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Homogeneous metathesis for the production of propene from butene
Applied Catalysis A: General 340 (2008) 236–241
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Homogeneous metathesis for the production of propene from butene Wolfgang H. Meyer *, M.M. Daphne Radebe, D. Wynand Serfontein, Umesh Ramdhani, Maria du Toit, Christakis P. Nicolaides Sasol Technology (Pty) Ltd., Research & Development, P.O. Box 1, Sasolburg 1947, South Africa
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 January 2008 Received in revised form 13 February 2008 Accepted 14 February 2008 Available online 10 March 2008
The cross-metathesis of 1-butene and 2-butene is described for the production of propene and 2-pentene using Phobcat, a homogeneous Grubbs first generation-type catalyst bearing 9-cyclohexyl-9phosphabicyclo-[3.3.1]-nonane as ligand. In a closed system at 20 bar and 50 8C, the reaction mixture composition at different 1-butene:2-butene feed ratios is in fair agreement with the calculated composition at equilibrium based on thermodynamic data. Compared to heterogeneous metathesis over a WO3/SiO2 catalyst, the homogeneous reaction exhibits better reaction control and selectivity to the desired products. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Propene Butene Metathesis Homogeneous Ruthenium Phoban
1. Introduction In the classic Phillips triolefin process, 2-butene and ethene are converted to propene over a heterogeneous metathesis catalyst. This technology is currently widely used to manipulate the propene:ethene ratio of a cracker in a downstream metathesis operation using the Olefin Conversion Technology licensed by ABB Lummus. The commonly used heterogeneous metathesis catalyst tungsten oxide on silica operates at temperatures above 260 8C. Coking deactivates the catalyst which can, however, be successfully regenerated [1]. An alternative way to produce propene is the auto-metathesis of 1-butene over such a heterogeneous catalyst. At elevated temperatures, the 1-butene feed is partly isomerised to 2-butene – assisted or non-assisted by the presence of an isomerisation catalyst – and the cross-metathesis of the two butenes results in the formation of propene and 2-pentene [2–4]. The homogeneous Grubbs first generation catalyst [5] has been intensively studied in the homometathesis and ethenolysis of methyl oleate as a renewable monomer in the laboratories of The Dow Chemical Company with methyl 9-decenoate and 1-decene as the products of the ethenolysis [6]. For the economical manufacture of such or other bulk chemicals, however, a suitable application of homogeneous metathesis catalysts has not yet been found.
* Corresponding author. Tel.: +27 169604715; fax: +27 115222034. E-mail address: [email protected] (W.H. Meyer). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.02.019
In this light, the evaluation of homogeneous ruthenium catalysts in the above described cross-metathesis of 1-butene and 2-butene seems attractive: the reaction temperature required is lower, the reaction control is better and the selectivity is possibly higher than in the heterogeneously catalysed reaction where the formation of higher molecular weight metathesis products and byproducts is observed. These unwanted products arise mainly from secondary metathesis reactions as well as alkene interconversions and hydrogen transfer reactions, respectively, over the heterogeneous catalysts, as described later in this work. The present paper reports on the cross-metathesis of 1-butene and 2-butene using the homogeneous Grubbs first generation-type ruthenium catalyst 1 [7–9] and includes a comparison to the heterogeneous catalyst, tungsten oxide on silica. 2. Experimental 2.1. General Phobcat, [benzylidene dichlorobis(9-cyclohexyl-9-phosphabicyclo-[3.3.1]-nonane)ruthenium], was synthesized as previously described [7]. Rhodium(III)chloride hydrate as well as toluene and n-nonane were purchased from Sigma–Aldrich and 1-butene from Air Liquide. SafolTM23 was available in-house. All reactions were carried out under inert conditions using dried and oxygen-free solvents and compounds. An Agilent 6890N GC instrument equipped with a Pona column was used for the analysis of the gaseous and liquid reaction mixtures of the homogeneously
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catalyzed runs. For the heterogeneous metathesis experiments, an HP 6890 GC instrument equipped with a Pona column was utilised for the liquid phase analysis, while an Agilent 6850 GC instrument equipped with a Al2O3/KCl Plot column was used for the analysis of the gas mixtures. For the thermodynamic calculation, the process simulation program ‘‘Aspen Plus’’ from Aspen Technology Inc. was used. Equilibrium compositions were calculated subject to the reaction Eqs. (1)–(6) of Scheme 1. 2.2. Isomerisation of 1-butene Into a 300 mL Parr autoclave containing 10 mg of RhCl3xH2O and 20 mL SafolTM23, about 150 mL of liquefied 1-butene was added through a sample bomb at sample bomb pressure (2–3 bar). The reaction was stirred at room temperature and the head space monitored via GC until the 2-butene content was 90–95% (1–3 days). The gas mixture was condensed out of the autoclave into a stainless steel sample bomb cooled to about 10 8C and was used without further manipulation.
containing the activated WO3/SiO2 catalyst (4 g) at the reaction temperature. The preparation and activation of the catalyst have been described previously [10]. The exiting gas stream was cooled down to room temperature to give a gas phase and a liquid phase, both of which were analyzed by GC.
