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Hydrogenation of Propyne in Palladium-Containing Polyacrylic Acid Membranes and Its Characterization Lothar Gro1 schel,† Rami Haidar,† Andreas Beyer,† Helmut Co1 lfen,‡ Benjamin Frank,† and Reinhard Schoma1 cker*,† Institute of Chemistry, Technical University of Berlin, Strasse des 17. Juni 124, 10623 Berlin, Germany, and Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, 14476 Potsdam, Germany
In this contribution, catalytically active membranes based on poly(acrylic acid) networks containing palladium nanoparticles are presented as a suitable catalyst for a gas-phase hydrogenation reaction. The palladium particles were prepared in sizes of a few nanometers from Pd(OAc)2 in the presence of a block copolymer in organic solutions with reducing agents such as NaBH4 or LiAlH4. After the metal dispersion had been mixed with a polymer dispersion with a defined amount of poly(acrylic acid), catalytically active membranes were obtained by cross-linking the dispersion with a difunctional epoxide. Membranes with defined porosities and amounts of palladium were characterized for their catalytic activity. The partial hydrogenation of propyne to propene was chosen as the model reaction. To benchmark the activity and selectivity of the prepared membranes, the hydrogenation was also studied in a fixed-bed reactor filled with similar amounts of commercially available porous or egg-shell catalysts. Simulations of the reaction in membranes were performed using numerical tools in order to distinguish between kinetic and mass-transfer control of the reaction. Introduction Industrial-scale hydrogenation reactions are generally performed with supported noble metal catalysts. Very often, an intermediate represents the desired product. To realize high conversion and selectivity, mass-transfer limitations inside the catalytic layer of the heterogeneous catalysts have to be avoided. Therefore, the use of egg-shell catalysts with a defined thickness of the active layer is the state of the art. These catalysts have to be newly developed for every single reaction because mass transport and reaction inside the catalyst pellets have to be adjusted precisely. A new approach to performing heterogeneous catalytic hydrogenation reactions is the use of catalytically active membranes. Reports on heterogeneous catalysis using inorganic membranes with different noble metals can be found in the literature,1-6 in addition to publications on dense organic membranes with incorporated catalysts.7,8 The motivation for using membranes instead of pellets as catalyst support materials is that mass transport within membranes is by convective flow instead of by diffusion in porous pellets. Convective flow is easier to control and to adjust to the kinetics of the reaction than pore diffusion. To date, only a few publications on porous polymer membranes are available.9-13 Some previously published results regarding the hydrogenation of propyne using different types of membrane reactors and reaction conditions are summarized in Table 1. Under different reaction conditions the results of the hydrogenation reaction can differ greatly. Ziegler et al.10 hydrogenated a stream of 5 vol % propyne in propene, Liu et al.12 and Jackson et al.26 used a nitrogen matrix * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +49 30 31422261. † Technical University of Berlin. ‡ Max Planck Institute of Colloids and Interfaces.
for propyne hydrogenation. Because of the different materials and conditions, no general conclusions can be drawn that allow the design of a membrane reactor and the selection of a suitable membrane. The preparation of the membranes that are discussed herein and the incorporation of palladium particles into these membranes are described in detail in our previous papers.14,15 The applied preparation is a combination of methods for immobilizing Pd particles on different supports that can be found in the literature.16-21 In this work, the characterization of catalytically active membranes is presented. The polymeric catalyst support is a porous material formed by cross-linking of a poly(acrylic acid) (PAA) dispersion. The active species inside this three-dimensional network are immobilized Pd nanoparticles on the scale of a few nanometers. For the investigation of the catalytic behavior of these membranes, the partial hydrogenation of propyne in the gas phase has been chosen as a suitable model reaction. More information on this reaction performed with other catalysts can be found in the literature.22-30 The effects of the porosity and the catalyst loading of the membrane as well as flow rate of the reactant mixture on the catalytic behavior of the membranes are investigated. Experimental Section Chemicals. 1,2,4-Trimethylbenzene (TMB) (99%); toluene (99%); acrylic acid (99%); NaBH4 (0.5 M solution in 2-methoxyethyl ether); LiAlH4 (0.5 M solution in THF); and the catalysts, 0.15 wt % Pd on SiO2, 0.5 wt % Pd on active carbon, and 0.15 wt % Pd on Al2O3 were obtained from Sigma-Aldrich. 2,2′-Azobis(2,4-dimethylvaleronitrile) initiator was obtained from Wako Pure Chemicals. The block copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO; Mpolystyrene ) 3000, Mpoly(ethylene oxide) ) 1000) was purchased from Goldschmidt. Propyne (98%) and palladium(II) acetate were obtained from Merck and hexanediol diglycidyl ether (66%) was ob-
