Synthesis of copper mesostructured silica and functionalization by trimethyl chlorosilane (TMCS)

2.1 Synthesis of MCM-41

A typical gel of synthesis for the sample MCM-41 was prepared by adding 150 g of colloidal silica (ludox 40%, Prolabo) to an aqueous solution containing 73.63 g TEAOH ((C₂H₅)₄NOH, 35% solution, Aldrich) and 91.115 g of cetyltrimethylammonium bromide (CTMABr, Biochem), under agitation for 1 h. The resulting gel was brought to a temperature of 100 °C for 48 h. The formed solid is filtered, washed with distilled water and dried at 80 °C during one night. In the end, the material obtained is calcined under air flow at 550 °C for 6 h. For the Al-MCM-41 synthesis we used the same molar composition given for the synthesis of MCM-41with the following steps: preparing three solutions S1, S2 and S3, respectively, which contain a source of aluminium (NaAlO₃) mixed with the TEAOH (S1), an aqueous silica solution (S2) and an aqueous solution which contains CTABr (S3). The Si/Al molar ratio is equal to 50. The mixture obtained after homogenization of 2 h is carried to 100 °C for 2 days. The products are calcined at 550 °C for 6 h.

2.2 Preparation of Cu-supported MCM-41 catalysts

The used method is called TIE or Tensioactif Ionic Exchange method (Decyk et al., 2001): 3 g of as-synthesized MCM-41 is added to 200 ml of an ethanolic solution of Cu(NO₃)₂ (0.1 M) under agitation. After 30 min exchange period at 60 °C, the products are filtered, washed by ethanol then dried at 80 °C. Finally, the samples are calcined at 550 °C during 6 h.

2.3 Preparation of (TMCS)-Cu-MCM-41 catalysts

A monolayer of trimethylsilyl (TMS) groups is covalently attached to the pore surface of MCM-41. Approximately 2 g of template-extracted MCM-41 was outgassed in a vacuum system with a residual pressure of less than 5 · 10⁻⁴ Torr at a temperature of 450 °C for a given time and then subsequently soaked in a TMCS solution in toluene with a concentration of 5 wt% (solid:liquid) at 70 °C under stirring for a desired time. The mixture was then extensively washed with toluene and acetone to rinse away any residual chemicals. Finally, the powder was dried at 50 °C.

2.4 Oxidation of phenol

The catalytic properties were evaluated in a typical oxidation system. The experimental runs were carried out in a glass, semi-batch reactor, equipped with a magnetic stirrer. The temperature of the reaction vessel was maintained constant 70 °C using a round oil-heated jacket. Amounts of catalyst were added to the substrate solution. An aqueous H₂O₂ solution (30% w/v) was added after the desired temperature was reached. We employed a constant 3:1 phenol:H₂O₂ molar ratio and the reaction medium is water. Samples were filtered in order to remove any catalyst particles. The analyzes of samples were performed by gas chromatography.

2.5 Characterization of catalyst

Nitrogen adsorption was performed at −196 °C in a TriStar 3000 V6.04 A volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3 × 10⁻³ Torr static vacuum was reached. The surface area was calculated by the Brunauer–Emmett–Teller (BET) method, the distribution of pores was evaluated by BJH method (Barrett et al., 1951). Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Bruker AXS D-8 diffractometer with Cu Kα radiation. Fourier- transform infrared (FT-IR) spectra of samples in KBr pellets were measured on a Bruker Vector 22 spectrometer. UV spectra were recorded on a UV/Vis Perking Elmer – Lamda 14 spectrometer, and chemical analyzes were carried out by atomic absorption (Varian Spectra AA 220).

3.1 Characterization of MCM-41

Powder low-angle X-ray diffraction (XRD) patterns of Si-MCM-41 before and after modification are depicted in Fig. 1A and B which clearly indicate characteristic peaks of mesoporous materials.

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Figure 1

X-ray powder diffraction pattern of (A) Si-MCM-41 and (B) Al-MCM-41.

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The XRD pattern of Si-MCM-41 exhibited a well ordered structure and the peaks are indexed on a hexagonal lattice which correspond to (1 0 0), (1 1 0), (2 0 0) reflections.

For the aluminium form (Fig. 1), corresponding to a ratio of Si/Al = 50, the diffractogram pattern is similar to the Al-MCM-41 prepared by Cesteros (Cesteros and Haller, 2001). The decrease of signals intensities and the lack of d₂₀₀ reflection relative to pure MCM-41 were caused by the participation of aluminium in the formation of the crystalline phase.

The nitrogen adsorption–desorption isotherm of the MCM-41 sample (Fig. 2) is of type IV, typical of mesoporous solids according to the Brunauer classification, the curves of adsorption and desorption are practically confused which indicates pores’ uniformity with diameter < 4 nm and the specific area is high (1294 m²/g) (Table 1). The Al-MCM-41 isotherm is shown in Fig. 2, it is also of type IV with a surface area of (1053 m²/g) characteristic of mesoporous materials.

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Figure 2

N₂ physisorption isotherms of (A) Si-MCM-41 and (B) Al-MCM-41.

