Abstract
The Na+-form (Na-RHO) of zeolite rho has emerged as a potential CO2 adsorbent with excellent selectivities toward CH4 and N2, but the very slow CO2 adsorption kinetics hinders its practical use. Here we report that mesoporous Na-RHO, prepared by NH4F/H2O2 etching, shows remarkably enhanced CO2 adsorption kinetics (ca. 7 min vs > 2 h) compared to the Na-RHO, which was also observed for the NH4F/H2O2-etched, Cs+-form (Cs-MER) of merlinoite. A combination of SEM, TEM, and N2 adsorption measurements demonstrates that such kinetics enhancement originated from the mesopore creation by fluoride medium etching, while minimizing the loss of zeolite crystallinity. The observed differences in the zeolite crystal morphology of Na-RHO and Cs-MER and in the characteristics of their intrinsic defect sites appear to be responsible for those in the shape and average size of the secondary mesopores created. The overall results of this study suggest that NH4F/H2O2 etching can be also applied to other small-pore zeolites to improve CO2 adsorption kinetics.
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