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C NMR Characterization of the Organic Constituents in Ligand-Modified Hexagonal Mesoporous Silicas: Media for the Synthesis of Small, Uniform-Size Gold Nanoparticles Edward W. Hagaman,* Haoguo Zhu, Steven H. Overbury, and Sheng Dai Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received November 14, 2003. In Final Form: July 12, 2004 This paper reports the 13C NMR characterization of functionalized MCM-41’s and describes the chemistry that occurs in the pores of these materials in the process of forming gold nanoparticles. Nanoparticles formed on hexagonal mesoporous silica (MCM-41) by hydrogen reduction of chloroauric acid have little affinity for pure silica surfaces. The gold can be removed from the support with very mild treatment, for example, solvent extraction. The loss of gold from the substrate can be prevented using a pore functionalization methodology that entails synthesis of the silica containing polydentate amine functionality chemically bound in the mesopores. The synthetic scheme introduces solvents and templating reagents (surfactants) into the mesopores that are chemically reactive under the conditions required for gold particle formation. Extensive base-catalyzed elimination and nucleophilic substitution reactions involving the tetraalkylammonium surfactant occur during the reduction of chloroauric acid to gold.
Introduction Highly dispersed gold nanoparticles have been demonstrated to be very active catalysts for a number of important chemical reactions ranging from oxidation1-6 to hydrogenation,7 hydrogen production,8 and hydrochlorination.9 Catalytic activities of gold strongly depend on its particle size. It is necessary to have homogeneous distributions of small gold nanoparticles with diameters between 2 and 5 nm for excellent catalytic activities. To achieve the controlled synthesis of dispersed gold nanoparticles in the narrow size range, several methods have been developed. These methodologies include coprecipitation from an aqueous solution of HAuCl4,1,2 depositionprecipitation using precipitation agents,7,8,10,11 cosputtering of gold and metal oxide on a substrate,12 and chemical vapor deposition of gold nanoparticles on porous matrixes.3 The key drawback associated with these synthetic processes is the difficulty controlling both location and size of the gold nanoparticles in oxide matrixes. Gold nanoparticles are normally formed either on external surfaces of the oxide particles or embedded in microporous oxide matrixes. Gold nanoparticles on external surfaces are susceptible to aggregation because of the decreased (1) Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batista, J.; Hocevar, S.; Matralis, H. K. Catal. Today 2002, 75, 157-167. (2) Harut, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301-309. (3) Okumura, M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 315-316. (4) Guzman, J.; Gates, B. C. J. Phys. Chem. B 2002, 106 (31), 76597665. (5) Lopez, N.; Nørskov, J. K. J. Am. Chem. Soc. 2002, 124 (38), 1126211263. (6) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124 (25), 7499-7505. (7) Okumura, M.; Akita, T.; Haruta, M. Catal. Today 2002, 74, 265269. (8) Boccuzzi, F.; Chiorino, A.; Manzoli, M.; Andreeva, D.; Tabakova, T.; Ilieva, L.; Iadakiev, V. Catal. Today 2002, 75, 169-175. (9) Bond, G. C. Catal. Today 2002, 72, 5-9. (10) Uphade, B. S.; Yamada, Y.; Akita, T.; Nakamura, T.; Haruta, M. Appl. Catal., A 2001, 215, 137-148. (11) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B 2002, 106 (31), 7634-7642. (12) Kobayashi, T.; Haruta, M.; Tsubota, S.; Sano, H. Sens. Actuators 1990, B1, 222.
melting point of nanoparticles13 and the lack of space confinement, while gold nanoparticles embedded in the microporous matrixes are inaccessible to reactants for effective catalytic reactions. Alternative methodologies for synthesis of gold nanoparticles inside the pores of mesoporous materials are summarized in our previous work.14 We have recently developed a cosynthesis methodology for the preparation of gold-containing mesoporous silica materials. The essence of this sol-gel cosynthesis method is to combine surfactant template synthesis of mesoporous silica materials with the introduction of metal ions via bifunctional amino-silane ligands, so that the formation of mesostructures and metal-ion doping occur simultaneously.14 The purpose of the amine functionality is to complex and stabilize the gold(III) precursors and the gold nanoparticles.15-17 This strategy does work and allows the preparation of small uniform (2-5 nm) gold particles on silica. Fumed silica18 and silica gels19,20 have been studied extensively by solid-state NMR methods, as have ligandmodified silicas.21-23 Amine24 and polydentate amine (13) Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. Chem. Commun. 2002, 1334-1335. (14) Zhu, H.; Lee, B.; Dai, S.; Overbury, S. H. Langmuir 2003, 19, 3974-3980. (15) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 47234730. (16) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277-6282. (17) Brown, L. O.; Hutchison, J. F. J. Am. Chem. Soc. 1999, 121, 882-883. (18) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 51035119. (19) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 52085219. (20) Fyfe, C. A.; Zhang, Y.; Aroca, P. J. Am. Chem. Soc. 1992, 114, 3252-3255. (21) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 18481851. (22) Wang, L.-Q.; Exarhos, G. J.; Liu, J. Adv. Mater. 1999, 11, 13311341. (23) Maciel, G. E.; Bronnimann, C. E.; Zeigler, R. C.; Chuang, I.; Kinney, D. R.; Keiter, E. A. In The Colloid Chemistry of Silicas; Bergna, H. E., Ed.;ACS Advances in Chemistry Series, Vol. 234; American Chemical Society: Washington, DC, 1994; p 269.
