Ligand-modified mesoporous silica SBA-15/silver hybrids for the catalyzed reduction of methylene blue

Abstract

Mesoporous silica SBA-15 was prepared and functionalized by (3-glycidyloxypropyl)trimethoxysilane in order to bind 2-aminothiazole and aminopropyl-triazole ligands. These groups were immobilized through two distinct chemistry routes: reaction of epoxy with amine resulting in AMT-SBA-15 for the former, and epoxy ring opening followed by 1,3-dipolar cycloaddition click reaction providing Tr-SBA-15, for the latter. These ligand-functionalized SBA-15 materials served as reactive platforms for the in situ deposition of silver nanoparticles from the reduction of silver nitrate. The silver-decorated hybrid Ag/AMT-SBA-15 and Ag/Tr-SBA-15 samples, as well as their precursors, were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption isotherms, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The silver-decorated hybrid materials were evaluated as catalysts for the reduction of methylene blue; the results showed excellent catalytic efficiencies with reduction rate constants of 14.3 × 10⁻³ and 9.4 × 10⁻³ s⁻¹ for Ag/Tr-SBA-15 and Ag/AMT-SBA-15, respectively. These values are comparable or higher than those claimed in the literature for similar application.

---

1. Introduction

Mesoporous silica exhibits remarkable textural, morphological and mechanical properties which have raised enormous interest within the materials science community.¹ These properties can be controlled by the combination of several parameters: sol–gel and emulsion chemistries, the nature and concentration of the template, the acid–base properties of the synthesis medium.² After nearly three decades, mesoporous silicas were proved to be ideal materials for numerous applications in heterogeneous catalysis,^3,4 selective adsorption,⁵ separation,^6,7 design of optical and electrical devices,⁸ electrochemical sensors^9,10 and drug delivery,¹¹ to name but a few. Particularly, SBA-15 (one of the ordered mesoporous silicas) has relatively high thermal and mechanical stability as well as excellent catalytic performances. In addition, it possesses hexagonal arrays of uniform pores with high specific surface areas, large pore volumes and reactive functional groups on the surface, which make it an ideal material with tunable properties for a large variety of purposes. For example, one can take advantage of the versatile chemistry and adsorptive capability of mesoporous silica to make hybrid catalytic mesoporous silica/metal nanoparticle systems. Towards this end, Pt,^12,13 Pd,^14,15 Ag,^16,17 and Au nanoparticles^18,19 were immobilized on silica via two main routes: either (i) by doping metal salts into the pores (direct incorporation)^20,21 or (ii) by grafting alkoxysilyl compounds for the complexation of the noble metals. The latter option is usually conducted in two steps where the synthesis is followed by post-functionalization of the inner surface. It is worth noting that as for pristine mesoporous silicas, their hybrid organic–inorganic derived materials have also several figures of merit: excellent mechanical properties, high chemical and thermal stability, and remarkable sensitivity in sensor applications.^22,23 These distinct properties are highly desirable in the development of solid supports for heterogeneous catalysis.

Several ligands containing N, S or O donor atoms were successfully immobilized onto mesoporous silica and used for heavy metal extraction and preconcentration, namely: 5-mercapto-1-methyltetrazole,²⁴ aminobenzenesulfonamide,²⁵ diphenylpyrazole,²⁶ 4-(2-pyridyl)-1,2,3-triazole,²⁷ phloroglucinol diimine,²⁸ amidoxime,¹⁴ melamine–pyridine,²⁹ cationic 4,4-bipyridinium (viologen),¹⁹ imidazole ionic liquid,³⁰ and N-phenylthiazolium salts.³¹ Some of these reports concerned environmental applications i.e. removal of toxic heavy metals^24,25,27 whereas other publications described the use of ligand-modified mesoporous silica for the immobilization of catalytic metal nanoparticles (NPs).^{14,19,26,28–31}

As far as the catalytic noble metals are concerned, there has been much interest in the synthesis of silver nanoparticles for the development of electrochemical sensors,^32,33 antibacterial materials,^34–36 Raman-based optical sensors³⁷ and heterogeneous catalysis.^38–40

In this work, we used selective chelating mesoporous silicas, modified by 2-aminothiazole groups containing both nitrogen and sulfur, on the one hand, and aminopropyltriazole groups (Tr) on the other hand for the immobilization of silver nanoparticles in aqueous medium. 2-Aminothiazole was incorporated in the mesoporous silica surface by reaction with 3-glycidoxy groups already tethered to mesoporous silica,⁴¹ resulting in the final product aminothiazole-functionalized silica (AMT-SBA-15). The incorporation of aminopropyltriazole (3) was obtained by 1,3-dipolar cycloaddition reactions between organic azides and terminal alkynes (propargylamine);^42,43 the final product will be denoted Tr-SBA-15. Fig. 1 displays the steps to obtain ligand-modified mesoporous silica: (i) modification of SBA-15 by (3-glycidyloxypropyl)trimethoxysilane (GPTMS), (ii) grafting of 2-aminothiazole, (iii) opening of the epoxy groups by sodium azide and (iv) triggered click coupling reaction between azido groups from N₃-SBA-15 and the alkyne group from propargylamine.

Fig. 1 Schematic representation of the preparation of AMT-SBA-15 and Tr-SBA-15 hybrid supports.

The hybrid materials were characterized using Fourier transform infrared (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD), low-angle XRD, X-ray photoelectron spectroscopy (XPS) and BET surface area measurements. The reduction of methylene blue was taken as a model reaction to judge the performance of the ligand modified SBA 15 supported Ag nanoparticles.

