Ind. Eng. Chem. Res. 2001, 40, 5317-5325
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Evaluation of Catalyst Support Effects during Rhodium-Catalyzed Hydroformylation in Supercritical CO2 Greg Snyder, Andrew Tadd, and Martin A. Abraham* Department of Chemical Engineering, University of Toledo, Toledo, Ohio 43606
Hydroformylation chemistry is commercially practiced for the production of aldehyde compounds, which are used as precursors to surfactants and plasticizers. Current technology using the aqueous-phase process is limited by the low solubility of the olefin in water. New techniques using supercritical fluid solvents are in development but rely on the modification of a homogeneous catalyst to increase its solubility in supercritical carbon dioxide (scCO2). An alternative approach for the use of scCO2 is to use the solvent only as a means of bringing all of the reactants into a single fluid phase, combined with a heterogeneous catalyst. The current paper reports on the development of a heterogeneous catalyst for hydroformylation in scCO2, wherein the solid catalyst has been specifically designed to take advantage of the unique properties of this benign solvent. We demonstrate that catalyst tailoring can be achieved by promoting specific fluid-solid interactions that impact the rate of the reaction. This development may allow us to develop new heterogeneous catalysts that further target the fluid-solid interactions to control issues of selectivity and leaching that have been problematical in the development of commercially viable heterogeneous catalysts for the hydroformylation reaction. Introduction Hydroformylation, or “oxo synthesis”, was first discovered by Otto Roelen in 1938. In this reaction aldehydes are produced from olefins, carbon monoxide, and hydrogen, with isomeric aldehydes forming for C3 and higher olefins. Roelen observed the formation of propi-
onaldehyde when ethylene was reacted with CO and H2 using a cobalt-thorium catalyst at elevated pressures and temperatures. Currently, nominal production capacities for hydroformylation are more than 7 million tons/year.1 Butanal accounts for about 73% of aldehyde production; about 75% of the n-butanal is converted into 2-ethylhexanol, which is converted to phthalate ester (a plasticizer) for poly(vinyl chloride) production. All commercial hydroformylation processes in operation today use a homogeneous catalyst. The catalyst is commonly dissolved in an organic solvent, such as toluene or an alkane, or sometimes it is dissolved in the reaction products themselves, usually the higher condensation products. Cobalt and rhodium are the metals used almost exclusively, although the latest hydroformylation processes are generally based on rhodium. The addition of phosphine ligands to the homogeneous catalyst provides an increase in reactivity, stability, and selectivity. Homogeneous catalysts are used in hydroformylation because they provide better activity and selectivity when compared to heterogeneous catalysts. However, homogeneous catalysts are typically soluble transition-metal salts or complexes that are dissolved in a suitable organic solvent that must be eliminated as a waste from the process. * Correspondingauthor.E-mail:
[email protected].
The Pollution Prevention Act of 1990 states that the option of first choice is to prevent the formation of wastes at the source.2 The development of a watersoluble catalyst by Rhone-Poulenc has led to a more efficient and environmentally benign process3 and improved catalyst recovery. However, the reactor contains both an organic and an aqueous liquid phase, with the butanals dissolved in the organic phase and the catalyst dissolved in the aqueous phase. In addition, the solubility of the olefin becomes increasingly small at higher molecular weights, until insufficient olefin can be dissolved into the aqueous phase to make this process commercially viable. In addition, the multiphase nature of the reaction system leads to inherent difficulties due to mass transfer and/or solubility of the reactants and the products in the catalyst-containing liquid phases. Several researchers have attempted to develop a heterogeneous catalyst for vapor-phase hydroformylation. For example, Rode et al.4 studied rhodium supported on zeolites for use in vapor-phase propylene hydroformylation and achieved a regioselectivity (naldehyde/isoaldehyde) of about 2. Many investigators have used silica as the support, to capitalize on its acidity. Huang et al.5 impregnated SiO2 with solutions of RhCl3‚nH2O, Co(NO3)2‚6H2O, Co2(CO)8, Rh4(CO)12, or RhCo3(CO)12 and found that rhodium catalysts gave the highest activities (approximately 3.3 mol of propanal/ mol of catalyst/min) and selectivity toward oxygenates. Chuang and co-workers6-8 and Brundage et al.9 have used in situ IR spectroscopy and determined that linearly adsorbed CO on Rh+ sites is more active than linearly adsorbed CO on Rh0 sites. Fukuoka et al.10 used various bimetallic catalysts for vapor-phase ethylene hydroformylation and CO hydrogenation. Reaction rates were improved by factors of 62-110 on catalysts derived from rhodium-iron clusters with the metal composition of FeRh5, FeRh4, and Fe3Rh2 as compared to Fe-free Rh4/SiO2. Polymer-supported rhodium catalysts have
10.1021/ie0010722 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/19/2001
