Ind. Eng. Chem. Res. 2008, 47, 8037–8042
8037
Isoprene and C5 Olefins Production by Oxidative Dehydrogenation of Isopentane† Rita X. Valenzuela,‡ Jose´ M. Mun˜oz Asperilla,§ and Vicente Corte´s Corbera´n* Institute of Catalysis and Petroleumchemistry (ICP), Spanish Council for Scientific Research (CSIC), Marie Curie 2, 28049 Madrid, Spain
The current limited availability of isoprene and isoamylenes (methyl butenes), which are the feedstock for a variety of petrochemical processes, could be boosted by converting the isopentane contained in the same refinery C5-fractions from which they are obtained. The oxidative dehydrogenation (ODH) of isopentane to isoprene and isoamylenes for this purpose has been studied very little to date. We present here a study of this reaction on oxide catalysts, using oxygen as an oxidant, without any dopant in the feed. After a broad catalyst formulations screening, a V-Mg oxide catalyst (15 wt % V2O5) was selected to study the influence of the reaction parameters (temperature, residence time, and feed composition). Selectivity values of ca. 25% to isoprene, and 70% to isoprene plus isomylenes, could be attained under selected reaction conditions. These results are promising for the direct conversion of the branched C5-hydrocarbons fraction to isoprene. Introduction Currently, most petrochemical processes use olefins or aromatics as raw materials. On the other hand, alkanes are largely abundant in refinery streams. However, except for n-butane, isobutane, and some long-chain (C12) paraffins, their current use is limited to energy carriers for combustion. Hence, a major challenge for the refining and petrochemical industry involves increasing the added value of light alkanes, by developing either new, energy-efficient dehydrogenation processes for the production of the corresponding olefins, or new technology, using the paraffin directly as building blocks for high-value chemicals. Isoprene (2-methyl-1,3-butadiene) and C5 olefins (amylenes) are used as feedstocks for a wide variety of end-uses (hydrocarbons resins, elastomers, and fine chemicals). High-purity isoprene is used almost entirely (over 95%) as monomer for production of elastomers such as polyisoprene, styrenic thermoplastic elastomer block copolymers (styrene-isoprene-styrene, SIS), and butyl rubber. Total world isoprene consumption reached 717 000 t in 2004. The main petrochemical use of isoamylenes (2-methyl-2-butene and 2-methyl-1-butene) is the production of tert-amyl-methyl ether (TAME), which is used as an octane booster in green gasoline. Although the ban of ethers in gasoline formulation in some areas of the United States may have an impact on its future demand, the European Union (E.U.) production of TAME reached 250 000 t in 2002 and is growing. However, the TAME potential capacity is strictly related to the reactive isoamylene present in the light gasoline cracking plants. Isoamylenes are also used, in Russia and Japan, to produce isoprene by dehydrogenation in a flow-through, fixed-bed, adiabatic reactor with iron oxide catalyst at a temperature of 600-640 °C in the presence of steam.1 These severe conditions (high dilution of raw materials with steam, high temperature) lead to gradual catalyst deactivation,2 with the result that the basic catalytic parameters of the catalyst fall in 1-2 years. * To whom correspondence should be addressed. Tel.: +34915854783. Fax: +34-915854760. E-mail address:
[email protected]. † Dedicated to the memory of Dr. J. A. Delgado. ‡ Present address: CIEMAT, Av. Complutense 22, 28040 Madrid, Spain. § Present address: Te´cnicas Reunidas S.A., Arapiles 13, 28015 Madrid, Spain.
