left. Subsequently, in case that 1-butene is extracted from the residue, C 4 raffinate-iii having a high level of 2-butene remains. Currently, the most 2-butene is consumed as fuel. If 2-butene, a low-price fuel contained in C 4 raffinate-ii or C 4 raffinate-iii, is isomerized, the high valueadded 1-butene can be produced [1]. The 2-butene positional isomerization has not been widely commercialized due to the fact that 2-butene is thermodynamically more stable than 1-butene at lower temperatures. Because the most serious problem during the process of 2-butene positional isomerization is the transformation of 2-butene to isobutene, heavy elements or light elements, it is necessary to instigate a selective reaction in the production of 1-butene [2-9]. As alumina interacts strongly with metals or metal oxides, it can distribute an active substance widely, support it, and has superior thermal stability, making it the most

2 Selective Positional Isomerization of 2-Butene over Alumina and La-promoted Alumina Catalysts 1063 commonly used support. In particular, alumina, having a wide surface area due to the well-developed pores, is used as a support. It is so thermally stable that the structure is maintained even at high temperature up to 700 o C. However, due to the presence of weak acidity, it cannot be used in reactions that may have side reactions caused by acid sites [10]. The objective of the study is to select the optimal alumina catalyst for the positional isomerization that converts 2-butene to 1-butene. In addition, this study investigated the effects of the kind of alumina catalyst and the amount of lanthanum as an additive on the acidic property of the catalyst and the performance of 2-butene isomerization. Experimental Catalyst Preparation Three different activated aluminas were purchased, basic, neutral and acidic types from Aldrich and Brockmann I. The mean pore size was 58 A and the particle size was below 150 mesh. η-alumina was prepared as follows; 200 mol (3600 ml) of distilled water and 1 mole ( g) of ASB (Alumina tri-sec-butoxide) were mixed. Drop ASB was added into a beaker containing refrigerated water while stirring and aged at the normal temperature for a day. After filtering and drying in the oven for 24 h, it was calcined at 550 or 600 o C for 24 h. La/alumina catalyst was prepared using the incipient wetness method. Solutions were prepared by adding 1.8, 2.57, 3.86 or 5.14 g of lanthanum nitrate into 6 ml of distilled water and stirring, and each solution was added drop by drop to 20 g of alumina neutral. Then, it was dried and calcined at 500 o C for 4 h. Catalyst Characterization The properties of the catalyst were analyzed using FT-IR (Fourier Transform-InfraRed spectroscopy), ammonia temperature-programmed desorption, XRD (X-ray diffractometer), and BET. The BET surface area of the catalyst was measured using an ASAP 2010 apparatus by Micromeritics. After a catalyst sample was dried, 0.3 g was taken. After 5 h outgassing at 250 o C and in a vacuum, nitrogen was supplied as an adsorption gas at the temperature of liquid nitrogen, and nitrogen adsorption-desorption isotherms and the BET surface area were obtained. The crystallinity of the catalysts was examined through XRD. The XRD device was obtained from Rigaku D/MAX-II. It used Cu Kα radiation energy. To analyze the acid sites of catalysts, FT-IR (PERKIN ELMER) was used [11]. This progressed as follows: g of catalyst was taken for analysis and formed into a disk under high pressure. The IR spectrum was obtained at 350 o C under Figure 1. Experimental set-up for 2-butene isomerization. vacuum, and the temperature was lowered to 35 o C. Pyridine was flowed at a constant rate and allowed to adsorb for 30 min under a vacuum. Under a rising temperature, the IR spectrum was obtained at 100, 150, 200, 250, and 300 o C. Ammonia temperature-programmed desorption (TPD) of the catalysts were carried out in a Micromeritics 2900 TPD apparatus [12]. The samples were outgassed under a He flow (50 ml/min) with a heating rate of 15 o C/min from room temperature to 550 o C. This final temperature was maintained for 1 h and, subsequently, the samples cooled to 180 o C and treated with an ammonia flow (30 ml/min) for 30 min. The physisorbed ammonia was removed from the sample by flowing He at 180 o C for 2 h. The chemically bonded ammonia was determined by increasing the temperature to 550 o C, with a heating rate of 15 o C/min. The ammonia presented in the effluent stream was continuously monitored using a thermal conductivity detector (TCD). Catalytic Activity The 2-butene (99.3 %) with a ratio of 74 (cis)/26 (trans) was purchased from Aldrich. The 2-butene isomerization reaction was performed using 0.13 g of the catalyst in a fixed bed reactor (Figure 1). The reaction temperature, pressure and the WHSV (weight hourly space velocity) of 2-butene were 450 o C, 1 atm and 70 h -1, respectively. To monitor the changes in the activity of the catalyst according to the reaction time, 0.3 µl of the product was analyzed each hour using directly connected GC (gas chromatography: Acme 6000 GC). From the GC results, the conversion, selectivity and yield of 1-butene were calculated.

