Ind. Eng. Chem. Res. 2005, 44, 9853-9859
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Stability and Kinetic Studies of Supported Ionic Liquid Phase Catalysts for Hydroformylation of Propene Anders Riisager,† Rasmus Fehrmann,† Marco Haumann,‡ Babu S. K. Gorle,‡ and Peter Wasserscheid*,‡ Department of Chemistry and Center for Sustainable and Green Chemistry, Technical University of Denmark, Building 207, DK-2800 Kgs. Lyngby, Denmark, and Lehrstuhl fu¨ r Chemische Reaktionstechnik, Universita¨ t Erlangen-Nu¨ rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany
Supported ionic liquid phase (SILP) catalysts have been studied with regard to their long-term stability in the continuous gas-phase hydroformylation of propene. Kinetic data have been acquired by variation of temperature, pressure, syngas composition, substrate concentration, and residence time. The activation energy was determined to be 63.3 ( 2.1 kJ mol-1, which is in good agreement with known results from biphasic hydroformylation. The results from the kinetic studies confirmed previously published results on the homogeneous nature of the heterogenised Rh-SILP catalyst. Long-term stability exceeded 200 h time on stream with no loss in selectivity. A small decrease in activity could be compensated by a vacuum procedure regaining the initial activity. Introduction The hydroformylation of alkenes, discovered in 1938 by Otto Roelen at Ruhrchemie, Oberhausen, Germany,1 is one of the largest applications of homogeneous transition metal catalysis in industry. Worldwide production capacities exceeded 8 mio t/year in 1995.2 Industrial hydroformylation catalysts are based on cobalt and rhodium complexes exclusively, with the rhodium processes operating at much milder conditions (typically 80 °C, 15 bar) than the cobalt processes (typically 200 °C, 300 bar). The aldehydes formed by the addition of hydrogen and carbon monoxide to the olefinic double bond are important precursors for, e.g., plasticizers and detergent alcohols. Besides the desired linear n-aldehydes, branched iso-aldehydes are obtained in a parallel reaction as depicted in Scheme 1. The selectivity of the catalyst can be influenced by the use of sterically demanding ligands such as sulfoxantphos 1 (see Scheme 1) and is referred to as n:iso ratio or linearity (in percentage). In 1984, a biphasic propene hydroformylation process, introduced by Ruhrchemie/Rhoˆne-Poulenc (RCH/RP), made use of the immiscibility between an aqueous phase containing the water-soluble Rh complex and the organic product phase.3 Simple product separation and quantitative catalyst recovery were achieved in a decantation unit, thus overcoming the drawbacks of homogeneous catalysis. Ever since, a variety of immobilization concepts for hydroformylation catalysts have been studied, ranging from biphasic fluorous phase4 over microemulsions5 to supported catalysis.6 Among the biphasic catalysis concepts the use of ionic liquids has gained significant interest over the past decade both in academia and industry.7 Ionic liquids consist entirely of ions and have no measurable vapor * To whom correspondence should be addressed. Phone: (+49) 9131-8527420. Fax: (+49) 9131-8527421. E-mail:
[email protected]. † Technical University of Denmark. ‡ Universita ¨ t Erlangen-Nu¨rnberg.