Until recently, the chemical industry’s demand for these C5 olefins and diene has been met by multiproduct complex separation processes of oil-refining feedstocks, such as pyrolysis gasoline stream and steam-cracking fractions. Usual compositions of refinery C5 fractions coming from steam crackers are as follows: isoprene, 15-17%; n-pentane and isopentane, 20-30%; mono-olefins, 15-25%; diolefins, 10-15%; and cyclic C5, 20-30%. The component with highest demand, isoprene, is isolated by extractive rectification. The current use of the remaining fraction as a component for gasoline requires partial hydrogenation after dilution in an inert stream, followed by separation from the diluent. This implies an additional cost for the use of this fraction, with no increase in the added value. The production of C5 olefins and diolefins as byproducts limits its availability (the isoprene yield is only 2-5% of the ethylene yield of the crackers3) and imposes lack of flexibility for the production. This very limited availability of isoprene, and isoamylenes, which could be used to produce it, is facing the needs of their increasing demand. However, this limitation could be overcome by converting the accompanying isopentane (the content of which is of comparable magnitude) into these olefins. Conceptually, the least-expensive and easiest route is through oxidative dehydrogenation (ODH), because (i) it overcomes the thermodynamic limitations of the endothermic dehydrogenation, and (ii) oxygen (pure or in air) is inexpensive and possesses the high reactivity needed to activate the saturated hydrocarbons. Therefore, it is quite surprising that, despite its industrial strategic importance, practically no fundamental studies have been reported in the literature concerning the ODH of isopentane (i-C5). A very limited number of open literature publications involve the dehydrogenation of C5 alkanes under oxidative conditions. Early works concern mainly Russian authors who, in the 1970s, studied the transformation of isopentane to isoprene over oxide catalysts4 and zeolites,5 often cofeeding a variety of halogens or hydrogen halides.6 Other publications, mainly U.S. patents that were submitted in the 1970s, include studies on the ODH of C5 alkanes. Note that even these patents generally include the ODH of a broad range of hydrocarbons, with C5 components being only one of the groups mentioned. Usually, the C5 reactions studied are (i) the production of isoprene from isopentane,7 from isoamylenes,8-10 and from mixed feeds consisting of isoamylenes, n-pentane, and isopentane;11 and (ii)
10.1021/ie800756p CCC: $40.75 2008 American Chemical Society Published on Web 09/25/2008
8038 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
the production of mono-olefins and diolefins from isopentane.12,13 A variety of catalysts have been proposed for these reactions. In most cases, they are prepared via precipitation and consist of two or, usually, more metals in a complex oxide form. Among the more frequently used elements in those catalysts are iron, cobalt, nickel, magnesium, vanadium, and uranium. Usual reaction temperatures are in the range of 450-650 °C. For example, Cichowski13 used the Co-Fe-P-O catalyst for the ODH of isopentane to a mixture of isoprene and isopentene, reporting a conversion of 25% i-C5. Tsailingold14 applied a V-Mo-O catalyst for the dehydrogenation of butane and isopentane, which provided a conversion of 44%. Academic research on the ODH of light alkanes increased substantially in the early 1990s, after the discovery of the V-Mg-O catalysts by Kung and co-workers,15 who studied the ODH of propane, isobutane, butane, pentane, and cyclohexane over Mg3(VO4)2-MgO and (VO)2P2O7 catalysts.16 