Surface Characterization of Supported Pd Catalysts Activated with

© Copyright 1999 American Chemical Society

AUGUST 3, 1999 VOLUME 15, NUMBER 16

Letters Surface Characterization of Supported Pd Catalysts Activated with Chiral Hydrogen Donors Marı´a A. Aramendı´a, Yolanda Avile´s, Victoriano Borau, Ce´sar Jime´nez,* Jose´ M. Marinas, and Jose´ R. Ruiz Departamento de Quı´mica Orga´ nica, Facultad de Ciencias, Universidad de Co´ rdoba, Avda San Alberto Magno s/n, E-14004 Co´ rdoba, Spain Received January 19, 1999. In Final Form: May 11, 1999 The activation of SiO2-supported Pd solids with chiral hydrogen donors such as (R)-(+)- or (S)-(-)limonene produces catalysts with a special feature, namely, the presence of metal-bound limonene species on the surface. A surface complex between limonene and Pd2+ ions is postulated from TPD/TCD, TPD/MS, XPS, and 13C CP/MAS NMR data. On the basis of the results, the configuration of the stereosite in the hydrogen donor is not altered as the surface-binding interaction does not involve the asymmetric site in limonene.

Introduction Some cyclic alkanes and alkenes have for several decades been known to be able to become aromatic by releasing hydrogen from their molecules. Thus, cyclohexene becomes benzene in the presence of Pt, Pd, Ru, or Rh1,2 at a low pressure. Hydrogen-transfer catalytic reduction is a powerful method for reducing functional groups3 and has been widely used to reduce compounds bearing carbonyl and imine groups to the corresponding alcohols and imines. Brieger and Nestrick published a comprehensive review of this topic.4 In hydrogen-transfer hydrogenations, the donor and acceptor (both adsorbed on the catalyst) exchange hydrogen. The most simple instance of this type of process is the disproportionation of cyclohexene, where one reactant molecule transfers a hydrogen molecule to another. The resulting products are benzene and cyclohexane.2 Carra´ et al.5 postulated a (1) Aramendı´a, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Santano, C.; Sempere, M. E. J. Mol. Catal. 1992, 72, 221. (2) Aramendı´a, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Sempere, M. E. J. Catal. 1987, 108, 487. (3) Nickon, A.; Kwasnik, H. R.; Mathew, C. T.; Swartz, T. D.; Willians, R. O.; DiGiorgio, J. B. J. Org. Chem. 1978, 43, 3904. (4) Brieger, G.; Nestrick, T. J. Chem. Rev. 1974, 74, 567. (5) Carra´, S.; Beltrame, P.; Racaini, V. J. Catal. 1964, 3, 353.

mechanism involving four palladium atoms for this process. Most of the catalysts used in transfer processes are previously activated in a hydrogen stream to reduce the precursor metal salt. The shape and structure of the crystallites formed depend on the particular synthesis conditions. Recently, various techniques have been used to locate organic molecules on catalytic solids. Prominent among such techniques are those based on adsorption isotherms and calorimetric measurements,6,7 which provide information about the thermodynamics of adsorbate-host interactions. Diffraction data furnish information concerning the site location of adsorbate molecules and the changes in lattice structure induced by both temperature and the presence of an organic adsorbate.8,9 Changes in host structure may also be observed by solid-state NMR analysis. Specifically, 13C CP/MAS NMR spectroscopy has lately been used to study various saturated and unsatur(6) Richards, E. E.; Rees, L. V. C. Zeolites 1998, 8, 35. (7) Talu, O.; Guo, C. J.; Hayhurst, D. T. J. Phys. Chem. 1989, 93, 7294. (8) Mentzen, B. F. Mater. Res. Bull. 1992, 27, 831. (9) Sacerdote, M.; Bossdot, F.; Mentzen, B. F. C. R. Acad. Sci. (Paris) 1991, 312 (II), 1513.

