3796
J. Phys. Chem. 1992, 96, 3796-3799
Platlnum Cluster Catalysts Supported on Porous Chelate Resin-Metal Complexes: Effect of Resln Porosity on Catalytic Activity Naoki Toshima,* Toshiharu Teranishi, Hiroyuki Asaauma, and Yasukazu Saito Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan (Received: October 7, 1991)
Porous chelate resin-metal complexes were prepared from a commercial chelate resin having iminodiacetic acid moieties. This was accomplished by complexing with multivalent cations such as A13’ and drying after washing with ethanol. The supported platinum cluster catalysts, formed by reducing the platinum ions on the porous chelate resin-metal complexes, had a mean diameter of 26 A, occupying pores approximately 40 A in diameter of the chelate resin-metal complexes. These systems were then used as catalysts during the hydrogenation of olefin or diene. The catalytic activity was found to depend on the type of metal ion used in the chelate resin-metal complexes, and it increased in the order of Nat C Mg2+C Al”. The surface area of the chelate resin-metal complex, measured by a BET method, also increased in the same order, suggesting that the surface area is related to the catalytic activity.
Introduction Homogeneous dispersions of platinum c l ~ s t e r s l -have ~ been prepared by the reduction of hexachloroplatinic acid under mild conditions in the presence of water-soluble polymerse6 or surfactant~.’.~The cluster particles prepared in such a manner are protected by polymers or micelles. They have the advantage of being small in size with a narrow size distribution compared with metal particles on inorganic supports. They work as highly active and selective catalysts for the hydrogenation of o l e f i n ~ ~and .~J~ the photochemical evolution of hydrogen from water.s.8.11,12 However, it is difficult to separate the dispersion of clusters from the reaction mixtures during repeated usage, which is important, especially when making use of expensive noble metal catalysts. To overcome this problem, numerous attempts have been made to support the platinum clusters, such as on active carbon or inorganic ~ubstrates.’~” However, only a few studies have been conducted on the support of polymer-protected metal clusters. In order to form the polymer-protected metal clusters on a solid support, cross-linked polymers can be used in which the main constituents form the solid, while the rest of the polymer on the surface expands in the solution. An example of this type of support is the polystyrene-based chelate resin (CR) functionalized by iminodiacetic acid moieties. The palladium ions in the chelate can be reduced by refluxing in alcohol, resulting in the formation (1) For reviews: Schmid, G. Srrucr. Bonding 1985, 62, 51. Hirai, H.; Toshima, N. In Tailored Metal Caralysrs; Iwasawa, Y . , Ed.; Reidel: Amsterdam, 1986;p 87. Walther, B. Z.Chem. 1989, 29, 117. (2) Dunworth, W. P.;Nord, F. F. Adv. Caral. 1954, 6, 125. (3) Aika, K.; Ban, L. L.; Okura, I.; Namba, S.; Turkevich, J. J . Res. Inst. Caral. Hokkaido Vniv. 1976, 24, 54. (4) Hirai, H.; Nakao, Y.; Toshima, N. J . Macromol. Sci.-Chem. 1979, A13, 727. (5) Toshima, N.; Kuriyama, M.; Yamada, Y.; Hirai, H. Chem. Lerr. 1981, 193. (6)Hirai, H.; Chawanya, H.; Toshima, N. Bull. Chem. SOC.Jpn. 1985, 58, 682. (7) Toshima, N.; Takahashi, T. Chem. Lerr. 1988, 573. (8) Toshima, N.; Takahashi, T.; Hirai, H. J . Macromol. Sci.-Chem. 1988, A25, 669. (9)Hirai, H. J. Macromol. Sci.-Chem. 1979, A13, 633. (10) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1985, 3, 127. (11) Nishijima, T.; Nagamura, T.; Matsuo, T. J . Polym. Sci., Polym. Leu. Ed. 1981, 19, 65. (12)Ohsaka, T.; Sakamoto, T.; Matsuo, T. Chem. Lerr. 1983, 1675. (13) Nakao, Y.;Kaeriyama, K. J . Colloid InrerfaceSci. 1989, 131, 186. (14) Alberts, D.; Seibold, K.; McEvoy, A. J.; Kiwi, J. J . Phys. Chem. 1989, 93, 1510. (15) Vannice, M. A.; Sen, B. J . Catal. 1989, 115, 65. (16)Subramanian, S.; Schwarz, J. A. Appl. Catal. 1990, 61, L15. (17) Primet, M.;El Azhar, M.;Guenin, M. Appl. Catal. 1990, 58, 241.
