1-Heptyne Selective Hydrogenation over Pd Supported Catalysts

Cecilia R. Lederhos, Pablo C. L’Argentiere,* and Nora S. Fıgoli

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 duringthe 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 theclassic Lindlar catalyst. The reduction and operating temperatures were found to play animportant role in the catalytic behavior of the catalysts studied. The XPS results show thatpalladium in Pd/Al2O3 and Pd/C is electron-deficient. Within certain limits, the electron-deficientPd species do not favor the further hydrogenation of 1-heptene to heptane, thus raising theselectivity to 1-heptene. The differences observed in the catalytic behavior of Pd/Al2O3 and Pd/Ccould be attributed, at least partially, to the differences in the support porosity.

Introduction

The catalytic selective semihydrogenation of acety-lenes using either homogeneous or heterogeneous cata-lysts has been widely studied in the past several years,because of its academic and industrial interest.1 Thehydrogenation of an alkyne can be virtually stopped atthe semihydrogenation stage because the alkyne is morestrongly bound than the alkene and competes effectivelyfor the catalytic sites, thus blocking readsorption of thealkene or displacing it. Many products obtained throughthis kind of reactions are useful in the synthesis ofnatural products, such as biologically active com-pounds.2 One of the most studied catalytic systems forthese kind of reaction is the Lindlar catalyst [Pd/CaCO3modified with Pb(OAc)2], developed in 1953. Othercatalysts, mono- or bimetallic as well as complexes ofseveral transition metals, have also been proposed forthe reaction.2-8 Several materials have been used assupports, and they are usually classified as organic(macroreticular/macroporous polymers) or inorganic(silica, alumina, zeolites, clays) supports.9 Another kindof material, not clearly included in any of these groups,is carbon, whose outstanding properties as a catalystsupport are well recognized.10 It is known that carbonoffers very interesting properties as a catalystsupport.11-13 Among them, are the possibility of modify-ing the specific surface area, porosity, and surfacechemistry; moreover, carbon supports present the ad-vantage of being inert in liquid reaction media.

Little information is available in the literature re-garding, in particular, the selective hydrogenation of1-heptyne carried out in heterogeneous conditions. Theobjective of this paper is to study the influence of thereduction and reaction temperatures and of the supportin the selective hydrogenation of 1-heptyne using Pdcatalysts supported on γ-Al2O3 and on an activatedcarbon.

Experimental Section

Catalyst Preparation. Al2O3 (Ketjen CK 300, cyl-inders of 1.5-mm diameter) and a pelletized commercial

carbon (GF-45, from NORIT) were used as supports.Both materials were impregnated by the incipientwetness technique using PdCl2 solutions of concentra-tions sufficient to obtain 5% Pd on the final catalysts.Pd/Al2O3 was then calcined at 773 K, whereas Pd/C wascalcined at 373 K. Both catalysts were reduced under ahydrogen stream at 373 and 573 K to study theinfluence of the reduction temperature on the catalyticbehavior. For comparative purposes, a commercialLindlar catalyst provided by Aldrich (catalog number20 503-6, 5 wt % Pd on calcium carbonate, poisoned withlead) was used.

Catalysts Characterization. Physical adsorption ofgases (N2 at 77 K and CO2 at 273 K) and mercuryporosimetry were used to analyze the porous texturesof the two supports.1 Elemental analysis was used toassess the organic sulfur in GF-45.14

Palladium dispersion was measured by hydrogenchemisorption in a volumetric apparatus at 373 K.Determinations were made using the method of thedouble isotherm proposed by Benson et al.15

The electronic state of Pd was studied by X-rayphotoelectron spectroscopy (XPS), following the Pd 3d5/2peak binding energy (BE). Determinations were carriedout on a Shimadzu ESCA 750 electron spectrometercoupled to a Shimadzu ESCAPAC 760 Data System. Tocorrect possible deviations caused by electric chargingof the samples, the C 1s line was taken as an internalstandard at 285.0 eV, as previously described.16 Thesamples were introduced into the XPS sample holderfollowing the operating procedure described by otherauthors17 to ensure that there was no modification ofthe electronic states of the species analyzed.18 Regard-less, exposing the samples to the atmosphere for dif-ferent periods confirmed that there were no electronicmodifications. Cl/Pd surface atomic ratios on Pd/Al2O3and Pd/C were determined by comparing the areasunder the peaks after background subtraction andcorrections due to differences in escape depths and inphotoionization cross sections.19

Catalytic Evaluation. The 1-heptyne selective hy-drogenation was carried out in a stirred-tank reactorequipped with a magnetically driven stirrer. The stirrerhas a special design so as to obtain a good mixing. Theinner wall of the reactor was completely coated with

* To whom correspondence should be addressed. E-mail:plargent@fiqus.unl.edu.ar.

