1-Heptyne Selective Hydrogenation over Pd Supported Catalysts

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1-Heptyne Selective Hydrogenation over Pd Supported Catalysts Cecilia R. Lederhos, Pablo C. L’Argentie` re,* and Nora S. Fı´goli INCAPE (FIQ, UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina

The catalytic behavior of Pd supported on γ-Al2O3 and on an activated pelletized carbon during the selective hydrogenation of 1-heptyne to 1-heptene under mild reaction conditions was studied. Pd/Al2O3 and Pd/C are good catalysts for this reaction, with the former showing better behavior. Under the same operating conditions, Pd/Al2O3 also presents a better performance than the classic Lindlar catalyst. The reduction and operating temperatures were found to play an important role in the catalytic behavior of the catalysts studied. The XPS results show that palladium in Pd/Al2O3 and Pd/C is electron-deficient. Within certain limits, the electron-deficient Pd species do not favor the further hydrogenation of 1-heptene to heptane, thus raising the selectivity to 1-heptene. The differences observed in the catalytic behavior of Pd/Al2O3 and Pd/C could be attributed, at least partially, to the differences in the support porosity. Introduction The catalytic selective semihydrogenation of acetylenes using either homogeneous or heterogeneous catalysts has been widely studied in the past several years, because of its academic and industrial interest.1 The hydrogenation of an alkyne can be virtually stopped at the semihydrogenation stage because the alkyne is more strongly bound than the alkene and competes effectively for the catalytic sites, thus blocking readsorption of the alkene or displacing it. Many products obtained through this kind of reactions are useful in the synthesis of natural products, such as biologically active compounds.2 One of the most studied catalytic systems for these kind of reaction is the Lindlar catalyst [Pd/CaCO3 modified with Pb(OAc)2], developed in 1953. Other catalysts, mono- or bimetallic as well as complexes of several transition metals, have also been proposed for the reaction.2-8 Several materials have been used as supports, and they are usually classified as organic (macroreticular/macroporous polymers) or inorganic (silica, alumina, zeolites, clays) supports.9 Another kind of material, not clearly included in any of these groups, is carbon, whose outstanding properties as a catalyst support are well recognized.10 It is known that carbon offers very interesting properties as a catalyst support.11-13 Among them, are the possibility of modifying the specific surface area, porosity, and surface chemistry; moreover, carbon supports present the advantage of being inert in liquid reaction media. Little information is available in the literature regarding, in particular, the selective hydrogenation of 1-heptyne carried out in heterogeneous conditions. The objective of this paper is to study the influence of the reduction and reaction temperatures and of the support in the selective hydrogenation of 1-heptyne using Pd catalysts supported on γ-Al2O3 and on an activated carbon. Experimental Section Catalyst Preparation. Al2O3 (Ketjen CK 300, cylinders of 1.5-mm diameter) and a pelletized commercial * To whom correspondence should be addressed. E-mail: [email protected].

carbon (GF-45, from NORIT) were used as supports. Both materials were impregnated by the incipient wetness technique using PdCl2 solutions of concentrations sufficient to obtain 5% Pd on the final catalysts. Pd/Al2O3 was then calcined at 773 K, whereas Pd/C was calcined at 373 K. Both catalysts were reduced under a hydrogen stream at 373 and 573 K to study the influence of the reduction temperature on the catalytic behavior. For comparative purposes, a commercial Lindlar catalyst provided by Aldrich (catalog number 20 503-6, 5 wt % Pd on calcium carbonate, poisoned with lead) was used. Catalysts Characterization. Physical adsorption of gases (N2 at 77 K and CO2 at 273 K) and mercury porosimetry were used to analyze the porous textures of the two supports.1 Elemental analysis was used to assess the organic sulfur in GF-45.14 Palladium dispersion was measured by hydrogen chemisorption in a volumetric apparatus at 373 K. Determinations were made using the method of the double isotherm proposed by Benson et al.15 The electronic state of Pd was studied by X-ray photoelectron spectroscopy (XPS), following the Pd 3d5/2 peak binding energy (BE). Determinations were carried out on a Shimadzu ESCA 750 electron spectrometer coupled to a Shimadzu ESCAPAC 760 Data System. To correct possible deviations caused by electric charging of the samples, the C 1s line was taken as an internal standard at 285.0 eV, as previously described.16 The samples were introduced into the XPS sample holder following the operating procedure described by other authors17 to ensure that there was no modification of the electronic states of the species analyzed.18 Regardless, exposing the samples to the atmosphere for different periods confirmed that there were no electronic modifications. Cl/Pd surface atomic ratios on Pd/Al2O3 and Pd/C were determined by comparing the areas under the peaks after background subtraction and corrections due to differences in escape depths and in photoionization cross sections.19 Catalytic Evaluation. The 1-heptyne selective hydrogenation was carried out in a stirred-tank reactor equipped with a magnetically driven stirrer. The stirrer has a special design so as to obtain a good mixing. The inner wall of the reactor was completely coated with