pressure which makes them attractive as alternative solvents for homogeneous catalysis. Their polar nature allows the stabilization of ionic transition metal complexes as well as complexes being susceptible to hydrolysis. The use of ionic liquids in catalysis has been reviewed recently.8 However, besides their advantageous properties as solvents, ionic liquids are relatively expensive, even though being commercially available now.9 Furthermore, ionic liquids are normally highly viscous solvents, thus having low diffusion coefficients for hydrogen and carbon monoxide.10 In case of fast chemical reactions this may result in mass transfer limitations, where only the catalyst in the diffusion layer of the biphasic system is utilized. In an ideal system, a thin film of catalyst containing ionic liquid the size of the diffusion layer would make use of all of the catalyst and ionic liquid (see Figure 1). Such a film can be immobilized by physisorption, tethering or covalent anchoring of ionic liquid fragments on a highly porous material, thus creating a large reaction surface. Furthermore, a solid catalyst that can be applied in conventional fixed-bed reactors is preferred from an industrial point of view. Such supported ionic liquid-phase (SILP) catalyst systems have recently been reported by us and other researchers for hydroformylations,11 hydrogenations,12 hydroaminations,13 and Heck reactions.14 The SILP catalytic concept offers a very efficient ionic liquid utilization and provides relatively short diffusion distances of reactants compared to conventional two-phase organic-ionic liquid catalyst systems. In addition, the negligible vapor pressure, large liquid range, and thermal stability of ionic liquids ensure that the solvent is retained on the support in its fluid state at common reaction conditions, e.g., elevated temperatures, which makes SILP catalysts highly suitable for continuous processing. Moreover, the ionic liquid solvent properties are tuneable by the choice of the cation/anion combination, which further leads to advantages of the SILP concept such as, e.g., compatibility with hydrolytically labile catalyst complexes,
10.1021/ie050629g CCC: $30.25 © 2005 American Chemical Society Published on Web 11/11/2005
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Scheme 1. Propene Hydroformylation Using Rh-Sulfoxantphos Catalyst Complex
specific solubility properties, and possible catalyst activation by liquid-catalyst interaction. We have recently demonstrated15 the applicability of selective (n:iso ratios up to 23.7, i.e., 96.0% linearity) unmodified silica gel SILP rhodium catalysts (Rh-SILP) containing the bisphosphine sulfoxantphos 1 (Figure 1) with physisorbed hexafluorophosphate and n-octyl sulfate ionic liquids [BMIM][X] (X ) PF6 or n-C8H17OSO3) for the gas-phase propene hydroformylation in a continuous-flow process. The ionic liquid film proved to be a prerequisite for an active, selective, and stable catalyst. Experiments without the ionic liquid revealed a deactivation of the catalyst within 10 h time on stream, indicated by a loss of both activity and selectivity. The homogeneous nature of the Rh-SILP catalyst was verified by in situ FTIR experiments. A proper thermal pretreatment, reducing the content of surface silanol groups of the support material, has been identified to be crucial for long-term stability of the catalyst. NMR measurements on the Rh-SILP catalyst indicated that more than 50% of the excess ligand was bound to the remaining acidic sites of the support, thus making a higher L:Rh ratio than in biphasic catalysis a prerequisite for obtaining a stable and selective catalyst system. In this work we present further evidence for the homogeneous nature of the Rh-SILP catalyst based on kinetic results. The results strongly support the previous spectroscopic results. Furthermore, the catalyst performance has been studied for more than 200 h time on stream in order to verify the long-term stability of the Rh-SILP catalyst. Experimental Section Catalyst Preparation. All syntheses were carried out using standard Schlenk techniques under prepurified argon. The silica gel 100 (particle size 0.06-0.2 mm, pore volume 0.966 mL g-1, Merck), was calcinated in
air at 450 °C for 24 h and stored under argon prior to use. The ligand sulfoxantphos 1 has been synthesized by sulfonation of xanthene (Aldrich) according to literature procedures.16 Rh(CO)2(acac) (0.0512 g, 0.2 mmol) (Aldrich), was dissolved in 20 mL of dried MeOH and stirred for 10 min. Ligand 1 (1.57 g, 2 mmol) were added, and the orange solution was stirred for another 10 min. Afterward, 1 mL (1.06 g) of ionic liquid [BMIM][n-C8H17O-SO3] (pH ) 8.3, H2O < 500 ppm, Cl- < 100 ppm, Solvent-Innovation) was added to the solution. After being stirred for 30 min, 10 g of calcinated silica were added and the solution was stirred for 60 min. The MeOH was removed in vacuo, and a pale red powder was obtained. The supported ionic liquid-phase catalyst was stored under argon until further use. Kinetic Experiments. The long-term stability and kinetic studies were carried out in a continuous fixed bed reactor setup depicted in Chart 1. The SILP catalyst was filled into the reactor (4), and the complete rig was evacuated at room temperature. The rig was pressurized with 50 bar helium and left under pressure for 30 min while monitoring the pressure. If no pressure drop was observed, the reactor was heated to reaction temperature under helium pressure. The complete setup was evacuated and flushed with helium three times before syngas and propene were fed into the system. Propene (2.8, Linde) was taken out of a reservoir in the liquid state and fed into a heated evaporator (2) via a HPLC pump (1) (Knauer) to control the molar flow of the substrate. Both carbon monoxide (3.5, Linde) and hydrogen (5.0, Linde) flows were adjusted by means of mass-flow controllers (MFC, 5850 S series, Brooks Instruments). The preheated gases were combined with the propene in the mixing unit (3), which was filled with glass beads in order to ensure proper mixing and isothermal conditions. The gas mixture could then either enter the reactor (4) or exit the system via a bypass. The reactor (4) consisted of a stainless steel tube (10 mm diameter, 220 mm length) equipped with a bronze sinter plate for catalyst placement. After the reactor, the gas mixture passed a 7-µm filter in order to avoid Chart 1. Flow Scheme of the Continuous Hydroformylation Reactor Setup
Figure 1. Schematic principle of SILP catalysis.