1752 Ind. Eng. Chem. Res. 2005, 44, 1752-1756

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

PTFE so that the catalytic action of the steel of thereactor could be neglected, as found by other authors.20

The reaction was carried out at 750 rpm using a volumeof liquid of 100 mL and 0.8 g of catalyst. The hydrogenpressure in all experiments was 150 kPa; it is wellestablished in the literature3 that high alkene selectivi-ties require low hydrogen pressures. A 5% (v/v) solutionof 1-heptyne in toluene was used as the feed. In thecatalytic evaluation of the Lindlar catalyst, an adequatemass was suspended in the reactant solution to obtainthe same amount of Pd as in Pd/Al2O3 and Pd/C.

The possibility of diffusional limitations during thecatalytic tests was investigated following procedurespreviously described.21 Experiments were carried out atdifferent stirring velocities in the range of 180-1400rpm. The constancy of the activity and selectivity above500 rpm ensured that external diffusional limitationswere absent at the rotary speed selected (750 rpm). Onthe other hand, to ensure that the catalytic results werenot influenced by intraparticle mass-transfer limita-tions, the catalysts were crushed to one-fourth of theoriginal 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 valuesobtained were the same as those for the catalyst thatwas not crushed. Hence, it can be accepted that internaldiffusional limitations were absent in the operatingconditions of this work.

Reactant and products were analyzed by gas chro-matography using a flame ionization detector and aChrompack CP WAX 52 CB capillary column.

Results and Discussion

Table 1 presents the Brunauer-Emmett-Teller (BET)surface area and the supermicro-, micro-, meso-, andmacropore volumes, SBET, Vsm, Vmicro, Vmeso, and Vmacro,respectively. These results have been previously re-ported,14 but they are included here because they areimportant for the discussion of this paper. It can beobserved that the activated carbon includes almost thesame amounts of the four type of pores, with a largeproportion of pore volume in the range of micro-,supermicro-, and mesopores, the so-called transportpores. γ-Al2O3 is a mesoporous solid having a poorcontribution of supermicro-, micro-, and macropores.

The possible sulfur content of GF-45 is undetectableby the experimental technique used. This is importantbecause sulfur could have a negative influence on thecatalytic performance.

Palladium dispersion in Pd/Al2O3 was 28%, and it was32% in Pd/C after reduction at 573 K. Taking intoaccount the similar dispersion values of the two cata-lysts, their catalytic behavior was compared usingconversions instead of turn over frequencies.

Table 2 summarizes the BE and full width at half-maximum intensity (fwhm) values for the Pd 3d5/2 peakand also the Cl/Pd superficial atomic ratios. The XPSresults indicate that the reduction temperature influ-ences the electronic state of Pd on Pd/Al2O3 as well ason Pd/C. According to the literature,22 the Pd 3d5/2 peak

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 amountsof electron-deficient palladium species (Pdn+) on thesurface. The presence of electron-deficient metal specieson the surface of reduced catalysts prepared from acidsolutions of PdCl2 was reported in the literature manyyears ago.23,24 In a previous work,25 we verified experi-mentally the stoichiometric reduction of unsupportedPdCl2 at room temperature. In supported catalysts,however, some stable species Pdn+ remain on thesurface, even in carefully reduced samples. Theseoxidized forms of Pd might be palladium chloride orpalladium 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 eVfor the sample reduced at 373 K and at 336.5 eV forthe one reduced at 573 K, thus indicating that the Pdn+/Pd0 superficial atomic ratio in the catalyst decreaseswhen the reduction temperature increases from 373 to573 K. For Pd/C, a decrease in the Pd BE (from 337.2to 336.8 eV) is also observed when the reductiontemperature is increased. It must be noted that pal-ladium in Pd/Al2O3 as well as in Pd/C is not presentonly as Pd0 after the reduction treatments. This can beattributed, considering the preparation conditions, tothe influence of chlorine, which is not completelyeliminated after the calcination and reduction pretreat-ments, as can also be observed in Table 2. The Pd 3d5/2peak BE in the Lindlar catalyst was 337.2 eV, thusindicating that Pd is not completely reduced. The fwhmvalues, higher than 2.0 eV, are indicative that morethan one Pd species might be present in Pd/Al2O3 andPd/C.

