Ligand-free gold nanoclusters confined in mesoporous silica nanoparticles for styrene epoxidation

We present a novel approach to produce gold nanoclusters (Au NCs) in the pores of mesoporous silica nanoparticles (MSNs) by sequential and controlled addition of metal ions and reducing agents. This impregnation technique was followed to confine Au NCs inside the pores of MSNs without adding external ligands or stabilizing agents. TEM images show a uniform distribution of monodisperse NCs with an average size of 1.37 ± 0.4 nm. Since the NCs are grown in situ in MSN pores, additional support and high temperature calcination are not required to use them as catalysts. The use of Au NC/MSNs as a catalyst for the epoxidation of styrene in the presence of tert-butyl hydroperoxide (TBHP) as a terminal oxidant resulted in an 88% conversion of styrene in 12 h with a 74% selectivity towards styrene epoxide. Our observations suggest that this remarkable catalytic performance is due to the small size of Au NCs and the strong interaction between gold and the MSNs. This catalytic conversion is environmentally friendly as it is solvent free. We believe our synthetic approach can be extended to other metal NCs offering a wide range of applications.

inlet temperature = 250 °C, detector temperature = 250 °C, temperature program = 40 °C/min), 300 °C (3 min). GC−MS measurements were performed with an Agilent 7890A series coupled with Agilent 5975C series. GC/MS equipped with a capillary column coated with nonpolar stationary phase HP-5MS was used for molecular weight determination and identification that allowed the separation of hydrocarbons according to their boiling point differences. The BET surface area, total pore volume and pore size of MSN-NH 2 and MSN-NH 2/ Au NCs were determined with the Micromeritics ASAP 2420 instrument. X-ray photoelectron spectroscopy (XPS) studies were carried out in a Kratos Axis Ultra DLD spectrometer, equipped with a monochromatic Al Kα X-ray source (hν= 1486.6 eV) operating at 150 W, a multi-channel plate and delay line detector under a vacuum of ~10 -9 mbar. The survey and high-resolution XPS spectra were analyzed at fixed analyzer pass energies of 160 eV and 20 eV, respectively. The material was dispersed in MeOH then dropcast over silicon substrate. Powder X-ray diffraction was recorded on a Bruker AXS D8 diffractometer using Cu-Kα radiation. One-dimensional 1H MAS and 13C CP/MAS solid state NMR spectra were recorded on Bruker AVANCE III spectrometers operating at 600 MHz resonance frequencies for 1H. Experiments at 600 MHz employed a conventional double-resonance 3.2 mm CP/MAS probe. Dry nitrogen gas was utilized for sample spinning to prevent degradation of the samples. NMR chemical shifts are reported with respect to the external references TMS and adamantane. For 13C CP/MAS NMR experiments, the following sequence was used: 90° pulse on the proton (pulse length 2.4 s), then a cross-polarization step with contact time of typically 2 ms, and finally acquisition of the 13C signal under high-power proton decoupling. The delay between the scans was set to 5 s to allow the complete relaxation of the 1H nuclei, and the number of scans ranged between 10000 and 20000 for 13C and was 32 for 1H. An exponential apodization function corresponding to a line broadening of 80 Hz was applied prior to Fourier transformation. The 2D 1H−13C heteronuclear correlation (HETCOR) solid state NMR spectroscopy experiments were conducted on a Bruker AVANCE III spectrometer using a 3.2 mm MAS probe. The experiments were performed according to the following scheme: 90° proton pulse, t1 evolution period, CP to 13C, and detection of the 13C magnetization under TPPM decoupling. For the cross-polarization step, a ramped radio frequency (RF) field centered at 75 kHz was applied to the protons, while the 13C channel RF field was matched to obtain an optimal signal. A total of 64 t1 increments with 2000 scans each were collected. The sample spinning frequency was 15 kHz. Using a short contact time (0.2 ms) for the CP step, the polarization transfer in the dipolar correlation experiment was verified to be selective for the first coordination sphere to lead to correlations only between pairs of attached 1H−13C spins (C−H directly bonded).

