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).