2.3. Homogeneous metathesis of butenes
3. Results and discussion
The set-up depicted in Fig. 2 was used for the homogeneous metathesis runs. Typically, 13.2 mg of the homogeneous ruthenium catalyst was dissolved in 10 mL (25 mL) toluene and the solution introduced into the autoclave through the designated port. The total liquid butenes between 10 and 14 g (or 23.0 and 50.5 g) contained in the loading bombs of different length to achieve the desired 1-butene:2-butene ratio were injected into the autoclave using a nitrogen pressure of about 25 bar until the autoclave was pressurized to 20 bar. The mixture was stirred at 1000 rpm, heated to reaction temperature within 10 min and kept at the same temperature for the duration of the run. Head space samples were drawn at regular intervals via the sample valve (Fig. 2) and analysed. The reaction was considered either ceased or at thermodynamic equilibrium when no change in the head space composition was observed within experimental error. The autoclave was cooled to 5–10 8C by means of an ice bath. The autoclave was carefully deflated to ambient pressure into a gas bag. The mass of the captured gas was determined by weighing the autoclave before and after expansion. The remaining liquid contained a considerable amount of dissolved butenes as their escape could be noticed from the bubbling liquid. Both gas and liquid were analysed via GC.
Since complex 1 (Fig. 1) is known to have no or only very low isomerisation activity [7], liquid 1-butene was first isomerized to 2-butene using rhodium(III) chloride dissolved in SafolTM23 – a mixture of C12 and C13 alcohols – in a 300 mL Parr autoclave at room temperature before mixtures of 1-butene and 2-butene were metathesized. Only six different compounds (1-butene, 2butene, ethene, propene, 2-pentene and 3-hexene) from the primary metathesis reactions between the two butene isomers (Eq. (1), Scheme 1) or from the self-metathesis of 1-butene (Eq. (2)) are expected to be present in the reaction mixture, while neither the self-metathesis of 2-butene nor all other possible secondary metathesis reactions will form new products (Eqs. (3)– (6), Scheme 1). The metathesis reaction was carried out in the set-up as depicted in Fig. 2. The catalyst is introduced as a toluene solution through the catalyst injection port into a 50 or 300 mL Parr autoclave under inert conditions. The loading bombs for the two butene isomers are individually charged with the liquefied butenes by drawing the liquids into the evacuated bombs from the storage vessels in which the gases were previously condensed. The loading bombs were manufactured from 3/4 in. steel tubing of different length to allow for different amounts and ratios of 1-butene and 2-butene to be injected into the autoclave. The liquid butenes are pushed into the autoclave with nitrogen gas to a pressure of 20 bar. At such pressures and the chosen temperatures below 60 8C, the butenes and the toluene solution of the catalyst form one liquid phase.
2.4. Heterogeneous metathesis of 1-butene Liquid 1-butene was pumped at the desired feed rate into a tubular fixed-bed reactor with an inner diameter of 15 mm
Fig. 1. Phobcat, 1 (Cy, cyclohexyl; Ph, phenyl).
Scheme 1. Butene metathesis with pre-isomerisation.
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Fig. 2. Experimental set-up for homogeneous butene metathesis.
Along with nitrogen, the head space of the autoclave is composed mainly of the formed ethene and propene, and to a certain extent of butenes, 2-pentene and 3-hexene, according to their partial pressures. This composition is monitored over time using gas chromatography. The samples are obtained by releasing a small amount of gas through the dedicated gas sampling outlet. When the head space composition of two samples remained approximately constant over three consecutive measurements (total of at least 90 min), the reaction was considered to have reached equilibrium. Carefully releasing the gas into a gas bag after cooling down to 10 8C and retrieving the liquid after full expansion to ambient pressure allowed for the quantification of all reactants and products. The determined mass losses were typically in the range of 15–20 mass% and were considered as butenes dissolved in toluene which then evaporated from the solvent before GC analysis of the liquid. Apart from the solvent (toluene) and the internal standard (n-nonane), only the six components depicted in Scheme 1 were present (99+%). Fig. 3 shows the change in the composition of the head space versus the reaction time at an initial feed ratio of 1-butene:2butene of 64:36. The reaction is close to equilibrium after about 60 min, after which only small changes in composition are noticeable. Nevertheless, the metathesis reactions were run for 2.5 or 3 h to allow for a closer approach to equilibrium. The results obtained with different starting compositions are shown in Table 1 (runs 1–7) and Fig. 4. With pure 1-butene feed, the fractions of ethene and 3-hexene should be equal. The experimental relative difference of about 6% indicates the experimental error of the method which assumes mass losses solely due to the mentioned evaporation of the butenes.
