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The surface structure of catalysts activated with hydrogen donors as elucidated by multinuclear solid-state NMR
Solid State Nuclear Magnetic Resonance 16 Ž2000. 217–224 www.elsevier.nlrlocatersolmag
The surface structure of catalysts activated with hydrogen donors as elucidated by multinuclear solid-state NMR Marıa ´ A. Aramendıa, ´ Victoriano Borau, Cesar ´ Jimenez ´ ) , Jose´ M. Marinas, Jose´ R. Ruiz, Francisco J. Urbano Departamento de Quımica Organica, Facultad de Ciencias, UniÕersidad de Cordoba, AÕda San Alberto Magno s r n, ´ ´ ´ E-14004 Cordoba, Spain ´ Received 23 November 1999; received in revised form 12 January 2000; accepted 21 January 2000
Abstract 1
H, 27Al and 31 P MAS, and 13C and 29 Si CPrMAS NMR spectroscopies, were used to characterize catalysts of Pd supported on various solids including SiO 2 , AlPO4 and Mg 3ŽPO4 . 2 that were activated with the chiral hydrogen-donor limonene. The above-mentioned techniques were used to check for the formation of an organopalladium complex between Pd 2q atoms and the olefin bonds in the limonene molecule on the catalyst surface. The results are compared with those obtained for catalysts activated in a hydrogen stream. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Pd catalysts; MAS and CPrMAS NMR; Limonene
1. Introduction The reduction of multiple bonds with molecular hydrogen and a metal catalyst is a well-known and widely documented process. By contrast, the ability to effect the process with the assistance of an organic molecule acting as a hydrogen donor, in the presence of a catalyst, has scarcely been exploited. In this process, which is referred to as catalytic transfer of hydrogen, a cyclic alkane or alkene is aromatized with release of hydrogen. In this way, cyclohexene becomes benzene in the presence of Pt, Pd, Ru and
)
Corresponding author. Fax: q34-57-218-606.
Rh catalysts w1,2x at low pressure. In hydrogen-transfer hydrogenations, the donor and the acceptor Žboth adsorbed on the catalyst. exchange hydrogen. The simplest instance of this process is the disproportionation of cyclohexene, where one reactant molecule transfers a hydrogen molecule to another. The resulting products are benzene and cyclohexane w1x. In recent years, a variety of techniques have been used to identify various organic molecules on different types of solids. Such techniques include adsorption isotherms and calorimetric measurements w3,4x that have provided insight into the thermodynamics of adsorbate–host interactions. Changes in the host structure can also be examined by solid-state NMR analysis. Thus, 13 C CPrMAS NMR spectroscopy is currently being used to study various hydrocarbons, both saturated and unsaturated, adsorbed on different
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types of solids w5–8x including supported metal catalysts w9,10x. In previous work, we found Pd catalysts supported on silica and activated with Ž R .-Žq.- and Ž S .-Žy.-limonene to form a complex via Pd 2q atoms on the solid surface w11x. In this work, we used multinuclear solid-state NMR spectroscopy to characterize catalysts of Pd supported in SiO 2 , AlPO4 and Mg 3 ŽPO4 . 2 , and activated with Ž R .-Žq.limonene. This technique is highly useful for the structural elucidation of solids — particularly amorphous ones. Thus, it has been extensively used recently to characterize catalytically active solids of variable nature such as Al 2 O 3 w12x, TiO 2 –SiO 2 w13x, sepiolites w14x and PtrMgO systems w15x.
2. Experimental 2.1. Preparation of the catalysts Aluminum orthophosphates at a unity PrAl ratio were prepared from a solution containing 198 g of AlCl 3 P 6H 2 O in 840 ml of distilled water that was supplied with 85 ml of 85% wrw H 3 PO4 . The mixture was homogenized and cooled to 273 K, after which an aqueous solution of 0.1 M ammonia was added in small portions with vigorous stirring in order to maintain the pH of the reacting mass as constant as possible. Gel precipitated above pH 4 and additions were finished at pH 6.2. The gel was then allowed to stand for 24 h, washed with isopropyl alcohol, dried in a stove at 383 K for 24 h and calcined at 923 K for 3 h. The AlPO4 solid thus obtained was labeled AP. The pure silica solid was obtained by calcining commercially available silica gel ŽMerck, ref. 9385. at 873 K for 6 h and was labeled S. Mg 3 ŽPO4 . 2 was obtained by calcining commercially available Mg 3 ŽPO4 . 2 P 8H 2 O ŽAldrich ref. 34, 470-2. at 773 K for 3 h and was labeled MP. Once the solids to be used as supports were prepared, palladium was deposited on them using the impregnation method: a volume of 50 ml of N, N-dimethylformamide was used to dissolve the amount of impregnating salt, PdŽNO 3 . 2 P 2H 2 O, required for the system to contain 3% wrw metal. An amount of
10 g of support and the previously made palladium salt solution were placed in a 250 ml flask and the mixture was stirred for 24 h, after which the solvent was evaporated and the resulting solid calcined at 573 K for 1 h, using a 1 Krmin gradient. The catalysts were activated using two different procedures, as discussed below. Ža. With gaseous dihydrogen. The previously deposited metal was reduced in a dihydrogen stream at 573 K at a flow-rate of 20 mlrmin for 1 h. Then, a nitrogen stream was passed at room temperature at 20 mlrmin for 1 h in order to stabilize the solid. The catalysts thus obtained were designated PdAP, PdS and PdMP. Žb. With Ž R .-Žq.-limonene at 449 K for 1 h. Under these conditions, limonene was converted into p-cymene and the hydrogen released reduced impregnated PdŽII. to PdŽ0. Žsee Scheme 1.. After the mixture was cooled down, the solid was filtered off and washed with cyclohexane and methanol. The catalysts thus obtained were labeled PdAP-Žq.-L, PdS-Žq.-L and PdMP-Žq.-L. 2.2. Chemical and textural properties of the catalysts The specific surface area Ž SBET ., pore volume Ž Vp . and average pore radius Ž rp . were measured. The first parameter was obtained using the BET method w16x while pore distributions were determined by implementing the BJH method w17x on a Micromeritics ASAP 2000 analyzer. All samples were degassed at 423 K at a pressure below 0.1 Pa prior to analysis. Acid and basic sites were quantified from the retention isotherms for two different titrants, viz. cyclohexylamine and phenol, respectively, dissolved in cyclohexane. The amount of titrant retained by each solid was measured spectrophotometrically Žat a
Scheme 1.
