DOI: 10.1002/chem.201303167

Full Paper

& Mesoporous Materials

Ethane-Bridged Periodic Mesoporous Organosilicas Functionalized with High Loadings of Carboxylic Acid Groups: Synthesis, Bifunctionalization, and Fabrication of Metal Nanoparticles Juti Rani Deka,[a] Hsien-Ming Kao,*[a] Shu-Ying Huang,[a] Wei-Chieh Chang,[a] ChunChiang Ting,[a] Purna Chandra Rath,[a] and Ching-Shiun Chen*[b]

Abstract: Well-ordered periodic mesoporous organosilicas (PMOs) functionalized with high contents of carboxylic acid (COOH) groups, up to 85 mol % based on silica, were synthesized by co-condensation of 1,2-bis(triethoxysilyl)ethane (BTEE) and carboxyethylsilanetriol sodium salt (CES) under acidic conditions by using alkyl poly(oxyethylene) surfactant Brij 76 as a structure-directing agent. A variety of techniques including powder X-ray diffraction (XRD), nitrogen adsorption/desorption, Fourier-transformed infrared (FTIR), transmission electron microscopy (TEM), 13C- and 29Si solid-state nuclear magnetic resonance (NMR) were used to character-

ize the products. The materials thus obtained were used as an effective support to synthesize metal nanoparticles (Ag and Pt) within the channel of 2D hexagonal mesostructure of PMOs. The size and distribution of the nanoparticles were observed to be highly dependent on the interaction between the carboxylic acid functionalized group and the metal precursors. The size of Pt nanoparticles reduced from 3.6 to 2.5 nm and that of Ag nanoparticles reduced from 5.3 to 3.4 nm with the increase in the COOH loading from 10 to 50 %.

Introduction

synthesized within the channel of SBA-15 by the reduction of the metal salt, which was incorporated by incipient wetness impregnation or the ion-exchange method. Krawiec et al. has reported the in situ-preparation of well-dispersed Pt nanoparticles (  4.2 nm) inside the pores of MCM-41.[18] Naik et al. prepared highly dispersed spherical Ag nanoparticles (  7 nm) and nanorods (diameter  7 nm, length  60 nm) inside the channels of SBA-15 through the ion-exchange reaction of Ag + ions with negatively charged SiO2 surface followed by their reduction with trisodium citrate.[3] The key drawback associated with the direct use of mesoporous silicas such as MCM-41 and SBA-15 is the difficulty in controlling the location of the growth of nanoparticles due to the poor interaction between the surface silanol groups and the metal ions. To ensure adequate incorporation and appropriate interaction between the porous support and metal nanoparticles, additional surface modifications of mesoporous silica with desirable organic functional groups are accomplished in the second method. The precursor compound is then loaded into the functionalized mesoporous silicas through the interaction between the functional groups and the metal ions.[19] Mesoporous silicas have been functionalized with various functional groups, such as amine,[20, 21] thiol,[10] and carboxylic acid[22, 23] to synthesize Pt, Au, and magnetite nanoparticles. However, the uniform distribution of the metal content is a challenging task because the organic functional group is anchored inhomogeneously in the pore surface, which ultimately results in the inhomogeneous distribution of metal nanoparticles.

The advent of mesoporous silica materials such as MCM-41 and SBA-15 has begun an era of research into exploiting these materials as efficient templates for the synthesis of metal nanoparticles, nanowires, and nanowire networks.[1–10] Metal nanoparticles have gained considerable interest in recent years because they possess unique optical, electrical, and magnetic properties, which are strongly dependent on the size and shape of the particle. The ordered mesoporous silicas with uniform mesostructures, high surface areas, and tunable pore sizes have been used as promising templates to control the shape and size of metal nanoparticles.[11–15] Two methodologies are generally adopted to transport precursor molecules or ions for assembly of nanoparticles inside the channels of mesoporous silicas. The first methodology involves the direct impregnation of mesoporous materials with precursor molecules or ions.[1, 5, 16] Nanostructures of Pt, Ag, Au,[5] and Pd[17] have been [a] Dr. J. R. Deka, Prof. H.-M. Kao, S.-Y. Huang, W.-C. Chang, Dr. C.-C. Ting, P. C. Rath Department of Chemistry, National Central University Chung-Li, 32054, Taiwan (R.O.C.) Fax: (+ 886) 3-4227664 E-mail: [email protected] [b] Prof. C.-S. Chen Center for General Education, Chang Gung University 259 Wen-Hwa 1st Road, Kwei-Shan Taoyuan, Taiwan 333 (R.O.C.) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303167. Chem. Eur. J. 2014, 20, 894 – 903