The experimental curves of composition are in fair agreement with those predicted from the thermodynamic calculations using the Aspen program (Figs. 4 and 5). The highest propene fraction of 26 mol% is relatively close to the predicted value of 29.4 mol% at an initial butene isomer feed ratio of about 50:50. Scaling the reaction upto a feed:Ru ratio of 23,000 (Table 1, run 8) and 210,000 (Table 1, run 9) gave satisfactory agreement to run 4 within experimental error. The thermodynamic and experimental conversions compare favourably and peak at an initial 2-butene content of 30–50% (Fig. 6). Figs. 4 and 5 illustrate that with an increasing 2-butene fraction, propene and 2-pentene become the sole products approaching a 1:1 molar ratio due to the decreasing selfmetathesis of 1-butene. The theoretically highest selectivity to propene is 50 mol% or 37.5 mass% which is approached experimentally as shown in Fig. 7. Due to the increasing propene selectivity, the maximum propene yield is observed at an initial 2butene fraction of about 50% whilst the highest conversion is located at about 40%. A further increase in the initial 2-butene fraction, e.g. from 50 to 60%, results in a decrease of propene yield at thermodynamic equilibrium but a simultaneous slight increase of the propene selectivity. Reducing the initial 2-butene fraction to 35% results in the same decrease of propene yield but a decrease of the propene selectivity. Considering a continuous process with the recycle of non-reacted butenes, it is clearly advantageous for the maximum propene production to perform the metathesis reaction in a regime of an initial 2-butene fraction of above rather than below 50%. Beyond 60%, however, the improvement in the selectivity to propene is too marginal to justify a further increase in the recycle of butenes which would be necessary due to the sharp drop in conversion.
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Fig. 3. Molar head space composition over time at an initial feed ratio of 1-butene:2-butene = 64:36.
Fig. 4. Experimental molar composition at thermodynamic equilibrium at different butene feed ratios.
The Olefin Conversion Technology licensed by ABB Lummus applies a heterogeneous WO3/SiO2 catalyst for the classic metathesis reaction of ethene, 2-butene and propene [1]. In the heterogeneous metathesis of 1-butene at 400 8C and 20 bar on a
WO3/SiO2 catalyst, 1-butene is partly isomerised to 2-butene followed by the cross-metathesis to propene and 2-pentene (Table 2). Although the mol fraction of propene is 30.1%, which is higher than the best homogeneous metathesis result and equal
Fig. 5. Calculated molar composition at thermodynamic equilibrium at different butene feed ratios.
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Table 1 Composition of feed and reaction mixture at end-of-run Run
Feed (g)
1-C¼ 4;0 (mol%)
2-C¼ 4;0 (mol%)
C¼ 2 (mol%)
C¼ 3 (mol%)
1-C¼ 4 (mol%)
2-C¼ 4 (mol%)
2-C¼ 5 (mol%)
3-C¼ 6 (mol%)
Conversion (%)
Spropene (mol%)
1 2 3 4 5 6 7 8 9
10.6 12.9 13.4 12.0 13.5 13.8 11.0 23.0 50.5
19 26 41 51 64 81 100 47 48
81 74 59 49 36 19 0 53 52
0.1 0.5 1.6 3.1 5.4 9.4 16.0 3.2 2.8
9.7 15.5 22.8 25.9 23.7 14.0 0.2 26.1 25.9
0.9 3.3 10.0 17.7 30.4 50.8 68.4 13.0 15.1
79.4 66.0 43.8 28.5 15.9 4.6 0.4 33.6 35.0
9.6 13.6 18.6 19.6 16.7 9.4 0.1 19.5 16.9
0.4 1.1 3.2 5.2 7.9 11.8 15.0 4.5 4.3
19.7 30.7 46.2 53.8 53.7 44.6 31.2 53.3 49.9
49.2 50.5 49.4 48.1 44.1 31.4 0.6 49.0 51.9
¼ 1-C¼ 4;0 and 2-C4;0 refer to the ratio at t = 0 s, other at t = end of run. Runs 1–7: 10 mL toluene, butene:Ru = 10,000–14,000, 50 8C. Run 8: 25 mL toluene, butene:Ru = 23,000, 50 8C. Run 9: 25 mL toluene, butene:Ru = 210,000, 30 8C.
