Dispersed gold nanoparticles supported in the pores of flower-like macrocellular siliceous foams based on an ionic liquid as catalysts for reduction

Abstract

A facile method has been developed for the synthesis of a flower-like macrocellular siliceous foam with a large and uniform pore size, using P123 and protic ionic liquid as the co-templates under acidic conditions. The influence of the protic ionic liquid concentration and the hydrothermal temperature on the synthesis of the macrocellular siliceous foam is systematically investigated. The structures of all the composites were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray powder diffraction (XRD), UV-vis diffuse reflection spectroscopy, and N₂ gas sorption. The results showed that the flower-like macrocellular siliceous foam possessed about 100 nm sized large pores, which was appropriate for it to be applied in catalytic reactions. Moreover, Au NPs were immobilized in the pores of the flower-like macrocellular siliceous foam through a self-assembly procedure. The obtained NH₂-S-6-393/Au sample exhibited a remarkably higher catalytic activity in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄. The macrocellular siliceous foams are suitable supports for catalytic reactions on account of their special structure and can be highly beneficial for a wide range of applications.

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1. Introduction

Porous silica materials with uniform large pores are a kind of desirable support for chemical reactions involving large molecules, which have aroused worldwide attention.^1–3 During the past decades, various methods have been employed to synthesize porous silica materials, including a postsynthetic demetalation process and template synthesis.^4,5 In general, the cosolvents including cosurfactants and oils act as the expanding agents to synthesize silica materials with large pores.⁶ Mesocellular siliceous foams (MCF) with uniform and large pore systems are compelling representatives. Zhao et al.⁶ synthesized mesocellular siliceous foams (MCF) with uniformly sized cells and windows in aqueous acid by using P123 as the template in the presence of 1,3,5-trimethylbenzene (TMB). Zhou et al.⁷ obtained hierarchical porous silicas with pore structures of disordered-SBA-15-type together with mesocellular foams by using a P123/cosurfactant/1,3,5-trimethylbenzene/water system. Hsu et al.⁸ synthesized the mesocellular foam-like silica mesophases using carboxy-terminated triblock copolymer Pluronic P123. And these silica materials were used as hard template to obtain the MCF-like carbon material through a nanocasting route. And the mean pore size of MCF obtained by TMB is about 10–50 nm, which is good for the reaction with large molecules and making MCF a suitable support for the immobilization of small and well-defined NPs.^9–12

Gold nanoparticles (Au NPs) have attracted considerable attention owing to the special catalytic properties since the seminal work of Haruta.¹³ And Au NPs were applied in all kinds of reactions, such as hydrocarbon combustion, methanol synthesis, water gas shift reaction, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and so on.^14–17 It is known that there is a correlation between nanoparticle size and catalytic activity, moreover, catalysts with Au particles size below 6 nm can display good activity.^18–21 However, Au NPs are unstable because of the high surface energy and tend to aggregate into larger clusters during the calcination and catalytic reaction.²² Hence, it is very important to develop the methods for the preparation of small and stable Au NPs. Nowadays, it is very popular to support Au NPs onto hierarchical porous silica materials, which is beneficial to the catalyst recovery.^23,24 And this method can prevent Au NPs from agglomerating during the reaction. A number of hierarchical porous silica materials have been used as supports for Au NPs, including SBA-15, filter paper, PMMA and siliceous mesocellular foam (MCF) which is obtained by TMB as reaming agent.^25–28 Eriksson et al.²⁹ successfully obtained the production of dispersed thiol stabilized gold nanoparticles in the pores of MCF by electrochemical method. Sobczak et al.²⁶ prepared Au supported on MCF doped with Nb or Zr for oxidation of methanol. Therefore, silica porous material with large pore size is a suitable support for Au NPs.

In our past work, we have obtained the silica material like mesocellular siliceous foam with P123 and protic ionic liquid (triethylamine acetate) as the templates when the pH is 3 and demonstrated that this material constituted an excellent support for 12-tungstophosphoric acid (HPW) based catalysts in Friedel–Crafts alkylation of o-xylene and styrene.³⁰ 1-Phenyl-1-xylyl-ethane (PXE), which is the product of Friedel–Crafts alkylation of o-xylene and styrene, is an appropriate solvent for many materials.^31–35 On the basis of our past work, we study further about the influence of ionic liquid content and hydrothermal temperature to the structure of the final sample, and successfully synthesized a series of flower-like macrocellular siliceous foams (mean pore size is about 90 nm) which is different from the conditional mesocellular siliceous foam (mean pore size is about 10–50 nm) induced by P123 and TMB. Owing to the special structure of the macrocellular siliceous foam, they can be applied to a massive of catalytic reactions. Moreover, an amount of Au NPs were absorbed on the macrocellular siliceous foams. And the obtained catalyst NH₂-IL-6-393/Au was used in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and exhibited an enhanced catalytic activity.