The analytical strategy devised herein employs GPTMS as a first step of modification of silica. It is a very interesting silane bearing a highly reactive epoxy group which undergoes ring opening reaction with many chemical compounds. We explore this particular feature of GPTMS to design and tune the chemical composition of the modified mesoporous silica prior to the in situ immobilization of silver nanoparticles. This strategy is simple, efficient and has not been reported before, hence the motivation for this work.

2. Experimental section

2.1. Materials

Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) [EO20PO70EO20, Pluronic P123, M_w = 5800], tetraethyl orthosilicate (TEOS, 98%), (3-glycidyloxypropyl)trimethoxysilane (GPTMS), sodium ascorbate, sodium azide (NaN₃), ammonium chloride (NH₄Cl), copper(II) sulfate pentahydrate (CuSO₄·5H₂O), 2-amino-1,3-thiazole, propargylamine, toluene, isopropyl alcohol, dimethylformamide, HCl and ethanol were of high purity grade and used as received (all Aldrich-Sigma products). Water was deionized using the Milli-Q system of Millipore.

2.2. Synthesis of SBA-15 mesoporous silica

The mesoporous SBA-15 was synthesized according to Zhao et al.⁴⁴ First, 3 g of Pluronic P123 was dissolved in 16.5 ml of HCl (12 M) and 112 ml of water. The solution was vigorously stirred for 3 h at 40 °C. Second, after complete dissolution of the nonionic triblock copolymer, 7.427 g of TEOS was rapidly added to the acidic solution under continuous stirring. A gel was obtained after resting for 2 h at 40 °C; the mixture was then transferred to an oven for further condensation at 90 °C under static conditions for 24 h. The precipitate obtained after filtration was washed and dried. Finally, the template was removed by calcination at 550 °C during 8 h.

2.3. Preparation of ligand-modified mesoporous silver/silica SBA-15 hybrids

2.3.1. Synthesis of G-SBA-15. The glycidyl-functionalized mesoporous silica (G-SBA-15) was prepared by stirring 1 g of mesoporous silica in 50 ml of anhydrous toluene for 30 min at 110 °C. Then, 2 ml of GPTMS was added to the solution, maintaining the stirring for another 24 h under reflux and nitrogen atmosphere. The resulting material was filtered off, washed several times with ethanol and dried at 100 °C.

2.3.2. Azidation of G-SBA-15. 500 mg of G-SBA-15 were dissolved in 10 ml dry DMF, and once dissolved, 40 mg of NH₄Cl was added. The mixture was stirred for 30 min at 52 °C, and then 34 mg of NaN₃ were added to the solution. The mixture was stirred under reflux for 24 h and the process was continued under reflux for another 24 h. The product was filtered off and dried under vacuum for 24 h at 70 °C.

2.3.3. Synthesis of AMT-SBA-15. 2-Amino-1,3-thiazole (100 mg, 1 mol) was dissolved in 10 ml of isopropyl alcohol and 2 ml of trimethylamine was added. To this solution, 0.5 g of G-SBA-15 was added. The mixture was stirred under reflux for 6 h at 82 °C. The product was then filtered off, washed several times with isopropyl alcohol and water, and finally dried under vacuum at 70 °C.

2.3.4. Synthesis of Tr-SBA-15. Tr-SBA-15 was obtained by 1,3-dipolar cycloaddition reaction as follows: the azido-G-SBA-15 (0.25 g), CuSO₄ (0.25 g), and sodium ascorbate solution (0.20 g, 0.1 M) was added to a solution of propargylamine (1.11 ml, 1 M) in DMF (10 ml) and the solution was stirred at 52 °C for 24 h. The reaction mixture was then poured into DMF, and the resulting product was filtered off and dried under vacuum for 24 h at 70 °C.

2.3.5. Immobilization of silver nanoparticles on the surface of modified SBA-15. Typically, silver nanoparticles were prepared by adding a 50 mg portion of AMT-SBA-15 or Tr-SBA-15 to 20 ml of silver nitrate solution (10⁻³ M), and the mixture was stirred for 12 h at room temperature under nitrogen atmosphere. Then 5 ml of 0.1 mol L⁻¹ sodium borohydride solution was added into the above solution, followed by stirring until the color of the solution remained unchanged. The products were separated and washed with ultrapure water and dried under vacuum for 24 h at 25 °C.

2.4. Catalytic reduction of methylene blue

A 5 mg portion of catalyst was added to 10 ml of MB aqueous solution (pH = 5.8, 40 mg L⁻¹). Subsequently, 0.5 ml of fresh NaBH₄ aqueous solution (0.1 mol L⁻¹) was injected into the solution under stirring. The blue color of MB vanished gradually. The catalytic process was monitored by measuring the absorbance values at 665 nm using a UV-vis spectrophotometer. For comparison we have also conducted similar experiments with AMT-SBA-15 and Tr-SBA-15 as reference materials, without any supported AgNPs.

The catalytic degradation efficiency of the as-synthesized sample was calculated as follows:

where A₀ is the initial absorbance before the addition of the catalyst and A_t is the absorbance at time ‘t’ after the addition of the catalyst. The reaction rate constant ‘k_app’ was determined using the first-order kinetics equation.

ln[A_t]/[A₀] = −k_appt (2)

2.5. Characterization

N₂ adsorption/desorption isotherms were measured at 77 K using a Belsorp Max apparatus from Bel (Japan). The samples were outgassed at 150 °C and 0.1 MPa for 12 h before measurements. The specific surface (S_BET) values were obtained using the Brunauer–Emmett–Teller (BET) method,⁴⁵ while the mesopore size of the materials was determined by the Barrett–Joyner–Halenda (BJH) method⁴⁶ using the desorption branch.