to the thermodynamically predicted value, the selectivity to propene is 45.8%. This value, however, is lower than the homogeneous metathesis runs with initial 2-butene mol fractions greater than 0.4 (Tables 1 and 2 and Fig. 7). The apparent waste of butene materializes itself in the fraction of C7+ compounds as well as some minor paraffin formation stemming from the metathesis of isomerised pentenes and hexenes, and from the alkene interconversions and hydrogen transfer reactions, respectively (Table 2). The applied heterogeneous catalyst shows satisfactory metathesis activity at 350 8C, at which temperature the isomer-
isation activity by the acidic centres [11] is already observed but lower than at 400 8C (Table 2). As expected, the C7+ fraction is lower at 350 8C. Some 2-butene is formed which almost completely reacts in the cross-metathesis with 1-butene to propene and 2-pentene. While homogeneous metathesis is able to deliver almost exclusively ethene and 3-hexene at pure 1-butene feed (S = 99 mol%), the WO3/SiO2 system produces considerable amounts of propene and 2-pentene due to its isomerisation activity—even at lower temperature. In our heterogeneous experiment at 350 8C the
Fig. 6. Calculated and experimental conversions at different butene feed ratios.
Fig. 7. Conversion, propene selectivity and yield at different butene feed ratios.
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Table 2 Composition of reaction mixture in 1-butene metathesis over WO3/SiO2 Run
T (8C)
C¼ 2 (mol%)
C¼ 3 (mol%)
1-C¼ 4 (mol%)
2-C¼ 4 (mol%)
C¼ 5 (mol%)
C¼ 6 (mol%)
C7+ (mol%)
C1–6 (mol%)
Conversion (%)
Spropene (mol%)
1 2
350 400
15.1 5.5
16.2 30.1
37.4 9.2
2.6 25.1
13.2 17.7
12.3 7.6
2.5 4.2
0.7 0.6
60.0 65.7
27.0 45.8
selectivity to combined ethene and 3-hexene is 46 mol%, and values of 73 and 89 mol% – the latter using a high purity silica support – were reported elsewhere [12]. The isomerisation activity can be reduced by the introduction of alkali-metal ions which is beyond the scope of this comparison [13]. 4. Conclusion Homogeneous metathesis can provide a product spectrum of ethene, propene, 2-pentene and 3-hexene from mixtures of 1butene and 2-butene with a distribution which is in fair agreement with the one predicted by thermodynamic calculations. Compared to the classic heterogeneous metathesis on a WO3/ SiO2 catalyst, the homogeneous reaction can be carried out at milder conditions and butene losses due to formation of higher products are not observed. Furthermore, the selectivity to propene is higher at the optimum feed ratios (1-butene:2-butene from 40:60 to 50:50) and pure 1-butene reacts to ethene and 3-hexene with a selectivity of 99 mol% versus only 48 mol% for the heterogeneous reaction at 350 8C. Considering the superior reaction control in the described homogeneous metathesis reaction, such a process could compete with heterogeneous metathesis provided that the higher catalyst
costs and the more complex process design can be successfully addressed.
References [1] J.C. Mol, J. Mol. Catal. A: Chem. 213 (2004) 39–45. [2] J.M. Botha, J.M.M. Mbatha, B.S. Nkosi, A. Spamer, J. Swart, WO 00/14038, Sasol Technology (Pty.) Ltd., 2000. [3] R.J. Gartside, M.I. Greene, Q.J. Jones, WO 03/076371 A1, ABB Lummus Global Inc., 2003. [4] P. Schwab, A. Ho¨hn, US 6271430 B2, BASF AG, 2001. [5] G.C. Fu, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc. 115 (1993) 9856–9857. [6] K.A. Burdett, L.D. Harris, P. Margl, B.R. Maughon, T. Mokhtar-Zadeh, P.C. Saucier, E.P. Wasserman, Organometallics 23 (2004) 2027–2047. [7] G.S. Forman, A.E. McConnell, M.J. Hanton, A.M.Z. Slawin, R.P. Tooze, W. Janse van Rensburg, W.H. Meyer, C.L. Dwyer, M.M. Kirk, D.W. Serfontein, Organometallics 23 (2004) 4824–4827. [8] G.S. Forman, A.E. McConnell, R.P. Tooze, C.L. Dwyer, D.W. Serfontein, ZA 03/00087, Sasol Technology, UK, 2003. [9] C.L. Dwyer, M.M. Kirk, W.H. Meyer, W. Janse van Rensburg, G.S. Forman, Organometallics 25 (2006) 3806–3812. [10] C. van Schalkwyk, A. Spamer, D.J. Moodley, T.I. Dube, J. Reynhardt, J.M. Botha, Appl. Catal. A: Gen. 255 (2003) 121–131. [11] A.J. van Roosmalen, J.C. Mol, J. Catal. 78 (1982) 17–23. [12] R.J. Gartside, M.I. Greene, A.M. Khonsari, L.L. Murrell, US 2003/0028063 A1 (2007). [13] A. Spamer, T.I. Dube, D.J. Moodley, C. van Schalkwyk, J.M. Botha, Appl. Catal. A: Gen. 255 (2003) 153–167.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Homogeneous metathesis for the production of propene from butene Wolfgang H. Meyer *, M.M. Daphne Radebe, D. Wynand Serfontein, Umesh Ramdhani, Maria du Toit, Christakis P. Nicolaides Sasol Technology (Pty) Ltd., Research & Development, P.O. Box 1, Sasolburg 1947, South Africa
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 January 2008 Received in revised form 13 February 2008 Accepted 14 February 2008 Available online 10 March 2008