10.1021/la0304161 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/28/2004
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ligands25 grafted onto silica surfaces have been prepared and characterized by NMR. This paper reports the solidstate 13C NMR characterization of MCM-41’s functionalized with propyldiethylenetriamine side chains bonded to the pore walls and describes the chemistry that occurs in the pores of these materials in the process of forming gold nanoparticles. Experimental Section Synthesis. Preparation of Amine-Derivatized MCM-41. The procedure follows that given in Zhu et al.,14 using 1N-[3(trimethoxysilyl)propyl]diethylenetriamine (TMSPdien), 1, for the side chain precursor amine in place of N-[3-(trimethoxysilyl)propyl]ethylenediamine. Preparation of Gold-Impregnated, Amine-Derivatized MCM41. To a solution of HAuCl4‚3H2O (0.083 g, 0.2 mmol, Aldrich) in 70 mL of H2O, 1 (0.3 mL, 1.2 mmol) was added slowly, with stirring. After the solution changed to a light yellow color, 1.6 g of CTAB and 20 mL of 1.0 M aqueous KOH were added under vigorous stirring. The solution was stirred vigorously for 30 min. Tetraethyl orthosilicate (6.0 g, 29 mmol, Aldrich) was added, and the mixture was stirred for 20 h at room temperature. The resulting precipitate was filtered, washed with deionized H2O, and dried for 1 day at room temperature under a vacuum. This material, denoted “as-synthesized”, was portioned, and separate lots were reduced at 50, 100, and 200 °C for 1 h using 4% H2 in Ar in an atmospheric-pressure flow-through hydrogenation reactor. These samples are referred to as “reduced” materials. Portions of the as-synthesized and the reduced materials were set aside for NMR analysis without further chemical processing. The organic material remaining in these products was then removed by solvent extraction: one gram of derivatized silica was stirred with 160 mL of 0.4 M HCl in ethanol (prepared by adding 5.3 mL of 37% HCl to absolute ethanol to make a final volume of 160 mL), filtered, washed with ethanol, and dried in a vacuum. In an alternative extraction procedure, the surfactant was removed by ion exchange: one gram of derivatized silica was stirred with 160 mL of 0.8 M NH4Cl in ethanol, filtered, washed with ethanol, and dried in a vacuum. In Results and Discussion, all as-synthesized materials that have been subjected to further processing (reduction, extraction) are described by specifying the process the material has undergone. Collection of the Effluent from the Gas Stream during Hydrogen Reduction. In the H2 reduction of one sample of amine-derivatized MCM14 (prepared without the gold precursor) conducted at 200 °C for 2 h, the apparatus was modified to collect the effluent carried by the gas stream from the hot zone of the reactor into a trap cooled with ice water. At the end of the reduction, a waxy solid that melted on warming was obtained from the trap. It dissolved in deuteriochloroform to yield a homogeneous transparent solution that was analyzed by 13C NMR. Assignments for 1-hexadecene26 δ (TMS ) 0 ppm): 113.9 (CH2), C(1); 139.1 (CH), C(2); 33.7 (CH2), C(3); 28.8, 29.1, 29.3, 29.4, 29.6, C(4)-C(13), multiple degenerate resonances with no specific assignments; 31.8, C(14); 22.6, C(15); 14.0 (CH3), C(16). Assignments for N,Ndimethylhexadecylamine:27 45.4 (CH3), N-methyl; 59.9 (CH2), C(1); 27.4 (CH2), C(2); 27.4, (CH2), C(3); 28.8, 29.1, 29.3, 29.4, 29.6, C(4)-C(13), multiple degenerate resonances with no specific assignments; 31.8, C(14); 22.6, C(15); 14.0 (CH3), C(16). NMR. NMR measurements were performed on a CMX Infinity spectrometer using a 2.42 T magnet. 13C spectra were obtained at 25.89 MHz by 1H-13C cross polarization (CP) using conventional techniques.28 The magic angle spinning (MAS) speed was 4.5 kHz. The 1H 90° pulse width was set to 3.1 µs (H1H ) 81 kHz), and the Hartmann-Hahn CP match condition was satisfied at the first upper spinning sideband. The recycle delay was 1 s. (24) Chiang, C.-H.; Liu, N.-I.; Koenig, J. L. J. Colloid Interface Sci. 1982, 86, 26-34. (25) Yang, J. J.; El-Nahhal, I. M.; Chuang, I.-S.; Maciel, G. E. J. Non-Cryst. Solids 1997, 209, 19-39. (26) Stothers, J. B. Carbon-13 NMR Spectroscopy; Academic Press: London, 1972; p 81. (27) Eggert, H.; Djerassi, C. J. Am. Chem. Soc. 1973, 95, 3710-3718. (28) Yannoni, C. S. Acc. Chem. Res. 1982, 15, 201.
