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Gold/periodic mesoporous organosilicas with controllable mesostrcture by using compressed CO2 Xin Huang, Mengnan Zhang, Meijin Wang, Wei Li, Cheng Wang, Xiaojian Hou, Sen Luan, and Qian Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04020 • Publication Date (Web): 25 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Gold/periodic mesoporous organosilicas with controllable mesostrcture by using compressed CO2 Xin Huang,‡ Mengnan Zhang,‡ Meijin Wang,‡ Wei Li,* Cheng Wang, Xiaojian Hou, Sen Luan, and Qian Wang Department of Chemistry, Capital Normal University, Beijing, 100048 China. Tel: +86-10-68903086, E-mail: [email protected]

Scheme 1. Schematic illustration of formation of Au/PMOs composite with one-pot approach.

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ACS Paragon Plus Environment

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Figure 1. TEM images of the Au/PMOs composite nanomaterials synthesized with TESPTS/TMOS as organosilica sources and HAuCl4 as gold source at CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90 MPa; (f) HRTEM images of gold nanoparticle within PMOs.

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Figure 2. XRD patterns of Au/PMOs nanomaterials synthesized with different CO2 pressure.

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Figure 3. Nitrogen adsorption–desorption isotherms and pore size distributions of the Au/PMOs synthesized by TESPTS/TMOS as organosilica source and HAuCl4 as gold source with different CO2 pressures.

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Figure 4. TEM images of the single PMOs nanomaterials synthesized using TESPTS/TMOS as organosilica precursor with CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90MPa.

Figure 5. Nitrogen adsorption–desorption isotherms and pore size distributions of the obtained PMOs with different CO2 pressures.

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Figure 6. Solid-state (a) 29Si MAS and (b) 13C CPMAS NMR spectrum of the obtained PMOs prepared at different pressure.

Figure 7. FTIR of the prepared PMOs prepared at different pressures.

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Scheme 2. Schematic illustration of mesostructure transformations of PMOs by using compressed CO2.

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Figure 8. (a)Equation for the reduction of 4-NP to 4-AP; (b) UV–vis spectra of 4-NP (1) before and (2) after the addition of NaBH4; (c) UV–vis spectra of the reduction process of 4-NP in the presence of the Au/PMOs, reaction time: 30s (red), 60s (green), and 100s (blue); (d) ln(Ct /C0) versus time plot for 4-NP reduction; (e) The recyclability of the Au/PMOs with the CO2 pressure of 5.90 MPa as the catalyst for the reduction of 4-NP with NaBH4.

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Scheme 3. Schematic illustration of reaction pathway of the reactants with gold nanoparticle catalysts with different mesostructure of PMOs support.

Figure 9. (a) XRD patterns of fresh and used Au/PMOs catalyst, TEM images of the (b) used Au nanoparticle and (c) used Au/PMOs composites prepared at 5.90 MPa after running 5 cycles.

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Gold/periodic mesoporous organosilicas with controllable mesostrcture by using compressed CO2

Journal: Manuscript ID Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Langmuir la-2017-04020f Article 23-Nov-2017 Huang, Xin; Department of Chemistry, Capital Normal University Zhang, Mengnan; Department of Chemistry, Capital Normal University Wang, Meijin; Department of Chemistry, Capital Normal University Li, Wei; Capital Normal University, Department of Chemistry Wang, Cheng; Department of Chemistry, Capital Normal University, Hou, Xiaojian; Department of Chemistry, Capital Normal University, Luan, Sen; Department of Chemistry, Capital Normal University, Wang, Qian; Department of Chemistry, Capital Normal University


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Gold/periodic mesoporous organosilicas with controllable mesostrcture by using compressed CO2 Xin Huang,‡ Mengnan Zhang,‡ Meijin Wang,‡ Wei Li,* Cheng Wang, Xiaojian Hou, Sen Luan, and Qian Wang Department of Chemistry, Capital Normal University, Beijing, 100048 China. Tel: +86-10-68903086, E-mail: [email protected] ‡ These authors contributed equally to this manuscript. Abstract: Gold nanoparticles confined into the walls of periodic mesoporous organosilicas (PMOs) with controllable morphology have been successfully fabricated through a one-pot method by using different CO2 pressures. The synthesis can be easily conducted in a mixed aqueous solution by using HAuCl4 as gold source, bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPTS) and tetramethoxysilane (TMOS) as the organosilica precursor. P123 and compressed CO2 served as the template and catalytic/regulative agent, respectively. TEM, N2 adsorption, and XRD were employed to character the structural of the obtained composite materials. To further investigate the formation mechanism, a series of ordered PMOs with 1D nanotube, 2D hexagonal, vesiclelike and cellular foam structure were obtained by using different CO2 pressures without gold source. The mechanism for mesostructure evolution of PMOs with different CO2 pressures was proposed and discussed in detail. The catalytic performance of Au based PMOs were evaluated for the reduction of p-nitrophenol (4-NP). These obtained composite with different mesostructure not only exhibit excellent catalytic activity, high conversion rate and remarkable thermal stability, but also exhibit morphology-dependent reaction properties in the reduction of 4-NP. The possible reaction pathway of the reactants to embedded Au active sites was proposed and schemed. Keywords: Compressed CO2, one-pot synthesis, Au/PMOs composite, controllable mesostrcture, catalyst 1