2. Experimental section

2.1 Chemicals

P123 (PEO₂₀PPO₇₀PEO₂₀) was purchased from Sigma-Aldrich, the inorganic silica precursor was silicon(IV) tetraacetate (TEOS 97%, Fluka) and HCl (37% in water, Aldrich) was used as reaction catalyst, triethylamine acetate was obtained according to the literature.³⁶ Sodium borohydride (NaBH₄), HAuCl₄ (10 mg mL⁻¹), g-aminopropyl-triethoxysilane (APTES), were purchased from the China National Pharmaceutical Group Corp. All reagents were used without further purification.

2.2 Synthesis of macrocellular siliceous foams

In a typical synthesis, P123 (4 g) and a certain amount of protic ionic liquid (triethylamine acetate) were completely dissolved in 50 mL deionized water, its pH was controlled to 3 by HCl (2 M). 9 g of tetraethyl orthosilicate (TEOS) was then dripped slowly with strong stirring. After stirring at 313 K for 24 h, the whole solution was transferred into a Teflon-lined autoclave of 100 mL capacity and heated at an appropriate temperature for 2 days. After washed with water and ethanol, the white as-synthesized solid powders were then calcined at 823 K for 6 h in ambient air, with a heating rate of 2 K min⁻¹.

Similarly, a series of silica materials were synthesized by changing the ionic liquid content and the hydrothermal temperature during synthesis. The final products were denoted as S-X-Y, in which X represented the protic ionic liquid content, Y represented the hydrothermal temperature, respectively. As a comparison, samples either without P123 (denoted as S1) or using water to replace PIL (denoted as S2) were also prepared.

2.3 Synthesis of NH₂-S-6-393/Au

For the modification with amino groups, 0.2 g of silica material S-6-393 were transferred into a mixture of 100 mL of isopropanol and 1 mL APTES, and the temperature heated up to 353 K for 12 hours. The treated S-6-393 were washed with ethanol and then dried in vacuum at 323 K overnight. Amino-modified S-6-393 was re-dispersed in 80 mL of deionized water. And NH₂-S-6-393/Au was obtained by the reduction of HAuCl₄ with NaBH₄ under the protection of trisodium citrate. Typically, 80 mL of above amino-modified S-6-393 aqueous solution was mixed with 0.72 mL of HAuCl₄ and 1.2 mL of trisodium citrate (40 mg mL⁻¹). After stirring for 30 min, 9 mL NaBH₄ (0.1 M) was immediately added to the above solution, which corresponded to a color change of light yellow to light red. And the solution keeps stirring at room temperature for 2 h in order to insure that the Au colloids were completely absorbed. After collected by centrifugation, and washed with water to remove the trisodium citrate, the obtained NH₂-S-6-393/Au dried at 323 K for 12 h. Finally, the products were calcined in air at 423 K for 3 h.

2.4 Catalytic tests of NH₂-S-6-393/Au

The catalytic activity of NH₂-S-6-393/Au was evaluated by the reduction of 4-NP to 4-AP with a NaBH₄ aqueous solution at room temperature. Typically, 0.03 mL of the 4-NP aqueous solution (0.012 M), 0.5 mL of NaBH₄ aqueous solution (0.25 M) and 2 mL distilled water were added into a quartz cell. Then, 0.15 mL of aqueous dispersion of the NH₂-S-6-393/Au (0.5 mg mL⁻¹) was added into the above mixture at room temperature. The change of the mixture solution UV-vis absorption spectra was measured to evaluate the catalytic activity and stability. For the recycling property, the catalyst was collected, washed with water and ethanol, dried at 333 K overnight and reused in the next cycle.

2.5 Characterization

Transmission Electron Microscopy (TEM) was performed on JEM-2010 instrument with an accelerating voltage of 100 kV. Scanning electron microscopy (SEM) images were recorded on a JEOL JSM-5600L SEM Instrument with a working distance of 3–4 mm and an electron voltage of 3.0 kV. The N₂ physical adsorption and desorption isotherms were adopted at 77 K to obtain surface areas with ASAP 2020 apparatus (Micromeritics USA) by means of the Brunauer–Emmett–Teller (BET) method. The pore size distribution in mesopore range was analyzed by the BJH (Barrett–Joyner–Halenda) method using the Halsey equation for multilayer thickness. The X-ray diffraction patterns of the products were collected on a Bruker D8 Advance Diffractometer (Germany) with Cu-Kα radiation (λ = 1.5418 Å) at a scanning rate of 0.02 S⁻¹ in the 2θ range from 10° to 90°, with an operation voltage and current maintained at 40 kV and 40 mA.