Figure 1. Hexagonal mesoporous silica which contains the diethylenetriamine ligand bound through a propyl tether to a framework silicon site. The pore diameter of the materials discussed in this paper is 2.0-2.26 nm (unpublished results). Chemical shifts were referenced externally to the methyl resonance of hexamethylbenzene (17.3 ppm relative to TMS δ ) 0 ppm). The rate at which signal intensity accrues in a 1H-13C cross polarization experiment is dependent on the inverse sixth power of the internuclear distance, rCH-6. For proton-rich samples such as those considered here, all resonances cross polarize rapidly, in the absence of molecular motion. Rapid internal molecular reorientation diminishes the 1H-13C dipolar interaction strength, causing slow CP buildup rates. The molecular motion also slows the decay of the magnetization from the spin lock field. Consequently, it is possible to simplify spectra containing all signals by using a long CP contact time. During the long contact, resonances from static carbons build up and decay from the spectrum, leaving a signal that arises only from the more mobile carbon fraction. In the present work, a 20 ms CP contact time is used to generate these spectra. The rate at which the 13C signal decays is governed by proton relaxation in the rotating frame (1H T1F-1) and is estimated using variable contact time intensity data. The long contact time data are least-squares fit to a decaying exponential to determine the time constant, T1F.
Results and Discussion Hexagonal mesoporous silica which contains the diethylenetriamine ligand bound through a propyl tether to a framework silicon site is depicted schematically in Figure 1. The amine precursor, 1 (TMSPdien), is incorporated into the framework silica by hydrolysis and condensation of the trimethoxysilyl moiety during the synthesis of the mesoporous silica. The trimethoxysilyl function in 1 becomes a trisiloxysilyl function on the surface of the mesoporous silica. The propyldiethylenetriamine side chain is the only carbonaceous fragment that is bonded to the inorganic matrix. Solid-state NMR spectra are more complex than suggested by this simple expectation. The solid-state spectra of the derivatized MCM-41’s are interpreted on the basis of the following solution NMR studies of the triamine precursor.
NMR of 1 in Solution. The 13C NMR spectrum of 1 in water is dependent on triamine concentration (Figure 2). At 0.58 M, all resonances except the O-methyl resonance at 50 ppm show evidence of extensive heterogeneous broadening. The fine structure on the broad resonances indicates that multiple, stable conformations of the molecule coexist in solution. The subset of unique, conformationally based resonances for each carbon decreases upon dilution, suggesting they arise from intermolecular hydrogen-bonded conformers. Significant broadening remains evident at 0.1 M. Not until the concentration falls below 0.04 M does the spectrum become independent of concentration and display a single sharp resonance line for each carbon site. When 1 is bonded to a silica surface (vide infra) and participates in hydrogen-bonding interac-
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Figure 3. Titration plot of the side chain resonances of 1. The ordinate units are mole ratio of added acid (HCl) to 1, 0.04 M in water. Open triangles mark C(8), β to the primary amine, N(10). Open squares and diamonds mark C(6) and C(2), β to N(4). Solid squares and diamonds mark C(5) and C(9), β to N(7). The close chemical shift pairs, C(3) C(8), and C(5) C(6), are unambiguously assigned by their shift patterns.
Figure 2. 13C NMR spectra of 1 in water at (a) 0.58 M, (b) 0.14 M, and (c) 0.04 M. The asymmetry and fine structure of the resonance bands at higher concentrations is proof of heterogeneous broadening from multiple hydrogen-bonded conformers. Carbon assignments are indicated on the dilute spectrum. The sharp resonance at 67.4 ppm is from the internal chemical shift standard, 1,4-dioxane.
