Enhanced Brønsted acid strength of HCl-promoted SO42−/TiO2 solid composite superacid: Synergetic effect of hydrogen chloride with induced titanium site

Development of the strategy to regulate the acidic properties of solid superacids had been attracting widespread attention in the acidic catalysis-related fields. In the present work, an HCl-promoted SO42−/TiO2 solid composite superacid was synthesized by incorporation of hydrogen chloride into sulfated titania (SO42−/TiO2). The Brønsted acid strength of HCl-promoted SO42−/TiO2 was significantly increased compared with that of SO42−/TiO2. Friedel-Crafts alkylation reaction results of HCl-promoted SO42−/TiO2 showed an increase by 27% in reaction rate and decreased byproduct selectivity (1% versus 10% for HCl-promoted SO42−/TiO2 and SO42−/TiO2, respectively). Structural characterizations demonstrated that HCl had the synergetic effect with induced titanium ion of HCl-promoted SO42−/TiO2 to form a new type of Brønsted acidic site. The configuration of enhanced Brønsted acidic site verified by DFT calculations was more liable to release the proton than that of conventional Brønsted acidic site, consequently increasing the Brønsted acid strength of HCl-promoted SO42−/TiO2.