Acidic and neutral caesium salts of 12-molybdophosphoric acid supported on SBA-15 mesoporous silica. The influence of Cs concentration and surface coverage on textural and structural properties

Abstract

The Cs salts of molybdophosphoric acid Cs1H2PMo12O40 (Cs1PM) and Cs3PMo12O40 (Cs3PM), were supported on SBA-15 in the concentration of 20, 30 and 40wt.% loadings. Because Cs salts are insoluble, the SBA-15 supported Cs-acid salts were prepared by two-step sequential impregnation and in situ reaction on the support. The structure and texture of these CsPM/SBA-15 composites were studied by XRD, SEM-EDS, FT-IR and micro-Raman spectroscopy, UV–vis-DRS and N2 adsorption. Thermal stability was investigated by thermo gravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). The acidity of composites was studied by the adsorption of ammonia and its temperature programmed desorption – TPD using thermogravimetry. The evolved gases during the adsorption–desorption of ammonia on CsPM/SBA-15 composites were identified by online mass spectrometry coupled with thermal gravimetry technique. FT-IR and Raman studies demonstrated that CsPM maintained its Keggin structure after deposition on mesoporous SBA-15, regardless of the active phase concentration. For both Cs1H2PMo12O40 and Cs3PMo12O40-SBA-15 composites could be observed an important increase of band gap energy in comparison with pure salts. The values of specific surface area of pure CsPM were increased by deposition on mesoporous silica support. The immobilization of CsPM on mesoporous SBA-15 obviously increases the thermal stability and the total acidity of the strong acidic sites of the Keggin structures in comparison with their parent bulk Cs salts.