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also been used11 but were observed to have significant rhodium leaching during hydroformylation of 1-octene. As an intermediate to the liquid- and vapor-phase reactions, it is possible to conduct the reaction in a supercritical fluid (SCF), which possesses properties between those of a liquid and a gas. The densities of a SCF are high enough for the fluid to have substantial dissolution strength while the diffusivities of solutes in SCFs are higher than those in liquids and the viscosities are lower, enhancing mass transfer. Compounds that are insoluble in a fluid at ambient conditions can become soluble at supercritical conditions. Also, SCFs have nearly zero surface tension, which allows penetration into micropores of solid media and improves the masstransfer capability.12 SCFs have also exhibited tunable solvent properties that affect the selectivity of the reactions.13 In their pioneering work, Rathke et al.14 demonstrated propylene hydroformylation in scCO2 at 80 °C, a fluid density of 0.5 g/mL, and a Co2(CO)8 catalyst with a regioselectivity of 88%; aldehydes were the only products detected. In their examination of pressure effects during propylene hydroformylation, Guo and Akgerman15 observed only the linear and branched butanals and an increase in the rate constant and regioselectivity with increasing pressure at constant temperature. Attempting to increase the solubility of the catalyst, Palo and Erkey16 modified the rhodium catalyst with fluorinated arylphosphine ligands and demonstrated hydroformylation of 1-octene in scCO2 with a regioselectivity of approximately 2.4. Koch and Leitner17 demonstrated that a range of ligands and ligand modifiers were also effective for hydroformylation, although the reaction also proceeded in scCO2 with unmodified rhodium catalysts. In our previous work,18 we demonstrated the effectiveness of rhodium supported on activated carbon for the hydroformylation of propylene. However, the yield of aldehyde was substantially below the propylene conversion, leading us to conclude that aldehyde product remained irreversibly adsorbed in the activated carbon support. We therefore wanted to consider an alternative support with lower surface area, to minimize the adsorption of the products. Moreover, we attempted to modify the surface of the support to enhance the product desorption. Within this paper, we describe the results of these experiments and discuss the effectiveness of surface modification on the heterogeneously catalyzed hydroformylation reaction performed in scCO2. Experimental Section Catalytic runs were performed in a batch reactor to determine the effect of catalyst support on the reaction rate and product selectivity. The experimental system and procedures are described next. Additional experiments were performed using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) as a means of evaluating the effect of the support on individual steps within the mechanism. Catalysts were prepared using the procedures described below. Reactor System. The reactor used was an Autoclave Engineers batch reactor, consisting of a cap and body assembly and a magnetically driven shaft with an impeller fitted at the bottom of the shaft, as shown in Figure 1. The reactor volume was 282 cm3, with a maximum working pressure of 372 bar. A pressure transducer (Sensotech Inc., model THE-743-11) mea-
Figure 1. Schematic diagram of the experimental batch reactor and the on-line analysis system.
sured pressure inside the reactor. A temperature controller (LFE Instruments, model 1410), that supplies power to a heating jacket surrounding the body section of the reactor, maintained the temperature of the reactor. Removal of heat from the reactor was accomplished by use of a cooling coil extending out from the reactor cap. Room temperature water was pumped through the cooling coil using a peristaltic pump (ColeParmer Masterflex). A rupture disk was attached to the reactor with a burst pressure of 340 bar. Use of a heterogeneous catalyst was accomplished by use of a basket assembly, which was fitted onto the stirring shaft. The catalyst basket consisted of a stainless steel cage with an inner lining of stainless steel mesh held in place by two stainless steel disks, which are held in place using setscrews. Both disks have paddles that protrude into the basket to provide good agitation of the reaction medium while ensuring that there is no slippage of the catalyst inside the basket assembly. Two different sections of 1/4 in. stainless steel tubing were used to feed the reactants into the reactor. One line was used to fill both hydrogen (AGA Gas, Inc., 99.5%) and carbon monoxide (AGA Gas, Inc., 99.95%). One valve was opened while the opposite was closed (Parker Valve) to allow flow of one gas into the reactor, filling it to a predetermined partial pressure, monitored inside the reactor, at the reactor temperature. The valve positions were reversed, and the procedure was repeated with the other gas. The Peng-Robinson equation of state was used to convert the partial pressures of each reactant into the molar amounts loaded into the reactor. Carbon dioxide and propylene were purchased as compressed liquids (AGA Gas, Inc., 99.99% and 99.0%, respectively) and pumped into a 150 mL weighing vessel (Whitey) using a liquid pump (LDC Analytical, model 796). The fluids were maintained as liquids by immersing the tubing leading up to the pump, the pump heads, and some of the tubing after the pump in an ice bath. A three-way needle valve (HIP) was switched to allow the vessel to fill without fluid flow into the reactor. The vessel was then closed off with a needle valve, disconnected, weighed, and reinserted. The three-way valve was then switched to allow flow into the reactor. Finally, the vessel was again closed off, disconnected, and weighed. The loss of weight of the fluid was equivalent to the amount added to the reactor and was converted to molar quantities using the molecular weight of each component. For the reactions reported here, the starting concentrations were 0.53 mol/L propylene, 0.71 mol/L H2 and CO, and 4.96 mol/L CO2 as the solvent.