10.1021/ie040187t CCC: $30.25 © 2005 American Chemical Society Published on Web 02/11/2005

Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1753 Table 1. BET Surface Area and Pore Volumes of the Supports sample

SBET (m2 g-1)

Vsm (mL g-1)

Vmicro (mL g-1)

Vmeso (mL g-1)

Vmacro (mL g-1)

GF-45 γ-Al2O3

1718 180

0.498 0.030

0.345 0.048

0.449 0.487

0.400 0.094

PTFE so that the catalytic action of the steel of the reactor could be neglected, as found by other authors.20 The reaction was carried out at 750 rpm using a volume of liquid of 100 mL and 0.8 g of catalyst. The hydrogen pressure in all experiments was 150 kPa; it is well established in the literature3 that high alkene selectivities require low hydrogen pressures. A 5% (v/v) solution of 1-heptyne in toluene was used as the feed. In the catalytic evaluation of the Lindlar catalyst, an adequate mass was suspended in the reactant solution to obtain the same amount of Pd as in Pd/Al2O3 and Pd/C. The possibility of diffusional limitations during the catalytic tests was investigated following procedures previously described.21 Experiments were carried out at different stirring velocities in the range of 180-1400 rpm. The constancy of the activity and selectivity above 500 rpm ensured that external diffusional limitations were absent at the rotary speed selected (750 rpm). On the other hand, to ensure that the catalytic results were not influenced by intraparticle mass-transfer limitations, the catalysts were crushed to one-fourth of the original size of the pellets used as the support. Then, several runs using the crushed catalyst were carried out. In every case, the conversion and selectivity values obtained were the same as those for the catalyst that was not crushed. Hence, it can be accepted that internal diffusional limitations were absent in the operating conditions of this work. Reactant and products were analyzed by gas chromatography using a flame ionization detector and a Chrompack CP WAX 52 CB capillary column. Results and Discussion Table 1 presents the Brunauer-Emmett-Teller (BET) surface area and the supermicro-, micro-, meso-, and macropore volumes, SBET, Vsm, Vmicro, Vmeso, and Vmacro, respectively. These results have been previously reported,14 but they are included here because they are important for the discussion of this paper. It can be observed that the activated carbon includes almost the same amounts of the four type of pores, with a large proportion of pore volume in the range of micro-, supermicro-, and mesopores, the so-called transport pores. γ-Al2O3 is a mesoporous solid having a poor contribution of supermicro-, micro-, and macropores. The possible sulfur content of GF-45 is undetectable by the experimental technique used. This is important because sulfur could have a negative influence on the catalytic performance. Palladium dispersion in Pd/Al2O3 was 28%, and it was 32% in Pd/C after reduction at 573 K. Taking into account the similar dispersion values of the two catalysts, their catalytic behavior was compared using conversions instead of turn over frequencies. Table 2 summarizes the BE and full width at halfmaximum intensity (fwhm) values for the Pd 3d5/2 peak and also the Cl/Pd superficial atomic ratios. The XPS results indicate that the reduction temperature influences the electronic state of Pd on Pd/Al2O3 as well as on Pd/C. According to the literature,22 the Pd 3d5/2 peak