10.1021/la990053y CCC: $18.00 © 1999 American Chemical Society Published on Web 07/09/1999

5184 Langmuir, Vol. 15, No. 16, 1999 Scheme 1. Reduction of Pd2+ with Limonene as the Hydrogen Donor

Letters Table 1. Experimental Parameter Values Used in the XPS Experiments and Pd Composition of the Catalysts catalyst

ated hydrocarbons adsorbed on different supports,10-12 including supported metal catalysts.13,14 In this work, several previously unreported catalysts obtained by activation with the same substance subsequently employed as a donor in the hydrogen transfer were studied. The catalysts were characterized by using various instrumental techniques including adsorption isotherms, X-ray photoelectron spectroscopy, solid-state nuclear magnetic resonance, and temperature-programmed desorption coupled to mass spectrometry. Experimental Section Preparation of Catalysts. The new catalysts were prepared on a support consisting of commercially available silica (Merck reference no. 7734) that was calcined at 650 °C for 3 h and called S. Palladium was deposited onto it by impregnation in a Pd(NO3)2‚2H2O solution in N,N-dimethylformamide. The amount of precursor salt used was that required to obtain a Pd content of 3 wt % in the final catalyst. The solution was supplied with the support and stirred for 24 h, after which it was evaporated to dryness and calcined at 300 °C. The catalyst was activated by two different procedures, namely, with the following: (a) With gaseous dihydrogen: The previously deposited metal was reduced in a dihydrogen stream at 300 °C at a flow rate of 20 mL/min for 1 h. Then, a nitrogen stream was passed at room temperature at 20 mL/min for 1 h to stabilize the solid. The catalyst thus obtained was called PdS. (b) With (R)-(+)- or (S)-(-)-limonene: An amount of 1 g of silica containing the deposited metal was refluxed with 18 mL of (R)-(+)- or (S)-(-)-limonene at 176 °C for 1 h. Under these conditions, limonene was converted into p-cymene and the hydrogen released reduced impregnated Pd2+ to Pd0 (see Scheme 1). After the mixture was cooled, the solid was filtered off and washed with cyclohexane and methanol. The catalysts thus obtained were labeled PdS-(+)-L and PdS-(-)-L. Chemical Textural Properties. The specific surface area (SBET), pore volume (Vp), and mean pore diameter (dp) of the support and catalysts were determined by using the BET method15 on a Micromeritics ASAP 2000 analyzer. Acid and basic sites were characterized from the retention isotherms obtained with cyclohexylamine and phenol, respectively, both dissolved in cyclohexane, as described elsewhere.16 TPD-TCD Experiments. Temperature-programmed desorption experiments were carried out on a Micromeritics TPD/ TPR 2900 instrument. Argon at a flow rate of 50 mL/min was used as both the analysis and reference gas. An amount of 100 mg of sample was placed in a U-shaped reactor that was introduced into an oven equipped with temperature programming. A cold trap was placed at the oven outlet to retain water in the catalyst and prevent it from reaching the detector. Prior to the tests, the catalysts were flushed with argon at room temperature at 50 mL/min until a stable baseline was obtained. (10) Su, B.-L.; Norberg, V. Langmuir 1998, 27, 831. (11) Nicholas, J. B.; Kheir, A. A.; Xu, T.; Krawietz, T. R.; Haw, J. F. J. Am. Chem. Soc. 1998, 120, 10471. (12) Ebner, M.; Franke, V.; Gu¨nter, H. Fresenius J. Anal. 1997, 357, 505. (13) Grifftis, J. M.; Bell, A. T.; Reimer, J. A. J. Phys. Chem. 1994, 98, 1918. (14) Ivanova, I. I.; Pasau-Claerbout, A.; Seirvert, M.; Blom, N.; Derouane, E. G. J. Catal. 1996, 158, 521. (15) Brunauer, S.; Emmet, P. H.; Teller, E. J. J. Am. Chem. Soc. 1938, 60, 309. (16) Aramendı´a, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Rodero, F. Colloids Surf. 1984, 12, 227.