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
maximum absorption wavelength of 226 nm for cyclohexylamine and 271 nm for phenol.. That of titrant adsorbed in monolayer form was obtained from the Langmuir equation, using a previously reported method w18x.
check this assumption, the spectra for solids PdS and PdS-Žq.-L were recorded immediately after preparation and a few days after being transferred to the rotor; both spectra were identical for the two solids, so the presence of atmospheric water in the rotor was excluded. 1 H residual resonance from the probe and rotor, identified via an 1 H MAS NMR spectrum obtained in the absence of sample, was found to consist of a weak, broad band. All 1 H spectra were thus corrected for this residual resonance by subtracting the blank spectrum for the empty rotor.
2.3. NMR spectroscopy Solid state 1 H, 27Al and 31 P MAS NMR spectra were recorded at 400.13, 104.26 and 161.96 MHz, respectively, at room temperature, on a Bruker ACP400 spectrometer. The excitation pulse and recycle time for 1 H were 5 ms Žpr2 pulse. and 10 s Ž1000 scans., respectively; 0.6 ms Žpr8 pulse. and 0.5 s Ž256 scans. for 27Al; and 5 ms Žpr2 pulse. and 10 s Ž500 scans. for 31 P. The cross-polarization technique ŽCPrMAS. was used with 13 C and 29 Si. The contact times for the magnetization transfer between protons and 13 C and 29 Si nuclei were 8 and 6 ms, respectively, with an accumulation of 50 000 scans for 13 C and 20 000 scans for 29 Si. A sample rotation frequency of 3500 Hz was used in all instances. 1 H, 13 C and 29 Si spectra are referred to tetramethylsilane ŽTMS.; 27Al spectra to a 1 M AlŽH 2 O. 63q solution; and 31 P spectra to 85% wrw H 3 PO4 . All samples were evacuated to 3 mm Hg at 373 K for 8 h before their spectra were recorded. 1 H MAS NMR spectra were recorded following degassing of the samples, which were transferred to the rotor under a moisture-free nitrogen atmosphere. Spectra were interpreted on the assumption that no atmospheric water penetrated the rotor. In order to
3. Results and discussion 3.1. Chemical textural properties of the supports and catalysts Table 1 summarizes the textural properties of the solids used as Pd supports, which were determined from the corresponding isotherms. As can be seen, the solids exhibit a high specific surface area and an also high cumulative pore volume. By exception both parameters are very low in solid MP, which, however, possesses very large pores. The textural properties of the metal systems were determined similarly to those of the supports. No especially large differences between the isotherms of the metal catalysts and their respective supports were found. Thus, the mean pore diameter was scarcely affected by the particular reduction method used.
Table 1 Chemical textural properties of the catalysts Catalyst
S BE T Žm2 gy1 .
Vp Žml gy1 .
d p ŽA.
X ma Ž10 5 rmol gy1 .
X bb Ž10 5rmol gy1 .
S AP MP PdS PdS-Žq.-L PdAP PdAP-Žq.-L PdMP PdMP-Žq.-L
384 207 7 375 364 167 155 5 5
0.69 1.03 0.05 0.65 0.66 0.87 0.85 0.03 0.03
53 141 249 51 49 143 146 277 267
69.7 63.5 1.6 72.1 41.9 63.1 52.1 1.6 1.5
2.7 4.4 0.2 2.0 0.3 4.3 0.9 0.1 0.0
a
Acidity against cyclohexylamine. Basicity against phenol.