X-Ray photoelectron spectra (XPS) were recorded using a K Alpha apparatus (Thermo) fitted with a monochromatic Al-Kα X-ray source (1486.6 eV, spot size: 400 μm). The pass energy was set to 200 and 50 eV for the survey and the narrow regions, respectively. The spectra were calibrated against the C–C/C–H C 1s component set at 285 eV. The composition was determined using the manufacturer sensitivity factors.

XRD data were recorded on an Empyrean diffractometer equipped with Cu-Kα radiation source (λ = 0.1540526 nm). PXRD measurements were performed from 0.5 to 5 (2θ) with a step size of 0.02 with a dwell time of 10 s. Wide-angle powder XRD data (from 10 to 90° (2θ)) was carried out on a Siemens model D-500 diffractometer with Cu-Kα radiation.

Fourier transform infrared (FTIR) spectra of the modified surface mesoporous silicas were recorded using a Thermo-Scientific Nicolet 8700 spectrometer ranging from 4000 to 400 cm⁻¹ with 50 scans and 4 cm⁻¹ resolution.

Microstructural analyses were performed by ex situ TEM (TEM-Tecnai F20 with a Field Emission Gun 200 kV, punctual resolution 0.24 nm and Energy Filtering GIF). For TEM measurements, a small amount of the sample was dispersed in ethanol under ultrasonication for 30 min and a drop of the dispersion was spread on a carbon-covered copper grid and then ethanol was evaporated.

The residual concentration of methylene blue was determined at the maximum absorbing wavelength (λ = 665 nm) using an Agilent Cary 60 UV/Vis spectrophotometer.

3. Results and discussion

3.1. Physicochemical properties of pristine and ligand-modified SBA-15 materials

3.1.1. X-Ray powder diffraction. Fig. 2 displays small angle X-ray diffraction patterns of pristine and ligand-modified SBA-15. Three diffraction peaks (one intense reflection centered at ∼2θ = 0.93° and two low intensity peaks at about 2θ = 1.6 and 1.86°) indexed to (100), (110) and (200), respectively, account for the hexagonal crystal structure of SBA-15. The sharp peak centred at 2θ = 0.93° corresponds to a d-spacing of 110.2 Å which in turn corresponds to a large unit-cell parameter a₀ = 127.3 Å. The hexagonal pore structure of Ag/Tr-SBA-15 is retained after ligand-functionalization and further immobilization of the Ag NPs. Indeed the reflections corresponding to the (110) and (200) planes cannot be easily distinguished for Ag/AMT-SBA-15. This indicates that the sample is less structured than the other samples. The decrease of peak intensity could be due to the presence of the organic component and Ag clusters; this is in line with results reported in the literature.⁴⁰ As far as the peak at low angle is concerned (XRD pattern of Ag/Tr-SBA-15), one can note an additional diffraction peak which could probably be due to the contrast of scattering between the surface silica framework organic moieties and silver nanoparticles located inside the channels of SBA-15.

Fig. 2 Small-angle XRD patterns of SBA-15, Ag/3-AT-G-SBA-15 and Ag/AMT-G-SBA-15.

The high-angle XRD patterns of silver/ligand modified mesoporous silica are displayed in Fig. 3. For all samples four sharp diffraction peaks are centered at 2θ = 38.23, 44.47, 64.48 and 77.31° and assigned to (111), (200), (220) and (311) planes, respectively. The signals are due to the face centered cubic structure of silver crystals existing inside the solid support. The formation of silver nanoparticles inside the functionalized mesoporous silica is a result of reduction of silver ions which are attracted by the functional groups. The reduction reaction results owing to the decomposition of NaBH₄:⁴⁷

NaBH₄ + 2H₂O → NaBO₂ + 4H₂.

Fig. 3 High-angle XRD patterns of Ag/AMT-SBA-15 and Ag/Tr-SBA-15.

The intensities of the high-angle peaks decrease for Ag/AMT-SBA-15 when the amount of silver anchored decreases.

The crystallite size of the silver nanoparticles can be estimated using the Scherrer equation:⁴⁸

where d is the average nanoparticle size, K is the shape-dependent Scherrer’s constant correlating to the true shape of the crystallite (0.9), λ is the radiation wavelength (1.5406 Å), β is the full peak width (given in radians), and θ is the diffraction angle.

The average size of the cubic silver crystallites is ≈11 and 7 nm for Ag/Tr-SBA-15 and Ag/AMT-SBA-15, respectively, as determined from four diffraction peaks displayed in Fig. 3.

3.1.2. Nitrogen adsorption–desorption isotherms. Nitrogen physisorption isotherms and pore size distribution for the pristine and ligand-modified SBA-15 samples are shown in Fig. 4. It can be observed that the precursor SBA-15 and modified SBA-15 show type IV isotherms according to the IUPAC classification hysteresis loop at high relative pressure values (P/P₀ = 0.65).⁴⁹ The isotherms are as expected for mesoporous materials and are indicative for a successful mesophase synthesis. The hysteresis loops indicate narrow pore size distributions (Fig. 4) and an intact porous framework during the functionalization steps, which is also confirmed by TEM images (see below).

Fig. 4 Nitrogen adsorption/desorption isotherms of SBA-15, G-SBA-15, AMT-SBA-15, Tr-SBA-15, Ag/AMT-SBA-15 and Ag/Tr-SBA-15.