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Full Paper Periodic mesoporous organosilicas (PMOs) with organic fragments embedded in silica walls could greatly overcome these problems. PMOs, a new class of organic/inorganic hybrid organosilicas represented by the general formula (R’O)3Si-R-Si(R’O)3 is generally synthesized by the simultaneous use of a soft template and a hydrolysable bis-silane that holds an organic functional linker (R) between the silicon atoms, condensing around the template similar to the procedure used for the preparation of ordered mesoporous silicas.[24–30] The chemical and physical properties of PMOs can be easily tuned by changing the organic functionalities inside the framework, which offers opportunities for their application.[31] The functional groups are distributed homogenously throughout the framework of the materials, unlike functionalized mesoporous silicas in which the organic groups are anchored onto the pore walls. However, most of the bridging organic groups are chemically inert, which limits the application of PMOs. Hence, the focus is given on the synthesis of bi-functionalized PMOs (BPMOs) with pendant organic groups within the mesopore for wider applications. A second functional group like thiol,[32] vinyl,[33] sulfonic acid,[34] amino,[35, 36] and carboxylic acid[37] groups have been incorporated into PMOs to enrich the composition, while maintaining the unique mesoporous framework. Although there are many reports on the synthesis of PMOs, the number of applications involving PMO-supported noble metal nanoparticles is quite limited. In an early example, Kapoor et al.[38] exploited the hydrophobic character of PMOs to improve the catalytic performance of Au particles in the vapor-phase epoxidation of propene to propene oxide by using H2 and O2. Large-pore PMOs with ethylene and phenylene bridges were used as supports to synthesize Au nanoparticles with an average size from 3 to 15 nm.[39] Pd nanoparticles of 1.9 nm in size have been supported on a phenylene-bridged PMOs by the impregnation method, which showed better catalytic activity than the silica counterpart.[40] Recently, Zhu et al. has reported on the synthesis of fine Au nanoparticles through in situ-reduction of Au3 + with HS/SO3H-functionalized PMOs.[41] Among the various functional groups that could be introduced into PMOs to use as supports for the synthesis of metal nanoparticles, carboxylic acid (COOH) is one of the most attractive reactive groups and possesses the ability to form hydrogen bonds with organic or inorganic species. The synthesis of ordered mesoporous materials with a high content of organic functional groups, such as COOH groups, uniformly distributed over the ordered structure is highly desirable, because it determines many important properties of PMOs, such as adsorption capacity for metal ions, enhanced hydrothermal stability, and surface reactivity and hydrophobicity.[42, 43] Moreover, the carboxylic acid group can be easily deprotonated in neutral and basic environments to form negatively charged moieties. The formed negative moieties have proved suitable as effective binding sites for binding biomolecules, selective adsorption of metals,[44, 45] synthetic ion channel,[46] and polypeptide synthesis.[47] To broaden the scope of PMOs and to exploit their potential applications, this work describes a successful synthesis of well-ordered COOH-functionalized ethanebridged PMOs and its effectiveness as a support for the synChem. Eur. J. 2014, 20, 894 – 903

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thesis of metal nanoparticles. The presence of ethane-bridging groups in the silica support could prevent the aggregation of nanoparticles owing to the enhanced surface hydrophobicity. Although ethane-bridged PMOs are the most widely studied organic linker due to its extensive availability, unfortunately the ethane-bridging group offers few possibilities for further chemical modification in comparison to other organic-bridged silicas such as benzene-silicas. Recently, our group has synthesized highly ordered benzene-bridged PMOs functionalized with high loadings (up to 60 mol % based on silica) of COOH groups.[37] It still remains a challenge to develop a simple and direct synthesis of ethane-bridged PMOs that are functionalized with a high loading of organic groups. Ethane-bridged PMOs with exceptionally high loadings of pendant COOH group (up to 80 %) have been synthesized by co-condensation of 1,4-bis(trimethoxysilyl)ethane (BTME) and carboxyethylsilanetriol sodium salt (CES) by using the triblock copolymer Pluronic P123 as the template and KCl as an auxiliary agent under acidic conditions.[48] In this study, ethane-bridged PMOs with exceptionally high loading (up to 85 %) of a pendant COOH group, denoted as EC-x, in which x represents the molar percentage ratio of CES/(BTEE + CES), with a high degree of mesostructural order was synthesized by co-condensation of 1,2bis(triethoxysilyl)ethane (BTEE) and CES in the presence of nontoxic and biodegradable nonionic surfactant Brij 76 under acidic conditions. Carboxylic acid-functionalized ethanebridged PMOs with a high degree of mesostructural order, large specific surface areas, and narrow pore-size distribution can also be obtained by using BTEE as the silicon precursor and Brij 76 as the structure-directing agent in comparison with those synthesized using BTME as the precursor and P123 as the template. Additionally, BTEE is relatively cheaper than BTME. The prepared COOH-functionalized ethane-bridged PMOs were used as the supports to synthesize Ag and Pt nanoparticles within the channels by using the impregnation method, followed by thermal decomposition. The dependence of the size of the metal nanoparticles with the contents of the incorporated COOH groups was also investigated.