b
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The deposition of the metal decreased both the specific surface area and the cumulative pore volume of the three supports, whichever the reduction method employed. As regards chemical textural properties ŽTable 1., all the solids used as supports — MP excepted — exhibited comparable acidity. This was not the case with basicity; thus, solid AP exhibited the largest population of surface basic sites, even though, on the whole, the population of surface acid sites was markedly greater than that of basic sites, so the solids were essentially acidic. This was not so outstanding in support MP, where acid and basic sites were more similar in number. The metal systems were not analyzed for basicity owing to the low basic character of the starting supports. Thus, the deposition of palladium on the solids activated in a hydrogen stream had little effect on surface acidity relative to that of the respective supports. However, the solids that were activated with the hydrogen donor underwent a decrease in surface acidity, which suggests that these catalysts possess differential features with respect to those activated with dihydrogen and that their original acid sites are blocked. 3.2. NMR analysis of the supports Fig. 1 shows the 1 H MAS NMR spectra for the three solids used as supports. As can be seen, the spectrum for solid S ŽFig. 1A., which consisted of pure silica, exhibits a signal at 1.8 ppm that is slightly asymmetric in its low-field portion. This spectrum is similar to those previously obtained by CRAMPS w19,20x and MAS w21x. The signal at 1.8 ppm was assigned to isolated SI–OH groups Žsilanols., and the low-field asymmetry in the signal to hydrogen binding via the more tightly packed OH groups and silanodiol groups w22x. The spectrum for solid AP ŽFig. 1B., consisting of AlPO4 , exhibits two signals at y0.1 and 3.3 ppm that suggest the presence of two different types of OH groups. A third broader signal at 5.2 ppm was assigned to physisorbed water, which was not completely removed under the conditions used in the treatment preceding the recording of spectra. Based on the w23x for aluminas, the resonance results of Knozinger ¨ at y0.1 ppm can be assigned to the more basic OH
Fig. 1. 1 H MAS NMR spectra for supports S ŽA., AP ŽB. and MP ŽC. Ž ) denote ssb..
groups. The signal at 3.3 ppm can be ascribed to P–OH groups, consistent with the results of other authors for AlPO4 –Al 2 O 3 systems and AlPO4 molecular sieves, which exhibit signals at 3 ppm that have been ascribed to such groups w22,24x. Finally, the spectrum for solid MP ŽFig. 1C. exhibits three signals at 3.1, 1.2 and y0.1 ppm that were previously assigned w25x to P–OH, Mg–OH and basic Mg–OH groups, respectively. Fig. 2 shows the 29 Si CPrMAS NMR spectrum for solid S. As can be seen, it exhibits three signals
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
Fig. 2. 29 Si CPrMAS NMR spectrum for support S.
at about y110, y101 and y92 ppm which were previously assigned to silicon atoms in Si–O tetrahedra of the SiO 2 structure, silanol groups and silanodiol groups, respectively w26,27x. 27 Al MAS NMR spectroscopy allows one to elucidate the local environment of aluminum in both amorphous and crystalline orthophosphates w28,29x. In the former, this technique has revealed the presence of tetrahedral ŽAl Td . and octahedral aluminum ŽAl Oh .. The chemical shifts for isolated AlO6 octahedra appear at ca. y12 ppm, whereas those for isolated AlO4 tetrahedra are observed at ca. 39 ppm. Consequently, support AP, the spectrum for which only exhibits a signal at 38.5 ppm ŽFig. 3., can be assumed to contain Al Td only. The strong shift in this signal with respect to other aluminum oxides
27
Fig. 3. Al MAS NMR spectrum for support AP
Ž)
denote ssb..
221
was ascribed by Muller ¨ et al. w30x to the influence of phosphorus atoms in the second coordination sphere. Fig. 4 shows the 31 P MAS NMR spectra for supports MP and AP. In previous work, solid MP was found to contain two types of P sites w25x corresponding to crystalline magnesium orthophosphate, farringtonite phase Žsharp signal at y0.5 ppm., and amorphous magnesium orthophosphate Žbroad signal at 0.5 ppm.. Support AP exhibits a single broad band in addition to a symmetric set of spinning side bands Žssb.. The chemical shift, y26.8 ppm, corresponds to P atoms in tetrahedral coordination with P–O–Al bonds wPŽOAl.4 environmentsx w28,31x. 3.3. NMR analysis of the catalysts In previous work w11x we found a complex between limonene and Pd 2q ŽScheme 1. to be formed
Fig. 4. 31 P MAS NMR spectra for supports MP ŽA. and AP ŽB. Ž ) denote ssb..
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dihydrogen-activated solids Žnot shown. were similar to those for the supports, those for the limoneneactivated catalysts exhibited overlapped signals that were inclusive except for the facts that they were concentrated in the alkyl proton region and that no signals for olefinic 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. Fig. 6 shows the 13 C CPrMAS NMR spectra for the catalysts PdS-Žq.-L, PdAP-Žq.-L and PdMPŽq.-L. Palladium organometals have scarcely been examined by 13 C CPrMAS NMR spectroscopy, which affords deeper, more precise study of chemical shifts in the signals than does the parent technique. A comparison of the 13 C NMR spectrum for dissolved limonene w32x with that for chemisorbed limonene reveals that the signals at 121.5, 133.6, 101.2 and 144.9 ppm, assigned to the four unsatu-
Fig. 5. 1 H MAS NMR spectra for catalysts PdS-Žq.-L ŽA., PdAP-Žq.-L ŽB. and PdMP-Žq.-L ŽC. Ž ) denote ssb..
