D-2 and H-2 adsorption capacity and selectivity in CHA zeolites: Effect of Si/Al ratio, cationic composition and temperature

The work deals with the effect of composition of CHA zeolites on the adsorption and separation of H-2 and D-2 under cryogenic temperatures. In the first part of this work the effect of Si/Al ratio and cationic composition on single gas adsorption of H-2 and D-2 was studied at 77.4 K. It was found that the adsorption capacities increase with Al content up to Si/Al = 2.1. Unexpectedly, Na-CHA zeolite with the highest Al content (Si/Al = 1.1) adsorbs only negligible amount because of the collapse of the zeolite structure upon dehydration at 400 degrees C. The Na- and Li-containing chabazites with Si/Al = 2.1 possess similar adsorption capacities. In contrast, progressive replacement of Na+ with K+ results at 77.4 K in decreasing H-2 and D-2 adsorbed amount which becomes negligible for K+ content higher than similar to 60 mol. %. In the second part of the work the D-2/H-2 thermodynamic selectivity for Na-, Li- and K-Na chabazites (Si/Al = 2.1) was measured using coadsorption technique at 40-77.4 K in the pressure range 530-650 hPa corresponding to high loading. The cationic composition is found to have only minor effect on the D-2/H-2 selectivity which increases exponentially with lowering temperature. For example in Na3.9Al3.9Si8.1O24 chabazite it rises from 2.6 at 77.4 K to 13 at 40 K. It is found that variation of D-2/H-2 selectivity with temperature is in a fair agreement with a simple quantum sieving model making use of the aperture size of the chabazite cavity as a unique adjustable parameter.