Table 2. XPS Results for Pd/Al2O3 and Pd/C catalyst Pd/Al2O3 Pd/C

reduction temperature (K)

Pd 3d5/2 (eV)

fwhm (eV)

Cl/Pd (at/at)

373 573 373 573

337.0 336.5 337.2 336.8

2.8 2.3 2.9 2.3

0.6 0.2 0.7 0.3

BE for Pd0 is 335.1 eV. However, for Pd/Al2O3 and Pd/ C, the Pd 3d5/2 peak BE appears shifted to higher values, suggesting that these catalysts have different amounts of electron-deficient palladium species (Pdn+) on the surface. The presence of electron-deficient metal species on the surface of reduced catalysts prepared from acid solutions of PdCl2 was reported in the literature many years ago.23,24 In a previous work,25 we verified experimentally the stoichiometric reduction of unsupported PdCl2 at room temperature. In supported catalysts, however, some stable species Pdn+ remain on the surface, even in carefully reduced samples. These oxidized forms of Pd might be palladium chloride or palladium oxide (Pd 3d5/2 BE ) 337.5 or 336.9 eV, respectively22) or a mixture of the two. For Pd/Al2O3, the Pd 3d5/2 peak appears at 337.0 eV for the sample reduced at 373 K and at 336.5 eV for the one reduced at 573 K, thus indicating that the Pdn+/ Pd0 superficial atomic ratio in the catalyst decreases when the reduction temperature increases from 373 to 573 K. For Pd/C, a decrease in the Pd BE (from 337.2 to 336.8 eV) is also observed when the reduction temperature is increased. It must be noted that palladium in Pd/Al2O3 as well as in Pd/C is not present only as Pd0 after the reduction treatments. This can be attributed, considering the preparation conditions, to the influence of chlorine, which is not completely eliminated after the calcination and reduction pretreatments, as can also be observed in Table 2. The Pd 3d5/2 peak BE in the Lindlar catalyst was 337.2 eV, thus indicating that Pd is not completely reduced. The fwhm values, higher than 2.0 eV, are indicative that more than one Pd species might be present in Pd/Al2O3 and Pd/C. Figures 1 and 2 present the results of total conversion and selectivity to 1-heptene as a function of time for the two reduction temperatures for Pd/Al2O3 and Pd/C,

Figure 1. Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd/Al2O3 reduced at two temperatures. Reaction temperature: 303 K. Solid symbols, reduced at 573 K; open symbols, reduced at 373 K. 2, total conversion; [, selectivity to 1-heptene.

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Figure 2. Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd/C reduced at two temperatures. Reaction temperature: 303 K. Solid symbols, reduced at 573 K; open symbols, reduced at 373 K. 2, total conversion; [, selectivity to 1-heptene.

respectively. The reaction temperature was 303 K. It can be noted that the shape of the curves is not completely linear. This can be assigned to the preferential adsorption of the alkyne as compared to hydrogen. Following the Langmuir adsorption isotherm, as the alkyne concentration is decreased, there will be a decrease in the number of sites occupied by the alkyne and an increase in the adsorbed hydrogen concentration. This situation induces an increase in the rate of reaction with time on stream. It can be noted that, for both supports, the total conversion is higher at the highest reduction temperature, whereas the selectivity to 1-heptene is slightly higher when the lower reduction temperature was used. These results can be explained by taking into account the electronic state of Pd, which was more electron-deficient when the lower reduction temperature was used. The electron-deficient Pd species could be less active for the hydrogenation of 1-heptyne, but they probably inhibit the interaction between Pd0 and 1-heptene by an electronic effect, decreasing its electron-donor character. Hence, it can be stated that the presence of electron-deficient Pd species is positive from the point of view of selectivity. It seems likely that the role of chlorine remaining after the heat treatments could be to stabilize the positively charged palladium structures, resulting in less active but more selective catalysts. Therefore, for both supports used in this work, our results suggest a correlation between temperature of reduction, concentration of electron-deficient palladium species, and total conversion and selectivity. Mallat et al.26 also found an increase in selectivity when smaller, more electron-deficient Pd clusters were used. As previously mentioned, Pd in the Lindlar catalyst is also electron-deficient (Pd 3d5/2 peak BE ) 337.2 eV, that is, 2.1 eV higher than that corresponding to Pd0). Studying the Lindlar catalyst, other authors27 found by H/D experiments that modification of palladium with lead acetate favors the interaction of the alkyne with the electron-deficient Pd species. It can also be considered that 1-heptene is more weakly adsorbed than 1-heptyne on electron-deficient Pd species and, once formed, the 1-heptene molecules are more easily desorbed than 1-heptyne. This effect was previously found for other semihydrogenations on Ru supported catalysts.28

