Infrared Spectroscopic Study on Methane Adsorption Sites on Mesoporous Silicates

Abstract

Abstract Some MCM-41 materials substituted by trivalent metal ion (Met-MCM-41; Met = Al, Fe, and Ga) were prepared, and their methane adsorption sites were investigated by IR spectroscopy. Three adsorption sites on Met-MCM-41 with different strengths of electric field were observed. They were assigned to pore wall site (0 V m-1), silanol site (ca. 4×109 V m-1), and Na+ site (ca. 6×109 V m-1), respectively. The amounts of silanol site in Met-MCM-41 were estimated to 2—3 mmol g-1, while the amounts of the Na+ site were 0.1—0.2 mol g-1. Methane molecules were preferentially adsorbed on the electrostatic sites (silanols and Na+sites) at low methane pressures, but as methane pressure increased, the amount of adsorption on the pore wall site became comparable to that on the silanol sites. Since the amounts of Na+ site estimated from methane adsorption on the Met-MCM-41 samples were less than the theoretical ones calculated from the compositions of starting materials, most of the Na+ sites were thought to be inaccessible for methane molecules.