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decontamination of the tubing with catalyst or solid particles. A back-pressure regulator valve (5) (Samson) was used to maintain the desired reaction pressure and outlet gas flow. After the regulator valve, the gas stream was split and a minor flow was passed through a 134µL sampling loop mounted on a gas chromatograph (HP 5890 II plus). Samples were taken at regular intervals by injecting the volume of the sampling loop via a 6-port valve into the gas chromatograph. Gas Chromatography (GC). The conversion of propene as a function of process conditions was measured using on-line GC technique. A HP 5890 GC equipped with a Pona column (50 m, 0.2 mm diameter, 0.25 µm coating) and a flame ionization detector (FID) precalibrated for propene, n-butanal, and iso-butanal (allowing the peak areas to be transferred into propene conversion) was applied: Injector temperature 150 °C, split ratio 43:1, helium carrier gas flow 2.4 mL min-1, detector temperature 250 °C. To detect possible highboiling byproducts (heavies), the following temperature program was used: initial temperature 50 °C, initial time 5 min, heating ramp of 50 °C min-1, final temperature 150 °C, final time 3 min, cooling ramp 50 °C min-1, final temperature 50 °C. GC mass spectroscopy data have been recorded on a Varian Saturn 2100T equipped with a CP 8410 autosampler, ion trap detector, and Varian factor four capillary column (15 m, 0.25 mm diameter, 0.25 µm coating): Injector temperature 220 °C, split ratio 50:1, helium carrier gas flow 1 mL min-1. The following temperature program was used: initial temperature 50 °C, initial time 5 min, heating ramp of 10 °C min-1, final temperature 220 °C, final time 10 min. Inductively Coupled Plasma (ICP) Analysis. During the long-term stability experiments the exit gas streams from the GC and reactor were combined and collected in a cold trap filled with liquid nitrogen. The liquid samples were subsequently analyzed by ICP. Calculations. The residence time τ was calculated from the total amount of Rh in the catalyst divided by the total molar flow Ftotal of substrates
The turn-over frequency (TOF) was calculated from the molar flow of aldehydes divided by the amount of rhodium
Faldehyde FpropeneX -1 ) [h ] nRhodium nRhodium
The rate of reaction was calculated from the converted propene and the residence time
r)
entry
temperature (°C)
ppropene (bar)
conversion (%)
TOF (h-1)
n-butanal (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
65 65 65 82 82 82 100 100 100 120 120 120 130 130 130 140 140 140
0.9 1.8 3.2 0.9 1.8 3.2 0.9 1.8 3.2 0.9 1.8 3.2 0.9 1.8 3.2 0.9 1.8 3.2
0.86 0.91 0.93 2.13 2.09 2.54 6.54 7.48 9.82 20.5 21.1 21.4 27.6 32.8 28.1 21.0 26.9 28.1
4 8 17 9 19 45 29 67 175 91 188 381 123 292 501 94 240 500
95.9 95.9 95.8 95.7 95.6 95.4 95.2 95.2 95.1 94.1 93.8 93.5 93.4 92.9 90.1 92.5 92.3 87.7
a Standard conditions: 10 bar syngas pressure, H :CO ) 1:1, 2 nRhodium ) 8.17 × 10-5 mol, τ ) 0.9 s.