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1. Introduction Owing to the quantum size effect and surface effect, a growing interest in noble metal nanoparticles has been fully developed in recent years for their prominent properties, such as electronic, optical, magnetic and catalytic properties.1-4 However, the noble metal nanoparticles are easily agglomerated under reaction conditions for their high surface energy.5 It has greatly hampered the development of noble metal nanoparticles because their excellent properties are largely determined by the particle size.6 Therefore, some approaches have been proposed and carried out to prevent agglomeration and overgrowth of active nanoparticles.7 Thereinto, using solid support (e.g. polymer, carbonaceous material, and metal oxide) to isolate metal nanoparticles has been demonstrated to be an effective and efficient strategy.8 Meanwhile, the size and dispersion of nanoparticles have been greatly influenced by the morphologies and structures of solid support.9-13 Among numerous kinds of solid supports, ordered mesoporous silica with diverse morphology and large surface area has made them an ideal support of noble metal nanoparticals.14 On the basis of current research, mesostructure of the periodic mesoporous organosilicas (PMOs) is of great importance since they can alter optical, electronic, mechanical, and catalytic behavior of the embedded metal nanomaterials.15-17 Therefore, the mesostructure design of PMOs has been reported to be crucial for improving their catalytic activity and selectivity. The catalytic activity and selectivity of metal/PMOs composites are mainly determined by the PMOs' mesostructure characteristics (such as vesicle, mesocellular foam, spongelike, or tubular) which can influence the number of active sites, reactant/product diffusion, and accessibility of the active sites to reactants.18 To achieve the best catalytic performance, PMOs with different mesostructure should be prepared and precisely controlled.19-21 The evolution of the mesostructure is generally realized by changing the surfactant/silicate ratio, pH of the reaction medium, and adding inorganic salts or organic additives.22-26 We know that the addition of acid/base, inorganic salt or organic additives not only cause a deleterious effect on the environment, but also lead to extra cost and difficulties in post-treatment. Furthermore, although these methods

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can change the mesopore size of the nanomaterials, it is very difficult to transform the morphology and structure effectively. In addition, although numerous attempts have been done to load gold nanoparticles into PMOs, such as impregnation, electrochemical deposition, and chemical vapor deposition,27-30 most of them are multi-step synthetic methods of time-consuming and complex. Some excess and uncertain factors are involved in the synthesis and many parameters need to be monitored. Therefore, a simple and sustainable method to fabricate well-dispersed noble metal nanoparticals within PMOs that has a diversity of morphologies is still expected. Herein, we develop a novel one-pot method to synthesize gold/periodic mesoporous organosilicas (Au/PMOs) with controllable mesostructure by using compressed CO2. Supercritical or compressed CO2 has attracted great attention for its unique characteristics, such as readily available, inexpensive, nontoxic, and nonflammable. Consequently, it is widely employed in extensive fields including extraction and fractionation,31,

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chemical reactions,33 material science,34 and microelectronics.35

Moreover, compressed CO2 can control the properties of a variety of surfactant assemblies flexibly.36 Recently we found that compressed CO2 could work as catalyst for synthesis of PMOs, and tune the pore size of the obtained PMOs.37-40 It is attributed to the carbonic acid formed by dissolving CO2 into water, which can serve as catalyst in the hydration of organosilica precursors to form PMOs nanomaterials. Moreover, CO2 can penetrate into the hydrocarbon-chain region to expand the volume region of the micelles. Therefore, we try to achieve a precise control of the mesostructure transformations of PMOs through a facile one-pot process by using compressed CO2. The synthesis can be easily conducted in a neutral mixed solution by using HAuCl4 as gold source, TESPTS and TMOS as the organosilica precursor, P123 as the template, and compressed CO2 as the catalytic and regulative agent. Thus, TEOS and TESPTS can co-condense and form the framework, where the thioether group of TESPTS can anchor Au3+ to facilitate the gold impregnation. After calcination, the template was removed and the anchored Au3+ was reduced to gold nanoparticles. Consequently, the reduced gold nanoparticles are embedded into the walls of PMOs, 3


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making the channels less likely to be blocked and the gold nanoparticles are sintering free under calcination or reaction conditions. Meanwhile, the PMOs with different mesostructure can be achieved by using different CO2 pressures. To the best of our knowledge, the one-pot synthesis of Au/PMOs with tunable mesostructure has rarely been reported. More specifically, the structure transformation from a one-dimensional nanotube via two-dimensional hexagonal nanoparticle to three-dimensional cellular foams can be easily realized by adjusting the CO2 pressure. N2 adsorption, solid-state NMR and FTIR were employed to investigate the structural characterization of PMOs. The mechanism for mesostructure evolution of PMOs by using compressed CO2 was discussed in detail. These obtained Au/PMOs composite as catalysts exhibit excellent catalytic activity and high conversion rate in the reduction of 4-nitrophenol. The method proposed in this study and the investigation of the relationship between mesostructure of the PMOs support and catalytic performance of the embedded gold nanoparticles provide a new avenue to develop an architectural design of mesoporous silica in green and sustainable chemical processes.