of a palladium cluster bound by the chelate resin.18 However, the activity of the catalyst per amount of charge of the metal ions is low as compared to homogeneous dispersions. Immobilization of polymer-protected metal clusters on polymer gels by chemical bonding has been successfully carried out, where both polymers have reactive groups.I9 While such metal clusters show catalytic activity and selectivity similar to the corresponding homogeneous dispersions, their formation is somewhat difficult for practical purposes. Chelate resins with iminodiacetic acid moieities have some characteristic properties, such as their porosity in the dry state, which support metal clusters. Recently, it has been determined that the surface area of the chelate resin was increased by 3 orders of magnitude after complexation with multivalent cations such as A13+ and also by drying after the resin was washed with an organic solvent miscible with water.M These porous chelate resin complexes are useful in gas separation processes20,21as well as catalysts.22 In this paper, chelate resin-metal complexes dried from ethanol are used as support for platinum clusters, which are produced by the reduction of the PtCb2-ions on the resin complexes. Sodium, magnesium, and aluminum ions are used for the complexation with the chelate resin. Thus, the chelate resins always contain two kinds of metals; the platinum clusters act as the catalyst, whereas sodium, magnesium, or aluminum ions make the chelate resin porous. The catalytic activities of the platinum clusters supported on the chelate resin-metal complexes are examined during the hydrogenation of olefins and diene, as affected by the metal ions complexed with the chelate resin and by the surface area of the support.
Experimental Section Materials. A commercial chelate resin (Diaion CR-10, with a mean particle diameter of 0.5 mm), having the structure shown in Figure 1 (Mitsubishi Chemical Industry Co. Ltd.), expands with the addition of water. The resin was used either in the swollen state or in a dry state which resulted from drying after being (18) Hirai, H.; Komatsuzaki, S.; Toshima, N. Bull. Chem. Soc. Jpn. 1984, -57. . , 4RR .- - . (19) Ohtaki, M.; Toshima, N.; Komiyama, M.; Hirai, H. Bull. Chem. Soc. Jpn. 1990, 63, 1433. (20)Toshima, N.; Asanuma, H.; Hirai, H. Bull. Chem. SOC.Jpn. 1989, 62, 893. (21)Asanuma, H.; Toshima, N. J. Polym. Sci., Part A: Polym. Chem. . 1990, 28, 907. (22)Toshima, N.; Teranishi, T.; Asanuma, H.; Saito, Y. Chem. Lerr. 1990, 819. (23)Asanuma, H.; Toshima, N. J. Chem. SOC.,Chem. Commun. 1989, 1075.
0022-365419212096-3796%03.00/0 0 1992 American Chemical Societv
Pt Catalysts on Porous Chelate Resin-Metal Complexes
y '
The Journal of Physical Chemistry, Vol. 96, No. 9, 1992 3797 TABLE I: Concentration of PtCbz- Ions in Water before and after Contact with Various Resin-Metal Complexes concn/ mol dm-)
,CH2COOH
CH2N\CH2C00H Figure 1. Schematic illustration of the chemical structure of the chelate resin.