In fact, vanadium-containing catalysts have been the most investigated for the ODH of light alkanes, and some controversy existed on the optimal catalyst composition and the mostselective V-Mg-O active phase.17 However, despite the large number of studies on this system, no data on isopentane ODH on this system have been reported. It is noticeable that practically all papers on the ODH of C5-alkanes are devoted to the linear n-pentane and its conversion to maleic and phthalic anhydrides18-21 or to complex mixtures of olefins and oxygenates, working at high temperatures and very short contact times.22,23 Concerning specifically the ODH of isopentane, Rizayev et al.24 found that the most effective system was an aluminasupported Ni-V-Sb oxide. Note that the best results were obtained at temperatures of >600 °C, with a reacting mixture of isopentane, oxygen, and water vapor (in a molecular ratio of 1:1:20, which is not much different than that used in anaerobic dehydrogenation). These high steam contents are limiting the practical application of the process and also involve a high energy cost. The final objective of the present research was to explore the valorization of refinery C5 fractions after isoprene has been extracted, a mixture of paraffins (30-40%), olefins (30-40%), and others (40-20%), via the ODH of the alkanes to isoprene and linear diolefins (piperylenes). In the present paper, we have investigated the ODH of isopentane to isoprene and isoamylenes, using molecular oxygen as an oxidant and without the use of water or gaseous dopes (halogens, etc.) in the feed. Based on a broad screening of catalyst formulations, a vanadium magnesium oxide catalyst was selected for optimization of the reaction conditions. Experimental Section Catalyst Preparation. Based on previous reports in the literature, a broad range of catalyst compositions was prepared, using mainly two synthesis methods: impregnation and the homogeneous citrate method. They are summarized in Table 1. The starting materials were the nitrates of the component cations (Zn, Fe, Cr, etc.), but for vanadium, different sources were used (for example, ammonium metavanadate (NH4VO4) for impregnation, or vanadium pentoxide (V2O5) for preparing vanadium oxalate or citrate). Bulk oxides were prepared using the homogeneous citrate method, by mixing aqueous solutions of the component cations in adequate ratios, and adding a stoichiometric quantity of citric acid (C6H8O7) (one acid molecule per three metal ions), followed by evaporation at 30 °C under reduced pressure, until forming a syrup-like liquid,
Table 1. Catalysts Prepared and Tested sample code
elemental composition
preparation method
BET surface area (m2/g)
1-S 3-S 5-S 6-S 7-S
citrate citrate citrate citrate citrate
10 12 5.2 26 10 n/aa n/aa 112 28 n/aa 134 81 159 94 12 36 11 3.1 56 86 39 78
21-J 23-J 25-J 28-J 29-J
impregnation impregnation impregnation impregnation impregnation vanadium oxalate vanadium oxalate vanadium citrate vanadium citrate citrate citrate citrate citrate citrate precipitation Li citrate Li citrate and Sn oxalate citrate citrate citrate impregnation impregnation
ZnFe2O4 MnFe2O4 MnFe1.9Cr0.1O4 ZnFe1.9Cr0.1O4 Co-Fe-P-O (Fe/Co ) 0.07, P/Co ) 0.27) 1-R V-Mo-Mg-O (V/Mo ) 21) 2-R V-Mo-Mg-O (V/Mo ) 0.11) 1-RX V-Mg-O (15 wt % V2O5) 2-RX V-Mg-O (50 wt % V2O5) 3-RX V-Mg-O (24 wt % V2O5) 1-J V-Mg-O (25 wt % V2O5) 2-J V-Mg-O (12.5 wt % V2O5) 3-J V-Mg-O (25 wt % V2O5) 4-J V-Mg-O (12.5 wt % V2O5) 6-J Mo-Mg-O (Mo/Mg ) 1) 8-J NdVO4 10-J CeVO4 12-J Mo-Mg-O (Mo/Mg ) 2) 14-J MgCr2O4 16-J MgO (support) 17-J Li-Mg-O (7% Li) 18-J Li-Sn-Mg-O (16% Li, 4% Sn)
a
Mg(P2O7) CrVO4 Cr2(P2O7)3 V-Mg-O (10 wt % V2O5) V-Mg-O (12.5 wt % V2O5)
73 8.0 96 n/aa n/aa
Not available.
