Titania aerogels: Influence of synthesis parameters on textural, crystalline, and surface acid properties

Abstract

A series of titanium oxides was obtained by the sol–gel method, by controlling different synthesis parameters, such as alcohol/alkoxide and water/alkoxide molar ratios, type of catalyst (acid or basic) and use of complexing agents. The gels were dried with supercritical carbon dioxide (aerogels), and further calcined in air at 500 °C to obtain the oxides. The samples were characterized by nitrogen adsorption, X-ray diffraction, thermal analysis, and titration with n-butylamine. Most of the aerogels and their corresponding oxides are mesoporous solids, the titanium oxide prepared with acetic acid being the most mesoporous one of the series. However, their pore size distributions are very different, depending on the synthesis conditions. The amount of water is a factor of great influence on the formation of meso- and micropores in the aerogels. Most of the titanium oxides contain the anatase phase. Those prepared with higher amounts of water possess however a mixture of anatase and rutile. The acid strength of the surface sites of the titanium oxides is quite different, depending on the synthesis parameters. The oxide prepared with nitric acid and the lowest amount of water contains the sites with the highest acid strength. That synthesized with ethylenglycol shows the widest distribution of acid sites of all the series. The textural and crystalline characteristics, as well as the surface acidity of the titanium oxides, which can be used as catalytic supports, can be tailored by changing the synthesis conditions.