10.1021/ie050426s CCC: $30.25 © 2005 American Chemical Society Published on Web 11/01/2005
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9065 Table 1. Literature Data on the Hydrogenation of Propyne over Pd Catalysts ref
reactor
temperature (K)
partial pressure of propyne (bar)
contact time (s)
diameter (cm)
mass of Pd (mg)
conversion (%)
selectivity (%)
10 12 26 27
poly(amideimide) membrane hollow-fiber reactor pulse-flow reactor pulse-flow reactor
300 313 333-673 300-673
0.05 0.2 0.25 0.5
12 8
2 3.0 2.5 2.5
2,2-8,6 0.8-1.7 10-1500 500
100 15-85 25-70 68
81-99 91-98 37-100 100
tained from Witco. Hydrogen and nitrogen were obtained from Linde. Preparation of the Catalytically Active Membranes. Porous membranes with porosities of 33%, 40%, 57%, 64%, and 73% were produced as described in ref 14. The preparation of metal colloids and the crosslinking of the polymer-metal dispersion with hexanediol diglycidyl ether were performed as described in ref 15. Characterization. Fragments of the catalytically active membranes were characterized by means of scanning electron microscopy (SEM) using a Hitachi S 4000 instrument at 20 kV. Transmission electron microscopy (TEM) was performed with a JEOL-JTSEM 200B electron microscope operating at 150 kV. The samples of liquid dispersions were diluted with toluene (1/120) and subsequently treated in an ultrasonic bath to prevent agglomeration of the polymer particles. The well-mixed dispersion was evenly distributed on a copper sample grid, and the solvent was evaporated. Catalytic Tests. To determine the catalytic activity and the selectivity of the produced membranes, the gasphase hydrogenation of propyne to propene accompanied by several byproducts was chosen as the model reaction:
In general, the selectivity decreases with increasing conversion of propyne. The decrease of the selectivity for propene of such a consecutive reaction is more pronounced the higher the ratio of the rate constants k2/k1 is. To test the selectivity of this reaction in a membrane reactor, the following experimental setup was designed (see Figure 1). The membrane reactor was made of steel and constructed to carry a membrane with an area of 32 cm2 and a thickness of about 1 mm (3). The reactants,
Figure 1. Experimental setup: Membrane reactor with fixedbed reactor in bypass.
propyne and hydrogen, were fed from their storage vessels to the reactor through flowmeters. The stoichiometric gas mixture passed through the membrane fixed on a sintered support made of inert material (4). Parallel to the membrane reactor, a fixed-bed reactor was installed for comparisons with reference catalysts. The product stream was analyzed by gas chromatography (Chrompack 9003; propyne analysis, PLOT fused silica + Al2O3/KCl column with helium as the carrier gas). The conversion, X, and selectivity, S, of propyne hydrogenation were defined as follows
X)
n˘ propyne n˘ propyne,0 - npropyne
(1)
S)
n˘ propene n˘ propyne,0 - npropyne
(2)
Experimental errors in partial pressure measurements were (2% for conversion and selectivity. Results and Discussion Microscopic Characterization. A standard analytical method for the characterization of membrane surfaces is the SEM technique. Using this method, a network structure of the membrane formed from poly(acrylic acid) particles could be visualized. In Figures 2 and 3, an SEM image (100 000× enlargement) and an electron-dispersed micrograph (EDM) of a membrane prepared from about 1 g of PAA and 5 mg of Pd are shown. The porosity of the membrane is 58%. This value was calculated by comparing the membrane density to the density of crystalline PAA. As a result of the higher electron density of the metal compared to that of the organic polymer, electron backscattering is the most suitable method for the identification of palladium in such systems.
Figure 2. SEM micrograph of the surface of a PAA membrane ( ) 58%, mPd ) 5 mg).
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Figure 5. Double-sector sampleholder of the AUC with sample of dispersion. r is the distance from the sample to the rotation axis (m), and ω is the angular velocity (s-1).
