A second direction studied by many groups is the reliance on framework flexibility to achieve molecular sieving and controlling the sieving performance of a given material through balance of host-host and host-guest interactions. For example, Qazvini et al. demonstrated this effect in dynamic cobalt- based MUF-15-OMe MOF : the framework responds to guest uptake with a reversible, pore-expanding structural transformation. In this expanded structure, MUF-15-OMe then has improved properties for CO 2 /N 2 separation and enables kinetic quantum sieving, since D 2 diffuses faster than H 2 in the newly created void space.