Synthesis of Mesoporous Titania with Surfactant and its Characterization

A mesoporous titania was obtained by gelation from Ti-alkoxide in acidic solutions with addition of surfactant cetyltrimetylammonium bromide (CH3(CH2)15N(CH3)3Br) using a sol-gel process. The effects of surfactant concentration on synthesis of mesoporous titania were studied. The structural characterisation was studied by differential thermal analysis, infrared spectroscopy, X-ray diffraction. Studies by X-ray diffraction showed that crystallisation of TiO2 powder occurs at 200°C, above 200°C we obtained a mixture of two forms-Anatase and rutile. The textural characterisation by nitrogen adsorption-desorption allowed us to observe the variation of the surface area, porous volume and pore diameters according to temperature and [CTAB]/[Ti-alkoxide] molar ratio. The analysis of the results shows that addition of surfactant residue increases considerably its pore diameters. The deposit thin layers has been realized with a sol prepared with the destabilization of colloidal solutions process. Scanning electron-spectroscopy observation for thermally treated (at 400 and 600°C) samples, showed homogeneous layers without cracking.


INTRODUCTION
In recent years, the use of TiO 2 , as membrane inorganic has aroused great interest in several industrial applications. This is because they offer many advantages over organic counterparts in separation process due to their thermal, mechanical and chemical characteristics.
With this method a network of partially hydrolyzed and polycondensed monomers diluted in the solvent is formed. A mesoporous structure is obtained upon layer application (coating), drying and calcination.
The aim of this research is the use of sol-gel process to preparation of mesoporous Titania to contribute to synthesis of membrane. The effects of addition of surfactant (CH 3 (CH 2 ) 15 N(CH 3 ) 3 Br) on pore diameter and sol stability have been investigated.
Additionally, the thermal evolution, structural characteristics of calcined powders and films were studied.

) 3 Br) = CTAB
The synthesis mode was chosen as a maximum of connection formation: Ti-O-Ti. Thus, in a first step the idea is to use hydrolysis water by forming a maximum of connection of groupings Ti-OH.
Characterization: Powder XRD data were carried out with a Philips PW 1830 diffractometer with CuK radiation ( =1.5406Å). TGA curves were obtained in flowing nitrogen on TGA 2050 with a temperature increasing rate of 10°C/mn. N 2 adsorption-desorption isotherms were recorded on a Micromeritics ASAP 2010 automated sorption analyzer. The samples were outgassed at 150°C before the analysis.The FTIR spectra of samples were obtained using the KBr wafer technique.We have applied the Barret-Joyner-Halende (BJH) method to the determination of pore size.
Scanning electron microscopy is used to characterize membrane morphology, that is, the thickness and its homogeneity along the support.

RESULTS AND DISCUSSION
Infra-red spectra were recorded for every powder prepared with molar ratio [CTAB]/[Ti-alkoxide] equal to 0-0.5 and 1) treated at different temperatures (Fig. 2). Infra-red studies show that some OH remains in the titanium oxide. These OH groups are responsible of the membrane reactivity. The Ti-OH groups may be characterized by an absorption band situated at 500 cm −1 . Above temperature 450°C, we notice only Thermogravimetric analysis: The TGA of the assynthesized samples under N2 showed the loss of water below 120°C and surfactant loss started at 180°C and was completely removed at about 350°C (Fig. 3). The analysis of as-synthesized sample on heating to 450°C, revealed 30.52% total weight loss for a sample without surfactant, ratio[CTAB]/[Ti-alkoxide] = 0 (Fig. 3a) and 52% total weight loss with ratio equal to 1 (Fig. 3b).The first effect is attributed to the release of adsorbed water, the second to desorption and decomposition of the surfactant and the third to dehydroxylation of the surface and removal of little residual surfactant. It can be seen that the material without CTAB retains amorphous structure until 60°C, while at 200°C, a phase transformation into the crystalline anatase phase occurred.
Above this temperature 200°C we obtained a mixture of two forms anatase and rutile. While a patterns of powders containing CTAB present a broad peak, charasterising of Anatase up to 300°C. Only the changes of phase content Anatase: Rutile were observed, when a both ratio of CTAB and heattreatment increase. Above the temperature 400°C the transformation of phase anatase to rutile according CTAB/Ti-alkoxide molar ratio and heat-treatment were similar.
Anatase and rutile are the two forms of titanium dioxide produced in laboratory at atmospheric pressure.

TEXTURAL CHARACTERIZATION BY NITROGEN ADSORPTION-DESORPTION
N 2 -sorption isotherms were recorded for 350, 400, 500 and 600°C calcined mesoporous structured samples and are shown in (Fig. 5). The shape of these curves is of type IV, typical of mesoporous materials. The physisorption isotherm of the sample with and without surfactant treated at 350°C is similar as sample treated at 600°C. Table 1 lists the specific surface area, the pore volume and the pore size data measured from N 2 adsorption-desorption isotherms for titanium oxide samples as-synthesized and heat-treated at various temperature. With increasing of calcined temperature, the specific surface area and the pore volume began to decrease as shown in Fig. 6, 7. The mesostructured produced after calcination at 350°C has a Brunuer-Emmett-Teller (BET) surface area of 101 m 2 g −1 .
Between 350 and 600°C, the specific surface area  With increasing of CTAB concentration, the specific surface area and pore volume decrease. While in the same time we notice (Fig. 6, 7, 8) that the pore diameter increased from 24A to 99A°C and from 36A to 199A°C by increasing of [CTAB]/[Ti-alkoxide] ratio from 0 to 1 on heating at respectively 400 and 600°C (Table 1).
These results indicated that as the calcination temperature increased, the number of pore decreased as a result of sintering while at the same time the pore size increased.

Deposits of thin porous layers:
After the sol has been prepared, the titania layers were deposited by coating technique on mesoporous α alumina substrate presenting the pore diameter of 0.2 µm on the usable face for deposit. The dried membranes were calcined at temperature 400 and 600°C respectively during 3 h. (Fig. 9, 10, ).
It was found that the layer thickness could be varied in the range 1.85 to 1.25 µm respectively for temperature 400 and 600°C. This is a very interesting fact because as indicated at results the mesoporous structure of titania oxide is in the range of ultrafiltration application.

CONCLUSION
In this study, we investigated the synthesis of mesoporous titania using titanium isopropoxide as precursor under acidic conditions. The effects of [CTAB]/[Ti-alkoxide] ratio and the heat-treatment on structured titanium oxide as-synthesized have been studied. The changes in the molar ratio of CTAB/Ti led to different value of pore diameter, specific surface area and pore volume.
It was found that polymeric sols could be obtained only when the hydrolysis conditions were strictly controlled.
We note increase of the pore diameters according to the concentration of surfactant while porous volume and specific surface decrease. The control of sol-gel transition as well as thin layer deposit on a porous support α alumina has allowed us to contribute at the realization of ultrafiltration inorganic membranes.