2. Experimental CO2(>99.95%)

was

provided

by

Beijing

Analysis

Instrument

Factory.

Bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPTS) and tetramethoxysilane (TMOS) were purchased from Adamas Reagent Co., Ltd. The nonionic block copolymer surfactant Pluronic P123 (EO20-PO70-EO20, 96%) was obtained from Sigma-Aldrich. Absolute ethanol (99.98%) and hydrochloric acid (98%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). HAuCl4·4H2O and NaBH4 (98%) were provided by Shenyang Jinke Reagent Factory and Alfa Aesar. 4-nitrophenol (4-NP, A. R.) was purchased from Beijing Chemical Reagent Company. All the reagents were used without further purification and the solutions were prepared with deionized water. 2.1 Synthesis of PMOs In a typical synthesis, 1.0 g P123 was first dissolved in 29.0 g of ethanol and water mixture (the mass ratio is 0.062:1) at 40°C under vigorous stirring. Then 2.458 g

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TMOS and 0.458 g TESPTS were added to the above solution under stirring at 40 °C for about 15 min. The reaction mixtures were then transferred into a stainless steel autoclave (50.0 mL) and kept for 20 h at 40°C under vigorous stirring (1200 rpm). Meanwhile, CO2 was charged into the autoclave to reach a range of pressure. After that, the solution was heated to 100 °C and kept for an additional hydrothermal treatment for 24 h. The pressure of the autoclave was kept constant in the hydrothermal process. After depressurization, the obtained precipitate was filtered and dried in air overnight. Removal of the surfactant was accomplished by extraction with a mixture of ethanol−/HCl (100:3; v/v) using a Soxhlet apparatus for 48 h with subsequent drying of the product in automatic thermostat at 60°C. 2.2 Synthesis of Au/PMOs In a typical synthesis, 1.0 g P123 was dissolved in a solution containing 27.3 g of deionized water and 1.69 g of ethanol at 40°C under vigorous stirring. Subsequently, 2.458 g TMOS and 0.458 g TESPTS were added to the above solution with continuous stirring. Then the solution was transferred into a stainless steel autoclave and charged CO2 to reach a suitable pressure. After stirring for 24 h at 40°C, 1.0 mL of HAuCl4 (0.05M) aqueous solution was added and stirring for additional 1h. After that, the mixture was aged for 24 h at 100°C. Meanwhile, the pressure of the progressing was kept constant. The sample was centrifuged and washed 3 times with deionized water, and then dried at 80°C for 12 h. Finally, the as-prepared sample was calcined at 500°C for 5 h to remove the template and reduce Au3+. 2.3 Catalytic tests Reduction of 4-NP by the obtained Au/PMOs composite was monitored by UV–vis absorption spectra. In a typical procedure, 0.5 mL of aqueous solution of NaBH4 (0.2 M) was firstly added to a quartz cuvette. Subsequently, 0.05 mL aqueous solution of 4-NP (0.005 M) and 2.5 mL of deionized water were added to the above solution. After sonication for 2 min, 0.4 mL of an aqueous dispersion of Au/PMOs catalyst suspension (0.0125 wt %) was added to the mixed solution. In order to explore the recyclability of catalysts, the solid catalyst was separated from the liquid phase by centrifugation and washed by using absolute ethanol and deionized water orderly. The 5


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reduction reaction was repeated 5 times by using recovered catalyst at same conditions. 2.4 Characterization Transmission electron microscopy (TEM) images were obtained with a JEOL JEM2100F at an accelerating voltage of 80 kV. The Fourier transform infrared (FTIR) spectra were recorded on a Bruker-Vector 22 FTIR spectrometer. Nitrogen adsorption isotherms of samples were carried at -196 °C by a Quantachrome Belsorb-MAX system. All samples were degassed at 275°C over 3 h before the measurements. The BET specific surface area (SBET) were calculated from p/p0=0.05-0.3 while the pore size distributions were calculated by the BJH method from the desorption branch. The UV-vis measurements were performed on Persee TU-1800 UV-vis spectrophotometer. Powder X-ray diffraction (XRD) measurements were performed on a Bruker NanoStar at 40 kV and 35mA with Cu Kα radiation. Solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were collected on a Bruker DRX400 MHz FT-NMR spectrometer with a MAS speed of 8 kHz. Cross-polarization (CP) technique was used for both 13C and 29Si spectra, which were referenced to tetramethylsilane. The loading content of Au was collected on an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Varian VISTA-MPX).

