Effect of heteroatom doping on surface acidity and hydrophilicity of Al, Ti, Zr-doped mesoporous SBA-15

Syntheses for achieving Al, Ti, or Zr-doped mesoporous SBA-15 under strongly acidic conditions optimised by using additional hydrothermal treatment have been repeated so as to get at least 20 g of each material and the impact of doping procedure on surface acidity and hydrophilicity of the resulting samples has been studied, paying special attention to the use of interfacial phenomena in flow regimes. The textural parameters of the calcined materials were inferred from nitrogen adsorption–desorption isotherms at 77 K and powder X-ray diffraction patterns. The energy-dispersive X-ray spectroscopy was used to determine the molar ratio of silicon to heteroatom in the final product. These results gave indications for increasing heterogeneity of mesopore distribution with the heteroatom incorporation, especially for Al and Ti-doped materials. Stoichiometric heteroatom insertion was possible for zirconium and titanium. Surface acidity and hydrophilic character of the materials were evaluated by means of two-cycle gaseous ammonia adsorption at 373 K, gas flow-calorimetry measurements of ammonia adsorption at 353 K, liquid-flow-calorimetry measurements of the competitive adsorption of butan-1-ol from a 2 g L−1 solution in n-heptane, as well as potentiometric titration of powdered samples suspended in a 10−1 mol L−1 NaNO3 aqueous solution. Doping procedure enhanced both surface acidity and hydrophilicity, although the two aspects of surface activity did not necessarily follow the same tendency for the various samples. A large increase in the two parameters was observed only for Al-SBA-15. The particular behaviour of these samples could be explained based on the LUMO energy-levels evaluated using DFT modelling.