Palladium nanoparticles stabilized by poly (N-vinyl-2-pyrrolidone) (PVP) can be synthesized by corresponding Pd(acac)2 (acac = acetylacetonate) as precursor in methanol at 80°C for 2？h followed by reduction with NaBH4 and immobilized onto SiO2 prepared by sol-gel process under acidic conditions (HF or HCl). The PVP/Pd molar ratio is set to 6. The effect of the sol-gel catalyst on the silica morphology and texture and on Pd(0) content was investigated. The catalysts prepared (ca. 2% Pd(0)/SiO2/HF and ca. 0,3% Pd(0)/SiO2/HCl) were characterized by TEM, FAAS, and SEM-EDS. Palladium nanoparticles supported in silica with a size 6.6 ± 1.4？nm were obtained. The catalytic activity was tested in hydrogenation of alkenes. 1. Introduction Poly (N-vinyl-2-pyrrolidone) stabilized metal nanoparticles have attracted considerable interest when it comes to preventing coagulation and precipitation of the metal nanoparticles. Polymer protecting agents allow preparation of metal colloids that can be stable for months with reasonable control over size as well as shape [1]. The nanoparticles are kinetically unstable with respect to aggregation or the bulk metal and should be stabilized by electrostatic or steric protection [2, 3]. Some of the protecting agents provide steric stabilization such as surfactants [4], ionic liquids [5–8], and polyoxoanions [9]. Their main disadvantage, however, is the problematic separation of the catalytic particles from the product and unused reactants at the end of the reaction. Immobilization of the particles on a solid support can facilitate the separation process, but may simultaneously lead to a decrease in activity. The nanoparticle synthesis involves addition of a polymer to the metal salt followed by chemical or thermal reduction to produce a stable black suspension of Pd(0) particles. The types of stabilizers and concentration, the solvent polarity [10], and the aging time of colloidal suspensions [11] can have an effect on the size, shape and catalytic activity of palladium nanoparticles. Platinum nanoparticles protected by PVP have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis [12]. There have been several very recent reports in the literature of the catalytic properties of nanoparticles of Ag [13, 14], Rh [15], Pt [16, 17] and Pd [18] in PVP. Palladium colloid solutions stabilized by poly(N-vinyl-2-pyrrolidone) can be used as catalysts with high reactivity in C–C bond formation microwave-assisted reactions as shown by Heck [19] and Suzuki [20]. Catalytic
References
[1]
T. Teranishi and M. Miyake, “Size control of palladium nanoparticles and their crystal structures,” Chemistry of Materials, vol. 10, no. 2, pp. 594–600, 1998.
[2]
J. D. Aiken III and R. G. Finke, “A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis,” Journal of Molecular Catalysis A, vol. 145, no. 1-2, pp. 1–44, 1999.
[3]
A. Roucoux, J. Schulz, and H. Patin, “Reduced transition metal colloids: a novel family of reusable catalysts?” Chemical Reviews, vol. 102, no. 10, pp. 3757–3778, 2002.
[4]
V. Mévellec, A. Roucoux, E. Ramirez, K. Philippot, and B. Chaudret, “Surfactant-stabilized aqueous iridium(0) colloidal suspension: an efficient reusable catalyst for hydrogenation of arenes in biphasic media,” Advanced Synthesis & Catalysis, vol. 346, no. 1, pp. 72–76, 2004.
[5]
J. Dupont and J. D. Scholten, “On the structural and surface properties of transition-metal nanoparticles in ionic liquids,” Chemical Society Reviews, vol. 39, no. 5, pp. 1780–1804, 2010.
[6]
P. Migowski and J. Dupont, “Catalytic applications of metal nanoparticles in imidazolium ionic liquids,” Chemistry, vol. 13, no. 1, pp. 32–39, 2007.
[7]
J. D. Scholten, B. C. Leal, and J. Dupont, “Transition metal nanoparticle catalysis in ionic liquids,” ACS Catalysis, vol. 2, no. 1, pp. 184–200, 2012.
[8]
X.-D. Mu, D. G. Evans, and Y. Kou, “A general method for preparation of PVP-stabilized noble metal nanoparticles in room temperature ionic liquids,” Catalysis Letters, vol. 97, no. 3-4, pp. 151–154, 2004.
[9]
J. D. Aiken III and R. G. Finke, “Polyoxoanion- and tetrabutylammonium-stabilized Rh(0)(n) nanoclusters: unprecedented nanocluster catalytic lifetime in solution,” Journal of the American Chemical Society, vol. 121, no. 38, pp. 8803–8810, 1999.