3. Results and discussion The synthesis procedure of the Au/PMOs composite material by using one-pot approach with compressed CO2 is shown in Scheme 1. Firstly, P123 were added into the deionized water to formed micelles. Subsequently, the organosilica precursors of TMOS/TESPTS were added. As CO2 was charged into the solution and hydrothermal treatment, TMOS/TESPTS interacted with each other and bounded with hydrophilic PEO segments of P123 through hydrogen-bond interactions to self-assemble the mesostructures.38. With the addition of HAuCl4 solution, thioether group of TESPTS can effectively anchored Au3+ via coordination effect.41 After calcination, surfactant could be removed and the anchored Au3+ was reduced to Au nanoparticle. Thus, PMOs with highly dispersed gold nanoparticles could be achieved. Structural characterization, compositional information, and catalytic performance of the obtained Au/PMOs

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composite samples prepared with different CO2 pressures were conducted and the possible mechanism was proposed.

Scheme 1. Schematic illustration of formation of Au/PMOs composite with one-pot approach. 3.1 Structural characterization Typical TEM images of the obtained Au/PMOs samples at different CO2 pressures are shown in Figure 1. Notably, no aggregation of gold nanoparticles was observed in all samples, indicating that the well-dispersed gold particles are confined in the PMOs matrix. Figure 1a shows that the synthesized PMOs support presents tubular structure at the CO2 pressure of 2.90 MPa, and the average diameter of the inserted gold nanoparticles is ~ 8.0 nm (Figure S1). As the pressure is raised to 3.90 MPa, PMOs with wormlike mesostructure and the embedded gold nanoparticles with the size about 7.8 nm are observed from the TEM images (Figure 1b). The more ordered wormlike PMOs framework is obtained when the pressure increases to 4.50 MPa (Figure 1c). As shown in Figure 1d, the prepared PMOs framework possess 2D hexagonal mesostructure at 4.90 MPa, and they are loaded with gold nanoparticles with an average size about 8.0 nm. With increase of CO2 pressure to 5.90 MPa, the 2D hexagonal structure disappeared and the hollow foam with the diameter of ~15 nm could be clearly observed from the PMOs matrix (Figure 1e). Moreover, the size distribution of the gold nanoparticles confined in the wall of PMOs is centered at 8.0 nm (Figure S1). It can be concluded from the results that the mesostructure of the prepared PMOs support generally changes from 1D tubular via 2D wormlike and 7


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hexagonal to 3D foam-like nanostructure with an increase of CO2 pressure from 2.90 to 5.90 MPa. Interesting, the reduced gold particles are uniformly distributed within the wall of PMOs support and the sizes are almost unchanged with different pressures. Furthermore, the characteristic lattice fringes of 0.202 nm [200] and 0.144 nm [220] of Au shown in Figure 1f-1 and 1f-2 respectively reveal the face-centered cubic (fcc) structure of the obtained Au nanoparticles.34

Figure 1. . TEM images of the Au/PMOs composite nanomaterials synthesized with TESPTS/TMOS as organosilica sources and HAuCl4 as gold source at CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90 MPa; (f) HRTEM images of gold nanoparticle within PMOs. To further imply that the reduced gold nanoparticles have been loaded into the PMOs matrix successfully, XRD has been employed to character the structure of the composite materials. Figure 2 shows the XRD patterns of Au-embedded PMOs composite nanomaterials synthesized with different pressures. All samples reveal five peaks with different intensity, demonstrating the existence of the gold nanoparticles. A strong diffraction peak at ca. 24° are assigned to the amorphous mesostucture of silica.34 Moreover, the peaks at 2θ of 38°, 44°, 64°, 78° are related to (111), (200), (220), and (311) planes of the face centered cubic structure of gold (JCPDS PDF-04-0784),42 respectively. The above result indicates that the gold particles have been formed and confined in the wall of PMOs by the reduction of the HAuCl4 during the synthesis. 8


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N2 adsorption-desorption isotherms and the corresponding BJH pore size distributions of the obtained Au/PMOs composite are shown in Figure 3. All samples exhibit typical type IV isotherms according to IUPAC classification. The specific surface area and pore-size distribution, which determined by the BET method and BJH model respectively, are presented in Table 1. The isotherm pattern of Au/PMOs-2.90 is typical type IV with a H2 hysteresis loop. The corresponding pore size distributed at 7.0 nm, indicating the diameter of the tubular structure. When the pressure increases to 3.90 and 4.50 MPa, the curve of the adsorption isotherms and the corresponding pore size distributions of the two samples are almost unchanged. Meanwhile, two peaks presented in the BJH pore size distributions correspond to the wormlike nanostructure and the void space. That is, the samples of Au/PMOs-3.90 and Au/PMOs-4.50 have a pore size distribution of approximately 6.3 nm, which is typically associated with worm-like mesoporous structure. For the sample of 4.90 MPa, a H1 hysteresis

loop and a calculated pore size of approximately 7.1 nm are presented. This result shows that Au/PMOs-4.90 has an ordered hexagonal structure, which is consistent with the TEM results. The pore size distributions pattern of Au/PMOs-5.90 shows two peaks, centred at 3.8 nm and 12.2 nm, corresponding to the size of the mesopores in the shell and the hollow foam-like structure,

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respectively. The slight steep at p/p0 close to 1.0 is attributed to the cavity of the cellular foam-like structure which was shown in Figure 1e.

