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Synthesis, Characterization, and Reaction Studies of a PVP-Capped Platinum Nanocatalyst Immobilized on Silica† Oliver D. Lyons, Nathan E. Musselwhite, Lindsay M. Carl, Kimberly A. Manbeck, and Anderson L. Marsh* Department of Chemistry, Lebanon Valley College, 101 North College Avenue, Annville, Pennsylvania 17003 Received April 7, 2010. Revised Manuscript Received June 1, 2010 An immobilized platinum nanocatalyst was prepared by first functionalizing the surface of activated silica with poly(vinylpyrrolidone) (PVP) and then reducing encapsulated platinum ions in the presence of these functionalized supports to form nanoparticles. Surface functionalization was monitored by infrared spectroscopy and surface area measurements, and the resulting nanocatalyst was characterized using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Platinum nanoparticle size was determined to be approximately 5 nm based on TEM and XRD measurements. Catalytic activity of this material for the hydrogenation of cyclohexanone was found to be greater than that of unsupported colloidal PVP-capped platinum nanocatalysts. In addition, the immobilized nanocatalyst displayed no change in activity after being recycled. Taken together, these results clearly indicate advantages in the design of catalytic materials with desired properties.
Introduction Selective hydrogenations using platinum catalysts play a valuable role in many chemical industries, ranging from petrochemicals to pharmaceuticals.1,2 Many of these reactions are operated in the liquid phase at much lower temperatures than gas-phase reactions, thus increasing selectivity due to a decrease in the occurrence of side reactions.3 Achieving 100% selectivity is important to eliminate waste from unwanted byproducts, thereby streamlining processes by removing separation steps.4 Control of selectivity in catalytic reactions may be gained by tuning activation energy barriers through consideration of catalyst properties (such as particle size), reactant structure, and reaction conditions.5 In order to gain a better understanding of catalyst properties that influence selectivity, more realistic materials with well-controlled particle sizes and structures must be employed. Preparation of catalytic materials using synthetic methods from nanoscience allows for this control.6,7 For example, the size and shape of transition metal nanoparticles may be finely tuned using solution-based colloidal chemistry.8,9 Unlike metal particles in traditional supported catalysts, these nanoparticles have a narrow size distribution. In order to prevent aggregation in solution, the nanoparticles are typically capped with a polymer or dendrimer. These colloidal nanocatalysts are active for a variety of reactions; however, their stability during reactions may be cause for concern.10-13 †
Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. Telephone: 717-867-6149. Fax: 717-867-6075. E-mail:
[email protected]. (1) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH Publishers, Inc.: New York, 1997. (2) M€aki-Arvela, P.; Hajek, J.; Samli, T.; Murzin, D. Y. Appl. Catal., A 2005, 292, 1. (3) Singh, U. K.; Vannice, M. A. Appl. Catal., A 2001, 213, 1. (4) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686. (5) Zaera, F. J. Phys. Chem. B 2002, 106, 4043. (6) Bell, A. T. Science 2003, 299, 1688–1691. (7) Somorjai, G. A.; Rioux, R. M. Catal. Today 2005, 100, 201. (8) Roucoux, A.; Schulz, J.; Patin, H. Chem. Rev. 2002, 102, 3757. (9) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (10) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (11) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2003, 107, 12416. (12) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 5726. (13) Narayanan, R.; El-Sayed, M. A. Langmuir 2005, 21, 2027.
