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Effect of Flow Rate of a Biphasic Reaction Mixture on Limonene Hydrogenation in High Pressure CO2 Ewa Bogel-Łukasik,* Rafał Bogel-Łukasik, and Manuel Nunes da Ponte REQUIMTE, Departamento de Quı`mica, Faculdade de Cieˆncias e Tecnologia, UniVersidade NoVa de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal
In this work the effect of the overall flow rate of the biphasic reaction mixture on hydrogenation distribution products is reported. As it was already presented by us, the reaction rate strongly correlates with the phase equilibrium existing in the system. On the other hand, carbon dioxide is nonreactive; nevertheless by its presence it changes the energy balance. The catalytic performance in four kinds of overall flow rate conditions (1.3, 3.3, 5.3, and 7.3 mL/min) was compared under fixed hydrogen (2.5 MPa) and total pressure (12.5 MPa). The appearance of isomers of limonene and partially hydrogenated products significantly rely upon the flow rate used in the reaction. The 1% Pd SCN catalyst used in the reaction gave the final products trans- and cis-p-menthane in a 2/3 to 1/3 molar ratio. 1. Introduction Reactions in supercritical fluids (SCFs) are an innovative and challenging field of research, which offers a number of important advantages in the development of new technologies for the chemical and biochemical process industries. The replacement of the volatile organic solvents used as reaction medium with environmentally benign solvents such as SCFs, especially carbon dioxide and water, is one of the preliminary objectives of current research in chemistry and principles of green chemistry.1 Hydrogenation of organic compounds under supercritical conditions can be carried out as batch reactions2,3 in sealed autoclaves or, due to the gas-like nature of supercritical fluids, in continuous flow reactors.4,5 One of the reports about the hydrogenations occurring under supercritical conditions present the selective hydrogenation of 2-cyclohexen-1-one over Pt-MCM-41. The hydrogenation proceeded at a very high rate and produced cyclohexanone with a selectivity of 100% in a batch reactor.2 The total conversion in hydrogenation of itaconic acid of the batchwise recycling reached a maximum after 1 h of batch hydrogenation while in semicontinuous reaction it took more than 2 h.6 The scCO2 flow system in hydrogenation of rac-sertraline imine suggested that the excellent heat transfer properties of scCO2 help to maintain exceptional levels of chemoselectivity.7 The reaction rate and product distribution in the hydrogenation of citral was strongly influenced by the continuous8 or batch3,8 mode of the reaction. In the cycloaddition of CO2 and ethylene oxide in a continuous flow system, increasing the flow rates of liquid CO2 and ethylene oxide results in a decrease in conversion, but the formation rate of ethylene carbonate increases.9 The continuous10 catalytic hydrogenations in scCO2 guides the hydrogenation of the CdC bonds without affecting the CdO bonds. The use of scCO2 was crucial because it lowered the gas-liquid mass transfer resistance and enhanced the diffusion through the external fluid film of the catalyst. In principle, the flow reactors can be generally smaller than the corresponding batch reactors and they ensure a comparable efficiency of the reaction and quantity of the product produced at once. This reduction in size is particularly attractive for supercritical fluid systems because it diminishes both the cost and the safety problems of high pressure equipment.11 Besides * To whom correspondence should be addressed. Tel: (351) 212948353. Fax: (351) 212948385. E-mail:
[email protected].