Results and Discussion Structural ordering of EC-x The small-angle XRD patterns of template-extracted EC-x (x = 0–85 %) are shown in Figure 1. The three well-resolved diffraction peaks observed in the region of 2q = 1–3 8 can be indexed to the (100), (110), and (200) diffractions, characteristic reflections of long-range ordered p6mm hexagonal symmetry. This result demonstrates that at most, 85 mol % CES can be incorporated in the initial synthetic mixture to obtain highly ordered COOH functionalized ethane-bridged PMOs. The d-spacing and lattice parameters (a0) of EC-x were calculated from the position of the (100) diffraction peak and the results are given in Table 1. The d-spacing and a0 values were increased from 6.1 to 6.7 nm and increased from 7.0 to 7.7 nm, respectively, with the increase in the CES percentage from 0 to 85 %. 895

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Full Paper the bifunctionalized PMOs lose its ordered structure when the pendant organic group loading exceeds 20–30 %. This wide range of COOH loadings on the ethane-bridged PMOs is advantageous for the controlled growth of metal nanoparticles.

Textural properties The nitrogen adsorption/desorption isotherms of template-extracted EC-x (x = 0-85 %) are displayed in Figure 2 and their cor-

Figure 1. XRD patterns of template-extracted EC-x, in which x = (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60, (h) 70, (i) 80, and (j) 85.

Table 1. Textural properties of EC-x after template extraction, in which x represents the molar percentage ratio of CES/(BTEE+CES). x

d100 [nm]

a0 [nm][a]

ABET Vp Pore size Wall thickness [nm] [m2 g1][b] [cm3 g1][c] [nm]

0 10 20 30 40 50 60 70 80 85

6.1 (6.2)[d] 6.1 (6.2) 6.1 (6.2) 6.3 (6.3) 6.3 (6.4) 6.5 (6.5) 6.5 (6.6) 6.7 (6.6) 6.7 (6.7) 6.7 (6.6)

7.0 (7.1) 7.0 (7.1) 7.0 (7.1) 7.2 (7.2) 7.2 (7.4) 7.5 (7.5) 7.5 (7.6) 7.7 (7.6) 7.7 (7.7) 7.7 (7.6)

894 875 774 670 625 622 595 500 273 310

1.04 0.94 0.94 0.79 0.69 0.61 0.65 0.55 0.45 0.39

4.1 4.1 3.7 3.7 3.6 3.9 3.9 3.5 3.1 2.9

2.9 2.9 3.3 3.5 3.6 3.6 3.6 4.2 4.6 4.8

[a] Lattice parameters a0 were calculated based on the formula a0 = 2d100/ p 3. [b] ABET: BET surface area. [c] Vp : Total pore volume. [d] The numbers in parenthesis are for as-synthesized samples.

Figure 2. N2 adsorption/desorption isotherms of template-extracted EC-x, in which x = (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60, (h) 70, (i) 80, and (j) 85. Filled/empty circles = adsorption/desorption.

This high loading of COOH groups can be attributed to the similar anionic charge density on the organosilicate species that were formed during the hydrolysis of BTEE and CES, and comparable hydrolysis rate of both the organosilane precursors. These factors promote the self-assembly interactions of nonionic Brij 76 with the negatively charged silicates and, consequently produced highly COOH group-loaded ethanebridged PMOs. Ethane-bridged PMOs with a COOH loading up to 80 % was previously synthesized by our group through co-condensation of 1,4-bis(trimethoxysilyl) ethane (BTME) and CES using Pluronic P123 as the template.[48] Usually, most of

responding structural properties are listed in Table 1. All the isotherms are of type IV according to IUPAC classifications and show a large increase in volume at a relative pressure of 0.4– 0.6 due to capillary condensation in the mesopores. EC-x has a representative H1-type hysteresis of cylindrical pores, similar to that reported for pure mesoporous silicas without addition of organic functional groups.[49, 50] This observation shows that the addition of COOH functional groups does not affect the hierarchical structures of the resultant materials. The BET surface area and pore volume decreased with increasing CES contents due to the occupation of the mesopores by the COOH groups.

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Figure 4. TEM images of EC-x, in which x = (a) 40, (b) 60, and (c) 80 along parallel (left) and perpendicular (right) to the pore axis.

Figure 3. SEM images of EC-x, in which x = (a) 0, (b) 20, (c) 40, (d) 50, (e) 60, and (f) 80.

ordered 2D hexagonal mesostructure. The pore size and wall thickness of the samples were measured from the bright/dark contrast in the images and were estimated to be 3–4 nm in size and 3–4 nm in thickness, which are in good agreement with the nitrogen sorption data presented in Table 1.