Figures 1 and 2 present the results of total conversionand selectivity to 1-heptene as a function of time for thetwo reduction temperatures for Pd/Al2O3 and Pd/C,

Table 1. BET Surface Area and Pore Volumes of theSupports

sampleSBET

(m2 g-1)Vsm

(mL g-1)Vmicro

(mL g-1)Vmeso

(mL g-1)Vmacro

(mL g-1)

GF-45 1718 0.498 0.345 0.449 0.400γ-Al2O3 180 0.030 0.048 0.487 0.094

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

catalystreduction temperature

(K)Pd 3d5/2

(eV)fwhm(eV)

Cl/Pd(at/at)

Pd/Al2O3 373 337.0 2.8 0.6573 336.5 2.3 0.2

Pd/C 373 337.2 2.9 0.7573 336.8 2.3 0.3

Figure 1. Total 1-heptyne conversion and selectivity to 1-hepteneas 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; [, selectivityto 1-heptene.

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respectively. The reaction temperature was 303 K. Itcan be noted that the shape of the curves is notcompletely linear. This can be assigned to the prefer-ential adsorption of the alkyne as compared to hydrogen.Following the Langmuir adsorption isotherm, as thealkyne concentration is decreased, there will be adecrease in the number of sites occupied by the alkyneand an increase in the adsorbed hydrogen concentration.This situation induces an increase in the rate of reactionwith time on stream. It can be noted that, for bothsupports, the total conversion is higher at the highestreduction temperature, whereas the selectivity to 1-hep-tene is slightly higher when the lower reduction tem-perature was used. These results can be explained bytaking into account the electronic state of Pd, which wasmore electron-deficient when the lower reduction tem-perature was used. The electron-deficient Pd speciescould 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 itselectron-donor character. Hence, it can be stated thatthe presence of electron-deficient Pd species is positivefrom the point of view of selectivity. It seems likely thatthe role of chlorine remaining after the heat treatmentscould be to stabilize the positively charged palladiumstructures, resulting in less active but more selectivecatalysts. Therefore, for both supports used in this work,our results suggest a correlation between temperatureof reduction, concentration of electron-deficient pal-ladium species, and total conversion and selectivity.Mallat et al.26 also found an increase in selectivity whensmaller, more electron-deficient Pd clusters were used.As previously mentioned, Pd in the Lindlar catalyst isalso 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 byH/D experiments that modification of palladium withlead acetate favors the interaction of the alkyne withthe electron-deficient Pd species. It can also be consid-ered that 1-heptene is more weakly adsorbed than1-heptyne on electron-deficient Pd species and, onceformed, the 1-heptene molecules are more easily de-sorbed than 1-heptyne. This effect was previously foundfor other semihydrogenations on Ru supported cata-lysts.28

As previously mentioned, two reaction temperatureswere used for each catalyst: 280 and 303 K; the resultsare shown in Figures 3 and 4 for Pd/Al2O3 and Pd/C,respectively. The reduction temperature was 573 K. Forthe two catalysts, the total conversion was higher whenthe reaction was carried out at 303 K. The selectivitydid not change in the case of Pd/Al2O3. For Pd/C, a slightdifference in selectivity at low conversion values wasobserved, whereas the selectivities were almost thesame at the highest conversion values. Other authors29

have found similar results while studying the semi-hydrogenation of several alkynes.

It is important to compare the supports employed forthe catalysts reduced at 573 K and run at 303 K.Analyzing the data presented in Figures 1-4, it can benoted that the better performance, from the point ofview of conversion, is achieved when Al2O3 is used asthe support, and the selectivity is slightly higher at highconversion values for Pd/C. Although the results men-tioned in the previous paragraphs draw attention to therelationship existing between the surface chemical stateof palladium and activity and selectivity, upon compari-

Figure 2. Total 1-heptyne conversion and selectivity to 1-hepteneas 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; [, selectivityto 1-heptene.