The cross-metathesis of 1-butene and 2-butene is described for the production of propene and 2-pentene using Phobcat, a homogeneous Grubbs first generation-type catalyst bearing 9-cyclohexyl-9phosphabicyclo-[3.3.1]-nonane as ligand. In a closed system at 20 bar and 50 8C, the reaction mixture composition at different 1-butene:2-butene feed ratios is in fair agreement with the calculated composition at equilibrium based on thermodynamic data. Compared to heterogeneous metathesis over a WO3/SiO2 catalyst, the homogeneous reaction exhibits better reaction control and selectivity to the desired products. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Propene Butene Metathesis Homogeneous Ruthenium Phoban
1. Introduction In the classic Phillips triolefin process, 2-butene and ethene are converted to propene over a heterogeneous metathesis catalyst. This technology is currently widely used to manipulate the propene:ethene ratio of a cracker in a downstream metathesis operation using the Olefin Conversion Technology licensed by ABB Lummus. The commonly used heterogeneous metathesis catalyst tungsten oxide on silica operates at temperatures above 260 8C. Coking deactivates the catalyst which can, however, be successfully regenerated [1]. An alternative way to produce propene is the auto-metathesis of 1-butene over such a heterogeneous catalyst. At elevated temperatures, the 1-butene feed is partly isomerised to 2-butene – assisted or non-assisted by the presence of an isomerisation catalyst – and the cross-metathesis of the two butenes results in the formation of propene and 2-pentene [2–4]. The homogeneous Grubbs first generation catalyst [5] has been intensively studied in the homometathesis and ethenolysis of methyl oleate as a renewable monomer in the laboratories of The Dow Chemical Company with methyl 9-decenoate and 1-decene as the products of the ethenolysis [6]. For the economical manufacture of such or other bulk chemicals, however, a suitable application of homogeneous metathesis catalysts has not yet been found.
* Corresponding author. Tel.: +27 169604715; fax: +27 115222034. E-mail address: [email protected] (W.H. Meyer). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.02.019
In this light, the evaluation of homogeneous ruthenium catalysts in the above described cross-metathesis of 1-butene and 2-butene seems attractive: the reaction temperature required is lower, the reaction control is better and the selectivity is possibly higher than in the heterogeneously catalysed reaction where the formation of higher molecular weight metathesis products and byproducts is observed. These unwanted products arise mainly from secondary metathesis reactions as well as alkene interconversions and hydrogen transfer reactions, respectively, over the heterogeneous catalysts, as described later in this work. The present paper reports on the cross-metathesis of 1-butene and 2-butene using the homogeneous Grubbs first generation-type ruthenium catalyst 1 [7–9] and includes a comparison to the heterogeneous catalyst, tungsten oxide on silica. 2. Experimental 2.1. General Phobcat, [benzylidene dichlorobis(9-cyclohexyl-9-phosphabicyclo-[3.3.1]-nonane)ruthenium], was synthesized as previously described [7]. Rhodium(III)chloride hydrate as well as toluene and n-nonane were purchased from Sigma–Aldrich and 1-butene from Air Liquide. SafolTM23 was available in-house. All reactions were carried out under inert conditions using dried and oxygen-free solvents and compounds. An Agilent 6890N GC instrument equipped with a Pona column was used for the analysis of the gaseous and liquid reaction mixtures of the homogeneously
W.H. Meyer et al. / Applied Catalysis A: General 340 (2008) 236–241
237
catalyzed runs. For the heterogeneous metathesis experiments, an HP 6890 GC instrument equipped with a Pona column was utilised for the liquid phase analysis, while an Agilent 6850 GC instrument equipped with a Al2O3/KCl Plot column was used for the analysis of the gas mixtures. For the thermodynamic calculation, the process simulation program ‘‘Aspen Plus’’ from Aspen Technology Inc. was used. Equilibrium compositions were calculated subject to the reaction Eqs. (1)–(6) of Scheme 1. 2.2. Isomerisation of 1-butene Into a 300 mL Parr autoclave containing 10 mg of RhCl3xH2O and 20 mL SafolTM23, about 150 mL of liquefied 1-butene was added through a sample bomb at sample bomb pressure (2–3 bar). The reaction was stirred at room temperature and the head space monitored via GC until the 2-butene content was 90–95% (1–3 days). The gas mixture was condensed out of the autoclave into a stainless steel sample bomb cooled to about 10 8C and was used without further manipulation.
containing the activated WO3/SiO2 catalyst (4 g) at the reaction temperature. The preparation and activation of the catalyst have been described previously [10]. The exiting gas stream was cooled down to room temperature to give a gas phase and a liquid phase, both of which were analyzed by GC.
