Impregnation of mesoporous silica for catalyst preparation studied with differential scanning calorimetry

Abstract

Aqueous impregnation of mesoporous silica as a first step in catalyst preparation was studied to investigate the distribution of the metal-precursor solution over the support. The degree of pore-filling after impregnation was determined using the freezing point depression of confined liquids. A separate bulk melting transition observed with differential scanning calorimetry in combination with TGA and N2-physisorption allowed rigorous quantification of the extent of pore-filling of the support. The micro- and mesoporous volumes of different mesoporous silica were filled up to 90−100% with water. With KCl solution, at maximum 75−85% of the pores were filled. However, the total pore-filling reached 85−90% when the estimated volume of physisorbed water was included. As a case study for catalyst preparation, silica supports were also impregnated with an aqueous Ni(NO3)2 solution. Generally, the solution occupied up to 80−90% of the pore volume, similar to the pore-filling with KCl. This study shows that ordered mesoporous silica, as well as silica-gel, are filled spontaneously and largely with aqueous solutions by capillary forces. Just up to 10% empty pores are expected to be present after a so-called incipient wetness impregnation. Therefore, a largely uniform distribution of the metal precursor over the support is achieved by aqueous impregnation.