[10]
N. Gacemand and P. Diao, “Effect of solvent polarity on the assembly behavior of PVP coated rhodium nanoparticles,” Colloids and Surfaces A, vol. 417, pp. 32–38, 2013.
[11]
I. Miguel-García, A. Berenguer-Murcia, T. García, and D. Cazorla-Amorós, “Effect of the aging time of PVP coated palladium nanoparticles colloidal suspensions on their catalytic activity in the preferential oxidation of CO,” Catalysis Today, vol. 187, no. 1, pp. 2–9, 2012.
[12]
H. Song, R. M. Rioux, J. D. Hoefelmeyer et al., “Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: synthesis, characterization, and catalytic properties,” Journal of the American Chemical Society, vol. 128, no. 9, pp. 3027–3037, 2006.
[13]
P. S. Mdluli, N. M. Sosibo, P. N. Mashazi et al., “Selective adsorption of PVP on the surface of silver nanoparticles: a molecular dynamics study,” Journal of Molecular Structure, vol. 1004, no. 1–3, pp. 131–137, 2011.
[14]
S. W. Kang and Y. S. Kang, “Silver nanoparticles stabilized by crosslinked poly(vinyl pyrrolidone) and its application for facilitated olefin transport,” Journal of Colloid and Interface Science, vol. 353, no. 1, pp. 83–86, 2011.
[15]
N. Yan, Y. Yuan, and P. J. Dyson, “Rhodium nanoparticle catalysts stabilized with a polymer that enhances stability without compromising activity,” Chemical Communications, vol. 47, no. 9, pp. 2529–2531, 2011.
[16]
M. Liu, J. Zhang, J. Liu, and W. W. Yu, “Synthesis of PVP-stabilized Pt/Ru colloidal nanoparticles by ethanol reduction and their catalytic properties for selective hydrogenation of ortho-chloronitrobenzene,” Journal of Catalysis, vol. 278, no. 1, pp. 1–7, 2011.
[17]
V. L. Nguyen, M. Ohtaki, V. N. Ngo, M. T. Cao, and M. Nogami, “Structure and morphology of platinum nanoparticles with critical new issues of low high-index-facets,” Advances in Natural Science, vol. 3, no. 2, pp. 1–4, 2012.
[18]
V. L. Nguyen, D. C. Nguyen, H. Hirata, M. Ohtaki, T. Hayakawa, and M. Nogami, “Chemical synthesis and characterization of palladium nanoparticles,” Advances in Natural Science, vol. 1, pp. 1–5, 2010.
[19]
D. D. L. Martins, H. M. Alvarez, L. C. S. Aguiar, and O. A. C. Antunes, “Heck reactions catalyzed by Pd(0)-PVP nanoparticles under conventional and microwave heating,” Applied Catalysis A, vol. 408, no. 1-2, pp. 47–53, 2011.
[20]
D. de Luna Martins, H. M. Alvarez, and L. C. S. Aguiar, “Microwave-assisted Suzuki reaction catalyzed by Pd(0)-PVP nanoparticles,” Tetrahedron Letters, vol. 51, no. 52, pp. 6814–6817, 2010.
[21]
M. A. Gelesky, S. S. X. Chiaro, F. A. Pavan, J. H. Z. Dos Santos, and J. Dupont, “Supported ionic liquid phase rhodium nanoparticle hydrogenation catalysts,” Dalton Transactions, no. 47, pp. 5549–5553, 2007.
[22]
M. A. Gelesky, C. W. Scheeren, L. Foppa, F. A. Pavan, S. L. P. Dias, and J. Dupont, “Metal nanoparticle/ionic liquid/cellulose: new catalytically active membrane materials for hydrogenation reactions,” Biomacromolecules, vol. 10, no. 7, pp. 1888–1893, 2009.
[23]
F. Durap, ？. Metin, M. Aydemir, and S. ？zkar, “New route to synthesis of PVP-stabilized palladium(0) nanoclusters and their enhanced catalytic activity in Heck and Suzuki cross-coupling reactions,” Applied Organometallic Chemistry, vol. 23, no. 12, pp. 498–503, 2009.
[24]
C. J. Brinker and G. W. Scherer, The Physics and Chemistry of Sol-Gel Processing, Academic Press, London, UK, 1990.