3. Results and discussion

3.1 Characterization of flower-like macrocellular siliceous foam

The TEM images of the resultant silica samples obtained at 393 K, as presented in Fig. 1, show that all the materials exhibit a foam-like or flower-like structure with an average pore size of 90–110 nm. When the ionic liquid content is low, the sample S-2-393 possesses a little disordered structure than other silica material, owing to the ionic liquid's roles of cosolvent, cosurfactant as well as salt in the system. With the increase of ionic liquid content, the obtained silica materials tend to be more stable with a well-defined macroporous structure and uniform pore size. From the size distribution histograms inset of the TEM images, it can be shown that the diameter of the pores is about 90 nm, indicating the macroporous structure of the silica materials. It is worth noticing that there is a little increase of pore size when the ionic liquid weight changes from 4 g to 10 g, which may be due to the fact that the mixed micelles of P123 and ionic liquid with the large diameter are formed. In contrast, it can be seen that the samples S2 and S1 possess the completely different morphology from the Fig. 2. The sample S2 shows a very ordered mesoporous structure and these mesopores are arranged in a hexagonally ordered array. While, the sample S1 has a disordered and ball-like structure. It gives evidence that the interaction between P123 micelles and ionic liquid is the main point to induce the macrocellular siliceous foam.

Fig. 1 TEM images of silica materials obtained by different protic ionic liquid at 393 K. Insets in each picture were the size distribution histograms.

Fig. 2 TEM images of (A) S2, (B) S1.

Fig. 3 shows the SEM images of the macrocellular siliceous foam. As can be seen, the silica materials with flower-like structure are built up by a three-dimensional network of pores, which benefits for a large area to carry active ingredient. It can be seen that the pore size of all the materials is about 90–110 nm, which is agreement with the results of TEM. Saving, it is worth noticing that the structure is destroyed a little with the increasing of the protic ionic liquid content. The interconnected structure of the macropores makes these silica materials promising candidates for supports for catalysts and in reactions involving large molecules. It is known that the mesopores of MCM-41 or SBA-15 are too small to allow a convenient introduction of Au NPs for catalysis due to the possibility of causing pore blocking. Hence, flower-like macrocellular siliceous foams with large pore size are suitable catalyst support for Au NPs to be applied in all kinds of reactions. Moreover, the large pore can increase the diffusion rate of reactants and a number of active sites are exposed, which will improve their catalysis activity.

Fig. 3 SEM images of (A and B) S-2-393, (C and D) S-6-393, (E and F) S-10-393.

N₂ adsorption isotherm of the flower-like macrocellular siliceous foam and corresponding pore size distributions of the sample S-6-393 are shown in Fig. 4, which strongly support the three-dimensional network structure of pores. The isotherms exhibit the pronounced hysteresis loop at very high relative pressure p/p₀ characterizing the presence of the macropores. And the sizes of the cells and windows which determined by the adsorption and desorption branch of the N₂ adsorption isotherm are about 110 nm and 30–50 nm, respectively. It is consistent with the results of TEM images. The calculated BET the surface area is 273 m² g⁻¹, which is less than conditional silica materials owing to the presence of large pores.

Fig. 4 N₂ adsorption–desorption isotherms and corresponding pore size distributions (inset) of the sample S-6-393.

TEM images of silica materials obtained at different temperature are displayed in Fig. 5. It can be seen more clearly that there is a significantly difference among these silica materials. And the sample S-6-353 shows a disordered sheet structure with some macropores inset. While, S-6-373 has a leaf-like structure which is similar with S-6-393 apart from a uniform cell pore size. And this kind of leaf-like structure is loose than the sample S-6-393. That certifies the fact that the silica materials with different morphology can be obtained under different conditions and the temperature is a very important factor to obtain the flower-like macrocellular siliceous foams.

Fig. 5 TEM images of silica materials obtained at different temperature.

In order to certify the flower-like structure of the silica material, the SEM images of silica materials induced by different temperature are obtained. From Fig. 6, it can be clearly seen that the sample macrocellular siliceous foam S-6-393 exhibits a uniform flower-like structure with the large pores. While, the silica material prepared at 373 K (S-6-373) shows a disordered flower-like morphology. A number of macropores can be found with the temperature decreases to 353 K, which may be caused by the reason that the interaction between P123 micelles and protic ionic liquid is influenced by the low temperature. In conclusion, the temperature is significant for the formation of flower-like macrocellular siliceous foam.

