Crystallization of mesoporous silica SBA-15 in a high pressure hydrothermal environment

ABSTRACT

Mesoporous silica SBA-15 (with ∼6 nm pore size and ∼6 nm wall thickness) was exposed to a hydrothermal environment at 2 and 5 GPa. The p,T quenched products were investigated by powder X-ray diffraction and transmission electron microscopy. Infrared spectroscopy and thermogravimetric analysis of a sample subjected to 5 GPa at room temperature suggests functionalization of both inner and outer pore surface by silanol. Partial transformation to nano-sized (20–50 nm) coesite crystals with nonfaceted morphology was observed during short equilibration times of 2 h at 125°C, which is significantly below the melting point of water (∼250°C). Untransformed SBA-15 maintained intact pore structure. At 175°C and during 8 h, SBA-15 transformed completely into faceted coesite crystals with dimensions 100–300 nm, suggesting Ostwald ripening and thus significant mass transport in the solid water environment. At 2 GPa the melting point of water is near 70°C. Partial transformation to nano-sized α-quartz was observed at 65°C and during 2 h. Untransformed SBA-15 partially pore collapsed. The reduced pore stability of SBA-15 at 2 GPa is attributed to the presence of liquid water in the pores due to melting point depression of confined water.

KEYWORDS:

• SBA-15
• high pressure hydrothermal environment
• multianvil
• piston-cylinder

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was partially supported by the Swedish Research Council.