Ind. Eng. Chem. Res. 2005, 44, 4717-4720
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SEPARATIONS Selective Absorption of Isoprene from C5 Mixtures by π Complexation with Cu(I) Seung J. Son,† Hyun W. Choi,† Dae K. Choi,*,† Sang D. Lee,† Hoon S. Kim,*,‡ and Seung W. Kim§ Environment & Process Technology Division, Korea Institute of Science & Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Korea, Department of Chemistry, Kyung Hee University, 1 Hoegidong, Dongdaemun-gu, Seoul 130-701, Korea, and Department of Chemical & Biological Engineering, Korea University, 1 5-Ka Anamdong Sungbuk-gu, Seoul 136-701, Korea
Selective absorption of isoprene from n-pentane mixtures was performed via reversible π complexation with an aqueous solution of copper(I) nitrate, which was prepared by the disproportionation reaction between copper powder and cupric nitrate trihydrate [Cu(NO3)2‚ 3H2O] in H2O in the presence of pyridine and nitric acid as stabilizers. The absorption of isoprene by the Cu(I) solution increased with an increase in the molar ratio of isoprene/Cu(I) up to 1 and remained nearly constant thereafter. The maximum isoprene pickup achieved at the molar ratio of isoprene/Cu(I) ) 1 was around 0.5 equiv of Cu(I), implying that isoprene was complexed with Cu(I) in a η2 mode. The absorption ability of the Cu(I) solution was strongly dependent on the concentrations of the stabilizers, which prevent the oxidation of Cu(I) to Cu(II). The optimal molar composition of the solution was found to be Cu(I)/pyridine/HNO3 ) 1/2.7/1. 1. Introduction Olefin/paraffin separation is one of the most important and costly classes of separations in the chemical and petrochemical industries.1-3 The conventional method for separating olefin/paraffin mixtures is highly energy-intensive cryogenic distillation.3,4 Accordingly, considerable effort has been made to develop alternative separation methods including extractive distillation, molecular sieve adsorption, absorption, facilitated transport membranes, etc.4-6 Among various methods, the separation using facilitated transport technology has attracted much interest because of its low energy consumption and simple operation. The basis for the separation is the ability of metal ions, particularly Ag(I) and Cu(I), to interact reversibly with olefins, forming silver(I) olefin or copper(I) olefin π complexes.7,8 The use of silver nitrate or cuprous diketonate as a complexing agent to separate olefins from paraffin mixtures has appeared in patents and literatures.4,9-14 Upon contact with the complexing agent, olefin forms a silver(I) olefin or copper(I) olefin complex, which, in turn, liberates olefins by a combination of heating and pressure reduction.1,7,8,13 Recently, BP Petrochemicals developed a new technology, called selective olefin recovery (SOR), to recover ethylene or propylene from the cracked gas.9 The major advantage of this process is the use of copper(I) nitrate instead of expensive silver(I) salts. However, no * To whom correspondence should be addressed. Tel.: +822-958-5872. Fax: +82-2-958-5879. E-mail:
[email protected] ((D.K.C.). Tel.: +82-2-961-0432. Fax: +82-2-966-3701. Email:
[email protected] (H.S.K.). † Korea Institute of Science & Technology. ‡ Kyung Hee University. § Korea University.
report has been published on the separation of dienes using the SOR process, This paper describes the application of the SOR process to the selective absorption of isoprene from n-pentane mixtures. Various factors affecting the isoprene absorption are also discussed. An isoprene/npentane mixture has been chosen as the feed mixture because isoprene is known to form an azeotrope with n-pentane.15 2. Theory The structure and bonding of π complexes were discussed in particular by Winstein and Lucas and by Dewar.3,4,14,16,17 Two types of bonding exist between the olefin and a metal ion, especially the silver ion: σ and π bonds. A σ bond forms through the donation of π electrons from the occupied 2p bonding orbital of the olefin into the vacant 5s orbital of the silver ions (Figure 1 a). π-bond formation is the result of the back-donation of d electrons from the occupied 4d orbitals of the silver ion to the unoccupied π*-2p antibonding orbitals of the olefin (Figure 1b). The stability of π complexes, largely determined by the degree of back-donation, is directly related to the reversibility of metal-olefin. In general, the reversibility of metal-olefin complexes increases with a decrease in the stability. Compared with the olefin complexes of Pt(I) and Pd(I) with fully occupied d orbitals, the olefin complexes of Ag(I) and Cu(I) are known to be considerably less stable because Ag(I) and Cu(I) have higher electron affinity and lower promotion energy in comparison with Pt(II) and Pd(II).13 In other words, the dissociation of olefins from the olefin complexes of Ag(I) and Cu(I) is much easier than that of Pt(II) and Pd(II). Because the basis of the
10.1021/ie049358k CCC: $30.25 © 2005 American Chemical Society Published on Web 05/26/2005
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Figure 1. Dewar-Chatt model of π-bond complexation.