during the activation of a SiO 2-supported Pd catalyst with this hydrogen donor. The structure was elucidated from TPD-MS data that were confirmed by 13 C CPrMAS NMR spectroscopy. The 27Al, 31 P and 29 Si NMR spectra for the catalysts Žnot shown. were similar to those for the supports, whether these were activated with dihydrogen or Ž R .-Žq.-limonene. This confirms that this type of surface site is not altered during the process by which the catalysts are obtained. On the other hand, 1 H MAS NMR spectra ŽFig. 5. were rather different. Thus, while the spectra for the
Fig. 6. 13 C CPrMAS NMR spectra for catalysts PdS-Žq.-L ŽA., PdAP-Žq.-L ŽB. and PdMP-Žq.-L ŽC..
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
rated carbons, are shifted upfield when the compound is chemisorbed on the catalyst surface. This strong shielding in the carbon atoms must be similar to that in metal-bonded protons w33x, so we can assume the four unsaturated carbon atoms 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. As a result, the metal surface contains structures where Pd 2q, a part of a surface palladium oxide that results from the decomposition of impregnated palladium nitrate, is bonded to allyl species resulting from the limonene molecule via the original double bonds ŽScheme 2.. In a similar process, Romrachev et al. w34x obtained SiO 2-supported Pd films by thermal decomposition of Pd 2q allyl complexes of the type wAllPdClx 2 above 513 K. The type of metal crystallite obtained depends on the structure of the complex ligand w35x. In order to check whether the synthetic method used with these surface complexes also applied to other hydrogen donors, the silica-supported catalyst was activated with alternative compounds such as a-phellandrene and vinylcyclohexene. Fig. 7 shows the 13 C CPrMAS NMR spectra thus obtained and those for the donors in solution. A comparison of the spectra for the compounds in adsorbed and dissolved form reveals, similarly to limonene, that the signals assigned to the unsaturated carbons are strongly shielded, with shifts from 125–145 to 70–80 ppm.
223
Fig. 7. 13 C CPrMAS NMR spectra for a catalyst supported on solid S and activated with vinylcyclohexene ŽA. and a-phellandrene ŽB..
Again, we can assume the formation of the complexes, similar to that shown in Scheme 2.
4. Conclusions 27
Scheme 2.
Al, 31 P MAS and 29 Si CPrMAS NMR spectroscopies revealed that the deposition of palladium on solids used as catalytic supports Žsilica, aluminum orthophosphate and magnesium orthophosphate. does not alter them in any way. The catalyst activation step, effected by both dihydrogen and a hydrogen donor, causes surface changes in the catalysts that depend on the particular treatment. Thus, the activation with dihydrogen only alters — to a small extent — the values of the surface parameters; on the other hand, the activation with limonene results in the formation of a surface complex between Pd 2q atoms and the unsaturated bonds of limonene itself, thereby endowing the catalyst with special properties. The structure of the surface complex has been elucidated by 13 C CPrMAS NMR spectroscopy.
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Acknowledgements The authors wish to express their gratitude to Spain’s Direccion Superior e ´ General de Ensenanza ˜ Investigacion ´ del Ministerio de Educacion ´ y Cultura ŽProject PB97-0446. and to the Consejerıa ´ de Educacion ´ y Ciencia de la Junta de Andalucıa ´ for funding this work. The staff of the Nuclear Magnetic Resonance Service of the University of Cordoba are also ´ gratefully acknowledged for their valuable assistance in recording the NMR spectra.
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w14x M.A. Aramendıa, J.M. Marinas, J.R. ´ V. Borau, C. Jimenez, ´ Ruiz, Solid State NMR 8 Ž1997. 251. w15x M.A. Aramendıa, J.M. ´ J.A. Benitez, V. Borau, C. Jimenez, ´ Marinas, J.R. Ruiz, F.J. Urbano, Langmuir 15 Ž1999. 1192. w16x S. Brunauer, P.H. Emmet, E.J. Teller, J. Am. Chem. Soc. 60 Ž1951. 73. w17x E.P. Barret, L.S. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 Ž1964. 373. w18x M.A. Aramendıa, J.M. Marinas, F. ´ V. Borau, C. Jimenez, ´ Rodero, Colloid Surf. 12 Ž1984. 227. w19x C.E. Bronnimann, R.C. Zeigler, G.E. Maciel, J. Am. Chem. Soc. 110 Ž1988. 2023. w20x C.E. Bronnimann, I.-S. Chuang, B.L. Hawkings, G.E. Maciel, J. Am. Chem. Soc. 109 Ž1987. 1562. w21x S.F. Rec, C.E. Bronniman, R.E. Wind, G.E. Maciel, J. Magn. Reson. 82 Ž1989. 454. w22x V.M. Mastikhin, I.L. Mudrakovski, A.V. Nosov, Prog. Nucl. Magn. Reson. Spectrosc. 