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One possible method of stabilizing these nanocatalysts is by supporting them on a high surface area material.14 The research on supported polymer-capped or dendrimer-capped platinum nanocatalysts has largely focused on making the colloidal metal nanoparticles and then embedding them within a high surface area material through several routes. For example, platinum nanoparticles may be synthesized using colloidal chemistry techniques and then impregnated into a high surface area support.15-21 More specifically, platinum nanoparticles with a narrow size distribution were prepared and then embedded into mesoporous SBA-15 silica.18,20 To remove the polymer capping agent, the resulting catalytic material was calcined and reduced, leaving bare platinum nanoparticles that were active for hydrogenation reactions. Polymercapped noble metal colloids were also immobilized on silica in the presence of organic acids.22 Other routes involved embedding colloidal nanoparticles in a high surface area oxide through a modified sol-gel procedure.23,24 These embedded nanocatalysts were then tested and found active for various hydrogenation reactions. More recent approaches utilized the colloidal nanoparticles as templating agents for the formation of mesoporous oxide materials, and after calcinations were tested as catalysts in various hydrogenation reactions.25-27 In each of these studies, the colloidal platinum nanoparticles were first synthesized and then employed in (14) Narayanan, R.; El-Sayed, M. A. J. Catal. 2005, 234, 348. (15) Yu, W.; Liu, H.; An, X. J. Mol. Catal. A 1998, 129, L9. (16) Yoo, J. W.; Hatchcock, D. J.; El-Sayed, M. A. J. Catal. 2003, 214, 1. (17) Lang, H.; May, R. A.; Iversen, B. L.; Chandler, B. D. J. Am. Chem. Soc. 2003, 125, 14832. (18) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192. (19) Serrano-Ruiz, J. C.; Lopez-Cudero, A.; Solla-Gullon, J.; Sepulveda-Escribano, A.; Aldaz, A.; Rodrı´ guez-Reinoso, F. J. Catal. 2008, 253, 159. (20) Grass, M. E.; Rioux, R. M.; Somorjai, G. A. Catal. Lett. 2009, 128, 1. (21) Lee, I.; Zaera, F. J. Catal. 2010, 269, 359. (22) Wang, Q.; Liu, H.; Wang, H. J. Colloid Interface Sci. 1997, 190, 380. (23) Lange, C.; De Caro, D.; Gamez, A.; Storck, S.; Bradley, J. S.; Maier, W. F. Langmuir 1999, 15, 5333. (24) Beakley, L. W.; Yost, S. E.; Cheng, R.; Chandler, B. D. Appl. Catal., A 2005, 292, 124. (25) Lin, K.-J.; Chen, L.-J.; Prasad, M. R.; Cheng, C.-Y. Adv. Mater. 2004, 16, 1845. (26) Ma, J.; Reng, S.; Pan, D.; Li, R.; Xie, K. React. Funct. Polym. 2005, 62, 31. (27) Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz, K.; Grass., M.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3027.
Published on Web 07/09/2010
DOI: 10.1021/la101383s
16481
Article
Lyons et al.
the preparation of embedded nanocatalysts. In some instances, the size and shape of the platinum nanoparticles were altered after formation of the supported nanocatalyst. A more efficient approach could involve the synthesis of the platinum nanoparticles in the presence of a functionalized high surface area support. In this work, poly(vinylpyrrolidone) (PVP) was synthesized on the surface of activated silica by first modifying the surface with 3-(trimethoxysilyl)propyl methacrylate (MPS) and then polymerizing 1-vinyl-2-pyrrolidinone in the presence of azobisisobutyronitrile (AIBN). Functionalization of the surface after each step of the synthesis was confirmed through surface area measurements and infrared spectroscopy. Silica-supported PVP-capped platinum nanocatalysts were prepared by reduction of platinum ions encapsulated on the surface of the support material. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements were obtained, and particle size was determined to be 4.6 nm and 4.9 nm, respectively. The prepared nanocatalyst was more active for the hydrogenation of cyclohexanone as compared to unsupported PVP-capped Pt nanocatalysts and displayed no change in turnover frequency after being recycled.
Experimental Section Materials. Azobisisobutyronitrile (AIBN), 3-(trimethoxysilyl)propyl methacrylate (MPS, 98%), sodium borohydride (99%), silica gel (200-425 mesh), and 1-vinyl-2-pyrrolidinone (>99%), and poly(vinylpyrrolidone) (MW=29 000 g/mol) were purchased from Sigma-Aldrich and used as received. Dihydrogen hexachloroplatinate hexahydrate (99.9%) was purchased from Alfa Aesar and used as received.