washed with a large amount of ethanol. It was determined by pH titration that 1 g of the dry resin contained 2.57 mmol of iminodiacetic acid (IDA). A commercial poly(acry1ic acid) resin (Diaion WK-20 Mitsubishi Chemical Industry Co. Ltd.) was used for comparison purposes. Preparation of the Catalysts. A. Preparation of the Chelate Resin-Metal Complexes. The chelate resin, composed almost entirely of the resin-sodium(1) complex (CR-Na), was washed once with an aqueous solution of sodium hydroxide and twice with 50 cm3 of distilled water. The CR-Na, containing the iminodiacetic acid moiety (25.7 mmol per 10.0 g in a dry state) and 38.6 mmol of MgC12 or A1Cl3.6H20 in 50 cm3 of water, was thoroughly mixed with a mechanical shaker for 1 day to form Mg2+or AI3+ complexes with the resin beads (CR-Mg and CRAl, respectively). The separated solids were washed four times with 50 cm3of ethanol, followed by drying in a vacuum at 50 OC. B. Binding of PtCb2-on the Resin Complexes. An aqueous solution of hexachloroplatinic acid (1.93 X mol dm-3 X 10.7 cm3 = 20.6 pmol) was added to each chelate resin-metal complex (0.8g in the dry state, containing 2.06 mmol of the iminodiacetic acid). The mixtures were mechanically agitated for 1 day to immobilize the PtC1b2-. The amount of bound PtC16'- was determined from the change in the absorbance at 260 nm of the supernatant solution. It was found that the platinum ions were immobilized only by the CR-AI. C. Preparation of the Platinum Clusters Supported on the Chelate Resin-Metal Complexes. The platinum ions on 0.8 g of CR-A1 were reduced by washing with 25 cm3 of an aqueous solution of LiBH., (4.70 mmol), resulting in platinum clusters supported on the resin (CR-AI-Pt). To form the platinum cluster catalysts supported on the CR-Na and CR-Mg (CR-Na-Pt and CR-Mg-Pt, respectively), A13+ions were leached from the CRAI-Pt by rinsing with the 3 mol dm-3 HCl solution. Aqueous solutions of NaOH (6.17 "01) or MgC12 (3.08 "01) were then added to the CR-H-Pt resin, yielding CR-Na-Pt and CRMg-Pt, respectively. Each catalyst was washed four times with 50 cm3 of ethanol and dried at 50 OC under vacuum conditions. Hydrogenation of Olefms and Diene. Catalytic hydrogenation reactions of various olefins and dienes under atmospheric pressure were conducted as follows: A 0.8-g sample of the dry-type platinum cluster catalyst supported on the CR-metal complex (containing 20.6 pmol of platinum) was added to 19 cm3 of ethanol. The mixture was stirred in a 50-cm3flask of hydrogen at 30 OC and atmospheric pressure until no further uptake of hydrogen was detected. The reaction was then started by the addition of 1 cm3 of the ethanol solution containing 0.1 mmol of the substrate. The hydrogen uptake was traced with a gas buret to determine the initial rate of hydrogenation. Merwnwneot. The concentration of PtCl2- in the solution was determined spectrophotometrically using the absorption peak at 260 nm. The BET surface area of the dry catalyst was measured nitrogen adsorption at 77 K with the Micromeritics Instrument Co., Ltd. Model 2200A rapid surface-area analyzer. Before measurements, the sample was completely dried at 90 OC for 40 min under a stream of nitrogen. The pore size distributions of dried CR-AI and CR-AI-Pt were obtained by Dollimore's method from the isothermal line at 77 K nitrogen adsorption and desorption with the Carloerva Co., Ltd. Series 1800 sorptomatic. For scanning electron microscopy (SEM) and X-ray microanalysis (EPMA), the resin beads were cleaved close to the
resin complex" CR-Na CR-Mg CR-AI WK-A1
before 1.93 1.93 1.93 1.93
after 2.54 2.23 0.181 1.93
immobilization no no
Yes no
"CR, chelate resin with iminodiacetic acid moieties; WK, poly(acrylic acid) resin.
TABLE II: Surface Area of the Chelate Resin-Metal Comdexes
(I
complex
water"
CR-Na CR-Mg CR-AI
CO.1 2.5 4.1
surface area/m2 g-' EtOH" without Pt with Pt 2.51 1.49 21.3 13.7 83.9 16.2
Solvent used in washing the resin-metal complexes.
maximum diameter with a stainless steel cutter and then coated with carbon in order to view the cross-sectional areas. The distributions of Na, Mg, and A1 atoms were evaluated through the intensity of the characteristic X-rays.