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5319
Figure 2. Experimental system showing details of the diffuse reflectance chamber for surface analysis of the catalyst under high pressure.
The reactor contents were discharged through a reactor outlet that leads to a three-way valve. One direction emptied the contents into a fume hood, while the other sent the sample through capillary tubing (1/16 in. o.d. × 0.005 in. i.d., Valco Instruments) for flow reduction to a six-way valve (Valco Instruments) for sampling. The reactor outlet tubing and the capillary tubing were heated to an approximate temperature of 100 °C by use of a heating tape (Cole-Parmer). The sixway valve was set in an oven that was maintained at 150 °C. The sample was then sent from the six-way valve to a gas chromatograph (GC; HP-5890) for analysis. The GC was equipped with a thermal conductivity detector and a 6 ft × 0.85 in. i.d. Porapak T (80/100 Alltech) packed column. Helium was used as the carrier gas at a flow rate of 25 mL/min. Prior to any experiments, the retention times for the reactants and products were determined by analyzing known amounts of each species, respectively. To evaluate experimental results, it is important that the phase behavior of the reaction system is known. On the basis of the initial conditions, phase behavior estimates according to the Peng-Robinson equation of state indicated that the contents of the reactor remained in the supercritical state throughout each experiment. Experimental confirmation of a single phase for reaction conditions was through experiments in the unstirred autoclave. At reaction conditions with no agitation, samples were taken from the top and bottom of the reactor and compared via GC. No differences were observed. Had a phase separation occurred, density differences in the two phases would have resulted in a concentration difference that would have been observed on the GC. DRIFTS Experiments. Adsorption of reactants and products was studied using a high-pressure, hightemperature DRIFTS cell manufactured by SpectraTech (Figure 2), mounted in a Nicolet Impact 400D FTIR spectrophotometer. The DRIFTS cell was equipped with ZnSe windows and was capable of operation to 100 bar and 400 °C. Approximately 25 mg of catalyst powder was loaded into the DRIFTS sample cup. The cup was equipped with a K-type thermocouple and an electrical resistance heater controlled with a Dynapar T506 PID controller. The inlets and outlets to the cell were configured such that the incoming gas was forced to pass through the catalyst to exit the cell. To investigate single-component adsorptions, the catalyst was first heated to 100 °C under flowing nitrogen or helium so that a background spectrum could be measured. This spectrum was used as the baseline for subsequent spectra, eliminating the need to perform subtractions between spectra. To investigate the adsorption behavior of CO in the presence of scCO2, the catalyst was preheated to 100 °C under flowing nitrogen
or helium. The gases were added by mass to a 150 mL cylinder, heated to 100 °C until a pressure of 100 bar was obtained, and admitted to the cell. After 70 min, the vapor phase was swept at constant pressure by nitrogen or helium to allow collection of the spectrum of the adsorbed species. Reactions were conducted in the DRIFTS cell by modification of the procedure outlined above for adsorption studies. The reactant gases were mixed in a 150 mL cylinder and heated to 100 °C. They were then passed into the cell to contact the preheated catalyst sample. For the reactions, spectra were collected at 100 bar without removal of the vapor phase. At the end of the reaction, the vapor phase was swept out at constant pressure by nitrogen or helium, and a spectrum was collected to look for any adsorbed species. Catalyst Preparation. The supports used in this research were made using soluble gel techniques. The procedure for each support is identical except for the amount and types of each starting material and is described schematically by Figure 3. For the SiO2 support, 75 mL of a mixture of 95% ethanol and 5% water was placed into a 250 mL Erlenmeyer flask and 0.120 mol of tetraethoxysilane was added; for the hydrophobic support [SiO2 with 10% CH3(CH2)9SiO2], 70 mL of a mixture of 95% ethanol and 5% water was placed into a 250 mL Erlenmeyer flask and 0.108 mol of tetraethoxysilane and 0.016 mol of decyltriethoxysilane were added; and for the CO2-philic support [SiO2 with 10% CF3(CF2)7(CH2)2SiO2], 40 mL of a mixture of 95% ethanol and 5% water was placed into a 250 mL Erlenmeyer flask and 0.0432 mol of tetraethoxysilane and 0.0048 mol of (heptadecafluoro1,1,2,2-tetrahydrodecyl)triethoxysilane were added. Next, the pH of the solution was adjusted to about 5.5 with acetic acid. The solution was then stirred for 2 days and heated at approximately 60 °C for about 8 h to evaporate the ethanol. As the solvent was boiled off, a solid gel formed and further dried at the current conditions. Last, the solid was dried overnight in an oven at 120 °C. Catalysts were prepared using an incipient wetness impregnation technique and the supports prepared locally, plus a commercially available silica gel (Aldrich Chemical). Before impregnation, the supports were reduced to a particle size of 40-60 mesh (0.25-0.42 mm) by crushing the supports. Solutions were made by dissolving the metal precursors in an appropriate solvent. Rhodium acetylacetonate [Rh(acac)3] and iron acetylacetonate [Fe(acac)3], purchased from Strem Chemicals, were dissolved in acetone (Aldrich Chemical). The solution was added dropwise, sufficient to wet the surface, to a previously weighed amount of support. The mixture was then allowed to dry in air at room temperature, thus precipitating the metal precursor. This process was repeated until all of the metal solution was placed onto the desired quantity of support material. The concentrations of the solutions and the amount of support were calculated beforehand in order to provide approximately 3 wt % rhodium on the catalyst after reduction. The dried catalyst matrix was then reduced under flowing hydrogen at 150 °C for 10-16 h in a stainless steel tube. The reduction temperature was chosen to be below the melting point for the Rh(acac)3 and Fe(acac)3 used in the impregnation step. The reactor loading and metal content for the catalysts used in these experiments are listed in Table 1.