Figure 3. EDM micrograph of the same sample and area displayed in Figure 2.
Figure 6. Sedimentation profile of the PAA-Pd dispersion.
Figure 4. TEM micrograph of a Pd-PAA dispersion diluted with toluene (1/120).
The PAA particles can be seen in Figure 2. Their size is approximately 100 nm. The markings show some catalytically active palladium nanoparticles adsorbed on the polymer particles, which are distributed randomly. From the EDM micrograph, the Pd particles can be seen as white dots with diameters of approximately 10 nm. The precursor of this presented cross-linked membrane is a PAA dispersion in TMB. For the investigation of the Pd particles in the precursor state, TEM studies were performed on these dispersions. The Pd particles appear in better contrast in comparison to the polymer. With a dilution of the dispersion (1/120), a monolayer of the PAA particles on the sample grid could be realized. Figure 4 shows the PAA dispersion with an enlargement of 100 000×. From this TEM image, it is clear that the size of the PAA particles is about 100 nm. The palladium particles show diameters of around 2-5 nm and are partly agglomerated but mostly distributed randomly on the surface of the polymer. Characterization Using an Analytical Ultracentrifuge. The analytical ultracentrifuge (AUC) is a suitable tool for the characterization of the stability of polymer-metal dispersions. The main interest of this
work is the strength of the interactions between the polymer and the metal particles. In this experiment, a highly diluted poly(acrylic acid)-palladium dispersion is placed in one sector and a sample of the solvent (TMB) in dialysis equilibrium with the sample is placed in the reference sector. A high-intensity Xe flash lamp is fired briefly as the selected sector passes the detector. The PAA particles are detected by Rayleigh interference optics, and the palladium particles are detected by light absorption. The experiment was performed at 3000 rotations/min. Figure 5 shows the apparatus schematically. From this experiment, the sedimentation coefficients of the PAA particles and the metal particles were calculated by the Svedberg eq 3
s)
u ω2r
(3)
where s is the sedimentation coefficient (s) and u is the sedimentation velocity (m s-1). The result of this measurement is illustrated in Figure 6. The nonuniform polymer particle sizes and shapes result in a distribution of sedimentation coefficients of both species. It is clear that the sedimentation coefficients of the polymer particles and the palladium particles are very similar. The smaller palladium particles are not separated from the larger polymer particles; otherwise, the sedimentation run would be much faster. This indicates strong attractive forces between the two species. The palladium particles are bonded strongly to the surface of the polymer particles. Catalytic Tests. Effect of the Flow Rate of the Gas Mixture. A series of catalytic studies on the hydrogenation of propyne were performed. Also, mea-
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Figure 7. Conversion of propyne and selectivity to propene versus flow rate of the gas phase ( ) 58%, mPd ) 2 mg, T ) 298 K, p ) 1 bar).
surements with commercial Pd catalysts in the fixedbed reactor were made to compare the membranes with these catalysts. In the initial experiments, a gas mixture of 50 vol % hydrogen and 50 vol % propyne was flowed through the membranes. The catalytic activity of a membrane with a porosity of 58% and a palladium content of 2 mg was investigated. The conversion and selectivity of this membrane as functions of the gas flow rate are shown in Figure 7. The range of the flow rate from 20 to 80 mL min-1 corresponds to an interval of the residence time from about 1 to 4 s. All measurements shown in this figure were made using the same membrane but randomly varying the flow rate. Steady-state conditions were assumed after a time on stream of 5 min. The measurements were taken after at least 20 min. From Figure 7, it is clear that an increasing flow rate (decreasing residence time) through the membrane results in a decreasing conversion and an increasing selectivity. This catalytic behavior is expected because the limiting factor of the reaction is the residence time. Effect of the Membrane Porosity. Further studies were done with membranes of different porosities. Highly porous polymer membranes contain pores with a larger diameter. The relation between porosity and pore size was shown in our previous paper.14 This means that, in catalytically active membranes with equal amounts of catalytic particles but with larger pore diameters, the catalyst is more widely distributed. At the same gas flow rate, the residence time in the more porous membrane will be longer. On the other hand, a better distribution of the catalytically active particles in membranes with low porosity will be expected, and faster conversion of the reactant will occur. These two effects-longer residence times and worse distribution vs short residence times and better distribution of the catalyst-compete with each other. Results of the experimental studies on membranes with different porosities are presented in Figure 8. The palladium content in each membrane was kept constant at 2 mg. It is clear that the conversion decreases with increasing porosity of the membranes. According to this result, it is surprising that an optimal porosity of the membrane exists, indicating an optimal catalyst distribution and an optimal residence time. This result was found to be independent of the flow rate and well reproducible. Effect of the Palladium Content of the Membranes. In the second series of experiments, membranes with Pd contents of 2.0, 2.7, and 5.5 mg were
Figure 8. Dependency of conversion on the porosity of the catalytically active membranes (mPd ) 2 mg, T ) 298 K, p ) 1 bar, flow rate ) 20 mL min-1).