tions with solvent and surface silanols, the interface region will be heterogeneous and will likely result in broadened resonances for the triamine residue in the solid. Several chemical shift assignments of 1 follow directly from gross electronegativity and multiplicity arguments [C(1), C(2), C(9), OCH3]. The secondary aminocarbon resonances are assigned from their chemical shift response as a function of amine protonation. Protonation of primary and secondary amines results in characteristic upfield shifts of resonances for sites β to the amine function, with only minimal shifts at sites R and γ to the amine function.29 In the absence of shift contributions resulting from conformational reorientation, protonation-induced shifts at the R and γ sites are 0.1 ppm. Figure 3 shows a plot of the resonances of 1 as a function of HCl added to an aqueous solution of the amine. The ordinate in Figure 3 is the HCl/amine mole ratio. The primary amine in 1 has the lowest pKa and will show the largest β protonation shift, ∆δβ. The free amine resonance at 50.7 ppm which (29) Hagaman, E. W. Structure Determination of Natural Products by 13C Nuclear Magnetic Resonance Spectroscopy. Thesis, Indiana University, Bloomington, IN, 1973. β protonation shifts in simple amines are 5-6 ppm for primary amines and 2-3 ppm for secondary amines.
displays the greatest initial slope with added acid and ∆δβ of 5 ppm is allocated to the carbon β to the primary amine, C(8). Accompanying the protonation of a secondary amine function, pairs of carbon resonances shift in unison as there are two carbons which have a β relationship to the nitrogen. Two pairs of resonances show this behavior in Figure 3, those at 40.5 and 47.8 ppm and the pair at 47.3 and 22.5 ppm. In each set, one resonance is uniquely assigned on gross chemical shift/electronegativity arguments. Hence, the high-field methylene at 22.5 ppm (no attached heteroatom) and the 47.3 ppm aminomethylene must each be in a β relationship to the same amine function, that is, N(4), and are allocated to C(2) and C(6), respectively. Similar arguments assign the 40.5/47.8 ppm resonances to C(9)/C(5), respectively, carbons β to N(7). Only the resonance at 51.9 ppm remains unassigned. It must be the C(3) resonance, R to N(4), and the only aminomethylene in the molecule not expected to show a β protonation shift. The C(3) chemical shifts at the pH extremes are similar, though the chemical shift profile of this resonance gives evidence that this carbon experiences two opposing perturbations. The high-field shift at low acid concentration is typical of ∆δR or ∆δγ. The downfield shift at low pH likely signifies a conformational reorientation of the side chain and an averaged reduction in γ interactions involving C(3). The perturbation occurs simultaneously with the protonation of N(7), as evidenced by the C(5) and C(9) titration profiles, and may signify a greater average elongation along the long chain axis to maximize charge separation. Chemical shift assignments of the free base and fully protonated form of 1 are listed in Table 1. In aqueous solution, chloroauric acid reacts with 1 along two reaction channels. The basic amine functions of 1 are simply protonated by the acid to form the triple ammonium salt 2, where the anion is the hydrated conjugate base of the acid. In the second reaction path, each of the three amine functions of 1 displaces a chloride ion from the inner coordination sphere of the gold tetrachloride anion forming a complex between one gold(III) cation and one
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13C
Hagaman et al. Chemical Shifts of 1 in Water and Grafted into Silicaa
phase
acid
form
1
2
3
5
6
8
9
OCH3
aqueous aqueous aqueous aqueous solid
none HCl HAuCl4 HAuCl4 HCl
free base triply protonated inner co-ord sphere complexb triply protonatedb triply protonated
11.2 9.6 10.1 9.7 9.5
22.5 20.5 21.5 20.5 19.8
51.9 51.4 62.0 51.5 51.2
47.8 43.8 58.0c 43.8 44.4
47.8 44.5 58.0c 44.5 44.4
50.7 45.7 56.5c 45.7 45.7
40.5 36.4 54.7c 36.5 36.4
49.8 49.9 50.0 50.0
a All chemical shifts with respect to TMS: δ ) 0 ppm. b The chemical shifts of these substances are determined simultaneously from an aqueous solution containing equimolar 1 and HAuCl4 (see Figure 4.) c Assignments may be interchanged.
Scheme 1
Figure 4. 13C NMR spectra of 1 in aqueous HAuCl4 solution. Chloroauric acid reacts with 1 to form acid/base salts and inner coordination sphere Au(III) amine complexes. Both reactions occur readily in aqueous solution. At HAuCl4/1 mole ratio ) 1, the uncomplexed amine is fully protonated and the resonances of the complex have reached their final value. The two species exist in approximately equal concentrations.
molecule of 1. At low pH, the fourth inner coordination site is occupied by chloride with two additional chloride ions occupying apical positions above and below the square planar complex. Both reactions occur simultaneously as demonstrated in Figure 4, which depicts the spectrum of 1 in water as a function of added HAuCl4. Figure 4a is the spectrum of the free base of 1 in water at 0.04 M. The addition of HAuCl4 such that the HAuCl4/1 mole ratio is