23 Ž1991. 211. w23x H. Knozinger, P. Ratnasamy, Catal. Rev. — Sci. Eng. 71 ¨ Ž1978. 31. w24x H. Pfeifer, in: NMR Basic Principles and Progress vol. 31 Springer-Verlag, Berlin, 1994, p. 31. w25x M.A. Aramendıa, J.M. Marinas, F.J. ´ V. Borau, C. Jimenez, ´ Romero, J.R. Ruiz, J. Solid State Chem. 135 Ž1998. 96. w26x G.E. Maciel, D.W. Sindorf, J. Am. Chem. Soc. 102 Ž1980. 7606. w27x E.T. Lipmaa, A.V. Samoson, V.V. Brei, Y.I. Gorlov, Dokl. Akad. Sci. USSR 259 Ž1981. 403. w28x C.S. Blackwell, R.C. Patton, J. Phys. Chem. 88 Ž1984. 6135. w29x W.F. Blean, P.R. Pfeffer, J.S. Frye, Phys. Chem. Miner. 16 Ž1989. 455. w30x D. Muller, I. Grunze, E. Hallas, G. Ladwig, Z. Anorg. Allg. ¨ Chem. 500 Ž1983. 80. w31x R.H. Meinhold, N.J. Tapp, J. Chem. Soc., Chem. Commun. Ž1990. 219. w32x M. Jautelat, J.B. Grutzner, J.D. Roberts, Proc. Natl. Acad. Sci. 65 Ž1970. 88. w33x H. Gunther, in: NMR Spectroscopy, Wiley, London, 1995, p. ¨ 99. w34x G.A. Romrachev, V.A. Varyukhin, B.A. Nesterov, React. Kinet. Catal. Lett. 22 Ž1983. 281. w35x G.A. Romrachev, E.V. Shitova, V. Yuvodzinski, O.N. Suvorova, V.A. Varyukhin, B.A. Nesterov, Dokl. Akad. Nauk SSSR 222 Ž1976. 1080.
The surface structure of catalysts activated with hydrogen donors as elucidated by multinuclear solid-state NMR Marıa ´ A. Aramendıa, ´ Victoriano Borau, Cesar ´ Jimenez ´ ) , Jose´ M. Marinas, Jose´ R. Ruiz, Francisco J. Urbano Departamento de Quımica Organica, Facultad de Ciencias, UniÕersidad de Cordoba, AÕda San Alberto Magno s r n, ´ ´ ´ E-14004 Cordoba, Spain ´ Received 23 November 1999; received in revised form 12 January 2000; accepted 21 January 2000
Abstract 1
H, 27Al and 31 P MAS, and 13C and 29 Si CPrMAS NMR spectroscopies, were used to characterize catalysts of Pd supported on various solids including SiO 2 , AlPO4 and Mg 3ŽPO4 . 2 that were activated with the chiral hydrogen-donor limonene. The above-mentioned techniques were used to check for the formation of an organopalladium complex between Pd 2q atoms and the olefin bonds in the limonene molecule on the catalyst surface. The results are compared with those obtained for catalysts activated in a hydrogen stream. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Pd catalysts; MAS and CPrMAS NMR; Limonene
1. Introduction The reduction of multiple bonds with molecular hydrogen and a metal catalyst is a well-known and widely documented process. By contrast, the ability to effect the process with the assistance of an organic molecule acting as a hydrogen donor, in the presence of a catalyst, has scarcely been exploited. In this process, which is referred to as catalytic transfer of hydrogen, a cyclic alkane or alkene is aromatized with release of hydrogen. In this way, cyclohexene becomes benzene in the presence of Pt, Pd, Ru and
)
Corresponding author. Fax: q34-57-218-606.
Rh catalysts w1,2x at low pressure. In hydrogen-transfer hydrogenations, the donor and the acceptor Žboth adsorbed on the catalyst. exchange hydrogen. The simplest instance of this process is the disproportionation of cyclohexene, where one reactant molecule transfers a hydrogen molecule to another. The resulting products are benzene and cyclohexane w1x. In recent years, a variety of techniques have been used to identify various organic molecules on different types of solids. Such techniques include adsorption isotherms and calorimetric measurements w3,4x that have provided insight into the thermodynamics of adsorbate–host interactions. Changes in the host structure can also be examined by solid-state NMR analysis. Thus, 13 C CPrMAS NMR spectroscopy is currently being used to study various hydrocarbons, both saturated and unsaturated, adsorbed on different
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M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
types of solids w5–8x including supported metal catalysts w9,10x. In previous work, we found Pd catalysts supported on silica and activated with Ž R .-Žq.- and Ž S .-Žy.-limonene to form a complex via Pd 2q atoms on the solid surface w11x. In this work, we used multinuclear solid-state NMR spectroscopy to characterize catalysts of Pd supported in SiO 2 , AlPO4 and Mg 3 ŽPO4 . 2 , and activated with Ž R .-Žq.limonene. This technique is highly useful for the structural elucidation of solids — particularly amorphous ones. Thus, it has been extensively used recently to characterize catalytically active solids of variable nature such as Al 2 O 3 w12x, TiO 2 –SiO 2 w13x, sepiolites w14x and PtrMgO systems w15x.