3 1064 Kyungah Lee, Jong-Ki Jeon, Eui-Hwan Hwang, Young Soo Ko, Young-Kwon Park, Seong Jun Lee, and Jae Ho Lee Table 1. BET Surface Area of the Catalysts Catalysts alumina (basic) alumina (neutral) alumina (acidic) 3.5 wt% La/alumina 5.0 wt% La/alumina 7.5 wt% La/alumina 10.0 wt% La/alumina η-alumina 3.5 wt% La/η-alumina BET Surface Area (m 2 /g) Results and Discussion Catalyst Characterization Table 1 shows the BET surface area of catalysts used in this research. The surface areas of activated alumina catalysts were in the range of m 2 /g. As the surface area decreased with the increase of the quantity of lanthanum added, it was 117 m 2 /g in case of 10 wt% La/alumina catalyst. The BET surface area of η-alumina catalyst was 287 m 2 /g, around twice larger than that of activated alumina. For the catalyst in which lanthanum was added to η-alumina, the surface area was reduced to 238 m 2 /g. Figure 2 shows the result of ammonia temperature-programmed desorption (NH 3 -TPD) of alumina catalysts. In case of the alumina (neutral) catalyst, the peaks are observed at around o C, meaning ammonia adsorbed into the weak acid sites [12,13]. The alumina (acidic) catalyst revealed the peaks at 330 and 450 o C, corresponding to the strong acid sites, in addition to the weak acid sites at around 250 o C. When La/alumina catalyst in which 3.5 wt% of lanthanum was added to the alumina (neutral) is compared with the alumina (neutral) catalyst, the peaks are observed at around o C in addition to the weak acid sites at around 200 o C. This suggests that strong acid sites were generated by the addition of lanthanum. According to the result of ammonia temperature programmed desorption of η-alumina, the size and area of peaks are similar to those of peaks from the weak acid sites observed in the result of ammonia temperature programmed desorption of alumina (neutral) catalyst. Accordingly, the quantity of acid sites is considered to be similar. However, for the alumina (neutral) catalyst the temperature at the highest point of peaks is around 250 o C, and for η-alumina it is around 300 o C, showing that η-alumina has stronger acidic sites. Figure 3 shows IR spectrum obtained for pyridine adsorbed into the alumina (basic) while raising temperature. The size of peaks observed at 1611, 1589 and 1442 cm -1 decrease considerably with the increase in desorption temperature. These peaks are pyridine in hydro- Figure 2. Ammonia temperature-programmed desorption curves for alumina catalysts. gen bond, corresponding to very weak acid sites (H) [14-17]. The size of peaks observed at 1448, 1490 and 1575 cm -1 does not decrease much compared to those above. These peaks are pyridine in dative bond, corresponding to Lewis acid sites (L). Figure 4 shows the results of pyridine IR analysis of the acidic, neutral and basic catalysts among activated alumina. The size of characteristic peaks showing H acid site and L acid sites decreased in order of acidic > neutral > basic alumina. Figure 4 compares the result of pyridine IR analysis of η-alumina and alumina (acidic), but no particular difference was observed. Figure 5 shows the result of pyridine IR analysis of La/alumina catalyst in which 10 wt% of lanthanum was added to alumina (neutral). In the results, the relative size of L acid sites compared to that of H acid sites is larger than that of alumina (neutral) catalyst. This result is consistent with the increase of strong acid sites in the result of ammonia temperature-programmed desorption mentioned earlier. Catalytic Activity Figure 6 shows the result of 2-butene isomerization using activated alumina (acidic, neutral, basic) catalysts and η-alumina. When 2-butene isomerization was performed using η-alumina catalyst, the yield of 1-butene at the reaction temperature of 450 o C, atmospheric pressure and WHSV of 70 h -1 was 21.3 wt%, which was higher than of activated alumina catalyst. According to

4 Selective Positional Isomerization of 2-Butene over Alumina and La-promoted Alumina Catalysts 1065 Figure 3. Pyridine IR over alumina [basic] under vacuum. Figure 5. Effect of lanthanum on pyridine-ir over alumina catalyst (150 o C). Figure 4. Pyridine IR over alumina catalysts (150 o C). the analysis result of catalyst property, η-alumina catalyst had a twice larger BET surface area than activated alumina, and its quantity of acid sites was almost the same but its acid strength was slightly higher. Sohlberg and coworkers. elucidated that η-alumina had greater Lewis acidity than that of γ-alumina, which was caused to different surface reconstruction [18]. No remarkable difference was observed in the results of