Figure 2. Differential analysis of the hydroformylation at various propene partial pressures and temperatures. 10 bar total pressure, H2:CO ) 1:1, nrhodium ) 8.17 × 10-5 mol, τ ) 0.9 s.
nRhodium τ) [s] Ftotal
TOF )
Table 1. Rh-SILP Hydroformylation of Propene at Various Substrate Concentrationsa
∆ppropene ppropene,0X bar ) τ τ s
[ ]
Results and Discussion Stability. In a first set of experiments, the Rh-SILP catalyst system has been applied for the hydroformylation of propene at 100 °C and 10 bar syngas pressure (H2:CO 1:1) over a period of 36 h time on stream. TOFs between 71 and 74 h-1 and selectivities for n-butanal of 95% (n:iso ) 19) were obtained in good agreement with previously published data.15 After 36 h time on
stream, no change in activity and selectivity was observed. On the basis of this preliminary result, the kinetic data were acquired within the first 36 h of a freshly installed catalyst sample. Variation of Propene Pressure. The hydroformylation has been performed at propene partial pressures of 0.9, 1.8, and 3.2 bar partial pressure and in the temperature range 65-140 °C with a residence time of 0.9 s. Table 1 compiles the results of these variations. At 65 and 82 °C, the reaction rates observed were low with TOFs in the range of 4-45 h-1 (entries 1 to 6). The highest TOFs were obtained at high propene partial pressure and high temperature (entries 15 and 18). However, the activities at 140 °C were not as high as could be expected assuming a normal Arrhenius temperature dependence. This might be attributed to the formation of high-boiling byproducts, namely, 2-ethylhexanal and 2-ethyl-hexanol (“heavies”), which can dissolve in the ionic liquid layer, thus lowering the effective catalyst concentration (for more details, see later Deactivation-Reactivation Studies). A differential analysis of all data was made by plotting ln(rate) against ln(ppropene). The reaction order n of propene was
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Figure 3. Integral analysis of propene hydroformylation at different residence times and temperatures. 10 bar syngas pressure, H2:CO ) 1:1, nrhodium ) 8.17 × 10-5 mol, ppropene ) 2.1 bar.
Figure 4. Activity of the Rh-SILP catalyst as a function of partial pressure of CO. 100 °C, nrhodium ) 7.98 × 10-5 mol, τ ) 0.8 s, phydrogen ) 1.3 bar, ppropene ) 1.8 bar (balanced with He).
calculated from the slope of the curves and the rate constant k from the intercept. Figure 2 shows the results of the differential analysis. The hydroformylation of propene with the Rh-SILP catalyst under investigation was found to be first order in propene and the activation energy was calculated from an Arrhenius plot (see Figure 6) to be 61.9 kJ mol-1 with a collision factor k0 of 3.7 × 107 s-1. Variation of Residence Time. At a constant propene partial pressure of 2.1 bar, the residence time inside the catalyst bed was varied by altering the total reactant flow. From the results in Table 2, it can be seen that shorter residence times generally resulted in lower conversions, whereas at longer residence times the conversions were higher. Under nondifferential conditions at higher conversions (entries 29-38), the observed TOFs decreased slightly with longer residence time representing the lower mean level of propene present in the reactor. The selectivity for the desired linear aldehyde decreased only slightly from 96.2-93.6% by increasing the residence time and temperature. At temperatures of 120 °C, TOFs between 320 and 470 h-1 could be achieved.
Figure 5. Activity of the Rh-SILP catalyst at different syngas compositions (H2:CO ratios) and total pressures of 10, 15, 20, and 30 bar. 100 °C, nrhodium ) 7.98 × 10-5 mol, τ ) 0.8 s.
Figure 6. Arrhenius plots based on propene pressure and residence time variations.