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Figure 3. Description of the method used to produce the catalysts used in this study. Table 1. Metal Content and Loading of Catalysts catalyst no. 1 2 3 4
5 6 7 8
support type
catalyst load (g)
percent metal (g/g of catalyst) unreduced reduced used
Catalysts Made with Rh and Fe silica gel 2.034 rhodium 1.84 iron 1.75 SiO2 with 10% 2.003 rhodium 2.94 CH3(CH2)9SiO2 iron 1.70 SiO2 with 10% 1.981 rhodium NA CF3(CF2)9 SiO2 iron NA SiO2 2.046 rhodium 2.31 iron 1.42 Catalysts Made with Rh Only silica gel 1.237 rhodium 2.91 SiO2 with 10% 1.228 rhodium 3.35 CH3(CH2)9 SiO2 SiO2 with 10% 2.053 rhodium 3.00 CF3(CF2)9 SiO2 SiO2 1.986 rhodium 2.4
3.16 2.2 2.79
1.88 0.58 0.19
1.52 2.57
0.67 0.10
0.93 3.34 0.76
0.60 0.02 0.33
0.44 0.07
0.61 0.10
0.05
0.14
0.61
0.07
The catalyst surface area was measured by the Brunauer-Emmett-Teller method using N2 as the adsorbate. Catalysts made with silica gel had a surface area of approximately 300 m2/g, while catalysts made from nonporous silica had a surface area of less than 1 m2/g. Metal impregnation on the nonporous supports was slow and required numerous wetting steps, indicating that minimal adsorption occurred. The amounts of rhodium and iron on each of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a PerkinElmer Plasma 1000/2000 emission spectrometer. A catalyst sample was placed in appropriate glassware, and the metal was allowed to dissolve from the support for about 1 day using aqua regia. The amounts of sample and solvent were both recorded. Known concentrations of standards of each metal were analyzed and used for calibration. Computations. The reaction rates (r) were calculated based on the mass of rhodium present on the catalyst after reduction and the change in the propylene concentration as a function of time. The concentration of propylene was plotted against time for that sample. A second-order polynomial was fitted to the data using EXCEL and then was differentiated at each sample point for each experiment, following the procedure
outlined by Fogler (p 37).19 Thus, the reaction rate at any time was obtained as
(
)
dCprop dt r)mRh
(1)
where mRh ) mass of rhodium ) % rhodium in the reduced catalyst × mass of catalyst used. The rates were compared at the initial time point for each reaction, because the catalysts rapidly deactivated. To quantitatively measure the role of the support, a mathematical model was needed to correlate the reaction rates for all catalysts. We have chosen to assume first-order decomposition
-rP ) kCP
(2)
where rP is the reaction rate based on the disappearance of propylene [mol/(h gcat)], CP is the concentration of propylene in the reactor (mol/L), and k is the first-order rate constant [(h gcat)-1]. Although concentrations of H2 and CO were not measured during the reaction, the concentration of these species can be assumed to be approximately constant because low conversion was achieved and H2 and CO were present in excess. This analysis, while not necessarily mechanistically significant, is consistent with previous reports for propylene hydroformylation4,15 and provides a quantitative measure through which the effect of catalyst support can be compared. All of the catalysts used in these experiments deactivated because of metal leaching, either during reduction or during reaction. The high surface area silica gel support produced the most stable catalyst, as determined by analyzing the percent of rhodium before and after reduction and that after reaction. Table 1 lists some results of ICP analysis for the metal content of several catalysts that were made using the Rh(acac)3 precursor. Catalyst 1, supported on the high surface area silica gel, had a much higher metal content after reaction than did the other catalysts. This was after nine reactions run, or after 5200 min of reaction time. All other catalysts were only used for one reaction, generally lasting approximately 600 min except for catalyst 2, which was run for two consecutive reactions
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5321
