Characterization of mesoporous silica and its pseudomorphically transformed derivative by gas and liquid adsorption

Abstract

Pseudomorphism is a term introduced by mineralogist to describe phase transformation that does not change the shape of a material. Pseudomorphic process, assisted by surfactants, for mesoporous pre-shaped silica particles allows to narrow the mesopore size distribution, to increase the surface area and the pore volume without changing the initial shape of silica particles. The textural, chemical and mechanical comparisons between commercial silica LiChrospher 60 (Merck) and its pseudomorphic transformation at constant particle size and morphology opens a unique opportunity to understand the effects of such mesoporosity transformation and judge the performance of both types of materials. On a mechanical point of view, commercial silica and its pseudomorphic transformation are stable until 40 MPa, which is enough to resist to chromatographic column packing. Liquid chromatographic tests reveal that column efficiency and mass transfer properties are maintained during the pseudomorphic transformation. Adsorption from polar probes and thermogravimetric measurements show that the surface chemistry is strongly modified by the pseudomorphic transformation, which is not pseudo-chemical. The surfactants use to generate well defined mesoporosity induct in one hand a low silanol density and in the other hand a more homogeneous silanol distribution on the silica surface, which opens the route towards a better control of surface functionalization. Moreover, grafted-mesoporous silicas exhibit an extremely high mechanical stability (>280 MPa). Pseudomorphic-silicas are remarkable candidates for applications needing morphology and surface chemistry control.