By assumption of a first-order reaction with respect to propene, integration of the rate expression
r)-
dppropene ) kpn dτ
resulted in
ln(ppropene) - ln(ppropene,0) ) -kτ Plotting ln(ppropene) - ln(ppropene,0) against the residence time τ resulted in a linear dependence as shown in Figure 3. The rate constants were determined from the slopes of the graphs, and the activation energy was found to be 64.8 kJ mol-1 in good agreement with the one obtained from variation of substrate partial pressure (see Figure 6). The collision factor k0 of 7.1 × 107 s-1 was higher than the one determined by substrate pressure variation. Variation of Total Pressure and Syngas Composition. At 100 °C, the partial pressure of CO was varied between 0.7 and 5.2 bar, while the hydrogen and propene partial pressures were kept constant at 1.3 and 1.8 bar, respectively. Helium was used as inert gas to
Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9857 Table 2. Rh-SILP Hydroformylation of Propene at Various Residence Timesa entry
temperature (°C)
residence time (s)
conversion (%)
TOF (h-1)
n-butanal (%)
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
65 65 65 65 65 84 84 84 84 84 100 100 100 100 100 120 120 120 120 120
2.0 1.5 1.2 1.0 0.8 2.0 1.5 1.2 1.0 0.8 2.0 1.5 1.2 1.0 0.8 2.0 1.5 1.2 1.0 0.8
0.72 0.96 1.02 0.82 0.68 4.66 3.67 3.11 2.13 1.75 10.3 8.57 7.31 5.46 5.57 34.3 29.1 25.5 23.1 18.7
7 12 16 16 17 44 46 49 48 44 97 108 115 103 140 324 367 401 437 471
95.3 95.0 95.4 96.2 96.2 95.4 95.5 95.4 95.4 95.7 95.2 95.2 95.1 95.1 95.1 94.5 94.4 93.9 93.8 93.6
a Standard conditions: 10 bar syngas pressure, H :CO ) 1:1, 2 nRhodium ) 8.17 × 10-5 mol, ppropene ) 2.1 bar.
maintain constant volume flow and total pressure of 10 bar. The activity decreased with increasing pCO as depicted in Figure 4, while the selectivity for n-butanal remained unchanged at 95%. The total syngas pressure was increased to 15, 20, and 30 bar with pCO varied in the range between 1.0 and 7.8 bar (ptotal ) 15 bar, pH2 ) 2.0 bar, ppropene ) 1.3 bar), 1.3 and 10.4 bar (ptotal ) 20 bar, pH2 ) 2.6 bar, ppropene ) 1.8 bar), and 2.0 to 15.6 bar (ptotal ) 30 bar, pH2 ) 3.9 bar, ppropene ) 2.7 bar), in all cases balanced with Helium gas. As can be seen from Figure 4, the activity increased with increasing total pressure. From the slopes of the linear fitting a negative order with respect to pCO of -0.4 can be calculated according to
r ) kpCOx (r expressed as TOF) In Figure 5, the activity, expressed as ln(TOF), has been plotted against ln(ptotal). The slope of the linear fitting should give the order m with regard to total syngas pressure according to
r ) kptotalm (r expressed as TOF) For pH2:pCO ratios of 0.25, 0.33, 0.67, and 2 an order m ) 0.4 has been calculated from the fitted graphs in Figure 5. Both the negative order in CO17 and the first order in substrate concentration18 are known from literature and can be derived from Wilkinson’s mechanism for modified rhodium-catalyzed hydroformylation.19 In Figure 6, the ln(k) was plotted against T-1 in an Arrhenius-type diagram based on substrate and residence time variation. The activation energies calculated from the independent experiments are similar and can be accounted to be 63.3 ( 2.1 kJ mol, clearly indicating that the RhSILP catalyst is operating under kinetically controlled reaction conditions. The collision factors k0 determined from the Arrhenius plots range between 2.7 × 107 s-1 (based on residence time) and 9.8 × 107 s-1 (based on propene pressure). In Table 3, the results from this study are compiled and compared with results for
