Acidic nickel salts of Keggin type heteropolyacids supported on SBA-15 mesoporous silica

Abstract

A series of heteropolyacids–mesoporous silica composites were prepared from nickel salt of molybdophosphoric acid Ni1HPMo12O40 (NiHPM) by supporting on SBA-15 mesoporous silica in different concentrations of active phase. The structure and texture of these NiHPM/SBA-15 composites were studied by XRD, FT-IR and micro-Raman spectroscopy, nitrogen physisorption at 77 K, UV–Vis–DRS and SEM–EDX. The effect of the support on thermal stability was investigated by TG-DTG, DTA and DSC methods. By impregnation of Ni salt on mesoporous silica the thermal stability of the Keggin structure increases in comparison with its parent bulk heteropolyacid. From XRD at small angles could be observed that NiHPM/SBA-15 composites with low surface coverage preserve the mesopore structure of silica, with well dispersed NiHPM on the surface of the pore walls for SBA-15. The adsorption of ammonia and its temperature programmed desorption using thermogravimetry were studied for acidity evaluation. The total acidity of the weak and strong acidic sites of NiHPM/SBA-15 composites is obviously increases in comparison with bulk Ni salt. The main products observed in the evolved gases during desorption of NH3 were NH3, CO2 and NO/NO2.