Organic-Free Synthesis of Merlinoite Zeolite with an Enhanced Si/Al Ratio and Its CO2 Adsorption Properties

Abstract

A channel-based small-pore aluminosilicate MER-type zeolite with Si/Al = 3.5, denoted as K-merlinoite, was synthesized using only K+ ion as a structure-directing agent. For the CO2 adsorption measurements, various alkali cation forms prepared from K-merlinoite with Si/Al = 3.5 are compared with not only each other but also with those of zeolite W, another MER-type zeolite, with Si/Al = 2.2, allowing us to identify effects of the type of extra framework alkali cations and Si/Al ratio on the CO2 adsorption properties of MER-type zeolite. More interestingly, we present unusual CO2 adsorption tendency, that is not linear but a convex tendency in CO2 adsorption capacity of NaK-merlinoite zeolites, plotted as a function of their Na+/K+ ratio. Specifically, NaK-merlinoite with Na+ content of 59 at. % shows the highest CO2 adsorption capacity at 298 K and 1.0 bar (4.63 mmol g-1). In case of NaK-merlinoite with Na+ content of 42 at. %, this material exhibits the highest selectivity of both CO2/N2 and CO2/CH4 in the VSA process amongst the various NaK-merlinoite zeolites. In addition, NaK-merlinoite zeolites have fast CO2 adsorption kinetics and long-term stability. These NaK-merlinoite zeolites are promising adsorbents for CO2 capture in flue gas (the mixture mostly consisting of CO2 and N2) and natural gas (the mixture mostly consisting of CO2 and CH4).