Figure 3. Nitrogen adsorption–desorption isotherms and pore size distributions of the Au/PMOs synthesized by TESPTS/TMOS as organosilica source and HAuCl4 as gold source with different CO2 pressures. Table 1. Structure properties of the Au/PMOs nanomaterial with different structure from nitrogen sorption measurements at different pressures. Pressure

BET surface

Pore volume

Pore diameter

(MPa)

area (m2/g)

(cm3/g)

(nm)

2.90

851

0.62

7.1 10


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3.90

867

0.92

6.2+13.8

4.50

858

0.59

6.2+14.3

4.90

865

0.57

7.1

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3.8+12.2

3.2 Structural and compositional information of the single PMOs prepared with compressed CO2 To further investigate the evolution of PMOs matrix, the single PMOs were also prepared without addition of HAuCl4 solution by using compressed CO2. Typical TEM images of the PMOs samples synthesized with different pressures are shown in Figure 4. It is observed from Figure 4a that the obtained nanoparticles are composed of uniform tubular structure with diameters around 10 nm at 2.90 MPa. When CO2 pressure is raised to 3.90 MPa, the prepared nanomaterials present both vesiclelike (hollow) and hexagonal structure (Figure 4b). Similarly, nanomaterials of hollow nanospheres coexisted with hexagonal structure were obtained at 4.50 MPa (Figure 4c). By further increasing pressure to 4.90 MPa, only hollow nanospheres could be found in the TEM images (Figure 4d). With increase of CO2 pressure to 5.90 MPa, the uniform cellular foamlike structure could be clearly observed (Figure 4e). From the results, we can see that the mesostructure of the prepared PMOs nanomaterials generally changes from nanotube via 2D hexagonal to hollow nanospheres and reticulated foams with an increase of CO2 pressure.

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Figure 4. TEM images of the single PMOs nanomaterials synthesized using TESPTS/TMOS as organosilica precursor with CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90MPa. N2 adsorption-desorption isotherms and the corresponding BJH pore size distributions of the extracted mesoporous materials are shown in Figure 5. The surface area, pore volume, and pore diameter of the samples are presented in Table 2. All samples represent type IV isotherm with hysteresis loop which reflects the mesoporous structure. PMOs material synthesized at 2.90 MPa possesses a type H2 hysteresis loop and the pore size centers at about 7.0 nm, indicating its hollow tubular structure. The curve of the samples synthesized at 3.90 MPa and 4.50 MPa exhibit H1 and H4 hysteresis loops at a medium relative pressure (p/p0) of 0.55-0.70 and 0.70-0.90. Meanwhile, the BJH pore size distributions exhibit triplet peaks, which mean the bimodal porous structure.43 The dominant peak at about 9.0 nm which corresponding to the SBA-15 pore structure indicates the existence of ordered two-dimensional hexagonal structure in the samples. The other two peaks center at 4.0 nm and 16.2 nm are attributed to the size of the mesopore in the shell and the void space between the loosely packed vesicles, respectively. Combined with the TEM, it could be concluded that the ordered hexagonal structure and vesicle-like structure coexist in these two samples. For the sample of 4.90 MPa, two peaks presented in the curve correspond to the size of the mesopores in the vesicle shell and the void space between them. It further demonstrates the existence of vesicle-like structure. A distinct steep increase of adsorption at 0.70-0.90 is presented for the sample of 5.90 MPa, demostrating the existence of the mesopores in the shell, and the corresponding pore size distributed at 6.2 nm. The other slight steep at p/p0 close to 1.0 is attributed to the cavity of the cellular foamlike structure which was shown in Figure 4e. Interestingly, the structure of the single PMOs does not absolutely match with that of the Au/PMOs composite samples even prepared at the same CO2 pressure. It is a common knowledge that the pH value plays as a crucial factor in the formation of the PMOs' mesostructure. Thus, we deduced that the addition of HAuCl4 is the major cause for the difference of the mesostructure. 12


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Figure 5 need to be moved to supporting information.