transition

PdS

Pd3d(5/2)

PdS-(+)-L

Pd3d(5/2)

PdS-(-)-L

Pd3d(5/2)

BEa (eV) 335.0 337.4 335.1 337.3 335.1 337.4

FWHMb

Pdc

2.1 1.9 2.0 2.4 2.0 2.4

Pd2+:0.07 Pd0:0.18 Pd2+:0.14 Pd0:0.56 Pd2+:0.15 Pd0:0.58

a Binding energy. b Full width at half-height. c Atomic percentages.

The thermal desorption process was conducted from room temperature to 600 °C, at a heating rate of 10 °C/min. Desorbed gases were monitored in a continuous manner, using a thermal conductivity detector. TPD-MS Experiments. Tests were carried out in an assembly similar to the previous one. The oven outlet was connected to the mass spectrometer probe to collect samples in a continuous fashion. The detector used was a VG Sensorlab quadrupolar mass spectrometer from Fisons Instruments. The carrier gas, nitrogen, was circulated at a flow rate of 50 mL/min. Prior to each test, the catalyst was flushed with the same gas at room temperature until a stable baseline was obtained. The temperature was raised from room level to 600 °C at a heating rate of 10 °C/min. The spectrometer was programmed to perform 6 scans/min, and a Faraday detector (detection limit 10-10 mbar) was used throughout. XPS Measurements. X-ray photoelectron spectroscopy (XPS) measurements were made on an Escalab 210 spectrophotometer. Pellets 13 mm in diameter were obtained by pressing at a low pressure. Because of the sample dimensions, experiments were conducted in the large-area XPS (LAXPS) mode. For this purpose, a double-anode X-ray gun was used at an average power of 100 W (10 kV × 10 mA). A vacuum in the main chamber was always better than 6 × 10-9 mbar. For each sample, data were accumulated in separate regions, using relatively long acquisition times. To achieve a high enough signal-to-noise ratio, different numbers of spectra, depending on the particular energy range, were accumulated and averaged. Spectral parameters (BE and fwhm) were determined by using the program Peakfit, included in the instrument’s bundled software. Depending on the particular energy interval, the estimated precision in the spectra ranged from 0.10 to 0.15 eV. Table 1 shows the experimental parameters examined and the proportion of Pd in the different oxidation states found. NMR Spectra. 1H, 29Si, and 13C NMR solid-state spectra were recorded at 400.13, 79.49, and 100.62 MHz, respectively, on a Bruker ACP-400 spectrometer. Samples were spun at the magic angle at 3.5 kHz. All measurements were made at room temperature. 1H MAS NMR spectra were obtained by using an excitation pulse of π/2 (5 ms) and a recycle time of 10 sslonger delay times had little effect on the relative intensities of the proton resonances. Overall, 1000 free induction decays were accumulated. Cross polarization (CP) was used to obtain 29Si and 13C spectra. For 29Si and 13C CP/MAS NMR experiments, the contact time for the transfer of magnetization between protons and 29Si and 13C were 6 and 5 ms (1000 and 15000 scans), respectively. Tetramethylsilane was used as a standard reference in all instances. Prior to measurement, samples were dehydrated by evacuation to a pressure below 10-6 mbar on a Micromeritics ASAP-2000 instrument at 120 °C overnight. Samples were then transferred, in a nitrogen atmosphere, to a moisture-free nitrogen glovebox and used to fill zirconia rotors. 1H MAS NMR spectra for a PdS sample were recorded immediately upon synthesis and a few days after being transferred to the rotor. The two spectra were identical and rather different from those for samples exposed to moisture, so the risk of water penetrating into the samples was excluded. The 1H MAS NMR background resonance from the probe itself, identified by recording the spectrum for an empty rotor, consisted of a broad, very weak resonancesapparently a static resonance. All 1H MAS NMR spectra were corrected for this background resonance by subtracting the empty rotor spectrum.