The N₂ physisorption data of all mesoporous SBA-15 materials are reported in Table 1. For the raw SBA-15, the specific surface area calculated by the BET equation (S_BET) is 670.8 m² g⁻¹, the average pore size is 5.42 nm measured by the BJH approach, and the pore volume is 0.74 cm³ g⁻¹ at P/P₀ = 0.97. After the surface modification by GPTMS, S_BET, the total pore volume and the average pore size decreased to 413 m² g⁻¹, 0.632 cm³ g⁻¹ and 5.06 nm, respectively. A decrease in the surface area is mainly due to the presence of GPTMS groups blocking the pores and thus lowering accessibility of N₂ to the inner surface.

Table 1 Specific surface areas, mean pore diameters and porous volumes for SBA-15 and ligand-modified SBA-15

Material	SBET/m^2 g^−1	D^BJHp^a/nm	Vp^b/cm^3 g^−1
a Mean pore diameter determined using the BJH method.b Total pore volume determined at P/P0 = 0.97.
SBA-15	670.8	5.42	0.740
G-SBA-15	413	5.06	0.632
AMT-SBA-15	379.3	4.40	0.608
Tr-SBA-15	162.2	4.05	0.227
Ag/AMT-SBA-15	304.3	3.70	0.423
Ag/Tr-SBA-15	129.6	3.42	0.125

---

Furthermore, after modification of SBA-15 with triazole and 2-amino-1,3-thiazole the surface area dropped to 162.2 and 379.3 m² g⁻¹, respectively. The decrease of the surface area of these supports is exacerbated after immobilization of the silver NPs, as it further dropped to 129.6 and 304.3 m² g⁻¹, respectively. Concomitantly, one can note the decrease in the pore volume after attachment of silver nanoparticles to the ligand-modified SBA-15 supports as result of the mesopore loading.

On the basis of BJH analysis, pristine, ligand-modified and silver-decorated SBA-15 materials have a pore size distribution between 5.42 and 3.42 nm (Fig. 5). It is worth noting that the average pore size and mesopore volume decrease due to the loading of functional groups and silver in mesoporous silica.

Fig. 5 Pore size distribution of SBA-15, G-SBA-15, AMT-SBA-15, Ag/AMT-SBA-15, Tr-SBA-15 and Ag/Tr-SBA-15.

3.1.3. FT-IR spectroscopy analysis. FT-IR spectroscopy was employed to characterize the chemical bonds and confirm the surface anchoring of the organic ligands and the silver doping. The FTIR spectra of the pristine SBA-15 (Fig. 6) exhibits only Si–O–Si symmetrical stretching vibration at 1077 cm⁻¹, Si–O–Si asymmetrical stretching vibration at 804 cm⁻¹, and the characteristic peak of Si–OH at 969 cm⁻¹. The peak at 3440 cm⁻¹ is attributed to the O–H vibration of Si–OH group. The vibration peak at 1648 cm⁻¹ corresponds to the physically adsorbed water molecules which are retained in the matrix even after drying.⁵

Fig. 6 FTIR spectra of SBA-15, G-SBA-15, azido-SBA-15, Tr-SBA-15 and AMT-SBA-15.

After successive modifications with GPTMS, IR absorption intensity of silanol groups at 3050–3700 cm⁻¹ were observed to decrease with the appearance of absorption peaks of C–H bonds (2833–2944 cm⁻¹), and Si–C bonds (1250 cm⁻¹), which indicates that most of the Si–OH bonds on the surface of SBA-15 have been occupied by the organic functionalities due to the modifications. Compared with the spectrum of G-SBA-15, the azido-SBA-15 displayed the characteristic absorption of azido group at 2113 cm⁻¹. C–N bending vibration (1450 cm⁻¹) and S–C bonds at (1490 cm⁻¹), which confirms the presence of 2-amino-1,3-thiazole groups in the SBA-15 mesostructure. The disappearance of characteristic vibration bands of the azido group and the concomitant appearance of C–N bending vibration, reveal that the functionalization with propargylamine was successfully achieved.

The N–H stretching bands are usually present within the range 3380–3310 cm⁻¹ coinciding with the broad band of O–H; this makes it difficult to determine the N–H peak position accurately.⁵⁰

3.1.4. XPS analysis. Fig. 7 displays high-resolution C 1s, N 1s and S 2p XP spectra of AMT-SBA-15 and its precursor G-SBA-15. For the latter, C 1s is fitted with three components centred at 285, 286.5 and 289 eV assigned to C–C/C–H, C–O and O–C=O, respectively. The high binding energy component O–C=O could originate from SBA-15; this component is invariably detected at the surface of high energy materials such as metals, metal oxides and ceramics.⁵¹ The intense C–O component is in line with the chemical structure of GPTMS. Note that for G-SBA-15, there is no N 1s peak, and this region is flat and noisy. After chemical reaction with 2-aminothiazole, S 2p is detected at 165 eV; it is a strong supporting evidence in favour of the attachment of aminothiazole to the G-SBA-15. The slight shift to higher binding energy compared to a pure thioether is probably due to the conjugation of the sulfur atom with the imine group. The C 1s region changes slightly in the sense that the second component at 286.5 eV is quite broad due to the existence of a C–N component overlapping the C–O component. The N 1s region is complex and fitted with three components whereas the attached complexing group has two nitrogen atoms. It is thus possible that one of the nitrogen atoms is partially protonated and this accounts for the high binding energy peak centred at 402.5 eV. The peaks centred at 399 and 400 eV are assigned to N–H and C=N, respectively. It follows that the extent of protonation is quite high.

Fig. 7 (a) C 1s, (b) N 1s and (c) S 2p XPS spectra of G-SBA-15 before and after attachment of aminothiazole.

