Ind. Eng. Chem. Res. 2007, 46, 8629-8637
8629
Biphasic Hydroformylation of 1,4-Diacetoxy-2-butene: A Kinetic Study Rashmi Chansarkar,† Ashutosh A. Kelkar,*,† and Raghunath V. Chaudhari*,‡ National Chemical Laboratory, Homogeneous Catalysis DiVision, Pune, India-411008, India, and Department of Chemical and Petroleum Engineering, Center for EnVironmentally Beneficial Catalysis, UniVersity of Kansas, Lawrence, Kansas 66047
Hydroformylation of 1,4-diacetoxy-2-butene was studied using a water-soluble Rh complex catalyst prepared in situ from [Rh(COD)Cl]2 complex and trisodium salt of tri-(m-sulfophenyl)phosphine (TPPTS) in a biphasic system. The sequence of addition of catalyst precursor, ligand, and reactant/solvent showed a significant effect on leaching of Rh from aqueous to organic phase, and hence, the procedure was optimized to develop a nonleaching and stable biphasic catalyst system. The only hydroformylation product (1,4-diacetoxy-2-formyl butane, DAFB) formed was found to deacetoxylate completely to 2-formyl-4-acetoxybutene (FAB), thus allowing a one-pot synthesis of FAB, an important intermediate for Vitamin A. Experimental data on the concentration-time and CO/H2 consumption-time profiles were obtained, and the effects of DAB concentration, CO partial pressure, H2 partial pressure, and catalyst concentration were studied in a stirred batch reactor over a temperature range of 338-358 K. The effect of aqueous phase holdup on the initial rate of hydroformylation and analysis of gas-liquid and liquid-liquid mass transfer effects were also investigated to identify the reaction rate data operating in a kinetic regime. A rate equation based on the known hydroformylation reaction mechanism was used to fit the experimental rate data and to evaluate kinetic parameters. The agreement between the model prediction and the experimental data was found to be excellent. The activation energy was calculated as 30.1 kJ/mol. The biphasic catalyst system reported here is not only efficient for catalyst-product separation but also provides a tandem synthesis of Vitamin A intermediate, FAB. 1. Introduction Hydroformylation of linear olefins has advanced considerably in the last few decades with the development of novel catalysts and ligands as well as a better understanding of the reaction kinetics and mechanism.1 Aldehydes obtained by the hydroformylation of substituted olefins are versatile chemical intermediates that can be easily converted to a variety of industrially important products with applications in fine chemicals and pharmaceuticals.2,3 One such important example is the hydroformylation of 1,4-diacetoxy-2-butene (DAB) to 1,4-diacetoxy2-formyl butane (DAFB),4 which is a key intermediate in Vitamin A acetate synthesis. The DAB hydroformylation has been described in patents separately by Fitton et al.4 and Himmele et al.,5 using homogeneous phosphine modified rhodium catalysts under high-pressure conditions (13.6 MPa). DAFB was recovered by high vacuum distillation and was reported to be thermally unstable. In our earlier work,6 we reported a systematic study on the product identification, selectivity behavior, and intrinsic kinetics of hydroformylation of DAB, using a homogeneous HRh(CO)(PPh3)3 catalyst under comparatively lower pressure conditions (6.8 MPa). Because the products of DAB hydroformylation (1,4-diacetoxy-2-formyl butane (DAFB) and 2-formyl-4-acetoxybutene (FAB)) are nonvolatile and thermally unstable, their separation from the catalyst poses a serious challenge. Therefore, it is important to develop a heterogeneous catalyst system to enable better catalyst-product separation. Biphasic hydroformylation of propylene provides a successful example of the commercial application of biphasic catalysis.7 This has led to the detailed * To whom correspondence should be addressed. Tel.: +91-2025902544. Fax: +91-20-25902621. E-mail:
[email protected] (A.A.K);
[email protected] (R.V.C). † National Chemical Laboratory. ‡ University of Kansas.