heterogeneous rather than homogeneous catalysis is experimentally simpler and usually is easier to scale up even in case of reactions carried out under high pressure conditions.12 The particular advantage of supercritical fluids is the ability to control conditions with great precision and, hence, to tune the selectivity of reactions.11 Additionally scCO2 exhibits a great potential for use of the biphasic systems, that is, in catalytic reactions.13 In the past, it was reported that hydrogenation in biphasic conditions in the presence of high-pressure carbon dioxide is faster than in monophasic ones.14,15 It can be explained by the fact that the concentration (mol/L) of substrate (as opposed to H2) under monophasic conditions is lower than in the two-phase system as it was resolved earlier.16 Our investigation focuses on hydrogenation of limonene using supercritical carbon dioxide in a biphasic (liquid and gas) system. Experimental setup used in this work allows simulating the reaction performed in batch-like and flow-like modes. For these reasons, four various overall flow rates of the biphasic reaction mixture were tested. Reactions were carried at 1.3, 3.3, 5.3, and 7.3 mL/min flow rates at 323.15 K. Temperature was determined experimentally as the lowest efficient and economically feasible operation temperature to carry out the hydrogenation of terpene in scCO2.16,17 Summarizing, on the basis of the great potential existing in the biphasic reactions in scCO2, the overall flow rate of the biphasic reaction mixture effect on hydrogenation distribution products is reported in this work. 2. Experimental Section 2.1. Materials. Chemicals. Hydrogen and carbon dioxide were supplied by Air Liquide, with a stated purity of 99.998 mol %. R-(+)-limonene (purity: 98%), (+)-p-menth-1-ene (purity: g97%), (+)-p-menth-3-ene (purity: g97%), terpinolene (purity: g97%), γ-terpinene (purity: g98.5%) and nonane (purity: g99%) were supplied by Fluka. cis-p-Menthane (purity: g97%) and trans-p-menthane (purity: g97%) were supplied by Fluorochem Limited. Catalyst. The spherical granulated activated carbon of the SCN type was synthesized by carbonization of vinylpyridine and divinylbenzene. Next, the catalyst particles were activated by contact with deaerated aqueous solution of tetrachloropalladate ions what led to the covering of their surface by porous
10.1021/ie900450m CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
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Figure 2. The main reaction pathways for hydrogenation of limonene.
Figure 1. The hydrogenation apparatus: (C) CO2 compressor, (TR) tubular reactor, (VC) view cell, (CP) circulation pump, (PT) pressure transducer, (T) temperature controller, (V) sample vessel, (HPLC) high pressure sampling valve, (S) syringe.
layer of 1 wt % Pd load. More detailed description of the method of the catalyst preparation was presented elsewhere.18 2.2. Reaction Experiments and Analytical Methods. The hydrogenations were performed with an apparatus consisting of one sapphire-windowed cell connected by a pump to a tubular reactor-short tube that encloses a catalyst bed. The hydrogenation apparatus is presented in Figure 1. The detailed description of the apparatus was giving in our previous paper.17 The hydrogenation of limonene was carried out in the presence of 0.2 g 1 wt % Pd catalyst, 1 mL of limonene at 323.15 K with 2.5 MPa of hydrogen. The fixed total pressure of experiment after charging the reactor with CO2 was 12.5 MPa because the reactions were carried out at conditions where both gas and liquid phases are present. Taking into account the solid catalyst, the three-phase reaction system had to be dealt with. Rheonik RHM 015 GNT flowmeter equipped with an electronic transmitter RHE 11 was used to measure the overall flow rate with repeatability better than 0.05% of the rate and accuracy better than 0.23%. The reactions were performed at four various flow rates, 1.3, 3.3, 5.3, and 7.3 mL/min. The reactants were continuously withdrawn from the bottom of the view cell, circulated through the catalyst bed, and sent back to the upper entrance of the cell. The liquid products were sampled at regular intervals through HPLC valve with a 100 µL sampling loop. The quantitative and qualitative analysis of the samples was performed with a HRGC-3000C gas chromatograph equipped with a flame ionization detector and a mass spectrum detector. A gas chromatography column (CP-Sil 8 CB from Varian Inc.) was used. Oven temperature program: 87-91 °C ramp at 0.5 °C/min, and 91-240 °C ramp at 20 °C/min. Injector and detector temperature was 250 °C. Nonane in hexane (1.5 mM) was used as external standard for GC analysis (response factor for R-(+)-limonene, 1.42; (+)-p-menth-1-ene,: 1.26; (+)-pmenth-3-ene, 1.26; cis-p-menthane, 1.44; and trans-p-menthane, 1.43; terpinolene, 1.44; and γ-terpinene, 3.76). To measure the Pd leaching, atomic absorption spectroscopy (AAS) analyses using a ThermoElectron atomic absorption
