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.