Letters

Langmuir, Vol. 15, No. 16, 1999 5185

Table 2. Chemical Textural Properties of the Catalysts Studied catalyst

SBET (m2/g)

Vp (mL/g)

dp (A)

Xma

Xmb

S PdS PdS-(+)-L PdS-(-)-L

384 375 364 348

0.69 0.65 0.66 0.61

53 51 49 51

69.7 72.1 41.9 36.8

2.7

a Acidity against cyclohexylamine (105 × mol/g). b Basicity against phenol (105 × mol/g).

Results and Discussion Table 2 summarizes the textural properties of the support and catalysts. No significant differences between the isotherms for the metal catalysts and the support were observed. Also, the pore diameter was not influenced by the choice of the reduction procedure. Metal deposition decreased both the specific surface area and the cumulative pore volume, albeit only slightly. Table 2 also summarizes the surface chemical properties of the support and catalysts. The metal-containing systems exhibited little basicity, consistent with the low basic character of the starting support. On one hand, deposition of palladium on the support had little effect on surface acidity relative to the support when the system was activated in a hydrogen stream. On the other hand, activation with (R)-(+)- or (S)-(-)-limonene decreased the original surface acidity. This suggests that the catalyst retains some form of limonene on its surface that alters its acidity relative to the original support. To obtain a deeper knowledge of these catalysts and confirm whether they contained any organic species on their surfaces, they were analyzed by using additional instrumental techniques. One such technique was thermal-programmed desorption (TPD) with thermal conductivity detection (TCD). Following routine calibration of the apparatus and cleanup of the catalyst, a profile containing several broad bands over the temperature range 300-600 °C was obtained (Figure 1a). Prior to activation with limonene, this catalyst had been calcined at 300 °C for 1 h because Pd(NO3)2 decomposes into palladium oxide and nitrous vapors at this temperature. The graph obtained exhibited signals above 300 °C that suggested the presence of physisorbed or chemisorbed forms of surface-nitrated palladium species which might have resisted calcination by virtue of the peculiar stabilizing effect of the surface. This synergetic phenomenon is known to occur in the decomposition of some hydroxides, carbonates, and sulfates, the decomposition temperature for which is altered by their mixing with or deposition onto other species.17 To identify the signals given by adsorbed organic matter, the impregnated solid was calcined at 650 °C instead of 300 °C. In this way, all of the nitrate would be thoroughly decomposed and any signal in the TPD recordings would be a result of the limonene activation treatment. After calcination, the system was subjected to the abovedescribed treatment in refluxing limonene. For comparison, a separate portion of the catalyst calcined at 650 °C was activated with dihydrogen; this sample exhibited no desorption bands between 25 and 600 °C (Figure 1b), so it contained no surface nitrate. Figure 1c shows the TPD recording for the limonene-activated system, which includes a strong signal at 515 °C and a weaker one at 583 °C; as confirmed by fitting a mass detector to the output (17) Domrachev, G. A.; Zhuk, B. V.; Karevin, B. S.; Neisterov, B. A.; Semenov, N. M.; Suvorova, O. N.; Obiedrov, A. M.; Khamylov, U. K. Organometal Compound to Prepare Metal and Oxide Coverages, Proceedings of the 2nd All. Union Conference, Gorki, 1977; p 21.

Figure 1. TPD-TCD profiles for the following catalysts: (a) PdS-(+)-L; (b) PdS calcined at 650 °C prior to activation with dihydrogen; (c) PdS-(+)-L calcined at 650 °C prior to activation with (R)-(+)-limonene.