SEM and TEM Studies

FTIR and TGA studies

The morphology of EC-x was studied by SEM. Figure 3 shows the SEM image of EC-x (x = 0–80). Pure ethane-bridged PMOs (EC-0) and EC-x (x = 10–50) showed regular rod-shaped morphologies with different lengths and sizes. Upon increasing the CES percentage from 0 to 80 %, the particle morphology changed significantly from a rod shape into a spherical shape and the particle size was reduced from 0.5 to 0.1 mm. Typically, the morphology of the particles depend on the concentration, molecular size, and hydrophilicity/hydrophobicity of the organosilane precursors.[51] The change in morphology from rod to spherical shape at higher COOH loadings in this study can be attributed to the larger amounts of sodium ions with increasing CES contents, which obstructed the formation of the rodshaped 2D micelles. Another important reason for preventing the formation of the rod-shaped micelles at higher CES contents was the presence of hydrophilic hydroxyl groups in CES, which allows CES to be more easily adsorbed onto the surface of the micelles than BTEE. The TEM images of EC-x (x = 40, 60, 80) recorded along the directions parallel and perpendicular to the pore are shown in Figure 4. The images revealed regular periodicity over a very large area, which gives clear evidence of the formation of well-

The incorporation of the COOH groups inside the channels of EC-x was confirmed by the FTIR analysis. Figure 5 shows the FTIR spectra of EC-x (x = 0–80). The strong band at 1720 cm1, typical of the C=O stretching vibrations of carboxylate species, gives direct evidence of the presence of COOH groups in EC-x (x = 10–80). The intensity of the band gradually increased with the increase in the CES content, suggesting quantitative incorporation of the COOH groups into the ethane-bridged PMOs. These results unambiguously confirmed the existence of carboxylic acid groups in the EC-x materials. In addition, well-known absorption bands due to the stretching vibration modes of surface silanol groups and water in the range of 3500–3740 cm1 and the CH stretching modes of CH2 in the range of 2900–3000 cm1 were also observed.[52–54] The concentration and the thermal stability of the COOH groups within EC-x were evaluated by TGA. All the samples (Figure S1, the Supporting Information) exhibited a small weight loss below 200 8C due to the adsorption of the water molecules within the materials. The second major weight loss between 480 and 720 8C for the pure ethane-bridged PMO (i.e., EC-0) was attributed to the decomposition of the ethanebridging groups in BTEE. For the case of EC-x (x = 10–80),

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Full Paper and CH2CH2 groups from BTEE. Other resonances at d = 28 and 178 ppm are assigned to the CH2 (C2) carbon atom and the C=O carbon atom of the COOH group, respectively. The intensity of the peak at d = 178 ppm increases gradually with an increase in the CES content, which confirms the quantitative incorporation of COOH groups on the mesopore surface. The decrease in the intensity of the peak at d = 7 ppm with the increase in CES indicated that the pendant COOH group on the surface of the mesopore was proportional to the amount of CES used in the initial synthetic mixture. The absence of any peak corresponding to Brij 76 suggested complete removal of the surfactant from EC-x. The 13C CP MAS NMR spectra of EC-x, as shown in Figure 6, also demonstrated the quantitative incorporation of COOH groups as the corresponding intensity of the peak at d = 178 ppm increased with the increase in the CES content. It can be concluded that both 13 C-based solid-state NMR techniques demonstrate quantitative incorporation of COOH groups into ethane-bridged PMOs. The connectivity of ethane, silicate, and COOH groups was confirmed by 29Si MAS NMR spectroscopy. Figure 7 shows the 29 Si MAS NMR spectra of the selected EC-x samples. The spectra revealed two dominant peaks at d = 58 and 67 ppm followed by a small shoulder peak at d = 49 ppm, corresponding to T2 [CSi(OSi)2(OH)], T3 [CSi(OSi)3] and T1 [CSi(OSi)(OH)2] species, respectively.[56] However, the accurate determination of the sites could not be made due to the pronounced overlapping of the Tm,E (from BTEE) and Tn,C (from CES) signals. We attempted to determine the fractions of the COOH groups in EC-x by deconvolution and separation of each spectrum, as

Figure 5. IR spectra of EC-x after template removal, in which x = (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60, (h) 70, and (i) 80.

which contains both ethane-bridging- and pendant carboxylic acid groups, the decomposition temperature was in the range of 200–700 8C. The decomposition in the range of 200–500 8C can be attributed to the disintegration of the COOH groups as observed from the differential thermal analysis (DTA) plots. The peaks observed in the range of 400–700 8C indicated the decomposition of the ethane-bridging groups in the pore walls. Because the weight losses due to COOH and ethane groups were severely overlapped, it was almost impossible to precisely determine the exact content of the incorporated COOH moieties in EC-x by TGA. Quantitative determination of functionalized  COOH group in ethane-bridged PMOs 13