2. Experimental 2.1. Preparation of the catalysts Aluminum orthophosphates at a unity PrAl ratio were prepared from a solution containing 198 g of AlCl 3 P 6H 2 O in 840 ml of distilled water that was supplied with 85 ml of 85% wrw H 3 PO4 . The mixture was homogenized and cooled to 273 K, after which an aqueous solution of 0.1 M ammonia was added in small portions with vigorous stirring in order to maintain the pH of the reacting mass as constant as possible. Gel precipitated above pH 4 and additions were finished at pH 6.2. The gel was then allowed to stand for 24 h, washed with isopropyl alcohol, dried in a stove at 383 K for 24 h and calcined at 923 K for 3 h. The AlPO4 solid thus obtained was labeled AP. The pure silica solid was obtained by calcining commercially available silica gel ŽMerck, ref. 9385. at 873 K for 6 h and was labeled S. Mg 3 ŽPO4 . 2 was obtained by calcining commercially available Mg 3 ŽPO4 . 2 P 8H 2 O ŽAldrich ref. 34, 470-2. at 773 K for 3 h and was labeled MP. Once the solids to be used as supports were prepared, palladium was deposited on them using the impregnation method: a volume of 50 ml of N, N-dimethylformamide was used to dissolve the amount of impregnating salt, PdŽNO 3 . 2 P 2H 2 O, required for the system to contain 3% wrw metal. An amount of
10 g of support and the previously made palladium salt solution were placed in a 250 ml flask and the mixture was stirred for 24 h, after which the solvent was evaporated and the resulting solid calcined at 573 K for 1 h, using a 1 Krmin gradient. The catalysts were activated using two different procedures, as discussed below. Ža. With gaseous dihydrogen. The previously deposited metal was reduced in a dihydrogen stream at 573 K at a flow-rate of 20 mlrmin for 1 h. Then, a nitrogen stream was passed at room temperature at 20 mlrmin for 1 h in order to stabilize the solid. The catalysts thus obtained were designated PdAP, PdS and PdMP. Žb. With Ž R .-Žq.-limonene at 449 K for 1 h. Under these conditions, limonene was converted into p-cymene and the hydrogen released reduced impregnated PdŽII. to PdŽ0. Žsee Scheme 1.. After the mixture was cooled down, the solid was filtered off and washed with cyclohexane and methanol. The catalysts thus obtained were labeled PdAP-Žq.-L, PdS-Žq.-L and PdMP-Žq.-L. 2.2. Chemical and textural properties of the catalysts The specific surface area Ž SBET ., pore volume Ž Vp . and average pore radius Ž rp . were measured. The first parameter was obtained using the BET method w16x while pore distributions were determined by implementing the BJH method w17x on a Micromeritics ASAP 2000 analyzer. All samples were degassed at 423 K at a pressure below 0.1 Pa prior to analysis. Acid and basic sites were quantified from the retention isotherms for two different titrants, viz. cyclohexylamine and phenol, respectively, dissolved in cyclohexane. The amount of titrant retained by each solid was measured spectrophotometrically Žat a
Scheme 1.
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
maximum absorption wavelength of 226 nm for cyclohexylamine and 271 nm for phenol.. That of titrant adsorbed in monolayer form was obtained from the Langmuir equation, using a previously reported method w18x.
check this assumption, the spectra for solids PdS and PdS-Žq.-L were recorded immediately after preparation and a few days after being transferred to the rotor; both spectra were identical for the two solids, so the presence of atmospheric water in the rotor was excluded. 1 H residual resonance from the probe and rotor, identified via an 1 H MAS NMR spectrum obtained in the absence of sample, was found to consist of a weak, broad band. All 1 H spectra were thus corrected for this residual resonance by subtracting the blank spectrum for the empty rotor.
2.3. NMR spectroscopy Solid state 1 H, 27Al and 31 P MAS NMR spectra were recorded at 400.13, 104.26 and 161.96 MHz, respectively, at room temperature, on a Bruker ACP400 spectrometer. The excitation pulse and recycle time for 1 H were 5 ms Žpr2 pulse. and 10 s Ž1000 scans., respectively; 0.6 ms Žpr8 pulse. and 0.5 s Ž256 scans. for 27Al; and 5 ms Žpr2 pulse. and 10 s Ž500 scans. for 31 P. The cross-polarization technique ŽCPrMAS. was used with 13 C and 29 Si. The contact times for the magnetization transfer between protons and 13 C and 29 Si nuclei were 8 and 6 ms, respectively, with an accumulation of 50 000 scans for 13 C and 20 000 scans for 29 Si. A sample rotation frequency of 3500 Hz was used in all instances. 1 H, 13 C and 29 Si spectra are referred to tetramethylsilane ŽTMS.; 27Al spectra to a 1 M AlŽH 2 O. 63q solution; and 31 P spectra to 85% wrw H 3 PO4 . All samples were evacuated to 3 mm Hg at 373 K for 8 h before their spectra were recorded. 1 H MAS NMR spectra were recorded following degassing of the samples, which were transferred to the rotor under a moisture-free nitrogen atmosphere. Spectra were interpreted on the assumption that no atmospheric water penetrated the rotor. In order to
3. Results and discussion 3.1. Chemical textural properties of the supports and catalysts Table 1 summarizes the textural properties of the solids used as Pd supports, which were determined from the corresponding isotherms. As can be seen, the solids exhibit a high specific surface area and an also high cumulative pore volume. By exception both parameters are very low in solid MP, which, however, possesses very large pores. The textural properties of the metal systems were determined similarly to those of the supports. No especially large differences between the isotherms of the metal catalysts and their respective supports were found. Thus, the mean pore diameter was scarcely affected by the particular reduction method used.
