Adsorption - Introduction to Carbon Capture and Sequestration - page 267

characterized by hysteresis, defi ned as the dependence of a system on
both its current and past environments. The observation of one or more
distinct steps in the adsorption isotherms represents a key feature of
fl exible frameworks. The MIL-53 series has been extensively investigated
for high pressure CO 2 and CH 4 adsorption [6.34].
Mesh-adjustable molecular sieves (MAMS) constitute another class
of materials for gas separations and are based on temperature-induced
gating phenomena [6.35]. MAMS represent a case where the dynamics
of substituents at pore openings allow some molecules to pass but not
others.
Surface-functionalized frameworks
The grafting of functional groups with a high affi nity for CO 2 onto the
surfaces of porous materials via ligand modifi cation or coordination to
unsaturated metal centers has been employed as a strategy to enhance
the capacity and selectivity for CO 2 adsorption. This approach has analo-
gies with other functionalized solid adsorbents such as amine-grafted
silicas; however, the crystalline nature of metal-organic frameworks pro-
vides for a molecular level of control of pores, assisting in the “tuning” of
frameworks for gas separations. Frameworks containing open metal sites
have been selectively grafted with molecules that have a high affi nity for
CO 2. This grafting can be done with highly polar pyridine derivatives
[6.36], and amine-functionalized ligands [6.37, 6.38].
Section 6
Amines versus MOFs
In the previous chapter, we discussed liquid absorption and in the pre-
sent chapter, we have been discussing adsorption. An important practi-
cal question is: so what is the best technology? It is actually not easy to
give a straightforward answer to this question. An easier question to
answer is which technology would be best to use if we were to build a