Environmental Engineering Reference
In-Depth Information
Figure 6.5.3
Example of a metal organic framework (Mg-MOF-74)
Graphic by Richard Martin. This animation can be viewed at:
http://www.worldscientifi c.
com/worldscibooks/10.1142/p911#t=suppl
Flexible frameworks
In contrast to the rigid frameworks discussed above which retain
their porosity upon adsorption and desorption, fl exible and dynamic
frameworks collapse upon removal of guest solvent molecules, but their
porous structures are restored by adsorption of gas molecules at high
pressures [6.33]. The adsorption isotherms are typically characterized by
a distinct step, at which point the material “opens up” as gas molecules
enter the pores. In some striking cases, almost no adsorption occurs
below a threshold or “gate-opening” pressure; once the threshold pres-
sure is exceeded, the pores of the material become accessible and per-
mit signifi cant adsorption. The desorption isotherms are also typically
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