C and 29Si solid-state NMR spectroscopy

The successful incorporation of both ethane and COOH groups in EC-x as well as the surfactant removal efficiency were confirmed by 13C-based solidstate NMR spectroscopy. 13C MAS NMR spectra with probe background suppression (i.e., DEPTH NMR)[55] was recorded because the 13C signals obtained from this study should be proportional to the exact content of the organic group present in the sample as long as the signals are well-resolved. The 13C DEPTH NMR spectra of EC-x, shown in Figure 6, display three distinct peaks at d = 178, 28, and 7 ppm. The peak at d = 7 ppm is due to the CH2 group (C3 carbon) directly bonded to the Si atom of the COOH group Chem. Eur. J. 2014, 20, 894 – 903

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Figure 6. 13C DEPTH (left) and CP MAS (right) NMR spectra of EC-x after template removal, in which x = (a) 0, (b) 20, (c) 40, (d) 60, and (e) 80.

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Full Paper Table 3. Total acidity capacity [mmol of H + per g of SiO2] of EC-x. Sample

Equivalent point of titration (pH)

Acidity capacity (calcd)[a]

Acidity capacity (observed)

EC-20 EC-40 EC-60 EC-80

6.7 7.0 6.9 7.1

0.6 1.1 2.4 3.6

0.6 1.1 2.3 3.5

[a] Expected total acidity capacity at 100 % incorporation of CES.

of the pH value versus the amount of NaOH, the acidic capacities of EC-x were calculated and the results are presented in Table 3. It can be realized from the data that the acidity capacity increased from 0.6 mmol H + g1 to 3.5 mmol H + g1 with increasing the molar ratio of CES/(CES+BTEE) from 0.2 to 0.8. Remarkably, the COOH group densities were much higher than that of the analogue, that is, COOH functionalized ethanebridged PMOs prepared with a CN-containing silane.[56] The results corroborated that the present simple synthesis route can efficiently produce highly COOH-functionalized ethanebridged PMOs. Moreover, the loading of the COOH group can be tuned ranging from 0 to 85 % based on silica for specific applications.

EC-x as a support for the synthesis of Pt and Ag nanoparticles

29

Figure 7. Si MAS NMR spectra of EC-x after template removal, in which x = (a) 0, (b) 20, (c) 40, (d) 60, and (e) 80. The dotted lines types show the components used in the spectral deconvolution.

Because of the importance of metal nanoparticles in the nanochemistry of ordered mesoporous silicas and their application in catalysis, we have tested the possibility of the prepared EC-x (x = 0, 10, 30, 50) as the support to synthesize Pt and Ag nanoparticles. The positively charged [Pt(NH3)4]2 + and Ag + ions were at first attracted onto the surface of mesoporous silica functionalized with COOH groups by electrostatic interaction and then reduced to Pt and Ag nanoparticles in the flowing mixture of the H2 and Ar atmosphere. The wide-angle XRD patterns of Pt-EC-x and Ag-EC-x are shown in Figure 8. Three peaks at 2q = 39.7, 46.3, and 67.4 8, corresponding to the (111), (200), and (220) reflections, respectively, were observed in the

Table 2. 29Si MAS NMR spectral deconvolution results of template-extracted EC-x. x

T3,E [%]

T2,E [%]

T1,E [%]

T3,C [%]

T2,C [%]

0 20 40 60 80

69.2 66.4 61.9 38.3 26.1

26.5 20.2 12.2 12.1 8.2

4.3 2.9 1.7 2.1 0

0 5.3 14.3 22.3 44.1

0 5.3 10.0 25.2 21.7

Tn,C/(Tm,E0.5+Tn,C) [%][a] 0 19.2 39.1 64.4 79.4

[a] Because each BTEE contained two silicon atoms, the intensities of Tm,E were multiplied by 0.5 to obtain the “real” amount of BTEE. The uncertainty in the deconvolution results was around  10 %.

shown as the dashed lines in Figure 7, and the results are given in Table 2. The 29Si NMR deconvolution results were in good agreement with the molar ratios of the organosilane precursors used in the initial reaction mixture. Acid/base titration To evaluate the incorporation efficiency of the COOH group, the total acidic capacity of EC-x was determined by potentiometric titration with an aqueous solution of NaOH (0.01 m). From the titration plots (Figure S2, the Supporting Information) Chem. Eur. J. 2014, 20, 894 – 903

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Figure 8. XRD patterns of (A) Pt-EC-x and (B) Ag-EC-x, in which x = (a) 0, (b) 10, (c) 30, and (d) 50.