or 975 min. Our data show the degree of metal loss after a reaction has been run with the catalysts. The data in Table 1 shows clearly that the bimetallic catalysts made with rhodium plus iron were more stable during reduction than the rhodium-only catalyst. Only during reaction did these catalysts significantly leach the metals. Minai et al.20 postulated that iron could act as an “anchor” for noble metals on refractory oxides, through the formation of Rh-Fe-O support bonds. In addition, all catalysts within this study were prepared with the Rh(acac)3 precursor, and if Fe was added, Fe(acac)3 was the precursor. Rogemann et al.21 recently studied the solubility of Fe(acac)3 in scCO2 with various cosolvents added. According to their results, at ideal conditions, all of the Fe(acac)3 used in the manufacture of our catalysts could be dissolved into scCO2. Because the reduction temperature was relatively mild (150 °C), it could be expected that some of the rhodium was not reduced to metallic form. Because both Rh(acac)3 and Fe(acac)3 exhibit similar degrees of solubility in various solvents (CRC),22 it would seem reasonable that Rh(acac)3 would have a solubility behavior similar to that of Fe(acac)3. Thus, we suspect that Rh(acac)3, which was condensed on the surface during impregnation, dissolved into the fluid phase during reaction. Because the catalysts deactivated throughout each experiment, it was necessary to include deactivation in the rate model. A common way to model reaction rates with catalyst deactivation is to assume that the reaction mechanism and the leaching mechanism are independent.19 Under these conditions, the rate expression becomes
-rP ) ka(t) CP
Figure 4. Experimentally measured values for the Rh-Fe catalyst supported on silica gel, compared to the confidence limits of the model predications. Table 2. Comparison of the Average Product Regioselectivity catalyst no. 1 2 3 4 5 6
(3)
7
where a(t) is a description of the time-dependent catalytic activity relative to an initial value. Unfortunately, the profile of the activity was not determined, and results are only available from fresh and spent catalysts. Thus, it was necessary to assume a decay order for the deactivation system. Both first- and second-order dependence was considered, with the data more closely approximated using the second-order model. Incorporation of the deactivation model into the rate expression provides a single kinetic expression that accounts for both reaction and deactivation
8c
-rP )
kCP 1 + kdt
(4)
where kd is the catalyst decay rate constant (h-1). This model was used to compare the performance of all catalysts described within this paper and the constants determined using the nonlinear regression analysis of POLYMATH.19 Results The concentrations of propylene and aldehyde products were measured as a function of time. Following each experiment, the propylene concentration data were converted to reaction rate using the methods described by eq 1. All of the reaction rates were normalized to the ICP results for the percent of rhodium on each catalyst following reduction. All experiments were performed at a constant temperature of 100 °C and a pressure of 170 bar.
support type
% conva % yielda regioselectivityb
Catalysts Made with Rh and Fe silica gel 13.52 14.41 SiO2 with 10% 10.84 10.87 CH3(CH2)9 SiO2 SiO2 with 10% 6.27 6.08 CF3(CF2)9 SiO2 SiO2 4.38 4.68 Catalysts Made with Rh Only silica gel 10.66 10.45 SiO2 with 10% 1.4 2.78 CH3(CH2)9 SiO2 SiO2 with 10% 5.78 3.86 CF3(CF2)9 SiO2 SiO2 3.22 3.87
1.28 ( 0.10 1.11 ( 0.02 1.2 ( 0.04 1.13 ( 0.02 1.02 ( 0.02 1.28 ( 0.06 1.16 ( 0.08 1.17 ( 0.05
a
Propylene conversion and total yield measured at 5 h. b Average value over 10 h of the experimental run.
Results from a typical experimental run are shown in Figure 4, which provides the conversion, aldehyde yield, and reaction rate as a function of time for the Rhonly catalyst supported on silica gel. Note that the total aldehyde yield closely parallels the conversion of propylene, indicating the approximate closure of the material balance. The curves in the figure represent the predictions of the model (see eq 4) in terms of the 95% confidence limit of the rate constants. Both the conversion and the reaction rate (calculated from the experimental data through eq 1) reside mostly within the 95% confidence limits of the model parameters. Thus, we observe that the reaction model chosen adequately represents the experimental data. Conversion, Selectivity, and Material Balance. Table 2 provides a summary of the experimental results for catalysts considered in this paper. Reasonable conversion was obtained for most catalysts, with the highest conversion at 5 h of 13.5% obtained from catalyst 1, Rh-Fe/silica gel. Catalyst 6, the hydrophobic rhodium-only catalyst, had the lowest conversion of 1.4%, only slightly greater than a measured background conversion, likely due to the very low level of rhodium remaining on the catalyst following reduction. Note further that the yield of aldehyde closely corresponds to the propylene conversion, indicating that the entire product was collected and measured, within the limits of the experimental error. The single point at 5 h, representing the midpoint of each experiment, was
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Figure 5. Comparison of the reaction rates measured over Rh/ Fe catalysts supported on modified silica for the hydroformylation of propylene in scCO2 at 100 °C and 2500 psia.