pyridine IR analysis. Very weak acid sites are more advantageous to the positional isomerization of 2-butene. If the strength of acid sites is too high, side reactions, such as skeleton isomerization and cracking reaction, could be fostered, and as a consequence the selectivity goes down. Accordingly, because η-alumina catalyst has a wide surface area while not showing strong acid sites, it is considered advantageous to positional isomerization of 2-butene. Figure 7 shows the effects of the addition of lanthanum to activated alumina and η-alumina on the isomerization of 2-butene. In the presence of lanthanum the yield of 1-butene decreased and this tendency was observed for Figure 6. The results of 2-butene isomerization over alumina catalysts [WHSV: 70 h -1, Temp.: 450 o C]. both activated alumina and η-alumina. According to the analysis result of catalyst property, the addition of lanthanum resulted in a slight decrease in BET surface area. In the results of the ammonia temperature programmed desorption experiment and pyridine IR analysis, the addition of lanthanum resulted in the increase of Lewis acid sites compared to weak acid sites, and consequently, increased acid strength. This is opposite to the tendency that the addition of lanthanum reduces the activity of positional isomerization of 2-butene. This suggests that the activity of positional isomerization of 2-butene is not proportional to the acid strength, but it can take place only with weak acid sites rather than Lewis acid sites. The yield of i-butene, cracked products, and C 5+ over η-alumina is 1.0, 0.0, and 0.2 wt%, respectively. In the case of La- promoted η-alumina, the yield of i-butene, cracked products, and C 5+ is 0.1, 0.3, and 0.2 wt%, respectively. Even though the yield of i-bu-

5 1066 Kyungah Lee, Jong-Ki Jeon, Eui-Hwan Hwang, Young Soo Ko, Young-Kwon Park, Seong Jun Lee, and Jae Ho Lee Acknowledgments This work was supported by Energy Technology R&D Grant of the Korea Energy Management Cooperation (2005-E-ID11-P ). References Figure 7. Effect of lanthanum on 2-butene isomerization over alumina catalysts [WHSV: 70 h -1, Temp.: 450 o C]. tene decreased with lanthanum addition, the products by cracking and oligomerization over strong acid site increased slightly. In the case of neutral alumina, the yield of the products by cracking and oligomerization increased from 0.1 to 0.3 wt% with addition of lanthanum. Conclusion The BET surface area of η-alumina catalyst was 287 m 2 /g, and two times larger than that of activated alumina. η-alumina catalyst showed slightly higher acid strength but did not show strong acid sites, so it is considered to be more advantageous to the production of butene-1 through the positional isomerization of 2-butene. In the presence of lanthanum in η-alumina and activated alumina, the relative number of Lewis acid sites was larger than that of weak acid sites. The addition of lanthanum rather reduced the activity of positional isomerization of 2-butene. This suggests that the activity of positional isomerization of 2-butene is not proportional to the increase of acid strength but the weak acid sites could be mainly active for the positional isomerization compared to Lewis acid sites. 1. B. V. Vora, US Patent, 6,005,150 (1999). 2. H. H. Mooiweer, K. P. Jong, B. Kraushaar-Czarnetzki, W. H. J. Stork, and B. C. H. Krutzen, Stud. Surf. Sci. Catal., 84, 2327 (1999). 3. G. Seo, Catalysis Surveys from Asia, 9, 139 (2005). 4. J. W. Myers, US Patent, 4,289,919 (1980). 5. H. H. Hsing, US Patent, 5,043,523 (1990). 6. D. H. Powers, US Patent, 6,768,038 (2002). 7. J. K. Jeon, J. H. Yim, J. H. Lee, and Y. S. Kim, Catalysis, 21, 13 (2005). 8. F. Ancillotti, O. Forlani, B. Jover, G. Resofszki, and G. Gati, Applied Catalysis, 67, 249 (1991). 9. A. Moronta, J. Luengo, Y. Ramrez, J. Quiñónez, E. González, and J. Sánchez, Applied Clay Science, 29, 117 (2005). 10. H. Chon and G. Seo, Introduction of Catalyst, 4th Edn., pp , Hanrimwon Publishing Co., Seoul (2002). 11. B. Chakraborty and B. Viswanathan, Catal. Today, 49, 253 (1999). 12. S. K. Song, Y. Wang, and S. K. Ihm, Catal. Today, 111, 194 (2006). 13. Y. K. Park, S. W. Baek, and S. K. Ihm, J. Ind. Eng. Chem., 7, 167 (2001). 14. H. A. Benesi, J. Am. Chem. Soc., 78, 5490 (1956). 15. Z. M. El-Bahy, R. Ohnishi, and M. Ichikawa, Catal. Today, 90, 283 (2004). 16. J. Zhang, J. Chen, J. Ren, and Y. Sun, Appl. Catal. A: General, 243, 121 (2003). 17. L. I. Darvell, K. Heiskanen, J. M. Jones, A. B. Ross, P. Simell, and A. Williams, Catal. Today, 81, 681 (2003). 18. K. Sohlberg, S. T. pantelides, and S. J. Pennycook, J. Am. Chem. Soc., 123, 26 (2001).