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Full Paper XRD patterns of Pt-EC-x (part A of Figure 8), which confirmed the presence of Pt nanoparticles in the COOH-functionalized ethane-bridged PMOs. The three weak peaks at 2q = 38.0, 44.2, and 64.4 8, corresponding to the (111), (200), and (220) reflections of cubic structure of Ag,[57] were observed in the XRD patterns of Ag-EC-x (part B of Figure 8), indicating the formation of the Ag nanoparticles in the mesostructure. The intensities of the most intense peak at 2q = 39.7 8 for Pt-EC-x and at 2q = 38.0 8 for Ag-EC-x decreased with the increase in the COOH loading, suggesting the reduction of the particle size with the increase in the COOH content. The mean diameters of the Pt and Ag nanoparticles within the channels were estimated by using the Scherrer’s Equation from the peak width of the (111) reflection and the results are listed in Table 4. The reduction in size of Pt and Ag nanoparticles with the increase in the COOH loading from 0 to 50 % was also evidenced from the TEM images, as shown in Figure 9. The particle size of Pt and Ag nanoparticles determined from the TEM study are given in Table 4. The small variation between the particle size determined from the TEM analysis and the Scherrer’s Equation can be attributed to the inaccuracy in the evaluation of the line width of the XRD peak, which is influenced not only by the crystallite size but also by the structural strain; this is common for the materials at nanometer-size regime.[58] As determined from the Scherrer’s Equation from the XRD patterns, the average particle sizes were about 24.5 and 32.3 nm for Pt and Ag nanoparticles, respectively, when EC-0 was used as the support to synthesize metal nanoparticles. These particle sizes were much larger than the channel width of EC-0, which implied the formation of the nanoparticles on the external surface. The formation of most of the nanoparticles on the external surface when EC-0 was used as the support can be attributed to the weak interaction between the metal ions and surface silanol groups, which leads to poor dispersion of the nanoparticles preferentially on the most accessible external surface of the support. The presence of silanol groups on the surface facilitated the adsorption of Pt2 + and Ag + ions on them and they acted as the nucleation sites for the growth of the nanoparticles during thermal reduction. Additionally, most of the nanoparticles aggregated on the external surface due to the large surface energy. Similar uncontrolled growths of metal nanoparticles on the external surface of the SBA-15 were also

Figure 9. TEM images of (A) Pt-EC-x and (B) Ag-EC-x, in which x = (a) 0, (b) 10, (c) 30, and (d) 50.

realized by other researchers.[59, 60] Wang et al. has synthesized Pt nanoparticles of 6–8 nm in size by using SBA-15 as a support through the conventional incipient wetness impregnation followed by a new glow discharge plasma reduction.[59] Very recently, Strzałka et al. has synthesized Ag nanoparticles (  4 nm) inside SBA-15 by using Tollen’s reagent as the silver source.[60] However, due to the poor interaction between the surface silanol groups and the metal ions, it was difficult to control the location of the growth Table 4. Particle sizes, textural properties, and metal adsorption capacities of metal of the nanoparticles for both cases. On the other loaded EC-x. hand, the use of COOH-functionalized PMOs as the Pore Pore Metal Sample Particle Surface support avoids the growth of nanoparticles on the 2 1 3 1 size area [m g ] volume [cm g ] size [nm] adsorption external surface. The particles were distributed uni[mmol g1][b] [nm] formly within the channels of EC-x (x = 10–50) due to [a] Pt-EC-10 3.6 (3.9) 667 0.78 3.2 0.76 the homogenous distribution of the Pt-EC-30 3.3 (3.7) 519 0.45 2.7 0.84 COOH functional groups throughout the mesopore. Pt-EC-50 2.5 (2.6) 533 0.51 3.1 0.95 Ag-EC-10 5.3 (6.1) 671 0.78 3.2 1.37 The wide-angle XRD pattern (Figure S3, the SupportAg-EC-30 4.0 (4.4) 640 0.70 3.0 3.00 ing Information) and TEM image (Figure S4, the SupAg-EC-50 3.4 (3.4) 408 0.39 3.2 4.47 porting Information) of Ag-EC-80 showed the forma[a] The numbers in parenthesis are the average particle size of nanoparticles calculattion of large nanoparticles (  27 nm) on the surface ed by using the Scherrer Equation Dp = 0.94 l/(b1/2cosq). [b] Determined by ICP-AES. rather than inside the pore channel. The excessive