chosen arbitrarily but is representative of the results obtained throughout each run. One significant advantage of homogeneous hydroformylation relative to the heterogeneously catalyzed process is that the regioselectivity can be very high, usually in the range of 10-20. One of our goals when developing our supports was to improve the regioselectivity of the heterogeneous reaction. It was believed that the solvent interactions on the catalyst surface would lead to improvement in the selectivity. Table 2 lists the regioselectivity for the catalysts used in these experiments. The values listed are the average regioselectivity over the entire reaction run. The standard deviations were calculated using the EXCEL worksheet function with 95% confidence intervals and all of the data from the specific reaction run. The values were all slightly larger than 1, substantially lower than the value that can be achieved commercially. Interpretation of the results shows that none of the supports provided a statistically significant advantage in regioselectivity over the others. Despite literature reports that the addition of iron should have a positive effect on the regioselectivity,4,5 no such effect was observed in the current case. Thus, we conclude that the support modifications enhanced the rate of the reaction steps occurring prior to those that determine the selectivity of product formation or affected the rates of both isomeric pathways in an equivalent manner. Although not shown in Table 2, chemoselectivity also did not change among the nonporous supports. Butanals were almost exclusively produced for all reactions at 100 °C and 170 bar in scCO2. For the Rh/Fe catalysts, trace amounts of byproducts were observed at high propylene conversions. It was not possible to identify these byproducts. Analysis of known samples of n-butanol, isobutyl alcohol, 1-propanol, 2-propanol, and propane did not match retention times for the unknown peaks by GC analysis. Because of the low yields of butanals on the catalysts made without Fe, no byproducts were ever seen in these reactions. The catalyst prepared on silica gel also had high chemoselectivity with only trace amounts of byproducts; however, these byproducts were identified as n-butanol and isobutyl alcohol. Comparison of Catalyst Supports. The performances of rhodium-iron catalysts prepared on the nonporous silica supports are compared with that of catalyst prepared on porous silica gel in Figure 5, in which the reaction rate calculated from the measured conversion is shown as a function of the reaction time. The curves represent the model predictions, using the best-fit parameters reported in Table 3.
As can be seen from Figure 5, the Rh/Fe catalyst made with the fluorinated support (catalyst 3) had the highest initial reaction rates. However, this catalyst also lost its activity more rapidly than did the others and after 120 min produced very little additional product. The SiO2-supported Rh/Fe catalyst (4) had the lowest initial reaction rates but also slowly declined in activity. The hydrophobic-supported Rh/Fe catalyst (2) displayed reactivity between our other low surface area catalysts, and again the activity declined with increasing reaction time. The catalyst made with Rh/Fe on the silica gel (1) had the highest average reaction rates and had the smallest decline in activity out of all of the catalysts. Although not shown, the experimental data obtained from the Rh-only catalyst followed the same trends as those reported in Figure 5. The results are shown quantitatively in Table 3, which provides best-fit estimates according to the kinetic model of eq 4. Note that the value for each of the rate constants is normalized to the amount of rhodium measured on the catalyst by ICP following reduction. For both the Rh/Fe and the Rh-only catalysts, the CO2-philic support (catalysts 3 and 7) provided the highest values of the rate constant. However, these catalysts also had the highest values of the deactivation rate constant. This was seen qualitatively in Figure 5, in which the CO2-philic catalyst had the highest initial rate but also the greatest deactivation. Catalysts made on nonporous SiO2 had the lowest values for the rate constant but also the lowest deactivation rate constants. The higher rate constant for the silica gel catalysts relative to the unmodified SiO2 catalyst may be a result of the high surface area and thus greater availability of surface Rh. All of the catalysts deactivated over time, due to leaching of rhodium from the surface of the support. The high rate and minimal loss in activity for the silica gel catalyst is consistent with the lower amount of metal leaching that we observed with this catalyst. In addition, it provides an explanation for the high loss of rhodium observed for all of the catalysts. We used the density of rhodium metal (12.4 g/cm3)22 in order to estimate the specific volume of a rhodium atom as 0.38 × 10-23 cm3. If we assume that the Rh atoms are spherical, then the surface area projected for one atom is 6.95 × 10-16 cm2. In all cases, we attempted to place 0.1 g of Rh metal on the surface of the support; this corresponds to a required surface area of 4.1 × 105 cm2. Moreover, we used 2.5 g of support for the impregnation. The surface area of the nonporous supports was measured to be less than 1 m2/g, or an available surface area of less than 2.5 × 104 cm2. Thus, we attempted to put approximately 15 layers of metal onto the nonporous catalyst. However, the porous silica had a surface area of approximately 300 m2/g, or 7.5 × 106 cm2, which amounts to a rhodium coverage of less than one monolayer. We therefore conclude that the metal was essentially condensed onto the surface of the nonporous supports, and as soon as enough energy was provided, the metal could be displaced, leading to the high leaching rates observed. We propose that the experimental results demonstrate that modification of the catalyst surface can affect the local environment around the catalyst and influence the rate of the reaction. For example, the CO2-philic support would be expected to create a higher density of solvent near the surface of the catalyst. This high local
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5323 Table 3. Results from Kinetic Analysis catalyst no.