2.3. Homogeneous metathesis of butenes
3. Results and discussion
The set-up depicted in Fig. 2 was used for the homogeneous metathesis runs. Typically, 13.2 mg of the homogeneous ruthenium catalyst was dissolved in 10 mL (25 mL) toluene and the solution introduced into the autoclave through the designated port. The total liquid butenes between 10 and 14 g (or 23.0 and 50.5 g) contained in the loading bombs of different length to achieve the desired 1-butene:2-butene ratio were injected into the autoclave using a nitrogen pressure of about 25 bar until the autoclave was pressurized to 20 bar. The mixture was stirred at 1000 rpm, heated to reaction temperature within 10 min and kept at the same temperature for the duration of the run. Head space samples were drawn at regular intervals via the sample valve (Fig. 2) and analysed. The reaction was considered either ceased or at thermodynamic equilibrium when no change in the head space composition was observed within experimental error. The autoclave was cooled to 5–10 8C by means of an ice bath. The autoclave was carefully deflated to ambient pressure into a gas bag. The mass of the captured gas was determined by weighing the autoclave before and after expansion. The remaining liquid contained a considerable amount of dissolved butenes as their escape could be noticed from the bubbling liquid. Both gas and liquid were analysed via GC.
Since complex 1 (Fig. 1) is known to have no or only very low isomerisation activity [7], liquid 1-butene was first isomerized to 2-butene using rhodium(III) chloride dissolved in SafolTM23 – a mixture of C12 and C13 alcohols – in a 300 mL Parr autoclave at room temperature before mixtures of 1-butene and 2-butene were metathesized. Only six different compounds (1-butene, 2butene, ethene, propene, 2-pentene and 3-hexene) from the primary metathesis reactions between the two butene isomers (Eq. (1), Scheme 1) or from the self-metathesis of 1-butene (Eq. (2)) are expected to be present in the reaction mixture, while neither the self-metathesis of 2-butene nor all other possible secondary metathesis reactions will form new products (Eqs. (3)– (6), Scheme 1). The metathesis reaction was carried out in the set-up as depicted in Fig. 2. The catalyst is introduced as a toluene solution through the catalyst injection port into a 50 or 300 mL Parr autoclave under inert conditions. The loading bombs for the two butene isomers are individually charged with the liquefied butenes by drawing the liquids into the evacuated bombs from the storage vessels in which the gases were previously condensed. The loading bombs were manufactured from 3/4 in. steel tubing of different length to allow for different amounts and ratios of 1-butene and 2-butene to be injected into the autoclave. The liquid butenes are pushed into the autoclave with nitrogen gas to a pressure of 20 bar. At such pressures and the chosen temperatures below 60 8C, the butenes and the toluene solution of the catalyst form one liquid phase.
2.4. Heterogeneous metathesis of 1-butene Liquid 1-butene was pumped at the desired feed rate into a tubular fixed-bed reactor with an inner diameter of 15 mm
Fig. 1. Phobcat, 1 (Cy, cyclohexyl; Ph, phenyl).
Scheme 1. Butene metathesis with pre-isomerisation.
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Fig. 2. Experimental set-up for homogeneous butene metathesis.
Along with nitrogen, the head space of the autoclave is composed mainly of the formed ethene and propene, and to a certain extent of butenes, 2-pentene and 3-hexene, according to their partial pressures. This composition is monitored over time using gas chromatography. The samples are obtained by releasing a small amount of gas through the dedicated gas sampling outlet. When the head space composition of two samples remained approximately constant over three consecutive measurements (total of at least 90 min), the reaction was considered to have reached equilibrium. Carefully releasing the gas into a gas bag after cooling down to 10 8C and retrieving the liquid after full expansion to ambient pressure allowed for the quantification of all reactants and products. The determined mass losses were typically in the range of 15–20 mass% and were considered as butenes dissolved in toluene which then evaporated from the solvent before GC analysis of the liquid. Apart from the solvent (toluene) and the internal standard (n-nonane), only the six components depicted in Scheme 1 were present (99+%). Fig. 3 shows the change in the composition of the head space versus the reaction time at an initial feed ratio of 1-butene:2butene of 64:36. The reaction is close to equilibrium after about 60 min, after which only small changes in composition are noticeable. Nevertheless, the metathesis reactions were run for 2.5 or 3 h to allow for a closer approach to equilibrium. The results obtained with different starting compositions are shown in Table 1 (runs 1–7) and Fig. 4. With pure 1-butene feed, the fractions of ethene and 3-hexene should be equal. The experimental relative difference of about 6% indicates the experimental error of the method which assumes mass losses solely due to the mentioned evaporation of the butenes.
