Hiding extra-framework cations in zeolites L and Y by internal ion exchange and its effect on CO2 adsorption

Abstract

The fully La-exchanged form of the large pore zeolite L, La_3.0Al_9.0Si_27.0O₇₂, has been prepared by repeated aqueous ion exchange and internal cation exchange resulting from calcination to give a low silica zeolite (Si/Al = 3) with all cations hidden from available porosity. This results from the migration of La³⁺ from sites in the large 12-membered ring (12R) channel (exchangeable via aqueous solution) to ‘closed’ sites in the structure. The mechanism for this internal cation exchange, which has been elucidated by Rietveld analysis of powder X-ray diffraction data, comprises sequential La³⁺ migration, first to ste cages between cancrinite (can) cages in the framework and then into the can cages. In each case K⁺ cations must first leave the closed cages, in the first case through 8Rs, in the second via puckered 6Rs. The first migration step is achieved by 573 K, the second by 1073 K. Decrease of the K⁺ occupation in accessible main channel sites of zeolites K_9.0–, K_5.7La_1.1–, K_2.7La_2.1– and La_3.0–L strongly changes the shape of the CO₂ adsorption isotherms, as the strength of interaction decreases. The Henry law constant for K_9.0–L zeolite is 36.60(2) mol (kg bar)⁻¹, whereas for La_3.0–L it is 1.80(3) mol (kg bar)⁻¹, if an initial uptake of 0.1 mmol g⁻¹ is neglected. The isosteric heat of adsorption is correspondingly strongly reduced. A CO₂ isotherm shape similar to that observed for La–L is also shown by a fully-exchanged La–Y, La_18.7Al_56.0Si_136.0O₃₈₄, prepared from Na–Y by repeated aqueous and internal ion exchange. In this case the internal exchange requires the egress of Na⁺ cations from sodalite cages via planar 6R windows and so is possible at lower temperatures (<673 K). The ability to modify CO₂ isotherm shape in low silica zeolites by hiding cations in inaccessible cages can be of interest to tailor pressure swing adsorbents for a range of CO₂-containing gas streams.