Metal–organic frameworks in
the M/DOBDC series are known to contain a large number of coordinatively
unsaturated metal (M) sites. In this work, we study the influence
of various metal cations (M = Mg, Mn, Co, and Ni) in the framework
on its gas adsorption characteristics. The probe gases (viz. CO2, CO, CH4, C2H6, N2, and Ar) were carefully chosen to cover a wider range of polarity
and polarizability. While a significant impact of metal atom in the
framework is observed on adsorption of polar gases such as CO2 and CO, it has a negligible effect on adsorption of other
relatively nonpolar gases. On one hand, Henry’s constant of
CO2 for Mg/DOBDC is about 4–10 times higher than
that for other frameworks; on the other, Henry’s constant for
CO on Ni/DOBDC is about 100 times larger than that on Mn/DOBDC. The
pore volume of the framework governs adsorption capacity at higher
pressures. Each of the frameworks exhibits widely different adsorption
enthalpies for polar gases such as CO2 and CO. At pressures
below 15 bar, the Ideal Adsorbed Solution Theory predicts very good
selectivity for CO over all other studied gases on Ni and Co/DOBDC
frameworks, while Mg and Mn/DOBDC frameworks exhibit preferential
selectivity for CO2.
This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess methane adsorption isotherms on NIST Reference Material RM 8850 (Zeolite Y), at 25 °C up to 7.5 MPa. Twenty laboratories participated in the study and contributed over one-hundred adsorption isotherms of methane on Zeolite Y. From these data, an empirical reference equation was determined, along with a 95% uncertainty interval (Uk=2). By requiring participants to replicate a high-pressure reference isotherm for carbon dioxide adsorption on NIST Reference Material RM 8852 (ZSM-5), this interlaboratory study also demonstrated the usefulness of reference isotherms in evaluating the performance of high-pressure adsorption experiments.
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