The experimental curves of composition are in fair agreement with those predicted from the thermodynamic calculations using the Aspen program (Figs. 4 and 5). The highest propene fraction of 26 mol% is relatively close to the predicted value of 29.4 mol% at an initial butene isomer feed ratio of about 50:50. Scaling the reaction upto a feed:Ru ratio of 23,000 (Table 1, run 8) and 210,000 (Table 1, run 9) gave satisfactory agreement to run 4 within experimental error. The thermodynamic and experimental conversions compare favourably and peak at an initial 2-butene content of 30–50% (Fig. 6). Figs. 4 and 5 illustrate that with an increasing 2-butene fraction, propene and 2-pentene become the sole products approaching a 1:1 molar ratio due to the decreasing selfmetathesis of 1-butene. The theoretically highest selectivity to propene is 50 mol% or 37.5 mass% which is approached experimentally as shown in Fig. 7. Due to the increasing propene selectivity, the maximum propene yield is observed at an initial 2butene fraction of about 50% whilst the highest conversion is located at about 40%. A further increase in the initial 2-butene fraction, e.g. from 50 to 60%, results in a decrease of propene yield at thermodynamic equilibrium but a simultaneous slight increase of the propene selectivity. Reducing the initial 2-butene fraction to 35% results in the same decrease of propene yield but a decrease of the propene selectivity. Considering a continuous process with the recycle of non-reacted butenes, it is clearly advantageous for the maximum propene production to perform the metathesis reaction in a regime of an initial 2-butene fraction of above rather than below 50%. Beyond 60%, however, the improvement in the selectivity to propene is too marginal to justify a further increase in the recycle of butenes which would be necessary due to the sharp drop in conversion.
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Fig. 3. Molar head space composition over time at an initial feed ratio of 1-butene:2-butene = 64:36.
Fig. 4. Experimental molar composition at thermodynamic equilibrium at different butene feed ratios.
The Olefin Conversion Technology licensed by ABB Lummus applies a heterogeneous WO3/SiO2 catalyst for the classic metathesis reaction of ethene, 2-butene and propene [1]. In the heterogeneous metathesis of 1-butene at 400 8C and 20 bar on a
WO3/SiO2 catalyst, 1-butene is partly isomerised to 2-butene followed by the cross-metathesis to propene and 2-pentene (Table 2). Although the mol fraction of propene is 30.1%, which is higher than the best homogeneous metathesis result and equal
Fig. 5. Calculated molar composition at thermodynamic equilibrium at different butene feed ratios.
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Table 1 Composition of feed and reaction mixture at end-of-run Run
Feed (g)
1-C¼ 4;0 (mol%)
2-C¼ 4;0 (mol%)
C¼ 2 (mol%)
C¼ 3 (mol%)
1-C¼ 4 (mol%)
2-C¼ 4 (mol%)
2-C¼ 5 (mol%)
3-C¼ 6 (mol%)
Conversion (%)
Spropene (mol%)
1 2 3 4 5 6 7 8 9
10.6 12.9 13.4 12.0 13.5 13.8 11.0 23.0 50.5
19 26 41 51 64 81 100 47 48
81 74 59 49 36 19 0 53 52
0.1 0.5 1.6 3.1 5.4 9.4 16.0 3.2 2.8
9.7 15.5 22.8 25.9 23.7 14.0 0.2 26.1 25.9
0.9 3.3 10.0 17.7 30.4 50.8 68.4 13.0 15.1
79.4 66.0 43.8 28.5 15.9 4.6 0.4 33.6 35.0
9.6 13.6 18.6 19.6 16.7 9.4 0.1 19.5 16.9
0.4 1.1 3.2 5.2 7.9 11.8 15.0 4.5 4.3
19.7 30.7 46.2 53.8 53.7 44.6 31.2 53.3 49.9
49.2 50.5 49.4 48.1 44.1 31.4 0.6 49.0 51.9
¼ 1-C¼ 4;0 and 2-C4;0 refer to the ratio at t = 0 s, other at t = end of run. Runs 1–7: 10 mL toluene, butene:Ru = 10,000–14,000, 50 8C. Run 8: 25 mL toluene, butene:Ru = 23,000, 50 8C. Run 9: 25 mL toluene, butene:Ru = 210,000, 30 8C.