As far as triazole-functionalized SBA-15 is concerned, the C 1s and N 1s high-resolution spectra are displayed in Fig. 8 together with those of the precursor N₃-SBA-15.

Fig. 8 (a) C 1s and (b) N 1s XPS spectra of N₃-SBA-15 before and after click reaction with propargylamine.

The C 1s region of the azido-terminated surface is rather similar to that of G-SBA-15 as one carbon atom from the epoxy ring changes chemical environment from C–O–C to C–N₃ which has little effect on the binding energy position. After the click reaction the resulting Tr-SBA-15 has a high-resolution C 1s region with an intense peak centred at 286.7 eV due to additional C–N bonds from the triazole and the pendant NH₂ groups.

As far as the N 1s region is concerned, N 1s has two components centred at 401.0 and 404.7 eV in 2 : 1 ratio, assigned to the partially negatively charged external nitrogen atoms (–N=N⁺=N⁻ ⇔ –N⁻–N⁺≡N), and the positively charged central nitrogen atom (–N=N⁺=N⁻) in N₃, respectively.^52,53

After reaction with propargyl amine, the azide is transformed into triazole and the N⁺ peak component has vanished indicating complete reaction, a result that supports the FTIR findings. The N=N atoms have a single component centred at 399.5 eV and the sp³ nitrogen atom has a peak centred at 400.7 eV. The ([double bond splayed left]N–)/(N=N) peak ratio is 2.2 close to the theoretical ratio of 2.

3.1.5. TEM studies. The TEM image of SBA-15 silica clearly revealed a regular hexagonal array of uniform channels, indicative for the presence of a highly ordered pore structure of SBA-15. TEM provides strong supporting evidence of the immobilization of silver nanoparticles in/on the functionalized SBA-15 (Fig. 9a and b). TEM images indicate that channels are still structured despite the introduction of silver NPs and ligands in the mesopores. We can note that the Ag/AMT-SBA-15 is characterized by a good dispersion of silver nanoparticles (Fig. 9c and d). Further, no agglomeration of metallic silver nanoparticles is observed in the entire grid, suggesting uniform anchoring of silver species to the surface of the 2-aminothiazole-functionalized SBA-15 material.

Fig. 9 TEM micrographs of SBA-15 (a and b), Ag/AMT-SBA-15 (c and d) and Ag/Tr-SBA-15 (e and f).

Fig. 9e and f indicate that the small sized silver nanoparticles were well dispersed in the Tr-SBA-15; this proves the efficient stabilization with the aid of coordination effect between the tertiary amino groups and Ag⁺ ions. These micrographs indicate also that larger particles or aggregates are also observed on the external surface, whereas well dispersed nanoparticles are located on the inner surface of the solid. It is also clear that the channels are only partially occupied by silver nanoparticles. Silver particles with spherical shape are homogeneously distributed in the Tr-SBA-15 support.

3.2. Catalytic properties of the ligand-modified mesoporous silica/silver composites

Ag NP catalyzed reduction of dyes is well documented.^38,54,55 However, the catalytic efficiency and the stability of these nanoparticles are often hampered by aggregation or a congregation of particles due to interparticle interactions through van der Waals forces and high surface energies. The stability of the catalyst support can be enhanced by incorporating functional groups.⁵⁶

The reduction of methylene blue dye using NaBH₄ was selected as a model reaction⁵⁷ to test the catalytic activity of the Ag/Tr-SBA-15 and Ag/AMT-SBA-15 hybrids. The reduction reaction was monitored by UV-Vis spectrophotometry as well as by visual inspection of the color change of initially dark blue MB (Fig. 10). The dark blue solution exhibited a strong absorption peak at 665 nm (Fig. 11a and b). The colour of the solution is maintained when considering the effect of dilution on adding NaBH₄ solution, then became colorless after addition of Ag/AMT-SBA-15 or Ag/Tr-SBA-15 catalysts, in less than 6–8 min. This is due to the reduction of methylene blue to leucomethylene blue (colorless form) by transfer of an electron from metallic silver to methylene blue (oxidized form). The intensity of the characteristic peak at 665 nm gradually decreased with time until a colorless solution was obtained indicative of complete reduction of the dye.

Fig. 10 Mechanism of the catalyzed reduction reaction of MB to LMB. The digital photographs of the vials show blue pure MB and MB/NaBH₄ mixed solutions whereas the right hand solution containing MB, NaBH₄ and the silver hybrid catalyst becomes colourless due to the reduced form LMB.

Fig. 11 Successive UV-vis spectra for reduction of MB aqueous solution (12 mg L⁻¹) by NaBH₄ using Ag/Tr-SBA-15 (a) and Ag/AMT-SBA-15 (b) catalysts. First-order kinetics plots of catalytic reduction of MB in the presence of Ag/Tr-SBA-15 or Tr-SBA-15 (c), and in the presence of Ag/AMT-SBA-15 or AMT-SBA-15 (d).

Fig. 11 shows the time dependent UV-visible absorption spectra recorded during methylene blue reduction using Ag/Tr-SBA-15 (Fig. 11a) and Ag/AMT-G-SBA-15 (Fig. 11b) catalyst, respectively. A complete reduction of MB using Ag/Tr-SBA-15 was reached within 320 s. For comparison, under the same experimental conditions, the catalyzed reduction of MB using Ag/AMT-SBA-15 was complete after 480 s.

