Magali Bonne

Ecodesign of ordered mesoporous silica materials

Characterized by a regular porosity in terms of pore size and pore network arrangement, ordered mesoporous solids have attracted increasing interest in the last two decades. These materials have been identified as potential candidates for several applications. However, more environmentally friendly and economical synthesis routes of mesoporous silica materials were found to be necessary in order to develop these applications on an industrial scale. Consequently, ecodesign of ordered mesoporous silica has been considerably developed with the objective of optimizing the chemistry and the processing aspects of the material synthesis. In this review, the main strategies developed with this aim are presented and discussed.

Tuning the hydrophobicity of mesoporous silica materials for the adsorption of organic pollutant in aqueous solution

The ability of various as-prepared and organically modified MCM-41 and HMS mesoporous silica materials to behave as efficient adsorbents for organic pollutants in aqueous solution was investigated by using different surface functionalization procedures, so as to adjust their hydrophilic/hydrophobic balance. The hydrophilic and organophilic properties of the parent silica materials and their corresponding surface functionalized counterparts were studied by using water and toluene adsorption isotherms. Their quantification was determined by the hydrophobic static index value (HI(static)), as well as by the silanol and organic group densities after the functionalization step. A clear correlation could be found between the HI(static) values and either the superficial silanol density, or the amount of organic moieties grafted or incorporated to the silica materials. For the highly organically functionalized samples, the residual superficial silanol groups (<50%) are sufficiently isolated from each other so as to prevent the water capillary condensation within the pores, thereby leading to an increased hydrophobic character of the resulting mesoporous silica. Those hydrophobic samples, for which the water liquid meniscus formation within the mesopores was minimized or avoided, exhibited a storage capacity for an organic pollutant (N,N-diethyl-m-toluamide, DEET) in aqueous solution more than 20 times higher than that of the corresponding unmodified sample, independently of the silica nature (MCM-41 or HMS). For all calcined and silylated samples, the DEET maximum adsorption capacities determined by the Langmuir model could be correlated with the silica surface coverage by trimethylsilyl groups and thus with the remaining silanol amount.

Surface properties and thermal stability of SiO2-crystalline TiO2 nano-composites

TiO2/SiO2 nanocomposites are synthesized and fully characterized after thermal stabilization at temperatures between 400 °C and 800 °C. The control of the impregnation media is crucial to obtain nanocomposites of satisfactory quality, i.e. presenting no segregation of titania particles outside the silica pore structure. Characterization shows that pore size and pore volume decrease linearly with an increase in titania loading, and remain close to the theoretical values calculated assuming the formation of a non-porous coating. Surface area remains unchanged whatever the titania loading (always comprised between 450 and 480 m2 g−1), and micropore volume evolution suggests the formation of nanometric particles within the silica pores. While X-ray diffraction is inefficient to identify the titania phase, Raman spectroscopy showed the formation of anatase particles, with crystal sizes in the nanometric range (<4.5 nm, when stabilized at 400 °C). Satisfying thermal stability is obtained on the low titania loading nanocomposites (20 wt% TiO2), with only minor anatase crystal growth up to 800 °C. Further characterization by FT-IR of the surface chemical properties of the nanocomposites showed properties similar to that of conventional titania, while improved oxygen mobilities (as evaluated by the 18O/16O exchange reaction) are reported on the low titania loading, thermally stable, composites.

Tuning Hydrodesulfurization Active‐Phase Dispersion using Optimized Mesoporous Titania‐Doped Silica Supports

Solids composed of TiO2 nano‐crystallites (10–40 wt %) dispersed in an SBA‐15 matrix were synthesised and used as hydrodesulfurisation (HDS) active‐phase supports. Well‐dispersed polymolybdate species were detected by Raman spectroscopy on all the CoMo‐based oxidic precursors. Undesirable bulk CoMoO4 mixed oxide was also detected on the surface of low‐TiO2‐content materials, which suggests the enhancement of the Mo oxidic species dispersion as a result of the presence of TiO2. After activation under H2/H2S, the obtained catalysts were tested in the HDS of thiophene. An optimum conversion was observed for the catalyst supported on the solid that contained 20 wt % TiO2, which outperformed the homologous samples supported on TiO2 and SBA‐15. This clearly highlights the beneficial effect of using such nano‐TiO2‐SBA‐15 composite supports, in which SBA‐15 confers adequate textural properties to the system, and the active phase benefits from the dispersive effect of TiO2.

Influence of the porous texture of SBA-15 mesoporous silica on the anatase formation in TiO2–SiO2 nanocomposites

Two series of TiO2–SiO2 composites have been prepared by post-synthesis impregnation of a TiO2 precursor onto SBA-15 silica matrices and subsequent thermal treatment at 400 °C in air. The influence of the textural properties of the SBA-15 and the TiO2 content was evaluated upon the dispersion of TiO2 and its crystallization in anatase. The morphology of the samples before and after impregnation was observed by electronic microscopy while the textural and structural properties were evaluated by different techniques such as XRD, nitrogen sorption manometry, and Raman spectroscopy. The quantity of crystallized anatase in the composites was determined by XRD quantification using an internal standard. The bandgap energy (Eg) of the composites was evaluated by diffuse reflectance spectroscopy and the results show that Eg decreases as a function of the titania content. The photoactivity of the prepared materials was evaluated by following the degradation of methyl orange (MO) in water under UV irradiation. For the same amount of catalyst, the MO degradation rate in the presence of the composites containing 28 and 44 wt% of crystallized anatase was found to be similar to the one in the presence of commercial fully crystallized anatase.

Insights into the Formation and Properties of Templated Dual Mesoporous Titania with Enhanced Photocatalytic Activity

The one pot synthesis of dual mesoporous titania (2.3 and 7.7 nm) has been achieved from a mixture of fluorinated and Pluronic surfactants. The small and large mesopore networks are templated, respectively, by a fluorinated-rich liquid crystal and a Pluronic-rich liquid crystal, which are in equilibrium. After calcination at 350 °C, the amorphous walls are transformed into semicrystalline anatase preserving the mesoporous structure. Results concerning the photodegradation of methyl orange using the calcined photocatalysts highlight that the kinetic rate constant (k) determined for the dual mesoporous titania is 2.6 times higher than the k value obtained for the monomodal ones.