facilitated olefin transport is the reversible interaction between metal and olefin, Ag(I) and Cu(I) can be used as facilitating agents. 3. Experiment 3.1. Preparation of a Copper(I) Nitrate Solution. An aqueous solution of Cu(I)-olefin nitrate was used for the selective absorption of isoprene from C5 mixtures.9,18 A typical example for the preparation of a 2 M Cu(I) facilitator is as follows: To a 100-mL three-necked flask equipped with a gas inlet, a condenser, and a thermometer were added copper powder (0.95 g, 15 mmol), Cu(NO3)2‚3H2O (3.62 g, 15 mmol), and 13.1 mL of distilled water. After the complete dissolution of the copper(II) nitrate, pyridine (6.55 mL, 6.41 g, 81 mmol) was added with vigorous stirring while maintaining an argon purge to remove dissolved oxygen and to avoid contact with external air. After stirring for 1 h, the concentrated HNO3 (70 wt %, 2.7 g, 30 mmol) was slowly added using a dropping funnel. After the addition of HNO3, the resulting deep blue solution was heated to 333 K and reacted for 20 min to ensure the complete conversion of Cu(0) to Cu(I). 3.2. Absorption Experiment. Absorption experiments were carried out in an absorber by adding a mixture of isoprene and n-pentane to a solution containing Cu(I).18,19 The resulting solution was vigorously stirred at 600 rpm for 2-3 min to facilitate the absorption of isoprene into the aqueous layer containing Cu(I). After the absorption, two layers were separated. Both the organic and aqueous layers were analyzed using a gas chromatograph (HP model 5890 II) equipped with a flame ionization detector and a packed column (packing material: Unibead 2s 60/80, 3 m). 3.3. XRD Analysis. To confirm the in situ formation of Cu(I) species from the a disproportionation reaction of Cu(0) and Cu(II), the solution containing Cu(I) was exposed to air for 12 h, and the resulting red-orange crystalline solid was collected for X-ray diffraction (XRD) analysis.9 The XRD analysis was conducted on a Shimadzu XRD-6000 using a Cu source (λ ) 0.154 060 nm). 4. Result and Discussion 4.1. Preparation of Cu(I). As can be seen in eq 1, copper(I) nitrate was prepared from a disproportionation reaction between copper(0) and copper(II) nitrate in the presence of pyridine and HNO3.
Cu0 + Cu(NO3)2 f 2CuNO3
(1)
Pyridine and HNO3 were employed to facilitate the formation of Cu(I) and to stabilize the resulting CuNO3.
Figure 2. Effect of the molar ratio of feed isoprene/Cu(I) on the complexation of isoprene with Cu(I). Cu(I) ) 30 mmol. H2O ) 13.1 mL. Temperature ) 313 K. Isoprene(c) ) complexed isoprene. Isoprene(f) ) isoprene in the feed.
As shown in eq 2, pyridine is able to bind to copper(I) nitrate to form pyridine-coordinated copper(I) nitrate, thereby stabilizing Cu(I). The coordinated pyridine ligand can be easily replaced by an incoming isoprene molecule to form an isoprene-coordinated Cu(I) species because the bond strength of Cu(I)-pyridine is similar to that of Cu(I)-isoprene. 4.2. Effect of the Molar Ratio of Isoprene/Cu(I). To investigate the degree of isoprene complexation with Cu(I), the molar ratio of isoprene/Cu(I) was varied, and the results are shown in Figure 2. Binary mixtures containing equimolar amounts of isoprene and n-pentane were used as the feed. The amounts of Cu(I), pyridine, HNO3, and distilled water were set at 30 mmol, 81 mmol, 30 mmol, and 13.1 mL, respectively. The absorption of isoprene in the aqueous solution increased with an increase in the molar ratio of isoprene/Cu(I) up to 1 but remained nearly constant upon a further increase in the molar ratio. The maximum isoprene pickup achieved at the molar ratio of isoprene/ Cu(I) ) 1 was around 0.5 equiv of Cu(I), implying that isoprene was complexed with Cu(I) in a η2 mode. On the other hand, the separation factor for isoprene/ n-pentane decreased continuously with an increase in the molar ratio of isoprene/Cu(I). The separation factor is defined as
moles of isoprene per moles of n-pentane in the aqueous layer separation factor ) moles of isoprene per moles of n-pentane in the organic layer It is generally accepted that an olefin molecule forms a 1/1 π complex with Cu(I). However, the bonding mode of isoprene to Cu(I) seems to be different from that of a simple olefin molecule. As shown in Figure 3, two types
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Figure 3. Complexation modes of Cu(I) with isoprene.
