Gas adsorption by a crystalline silicic acid

Abstract

Crystalline silicic acids are prepared from alkali layer silicates by exchanging protons for the alkali ions. The acid H₂Si₂₀O₄₁ · xH₂O (parent material K₂Si₂₀O₄₁ · xH₂O) exhibits some outstanding gas adsorption properties which are related to the layer structure and the interlamellar microporosity. The external surface, about 20 m² g⁻¹, is estimated from nitrogen adsorption data after blocking the micropores. Slit-shaped ultramicropores (with diameters similiar to that of the nitrogen molecule) between the layers are widened to supermicropores near the crystal edges. During an adsorption run the nitrogen molecules penetrate more deeply into the ultramicropores. Nitrogen molecules strongly adsorbed in the ultramicropores are not desorbed at 77 K. Additional amounts of nitrogen are adsorbed by widening of the slit-shaped micropores at the crystal edges when pressure increases. This process proceeds slowly and is reversible.