J. Phys. Chem. C 2010, 114, 10823–10835
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Selective Hydrogenation of 1,3-Butadiene in the Presence of an Excess of Alkenes over Supported Bimetallic Gold-Palladium Catalysts Antoine Hugon, Laurent Delannoy,* Jean-Marc Krafft, and Catherine Louis Laboratoire de Re´actiVite´ de Surface, UMR 7197 CNRS, UniVersite´ Pierre et Marie Curie-UPMC, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed: January 18, 2010; ReVised Manuscript ReceiVed: May 3, 2010
Supported Au catalysts modified by the addition of low amount of Pd (Au/Pd atomic ratio g10), prepared by either codeposition-precipitation (DP) or coimpregnation in excess of solution (IES), result in the formation of bimetallic Au-Pd particles that promote the selective hydrogenation of butadiene in the presence of propene. The DP method appears more appropriate to obtain reproducible and homogeneous bimetallic catalysts with smaller nanoparticles than the IES method, although it does not allow one to perfectly control the Au/Pd ratio. Playing with the Au/Pd ratio, it is possible to modulate the catalytic properties, especially in the case of the DP samples, and to reach a satisfying compromise between activity and selectivity, with a very low amount of alkanes formed at complete conversion of butadiene. CO adsorption followed by DRIFTS indicates that bimetallic Au/Pd alloy particles are formed, except for the catalyst prepared by IES with the lowest Au/Pd atomic ratio of 10. Pd segregation to the surface of the nanoparticles upon exposure to CO is also observed, underlining that this bimetallic system evolves during exposure to the gas phase. However, the surface composition does not seem to change during butadiene selective hydrogenation performed at 50 °C since activity remains constant. I. Introduction Light alkenes produced by steam reforming contain a few percent of alkadienes or alkynes as impurities.1 For further polymerization processes, the impurity level must be as low as 10 ppm to avoid polymerization catalyst poisoning.1 One way to achieve this goal is to catalytically convert these molecules into alkenes while avoiding hydrogenation of the alkene stream. Palladium-based catalysts are so far used in industry for gasand liquid-phase selective hydrogenation. Palladium is highly selective when the concentration of impurity is still high, but for ultimate purification and elimination of traces of alkadienes or alkynes, i.e., at high conversion, palladium catalysts are not selective enough, and hydrogenation of alkenes cannot be avoided. Modifiers, such as Ag, must be added to palladium to limit these problems,2 and also to reduce the formation of “green oil”, responsible for catalyst deactivation.3 DFT studies showed that the alloying of palladium with another metal, such as silver, increases the favorability of the selective hydrogenation of alkynes or alkadienes by mainly decreasing the stability of the adsorbed intermediate alkenes (for instance, ethylene in the case of acetylene hydrogenation), facilitating the desorption of the alkene and thus limiting its hydrogenation.4–6 In our earlier paper,7 supported gold catalysts were investigated in the selective gas-phase hydrogenation of 1,3-butadiene in excess of propene (0.3% butadiene, 30% propene and 20% hydrogen), in order to simulate the process of purification of industrial alkene gas streams. A series of gold catalysts prepared by deposition-precipitation with urea (DP)8 on various oxide supports (alumina, titania, zirconia, ceria), and containing the same gold loading (1 wt % Au) with the same gold particle size (∼2 nm in average) exhibited similar catalytic properties. Under our experimental conditions, 100% of butadiene was converted at ∼170 °C into 100% butenes, with only a very small * Corresponding author. E-mail:
[email protected].