Figure 3. Consumption of limonene as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
spectroscope S series were performed. The level of metal content in the sample was below 2 ppm, which is the level of detection for this technique. 3. Results The influence of flow rate has been considered as an important factor in directing the rate of limonene disappearance during the hydrogenation process. The effect of flow rate has been examined at constant temperature, fixed initial hydrogen, and total pressures 2.5 and 12.5 MPa, respectively. These conditions guarantee the performance of the reaction in a two-phase (gas and liquid) reaction system.17 The main reaction pathways for hydrogenation of limonene are presented in Figure 2. As it can be seen in Figure 3 the speed of limonene consumption is remarkably different for the investigated flow rates. Obtained results indicate that the concentration of limonene decreases faster at lower flow rate and with the increase of the flow rate the speed of limonene consumption is lower. The p-menth-1-ene concentration profile depends on the flow rate of the reaction mixture as well. For all the investigated flow rates except the highest one, the concentration of p-menth1-ene peaks at 30 min, then diminishes and reacts completely after 3 h of the reaction. Maximum concentration of p-menth1-ene for 1.3 and 3.3 mL/min flow rates reaches 48.5%, while for 5.3 mL/min the maximum concentration gains only 20.5%. At the highest flow rate employed in the reaction, the intermediate was not detected, as Figure 4 illustrates. Results achieved
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Figure 4. Concentration profile for p-menth-1-ene as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
Figure 5. Concentration profile for p-menth-3-ene as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
for the second intermediate (p-menth-3-ene) are shown in Figure 5. The concentration profiles for various flow rates are in good coincident with those obtained for p-ment-1-ene. The most noticeable difference is the level of maximum concentration which is lower. For 1.3 and 3.3 mL/min it equals 20%, and for 5.3 mL/min, it reaches only 9%. At the highest flow rate the p-menth-3-ene intermediate was not observed. Additionally, the concentration profile for p-menth-3-ene is flattened comparing to p-menth-1-ene. The peak of the concentration profile for p-menth-3-ene is not so stepwise like for p-menth-1-ene, and it starts to diminish later than p-menth-1-ene. This behavior might be caused by the mechanism of the reaction, and the mechanism is studied and presented elsewhere. The sum of the concentrations of both partially hydrogenated products depends on the flow rate used in the reaction (Figure 6). For 1.3 and 3.3 mL/min flow rates there is no noteworthy difference. Although, for higher flow rates, the quantity of p-menth-1-ene and p-menth-3-ene decreases, and is counterbalanced by the increasing concentration of trans- and cis-pmenthane as presented in Figures 7 and 8. For the reaction performed at 5.3 mL/min flow rate, the intermediates were present in the analyzed samples, but finally for the highest considered flow rate (7.3 mL/min) even the partially hydrogenated products were not detected (Figure 6). The concentration profiles of cis- and trans-p-menthane show that both products are formed in 1:2 ratio as it is illustrated in Figures 7 and 8. Limonene isomers (terpinolene and γ-terpinene; not presented in Figures) were detected only within the first 15 min of the reaction performed at 1.3 and 3.3 mL/min in the concentration not higher than 5% in total.
Figure 6. Sum of concentration of p-menth-1-ene and p-menth-3-ene as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
Figure 7. Concentration profile for trans-p-menthane as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
Figure 8. Concentration profile for cis-p-menthane as a function of reaction time for (b) 1.3, (O) 3.3, (9) 5.3, and (0) 7.3 mL/min flow rates.