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Full Paper functionalized COOH groups in EC-80 collapsed the ordered mesoporous structure of EC-80 to some extent. As a result, it exhibited a significantly lower BET surface area, pore size, and pore volume (Table 1). This may have blocked the channels and resulted in the formation of large nanoparticles on the surface instead. The incorporation of metal nanoparticles inside the channels of EC-x can also be evidenced from the nitrogen adsorption/desorption isotherms of metalloaded samples, as shown in Figure 10 and Table 4. The reduction of the surface area as well as the pore volume and pore size of Pt-EC-x and Ag-EC-x corroborates the filling of the pores of EC-x by metal nanoparticles. Zhao et al. has synthesized Ag nanoparticles of size around 3 nm inside the channel of MCM-41 functionalized with thiol group by in situ-reduction of AgNO3.[61] However, the nanoparticles were distributed inhomogeneously because the thiol functional groups were anchored inhomogeneously on the pore surface. Zhang et al. has synthesized Pt nanoparticles of about 5 nm in size by using isocyanuratecontaining silsesquioxane (ICS)-bridged PMOs as support.[62] Wei et al. has synthesized Au nanoparticles (3 to 5 nm) by using aminopropyltrimethoxysilane (APTMS)-functionalized PMOs as the support.[63] The uniform distribution of the functional group within Figure 10. N2 adsorption/desorption isotherms of (A) Pt-EC-x and (B) Ag-EC-x in which the channels was found to be the prime advantage x = (a) 10, (b) 30, and (c) 50. Filled/empty circles = adsorption/desorption. The pore-size of using functionalized PMOs as supports to synthe- distribution curves are shown as insets. size metal nanoparticles. The size of Pt nanoparticles decreased from 3.6 to 2.5 nm and that of Ag nanoparticles from 5.3 to 3.4 nm with the increase in the COOH groups from 10 to 50 %. The decrease in size of the nanoparticles with the increase in the COOH groups can be attributed to the formation of larger numbers of the COO groups, which enhanced the ability to attract the oppositely charged [Pt(NH3)4]2 + and Ag + ions. Since the amounts of the metal precursors were fixed in the present study, and the numbers of nucleation centers for the formation of the nanoparticles were increased with an increase in the COOH groups, the size of the nanoparticles was decreased due to the increase in the reaction rate.[64] It can be inferred that the COOH group in EC-x worked as a stabilizer for the formation of Pt and Ag nanoparticles. The metal nanoparticles were prepared at pH 9 and the COOH group in EC-x was present in the form of COO under such basic conditions. Consequently, EC-x exhibited high adsorption capacities due to the strong electrostatic attraction between [Pt(NH3)4]2 + and Ag + 13 ions and the negatively charged surface of EC-x. It Figure 11. (A) C CP MAS NMR and (B) FTIR spectra of (a) Ag-EC-10, (b) Ag-EC-30, (c) AgEC-50, (d) Pt-EC-10, (e) Pt-EC-30, and (f) Pt-EC-50. demonstrates that carboxylic acid group plays a crucial role in binding with the metal ions. The possibility of the interaction between the COOH groups and the extent at the reduction temperature of 320 8C. 13C CP MAS metal nanoparticles during the reduction process was negligiNMR and FTIR spectra of Pt-EC-x and Ag-EC-x (Figure 11) furble because the carboxylic acid groups decomposed to a large ther confirmed the absence of interaction during reduction Chem. Eur. J. 2014, 20, 894 – 903

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Full Paper since almost no peak corresponding to the COOH groups was observed. The metal ion adsorption capacities of ECx were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and the results are listed in Table 4; it shows that EC-x has a high affinity for metal ions and the adsorption capacities systematically increased in proportion to the contents of the COOH groups. The adsorption capacity for Ag + was higher than that of Pt2 + . This could be due to the higher number of COO groups available for binding the monovalent Ag + ions relative to the divalent Pt2 + ions. Our results show that the interaction between the carboxylic acid groups and the metal precursors play an important role in the Pt2 + and Ag + reduction, and in controlling the size and dispersion of the nanoparticles within the channels of PMOs.

water, and finally air-dried at 70 8C. The obtained samples were denoted as EC-x. To remove the template, the as-synthesized EC-x material (0.5 g) was heated at reflux in H2SO4 (48 wt. %, 100 g) at 95 8C for 24 h. The product was then filtered and washed with acetone to obtain the template-extracted EC-x. To remove the ether groups, if any, formed due to H2SO4 treatment, the template-extracted EC-x (0.3 g) was treated again with HCl (2 m, 30 mL) in ethanol at 50 8C for 6 h. Subsequently, the sample was recovered by washing with water and finally dried at 70 8C.