amount of alkanes formed (∼100 ppm). DFT simulations revealed that the high selectivity of gold in the hydrogenation of acetylene in the presence of ethene or propene can be related to the adsorption of only acetylene on gold whereas both kinds of components compete for adsorption on palladium.9,10 The goal of the present study was to improve the activity of gold catalysts in the same reaction of selective hydrogenation of butadiene by addition of a small amount of palladium, without altering the high selectivity to alkenes. Hence, bimetallic Au-Pd catalysts supported on alumina with Au/Pd atomic ratio .1 were prepared. Regarding hydrogenation reactions involving bimetallic catalysts based on palladium, gold has been most often used as a dilutant, since it was considered as inactive.11–14 Only in a few cases, supported bimetallic gold-palladium catalysts have been investigated in selective hydrogenation of butadiene15 or alkynes.3,16–18 In those studies with bimetallic Au-Pd systems, the aim was to modify the properties of palladium by addition of gold, thus the Au/Pd ratios are lower or at most equal to 1. Even for the other types of reaction studies involving Au-Pd systems, such as selective oxidation, direct synthesis of hydrogen peroxide or decomposition of chlorinated molecules,19–22 the Au/ Pd ratios are close to 1. The originality of our study is to investigate bimetallic gold-palladium systems with Au/Pd . 1 in selective hydrogenation of diene in the presence of an excess of alkene. The preparation of bimetallic catalysts is usually not straightforward, and several methods have been reported for gold-based bimetallic catalysts:23 • preparations involving organo-bimetallic precursors such as Pt2Au4(CtCtBu)8;24 • those involving the deposition of preformed bimetallic particles: (i) colloidal Au-Pd particles stabilized by organic or polymers;25–31 (ii) bimetallic particles stabilized in dendrimers (Au-Pt on oxides32,33); (iii) deposition of
10.1021/jp100479b 2010 American Chemical Society Published on Web 05/26/2010
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bimetallic clusters obtained by low energy laser vaporization (Au-Ni/C);34,35 • the so-called redox methods developed by Barbier et al.36 for the preparation of Au-Pt37,38 and Au-Pd39–41 supported catalysts; • the more “classical” methods involving the simultaneous deposition of the two Au and Pd precursors, by coimpregnation24,42–44 or coadsorption of cations [Au(en)2]3+ and [Pd(NH3)4]2+ 45–47 or codeposition-precipitation.48 Of course, the results in terms of particle size and bimetallic features are at variance. In this work, the preparation methods retained were the same as those that were used for the preparation of the monometallic gold catalysts studied in the same catalytic reaction of selective hydrogenation,7,49 i.e., coimpregnation in excess of solution (IES), and codepositionprecipitation with urea (DP). II. Experimental Methods II.1. Preparation of Bimetallic Catalysts. Two methods were used to prepare the Au-Pd/Al2O3 samples with different Au/ Pd atomic ratios, between 5 and 50, the deposition-precipitation with urea (DP) and the impregnation in excess of solution (IES). The procedures described below refer to the synthesis of 3 g of catalyst containing 1 wt % Au and ∼550 ppm Pd, i.e., an Au/ Pd atomic ratio of 10. Parent aqueous solution of HAuCl4 · 3H2O (Acros Chemicals) (10 g · L-1) was prepared then stored in the dark, as well as a parent solution of PdCl2 (Sigma-Aldrich) (1.0 g · L-1, obtained by dissolution of PdCl2 at 50 °C in HCl aqueous solution). Alumina (AluC Degussa, 110 m2 · g-1, δ-type) was used as the oxide support. The IES-Au/Pd samples were prepared as follows. Three grams of alumina support were mixed with 24 mL of distilled water at RT in a flask. Then, 6.0 mL of a solution of HAuCl4 · 3H2O at 10 g · L-1, i.e., 60 mg of gold precursor, and 2.7 mL of a solution of PdCl2 at 1.0 g · L-1, i.e., 2.7 mg of palladium precursor, were added under vigorous stirring. After 1 h of stirring at room temperature (RT), the solution was evaporated with a rotating evaporator at 80 °C under vacuum for about 1 h. The preparation of DP-Au/Pd samples was performed as follows. A suspension of 3 g of alumina in 290 mL of distilled water was introduced into a glass reactor with double walls, allowing water circulation at 80 °C between the walls and therefore, a homogeneous and well-controlled temperature of 80 °C of the suspension. Then 6.0 mL of a solution of HAuCl4 · 3H2O at 10 g · L-1 (i.e., 60 mg of gold precursor) and 2.7 mL of a solution of PdCl2 at 1.0 g · L-1 (i.e., 2.7 mg of PdCl2) were added under vigorous stirring followed by the addition of 900 mg of solid urea. The suspension was stirred at 80 °C for 16 h in the absence of light. The solid was then separated by centrifugation (11000 rounds/min for 10 min) and washed with 300 mL of water. This procedure was repeated four times, and after the fourth washing, the addition of a few drops of silver nitrate to the supernatant never revealed any traces of chlorides. Finally, the samples were dried under vacuum at RT in a desiccator in the dark for about 12 h. Samples containing other Au/Pd ratios, i.e., 5, 20, and 50 for DP and 10 and 20 for IES, were prepared by adjusting the volume of PdCl2 to the desired amount. Reference samples of monometallic Au/Al2O3 with 1 wt % Au were also prepared by these two methods according to the same procedures as above, except for the absence of PdCl2. A reference Pd/Al2O3 sample with Pd loading of ∼300 ppm, which corresponds to the Pd loading in bimetallic samples with a Au/