Figure 5. Nitrogen adsorption–desorption isotherms and pore size distributions of the obtained PMOs with different CO2 pressures. Table 2. Structure properties of the PMOs nanomaterial with different structure from nitrogen sorption measurements at different pressures. Pressure

BET surface

Pore volume

Pore diameter

(MPa)

area (m2/g)

(cm3/g)

(nm)

2.90

798

0.96

7.0

3.90

804

1.01

9.0+4.0+16.2

4.50

900

1.02

9.2+4.0+16.2

4.90

730

0.83

3.9+16.2

5.90

775

0.98

6.2+13.9

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3.3 Compositional information of the single PMOs prepared with compressed CO2 Figure 6 displays the

29

Si magic-angle spinning (MAS) and

13

C cross-polarization

MAS (CP MAS) solid-state NMR spectra of the obtained single PMOs with different pressures. 29Si NMR spectrums of all PMOs samples in Figure 6a show both Q and T sites, well demonstrate the connectivity of the functional organic groups to the silica framework. These spectrums exhibit three nQ signals at −110, −101, and –91 ppm. The prominent signals at −101 ppm can be assigned to the silicon resonances of (OH)Si(OSi)3 (3Q, δ=−101ppm), representing silica with one terminal hydroxyl group. And two shoulder peaks at –110 ppm and –91 ppm can be assigned to the resonances of Si(OSi)4 (4Q, δ=−110 ppm) and (OH)2Si(OSi)2 (2Q, δ=−91 ppm), respectively.44 All these imply a high degree of TMOS crosslinking under the employed synthetic conditions. Some additional peaks originating from silicon bridged by the organic group can be observed in the range from –45 to –90 ppm. The spectrum of the obtained PMOs exhibits three mT signals at −56, −60, and −66 ppm, assigned to Si species covalently bonded to carbon atoms 1T [C–Si(OSi)(OH)2], 2T [C–Si(OSi)2(OH)], and 3T [C–Si(OSi)3], respectively.32 1T signal at −56 ppm corresponds to the incompletely hydrolyzed and condensed silicon species. Based on normalized peak areas, 1T/mT is about 0.43 at 2.90 MPa, indicating that ca. 43% of TESPTS was loosely crosslinked in the mesoporous framework because of its incomplete hydrolysis and condensation.44 Meanwhile, this ratio gradually decreases to 0.36, 0.34, and 0.29 when CO2 pressure increases to 3.90, 4.90 and 5.90 MPa. It showed that the hydrolysis of TESPTS was more thorough at the elevated pressure and more organic groups of TESPTS were bridged on the mesoporous framework. This maybe due to the decrease of pH value deriving from the dissolving and ionization of CO2 in water, as it is crucial for hydrolysis and condensation of organosilane precursor.45 Moreover, (3Q+4Q)/nQ of the four samples stays around 0.90, indicating the hydrolysis degree of TMOS hardly change with increase of CO2 pressure. In addition, the ratio of mT/(mT+nQ) calculated from the normalized peak areas of the four samples with different pressures are all 0.12, which is consistent with the TESPTS concentration in the initial mixture.

13

C

14


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cross-polarization (CP) MAS NMR spectrum gives more evidence of the existence of unhydrolyzed TESPTS prepared with different pressures. As seen in Figure 6b, the signals at 12, 24, and 42 ppm can be assigned to 1C, 2C, and 3C carbon species of Si–1CH2 2CH2 3CH2–S–S–S–S–3CH22CH21CH2–Si, respectively.46 The peaks at 16 and 58 ppm originate from the unhydrolyzed ethoxy groups (–OCH2CH3) of TESPTS and the ethoxy groups formed during the soxhlet extraction process. Similarly, these two peaks area account for ca. 37% of the total peak areas at 2.90 MPa. While it decreases to ca. 24% when the pressure increases to 3.90 MPa. The result is consistent with the findings that pH of the reaction mixture is not sufficient for the hydrolysis of organosilica precursors at lower CO2 pressure.44 Interestingly, this value is almost same at 3.90, 4.90, and 5.90 MPa. It indicates that the amount of unhydrolyzed TESPTS will not change when the pressure is above 3.90 MPa.

Figure 6. Solid-state (a) 29Si MAS and (b) 13C CPMAS NMR spectrum of the obtained PMOs prepared at different pressure. FTIR spectroscopy was also implemented to characterize the frame structure of the PMOs and shown in Figure 7. It further confirms that the organic groups of TESPTS have been bridged on the mesoporous framework with covalent bond. The strong band at around 3443 cm-1 result from the stretching and deformational vibrations of the residual water.34 The absorption peak at about 1085 cm-1 and the peak at 950 cm-1, which are attributed to the symmetrical stretching of Si−O−Si and Si−OH deformation vibration, respectively, indicate the formation of SiO2 in the framework. The weak band at around 798 cm-1 could be assigned the C–S stretching,35 proving the organic groups

15


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of TESPTS have bridged to the PMOs framework with covalent bond. All FTIR absorption curves show the similar trend, further indicating that the CO2 pressure has no adversely impact on the organosilicate structure of PMOs during the syntheses.