In the absence of silver nanoparticles, however, the colour of MB was stable over a very long period, far exceeding 320 or 480 s. The concentration of MB decreased marginally in the presence of Tr-SBA-15 or AMT-SBA-15, due to adsorption on the surface of the ligand-modified mesoporous silica. These observations clearly indicate that Ag NPs were the active sites when silver-decorated hybrid materials were used for the catalyzed reduction.

The pseudo-first-order rate constants (k) were calculated from the slope of the linear correlation between ln(A_t/A₀) and the reaction time (t) for Ag/Tr-SBA-15 (Fig. 11c) and Ag/AMT-SBA-15 (Fig. 11d). The average value of k_app was estimated to 14.3 × 10⁻³ and 9.4 × 10⁻³ s⁻¹ for Ag/Tr-SBA-15 and Ag/AMT-SBA-15, respectively.

The average value of k_app in the case of Ag/Tr-SBA-15 was significantly higher than that corresponding to the Ag/AMT-SBA-15 hybrid catalyst, probably due to a higher amount of immobilized AgNPs as judged from the XRD patterns. In Fig. 11c and d we also displayed ln(A_t/A₀) vs. time plots for Tr-SBA-15 and AMT-SBA-15 (Fig. 11d), respectively. Negligible changes are noted for both plots, indicating no reduction of MB in the absence of silver NPs supported on the hybrid SBA-15 materials.

On the basis of the catalytic reduction of MB schematized in Fig. 10 and the associated experimental results displayed in Fig. 11, a possible mechanism of catalyzed reduction can be as follows. Silver is a good electrical conductor and can transfer electrons between the donor species and acceptor substrate. It follows that the catalytic process can be interpreted in terms of a redox mechanism where the metal nanoparticles mediate the transfer of electrons from donor BH₄⁻ ions to the reactant (MB), which is the acceptor. This type of electron transfer phenomenon in which metal nanocrystals act as redox catalysts is known as the “electron relay effect” and has been already described in the case of the catalyzed reduction of other dyes.⁵⁸

Table 2 compares k_app values for the MB dye reductive degradation using our hybrid catalysts and those described in the literature. Clearly, the synthesis strategy devised so far for the hybrids under study achieved catalysis reaction rate constants which are higher or comparable to those reported in the literature. Note, however, that k_app depends on the MB/NaBH₄ initial ratio as well as the mass of hybrid catalyst. Indeed, combining a higher reducing agent relative concentration and two-fold higher mass of catalyst, we have achieved a higher k_app with Ag/Tr-SBA-15 (14.3 × 10⁻³ s⁻¹). In contrast, despite a high reducing agent relative concentration, a low k_app (∼0.90 × 10⁻³ s⁻¹) was obtained by Tang et al.⁶⁰ due to the use of a low mass of hybrid catalyst AgNPs/P(NIPAM-co-DMA).

Table 2 Comparison of rate constants for the degradation of MB by Ag NP supported onto various materials^a

Catalyst	mol MB/mol NaBH4 (T/°C)	Mass catalyst/volume/g L^−1	Rate constant, kapp/10^−3 s^−1	Ref.
a p-TSA: p-toluene sulfonic acid; PEI: polyethyleneimine; NIPAM: N-isopropylacrylamide; DMA: 2-(dimethylamino)ethyl methacrylate.
Ag (E)-SiO2	1/5 (25)	0.25	13.4	56
Ag(seed)-SiO2(p-TSA-)	1/5 (25)	0.25	4.03	56
AgNPs/SiO2-PEI	1/1700 (25)	—	7.65	59
AgNPs/P(NIPAM-co-DMA)	1/460 (25–40)	0.014–0.019	0.83–0.96	60
Ag/Tr-SBA-15	1/166 (25)	0.5	14.3	This work
Ag/AMT-SBA-15	1/166 (25)	0.5	9.4	This work

---

3.3. Recovery of the hybrid catalysts

Recyclability is an important factor in heterogeneous catalysis and has been addressed as follows. Ag/ligand-SBA-15 catalysts were prepared and served to catalyze the reduction of MB three times. After the first run, the catalysts were washed three times with ethanol, water and a 0.3 mol L⁻¹ solution of sodium carbonate in order to remove borates and any other residual reaction products from the previous run. The washed catalysts were then further employed to check the stability of their catalytic performances. Fig. 12 displays the recycling test of Ag/Tr-SBA-15 and Ag/AMT-SBA-15; a marginal decrease of the kinetic constant values k_app is noted. k_app in the third cycle was found to be as high as 12.5 and 8.5 × 10⁻³ s⁻¹ for Ag/Tr-SBA-15 and Ag/AMT-SBA-15, respectively, indicating that the catalyst exhibits a good stability against poisoning by the product of the reaction. The percentage of completion reduction (CR) was nearly the same in the first two cycles, decreasing only marginally (CR ∼ 92% for Ag/Tr-SBA-15 and ∼92% for Ag/AMT-SBA-15). The small decrease in the catalytic activity could probably be due to the leaching of Ag NPs from the ligand-modified mesoporous silica. Elsewhere, decrease in the surface activity was assigned to morphological modifications or masking of the catalytic sites by the formed product.⁵⁶ Herein, we attribute the small change in activity to possible decrease in the surface concentration of Ag NPs but rule out the second hypothesis linked to poisoning as the hybrid catalysts were thoroughly washed. Therefore, the experimental results indicate stable hybrid catalysts.

Fig. 12 Recycling test of Ag/Tr-SBA-15 and Ag/AMT-SBA-15 catalysts in water medium reduction of methylene blue coupling reaction.