Preparation of Silica-Supported PVP-Capped Pt Nanocatalyst. The silica-supported PVP-capped Pt nanocatalyst was prepared stepwise based on a procedure previously published for a Ru nanocatalyst.28 First, the silica surface was functionalized with MPS. To begin, 20.5 g of silica gel was activated by heating for 12 h at 150 °C. Next, 10 g of activated silica was added to a 500 mL multineck flask, along with 2.5 g of MPS and 250 mL of dry toluene. The mixture was refluxed under a nitrogen atmosphere at 111.1 °C for 24 h, after which the solution was filtered and the solid placed in a Soxhlet apparatus for 40 h to remove any unreacted MPS. The thimble was put in a vacuum desiccator at room temperature until dry, or for approximately 24 h. After functionalization of the silica surface with MPS, poly(vinylpyrrolidone) was formed through a free-radical polymerization across the end vinyl group. In a Schlenk flask, 1.0 g of MPS-functionalized silica, 5.0 g of 1-vinyl-2-pyrrolidinone, 0.1 g of AIBN initiator, and 25 mL of methanol were degassed by three freeze-pump-thaw cycles. The mixture was then heated for an excess of 24 h in an oil bath at 60 °C, after which it was diluted with 25 mL of methanol. The solid was removed by vacuum filtration and washed with 200 mL of methanol. Unreacted monomer and ungrafted PVP were removed by using a Soxhlet extraction with 300 mL of methanol for over 24 h. The final product was dried in a vacuum desiccator for over 24 h. To synthesize the platinum nanocatalyst, 0.033 g of H2PtCl6 3 6H2O and 0.784 g of silica-PVP were added with 1.6 mL of doubly distilled water (18.6 MΩ). This mixture was stirred for 10 min until uniform, after which it was placed in a vacuum desiccator for approximately 24 h to dry. Once dry, 0.6 g of Pt/silica-PVP was dispersed in 3.0 mL of twice-distilled water, and 0.0188 g of NaBH4 in 1.5 mL twice-distilled water was added dropwise (approximately 15 min) under vigorous stirring, during which time the mixture became brownish-black. After 2.5 h, the heterogeneous mixture was filtered and washed three times with doubly deionized (28) Zhou, X.; Wu, T.; Hu, B.; Jiang, T.; Han, B. J. Mol. Catal. A 2009, 306, 143.
16482 DOI: 10.1021/la101383s
H2O. The product was dried in a vacuum desiccator over 48 h. Roughly 0.5 g of product was created.
Synthesis of Unsupported PVP-Capped Platinum Nanocatalysts. The 3.6 nm PVP-capped platinum nanocatalysts were
prepared by a seeded growth method.18,29,30 First, 2.9 nm Pt nanocatalysts were made by an alcohol reduction method containing aqueous H2PtCl6 3 6H2O (20.0 mL, 6.0 mM), 180 mL of methanol, and 133 mg of PVP and refluxed for 3 h. The freshly prepared 2.9 nm Pt particles were dispersed in 100 mL of 90% methanol. Aqueous H2PtCl6 3 6H2O (10.0 mL, 6.0 mM) and methanol (90 mL) were added to the Pt colloidal solution and refluxed for 3 h. For 7.1 nm particles, glycol solutions of PVP (3 mL, 0.375 M) and H2PtCl6 3 6H2O (1.5 mL, 0.0625 M) were added to boiling ethylene glycol (2.5 mL) alternatively every 30 s for 16 min. The nanocatalysts were precipitated with the addition of acetone. Solvents were evaporated in each procedure, and the resulting residue was redispersed in water to obtain 1.2 mM solutions in terms of Pt concentration. Nanocatalyst Characterization. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Impact 420 spectrophotometer using KBr pellets. Surface area measurements were conducted on a Micromeritics ChemiSorb 2720 pulse absorption system under a flow of nitrogen gas. TEM images were obtained at The Penn State College of Medicine’s Electron Microscopy Laboratory using a JEOL JEM 1400 transmission electron microscope. Samples for TEM analysis were prepared by placing drops of nanocatalyst solution on copper grids coated with carbon/ Formvar films. XRD powder patterns were collected at Penn State University main campus using a Bruker D8 Advance instrument with a LynxEye detector and Cu KR radiation. Samples for XRD analysis were prepared by casting thin films onto quartz microscope slides. After carefully accounting for background contributions using the peak fitting routine in IGOR Pro, peak positions and peak widths were used to calculate particle size through the Debye-Scherrer equation: d ¼
Kλ β cos θ
ð1Þ
where d is the particle size, K is the shape factor (0.9), λ is the wavelength of the Cu X-rays (0.154 nm), β is the full-width at halfmaximum of the peak in radians, and θ is the position of the peak. Pt content for the silica-supported nanocatalysts was measured using a Perkin-Elmer AAnalyst 100 Flame atomic absorption spectrometer. Solutions were prepared by treatment of the silicasupported nanocatalyst with warm aqua regia, with a small amount of lanthanum(III) nitrate (Aldrich) added to aid in reduction of the platinum. The Pt content of the silica-supported nanocatalyst was determined to be 1.2% by mass. Cyclohexanone Hydrogenation. Hydrogenation reactions were performed in a Parr 4566 mini benchtop reactor.30 The reactor was charged by adding a selected amount of the Pt nanocatalyst to a known amount of cyclohexanone (Aldrich, 99.8%) dissolved in water to give a cyclohexanone to Pt mole ratio of 1000 to 1. After flushing with hydrogen (Airgas, Research purity), the reactor was heated to the reaction temperature and then pressurized to the selected hydrogen pressure. Small aliquots (