Results and Discussion Immobilization of PtCb2- on Various Chelate Resin-Metal Complexes. Table I gives the changes in the concentration of PtClS2-in solutions on equilibration with various chelate resinmetal complexes, which indicates that the platinum ions are bound only by the CR-A1 but not by CR-Na or CR-Mg. In CR-AI, 90.6% of available PtC162-ions was immobilized on the support. The reason for the difference is explained by the binding of cations to the resin. The amounts of Na+ and Mg2+ taken up exactly compensate the anionic group, whereas adsorbed A13+ exceeds the number of acetate groups, resulting in a surplus of positive charge. The latter is responsible for the interaction of the CR-A1 with the PtC16'- anions. When a poly(acry1ic acid) resin (Diaion WK-20) was used instead of the chelate resin, immobilization of PtCb2- did not occur. In this case, anionic groups on the resin fully neutralize A13+. Table I also indicates that the concentration of H2PtC16in the supernatant solution is higher after being in contact with the CR-Na and CR-Mg, which is expected to be due to the Donnan equilibrium. Preparation and Characterization of the Platinum Clusters Supported on the Chelate Resin-Metal Complexes. It was shown that the immobilized PtClS2-species could not be reduced either by hydrogen at rmm temperature or by boiling methanol. Instead, lithium borohydride reduction was applied to the CR-A1-PtCh2system, which resulted in a color change from light yellow to gray indicating the reduction of the platinum ions and cluster formation. It was also demonstrated that the platinum clusters were not removed during the conversion of CR-AI-Pt to CR-Na-Pt and CR-Mg-Pt by ion exchange as shown in the Experimental Section. In order to clarify whether the A13+ions were completely exchanged with other metal ions in the preparation of the CR-Na-Pt and CR-Mg-Pt catalysts, the distributions of the metal ions on the resins were determined by EPMA, which showed that Na+ and Mg2+ions were uniformly distributed over the resin particles of CR-Na-Pt and CR-Mg-Pt and that no A13+was detected in these complex resins. Thus, the A13+ ions were completely exchanged by the present procedure. Table I1 shows that the specific surface area increased in the order of Na+ < Mg2+< AI3+ and that it depended on the washing liquid. The specific surface areas of the resins with Pt clusters also increase in the order of Na+ < Mg2+ < A13+,although the values are lower. Figure 2 shows that the pore size distributions of CR-A1 and CR-AI-Pt have a maximum at -40 A. The difference between
Toshima et al.
3798 The Journal of Physical Chemistry, Vol. 94, No. 9, 1992
I
0.lOl-
0
20
40
4
0
200
100
d
Figure 2. Pore size distributions of CR-AI (solid line) and CR-AI-Pt (dashed line). A V represents the sum of the volumes of pores having diameters between 4 and 4 + A4.
10 t I min
20
Figure 5. Hydrogenation curves for 1,3-cyclooctadiene in ethanol catalyzed by the platinum clusters supported on the chelate resin-A13+ (open circles), -Mg2+ (triangles), and -Na+ (squares) complexes.
5
0
Figure 3. Representative transmission electron micrograph (TEM) of the platinum clusters in a slice of CR-AI-Pt beads. The magnification factor is 400000. 10
0
I
1
10
20
t lmin Figure 4. Hydrogenation curves for 1,3-cyclooctadiene in ethanol catalyzed by the platinum clusters supported on the chelate resin-A13+ complex dried from ethanol (solid circles) and from water (open circles).
the CR-AI and CR-AI-Pt resins strongly suggests that the platinum clusters occupy the pores of diameters of -40 of the chelate resin complex. The transmission electron micrograph (TEM) shown in Figure 3 indicates the presence of small platinum clusters in the resin, with an average size of 26 A. Catnlytic Activities in the Hydrogenationof Olefins and Diene. Platinum clusters supported on CR-metal complexes having high specific surface areas were applied as the catalysts for the hydrogenation of C-C double bonds under atmospheric pressure at 30 "C. Figure 4 illustrates the hydrogenation curves for 1,3cyclooctadiene (COD) in ethanol using the prepared CR-AI-Pt catalysts. The initial rate of hydrogen uptake by the catalyst dried directly from water (0.288 cm3 m i d ) is not as fast as that dried
20
40 60 t lmin Figure 6. As in Figure 5 except that the hydrogenation of IJ-cyclooctadiene was in cyclohexane.
