Effect of the SiO_2/TiO_2 ratio on the solid acidity of SiO_2-TiO_2/montmorillonite composites

摘要

SiO_2-TiO_2/montmorillonite composites with varying SiO_2/TiO_2 molar ratios were synthesized and the effect of the SiO_2/TiO_2 ratio on the solid acidity of the resulting composites was investigated. Four composites with SiO_2/TiO_2 molar ratios of 0, 0.1, 1 and 10 were synthesized by the reaction of colloidal SiO_2-TiO_2 particles prepared from alkoxides with sodium-montmorillonite at room temperature. The composites showed slight expansion and broadening of the XRD basal reflection, corresponding to the intercalation of fine colloidal SiO_2-TiO_2 particles into the montmorillonite sheets and incomplete intercalation to form disordered stacking of exfoliated montmorillonite and colloidal SiO_2-TiO_2 particles. The colloidal particles crystallized to anatase in the low SiO_2/TiO_2 composites but remained amorphous in the high SiO_2/TiO_2 composites. The specific surface areas (S_(BET)) of the composites measured by N_2 adsorption ranged from 250 to 370 m~2/g. considerably greater than in montmorillonite (6 m~2/g). The pore size increased with decreasing SiO_2/TiO_2 molar ratio of the composites. The NH_3-TPD spectra of the composites consisted of overlapping peaks, corresponded to temperatures of about 190 and 290 C.The amounts of solid acid obtained from NH_3-TPD were 186-338 μmol/g in the composites; these values are higher than in the commercial catalyst K10 (85μmol/g), which is synthesized by acid-treatment of montmorillonite. The present sample with SiO_2/TiO_2 = 0.1 showed the highest amount of acid, about four times higher than K10.