Synthesis of Pt and Ag nanoparticles using EC-x as support Pt and Ag nanoparticles were synthesized within the channels of EC-x by impregnation of [Pt(NH3)4(NO3)2] and [AgNO3], respectively, followed by thermal decomposition. In the typical synthesis of Pt nanoparticles, EC-x (100 mg) was dispersed in [Pt(NH3)4(NO3)2] (0.2 m, 10 mL), followed by stirring at room temperature for 3 h. The pH of the solution was adjusted to 9 by adding NaOH (0.1 m). The [Pt(NH3)4]2 + adsorbed solid was filtered, washed with water, and subsequently air-dried at room temperature. The product was then subjected to thermal treatment at 320 8C for 3 h under the flowing mixture of H2 (5 %) and Ar (95 %) to obtain ethane-silica loaded with Pt nanoparticles. The material thus obtained was denoted as Pt-EC-x. Similar procedures were followed to prepare Ag nanoparticle-loaded EC-x by using AgNO3 as the Ag precursor. However, the thermal reduction was carried out at 300 8C for 2 h. The obtained Ag incorporated EC-x was denoted as Ag-EC-x.

Conclusion We have demonstrated a successful synthesis of well-ordered PMOs functionalized with extremely high loadings of carboxylic acid groups (up to 85 % based on silica) through co-condensation of BTEE and CES by using Brij 76 as the structure-directing agent under acidic conditions. The effect of the relative molar ratios of CES/(CES + BTEE) on the order of the mesostructure was systematically investigated. The obtained COOHfunctionalized PMOs possess high surface areas, uniform poresize distribution and large pore volumes that make these materials ideal for various potential applications. The prepared COOH-functionalized ethane-bridged PMOs were used as ideal supports to synthesize Pt and Ag nanoparticles. Our results show that the interaction between the functional group and the metal precursors plays a key role in the distribution and growth of the nanoparticles within the channels of PMOs. By varying the COOH loading from 10 to 50 %, the size of Pt nanoparticles can be reduced from 3.6 to 2.5 nm and that of Ag nanoparticles from 5.3 to 3.4 nm.

Characterization Powder X-ray diffraction (XRD) patterns were collected on WigglerA beamline (l = 0.133320 nm) at the National Synchrotron Radiation Research Center in Taiwan. N2 adsorption/desorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The sample was degassed at 180 8C for 3 h before measurements. Specific surface areas were calculated by using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of P/P0 = 0.05–0.3. The pore-size distribution was obtained from the analysis of desorption branch of the isotherm by the Barrett– Joyner–Halenda (BJH) method. Pore volumes were obtained from the volumes of N2 adsorbed at P/P0 = 0.95 or in the vicinity. Thermogravimetric analysis (TGA) was carried out on a Perkin–Elmer TGA7 thermogravimetric analyzer with a heating rate of 10 8C min1 under air in a flow of 50 mL min1. Fourier transform infrared (FTIR) spectra of the powdered samples suspended in KBr pallets were acquired between 400 and 4000 cm1 on a JASCO4200 spectrometer. Solid-state 13C- and 29Si MAS (magic angle spinning) NMR spectra were recorded on a Varian Infinityplus-500 NMR spectrometer, equipped with a 7.5 mm Chemagnetics probe. The Larmor frequencies for 13C and 29Si nuclei are 125.7 and 99.3 MHz, respectively. 13C Cross-polarization magic angle spinning (CP MAS) NMR spectra were recorded by using a contact time of 1 ms. Single-pulse experiments with a p/6 pulse of 2 ms and a recycle delay of 200 s were used to acquire the quantitative 29Si MAS NMR spectra. Both 13C and 29Si chemical shifts were externally referenced to tetramethylsilane (TMS) at d = 0.0 ppm.

Experimental Section Materials BTEE, Brij 76, [AgNO3], and [Pt(NH3)4(NO3)2] were purchased from Sigma–Aldrich. CES (25 wt. % in water) was purchased from Gelest. All chemicals were used as received without any further purification.

Synthesis of carboxylic acid (COOH)-functionalized ethanebridged PMOs The synthetic procedures of COOH-functionalized ethane-silicas by co-condensation of BTEE and CES are as follows. Brij 76 (1 g) was dissolved in a mixture of HCl (2 m, 16 g) and H2O (8 g) and the solution was stirred at 50 8C to obtain a homogeneous solution and then BTEE was added. After 30 min, variable quantities of CES were added into the solution and vigorously stirred for 24 h at 50 8C. The milky reaction mixture was then hydrothermally treated at 100 8C for 48 h. The composition of the reaction mixture was varied in the range of 0.282 Brij 76: (1x) BTEE: x CES: 6.4 HCl: 237 H2O, in which x is the molar percentage ratio of CES/ (CES + BTEE). The resultant precipitate was filtered, washed with Chem. Eur. J. 2014, 20, 894 – 903

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Acknowledgements The financial support of this work by the National Science Council of Taiwan is gratefully acknowledged. 902

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Full Paper Keywords: carboxylic acids · nanoparticles · platinum · silver

mesoporous

materials

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Chem. Eur. J. 2014, 20, 894 – 903

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Received: August 11, 2013 Revised: October 15, 2013 Published online on December 11, 2013

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