Table 1 Chemical textural properties of the catalysts Catalyst
S BE T Žm2 gy1 .
Vp Žml gy1 .
d p ŽA.
X ma Ž10 5 rmol gy1 .
X bb Ž10 5rmol gy1 .
S AP MP PdS PdS-Žq.-L PdAP PdAP-Žq.-L PdMP PdMP-Žq.-L
384 207 7 375 364 167 155 5 5
0.69 1.03 0.05 0.65 0.66 0.87 0.85 0.03 0.03
53 141 249 51 49 143 146 277 267
69.7 63.5 1.6 72.1 41.9 63.1 52.1 1.6 1.5
2.7 4.4 0.2 2.0 0.3 4.3 0.9 0.1 0.0
a
Acidity against cyclohexylamine. Basicity against phenol.
b
219
220
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The deposition of the metal decreased both the specific surface area and the cumulative pore volume of the three supports, whichever the reduction method employed. As regards chemical textural properties ŽTable 1., all the solids used as supports — MP excepted — exhibited comparable acidity. This was not the case with basicity; thus, solid AP exhibited the largest population of surface basic sites, even though, on the whole, the population of surface acid sites was markedly greater than that of basic sites, so the solids were essentially acidic. This was not so outstanding in support MP, where acid and basic sites were more similar in number. The metal systems were not analyzed for basicity owing to the low basic character of the starting supports. Thus, the deposition of palladium on the solids activated in a hydrogen stream had little effect on surface acidity relative to that of the respective supports. However, the solids that were activated with the hydrogen donor underwent a decrease in surface acidity, which suggests that these catalysts possess differential features with respect to those activated with dihydrogen and that their original acid sites are blocked. 3.2. NMR analysis of the supports Fig. 1 shows the 1 H MAS NMR spectra for the three solids used as supports. As can be seen, the spectrum for solid S ŽFig. 1A., which consisted of pure silica, exhibits a signal at 1.8 ppm that is slightly asymmetric in its low-field portion. This spectrum is similar to those previously obtained by CRAMPS w19,20x and MAS w21x. The signal at 1.8 ppm was assigned to isolated SI–OH groups Žsilanols., and the low-field asymmetry in the signal to hydrogen binding via the more tightly packed OH groups and silanodiol groups w22x. The spectrum for solid AP ŽFig. 1B., consisting of AlPO4 , exhibits two signals at y0.1 and 3.3 ppm that suggest the presence of two different types of OH groups. A third broader signal at 5.2 ppm was assigned to physisorbed water, which was not completely removed under the conditions used in the treatment preceding the recording of spectra. Based on the w23x for aluminas, the resonance results of Knozinger ¨ at y0.1 ppm can be assigned to the more basic OH
Fig. 1. 1 H MAS NMR spectra for supports S ŽA., AP ŽB. and MP ŽC. Ž ) denote ssb..
groups. The signal at 3.3 ppm can be ascribed to P–OH groups, consistent with the results of other authors for AlPO4 –Al 2 O 3 systems and AlPO4 molecular sieves, which exhibit signals at 3 ppm that have been ascribed to such groups w22,24x. Finally, the spectrum for solid MP ŽFig. 1C. exhibits three signals at 3.1, 1.2 and y0.1 ppm that were previously assigned w25x to P–OH, Mg–OH and basic Mg–OH groups, respectively. Fig. 2 shows the 29 Si CPrMAS NMR spectrum for solid S. As can be seen, it exhibits three signals
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
Fig. 2. 29 Si CPrMAS NMR spectrum for support S.
at about y110, y101 and y92 ppm which were previously assigned to silicon atoms in Si–O tetrahedra of the SiO 2 structure, silanol groups and silanodiol groups, respectively w26,27x. 27 Al MAS NMR spectroscopy allows one to elucidate the local environment of aluminum in both amorphous and crystalline orthophosphates w28,29x. In the former, this technique has revealed the presence of tetrahedral ŽAl Td . and octahedral aluminum ŽAl Oh .. The chemical shifts for isolated AlO6 octahedra appear at ca. y12 ppm, whereas those for isolated AlO4 tetrahedra are observed at ca. 39 ppm. Consequently, support AP, the spectrum for which only exhibits a signal at 38.5 ppm ŽFig. 3., can be assumed to contain Al Td only. The strong shift in this signal with respect to other aluminum oxides
27
Fig. 3. Al MAS NMR spectrum for support AP
Ž)
denote ssb..