4. Discussion The quantitative information on liquid phase composition was obtained by applying the Peng-Robinson equation of state19 with the Mathias-Klotz-Prausnitz mixing rule20 using PE software.21 Concentration of hydrogen present in the liquid phase under reaction conditions equals 2.8 mol/L and the ratio H2 over limonene, calculated based on the correlation of the obtained VLE experimental data, equals 2.3.16 This ratio describes the
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composition of the initial reaction mixture and it changes in the course of the hydrogenation because limonene and H2 are consumed. Analyzing the profile of limonene disappearance presented in Figure 3, it can be concluded that limonene is consumed faster at lower flow rate, what can be recognized as a curiosum. But from the microscopic point of view, this unusual phenomenon could be explained by the fact that at higher flow rate, the residence time of limonene in the vicinity of catalyst is short. This may lead to the conclusion that the residence time is a rate controlling factor, because at two of the lowest (1.3 and 3.3 mL/min) flow rates when the residence time is the longest, limonene is consumed faster. On the other hand, an astonishing lack of difference in the products distribution profiles between 1.3 and 3.3 mL/min flow rates indicates that another parameter is a rate controlling factor as well. The H2 to limonene ratio is high and indicates that limonene should be hydrogenated without any mass transfer limitation; however, the ratio is provided to serve as a guide and corresponds only to the initial composition of the reaction mixture. This means that at the low 1.3 and 3.3 mL/min flow rates not only is the residence time important but also the H2 transfer limitation plays an important role, and partially hydrogenated products and isomers of limonene due to the deficiency of H2 in the liquid phase are formed.17 Similar conclusions can be taken from the reaction performed at 5.3 mL/min flow rate, albeit isomers of limonene were not detected at the beginning of the reaction, and the levels of intermediates concentration is much lower at 5.3 mL/min than at both previously considered flow rates (1.3 and 3.3 mL/min). Another observed difference is that at the 5.3 mL/min flow rate, the consumption of limonene is slower than at the 3.3 mL/min; however, a refreshing of the liquid layer by H2 is supposed to be faster than in the two lower flow rates. Nevertheless, the obtained results prove that it is not sufficiently effective to avoid the limitation of H2 transfer from a gas to the liquid phase through the interphase boundary. In the analyzed reactions, CO2 acts as a cosolvent and improves the solubility of H2 in the liquid phase. Nevertheless, the hydrogen content in the liquid phase is high although lower than required for the hydrogenation of limonene in the biphasic conditions.17 The high hydrogen content in the liquid phase makes the hydrogenation relatively quick but also reduces the hydrogen level faster than the fresh crosses the gas-liquid phase boundary. The fast consumption of H2 disturbs the existing equilibrium and the driving force leading to equilibrate the temporary lack of hydrogen is created. The mentioned driving force increases the speed of hydrogen migration from a gas phase to the liquid layer, which is supported by the higher flow rate which improves the H2 transfer through the phase boundary. For the 5.3 mL/min and lower flow rates, the transport is bounded, which affects the presence of the partially hydrogenated products in the analyzed samples. At the highest investigated flow rate, due to the disturbances caused by the flow rate applied, hydrogen from the gas phase ready to react is introduced. Because of this, intermediates are consumed as fast as they are formed, and for this reason they were not observed in the course of the reaction at the 7.3 mL/min flow rate (Figures 4-6). cis-p-Menthane and trans-p-menthane are produced by the hydrogenation of p-menth-1-ene and p-menth-3-ene. This may only be possible before the intermediates desorb from the catalyst surface and in the case of a larger amount of H2 present in the liquid reaction phase. Enrichment of the liquid phase by hydrogen is caused by the employment of higher a flow rate