TABLE III:
Initial Rate of Hydrogen Uptake over tbe Platinum Cluster Catalvsts S u ~ ~ o r t eond the Chelate Resin-Metal Complexes initial rate/cm3 min-' surface area/ catalvst m*n-' COD CHE CHoE COE 0.090 0.022 0.057 0.021 1.5 CR-Na-Pt 0.058 0.404 0.089 0.244 13.7 CR-Mg-Pt 0.260 0.982 0.543 0.615 CR-AI-Pt 16.2
after being washed with ethanol (0.543 cm3 min-I), which appears to be related to the respective specific surface areas of the catalysts. The hydrogenation rate also depends on the type of metal ions involved in the resin, as shown in Figure 5. The initial rate of hydrogen uptake increases in the order of Na+ < Mg2+< AI3+, which again follows the increasing order of the specific surface area, as indicated in Table 111. The hydrogenation of various olefins, such as cyclohexene (CHE), cycloheptene (CHpE), and cyclooctene (COE),was also performed using CR-M-Pt (M = Na, Mg, AI)dried from ethanol, and the initial rates show the same order (Table 111) as above. Usually, homogeneous dispersions of metal clusters, protected by a water-soluble polymer, are active catalysts in ethanol or a hydrophilic solvent, while being completely inactive in cyclohexane or hydrophobic solvents. If the polymer protecting the metal clusters is insoluble in a hydrophobic solvent, it will shrink and completely cover the supported metal particles, thus preventing contact between the substrates and the metal particles. Figure 6 illustrates the hydrogenation curves of COD with cyclohexane instead of ethanol as the solvent using all three catalysts. While the order is the same as in ethanol, the reaction rates are much lower. This result suggests that the platinum clusters in the present catalyst are probably not completely covered by the linear parts of the resin polymer.
J. Phys. Chem. 1992, 96, 3799-3806 Acknowledgment. We express our thanks to Dr. Eisuke Ogata of the Department of Reaction Chemistry, the University of Tokyo, for his technical advice on measuring the specific surface area. Thanks are also extended to Dr. Shigeru Ohtsuka of the Engineering Research Institute for his technical assistance in scanning electron microscopy and X-ray microanalysis measurement. This
3799
work was supported by a Grant-in-Aid for Scientific Research (C) (No. 01550611) from the Ministry of Education, Science, and Culture, Japan. Registry No. COD, 1700-10-3; CHE, 110-83-8; COE, 931-88-4; CHpE, 628-92-2; Pt, 7440-06-4.
SoiibState loSAgNMR Characterization of Silver Dispersed on Oxide Supports J. K. Plischke; A. J. Benesi,t and M. A. Vannice*,+ Department of Chemical Engineering, 107 Fenske Laboratory Building, and Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: October 16, 1991; Zn Final Form: December 23, 1991)
Magic angle spinning lo9Agsolid-state NMR spectroscopy, transmission electron microscopy, X-ray diffraction, and gas adsorption measurements were used to characterize Ag particles supported on SO2, 7-A120,, and zeolite A. The samples were prepared either by a standard incipient wetness technique or by ion exchange followed by drying and reduction in H2. IwAg NMR spectroscopy was used to follow the state of the silver throughout the preparation process. lwAg spectra were obtained for metallic Ag and for pure powder samples of AgNO, and AgCl as well as for Ag' in solution; however, no resonances were observed for powder samples of Ag20 and Ag202. Prior to drying, chemical shifts were found to be close to those of aqueous AgN0, solutions (-0 ppm), and after drying at 373 K, the chemical shift moved upfield to approach that of bulk AgNO,. After reduction, a single Knight-shifted metallic resonance at +5252 f 10 ppm was observed for all samples which contained Ag particles larger than 50 nm in diameter. Low-loading, well-dispersed samples with smaller Ag particles yielded no observable lo9Agresonance. It is proposed that the absence of an observable '"Ag signal in the latter samples is due to a surface boundary effect on conduction electrons, common to other metal conductors, which broadens the signal. The adsorption of 02,C12, and HC1 on the surface of the large Ag crystallites had no effect on the lo9Agspectrum.