Fig. 6 SEM images of silica materials obtained at different temperature.

From the results above, it is found that ionic liquids can enter the core of the P123 micelles to form the large mixed micelles which act as the templates of the macrocellular siliceous foam. With the removal of these templates, the foam-like or flower-like materials with large pore diameter are fabricated. In this situation, the main function of the protic ionic liquid is acting as the co-template. And the corresponding postulate formation mechanism of silica foams is introduced in Fig. 7.

Fig. 7 Postulate formation mechanism of hierarchical porous silicas.

3.2 Catalytic activity of NH₂-S-6-393/Au

The functional NH₂ group present on the wall of the macropores can act as a stabilizing agent to provide an anchoring surface, which gives a chance to the formation of highly dispersed Au NPs by in situ reduction of HAuCl₄ with NaBH₄. Furthermore, strong electrostatic interaction between S-6-393 and Au NPs due to the function of APTES, can protect Au NPs from sintering. TEM images (Fig. 8A and B) reveal that Au NPs immobilized on NH₂-S-6-393 are well dispersed. In order to clearly observe the Au NPs and the structure of functioned macrocellular siliceous foams, the high-resolution transmission electron microscopy (HRTEM) are used. It can be noted that Au NPs possess an average size of 4.1 nm. Moreover, the lattice fringes in Fig. 8D show the high crystallinity of the sample, including the crystalline with an interplanar spacing of 0.24 nm, 0.22 nm corresponds to the (111) and (200) plane of the Au NPs (Fig. 8E), respectively. And EDX spectra of the sample further confirms that the nanocomposite is composed of Si, O and Au element (Fig. 8F). And from the EDX analysis of Fig. 8F, it can be obtained that Au content of the synthesized NH₂-S-6-393/Au catalyst is 3.37%.

Fig. 8 TEM and HRTEM images (A to D), SAED pattern (E), energy-dispersive X-ray (EDX) pattern of NH₂-S-6-393/Au (F).

Fig. 9A shows the XRD patterns of NH₂-S-6-393 and NH₂-S-6-393/Au. It can be observed that the sample NH₂-S-6-393 presents a broad feature at 2θ = 22°, and the peaks at 2θ = 38.18°, 44.39°, 64.58°, 77.54° can be indexed as the (111), (200), (220), and (311) reflections of the face-centered cubic gold (JCPDS 65-2870). It is in accordance with the results of TEM. Adsorption studies of the catalyst NH₂-S-6-393/Au and the silica material IL-6-393 were carried out by the UV-vis spectra of the ethanol solutions. From the Fig. 9B, it can be shown that no obvious peaks are found in the sample S-6-393. While, there is a peak absorption wavelength at 524 nm which is the characteristic of nanocrystalline Au particle in NH₂-S-6-393/Au.³⁷

Fig. 9 (A) XRD patterns of (a) the sample NH₂-S-6-393, (b) NH₂-S-6-393/Au, (B) UV-vis absorption spectra of (a) S-6-393, (b) NH₂-S-6-393/Au.

It is known that metal NPs are usually used in the synthesis of 4-AP which is applied in analgesic and antipyretic drugs, corrosion inhibitor, and lubricant so on industrial production.^38–41 Therefore, the reduction of 4-NP to 4-AP at room temperature is chosen as a model reaction to quantitatively evaluate the catalytic performance of NH₂-S-6-393/Au. This process is monitored by UV-vis absorption spectroscopy because of the difference of the absorption peaks between 4-NP and 4-AP. The light yellow 4-NP solution possesses an absorption peak at 317 nm (Fig. 10A). After addition of NaBH₄, the peak moves to 400 nm, owing to the formation of the 4-nitrophenolate ion. Meanwhile, the color of the solution changes from light yellow to yellow-green. While, after addition of 0.15 mL catalyst solution, the absorption intensity of the peak at 400 nm decreases and the absorption peak of 4-AP at 300 nm appears, due to the successful conversion of 4-NP to 4-AP.

Fig. 10 (A) UV-vis spectra of 4-NP (black line) and after (blue line) adding NaBH₄ and 4-AP (red line) solution, UV-vis spectra of catalytic reduction of 4-NP to 4-AP, (B) NH₂-S-6-393/Au, (C) relationship of ln(C_t/C₀) and reaction time for the reduction of 4-NP catalyzed by NH₂-S-6-393/Au, (D) catalytic stability tests of NH₂-S-6-393/Au.

