On the pore forming mechanism of Upsalite, a micro- and mesoporous magnesium carbonate

Abstract

This work analyzes the pore forming mechanism and stability of Upsalite; an extraordinary moisture absorbing, high-surface area magnesium carbonate powder synthesised without the use of surfactants as pore forming agents. The pores in Upsalite were found to be created in a two-step process where the first step includes the formation of micropores by solvent evaporation and release of physically bound carbon dioxide, acting as an in situ pore-forming template. In the second step, the micropores expand to mesopores due to partial decomposition of organic groups on the surface of the pore walls when the material is stored in air at moderate temperatures (70°C). The resulting material has a narrow pore size distribution centered at 5nm, and the amorphous structure is stable upon storage in a humid atmosphere.It was further shown that calcination at temperatures above 250°C is required for complete removal of the organic surface groups in Upsalite. Prior to calcination, the organic groups present in the material act as barriers hindering water to induce crystallization of the bulk material. After calcination, however, Upsalite crystallizes into nesquehonite when stored at 100% relative humidity for several days. The results presented herein are expected to be useful for the development of novel surfactant-free synthesis routes of porous materials as well as for the understanding of the long-term performance of such materials.