drying at 80 °C overnight, decomposition of the thus-formed precursor at 300 °C for 16 h, and calcination at 550 °C for 20 h. Supported vanadium catalysts were prepared by impregnation of magnesium oxide with ammonium metavanadate aqueous solutions in the selected final atomic ratios.25 In some cases, the impregnation was made using solutions of vanadium citrate or oxalate (see Table 1), which were prepared by precipitation of aqueous solutions of V2O5 and the corresponding acids in the appropriate amounts. Impregnated catalysts were dried at 80 °C under vacuum and calcined in air at 550 °C for 6 h. Catalyst Characterization. Phases present in the samples were identified by X-ray diffraction (XRD) analysis that was performed on a Siemens D5000 diffractometer, using nickelfiltered Cu KR radiation. Specific surface areas were measured by Kr adsorption at -196 °C on a Micromeritics ASAP 2000 volumetric apparatus using the Brunauer-Emmett-Teller (BET) method. Catalytic Tests. Isopentane (Fluka, minimum purity of 99.5%), oxygen, and helium (N45, SEO) were used as reactants. The flow of gases was controlled by mass flow controllers (Brookhorst). Isopentane flow was controlled by mass flow controller for liquids (Brookhorst), with its liquid depot kept under pressure, which avoids the problems of uncontrolled evaporation, caused by its boiling point close to room temperature. Isopentane is evaporated into the helium flow in a preheated zone prior to the reactor. Catalytic tests are conducted in a fixed-bed, downflow quartz reactor. Reactants were fed separately through two entrances: isopentane diluted in helium is introduced through one coaxial and oxygen through a lateral one. This allows one to contact both reactants just in the prebed space, filled by silicon carbide (SiC) chips to facilitate the heat transfer and to reduce the empty volume (and, hence, the homogeneous reaction extension). The reaction temperature was monitored with a coaxial thermocouple, the end of which is placed in the center of the catalytic bed. Catalysts screening
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8039
Figure 1. Selectivity to (a) mono-olefins, (b) isoprene), and (c) total dehydrogenation, as a function of isopentane conversion on selected catalysts. Sample codes are as given in Table 1. Table 2. Influence of Residence Time and Reaction Temperature on Isopentane ODH over Catalyst 1-RX Selectivity, S (%) temperature, T (°C)
conversion, X (%)
375 400 425
11.6 21.2 36.1
375 400 425
6.5 12.1 21.6
350 400 450
1.9 7.0 22.2
monoolefins
isoprene
COx
cracking
12.6 11.8 11.0
61.5 70.8 78.0
1.2 1.0 0.9
12.8 13.7 14.0
51.7 59.6 67.3
1.3 1.3 1.1
12.1 17.0 19.0
31.1 44.8 58.0
0.5 1.1 1.2
W/F ) 159 24.7 16.6 10.3 W/F ) 72 34.3 25.4 17.8 W/F ) 40 56.4 37.3 22.0
tests for the ODH of isopentane were conducted in the temperature range of 300-500 °C (in some cases, 200-600 °C), with a feed molar composition of 2% isopentane, 4% oxygen, and the balance being helium, and a residence time of W/F ) 32 g cat · h/mol C5. Under these conditions, homogeneous reactions became detectable at 350 °C, but conversion remained 1, where oxygen concentration affects total conversion very little, as shown previously. Results are shown in Figures 5 and 6. At the three temperatures investigated, conversion increased significantly with increasing the i-C5 concentration up to 8 mol % and then stabilized (Figure 5). Under these conditions of excess oxygen, the higher the isopentane concentration, the lower the selectivity to total dehydrogenation. Concluding Remarks
Figure 6. Influence of isopentane concentration on the selectivity to total dehydrogenation on catalyst 1-RX. Oxygen concentration ) 25%. Legend shows the concentration of isopentane (in mol %).