support type
1 2 3 4 5 6 7 8
silica gel SiO2 with 10% CH3(CH2)9 SiO2 SiO2 with 10% CF3(CF2)9 SiO2 SiO2 silica gel SiO2 with 10% CH3(CH2)9 SiO2 SiO2 with 10% CF3(CF2)9 SiO2 SiO2
catalyst species Rh/Fe Rh/Fe Rh/Fe Rh/Fe Rh-only Rh-only Rh-only Rh-only
Figure 6. Adsorption of CO from a scCO2 medium and the effect of support on the adsorption bands.
solvent concentration should also enhance the concentration of each of the reactants and promote desorption of the aldehyde products into the fluid phase. This may account for the high initial rate observed for the CO2philic material. However, the very high deactivation rate constant of the catalyst made with the fluorinated support may also be a function of the high solubility of the catalyst precursors in scCO2, as we have now learned that the Rh(acac)3 precursor would be soluble in scCO2. The hydrophobic support was expected to increase the concentration of propylene at the catalyst surface, leading to a reaction rate higher than that of catalyst 4 but lower than that of catalyst 3, as observed experimentally. The hydrophobic support, however, does not impact the deactivation relative to SiO2, as it presumably does not alter the CO2 concentration near the surface of the catalyst. The unmodified support was not expected to alter the environment and thus provided no enhancement in reaction performance, which would explain the low rate of reaction relative to the catalysts made with modified supports. Underlying Chemistry. It was expected that we could gain insight into the mechanism of the reaction by probing the surface of the catalyst using DRIFTS analysis. Figure 6 compares the CO adsorption for the Rh/Fe catalysts supported on SiO2 with the hydrophobic and CO2-philic modified supports. Identification of adsorption bands at 2030 and 2075 cm-1 in the spectrum for the SiO2 material is indicative of linear CO and dicarbonyl adsorption on the rhodium.23,24 These adsorptions were recorded for CO adsorption from scCO2 and show that adsorption of CO onto the catalyst clearly occurs. Additional data for adsorption of CO from pure vapor-phase carbon monoxide indicated significant adsorption peaks at these wavenumbers and allowed us to positively identify the adsorption of CO onto these materials. Although we made no attempt to quantify the amount of adsorption, it was clear that the greatest CO adsorption occurred on the hydrophobic support. A fairly sharp
initial rate (mol/gcat h)
k [(h gcat)-1]
kd (h-1)
0.0465 0.0825 0.1455 0.0285 0.867 0.537 1.702 0.083
0.46 ( 0.01 0.55 ( 0.10 0.97 ( 0.10 0.19 ( 0.03 5.78 ( 0.55 3.58 ( 0.50 11.35 ( 1.07 0.55 ( 0.0007
0 0.21 ( 0.12 1.67 ( 0.54 0.20 ( 0.09 0.1 ( 0.035 -0.004 ( 5.72 × 10-5 0.137 ( 0.049 0.003 ( 0.0004
Figure 7. Development of aldehyde response under reaction conditions (CO + H2) in scCO2 at 100 °C and 100 psia and comparison of responses over different support media.