Synthesis of MSN.
Mesoporous silica nanoparticles (MSN) were prepared by dissolving CTAB (250 mg, 0.67 mmol) in deionized water (120 mL) then an aqueous solution of NaOH (875 L, 2 M) was added. The mixture was stirred for 30 min and heated, when the temperature reached up to 80 °C, TEOS (1.25 mL, 5.6 mmol) was slowly added. The sol-gel process was conducted for 2 h, and the solution was then cooled down to room temperature. Then collected by centrifugation and washed with water and ethanol to remove residual reactants. Subsequently, the product was dissolved in ethanol with 500 μl HCl, then kept under heating at 60°C for 6 hrs. in order to remove the template (CTAB). The product was collected via centrifugation and washed with ethanol and acetone. Then it was kept to dry under atmosphere pressure. Quantify the amount of hydroxyl group on the surface of silica. The interaction between methyllithium (CH 3 Li) and silica considered one of the most reliable method to estimate the number of OH group (the silanol number α OH ). 2 According to the stoichiometry in the equation of the reaction, the amount of the produced methane will be equivalent to the content of surface OH group.
The concentration of surface hydroxyl group (α OH(S) ) on SiO 2 surface, referred to the unit mass of the sample (mmol of OH/g of SiO 2 ). It was determined to be 4 ± 0.2 mmol OH/silica. (MSN-NH 2 ). 50 mg of MSN was dissolved in 5 ml dry toluene containing 12.5 µL APTES (3-aminopropyl) triethoxysilane and the solution was left under heating overnight at 120 o C. The solution was centrifuged and the precipitate was washed with toluene, THF and methanol, respectively. The precipitate was collected to dry under 60 o C. The CHN elemental analysis was performed for MSN before and after the amination process. MSN: 2% of C and ≃ 0 % of N, MSN-NH 2 : 2.16 % of N, 10.60 % of C and 4.42 % of H. The low percentage of C in MSN comes from unreacted TEOS, and the high carbon percentage in MSN-NH 2 related to the grafting of APTES on the surface of silica. According to the silanol number and the elemental analysis we can conclude that each one unit of APTES attached to 3 hydroxyl group on the surface of silica. Impregnation of Au NCs in the pores of MSN-NH 2 . The impregnation technique was followed in order to adjust the amount of the gold precursors needed to grow inside the pores of MSN. 3 So that, the volume of gold precursor solution will be equal to MSN-NH 2 pore volume. For low loading of gold NCs, an aqueous solution of HAuCl 4 (7L, 100 mM) was added to MSN-NH 2 (60 mg) dispersed in 1 mL of methanol followed by continuous stirring for 3 hours under ambient conditions. Afterwards, the mixture was centrifuged and washed twice with water then with methanol to remove unreacted HAuCl 4 and was dried overnight under reduced pressure at room temperature. Subsequently the dried MSN/Au particles was dispersed in 1.5 mL of methanol and cooled to 0 o C. This was followed by the addition of methanolic solution of NaBH 4 (14 mg, in 1 mL methanol) under vigorous stirring. The reaction was continued for 45 min for the complete reduction of impregnated Au(III) ions to Au(0) in the pores of MSN-NH 2 generating the Au nanoclusters. The color of the reaction mixture changed from light yellow to dark brown and the product so formed was collected by centrifugation and washed with water and methanol several times. In the same way, high loading of Au NCs was prepared but the concentration of gold precursor was ten times more than the previous one (70 L,100 mM).

Catalytic Experiments 1-Oxidation of styrene
With solvent: TBHP as an oxidant: MSN-NH 2 / gold catalysts (50 mg powder, 10 % wt loading of Au (0.025mmol)) were mixed with styrene (100 µL, 0.87 mmol), toluene (2 mL) and TBHP (0.5 mL, 5.5 mmol) in an ampoule. The ampoule was frozen outside using liquid nitrogen and sealed under vacuum. The sealed reactor was heated under vigorous stirring at 353 K or/and 333 K for 6, 12, and 24h. At the end of the reaction the ampules were taken outside, frozen under liquid nitrogen and then quenched with dichloromethane. The reaction mixture was filtered, the filtrate was collected and was analyzed by GC and GC-MS. 2-Solvent-free: TBHP as an oxidant: MSN-NH 2 / gold catalysts (50 mg powder, 10 % wt. loading of Au (0.025 mmol)) were mixed with styrene (100 µL, 0.87 mmol)), and TBHP (200 µL, 2.2 mmol) in an ampoule. The ampoule was frozen outside using liquid nitrogen and sealed under vacuum. The sealed reactor was heated under vigorous stirring at 353 K or/and 333 K for 6, 12, and 24h. At the end of the reaction the ampules were taken outside, frozen under liquid nitrogen and then quenched with dichloromethane. The reaction mixture was filtered, the filtrate was collected and was analyzed by GC and GC-MS.

3-Oxidation of cyclohexene
TBHP as an oxidant: MSN-NH 2 / gold catalysts (50 mg powder, 10 % wt loading of Au, 0.025 mmol) were mixed with cyclohexene (100 µL, 1mmol), and TBHP (200 µL, 2.2 mmol) in an ampoule. The ampoule was frozen outside using liquid nitrogen and sealed under vacuum. The sealed reactor was heated under vigorous stirring at 353 K for 12. At the end of the reaction the ampules were taken outside, frozen under liquid nitrogen and then quenched with dichloromethane. The reaction mixture was filtered, the filtrate was collected and was analyzed by GC and GC-MS.

4-Oxidation of 1-phenylpropene
TBHP as an oxidant: MSN-NH 2 / gold catalysts (50 mg powder, 10 % wt loading of Au, (0.025mmol)) were mixed with phenylpropene (100 µL, 0.77mmol), and TBHP (200 µL, 2.2 mmol) in an ampoule. The ampoule was frozen outside using liquid nitrogen and sealed under vacuum. The sealed reactor was heated under vigorous stirring at 353 K for 12. At the end of the reaction the ampules were taken outside, frozen under liquid nitrogen and then quenched with dichloromethane. The reaction mixture was filtered, the filtrate was collected and was analyzed by GC and GC-MS.

5-Oxidation of cis-stilbene
TBHP as an oxidant: MSN-NH 2 / gold catalysts (50 mg powder, 10 % wt loading of Au, (0.025mmol)) were mixed with stilbene (100 µL, 0.55), and TBHP (200 µL, 2.2 mmol) in an ampoule. The ampoule was frozen outside using liquid nitrogen and sealed under vacuum. The sealed reactor was heated under vigorous stirring at 353 K for 12. At the end of the reaction the ampules were taken outside, frozen under liquid nitrogen and then quenched with dichloromethane. The reaction mixture was filtered, the filtrate was collected and was analyzed by GC and GC-MS 6-Recyclability The catalyst stability was explored by its activity and recyclability. The reaction parameters applied for the recyclability test was the same for the one described in styrene epoxidation. Each run was done at 60 o C for 12h in the presence of TBHP as oxidizing agent. After the mixture cool down, the solid catalyst was isolated from the reaction mixture.