Figure 3. Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd/Al2O3 run at two temperatures. Reduction temperature: 573 K. Solid symbols, run at 303 K; open symbols, run at 280 K. 2, total conversion; [, selectivity to 1-heptene.

Figure 4. Total 1-heptyne conversion and selectivity to 1-heptene as a function of time for Pd/C run at two temperatures. Reduction temperature: 573 K. Solid symbols, run at 303 K; open symbols, run at 280 K. 2, total conversion; [, selectivity to 1-heptene.

As previously mentioned, two reaction temperatures were used for each catalyst: 280 and 303 K; the results are shown in Figures 3 and 4 for Pd/Al2O3 and Pd/C, respectively. The reduction temperature was 573 K. For the two catalysts, the total conversion was higher when the reaction was carried out at 303 K. The selectivity did not change in the case of Pd/Al2O3. For Pd/C, a slight difference in selectivity at low conversion values was observed, whereas the selectivities were almost the same at the highest conversion values. Other authors29 have found similar results while studying the semihydrogenation of several alkynes. It is important to compare the supports employed for the catalysts reduced at 573 K and run at 303 K. Analyzing the data presented in Figures 1-4, it can be noted that the better performance, from the point of view of conversion, is achieved when Al2O3 is used as the support, and the selectivity is slightly higher at high conversion values for Pd/C. Although the results mentioned in the previous paragraphs draw attention to the relationship existing between the surface chemical state of palladium and activity and selectivity, upon compari-

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The Lindlar catalyst is very often used for the selective hydrogenation of alkynes. Figure 5 compares the yield of 1-heptene (measured as the conversion to 1-heptene) for this catalyst with those corresponding to Pd/Al2O3 and Pd/C, the last two reduced at 573 K. The reaction temperature was 303 K. The comparison was made running the three catalysts at 303 K, the best temperature found for the catalyst having the highest conversion, i.e., Pd/Al2O3. A slightly better yield can be observed for Pd/Al2O3, which also has the advantage of being a pelletized material. The lowest yield corresponds to Pd/C. Conclusions

Figure 5. Conversion to 1-heptene vs time for the Lindlar catalyst and for Pd/Al2O3 and Pd/C, both reduced at 573 K. Reaction temperature: 303 K. 9, Pd/Al2O3; 4, Lindlar catalyst; b, Pd/C.

son of the two quite different supports, it appears that the chemical state of palladium is not the only factor that determines the catalytic behavior. In fact, in Pd/ Al2O3 and Pd/C reduced at 573 K palladium presents similar dispersions as well as electronic states; however, the two catalysts show different values of total conversion and selectivity at high conversion values. The influence of the support on the physicochemical properties and, therefore, on the catalytic behavior of metals is well-established in the literature.30 Specific support properties such as chemical nature, texture, pore structure, surface state, etc., can indeed modify the morphology and/or localization of the metal particles, electronic structure of the surface metal atoms, adsorption-desorption equilibria of reactants, etc., in different ways whereby different values of conversion and selectivity can arise. Thus, as our results suggest, the conversion and selectivity of palladium supported catalysts is a complex property of the whole catalyst and cannot be related to a single parameter. According to these considerations, the slightly higher selectivity to 1-heptene at the highest conversion values found for Pd/C could be a consequence of a shape selectivity induced by the porous supports. This might be because the 1-heptene molecule has a planar end, unlike the more voluminous end of the fully saturated heptane. If the active Pd species are located in narrow pores (micro and supermicropores), the formation of heptane will be hindered, thus increasing the selectivity to 1-heptene. If this is the case, it could also be suggested that the lower total conversion of Pd/C is due to the narrower porosity of the activated carbon, as it is probable that fewer 1-heptyne molecules could reach the active sites located in the supermicropores. On the other hand, if a significant fraction of the active species are located in pores of a particular size (larger supermicropores, practically absent in Pd/Al2O3, and mesopores), the concentration of 1-heptene in the neighborhood of the Pd species could be enhanced, thus favoring the consecutive hydrogenation of 1-heptene to heptane. Although the surface chemistry of GF-45 is quite unlike that of alumina, the similar dispersions and electronic states of Pd on Pd/Al2O3 and Pd/C reinforce the idea that their different catalytic behaviors are related to the differences in the support porosities.