adsorption peak was observed at 2075 cm-1 on the SiO2 support, corresponding to the linear CO adsorption; however, the dicarbonyl adsorption at 2030 cm-1 was somewhat reduced relative to the hydrophobic material. A very small adsorption response was observed at both bands on the CO2-philic (fluorinated) support. These data suggest that the hydrophobic modification to the support had little affect on CO adsorption, whereas the fluorinated side chains used in the CO2-philic support clearly diminished the ability of CO to adsorb onto the catalyst from the scCO2 solution. We propose that the reduced CO adsorption is a result of the high CO2 concentration that would be expected near the surface of the CO2-philic catalyst. Competitive adsorption of CO2 will decrease the availability of sites for CO adsorption. An enhanced local concentration of CO2 is not expected for the hydrophobic support, explaining why inhibition of the CO adsorption is not observed for this material. In addition to the adsorption experiments, we performed several reactions within the DRIFTS chamber, in an attempt to identify the time dependence of product formation. The spectra obtained from the modified supports are compared with the spectrum obtained over SiO2 in Figure 7. The concentration of feed materials was the same in all cases, and the reaction time was maintained at 90 min. Responses due to the propylene (1820-1840 cm-1) and aldehydes (1740 cm-1) were observed and assigned based on literature data23,24 as well as our own measurements using pure species. Because the propylene disappearance requires analysis of small changes in a large peak, it is easiest to evaluate the effect of the support by comparing the aldehydes peak. Qualitatively, the largest aldehyde response was obtained using the hydrophobic support, with decreasing response from the CO2-philic and SiO2 supports, respectively. This is in contrast to the reactor experiments, which revealed the greatest initial rates from the CO2-philic support. However, it must be noted
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Figure 8. Schematic diagram describing the interactions between the fluid and the solid support that can be envisioned through catalyst design.
that the results obtained within the DRIFTS cell represent the accumulation of 90 min worth of reaction product and do not represent the measured rate of reaction at 90 min. Careful examination of Figure 5, which reveals the rapid deactivation of the CO2-philic catalyst, also shows that the integrated rate over the first 90 min of reaction is actually lower for the CO2philic material compared to the hydrophobic. Thus, the DRIFTS experiments are in substantial agreement with the batch reactions. These results combine to demonstrate that the modifications made to the catalyst support do impact the performance of the catalyst in scCO2, as illustrated schematically in Figure 8. For the current materials, the CO2-philic support likely produces a local environment near the catalyst surface that is enriched in CO2, increasing the solubility of the aldehyde product and providing an increase in the intrinsic rate of the reaction. However, the rich CO2 environment also creates a region in which the rhodium metal is highly soluble, and the catalyst rapidly desorbs from the support and becomes inactive in the fluid phase. Thus, the catalyst deactivates and the benefits of the local environment are negated. However, the hydrophobic support provides a local environment in which CO adsorption and perhaps the aldehyde desorption are both enhanced. Because there are limited interactions between the CO2 and the hydrocarbon side chains on the support, there is no local density enhancement and thus no significant change in the rate of rhodium desorption, so the catalyst stability is not affected by this modification. Conclusions Propylene hydroformylation in scCO2 on unmodified and modified SiO2 was able to produce the desired aldehyde products. Although these catalysts were active and slightly selective, all catalysts rapidly deactivated because of leaching of rhodium metal into the fluid phase. The combination of the use of Rh(acac)3 as the precursor, the mild reduction conditions, and the low surface area of the supports contributed to the leaching. The addition of Fe to the catalyst decreased the amount of leaching during reduction but had little effect during reaction.
Modification of the catalytic supports led to enhanced reaction rates but did not significantly affect the product selectivity; catalyst stability was also affected. Addition of fluorinated side chains to the support produced a CO2philic catalyst that gave a high initial activity but was rapidly deactivated. Surface analysis revealed that low CO adsorption was obtained for this species, suggesting that the mechanism of enhancement was through increased solubility, and thus a higher desorption rate, of the aldehyde product. The rapid deactivation is also consistent with this assumption, because the mechanism of deactivation was through dissolution of the rhodium into the fluid phase. Thus, we conclude that modification of the support enabled the development of a local environment near the surface of the catalyst that was enriched in CO2 relative to the bulk concentration, which, in turn, altered the performance of the catalyst. It is thereby demonstrated that the fluid-solid interactions can be used to advantage for the control of heterogeneously catalyzed reactions performed in a supercritical medium. Acknowledgment This research was supported under the Lucent Technologies Fellowships in Industrial Ecology, National Science Foundation Grant BES-9873553. Additional financial assistance was received from the University of Toledo. Catalyst supports were prepared in the Department of Chemistry by Tom Barnard under the supervision of Prof. Mark Mason. ICP measurements were completed in the University of Toledo Instrumentation Center. We acknowledge their support in both personnel and equipment. Finally, we also acknowledge the assistance of Dr. Joan Brennecke and her research group in providing us with solubility data for iron acetylacetonate in scCO2. Literature Cited (1) Weissermel, K.; Arpe, H. Industrial Organic Chemistry; VCH Publishers: New York, 1993; pp 1-137. (2) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (3) Cornils, B.; Herrmann, W. Applied Homogeneous Catalysis with Organometallic Compounds; Wiley-VCH: New York, 1996; Vol. 1, pp 8-103.
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Received for review December 11, 2000 Revised manuscript received March 21, 2001 Accepted March 28, 2001 IE0010722