With respect to Fig. S1,† it is shown that S-6-393 has no catalytic activity for the reduction of 4-NP. Fig. 10B shows the absorption change of the reaction by the addition of the catalyst NH₂-S-6-393/Au. It can be noted that the reaction completely finished within 4 min by addition of 0.15 mL catalysts (0.5 g L⁻¹), which corresponded to a color change of yellow-green to colorless. Xu et al.¹⁶ found that 0.5 mL rod-like SiO₂/Au catalyst (0.5 g L⁻¹) were used in the reduction of 4-NP, and the absorbance of 4-nitrophenolate ions decreased to half after 10.5 min. Moreover, the size of Au NPs supported on the surface of the rod-like SiO₂ were more than 40 nm. Compared with this kind of catalyst, NH₂-S-6-393/Au with large pore size of the support and the small size of the Au NPs (∼4 nm) exhibited a much higher catalytic activity. Since the catalytic activity is related with the particle size of Au, the Au NPs with a diameter about 4 nm can show much higher catalytic activity, which certifying that the flower-like macrocellular siliceous foam templated by protic ionic liquid can be a suitable material to support the Au NPs and these Au NPs with small size can well disperse in the large pore of the macrocellular siliceous foam. Fig. 10C shows the linear relationships between ln(C₀/C_t) and reaction time. C_t and C₀ are 4-nitrophenol concentrations at time t and 0, respectively, is measured from the relative intensity of the respective absorbances at 400 nm. Linear relationships between ln(C₀/C_t) and reaction time are obtained in the reduction, which confirms the first-order reaction kinetics. Based on the linear relationship, the apparent rate constant (k) (obtained from the slope of the plots of ln(C₀/C_t) vs. time) of the obtained NH₂-IL-6-393/Au is 0.977 min⁻¹, which is higher than previous work.^16,17 And the high value is agreement with the high catalytic activity of the NH₂-S-6-393/Au. Additional, the stability to leaching of the active component and recyclability of the catalyst is very important question. As shown in Fig. 10D, NH₂-S-6-393/Au has a relatively stable and high catalytic activity after running for 5 cycles. Taken together, as-prepared flower-like macrocellular siliceous foam is a promising support to synthesis of nanocatalysts with excellent activity and stability. TEM image (Fig. S2A†) exhibited that Au NPs stayed small size (about 5 nm) after 8 cycles. Compared with the sample NH₂-S-6-393/Au calcined at 343 K (Fig. S2B†), whose particle size of Au NPs is about 3.3 nm, the catalyst NH₂-IL-6-393/Au calcined at 423 K maintains the small size of Au NPs. And all the results show that flower-like macrocellular siliceous foam is a suitable support for Au NPs and the obtained Au NPs with the small size are beneficial to the high catalytic activity. Furthermore, the Au NPs can enter into the channels of the macrocellular siliceous foam, which will be good for the stability of the Au catalysts. That is to say the encapsulation of Au NPs in macrocellular siliceous foam support can prevent the interaction between the particles and keep the catalysts the high catalytic activity even under relatively hash conditions.

4. Conclusion

The flower-like macrocellular siliceous foam with a large and relatively uniform pore size has been successfully fabricated by using P123 and protic ionic liquid (triethylamine acetate) as the co-templates under acidic conditions. Moreover, the influence of the protic ionic liquid concentration and the hydrothermal temperature on the synthesis of macrocellular siliceous foam is studied. The results showed that the macrocellular siliceous foam possessed about 100 nm sized large pores and was an appropriate support to be applied in catalytic reactions. Furthermore, we have synthesized the catalyst NH₂-S-6-393/Au obtained by constructing well-defined Au NPs supported on amino-modified macrocellular siliceous foam S-6-393 through a in situ reduction method. As for the catalyst NH₂-S-6-393/Au, it is found that Au NPs well-dispersed in the pores of macrocellular siliceous foam. Additional, NH₂-S-6-393/Au exhibits a remarkably higher catalytic activity and stability for the reduction of 4-NP to 4-AP by NaBH₄. From the results of the reduction of 4-NP, it can be concluded that the as-prepared macrocellular siliceous foam with the large pore size is a highly promising support to be used in a wealth of catalytic reactions.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant no. 21376051, 21106017 and 21306023), Natural Science Foundation of Jiangsu Province (Grant no. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant no. BA2014100), the Fundamental Research Funds for the Central Universities (3207046302, 3207046409 and 3207045421) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10124k

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