4 research outputs found
Adsorption and Separation of Carbon Dioxide Using MIL-53(Al) Metal-Organic Framework
In this work, we report adsorption
isotherms of various industrially
important gases, viz. CO<sub>2</sub>, CO, CH<sub>4</sub>, and N<sub>2</sub> on MIL-53Â(Al) metal organic framework (MOF). The isotherms
were measured in the range of 0–25 bar over a wide temperature
range (294–350 K). The structural transformation of the adsorbent
and the resulting breathing phenomenon were observed only in the case
of CO<sub>2</sub> adsorption at 294 and 314 K. Adsorption of CO (another
polar gas), N<sub>2</sub> and CH<sub>4</sub> did not induce any structural
transformation in this adsorbent for the experimental conditions considered
in this work. Since the CO<sub>2</sub> isotherms at 294 and 314 K
involve structural transformation and show a distinct step, a conventional
isotherm model cannot be used to describe such behavior. In order
to model these isotherms, a dual-site Langmuir-type equation (one
site each for the two structural forms, i.e., large pore phase and
narrow pore phase) that includes a normal distribution function to
account for structural transformation is proposed. This model successfully
mimics the Type-IV isotherm behavior of CO<sub>2</sub> on MIL-53Â(Al).
Henry’s constants and adsorption enthalpies of CO<sub>2</sub> on the two structural forms were calculated using this model. The
Ideal Adsorbed Solution Theory (IAST) was used to predict the selectivity
of CO<sub>2</sub> at 350 K over other gases studied in this work
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