4. Conclusion

In this study, novel catalytic ligand-modified mesoporous silica SBA-15/silver hybrids were successfully synthesized via a new and simple method consisting in the modification of the SBA-15 using epoxy silane (GPS) to obtain a versatile reactive mesoporous silica as a starting material. This reactive silane-modified silica was further functionalized in order to immobilize aminothiazole (AMT) and triazole (Tr) ligands, respectively. These ligands were used as an anchoring agent causing nucleation and to shape the nanoparticles at the silica surface. The process resulted in the preparation of the hybrid systems Ag/AMT-SBA-15 and Ag/Tr-SBA-15. FT-IR spectroscopy, nitrogen adsorption–desorption isotherm, XPS, XRD and TEM were employed for monitoring all modification steps for SBA-15. The catalyst characterization results showed successful anchoring of the ligands on the SBA-15 materials; interestingly the ordered mesoporous structure of SBA-15 silica remained almost unaltered after functionalization. The hybrid catalysts exhibited excellent catalytic efficiencies towards the reduction of methylene blue: the reduction rate constants were determined to be 14.3 × 10⁻³ and 9.4 × 10⁻³ s⁻¹ for Ag/Tr-SBA-15 and Ag/Tr-SBA-15, respectively. These values are comparable if not higher than those claimed in the literature for similar applications. In addition, the hybrid catalysts were found to be easily recyclable, i.e. they retained excellent catalytic performances after three reduction/washing cycles.

To summarize, we have described an efficient protocol to design silver-decorated ligand-modified mesoporous silica SBA-15 for the catalyzed reduction of methylene blue. The protocol could be extended to the catalyzed reduction of other dyes; it also paves the way to tether other ligands that would host silver among other catalytic metal nanoparticles.

Acknowledgements

AS wishes to thank the Tunisian Ministry of Higher Education and Scientific Research for the provision of travel grants to conduct research at Institut de Chimie et des Matériaux Paris-Est (ICMPE).