Introduction The reaction between ethylene and molecular oxygen has been catalyzed by Ag for decades to give economic yields of ethylene oxide;lJ however the precise role of silver in the catalytic formation of ethylene oxide still remains unclear. In an effort to elucidate the state of Ag, we have applied solid-state MAS lo9Ag N M R spectroscopy to characterize unreduced, impregnated Ag catalysts as well as reduced Ag crystallites supported on S O 2 , q-A120,, and zeolite A. Chemical shifts were correlated with the state of the unreduced Ag while Knight shifts and T I and T2 relaxation times were determined for dispersed silver metal particles. The two naturally occurring isotopes of Ag, Io9Ag (48.18% natural abundance) and lo7Ag (51.82% natural abundance), are Z = nuclei with absolute NMR sensitivities of 4.86 X and 3.43 X respectively, relative to 'H.Due to this low sensitivity and generally long T I relaxation times, only a limited number of lwAg NMR studies have been reported in the literature, and most of these have dealt with Ag compounds in solution. This study apparently represents the first attempt to examine supported Ag crystallites and their precursors using solid-state lWAgNMR. Due to the aforementioned physical properties of "Ag, the detection of lwAg resonances requires extensive signal averaging and can become prohibitively time-consuming, particularly if one is trying to observe low concentrations of surface atoms. In the solid state, these problems are compounded by chemical shift and Knight shift anisotropy, which can yield inhomogeneously broadened resonances which are difficult to detect above background noise. Also,the short T2relaxation times found in solids broaden resonances homogeneously and can make it difficult to detect NMR signals which decay by T2relaxation prior to probe r i n g d ~ w n . ~The latter phenomenon can be partially overcome with ringdown elimination pulse seq~ences.~ The intent *Towhom correspondence should be addressed. 'Department of Chemical Engineering. *Department of Chemistry. 0022-365419212096-3799$03.00/0
of this investigation was to monitor the genesis of reduced Ag catalysts and to determine if resonances could be obtained for very small ( < l o nm) Ag crystallites.
Experimental Section Sample Reparation. Ag/Si02 and Ag/q-A120, samples were prepared by a standard incipient wetness technique. The S O 2 (Davison, grade 57,220 m2/g) was ground and the 60/100 mesh cut was used for sample preparation. q-A1203(245 m2/g) was obtained from Exxon Research and Engineering Co. and had been prepared by the calcination in air of Davison &alumina trihydrate for 4 h at 863 K. Prior to impregnation both support materials were heated at 823 K for 2 h in air (Linde, dry grade) flowing at 0.472 L(STP) m i d to remove any organic impurities. Incipient wetness impregnation was conducted as follows. The appropriate amount of AgN03 (Aldrich Gold Label, 99.9999%) was dissolved in an amount of doubly distilled, deionized (DD) water which was just sufficient to fill the pore volume of the support (1.4 mL of H20/g S O 2 ,0.5 mL of H,O/g A120,). The solution was added dropwise to the support under vigorous stirring. In addition, a small amount of Fe was desired in some samples to act as an NMR relaxation agent. For these samples Fe(NO,), (Johnson-Matthey, Puratronic grade) dissolved in DD water was subsequently added dropwise to the support in a similar fashion. The samples were then dried in an oven overnight at approximately 373 K and stored in a desiccator until used. Ag loadings were determined by atomic absorption spectroscopy at the Mineral Constitution Laboratory of The Pennsylvania State University. Physical mixtures of Ag powder (Johnson-Matthey, Puratronic grade 99.999%) with Si02 or 7-A120, were prepared as references. Samples of Ag supported on zeolite A (Union Carbide, 3A) were prepared by ion exchange with aqueous AgNO, in a darkened (1) Van Santen, R. A.; Kuipers, H. P. C. E. Adu. C a r d . 1987, 35, 265. (2) Berty, J. M. In Applied Industrial Catalysis; Leach, B. E., Ed.; Academic Press: New York, 1983-4; Vol 1, p 207. (3) Benesi, A. J.; Ellis, P. D. J . Magn. Res. 1988, 78, 511.
0 1992 American Chemical Society