Pd/Al2O3 and Pd/C are good catalysts for the selective hydrogenation of 1-heptyne under mild reaction conditions, the former showing a better yield. Pd/Al2O3 also presents a better behavior than the classic Lindlar catalyst working at the same operating conditions (temperature, hydrogen pressure, and Pd/substrate ratio). Moreover, the Lindlar catalyst has the disadvantage that it cannot be pelletized and must be operated under slurry conditions; hence, the reactant solution must be purified after reaction by an expensive procedure to recover the catalyst. The reduction and operating temperatures were found to play an important role in the catalytic behavior of the catalysts studied. The XPS results showed that palladium in Pd/Al2O3 as well as in Pd/C is electrondeficient. Within certain limits, the electron-deficient Pd species do not favor the further hydrogenation of 1-heptene to heptane, thus raising the selectivity to 1-heptene. The differences observed in the catalytic behavior of Pd/Al2O3 and Pd/C can be attributed, at least partially, to the differences in the support porosities. Nevertheless, more work is necessary to reach a better understanding about the effect of the support on the catalytic behavior of the catalysts studied. Acknowledgment The experimental assistance of C. Ma´zzaro and the financial assistance of CAI+D (UNL), CONICET, and ANPCyT are greatly acknowledged. Literature Cited (1) L’Argentie`re, P. C.; Cagnola, E. A.; Liprandi, D. A.; Roma´nMartı´nez, M. C.; Salinas-Martı´nez de Lecea, C. Carbon-supported Pd complex as catalyst for cyclohexene hydrogenation. Appl. Catal. A 1998, 172, 41. (2) Ulan, J. G.; Maier, W. F. Mechanism of 2-hexyne hydrogenation on heterogeneous palladium. J. Mol. Catal. A 1989, 54, 243. (3) Lennon, D.; Marshall, R.; Webb, G.; Jackson, S. D. The effect of hydrogen concentration on propyne hydrogenation over a carbon supported palladium catalyst studied under continuous flow conditions. Stud. Surf. Sci. Catal. 2000, 130, 245. (4) Yu, R.; Liu, Q.; Tan, K.-L.; Xu, G.-Q.; Ng, S. C.; Chan, H. S. O.; Andy Hor, T. S. Preparation, characterization and catalytic hydrogenation properties of palladium supported on C60. J. Chem. Soc., Faraday Trans. 1997, 93, 2207. (5) Volpe, M. A.; Rodrı´guez, P.; Gı´gola, C. E. Preparation of PdPb/RAl2O3 catalyts for selective hydrogenation using PbBu4: The role of metal-support boundary atoms and the formation of a stable surface complex. Catal. Lett. 1999, 61, 27. (6) Guczi, L.; Schay, Z.; Stefler, G.; Liotta, L. F.; Deganello, G.; Venezia, A. M. Pumice-supported Cu-Pd catalysts: Influence of copper on the activity and selectivity of palladium in the hydrogenation of phenylacetylene and but-1-ene. J. Catal. 1999, 182, 456.

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Received for review June 23, 2004 Revised manuscript received November 19, 2004 Accepted November 19, 2004 IE040187T