References

1. A. Sayari and S. Hamoudi, Chem. Mater., 2001, 13, 3151–3168.
2. D. Y. Zhao, P. D. Yang, Q. S. Huo, B. F. Chmelka and G. D. Stucky, Curr. Opin. Solid State Mater. Sci., 1998, 3, 111–121.
3. A. Mavrogiorgou, M. Baikousi, V. Costas, E. Mouzourakis, Y. Deligiannakis, M. A. Karakassides and M. Louloudi, J. Mol. Catal. A: Chem., 2016, 413, 40–55.
4. K. C. W. Wu and Y. Yamauchi, J. Mater. Chem., 2012, 22, 1251–1256.
5. A. Saad, I. Bakas, J.-Y. Piquemal, S. Nowak, M. Abderrabba and M. M. Chehimi, Appl. Surf. Sci., 2016, 367, 181–189.
6. X. Liu, L. Zhou, X. Fu, Y. Sun, W. Su and Y. Zhou, Chem. Eng. Sci., 2007, 62, 1101–1110.
7. Y. Yamauchi, M. Sawada, T. Noma, H. Ito, S. Furumi, Y. Sakkad and K. Kuroda, J. Mater. Chem., 2005, 15, 1137–1140.
8. W. C. Molenkamp, M. Watanabe, H. Miyata and S. H. Tolbert, J. Am. Chem. Soc., 2004, 126, 4476–4477.
9. A. Badiei, P. Norouzi and F. Tousi, Eur. J. Sci. Res., 2005, 12, 39–45.
10. Y. Yamauchi, J. Ceram. Soc. Jpn., 2013, 121, 831–840.
11. F. Q. Tang, L. L. Li and D. Chen, Adv. Mater., 2012, 24, 1504–1534.
12. C. M. Yang, P.-H. Liu, Y. F. Ho, C. Y. Chiu and K. J. Chao, Chem. Mater., 2003, 15, 275–280.
13. H. Song, R. M. Rioux, J. D. Hoefelmeyer, R. Komor, K. Niesz, M. Grass, P. Yang and G. A. Somorjai, J. Am. Chem. Soc., 2006, 128, 3027–3037.
14. R. G. Vaghei, S. Hemmati and H. Veisi, J. Mol. Catal. A: Chem., 2014, 393, 240–247.
15. G. R. Bardajee, R. Malakooti, I. Abtin and H. Atashin, Microporous Mesoporous Mater., 2013, 169, 67–74.
16. K. Dinakaran, A. Chandramohan, M. R. Venkatesan, S. Devaraj, V. Devi and M. Alagar, Bull. Korean Chem. Soc., 2011, 32, 3861–3864.
17. M. Li, P. J. Pham, C. U. Pittman Jr and T. Li, Microporous Mesoporous Mater., 2009, 117, 436–443.
18. W. Yan, B. Chen, S. M. Mahurin, E. W. Hagaman, S. Dai and S. H. Overbury, J. Phys. Chem. B, 2004, 108, 2793–2796.
19. N. Fattori, C. M. Maroneze, L. P. D. Costa, M. Strauss, I. O. Mazali and Y. Gushikem, Colloids Surf., A, 2013, 437, 120–126.
20. X. Huang, G. Zhao, G. Wang, Y. Tang and Z. Shi, Microporous Mesoporous Mater., 2015, 207, 105–110.
21. J. Sun, D. Ma, H. Zhang, X. Liu, X. Han, X. Bao, G. Weinberg, N. Pfander and D. Su, J. Am. Chem. Soc., 2006, 128, 15756–15764.
22. J. I. Shi, Z. I. Hua and L. X. Zhang, J. Mater. Chem., 2004, 14, 795–806.
23. B. J. Melde, B. J. Johnson and P. T. Charles, Sensors, 2008, 8, 5202–5228.
24. D. Pérez-Quintanilla, A. Sánchez, I. del Hierro, M. Fajardo and I. Sierra, J. Hazard. Mater., 2009, 166, 1449–1458.
25. L. Hajiaghababaei, B. Ghasemi, A. Badiei, H. Goldooz, M. R. Ganjali and G. M. Ziarani, J. Environ. Sci., 2012, 24, 1347–1354.
26. J. Mondal, S. Sreejith, P. Borah and Y. Zhao, Chem. Eng., 2014, 2, 934–941.
27. P. Sharma and A. P. Singh, RSC Adv., 2014, 4, 43070–43080.
28. J. Mondal, P. Borah, A. Modak, Y. Zhao and A. Bhaumik, Org. Process Res. Dev., 2014, 18, 257–265.
29. H. Veisi, M. Hameliana and S. Hemmati, J. Mol. Catal. A: Chem., 2014, 395, 25–33.
30. P. Cruz, Y. Perez, I. d. Hierro and M. Fajardo, Microporous Mesoporous Mater., 2016, 220, 136–147.
31. S. Shylesh, Z. Zhou, Q. Meng, A. Wagener, A. Seifert, S. Ernst and W. R. Thiel, J. Mol. Catal. A: Chem., 2010, 332, 65–69.
32. A. Y. Khan and R. Bandyopadhyaya, J. Electroanal. Chem., 2014, 727, 184–190.
33. P. Kumar Sonkar and V. Ganesa, J. Solid State Electrochem., 2015, 19, 2107–2115.
34. Y. Tian, J. Qi, W. Zhang, Q. Cai and X. Jiang, ACS Appl. Mater. Interfaces, 2014, 6, 12038–12045.
35. D. V. Quanga, P. B. Sarawade, A. Hilonga, J. K. Kim, Y. G. Chai, S. H. Kim, J. Y. Ryu and H. T. Kim, Colloids Surf., A, 2011, 389, 118–126.
36. M. Liong, B. France, K. A. Bradley and J. I. Zink, Adv. Mater., 2009, 21, 1684–1689.
37. Z. Q. Tian, B. Ren and D. Y. Wu, J. Phys. Chem. B, 2002, 106, 9463–9483.
38. Y. Chi, L. Zhao, Q. Yuan, Y. Li, J. Zhang, J. Tu, N. Li and X. Li, Chem. Eng. J., 2012, 195–196, 254–260.
39. G. P. Yonga, D. Tian, H. W. Tong and S. M. Liu, J. Mol. Catal. A: Chem., 2010, 323, 40–44.
40. M. Boutrosa, J. M. Trichard and P. D. Costa, Appl. Catal., B, 2009, 91, 640–648.
41. S. Brase, C. Gil, K. Knepper and V. Zimmermann, Angew. Chem., Int. Ed., 2005, 44, 5188–5240.
42. R. Turgis, G. Arrachart, C. Delchet, C. Rey, Y. Barré, S. P. Rostaing, Y. Guari, J. Larionova and A. Grandjean, Chem. Mater., 2013, 25, 4447–4453.
43. O. D. L. Cobos, B. Fousseret, M. Lejeune, F. Rossignol, M. D. Colas, C. Carrion, C. Boissiere, F. Ribot, C. Sanchez, X. Cattoen, M. W. C. Man and J. O. Durand, Chem. Mater., 2012, 24, 4337–4342.
44. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. Fredrickson and B. Chmelka, Science, 1998, 279, 548–552.
45. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309–319.
46. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 73, 373–380.
47. R. E. Davis, E. Bromels and C. L. Kibby, J. Am. Chem. Soc., 1962, 84, 885–892.
48. S. P. Dubeya, M. Lahtinenb and M. Sillanpää, Process Biochem., 2010, 45, 1065–1071.
49. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti and J. Rouquerol, Pure Appl. Chem., 1985, 57, 603–619.
50. F. Sevimli and A. Yılmaz, Microporous Mesoporous Mater., 2012, 158, 281–291.
51. J. E. Castle and J. F. Watts, in Corrosion Control by Organic Coatings, ed. H. Leidheiser, NACE, Houston, 1981, p. 78.
52. C. M. Santos, A. Kumar, W. Zhang and C. Cai, Chem. Commun., 2009, 2854–2856.
53. A. Bensghaïer, Z. Salmi, B. Le Droumaguet, A. Mekki, A. A. Mohamed, M. Beji and M. M. Chehimi, Surf. Interface Anal., 2015 DOI:10.1002/sia.5924.
54. J. Han, P. Fang, W. Jiang, L. Li and R. Guo, Langmuir, 2012, 28, 4768–4775.
55. A. Molla, M. Sahu and S. Hussain, J. Mater. Chem. A, 2015, 3, 15616–15625.
56. N. Muthuchamy, A. Gopalan and K. P. Lee, RSC Adv., 2015, 5, 76170–76181.
57. K. Jlassi, A. Singh, D. K. Aswal, R. Losno, M. Benna-Zayani and M. M. Chehimi, Colloids Surf., A, 2013, 439, 193–199.
58. N. Gupta, H. P. Singh and R. K. Sharma, J. Mol. Catal. A: Chem., 2011, 335, 248–252.
59. A. Mignani, S. Fazzini, B. Ballarin, E. Boanini, M. C. Cassani, C. Maccato, D. Barreca and D. Nanni, RSC Adv., 2015, 5, 9600–9606.
60. Y. Tang, T. Wu, B. Hu, Q. Yang, L. Liu, B. Yu, Y. Ding and S. Ye, Mater. Chem. Phys., 2015, 149–150, 460–466.

---