Neutron Diffraction to Explain the ‘Trapdoor’ Mechanism of Zeolites for H2/D2 Separation

Separating hydrogen and deuterium is among the most formidable separation challenges. One method is using porous materials as Quantum Sieves, where chabazite zeolites (CHA) are highly effective at separating the two species due to the small pore windows within the crystalline framework. Purely siliceous (SiO2) CHA has poor D2/H2 selectivity. Extra-framework cations improve D2 selectivity in zeolites, specifically in CHA species where a “trapdoor mechanism” is suggested, where extra-framework cations sit in the centre of the CHA 8 membered ring. Our recent adsorption work on CHA with a potassium counterions (K-CHA) suggests that at ~159 K, the K+ species moves to one side, allowing a D2 molecule to slip past it, into the framework, hence acting as a “trapdoor”, whereas this does not occur with H2. This provides a unique opportunity to exploit this system for D2/H2 uptake at comparatively high temperatures.
Recently we have shown that combining Na+ and K+ cations in the same CHA system, leads to enhanced D2 uptake. In this work we will investigate the H2 and D2 uptake behaviour of K-CHA and NaK-CHA using Neutron Diffraction between 120 and 195 K. In doing so we will observe the distortion of the framework on introducing Na+ cations and contrast the behaviour of these two systems at different temperatures. Here the adsorbed H2 and D2 will contribute to the diffraction pattern, providing a further information about the adsorption and trapdoor mechanisms.