Figure 7. FTIR of the prepared PMOs prepared at different pressures. 3.4 Formation mechanism of single PMOs and Au/PMOs with controllable mesostructure by using compressed CO2 The mesostructure transformations of the PMOs support were achieved by the mesophase variations of the surfactant aggregation, and the mesophase transformations which caused by the self-assembly surfactant in the solution can be realized by adjusting the amount of compressed CO2. This is attributed to the pH decrease deriving from the acidification of CO2 in water at lower pressure and the penetration of CO2 molecules into the hydrophobic layer of the aggregation at higher pressures.46-48 In addition, the hydrolysis and condensation rate of TESPTS is much slower than that of TMOS.49 The acidification of CO2 can completely hydrolyse TMOS even at the low pressure, whereas is not enough for TESPTS. Therefore some unhydrolyzed TESPTS can penetrate into the core of the surfactant P123 micelles and realize the aggregation transitions.49 This is the main factor for the transformation of the surfactant aggregation at low pressure (2.90 MPa).37 Moreover, these unhydrolyzed TESPTS can be removed from the obtained materials with the surfactant during the extraction process. When the pressure increases, the continue ionization of CO2 accelerates the hydrolysis of TESPTS, and some CO2 molecules are able to penetrate into the hydrocarbon-chain 16


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region of the P123 micelles to swell the surfactant aggregation. Furthermore, we notice that the amount of the unhydrolyzed TESPTS is unchanged when the pressure is above 3.90 MPa. So it can be deduced that the insertion of CO2 gas molecules into the hydrophobic layer is main cause of the mesophase transformations in the surfactant solution at high pressures. According to the above discussion, we attempt to understand and reveal the mechanism on how compressed CO2 drives the mesostructure transformation of prepared PMOs, which is schematically presented in Scheme 2. Spherical micelles with PPO core and a corona of PEO chains are formed when the nonionic surfactant P123 dissolves in water with a certain concentration solution (Au/PMOs-4.90>Au/PMOs-4.50>Au/PMOs-3.90>Au/PMOs-2.90. It is clearly that the catalytic performance of foams-like Au/PMOs synthesized at 5.90 MPa was far exceed those of other Au/PMOs composites as catalysts. It has been reported that many factors of catalysts such as particle size, specific area and structure properties have important effects on their catalytic performance.52 Based on the above results, all samples prepared with different CO2 pressures almost have the same size of the gold nanoparticles embedded in the PMOs wall. Therefore, we can conclude that the unique mesostructure of Au/PMOs-5.90 plays a critical role in the catalytic performance. A possible morphology-dependent reaction mechanism for the reduction of 4-NP is proposed on the basis of the experimental results (Scheme 3). Au/PMOs-5.90 possess the 3D foam-like mesostructure and the larger pore diameter (Dp = 12.2 nm), which could favor the reactant molecules to react with the inner active 19


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sites easily.53 The reaction pathway of the reactants via the mesopores in the PMOs support with 3D foam-like mesostructure is the shortest among all samples and is easily accessible as well. In addition, the existence of porous in the silica wall of Au/PMOs-5.9 accelerates the access of reactant to the inner active sites. Thus, the product transportation can be greatly promoted by providing shorter diffusion pathways.54-56 For the catalyst of Au/PMOs-4.90, reactants can access to gold nanoparticles through the straight channel of the ordered 2D hexagonal PMOs. Meanwhile, the existence of conjoined channels of the PMOs support could facilitate the molecular reactants diffusion process. The crooked channels of Au/PMOs-4.50 with worm-like mesostructure may hinder the reactants diffusion to some extent in comparison to Au/PMOs-4.90. The less ordered of Au/PMOs-3.90 strengthen the difficulty to access the inner active site due to the longer reaction pathway of the reactants. That is, the ordered hexagonal channel is smoother than that of the worm-like channel and easier for the reactants to passed by. For Au/PMOs-2.90, the 1D twisted nanotube structure has only one channel, which makes the reactants more difficult to access the gold catalyst than that of the 2D worm-like mesostructure with internal connected multi-channel. A schematic drawing of the pathway of the reactants reacted with the embedded gold nanoparticle in the PMOs wall is presented in Scheme 3. In addition, the catalytic activities of the Au/PMOs-5.90 composites synthesized with compressed CO2, far exceeds those of the reported Au-based catalysts in the Table S2. It further confirmed that the mesostructure of the PMOs support could determine the rate of the catalytic reaction.

20


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Figure 8. (a)Equation for the reduction of 4-NP to 4-AP; (b) UV–vis spectra of 4-NP (1) before and (2) after the addition of NaBH4; (c) UV–vis spectra of the reduction process of 4-NP in the presence of the Au/PMOs, reaction time: 30s (red), 60s (green), and 100s (blue); (d) ln(Ct /C0) versus time plot for 4-NP reduction; (e) The recyclability of the Au/PMOs with the CO2 pressure of 5.90 MPa as the catalyst for the reduction of 4-NP with NaBH4. Table 3. Comparison of catalytic performance of the Au/PMOs nanomaterial with different structure at different pressures in the reduction reaction of 4-NP. Pressure(MPa)

2.90

Conversion(%)

98.5

k (s-1)

k' (s-1g-1)