Hugon et al. Pd ratio of 20, was prepared by IES. A Pd/Al2O3 sample with ∼300 ppm Pd was also prepared by deposition-precipitation with Na2CO3 and not with urea, for reasons that are explained in section III.1.a. Three grams of alumina was dispersed in 150 mL of distilled water, at RT, then 1.35 mL of PdCl2 solution (1.0 g · L-1) was added under stirring, and the pH was adjusted to 10.5 by dropwise addition of a solution of Na2CO3 (1 M). After 1 h of stirring, the solid was gathered by centrifugation and then washed four times in distilled water before vacuum drying at RT in a desiccator. Physical mixtures of monometallic samples, DP-Pd and DPAu, and IES-Pd and IES-Au were also prepared such as to get an Au/Pd atomic ratio of 20. Except when specifically mentioned, the Au/Pd atomic ratios reported in this paper are the experimental values, determined by chemical analysis, and not the nominal ones. For instance, DP-Au/Pd(20) is a gold-palladium catalyst supported on alumina, prepared by codeposition-precipitation with urea, with an experimental Au/Pd atomic ratio of 20. All the samples were stored under vacuum in a desiccator, in the dark in order to prevent any uncontrolled reduction of gold.50 Except when specifically mentioned, these as prepared Au-Pd samples were reduced under hydrogen at 500 °C (∼100 mg of catalyst under 100 mL · min-1 H2 with a heating rate of 3 °C · min-1 and then a 15 min plateau at 500 °C) and the gold catalysts at 300 °C. The reason for the different final temperatures is explained in section III.2.b. After reduction, the samples were cooled down to RT under nitrogen flow, and transfer in air before characterization. II.2. Techniques. Chemical analyses of Au, Pd, Cl and Al in the as prepared samples were performed by inductively coupled plasma atom emission spectroscopy at the CNRS Centre of Chemical Analysis (Vernaison, France). The metal weight loadings of the samples were expressed in weight percent, e.g., wt % Au ) (mAu/mmAl2O3) × 100. BET surface area, temperature programmed reduction (TPR) and H2 chemisorption were determined using the commercial CHEM-BET 3000 (Quantachrome) unit. The samples (50 to 100 mg) were loaded into a U-shaped Pyrex glass cell (10 cm × 4 mm i.d.) and heated in 17 cm3 · min-1 (Brooks mass flow controlled) 5% v/v H2/N2 to 300 °C (Au/Al2O3) or 500 °C (Pd/ Al2O3 and Pd-Au/Al2O3) at 3 °C · min-1. The effluent gas passed through a liquid N2 trap, and changes in H2 consumption were monitored by TCD with data acquisition/manipulation using the TPR Win software. The reduced samples were maintained at the final temperature for 2.5 h in a constant flow of H2, swept with 65 cm3 · min-1 N2 for 1.5 h, cooled to room temperature and subjected to H2 chemisorption using a pulse (10 µL) titration procedure. BET areas were recorded with a 30% v/v N2/He flow using pure N2 (99.9%) as internal standard. At least 2 cycles of N2 adsorption-desorption in the flow mode were employed to determine total surface area using the standard single point method. BET surface areas and H2 uptake values were reproducible to within (5%; the values quoted represent the mean. TEM analysis was performed on the reduced samples, using a JEOL JEM-100 CX II microscope operating at 100 kV. Metal particle size measurements were performed using software ITEM on digitized micrographs. Note that the particles were measured one by one and not automatically. The size limit for the detection of metal particles on Al2O3 is ca. 1 nm. The average metal particle sizes were determined from the measurement of at least 300 particles, and were expressed as the
Selective Hydrogenation of 1,3-Butadiene
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TABLE 1: Chemical Analysis and TEM Characterizations of the Bimetallic Au-Pd/Al2O3 Catalysts Prepared by DPU and IES targeted values catalyst code DP-Au DP-Pd DP-Au/Pd(10) DP-Au/Pd(20) DP-Au/Pd(90) DP-Au/Pd(>90) IES-Au IES-Pd IES-Au/Pd(10) IES-Au/Pd(20) a
Au/Pd ratio
Pd loading (ppm)
5 10 20 50
300 1080 540 270 108
10 20
300 540 270
experimental values Au loading (%)
Pd loading (ppm)
0.88 0.95 0.87 0.87 0.81 0.81 0.76 0.87
334 681 233 53 90), and by ∼100 °C between DP-Au and DP-Au/Pd(10) (Figure 8). The selectivity to alkenes is not as high as for monometallic gold catalysts since, for most of the DP-Au/Pd samples, the selectivity decreases when the temperature passes T100% (Figure 8). In the case of DP-Au/ Pd(10), the alkanes even start to form below T100%. However, for DP-Au/Pd(20), the amount of alkanes remains lower than
Selective Hydrogenation of 1,3-Butadiene 3000 ppm (