investigation of kinetics and catalysis of hydroformylation of a variety of olefins using water-soluble catalysts. Recently, Unveren and Schomacker8 reported use of a microemulsion as a reaction medium for the hydroformylation of different alkenes including DAB using surfactants in the presence of [Rh(acac)(CO)2]/tri-(m-sulfophenyl)phosphine (TPPTS) as a catalyst system. However, conversion of DAB (31.1%) as well as selectivity to FAB (59%) reported were very low. In this context, there is a need to develop a biphasic system with higher activity and selectivity for DAB hydroformylation. The biphasic hydroformylation of DAB represents a gas-liquid-liquid catalytic reaction, and hence, the overall performance of such a reaction would depend on reaction kinetics as well as interphase mass transfer and complex hydrodynamics associated with liquidliquid dispersion. A detailed study on the kinetics of biphasic hydroformylation of DAB would, therefore, be useful to investigate as a first step in understanding the optimum performance of the biphasic catalyst system. In this paper, hydroformylation of DAB to FAB, with a focus on catalysis and reaction kinetics, has been addressed. The stability of the biphasic catalyst and the role of sequence of addition of catalyst precursor, solvent, and ligand have also been studied, and optimum conditions for a nonleaching formulation have been determined. The experimental study was carried out using water-soluble [Rh(COD)Cl]2/TPPTS catalyst in which the effects of aqueous catalyst phase holdup, agitation speed, partial pressures of CO and H2, DAB concentration, and catalyst concentration were investigated over a temperature range of 338-358 K. These data were analyzed for the significance of gas-liquid and liquid-liquid mass transfer limitations. On the basis of the data observed in the kinetic regime, a rate equation has been proposed for this industrially important reaction system. The results would be valuable in arriving at optimum performance of the catalyst as well as the process.
10.1021/ie0700821 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
8630
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007
2. Experimental Section 2.1. Materials. Rhodium trichloride (RhCl3·3H2O) (Aldrich Chemicals, U.S.A.), triphenylphosphine (PPh3) (Loba, India), butenediol (Merck, India), and acetic anhydride (Merck, India) were used as received without further purification. Sulfuric acid, dimethyl formamide, acetyl acetonate, and sodium hydroxide (SD Fine Chemicals, India) were used as received. Oleum of 65% (w/w of SO3 in H2SO4) strength was prepared. For the synthesis of TPPTS, a procedure standardized by Bhanage et al.9 was used. [Rh(COD)Cl]2 was prepared according to a procedure reported by Chatt and Venanzi.10 DAB was prepared according to a procedure described by Brevet and Mori.11 Distilled, degassed water was used in all operations. Solvents, toluene, water, ethanol, cyclohexane, diethylether, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) were freshly distilled and degassed prior to use. Hydrogen and nitrogen supplied by Indian Oxygen Ltd., Bombay, and carbon monoxide (>99.8% pure) from Matheson Gas Co., U.S.A., were used directly from cylinders. The syngas with 1:1 ratio of H2/ CO was prepared by mixing H2 and CO in a reservoir. All operations were performed under argon atmosphere. 2.2. Instrumentation. 1H NMR and 31P NMR spectra were obtained on a Bruker AC-200 or MSL 300 spectrometer in CDCl3 at room temperature. Fourier transform infrared (FTIR) spectra were recorded on the Bio-Rad spectrophotometer. Liquid samples were analyzed using a Hewlett-Packard 6890 Series GC controlled by the HP Chemstation software and equipped with an autosampler unit, by using an HP-5 capillary column (30 m × 320 µm × 0.25 µm film thickness with a stationary phase of 5% phenylmethylsiloxane). The quantitative analysis was carried out by external standard method by constructing a calibration table for reactants and products in the range of concentrations studied. Since, in biphasic hydroformylation of DAB, two phases are involved (aqueous and organic phases), the two phases were separated and then analyzed separately. 