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3.2 Characterization of Cu-MCM-41

The X-ray powder pattern of Cu-MCM-41 showed the existence of three peaks of reflection (1 0 0), (1 1 0), and (2 0 0), characteristic of the hexagonal phase, thus the structure remains intact (Fig. 3). Peaks corresponding to copper oxide are visible at high angle diffraction (30° < 2θ < 40°). For Cu–Al-MCM-41 the structure remains also intact and we note also the appearance of peaks corresponding to copper oxide in the range (30° < 2θ < 40°). By comparison of the two spectra before and after calcinations, we can deduce that the copper oxide is formed after calcination (Fig. 3). UV spectra of the calcined samples Cu-MCM-41 (TIE) and Cu–Al-MCM-41 (TIE) are reported in Fig. 5. We note the appearance of a wide band from 200 to 600 nm corresponding to copper oxide CuO phase. The nitrogen isotherm adsorption of the samples Cu-MCM-41 and Cu–Al-MCM-41 are of the type IV (Fig. 4), the S_BET and V_meso decrease and the walls become thicker (Table 1) $$\text{a}=\frac{2}{\sqrt{3}}{d}_{100}\text{,}{D}_{\text{p}}=1.212522·d_{100}{\left(\frac{\rho ·V_{\mathit{meso}}}{1+\rho ·V_{\mathit{meso}}}\right)}^{1/2}$$ w = a − D_p.

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Figure 3

X-ray powder diffraction pattern of (A2) Cu–Si-MCM-41 and (B2) Cu–Al-MCM-41.

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Figure 5

DR-UV–vis spectra of: (A2) calcined Cu–Si-MCM-41 and (B2) calcined Cu–Al-MCM-41.

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Figure 4

N2 physisorption isotherms of (A) Cu–Si-MCM-41 and (B) Cu–Al-MCM-41.

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4.1 Characterization of grafted materials

The influence of the grafting is evaluated essentially by FTIR investigation. Fig. 6 shows the FTIR spectra of sample MCM-41(C) in the range 4000–400 cm⁻¹, we note the following bands: absorption band in the range 3200–3800 cm⁻¹ ascribed to free SiOH groups and another in the range 1500–2000 cm⁻¹ assigned to distortion vibrations of SiOH, and another in the range 1000–1100 cm⁻¹ ascribed to distortion vibrations of SiO⁻ and AlO⁻, and finally a band at 450 cm⁻¹ corresponding to distortion vibrations of T–O (T = Si and Al). These results are similar with those of the literature (Corma, 1995). After the grafting with TMCS groups (Fig. 7) we found the same bands with the disappearance of the vibration and distortion band of SiOH groups of the surface (–OH) and the apparition of two bands at 2964 cm⁻¹ and 845 cm⁻¹ assigned to the C–H vibrations.

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Figure 6

FTIR spectra of Cu–Al-MCM-41.

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Figure 7

FTIR spectra of TMCS-Cu–Al-MCM-41.

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According to the FTIR spectra of grafted Cu–Al-MCM-41, it is observed that a weak concentration of SiOH groups on the surface of these materials is due to their replacement by Si–O–Si(CH₃)₃ groups according to the following reaction: $$\text{Si}''–''\text{OH}+\text{Cl}''–''\text{Si}({\text{CH}}_3{\hspace{0.17em})}_3\to \text{Si}''–''\text{O}''–''\text{Si}({\text{CH}}_3{\hspace{0.17em})}_3+\text{HCl}$$ After treatment by TMCS groups, the solids present a hydrophobic surface and a more hydrothermal stability (Koyano et al., 1997).

5.1 Catalysis over Cu-MCM-41 and Cu Al-MCM-41

Table 2 presents the rate conversion of phenol by using the two catalysts (Cu-MCM-41 and Cu–Al-MCM-41). The reaction realized in aqueous media leads to different conversions of phenol for the two used catalysts and a different selectivity.

The MCM-41 doped by copper prepared by the TIE method presents a low rate of phenol conversion and the formation of polymers according to literature and other products according to literature but has selectivity for hydroquinone. Catechol and benzoquinone are not observed, probably because of the weak activity of the catalyst and the formation of polymers, on the other hand the Al-MCM-41 prepared by the same method presents a high activity but low selectivity to hydroquinone. The formation of polymers is provoked by the fast decomposition of oxygenated water this is induced by the active species (the copper in our case). It explains the high activity to the formation of polymers (Decyk et al., 2005).

The difference in activity can be attributed to the acidity of Al-MCM-41 materials assigned to weak sites of Brönsted and strong sites of LEWIS (Corma et al., 1994). Lewis’ acidity that develops these types of catalysts can play a role in the reaction (Ziolek et al., 1997).

5.2 Catalysis in the presence of TMCS-Cu–Al-MCM-41

To improve the activity of the copper mesoporous materials, we have done a second activation by grafting organic groups (TMCS).

By using Cu–Al-MCM-41 grafted by TMCS we find that the rate of conversion is very high (94%) with a negligible selectivity to phenol hydroxyls (catechol and hydroquinone). This high activity can be attributed to the modification of the materials surface, by the replacement of OH groups by hydrophobic groups that helps the adsorption of the phenol on the surface.