3. In the studied range of temperatures, if the molar ratio of O2 to i-C5 is 1, the increase of oxygen partial pressure has very little effect, showing just a very slight increase of conversion. Figure 4 shows the dependency of selectivity on conversion when varying the oxygen:isopentane ratio. As a general trend, for a given conversion, the total dehydrogenation selectivity (Figure 4, right) increased as the oxygen partial pressure decreased. The only exception is observed for O2:i-C5 ) 0.2, for which selectivity is surprisingly practically constant and even increases as conversion increases. This is probably due to oxygen exhaustion, even at low isopentane conversion, because of the high deficit
The results presented here show that the oxidative dehydrogenation (ODH) of isopentane could be an effective way to increase the availability of isoprene (and isoamylenes) from C5 fractions, making it feasible to increase the availability of these olefins. It may be operated at moderate temperatures (1, whereas below that ratio, the conversion increased markedly with oxygen content; as a general trend, the selectivity to total dehydrogenation (at isoconversion) increased as the oxygen partial pressure decreased. (4) In the presence of excess oxygen, conversion increased significantly as the i-C5 partial pressure increased, up to a certain value, and then stabilized. Summarizing, one may conclude that the best selectivity values could be obtained at high temperatures with O2:C5 ratios of ∼1 and lean feed (low isopentane concentration). Under selected conditions, selectivity values of ca. 25% to isoprene and 70% to isoprene plus isomylenes have been attained, which is quite promising. From the practical point of view, it would be convenient to submit the full isoprene-depleted C5 fraction (i.e., both alkanes and mono-olefins) to the ODH process. Preliminary results in our group have shown that the homogeneous gas-phase reaction of isopentene (2-methyl-2-butene) and oxygen starts already at 350 °C, with an isoprene selectivity close to 50%. This could contribute to increase the isoprene yield. Further studies of the ODH of the mono-olefins (isoamylenes) are needed to verify the feasibility of such a process. Acknowledgment The authors wish to thank Dr. J. A. Delgado and Prof. B. Delmon for their always constructive and enriching discussions, and the technical help of the members of Prof. Delmon’s group at UCL (Belgium) and of Mr. J. A. Carretero Carrio´n and Dr. M. I. Guijarro at ICP. Financial support by E.C. is also acknowledged. Supporting Information Available: Detailed results from catalysts screening tests are available as a table. This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Mukhlenov, I. P.; Dobkina, E. I.; Deryuzhkina, V. I.; Soroko, V. E. Tekhnologiya KatalizatoroV (Technology of Catalysts); Khimiya: Leningrad, 1979. (2) Kolesnikov, I. M., Kataliz i ProizVodstVo KatalizatoroV (Catalysis and Manufacture of Catalysts); Tekhnika TUMA GRUPP: Moscow, 2004. (3) Saltman, W. Isoprene. In Kirk-Othmer Concise Encyclopedia of Chemical Technology; Grayson, M., Ed.; John Wiley and Sons: New York, 1985; pp 674-675. (4) (a) Gusman, T. Ya.; Serebryakov, B. R.; Dalin, M. A. Oxidative dehydrogenation of isoamylenes on bismuth molybdates. Khim. Tekh. Topl. Masel 1967, 12, 1. (b) Lisova, N. N.; Shapolova, L. P.; Luk’yanenko, V. P. Effect of method of zinc-titanium catalyst preparation on its catalytic properties in oxidative dehydrogenation of isopentane. Neftepererab. Neftekhim. (KieV) 1984, 27, 15. (5) (a) Kuznetzov, A. V. Oxidative dehydrogenation of isopentane on zeolite catalysts. Khim. Khim. Tekhnol. Polim. Org. Sint., 1973, 49,CA 82: 98416f. (b) Okel’son, I. I.; Kuznetsov, A. V. Effect of cation nature on zeolite activity in the oxidative dehydrogenation of isopentane. Vpor. Kinetiki i Kataliza 1973, (1), 68.