Figure 9. Conversion and selectivity for different catalyst amounts and porosities of poly(acrylic acid) membranes ( ) 0.73/ 0.58, mPd ) 2.0/2.7/5.5 mg, T ) 298 K, p ) 1 bar).
prepared. PS-b-PEO block copolymer as a stabilizer and NaBH4 as a reducing agent were added in stoichiometric ratios with increasing Pd(OAc)2 amounts. The results of the measurements in two membranes with porosities of 58% and 73%, which are presented in Figure 9, show the dependency of the selectivity of propene on the conversion of propyne under isothermal conditions at 298 K. With increasing conversion, the selectivity decreases, as expected. Both membranes show a conversion ranging from 13% (2 mg of Pd, τ ) 1 s) to 78% (5.5 mg of Pd, τ ) 4 s). The selectivity decreases from 95% to 84%. The main reason for the decreasing selectivity is the formation of propane from propene in a consecutive reaction. Within experimental error, all selectivity results for different Pd contents fall on the same trajectory. This indicates that there is no change in the kinetics of the reaction due to mass-transfer effects caused by different membrane structures or loadings. Long-Term Stability. To test the long-term stability of the membranes, a membrane was kept under constant reaction conditions for about 6 days. A membrane with a porosity of 58% and a palladium content of 2.7 mg was chosen as a representative example. An equimolar reactant mixture at a flow rate of 20 mL/min was adjusted at a temperature of 298 K. These reaction conditions resulted in a propyne conversion of about 50%, so a change in the catalytic activity of the
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Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 Table 2. Reaction Orders for Propyne Hydrogenation ref
reaction order of propyne
reaction order of hydrogen
26 31 32
-0.02 -0.3 0
0.6 1.01 1
Figure 10. Conversion and selectivity in long-term propyne hydrogenation ( ) 58%, mPd ) 2.7 mg, T ) 298 K, p ) 1 bar, flow rate ) 20 mL min-1).
Figure 12. Numerical fit of the experimental data using a simplified reaction rate law (membrane ) 33/58/64%, mPd ) 2 mg, T ) 298 K, p ) 1 bar).
Hinshelwood mechanism. The following expressions for the rates of the reactions are given in the literature.4 r1 ) k1KP,propyneppropyneKP,hydrogenphydrogen Figure 11. Comparison of different catalysts regarding activity and selectivity in the partial hydrogenation of propyne.
membrane could be detected with a maximum sensitivity. The experimental results are shown in Figure 10. It can be seen clearly that no deactivation of the catalyst occurs over a time period of 140 h. Constant conversion and selectivity indicate no change in the catalytic behavior. Comparison with Other Catalysts. For benchmarking the PAA membranes, three commercially available catalyst pellets were used in a fixed-bed reactor. An egg-shell-type catalyst (Pd on Al2O3) and two porous catalysts (Pd on SiO2 and Pd on carbon) were used. In these experiments, a membrane with 2.7 mg of Pd and 58% porosity was used as a reference. The contents of Pd in the fixed bed of the catalyst supported by SiO2, active carbon, and Al2O3 were identical to that of the membrane (2.7 mg). The residence time of the gas flow was 4 s at a temperature of 298 K. The results of the measurements are shown in Figure 11. The porous catalysts, Pd/C and Pd/SiO2, exhibit very low conversions of 6 and 3.4%, respectively, and also a low selectivity. This was expected, because the reaction is very fast, so only a very small fraction of the pores will be utilized (high value of the Thiele modulus). In contrast to this result, the less-porous egg-shell catalyst exhibits a high catalytic activity, with an 85% yield and an 85% selectivity. The PAA membrane gives the same activity and selectivity. With both supports, an optimized utilization of the Pd is achieved. Simulation of the Hydrogenation Reaction. It is known that the catalytic reaction between hydrogen and propyne can be described sufficiently by the Langmuir-
(1 + KP,propyneppropyne + KP,propeneppropene + KP,hydrogenphydrogen)2
(4) r2 ) k2KP,propeneppropeneKP,hydrogenphydrogen (1 + KP,propyneppropyne + KP,propeneppropene + KP,hydrogenphydrogen)2
(5) where k1 is the rate constant for the hydrogenation of propyne to propene, k2 is the rate constant for the hydrogenation of propene to propane, and KP is the equilibrium constant for chemisorption. In our experiments, we kept the propyne/hydrogen molar ratio constant at 1.0. Therefore, the experimental data contained insufficient information for the determination of the adsorption constants. Thus, a simplified first-order rate law was used to fit our experimental data. With respect to the investigations of other research groups (Table 2), the following rate laws for sequential propene and propane formation were derived with zeroth-order dependence on the propyne and propene partial pressures and first-order dependence of the hydrogen partial pressure.