221
was ascribed by Muller ¨ et al. w30x to the influence of phosphorus atoms in the second coordination sphere. Fig. 4 shows the 31 P MAS NMR spectra for supports MP and AP. In previous work, solid MP was found to contain two types of P sites w25x corresponding to crystalline magnesium orthophosphate, farringtonite phase Žsharp signal at y0.5 ppm., and amorphous magnesium orthophosphate Žbroad signal at 0.5 ppm.. Support AP exhibits a single broad band in addition to a symmetric set of spinning side bands Žssb.. The chemical shift, y26.8 ppm, corresponds to P atoms in tetrahedral coordination with P–O–Al bonds wPŽOAl.4 environmentsx w28,31x. 3.3. NMR analysis of the catalysts In previous work w11x we found a complex between limonene and Pd 2q ŽScheme 1. to be formed
Fig. 4. 31 P MAS NMR spectra for supports MP ŽA. and AP ŽB. Ž ) denote ssb..
222
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dihydrogen-activated solids Žnot shown. were similar to those for the supports, those for the limoneneactivated catalysts exhibited overlapped signals that were inclusive except for the facts that they were concentrated in the alkyl proton region and that no signals for olefinic 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. Fig. 6 shows the 13 C CPrMAS NMR spectra for the catalysts PdS-Žq.-L, PdAP-Žq.-L and PdMPŽq.-L. Palladium organometals have scarcely been examined by 13 C CPrMAS NMR spectroscopy, which affords deeper, more precise study of chemical shifts in the signals than does the parent technique. A comparison of the 13 C NMR spectrum for dissolved limonene w32x with that for chemisorbed limonene reveals that the signals at 121.5, 133.6, 101.2 and 144.9 ppm, assigned to the four unsatu-
Fig. 5. 1 H MAS NMR spectra for catalysts PdS-Žq.-L ŽA., PdAP-Žq.-L ŽB. and PdMP-Žq.-L ŽC. Ž ) denote ssb..
during the activation of a SiO 2-supported Pd catalyst with this hydrogen donor. The structure was elucidated from TPD-MS data that were confirmed by 13 C CPrMAS NMR spectroscopy. The 27Al, 31 P and 29 Si NMR spectra for the catalysts Žnot shown. were similar to those for the supports, whether these were activated with dihydrogen or Ž R .-Žq.-limonene. This confirms that this type of surface site is not altered during the process by which the catalysts are obtained. On the other hand, 1 H MAS NMR spectra ŽFig. 5. were rather different. Thus, while the spectra for the
Fig. 6. 13 C CPrMAS NMR spectra for catalysts PdS-Žq.-L ŽA., PdAP-Žq.-L ŽB. and PdMP-Žq.-L ŽC..
M.A. Aramendıa ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 217–224
rated carbons, are shifted upfield when the compound is chemisorbed on the catalyst surface. This strong shielding in the carbon atoms must be similar to that in metal-bonded protons w33x, so we can assume the four unsaturated carbon atoms 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. As a result, the metal surface contains structures where Pd 2q, a part of a surface palladium oxide that results from the decomposition of impregnated palladium nitrate, is bonded to allyl species resulting from the limonene molecule via the original double bonds ŽScheme 2.. In a similar process, Romrachev et al. w34x obtained SiO 2-supported Pd films by thermal decomposition of Pd 2q allyl complexes of the type wAllPdClx 2 above 513 K. The type of metal crystallite obtained depends on the structure of the complex ligand w35x. In order to check whether the synthetic method used with these surface complexes also applied to other hydrogen donors, the silica-supported catalyst was activated with alternative compounds such as a-phellandrene and vinylcyclohexene. Fig. 7 shows the 13 C CPrMAS NMR spectra thus obtained and those for the donors in solution. A comparison of the spectra for the compounds in adsorbed and dissolved form reveals, similarly to limonene, that the signals assigned to the unsaturated carbons are strongly shielded, with shifts from 125–145 to 70–80 ppm.
223
Fig. 7. 13 C CPrMAS NMR spectra for a catalyst supported on solid S and activated with vinylcyclohexene ŽA. and a-phellandrene ŽB..
Again, we can assume the formation of the complexes, similar to that shown in Scheme 2.
4. Conclusions 27
Scheme 2.
Al, 31 P MAS and 29 Si CPrMAS NMR spectroscopies revealed that the deposition of palladium on solids used as catalytic supports Žsilica, aluminum orthophosphate and magnesium orthophosphate. does not alter them in any way. The catalyst activation step, effected by both dihydrogen and a hydrogen donor, causes surface changes in the catalysts that depend on the particular treatment. Thus, the activation with dihydrogen only alters — to a small extent — the values of the surface parameters; on the other hand, the activation with limonene results in the formation of a surface complex between Pd 2q atoms and the unsaturated bonds of limonene itself, thereby endowing the catalyst with special properties. The structure of the surface complex has been elucidated by 13 C CPrMAS NMR spectroscopy.
224
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Acknowledgements The authors wish to express their gratitude to Spain’s Direccion Superior e ´ General de Ensenanza ˜ Investigacion ´ del Ministerio de Educacion ´ y Cultura ŽProject PB97-0446. and to the Consejerıa ´ de Educacion ´ y Ciencia de la Junta de Andalucıa ´ for funding this work. The staff of the Nuclear Magnetic Resonance Service of the University of Cordoba are also ´ gratefully acknowledged for their valuable assistance in recording the NMR spectra.
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