2.3. Experimental Setup. All the hydroformylation experiments were carried out in a 50 mL (50 × 10-6 m3) microreactor, made of stainless steel and supplied by Parr Instruments Company, U.S.A. The reactor was provided with arrangements for sampling of liquid and gaseous contents, automatic temperature control, and variable agitation speed. The reactor was designed for a working pressure of 21 MPa and a temperature up to 523 K. The consumption of CO and H2 at a constant pressure was monitored by observation of the pressure drop in the gas reservoir as a function of time. 2.4. Experimental Procedure for Kinetic Study. In a typical experiment, a solution of [Rh(COD)Cl]2 (1.622 × 10-3 kmol/ m3) in toluene (15 × 10-6 m3) was added to a solution of TPPTS (16.22 × 10-3 kmol/m3) in water (10 × 10-6 m3). The rhodium complex in toluene was extracted with TPPTS into the aqueous phase, and the organic phase (toluene phase) was discarded (after ensuring that the entire rhodium complex had been extracted as indicated by decolorization of the organic phase). This aqueous catalyst solution containing Rh-TPPTS complex was charged into the reactor along with DAB (0.116 kmol/m3) in toluene (15 × 10-6 m3), which comprises the organic phase for the reaction. The contents were flushed with nitrogen and then with a mixture of CO and H2. Heating was started to attain a desired temperature, and then a mixture of CO and H2 (in a required ratio, 1:1) was introduced into the autoclave up to the desired pressure (6.8 MPa). A sample of the liquid mixture was withdrawn, and the reaction was started by switching the stirrer on. The reaction was then continued at a constant pressure of
Figure 1. Solubilities of H2 and CO in water (extrapolated values, 338358 K; literature values, 363, 373, and 383 K12,13).
Figure 2. Solubilities of H2 and CO in toluene (comparison of the experimental and literature16 values).
CO + H2 (1:1) by a supply of syngas from the reservoir vessel through a constant-pressure regulator. Since, in this study, the major product formed was an aldehyde, supply of CO + H2 in a ratio of 1:1 (as per stoichiometry) was adequate to maintain a constant composition of H2 and CO in the reactor as introduced in the beginning. This was confirmed in a few cases by analysis of CO content in the gas phase at the end of reaction. 3. Results and Discussion The objective of this work was to study the kinetics of hydroformylation of DAB using water-soluble [Rh(COD)Cl]2/ TPPTS complex catalyst. The effect of aqueous-phase holdup on the initial rate and gas-liquid and liquid-liquid mass transfer was investigated to ensure that the reaction was in the kinetic regime. The effects of catalyst concentration in the aqueous phase, hydrogen and carbon monoxide partial pressures, and DAB concentration on the initial rate of reaction were studied in a temperature range of 338-358 K. For the kinetic study, the solubilities of H2 and CO in the organic and aqueous phases were evaluated. The results are discussed in the following sections. 3.1. Solubility Data. Because a knowledge of the solubility of H2 and CO in the organic and aqueous phases is required for the kinetic study, the solubility data for the H2-water and CO-water systems (at 338-358 K temperature range) were estimated by extrapolation of the previously reported solubility data by Chaudhari and co-workers,12 Purwanto and Delmas,13 Wiebe and Gaddy,14 and Lange’s Handbook of Chemistry.15 The
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8631 Scheme 1 . Reaction Scheme for Hydroformylation of DAB
Table 1. Liquid-Liquid Equilibrium Data for DAB-Water-Toluene System
Scheme 2 . Effect of the Sequence of Addition on Leaching of Rh Complex
ratio of aqueous temperature DAB in organic DAB in aqueous to organic phase (K) phase (kmol/m3) phase (×103 kmol/m3) concs., MD × 103 338 348 358
0.116 0.116 0.116
1.02 1.08 1.10
8.8 9.3 9.5
