Characterization of Al- and Ti-modified MCM-41 using adsorption techniques

The influence of aluminium and titanium dopants on the structure and properties of MCM-41 silica has been evaluated and compared with previously published results. The acidity and basicity of the surface states (adsorption sites) were of special interest. The total acidity was studied by using temperature-programmed ammonia desorption (NH₃-TPD). Additionally, organic probe molecules with known Lewis and/or Brφnsted acid or base functional groups were adsorbed on the powder samples in dry cyclohexane. Both UV-vis spectroscopy and microcalorimety were used to quantify the adsorption. The adsorption data of the monofunctional probes were fitted with Langmuir-, Freundlich- and Henry-corrected Langmuir isotherms providing both the number of the surface sites and strength of probe molecule adsorption. The surface charge densities, relating to Brφnsted acidity and basicity, were determined for the doped samples in water using potentiometric titration. DRIFT spectroscopy was used to identify the type of the surface sites. The hexagonal pore structure of the doped MCM-41 samples was observed to remain roughly unaltered for all samples. The NH₃-TPD analysis indicates that the acidity increases as a result of modification. The probe adsorption showed that the number of sites increases and the strength of adsorption enhances with surface modification. It seems that the simple Langmuir-isotherm is sufficient for the surface state analysis in most cases. In a few cases, the Freundlich- or the Henry-corrected Langmuir isotherm gave, however, a superior fit to the experimental surface excess data. The DRIFT-analysis confirmed the existence of Lewis sites and the formation of Brφnsted type acid sites with enhanced Al- or Ti-doping, which was supported by the other methods used.