r1 ) k1phydrogen
(6)
r2 ) k2phydrogen
(7)
The general procedure involved the fitting of the parameters k1 and k2 to the experimental values. Figure 12 shows the experimental and simulated partial pressures of propyne, hydrogen, propene, and the byproduct propane in membranes with 58%, 64%, and 73% porosities, as functions of residence time. The symbols rep-
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9069
resent the experimental partial pressures of the reactants during the reaction, and the solid lines show the simulation according to the simplified rate law (eqs 6 and 7). With respect to the simulation, the following rate constants were obtained:
Acknowledgment
k1 ) 0.075 Pa-1 s-1
Literature Cited
k2 ) 0.005 Pa-1 s-1 It can be seen that the experimental data can be approximated sufficiently. Because of the simplified model, the numerical values of the rate constants are not meaningful, but the ratio shows that the first hydrogenation from propyne to propene occurs much more rapidly than the second one from propene to propane. Moreover, the kinetics were found to be nearly identical in all three membranes. With respect to Figure 8, we first assumed an influence of the porosity on the kinetic parameters. An exact calculation of the residence time inside the membrane pores led to the conclusion that there is no mass-transport limitation, so that the observed reaction rate is the intrinsic one. Porosity and gas flow velocity have no influence on the catalytic performance of the different membranes. Conversion and selectivity are determined only by the residence time of the gas mixture. Summary Catalytically active membranes can be used for selective partial hydrogenation reactions. In such membranes, prepared from cross-linked poly(acrylic acid), the reaction takes place at the surface of immobilized palladium nanoparticles. It was possible to detect the catalytically active species in the dispersion and on the surface of PAA by TEM and SEM. Both techniques showed a random distribution of Pd particles with sizes between 3 and 5 nm. By use of an analytical ultracentrifuge, it was possible to demonstrate that polymer-palladium dispersions are strongly bonded systems. From the experimental studies based on catalytic tests, various dependencies in this reaction system were observed. The porosity of the polymer membrane, the content of catalyst, and the residence time of the reaction mixture have an influence on conversion and selectivity. The membranes show similar conversions and selectivities as an egg-shell catalyst specially developed for partial hydrogenation reactions. However, the membrane has the advantage of a simpler adjustment to the reaction conditions in comparison to the eggshell catalyst. Use of a membrane involves an additional degree of freedom in controlling the mass transfer, namely, the flow rate across the membrane. Under the studied conditions, the kinetics of the hydrogenation reaction is not influenced by mass-transport phenomena of the reactants. The reproducibility of all experiments within a long experimental period is very good, because there is no leaching or deactivation of the catalyst. Simulation results show that the reaction of the hydrogenation of propyne in catalytically active membranes follows the same rate law in different porous membranes. One set of rate constants for main and side reactions could be determined. Conversion and selectivity in different porous membranes are determined solely by the residence time of the reactants.
This project is a part of SFB 448 “Mesoskopisch strukturierte Verbundsysteme” funded by the Deutsche Forschungsgemeinschaft.
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Received for review April 7, 2005 Revised manuscript received August 31, 2005 Accepted September 28, 2005 IE050426S
