3.5 Dye rejection and stability of hierarchical MOF lamellar membranes
Then, the structural stability and long-term operational stability of as-prepared hierarchical MOF lamellar membranes were conducted. Figure 6 shows that these membranes display favorable anti-fouling and anti-pressure performance. Preliminarily, five dyes with a series of sizes were filtered through the membranes to detect rejection ability (Figures 6a and S24). Before testing, membranes were compacted under 1 bar for 20 min to reach a complete equilibrium state. MOF-CH3@NH2 and MOF-CH3@CH3 membranes exhibit comparable dye rejection, because of their similar pore sizes. Specifically, for MB with small size of 1.4 × 0.58 nm, membranes show an invalid rejection (22%). Upon increasing the size of dyes to 1.6 × 1.4 nm (RR) and 2.2 × 1.7 nm (BB), membrane rejection experiences a sharp rise to almost 100%, verifying the precise sieving of membranes by intrinsic pores (1.02 nm). Meanwhile, to detect the dye adsorption on membrane rejection, PEGs with different molecular weight were filtered through the MOF membranes, and these membranes exhibited aMWCO of ~600 Da (Figure S25). According to Stokes equation,[64] the diameter is calculated to be 1.1 nm, which is well consistent with the pore size of membranes. This confirms that the dyes should be slightly adsorbed on membrane, which contribute to a higher rejection, while the size sieving effect is the main separation mechanism for these MOF membranes. In addition, cyclic operation (24 h) test displays that methanol permeance begins reducing at 16 h, and after washing, the permeance returns by 99% (Figure 6b and S26). This is ascribed to the dye molecules retained on the membrane surface block the transport of solvents to a certain extent, and after washing, the clean membrane regain its excellent performance. Moreover, pressure cyclic measurement results (Figure 6c) reveal that the lamellar structure of MOF membrane is able to bear transmembrane pressure as high as 6 bar without obvious collapse. This is supported by the negligible permeance fluctuation (below 5%) for methanol when elevating the pressure from 1 bar to 6 bar and adjusting back to 1 bar. Furthermore, long-term operation stability test (Figure 6d) displays that methanol permeance of MOF lamellar membranes experiences a moderate decline (below 5%) in the initial 2 h, and then maintains steady during the following 98 h. The slight decrease should be stemmed from the moderate compaction of adjacent nanosheets within lamellar structure, and similar observations are also reported for other lamellar membranes.[9,43] In general, the as-prepared MOF lamellar membranes exhibit satisfactory structural and operational stability, which is mainly contributed by the strong π-π stacking interactions within membrane structure,[65,66]holding promise for practical applications (Figure S27).