3.2 Solvent permeance of hierarchical MOF lamellar membranes
The solvent permeance of
hierarchical MOF lamellar membranes
was evaluated by a home-made dead-end cell under pressure of 1.0 bar
using seven polar solvents and five nonpolar solvents
(Figure S15). Results in Figure 2
reveal that hierarchical MOF lamellar membranes permit fast molecule
transport: 275.4 and 266.8 L m-2 h-1bar-1 for acetone and n-pentane, respectively. The
permeance is almost one order of magnitude higher than that of nonporous
graphene-based membranes (Figure S16). This is contributed by the
vertical channels throughout the membrane architecture, which greatly
shorten mass transfer distance and reduce transfer
resistance.[21,22] Furthermore, Figure 2a shows
that the permeance displays a good linear relationship with reciprocal
of solvent viscosity for MOF-CH3@BDC membrane, that is,
obeying Hagen–Poiseuille equation.[45] While for
MOF-CH3@CH3 and
MOF-CH3@NH2 membranes (Figures 2b and
c), molecule transport deviates obviously from Hagen−Poiseuille law,
implying that it is also affected by other factors expect for viscosity,e.g. molecular diameter and solubility
parameter.[46] In general, the permeance of polar
solvents is higher than that of nonpolar solvents for
MOF-CH3@NH2 membrane, while the
condition is inverse for MOF-CH3@CH3membrane. Taking acetone and n-hexane with similar viscosity as
examples, MOF-CH3@NH2 membrane gives a
permeance of 275.4 L m-2 h-1bar-1 for acetone, which is about 2 times higher than
that of n-hexane. While for MOF-CH3@CH3membrane, acetone permeance is 119.1
L m-2 h-1 bar-1,
about half of that of n-hexane permeance (214.8 L m-2h-1 bar-1). Since these hierarchical
lamellar membranes bear identical support layer, the variation of
molecule permeance should derive from distinct dissolution efficiency on
surface layer. Water and diiodomethane contact angles on the membrane
surfaces (Figure S17) show that water contact angle on hydrophilic
surface of
MOF-CH3@NH2membrane decreases quickly in the first 1 s. And then it continues to
drop moderately, confirming the positive tendency for water molecules
drilling into the subcutaneous tissue of membrane surface. In contrast,
hydrophobic MOF-CH3@CH3 membrane surface
permits diiodomethane to spread more smoothly than water, proving the
strong affinity toward nonpolar molecules.
However, the contact angles keep
almost constant after initial spreading, which implies that hydrophobic
pores give inert dissolution to both polar and nonpolar
solvents.[47] These observations deliver the fact
that molecular dissolution behavior is affected by the characteristics
of both molecule and membrane surface.