Figure 3. Total 1-heptyne conversion and selectivity to 1-hepteneas a function of time for Pd/Al2O3 run at two temperatures.Reduction temperature: 573 K. Solid symbols, run at 303 K; opensymbols, run at 280 K. 2, total conversion; [, selectivity to1-heptene.

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

1754 Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005

son of the two quite different supports, it appears thatthe chemical state of palladium is not the only factorthat determines the catalytic behavior. In fact, in Pd/Al2O3 and Pd/C reduced at 573 K palladium presentssimilar dispersions as well as electronic states; however,the two catalysts show different values of total conver-sion and selectivity at high conversion values.

The influence of the support on the physicochemicalproperties and, therefore, on the catalytic behavior ofmetals is well-established in the literature.30 Specificsupport properties such as chemical nature, texture,pore structure, surface state, etc., can indeed modify themorphology and/or localization of the metal particles,electronic structure of the surface metal atoms, adsorp-tion-desorption equilibria of reactants, etc., in differentways whereby different values of conversion and selec-tivity can arise. Thus, as our results suggest, theconversion and selectivity of palladium supported cata-lysts is a complex property of the whole catalyst andcannot be related to a single parameter.

According to these considerations, the slightly higherselectivity to 1-heptene at the highest conversion valuesfound for Pd/C could be a consequence of a shapeselectivity induced by the porous supports. This mightbe because the 1-heptene molecule has a planar end,unlike the more voluminous end of the fully saturatedheptane. If the active Pd species are located in narrowpores (micro and supermicropores), the formation ofheptane will be hindered, thus increasing the selectivityto 1-heptene. If this is the case, it could also besuggested that the lower total conversion of Pd/C is dueto the narrower porosity of the activated carbon, as itis probable that fewer 1-heptyne molecules could reachthe active sites located in the supermicropores. On theother hand, if a significant fraction of the active speciesare located in pores of a particular size (larger super-micropores, practically absent in Pd/Al2O3, and meso-pores), the concentration of 1-heptene in the neighbor-hood of the Pd species could be enhanced, thus favoringthe consecutive hydrogenation of 1-heptene to heptane.

Although the surface chemistry of GF-45 is quiteunlike that of alumina, the similar dispersions andelectronic states of Pd on Pd/Al2O3 and Pd/C reinforcethe idea that their different catalytic behaviors arerelated to the differences in the support porosities.

The Lindlar catalyst is very often used for theselective hydrogenation of alkynes. Figure 5 comparesthe yield of 1-heptene (measured as the conversion to1-heptene) for this catalyst with those corresponding toPd/Al2O3 and Pd/C, the last two reduced at 573 K. Thereaction temperature was 303 K. The comparison wasmade running the three catalysts at 303 K, the besttemperature found for the catalyst having the highestconversion, i.e., Pd/Al2O3. A slightly better yield can beobserved for Pd/Al2O3, which also has the advantage ofbeing a pelletized material. The lowest yield correspondsto Pd/C.

Conclusions

Pd/Al2O3 and Pd/C are good catalysts for the selectivehydrogenation of 1-heptyne under mild reaction condi-tions, the former showing a better yield. Pd/Al2O3 alsopresents a better behavior than the classic Lindlarcatalyst working at the same operating conditions(temperature, hydrogen pressure, and Pd/substrateratio). Moreover, the Lindlar catalyst has the disadvan-tage that it cannot be pelletized and must be operatedunder slurry conditions; hence, the reactant solutionmust be purified after reaction by an expensive proce-dure to recover the catalyst.

The reduction and operating temperatures were foundto play an important role in the catalytic behavior ofthe catalysts studied. The XPS results showed thatpalladium in Pd/Al2O3 as well as in Pd/C is electron-deficient. Within certain limits, the electron-deficientPd species do not favor the further hydrogenation of1-heptene to heptane, thus raising the selectivity to1-heptene.

The differences observed in the catalytic behavior ofPd/Al2O3 and Pd/C can be attributed, at least partially,to the differences in the support porosities. Neverthe-less, more work is necessary to reach a better under-standing about the effect of the support on the catalyticbehavior of the catalysts studied.

Acknowledgment

The experimental assistance of C. Mazzaro and thefinancial assistance of CAI+D (UNL), CONICET, andANPCyT are greatly acknowledged.

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

Accepted November 19, 2004

IE040187T

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