4 research outputs found
Adsorption Characteristics of Metal–Organic Frameworks Containing Coordinatively Unsaturated Metal Sites: Effect of Metal Cations and Adsorbate Properties
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. CO<sub>2</sub>, CO, CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, N<sub>2</sub>, 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 CO<sub>2</sub> and CO, it has a negligible effect on adsorption of other
relatively nonpolar gases. On one hand, Henry’s constant of
CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub>
Hydrogen Adsorption on Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-DOBDC Metal–Organic Frameworks
This work reports hydrogen adsorption
properties of four different
metal–organic frameworks (MOFs) namely Zn-BDC, Cr-BDC, Ni-DABCO,
and Mg-DOBDC. Gravimetric hydrogen adsorption measurements are performed
over a wide range of temperature (90 K to 298 K) and pressure (0 bar
to 100 bar). At the lowest experimental temperature (90 K to 100 K)
all the isotherms are saturated and the adsorption capacity is governed
by pore volume. On the other hand, at room temperature the isotherms
closely follow Henry’s law. Modeling of the excess isotherms
is also done. Net adsorption isotherms, which can directly indicate
the efficiency of porous adsorbent for storage, are also presented.
In terms of volumetric efficiency, Mg-DOBDC MOF exhibits best storage
capacity out of all the MOFs considered in this study
Effect of Adsorbent History on Adsorption Characteristics of MIL-53(Al) Metal Organic Framework
Structural transformation of MIL-53Â(Al)
metal organic framework
from large pore to narrow pore form (lp → np) or vice versa
is known to occur by adsorption of certain guest molecules, by temperature
change or by applying mechanical pressure. In this work, we perform
a systematic investigation to demonstrate that adsorbent history also
plays a decisive role in the structural transitions of this material
(and hence on its adsorption characteristics). By changing the adsorbent
history, parent MIL-53Â(Al) is tuned into its np domain at ambient
temperature such that it not only exhibits a significant increase
in CO<sub>2</sub> capacity, but also shows negligible uptake for CH<sub>4</sub>, N<sub>2</sub>, CO, and O<sub>2</sub> at subatmospheric pressure.
In addition, for the high pressure region (1–8 bar), we propose
a method to retain the lp form of the sample to enhance its CO<sub>2</sub> uptake