İsli SiO2/metal oksitlerin (MO=Al2O3, TiO2) hidrotermal koşullarda stabilitelerinin incelenmesi

Fumed metal/metalloid oxides, FMO (such as alumina, titania, silica etc.) are important raw materials for various industrial applications from chemical industries to food technologies. Due to extensive use of fumed SiO2 as a carrier in pharmaceutical applications, catalysis applications, cracking petrochemicals, and conversion for production of bio-renewables, hydrothermal stability is a critical requirement. Fumed silica based materials have high specific surface areas in the range of 50 and 400 m2/g which allows an efficient dispersion of active sites and large pore volume that favours the diffusion of bulky molecules in catalysis applications, however, their applications are limited by their poor stability due to the hydrolysis of their surfaces. Therefore; the research on developing hydrothermally stable mesoporous materials for catalytic applications has gained interest. To improve the hydrothermal stability of SiO2, many different methods are used such as adding salt, increasing pore wall thickness, and incorporating heteroatoms in situ or by post treatments. In the present study, the stability of fumed SiO2 materials and composites produced by flame hydrolysis is investigated under hydrothermal conditions applied for 24 hours and 7 days. Specifically, pure fumed silica, fumed alumina, and binary fumed mixed oxides of Al2O3- and TiO2-doped silica samples with various doping percentages are used for hydrothermal treatment. Surface areas of all untreated and hydrothermally treated samples are calculated by Brunauer-Emmett-Teller (BET) analysis and pore volume & pore maximum are calculated by Barrett-Joyner-Halenda (BJH) analysis by using nitrogen adsorption method. After hydrothermal treatment, BJH pore volume analysis shows decreases of 98 % for pure alumina, 75 % for pure silica and 26 % for Al2O3-doped silica in the pore volumes, respectively. According to powder X-ray Diffraction (XRD) analysis, all of the samples maintained their amorphous structure after the treatment. BET analysis indicates that after hydrothermal treatment, pure alumina and pure silica reveal the highest BET surface area reduction by 92 and 83 %, respectively. On the other hand, incorporation of Al2O3 and TiO2 into the silica matrix enhanced the surface stability and minimize the surface area reduction. Moreover, as doping amount increases (e.g. 0.8 to 1.0 wt %) the surface of the materials becomes more stable. Comparison of 1.0 % Al2O3- and TiO2-doped silica samples shows that incorporation of Al2O3 and TiO2 stabilizes the structure yielding BET surface area reduction by 31% and 26 %, respectively. The trend of surface area stabilization by doping follows the order: TiO2-doped silica > Al2O3-doped silica > pure silica. Titanium is a more electropositive dopant than aluminum and silicon itself and it has a higher affinity to the electrons of surface OH- groups. Therefore, in the TiO2-doped samples, surface OH- groups are resistant to water molecules resulting more stable samples in terms of surface area protection. Furthermore, due to the doping, the M-O-Si (M= Al, Ti) bonds are more polar and stronger than the pure silica case which in turn stabilizes the surface. The comparison of the Al2O3 doping on the surface (0.5 %) and into the bulk (1.0 %) of SiO2 reveals that the former shows an enhanced hydrothermal stability related to the higher concentration of the doping material on the surface. The results of this study shows that flame hydrolytically synthesized Al2O3- and TiO2-doped silica samples are promising materials not only due to their high purity and also due to their use under harsh hydrothermal conditions of catalysis applications.