D2 and H2 adsorption capacity and selectivity in CHA zeolites: Effect of Si/Al ratio, cationic composition and temperature

Abstract

Abstract The work deals with the effect of composition of CHA zeolites on the adsorption and separation of H2 and D2 under cryogenic temperatures. In the first part of this work the effect of Si/Al ratio and cationic composition on single gas adsorption of H2 and D2 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 °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 H2 and D2 adsorbed amount which becomes negligible for K+ content higher than ~60 mol. %. In the second part of the work the D2/H2 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 D2/H2 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 D2/H2 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.