posted on 2018-07-10, 00:00authored byPrzemyslaw Rzepka, Dariusz Wardecki, Stef Smeets, Melanie Müller, Hermann Gies, Xiaodong Zou, Niklas Hedin
Adsorption
technologies offer opportunities to remove CO2 from gas
mixtures, and zeolite A has good properties that include
a high capacity for the adsorption of CO2. It has been
argued that its abilities to separate CO2 from N2 in flue gas and CO2 from CH4 in raw biogas
can be further enhanced by replacing Na+ with K+ in the controlling pore window apertures. In this study, several
compositions of |Na12–xKx|-A were prepared and studied with respect to the
adsorption of CO2, N2, and CH4, and
the detailed structural changes were induced by the adsorption of
CO2. The adsorption of CO2 gradually decreased
on an increasing content of K+, whereas the adsorption
of N2 and CH4 was completely nulled already
at relatively small contents of K+. Of the studied samples,
|Na9K3|-A exhibited the highest CO2 over N2/CH4 selectivities, with α(CO2/N2) > 21 000 and α(CO2/CH4) > 8000. For samples with and without adsorbed
CO2, analyses of powder X-ray diffraction (PXRD) data revealed
that K+ preferred to substitute Na+ at the eight-ring
sites. The Na+ ions at the six-ring sites were gradually
replaced by K+ on an increasing K+ content,
and these sites split into two positions on both sides of the six-ring
mirror plane. It was observed that both the eight-ring and six-ring
sites tailored the maximum adsorption capacity for CO2 and
possibly also the diffusion of CO2 into the α-cavities
of |Na12–xKx|-A. The adsorption of CH4 and N2 on
the other hand appeared to be controlled by the K+ ions
blocking the eight-ring windows. The in situ PXRD study revealed that
the positions of the extra-framework cations were displaced into the
α-cavities of |Na12–xKx|-A on the adsorption of CO2.
For samples with a low content of K+, the repositioning
of the cations was consistent with a mutual attraction with the adsorbed
CO2 molecules.