(6) (a) For example, for HI, I2: Nikonov, V. I.; Nikonova, M. M.; Adel’son, S. V.; Liakumovich, A. G.; Rozenoer, S. S. Preparation of isoprene by oxidative dehydrogenation of isopentane. NoV. Neftekhim. Prod. Protsessy 1971, 6,CA 81:38601g. (b) For HCl: Adel’son, S. V.; Yudinson, R. N.; Lebedeva, N. I.; Paushkin, Y. M. Oxidative dehydrogenation of isopentane in the presence of hydrogen chloride. Khim. Prom. (Moscow) 1971, 47, 174. (c) For HBr, Br2: Sterligov, O. D.; Arinich, I. M.; Dudukina, T. V. Oxidative dehydrogenation of isopentane in presence of bromine. IzV. Akad. Nauk SSSR, Ser. Khim. 1971, (12), 2833. (d) Sterligov, O. D.; Arinich, I. M.; Dudukina, T. V. Oxidative dehydrogenation of isopentane in presence of hydrobromic acid. IzV. Akad. Nauk SSSR, Ser. Khim. 1972, (1), 1959. (7) Cichowski, R. S. Catalytic oxidative dehydrogenation of hydrocarbon feedstocks, U.S. Patent 3,758,609, 1973. (8) Cichowski, R. S. Oxidative dehydrogenation catalyst, U.S. Patent 3,862,910, 1975. (9) Colling, P. M.; Dean, J. C. Metal ferrites as catalysts for oxidative dehydrogenation of organic compounds, U.S. Patent 3,567,793, 1971. (10) Manning, H. E. Oxidative dehydrogenation utilizing manganese ferrite, U.S. Patent 3,671,606, 1972. (11) Manning, H. E. Chromium modified manganese ferrite oxidative dehydrogenation catalysts, U.S. Patent 4,026,920, 1977. (12) Walker, D. W.; Farha, F. E. Oxidative dehydrogenation catalyst, U.S. Patent 4,010,114, 1977. (13) Cichowski, R. S. Cobalt-, iron-, phosphorus- and oxygen-containing dehydrogenation catalysts, U.S. Patent 3,784,483, 1974. (14) Tsailingold, A. L.; Pilipenko, F. S.; Levin, V. A.; Vemova, T. P.; Stepanov, G. A.; Bashin, A. N.; Sivotkin, B. V.;, Kozin, V. V.; Basner, M. E. Mono- and diolefin hydrocarbons, Br. Patent 1,363,331, 1972. (15) Kung, H. H.; Chaar, M. A. Oxidative dehydrogenation of alkanes to unsaturated hydrocarbons, U.S. Patent 4,777,319, 1988. (16) Michalakos, P. M.; Kung, M. C.; Jahan, I.; Kung, H. H. Selectivity patterns in alkane oxidation over Mg3(VO4) 2-MgO, Mg2V2O7, and (VO) 2P2O7. J. Catal. 1993, 140, 226. (17) Mamedov, E. A.; Corte´s Corbera´n, V. Oxidative dehydrogenation of lower paraffins on vanadium-oxide-based catalysts: The present state and outlookssA review. Appl. Catal., A 1995, 127, 1. (18) Centi, G.; Lopez-Nieto, J. M.; Pinelli, D.; Trifiro, F. Synthesis of phthalic and maleic anhydrides from n-pentane. 1. Kinetic analysis of the reaction network. Ind. Eng. Chem. Res. 1989, 28, 400. (19) Korili, S. A.; Ruiz, P.; Delmon, B. Oxidative dehydrogenation of n-pentane on magnesium vanadate catalysts. Catal. Today 1996, 32, 229. (20) Ozkan, U. S.; Harris, T. A.; Schilf, B. T. The partial oxidation of C-5 hydrocarbons over vanadia-based catalysts. Catal. Today 1997, 33, 57. (21) Cavani, F.; Colombo, A.; Trifiro, F.; Sananes-Schulz, M. T.; Volta, J. C.; Hutchings, G. J. The effect of cobalt and iron dopants on the catalytic behavior of V/P/O catalysts in the selective oxidation of n-pentane to maleic and phthalic anhydrides. Catal. Lett. 1997, 43, 241. (22) Dietz, A. G.; Carlsson, A. F.; Schmidt, L. D. Partial oxidation of C-5 and C-6 alkanes over monolith catalysts at short contact times. J. Catal. 1998, 176, 459. (23) Iordanoglou, D. I.; Bodke, A. S.; Schmidt, L. D. Oxygenates and olefins from alkanes in a single-gauze reactor at short contact times. J. Catal. 1999, 187, 400. (24) Rizayev, R.G.; Talyshinskii, R. M.; Seifullareva, J. M.; Guseinova, E. M.; Panteleyeva, Yu. A.; Mamedov, E. A. Oxidative dehydrogenation of the C-4-C-5 paraffins over vanadium-containing oxide catalysts. In New DeVelopments in SelectiVe Oxidation II; Corte´s Corbera`n, V.; Vic Bello´n, S., Eds.;studies in Surface Science Catalysis 82; Elsevier: Amsterdam, 1994; p 125. (25) Chaar, M.; Patel, D.; Kung, H. H. Selective oxidative dehydrogenation of propane over V-Mg-O catalysts. J. Catal. 1988, 109, 463. (26) Valenzuela, R. X.; Corte´s Corbera´n, V. On the intrinsic activity of vanadium centres in the oxidative dehydrogenation of propane over VCa-O and V-Mg-O catalysts. Top. Catal. 2000, 11/12, 153.
ReceiVed for reView May 9, 2008 ReVised manuscript receiVed August 19, 2008 Accepted August 27, 2008 IE800756P