Figure 2. TPD-MS profile for the catalyst PdS-(+)-L.

of the TPD instrument, these two signals corresponded to H2 and 4-methyl-R-methylstyrene (m/z 132). The TPDMS profile for the catalyst PdS-(+)-L is shown in Figure 2, which was obtained by monitoring the previous two signals. As can be seen, this TPD-MS profile is very similar to the TPD-TCD recording (Figure 1c). The data provided by these techniques suggest the presence of some limonene derivative, strongly adsorbed on the surface,

5186 Langmuir, Vol. 15, No. 16, 1999

that is aromatized and eventually releases the previous two products when it reaches this temperature region. Although both signals pertain to the same reaction, they appear separately in the TPD-TCD and TPD-MS profiles. Consequently, the organic residue must remain on the surfaceseven after it is aromatizeds, bound to it via its exocyclic double bond, which does not take part in the decomposition of the surface complex. All this suggests the formation of a surface complex between the catalyst and limonene via their double bonds. As shown later on by the NMR experiments, a complex is formed on the catalyst surface that somehow involves the palladium. The chemical behavior of palladium toward allyl compounds basically involves the formation of stable Pd2+ complexes. These π-allyl complexes are more stable than σ-allyl complexes because the π electrons of the allyl group can be delocalized over four sites, even though the π-allyl/Pd2+ system is in dynamic equilibrium with the σ-allyl/Pd2+ one. These systems can occur as reaction intermediates for conjugate dienes and vinyl compounds, and also as crystalline, air-stable, readily isolated and characterized solids. Thus, there are reported instances of the formation of organopalladic complexes from olefins with structures similar to that of limonene (e.g., 1-methylcyclohexene18 and carvone19). Discriminating between Pd2+ and Pd4+ by this technique is rather difficult, owing to the similar bonding energies of both ions. Our energy values were between those for the two ions, so they can in principle be ascribed to palladium atoms in a +2 or +4 oxidation state. On one hand, however, reported data for palladium complexes18 led us to ascribe the abovementioned signals to Pd2+. On the other hand, the reductive atmosphere involved in the limonene treatment was seemingly incompatible with the presence of residual palladium in a high oxidation state. From the XPS results, it follows that Pd2+ is incompletely reduced and that the metal is present as Pd0 and, to a lesser extent, Pd 2+, both in the catalyst activated with dihydrogen and in those activated with limonene. The latter type of catalyst contains a higher proportion of Pd2+ because these surface Pd2+ atoms may form surface complexes with limonene. Table 1 shows the results obtained, the most salient of which are the Pd0 and Pd2+ contents in both systems. On the basis of them, the Pd0/Pd2+ ratios in the solids PdS, PdS-(+)-L, and PdS-(-)-L are 96/4, 80/20, and 80/20, respectively. To confirm the above hypotheses, MAS and CP/MAS NMR experiments were performed with a view to elucidating the structure of the palladium surface complex. The 1H MAS and 29Si CP/MAS spectra thus obtained for the support were similar to those previously reported for this type of solid.20-22 On one hand, the 29Si CP/MAS NMR spectra for the support and catalyst exhibited no appreciable differences, whether the latter was activated with dihydrogen or limonene (Figure 3a). This seemingly confirms that Si surface sites are not altered by the treatment. On the other hand, the 1H MAS NMR spectra were quite different (Figure 3b). While the spectrum for the dihydrogenactivated catalyst was similar to that for the support, the spectrum for the limonene-activated catalysts exhibited (18) Trost, B. M.; Metzner, P. J. J. Am. Chem. Soc. 1980, 102, 3572. (19) Trost, B. M.; Strege, P. E.; Weber, L.; Fullerton, T. J.; Dietsche, F. J. J. Am. Chem. Soc. 1978, 100, 3407. (20) Mastikhin, V. M.; Mudrkovsky, I. L.; Nosov, V. V. Prog. Nucl. Magn. Reson. Spectrosc. 1991, 23, 211. (21) Maciel, G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606. (22) Aramendı´a, M. A.; Borau, V.; Jime´nez, C.; Marinas, J. M.; Ruiz, J. R. Solid State NMR 1997, 8, 251.