solubility data obtained from the literature (363-383 K) and the extrapolated values at different temperatures (338-358 K) are shown in Figure 1. The solubilities of CO and H2 in toluene were determined experimentally at 338-358 K and compared with the literature16 (extrapolated) values (see Figure 2). It was found that the experimental values compare very well with the literature data. Also, it was observed that DAB concentration (up to 10% in toluene) had no effect on the solubilities of CO and H2 in toluene. The experimental data for liquid-liquid equilibrium for DAB in the toluene-water mixture are presented in Table 1. It was observed that temperature had only a marginal effect on the solubility of DAB in the aqueous phase. The Henry’s constants for hydrogen (HA) and carbon monoxide (HB) and the partition coefficients for DAB (given in Table 1) were used to evaluate concentrations of H2, CO, and DAB in the aqueous phase required for rate analysis. 3.2. Product Distribution and Selectivity. Several experiments on hydroformylation of DAB using water-soluble [Rh(COD)Cl]2-TPPTS catalyst in a two-phase system were carried out to assess the stability of biphasic catalytic system, material balance, and product distribution. The stoichiometric reaction is shown in Scheme 1. The results (concentration-time profile) of experiments carried out at three different temperatures (338-358 K) and 6.8 MPa total pressure of CO/H2 (1:1) are shown in Figure 3. These results represent concentrations in the organic phase. It was observed that the concentrations of DAB and FAB were negligibly small over the entire batch run in the aqueous phase. This indicates a complete extraction of the products in the organic phase after reaction in the aqueous phase. The analysis in the organic phase accounted for >95% of the material balance of DAB and FAB. It was observed that almost complete conversion of DAB was achieved in 60 min at 348 K for a typical case, and the material balance of CO or H2 and DAB consumed was in good agreement with the total amount of products (DAFB and FAB) formed. In the range of conditions investigated, the only product formed was the deacetoxylated aldehyde, FAB, since the major hydroformylation product, DAFB, was quantitatively converted to FAB by deacetoxylation in the range of conditions studied. No other side products were formed during the hydroformylation reaction. This is in contrast to the homogeneous HRh(CO)(PPh3)3 catalyzed hydroformylation of DAB,6 in which both DAFB (∼30%) and FAB (∼70%) are formed as the hydroformylation products.6 Probably the rate
Table 2. Effect of Solvents on the Activity and Selectivity of Biphasic Hydroformylation of DAB s.no.
solvent
1 2 3 4
toluene hexane cyclohexane ethanol
conv. (%) selectivity to FAB (%) Rh leaching (%) 99.9 99.9 99.9 20
100 100 100 38
11.6 Hz, the rate of reaction does not change. This observation indicates that the mass transfer effects are not important above 11.6 Hz. Figure 6 shows the effect of aqueous catalyst phase holdup () on the initial rate of hydroformylation of DAB at various agitation speeds. It was observed that the phase inversion takes place at an aqueous-phase holdup of about 0.6. For aqueous phase holdup < 0.6, the aqueous phase is the dispersed phase, as shown schematically in Figure 7a. In this case, the liquidliquid interfacial area is determined by aqueous-phase holdup (). For aqueous-phase holdup > 0.6, the organic phase is the dispersed phase, as shown schematically in Figure 7b. In this case, the liquid-liquid interfacial area will be determined by the organic-phase holdup (1 - ).
At an agitation speed of 18.3 Hz, a plot of initial rate vs aqueous (catalyst) phase holdup (Figure 6) shows that the rate first increases with the increase in aqueous-phase holdup and then passes through a maximum. In the kinetic regime, the rate per unit volume of the aqueous phase is expected to remain constant with linear dependence on aqueous-phase holdup. However, in the case where the reaction occurs essentially at the liquid-liquid interface, it would depend on the liquid-liquid interfacial area, which is governed by both agitation speed and holdup of the dispersed phase. The results at an aqueous-phase holdup < 0.5 indicate a kinetic regime. For higher aqueousphase holdup, the decreasing rate is a result of phase inversion with organic phase dispersed in continuous aqueous phase. In order to understand this effect, a more detailed analysis of mass transfer in biphasic hydroformylation is necessary. For the purpose of kinetic studies, the data below an aqueous-phase holdup of 0.5 (at 0.4) were used, wherein the kinetic regime prevails. 3.7. Gas-Liquid Mass Transfer Effect. The significance of gas-liquid mass transfer resistance was analyzed by comparing the initial rate of reaction and the maximum possible rate of gas-liquid mass transfer. The gas-liquid mass transfer resistance is negligible if a factor R1 defined as follows is