rhodium-catalyzed hydroformylations using sulfonated ligands in aqueous media and supported aqueous media. The activation energy of propene using Rh-SILP catalysts is slightly lower than the one determined by Mao et al. for HRh(CO)(TPPTS)3 in water.20 Compared with HRh(CO)(TPPTS)3 in supported aqueous phase (SAP) catalysis of higher alkenes,22,23 the activation energy for propene hydroformylation is in good agreement. This comparison provides additional evidence that the Rh-SILP catalyst truly acts as a homogeneous catalyst in the ionic film immobilized on the silica. Deactivation-Reactivation Studies. The stability of the Rh-SILP catalyst was initially studied at 100 °C and 10 bar syngas pressure (H2:CO ) 1:1) over a period of 36 h with no loss in activity or selectivity. This time on stream was extended to 180 h to test the longterm stability of the system. Figure 7 shows the results. The selectivity toward n-butanal remained constant around 95% (n:iso ) 19) during the reaction whereas, the TOFs slightly decreased from 74 to 61 h-1, corresponding to a total loss in activity of 17% or 0.1% per hour. Since the selectivity remained unchanged, it was assumed that the catalyst did not decompose, as observed in previous studies when using low L:Rh ratios.11,15 All ICP analyses of the exit gas streams condensed in liquid nitrogen showed rhodium contents below the detection limit of 3 ppm. Instead, the formation of high boiling side products dissolving in the ionic liquid layer is expected to be the cause of the slow deactivation. The effective rhodium concentration would be lowered by the dissolved byproducts. Furthermore, the film thickness might be increased and smaller pores can be flooded, which will lead to a lower reaction surface. To confirm this hypothesis, the gas flow was stopped after 180 h time on stream and the setup was evacuated for 10 min at 100 °C using a vacuum pump. When the experiment was continued after this procedure, the activity had indeed increased by 80% from 60 to 108 h-1. Within the next 20 h, the TOFs decreased again from 108 to 76 h-1 and the selectivity reestablished at 95% n-butanal. A second vacuum period of 10 min resulted in improved TOFs of 116 h-1, as depicted in Figure 8. In both cases, the observed “overshooting” of the activity by evacuation might be caused by simultaneous removal of CO ligand of the Rh complex leading to higher activity. A lower selectivity is expected and is indeed observed for a short while after evacuation. Thereafter, the catalyst solution is again saturated with CO gas, and both the activity and the selectivity are approaching the initial levels. These findings confirm the interpretation that the observed slight deactivation over time is not due to catalyst decomposition. Our previously published results clearly indicated that catalyst deactivation due to ligand degradation is accompanied with loss of selectivity,15 which is not observed in this case. In this context, we expected the formation of 2-ethyl-hexanal and 2-ethylhexanol to be of relevance. At higher temperatures, traces of these high-boiling side products were observed in the GC chromatograms of the gas outlet streams. To further support this interpretation of our results, the deactivated catalyst was removed from the reactor and the pale-yellow catalyst particles were placed in two round-bottom flasks containing (i) cyclohexane and (ii) ethanol. Immediately, the ethanol solution became orange in color, whereas the cyclohexane solution remained colorless. As expected the ethanol wash leads
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Table 3. Comparison of Results for Rh-SILP Hydroformylation with Literature Results
d
substrate
catalyst
EA (kJ mol-1)
solvent
support
ref
Propene Propene 1-octene 1-octene Linaloolb 1-dodecene
HRh(CO)2(1) HRh(CO)(TPPTS)3 HRh(CO)(TPPTS)3 HRh(CO)(TPPTS)3 HRh(CO)(TPPTS)3 HRh(CO)(TPPTS)3
63.2 77.0 65.9 71.0 60.7 72.8/70.9c
ILa water water water water waterd
SiO2
this work 20 21 22 23 24
a IL ) [BMIM][ n-C H O-SO ]. b Linalool ) 3,7-dimethyl-1,6-octadien-3-ol. 8 17 3 Cetyltrimethylammoniumbromide surfactant used.
c
SiO2 SiO2
Two different algorithms were used for data analysis.
(corresponding to the CdO stretching frequency), clearly indicating the presence of aldehydes in the catalyst. Conclusion
Figure 7. Long-term hydroformylation stability experiment. 100 °C, 10 bar syngas pressure (H2:CO ) 1:1), nrhodium ) 3.53 × 10-5 mol, ppropene ) 1.8 bar, τ ) 0.38 s.