Letters

Figure 3. NMR spectra for catalysts PdS and PdS-(+)-L: (a) 29Si CP/MAS; (b) 1H MAS; (c) 13C CP/MAS.

overlapped signals that were inconclusive except for the facts that they were concentrated in the alkyl proton region and that no signals for olefin or aromatic protons were observed. This means that the chemisorbed compound can only consist of an aliphatic portion and that the double bonds are involved in the adsorption. To complete the TPD experiments, the 13C CP/MAS NMR spectrum for the catalyst was recorded (Figure 3c). Organometallic compounds of Pd have scarcely been studied by 13C CP/MAS NMR spectroscopy; in fact, there are only a few reported 1H NMR spectra for dissolved organopalladiums that exhibit strong shielding for metalbound protons.23 The 13C CP/MAS NMR spectrum affords a deeper, more precise study of chemical shifts in the signals. A comparison of the 13C spectrum for dissolved24 and chemisorbed limonene reveals that the signals at 121.5, 133.6, 101.2, and 144.9 ppm in the former, ascribed to the four unsaturated carbon atoms, are markedly shifted upfield in the latter. This strong shielding effect must be similar to that of metal-bound protons,23 so one can assume the four unsaturated carbons in limonene to be involved in the formation of bonds with the metal since their weakening also shields the carbons and shifts their signals as far as those for aliphatic carbons. Consequently, the metal surface contains structures where Pd2+, a part of a surface palladium oxide that results from the decom(23) Gu¨nther, H. NMR Spectroscopy; John Wiley & Sons: London, 1995; p 99. (24) Jautelat, M.; Grutzner, J. B.; Roberts, J. D. Proc. Nat. Acad. Sci. 1970, 65, 88.

Letters Scheme 2. Pd2+ Complex Hypothetically Present on the Surface of Limonene-Activated Catalysts

position of impregnated palladium nitrate, is bonded to allyl species resulting from the limonene molecule via the original double bonds (Scheme 2). Heating above 500 °C causes these surface compounds to decompose and produces hydrogen and 4-methyl-R-methylstyrene. In a similar process, Romrachev et al.25 obtained SiO2-supported Pd films by thermal decomposition of Pd2+ allyl complexes of the type [AllPdCl]2 above 240 °C. The type (25) Romrachev, G. A.; Varyukhin, V. A.; Nesterov, B. A. React. Kinet. Catal. Lett. 1983, 22, 281.

Langmuir, Vol. 15, No. 16, 1999 5187

of metal crystallite obtained depends on the structure of the complex ligand.26 Further research on the structure of these complexes, which are strongly bound to the supporting surface and stable at high temperatures, is required. Other olefins might also form them under similar conditions. Should the surface-bound ligand preserve its chiralitysthe stereosite is not affected during the time of binding to the surfaces, which remains to be confirmed, specially modified catalysts for use in enantioselective hydrogenation processes could be prepared. There are several wellknown acid and basic modifiers for catalyst surfaces (e.g., tartaric acid and alkaloids, respectively); however, there are few references to neutral modifiers such as those studied in this work, which will be the subject of further research soon. Acknowledgment. We express our gratitude to Spain’s Direccio´n General de Ensen˜anza Superior e Investigacio´n del Ministerio de Educacio´n y Cultura (Project PB97-0446) and to the Consejerı´a de Educacio´n y Ciencia de la Junta de Andalucı´a for funding this work. The Nuclear Magnetic Resonance Service of the University of Co´rdoba and the Chemical Technology Institute of the Universitad Polite´cnica de Valencia are also gratefully acknowledged for their valuable assistance in performing and interpreting the XPS analyses. LA990053Y (26) Romrachev, G. A.; Shitova, E. V.; Yuvodzinski, V.; Suvorova, O. N.; Varyukhin, V. A.; Nesterov, B. A. Dokl. Akad. Nauk. SSSR 1976, 222, 1080.