to the thermodynamically predicted value, the selectivity to propene is 45.8%. This value, however, is lower than the homogeneous metathesis runs with initial 2-butene mol fractions greater than 0.4 (Tables 1 and 2 and Fig. 7). The apparent waste of butene materializes itself in the fraction of C7+ compounds as well as some minor paraffin formation stemming from the metathesis of isomerised pentenes and hexenes, and from the alkene interconversions and hydrogen transfer reactions, respectively (Table 2). The applied heterogeneous catalyst shows satisfactory metathesis activity at 350 8C, at which temperature the isomer-
isation activity by the acidic centres [11] is already observed but lower than at 400 8C (Table 2). As expected, the C7+ fraction is lower at 350 8C. Some 2-butene is formed which almost completely reacts in the cross-metathesis with 1-butene to propene and 2-pentene. While homogeneous metathesis is able to deliver almost exclusively ethene and 3-hexene at pure 1-butene feed (S = 99 mol%), the WO3/SiO2 system produces considerable amounts of propene and 2-pentene due to its isomerisation activity—even at lower temperature. In our heterogeneous experiment at 350 8C the
Fig. 6. Calculated and experimental conversions at different butene feed ratios.
Fig. 7. Conversion, propene selectivity and yield at different butene feed ratios.
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Table 2 Composition of reaction mixture in 1-butene metathesis over WO3/SiO2 Run
T (8C)
C¼ 2 (mol%)
C¼ 3 (mol%)
1-C¼ 4 (mol%)
2-C¼ 4 (mol%)
C¼ 5 (mol%)
C¼ 6 (mol%)
C7+ (mol%)
C1–6 (mol%)
Conversion (%)
Spropene (mol%)
1 2
350 400
15.1 5.5
16.2 30.1
37.4 9.2
2.6 25.1
13.2 17.7
12.3 7.6
2.5 4.2
0.7 0.6
60.0 65.7
27.0 45.8
selectivity to combined ethene and 3-hexene is 46 mol%, and values of 73 and 89 mol% – the latter using a high purity silica support – were reported elsewhere [12]. The isomerisation activity can be reduced by the introduction of alkali-metal ions which is beyond the scope of this comparison [13]. 4. Conclusion Homogeneous metathesis can provide a product spectrum of ethene, propene, 2-pentene and 3-hexene from mixtures of 1butene and 2-butene with a distribution which is in fair agreement with the one predicted by thermodynamic calculations. Compared to the classic heterogeneous metathesis on a WO3/ SiO2 catalyst, the homogeneous reaction can be carried out at milder conditions and butene losses due to formation of higher products are not observed. Furthermore, the selectivity to propene is higher at the optimum feed ratios (1-butene:2-butene from 40:60 to 50:50) and pure 1-butene reacts to ethene and 3-hexene with a selectivity of 99 mol% versus only 48 mol% for the heterogeneous reaction at 350 8C. Considering the superior reaction control in the described homogeneous metathesis reaction, such a process could compete with heterogeneous metathesis provided that the higher catalyst
costs and the more complex process design can be successfully addressed.
References [1] J.C. Mol, J. Mol. Catal. A: Chem. 213 (2004) 39–45. [2] J.M. Botha, J.M.M. Mbatha, B.S. Nkosi, A. Spamer, J. Swart, WO 00/14038, Sasol Technology (Pty.) Ltd., 2000. [3] R.J. Gartside, M.I. Greene, Q.J. Jones, WO 03/076371 A1, ABB Lummus Global Inc., 2003. [4] P. Schwab, A. Ho¨hn, US 6271430 B2, BASF AG, 2001. [5] G.C. Fu, S.T. Nguyen, R.H. Grubbs, J. Am. Chem. Soc. 115 (1993) 9856–9857. [6] K.A. Burdett, L.D. Harris, P. Margl, B.R. Maughon, T. Mokhtar-Zadeh, P.C. Saucier, E.P. Wasserman, Organometallics 23 (2004) 2027–2047. [7] G.S. Forman, A.E. McConnell, M.J. Hanton, A.M.Z. Slawin, R.P. Tooze, W. Janse van Rensburg, W.H. Meyer, C.L. Dwyer, M.M. Kirk, D.W. Serfontein, Organometallics 23 (2004) 4824–4827. [8] G.S. Forman, A.E. McConnell, R.P. Tooze, C.L. Dwyer, D.W. Serfontein, ZA 03/00087, Sasol Technology, UK, 2003. [9] C.L. Dwyer, M.M. Kirk, W.H. Meyer, W. Janse van Rensburg, G.S. Forman, Organometallics 25 (2006) 3806–3812. [10] C. van Schalkwyk, A. Spamer, D.J. Moodley, T.I. Dube, J. Reynhardt, J.M. Botha, Appl. Catal. A: Gen. 255 (2003) 121–131. [11] A.J. van Roosmalen, J.C. Mol, J. Catal. 78 (1982) 17–23. [12] R.J. Gartside, M.I. Greene, A.M. Khonsari, L.L. Murrell, US 2003/0028063 A1 (2007). [13] A. Spamer, T.I. Dube, D.J. Moodley, C. van Schalkwyk, J.M. Botha, Appl. Catal. A: Gen. 255 (2003) 153–167.