Figure 8. Reactivation of Rh-SILP catalyst by consecutive application of vacuum. 100 °C, 10 bar syngas pressure (H2:CO ) 1:1), nrhodium ) 3.53 × 10-5 mol, ppropene ) 1.8 bar, τ ) 0.38 s.
to the complete removal of the ionic catalyst phase from the support, while the cyclohexane wash does not. The ethanol solution was distilled in order to separate nonvolatile ionic liquid fragments and the distillate was analyzed by means of GC-MS. The cyclohexane solution was directly analyzed by GC-MS. In both cases, small amounts of byproducts 1-butanol, 2-ethyl-hexanal and 2-ethyl-hexanol were detected (the GC signals were further confirmed by injection of the respective pure compounds as references). These results indicate that relatively unpolar heavies were washed or extracted from the Rh-SILP catalyst by both ethanol and cyclohexane. Additionally, infrared analysis of both used and unused Rh-SILP catalysts was carried out. The used catalyst showed an absorbance band at 1730 cm-1
In this work, we have demonstrated the long-term stability of a homogeneous rhodium hydroformylation catalyst, which has been immobilized by the use of the new SILP technique. No loss in selectivity for linear butanal was observed during the experiments. The activity decreased by 17% within 180 h time on stream due to formation of high boiling side-products. These heavies could easily be removed from the catalyst by a vacuum procedure, after which the initial activity could be regained. The activation energy of 63.3 ( 2.1 kJ mol-1 added evidence that the catalyst is indeed a homogeneous complex dissolved in an ionic liquid film on a support. Furthermore, the Rh-SILP catalyst performed very similar to a homogeneous catalyst with regard to variation in syngas composition. This work further confirms the high technical potential of the SILP catalysis concept as it allows combining molecular defined homogeneous catalysis with heterogeneous, fixed-bed technology. In perspective, we believe that the knowledge gained in this and previously published work will accelerate significantly the successful development of new SILP catalysts and future SILP catalysis applications (e.g., other C-C couplings, hydrogenations, etc.). The SILP concept is most advantageous for continuous gas-phase reactions, where the combination of well-defined catalyst complexes, nonvolatile ionic liquids, and solid, porous supports can enhance the process economics with respect to product separation and catalyst recovery. If mechanical and chemical removal of the ionic liquid film can be prevented, the SILP technology can also be applied in multiphase slurry reactions. The higher utilization of the catalyst species and the ease of catalyst recycling by filtration will offer advantages compared to classical biphasic systems. We therefore anticipate that the SILP catalysis concept will help bridging the gap between homogeneous and heterogeneous catalysis and will lead to improved catalytic processes in the future. Acknowledgment This work was supported by the framework “Smart ligands - smart solvents” of ConNeCat financed by the German Federal Ministry for Education and Research (BMBF). Financial support (M. Haumann) by the Deutsche Forschungsgemeinschaft and by the Danish Research Council for Technology and Production is gratefully acknowledged. The authors thank Dipl.-Ing. Helmut Gerhard for support regarding the GC analysis, Ber-
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thold Melcher for doing the syngas variation experiments, and Dr. Christine Ernst for support with the catalyst synthesis. Dipl. Chem. Guido Henze (Universita¨t Dortmund, Lehrstuhl fu¨r Technische Chemie A) is acknowledged for the ICP analyses. Nomenclature acac ) Acetylacetonate BMIM ) n-Butyl-methyl-imidazole cation CO ) carbon monoxide EA ) Arrhenius activation energy (kJ mol-1) F ) Molar flow (mol s-1) GC ) gas chromatography GC MS ) gas chromatography coupled with mass spectroscopy H2 ) hydrogen ICP ) Inductively Coupled Plasma IR ) infrared spectroscopy k ) rate constant (for nth order reaction) ((mol L-1)1-n s-1) k0 ) collision factor (for nth order reaction) ((mol L-1)1-n s-1) m ) reaction order with respect to total syngas pressure MFC ) mass-flow controller MeOH ) methanol n ) reaction order with respect to propene n-butanal ) linear butanal n:iso ) linear-to-branched ratio of products NMR ) nuclear magnetic resonance spectroscopy nrhodium ) molar amount of rhodium in reactor (mol) p ) pressure (bar) PF6 ) hexafluorophosphate anion r ) reaction rate (mol L-1 s-1) RCH/RP ) Ruhrchemie/Rhoˆne-Poulenc (now part of European Oxo and Oxeno) Rh-SILP ) Rhodium sulfoxantphos immobilized by SILP SILP ) supported ionic liquid phase τ ) residence time (s) T ) temperature (°C) TOF ) turn-over frequency (h-1) x ) reaction order with respect to partial pressure of CO
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Received for review May 30, 2005 Revised manuscript received August 19, 2005 Accepted September 27, 2005 IE050629G