Figure 6. Effect of the temperature on the isoprene absorption. Cu(I) ) 30 mmol. Feed ) isoprene (30 mmol), n-pentane (26.4 mmol). H2O ) 13.1 mL. Isoprene(c) ) complexed isoprene.
Figure 4. Effect of the pyridine concentration on the complexation of isoprene with Cu(I). Cu(I) ) 30 mmol. Feed ) isoprene (30 mmol), n-pentane (26.4 mmol). H2O ) 13.1 mL. Temperature ) 313 K. Isoprene(c) ) complexed isoprene.
Figure 5. Effect of the HNO3 concentration on the complexation of isoprene with Cu(I). Cu(I) ) 30 mmol. Feed ) isoprene (30 mmol), n-pentane (26.4 mmol). H2O ) 13.1 mL. Temperature ) 323 K. Isoprene(c) ) complexed isoprene.
of bonding modes can be conceived for the complexation of isoprene with Cu(I): η2 and η4 modes. However, from the experimental results, it is believed that one molecule of isoprene binds to two Cu(I) ions simultaneously in a η2 mode.20,21 4.3. Optimization of the Pyridine/Cu(I) Molar Ratio. The amount of pyridine was varied to investigate the effect of pyridine on the complexation of isoprene with Cu(I). The concentration of Cu(I) and the molar ratio of HNO3/Cu(I) were fixed at 2 M and 1, respectively. The amount of complexed isoprene increased with an increase in the molar ratio of pyridine/Cu(I) up to 2.7 and decreased thereafter (see Figure 4). It seems that, at a higher molar ratio of pyridine/ Cu(I), the complexation of isoprene with Cu(I) ions is somewhat interfered with by the presence of large amounts of pyridine, possibly because of the competition of isoprene with pyridine for the coordination to Cu(I). The complexation of isoprene was also affected by the concentration of HNO3. As shown in Figure 5, the
Figure 7. XRD patterns: (a) copper powder; (b) copper(II) nitrate trihydrate; (c) red-orange crystalline solid after air treatment of a Cu(I) aqueous solution.
absorption of isoprene increased with an increase in the molar ratio of HNO3/Cu(I) up to 1 and thereafter
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decreased. At a molar ratio of HNO3/Cu(I) lower than 1, it seems that the formation of Cu(I) species is not completed, and therefore the amount of Cu(I) to complex with isoprene becomes reduced. On the contrary, at a higher molar ratio of HNO3/Cu(I), pyridine can be protonated by excess HNO3, thereby reducing the effective pyridine concentration. 4.4. Effect of the Temperature. The effect of the temperature on the separation of isoprene from the n-pentane mixture consisting of 30 mmol of isoprene and 26.4 mmol of n-pentane was investigated. The molar ratios of isoprene/Cu(I) and Cu(I)/pyridine/HNO3 were set at 1/1 and 1/2.7/1, respectively. As shown in Figure 6, the degree of isoprene complexation with Cu(I) increased very slowly with the temperature rise up to 338 K but decreased thereafter. It is likely that, at temperatures above 338 K, the rate of decomplexation of the Cu(I)-isoprene complex becomes significant and thus the net absorption of isoprene becomes smaller. 4.5. XRD Analysis. The presence of Cu(I) species was in the aqueous solution confirmed indirectly by the formation of Cu2O because Cu(I) species easily react with O2 to produce Cu2O as in eq 3.
1 2Cu(Ι) + O2 f Cu2O 2
(3)
The aqueous solution containing Cu(I) species was exposed to air, and the resulting red-orange crystalline precipitates were collected for XRD analysis. Figure 7 clearly shows the appearance of the peaks corresponding to Cu2O, confirming the presence of Cu(I) in the solution. 5. Conclusion The aqueous Cu(I) solutions, prepared from Cu(0), Cu(NO3)2‚3H2O, HNO3, and pyridine, were effective for the selective absorption of isoprene from n-pentane mixtures. The optimum molar ratio of Cu(I)/pyridine/HNO3 (70 wt %) was found to be 1/2.7/1. It was observed that the maximum absorption of isoprene in the aqueous solution of Cu(I) was around 0.5 equiv of Cu(I), implying that Cu(I) forms a complex with isoprene in a 2/1 ratio (η2 mode). Acknowledgment This research was performed for the Carbon Dioxide Reduction & Sequestration Center, one of 21st Century Frontier R&D Programs funded by the Ministry of Science and Technology of Korea.
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Received for review July 22, 2004 Revised manuscript received March 21, 2005 Accepted May 3, 2005 IE049358K
