Organic polymers as pore-regulating agents in TiO2-Al2O3 mixed oxide catalytic supports

The synthesis of TiO2-Al2O3 mixed oxide catalytic supports with a TiO2/(TiO2 + Al2O3) molar ratio equal to 0.5, was made by the co-precipitation of the corresponding metallic isopropoxides using different organic polymers as pore-regulating agents. The influence of the preparation parameters (type of polymeric additive, its amount, molecular weight and method of additive incorporation into the hydroxide precipitate) on the surface area and pore structure of the final solid were studied. It was found that organic polymers added during the hydrolysis step, in general, resulted in a significant increase in surface area and total pore volume. In all cases the pore-size distribution becomes clearly bimodal with two maxima in the pore-size distribution curve around 3 and 30-50 nm. In order to investigate the action mechanism of the polymeric additives, characterizations by Fourier transform-infrared, thermogravimetric/differential thermogravimetry and differential thermal analysis, as well as a TEM study of the hydroxide precipitates before calcination, were carried out. The results from these characterizations indicate that the textural changes induced by the polymeric additives can be rationalized in terms of a "filler" effect in the case where the additive is incorporated during the filtration step, and that when the additive is added during the hydrolysis step it is the chemistry of the polymer functional groups which determines the final texture of the solid. (C) 1998 Chapman & Hall.