Acidity in Titania–Alumina Mixed Metal Oxides

Abstract

Titania–alumina mixed oxides were prepared with varying titania content. The catalysts contained sulfate which was introduced via titanyl sulfate used as precursor during the synthesis. Samples were characterized by IR spectroscopy of adsorbed pyridine to determine Lewis and Bronsted acidity. At titania loadings below about 5 wt% no protonation of pyridine was detected by IR spectroscopy. However, 2-methyl-2-pentene isomerization indicated an increase in medium strength acidity in these samples relative to pure alumina and titania as inferred from an increased methyl shift activity. IR spectroscopy revealed the generation of fourfold coordinated Ti4+ sites and mildly acidic Ti–OH groups that may be responsible for the enhanced olefin isomerization reactivity of the mixed oxides. Addition of small amounts of titania also increased the concentration of the type Ia Al–OH group on the alumina surface characterized by an IR band at 3771 cm−1. At titania loadings of about 8 wt% and above Bronsted acidity was generated by sulfate groups as detected by the protonation of pyridine. Deposition of 0.3 wt% Pt on the titania–alumina mixed oxides generated finely dispersed Pt as measured by H2 chemisorption and TEM. The benzene hydrogenation activity on the Pt–TiO2–Al2O3 catalysts showed a maximum for the sample containing about 1 wt% titania and correlated with the relative abundance of the type Ia Al–OH groups on the alumina surface.