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and disturbance of the equilibrium existing in the reaction system. This facilitates the introduction of fresh hydrogen to the liquid phase. The apparatus used for these studies showed that the reactions performed at different flow rates could be considered as both the semicontinuous and semibatch systems. The highest flow rate (7.3 mL/min) simulates the process carried out in the batch due to a very fast circulation of the feed mixture and because of the large changes in the feed composition during reaction. As a proof we can quote the fact that at elevated flow rate only the fully hydrogenated products were observed in the analyzed samples. The slowest flow rates (1.3 and 3.3 mL/min) lead us to conclude that the reactions carried out at these conditions are continuous-like processes. The lower flow rate disturbs the feed minimally as it takes place in the continuous processes. In addition at 1.3 and 3.3 mL/min flow rates limonene isomers and intermediates were obtained, which also confirms our hypothesis. Formation of terpinolene and γ-terpinene could be explained by the Pd/C catalyzed isomerization of limonene via the π-allyl mechanism as depicted by Grau et al.22 5. Summary and Conclusion Hydrogenation of limonene performed at all measured flow rates, leads to the formation in the final stage of the fully hydrogenated products trans- and cis-p-menthanes in a 2/3 to 1 /3 ratio. Intermediates p-menth-1-ene and p-menth-3-ene were detected in measurable quantities in the reactions, when the flow rate was 1.3, 3.3, and 5.3 mL/min. Terpinolene and γ-terpinene (isomers of limonene) which were consumed in the progress of the reaction were noticed in the reactions when a 1.3 and 3.3 mL/min overall flow rate was applied. The formation of p-menth-1-ene, p-menth-3-ene, terpinolene, and γ-terpinene is possible via a straightforward addition of hydrogen to the CdC double bond and/or via a π-allyl mechanism which has been explained in our16 work and the work of Grau et al.22 Acknowledgment This work was supported by the European Commission in the frame of the Marie Curie Research Training Network SUPERGREENCHEM (EC Contract No.: MRTN-CT-2004504005). Literature Cited (1) Tang, S. L. Y.; Smith, R. L.; Poliakoff, M. Principles of green chemistry: Productively. Green Chem 2005, 7, 761–762. (2) Chatterjee, M.; Yokoyama, T.; Kawanami, H.; Sato, M. An exceptionally rapid and selective hydrogenation of 2-cyclohexen-1-one in supercritical carbon dioxide. Chem. Commun. 2009, 701–703. (3) Chatterjee, M.; Chatterjee, A.; Raveendran, P.; Ikushima, Y. Hydrogenation of citral in supercritical carbon dioxide using a heterogeneous Ni(II) catalyst. Green Chem. 2006, 8, 445–449. (4) Hyde, J. R.; Licence, P.; Carter, D.; Poliakoff, M. Continuous catalytic reactions in supercritical fluids. Appl. Catal., A 2001, 222, 119– 131. (5) Stephenson, P.; Kondor, B.; Licence, P.; Scovell, K.; Ross, S. K.; Poliakoff, M. Continuous asymmetric hydrogenation in supercritical carbon dioxide using an immobilised homogeneous catalyst. AdV. Synth. Catal. 2006, 348, 1605–1610. (6) Burgemeister, K.; Francio, G.; Gego, V. H.; Greiner, L.; Hugl, H.; Leitner, W. Inverted supercritical carbon dioxide/aqueous biphasic media for rhodium-catalyzed hydrogenation reactions. Chem.sEur. J. 2007, 13, 2798–2804. (7) Clark, P.; Poliakoff, M.; Wells, A. Continuous flow hydrogenation of a pharmaceutical intermediate, [4-(3,4-dichlorophenyl)-3,4-dihydro-2Hnaphthalenyidene]-methylamine, in supercritical carbon dioxide. AdV. Synth. Catal. 2008, 349, 2655–2659.
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(17) Bogel-Łukasik, E.; Fonseca, I.; Bogel-Łukasik, R.; Tarasenko, Y. A.; Nunes da Ponte, M.; Paiva, A.; Brunner, G. Phase equilibrium-driven selective hydrogenation of limonene in high-pressure carbon dioxide. Green Chem. 2007, 9, 427–430. (18) Tarasenko, Y. A.; Kopyl, S. A.; Lapko, V. F.; Lysenko, A. A.; Tomizuka, I. Role of chemisorbed oxygen in fixation of palladium by an activated carbon, SCN-3M, from aqueous solution of its complex ion. Electrochemistry 2002, 70, 316–321. (19) Peng, D. Y.; Robinson, D. B. A new two-constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59–64. (20) Pfohl, O.; Petkov, S.; Brunner, G. PE V2.9.9a - Software for Phase Equilibria Calculations; Technische Universita¨t Hamburg: Hamburg, Germany, 1998. (21) Mathias, P. M.; Klotz, H. C.; Prausnitz, J. M. Equation-of-state mixing rules for multicomponent mixtures: The problem of invariance. Fluid Phase Equilib. 1991, 67, 31–44. (22) Grau, R. J.; Zgolicz, P. D.; Gutierrez, C.; Taher, H. A. Liquid phase hydrogenation, isomerization, and dehydrogenation of limonene and derivatives with supported palladium catalysts. J. Mol. Catal., A 1999, 148, 203– 214.
ReceiVed for reView March 19, 2009 ReVised manuscript receiVed May 9, 2009 Accepted June 16, 2009 IE900450M