9.7×10-3

2.5×105

21


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3.90

97.3

8.3×10-3

3.2×105

4.50

96.5

4.5×10-3

4.3×105

4.90

98.3

10×10-3

13×105

5.90

99.3

53×10-3

17.9×105

Scheme 3. Schematic illustration of reaction pathway of the reactants with gold nanoparticle catalysts with different mesostructure of PMOs support. The recyclability of Au/PMOs prepared at CO2 pressure of 5.90 MPa shown in Figure 9 demonstrates that the obtained composite can be recycled 5 times without significant loss of activity, suggesting the good stability of the synthesized Au/PMOs. Meanwhile, the hollow foam-like mesostructure of the composites remain unchanged during the reaction and the aggregation of Au nanoparticles is inhibited. The stability of Au/PMOs can be deduced to the in situ reduction of Au nanoparticles during the heating process. Therefore, the gold nanoparticles can also be confined into the PMOs framework in the high-temperature reaction.

22


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Figure 9. (a) XRD patterns of fresh and used Au/PMOs catalyst, TEM images of the (b) used Au nanoparticle and (c) used Au/PMOs composites prepared at 5.90 MPa after running 5 cycles. 4. Conclusions In

summary,

gold/periodic

mesoporous

organosilicas

with

controllable

mesostrctures was obtained by using compressed CO2 with one-pot approach. The thioether-bridged mesoporous organosilicas with different mesostructures (nanotube, hexagonal, vesicles, and cellular foams) were controllably synthesized by simply tuning the CO2 pressure in the aqueous solution. The compressed CO2 have three advantages: (1) serve as catalyst in the hydration of organosilica precursors, (2) the regulative agent to control the morphology of obtained materials, and (3) penetrate into the hydrocarbon-chain region to expand the volume region of the micelles. The as-made Au/PMOs composite exhibits an almost complete conversion of reduction of 4-NP to 4-AP within 5 min. The high catalytic activity and superior recyclability can be assigned to aggregation-free gold nanoparticles and unique 3D hollow foam-like hollow mesostructure. It is anticipated that this facile and green approach will provide a new avenue to develop an architectural design of metal/mesoporous silica composites with various mesostructure and high catalytic activity, because the morphology and their corresponding catalytic performance can be readily manipulated by adjusting the pressure of compressed CO2. Acknowledgements This work was supported by the project of Beijing Natural Science Foundation (2142011). Notes and references:

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Gold/periodic mesoporous organosilicas with controllable mesostrcture by using compressed CO2 Xin Huang,‡ Mengnan Zhang,‡ Meijin Wang,‡ Wei Li,* Cheng Wang, Xiaojian Hou, Sen Luan, and Qian Wang Department of Chemistry, Capital Normal University, Beijing, 100048 China. Tel: +86-10-68903086, E-mail: [email protected]

Scheme 1. Schematic illustration of formation of Au/PMOs composite with one-pot approach.

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Figure 1. TEM images of the Au/PMOs composite nanomaterials synthesized with TESPTS/TMOS as organosilica sources and HAuCl4 as gold source at CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90 MPa; (f) HRTEM images of gold nanoparticle within PMOs.

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Figure 3. Nitrogen adsorption–desorption isotherms and pore size distributions of the Au/PMOs synthesized by TESPTS/TMOS as organosilica source and HAuCl4 as gold source with different CO2 pressures.

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Figure 4. TEM images of the single PMOs nanomaterials synthesized using TESPTS/TMOS as organosilica precursor with CO2 pressure of (a) 2.90, (b) 3.90, (c) 4.50, (d) 4.90, and (e) 5.90MPa.

Figure 5. Nitrogen adsorption–desorption isotherms and pore size distributions of the obtained PMOs with different CO2 pressures.

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Figure 6. Solid-state (a) 29Si MAS and (b) 13C CPMAS NMR spectrum of the obtained PMOs prepared at different pressure.

Figure 7. FTIR of the prepared PMOs prepared at different pressures.

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Scheme 2. Schematic illustration of mesostructure transformations of PMOs by using compressed CO2.

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Figure 8. (a)Equation for the reduction of 4-NP to 4-AP; (b) UV–vis spectra of 4-NP (1) before and (2) after the addition of NaBH4; (c) UV–vis spectra of the reduction process of 4-NP in the presence of the Au/PMOs, reaction time: 30s (red), 60s (green), and 100s (blue); (d) ln(Ct /C0) versus time plot for 4-NP reduction; (e) The recyclability of the Au/PMOs with the CO2 pressure of 5.90 MPa as the catalyst for the reduction of 4-NP with NaBH4.

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Scheme 3. Schematic illustration of reaction pathway of the reactants with gold nanoparticle catalysts with different mesostructure of PMOs support.

Figure 9. (a) XRD patterns of fresh and used Au/PMOs catalyst, TEM images of the (b) used Au nanoparticle and (c) used Au/PMOs composites prepared at 5.90 MPa after running 5 cycles.

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