224
G.-R. Xu
2D MOF nanosheets see little applications in solute separations. Zr-MOF
nanosheets were used as photocatalysts because of their ultrathin thickness of ~1.5 nm
and high exposed active sites [45].
3.2 Zeolite Nanosheets
The potential of zeolitic imidazolate framework (ZIF) in desalination was firstly
reported by Jiang et al. [46]. They further extended the research to five ZIFs
including ZIF-25 (dimethyl imidazolate), ZIF-71 (dichloro imidazolate), ZIF-93
(aldehydemethyl imidazolate), ZIF-96 (cyanide imidazolate), and ZIF-97 (hydroxymethyl imidazolate) with varied functionalization and diameters of 0.51, 0.54, 0.37,
0.55, and 0.35 nm [47]. Compared to ZIF-93 and 97, ZIF-25, 71, and 96 exhibited
higher water permeation because of larger aperture size (da). Meanwhile, water flux
difference for ZIF-25, 71, and 96 themselves was ascribed to functional groups
polarity rather than da. In spite of larger da, hydrophilic CH 3 group of ZIF-25
enabled them with higher water flux compared to ZIF-71 and 96. As for the salt rejections, ZIF-25 exhibited 97% and the others showed 100%. ZIF-25 was considered
comprehensively the most potential alternative.
In spite of great potential, desalination performance of zeolite nanosheets is still
currently limited in computer simulations with little experimental study. For example,
the structures and desalination performance of zeolite nanosheets were researched
by MD simulation [20]. It was believed that ultrathin-film nature and the versatile
pore structures enabled zeolite nanosheets to have a great opportunity in desalination. Influence of pore density, free energy barrier, and cages inclusion was studied.
It is indicated that desalination performance of zeolite nanosheets was derived from
their one-atom-thin thickness and unique water channels including one-dimensional
(1D), multi-dimensional, and cage-containing channels. Moreover, appropriate pore
diameter of 0.4–0.75 nm was also important. MD simulation results indicated that
zeolite nanosheets could exhibit salt rejection of 100% and water permeability one
time higher than that of currently used PA-TFC membranes. Water permeability
of zeolite nanosheet with one layer was as high as 40 L/(cm
2 ·day·MPa), similar
as that of other novel ultrathin membranes such as graphene. Even with thickness of 100 nm, zeolite nanosheets still demonstrated water permeability of 1.3
L/(cm
2 ·day·MPa), much higher compared to conventional membranes in the range
of 0.03–0.2 L/(cm
2 ·day·MPa).
Generally, heavy metal ions are always removed by adsorption. But membrane
filtration using nanosheets could also be used to realize heavy metal ions removal.
Potential of nanomembranes in removing heavy metal ions was Initially studied by
DFT [48]. The graphene structure was optimized to obtain the highest Cu
2+ and Hg
2+
removal. Moreover, they also investigate the ability of BN and graphene sheets in
removing Zn
2+ and indicated that pore structure played key roles [49]. Furthermore,
they found that only ions with energy barrier difference could be effectively separated. Besides the nanosheets above, MoS 2 nanosheets were also applied to separate
G.-R. Xu
2D MOF nanosheets see little applications in solute separations. Zr-MOF
nanosheets were used as photocatalysts because of their ultrathin thickness of ~1.5 nm
and high exposed active sites [45].
3.2 Zeolite Nanosheets
The potential of zeolitic imidazolate framework (ZIF) in desalination was firstly
reported by Jiang et al. [46]. They further extended the research to five ZIFs
including ZIF-25 (dimethyl imidazolate), ZIF-71 (dichloro imidazolate), ZIF-93
(aldehydemethyl imidazolate), ZIF-96 (cyanide imidazolate), and ZIF-97 (hydroxymethyl imidazolate) with varied functionalization and diameters of 0.51, 0.54, 0.37,
0.55, and 0.35 nm [47]. Compared to ZIF-93 and 97, ZIF-25, 71, and 96 exhibited
higher water permeation because of larger aperture size (da). Meanwhile, water flux
difference for ZIF-25, 71, and 96 themselves was ascribed to functional groups
polarity rather than da. In spite of larger da, hydrophilic CH 3 group of ZIF-25
enabled them with higher water flux compared to ZIF-71 and 96. As for the salt rejections, ZIF-25 exhibited 97% and the others showed 100%. ZIF-25 was considered
comprehensively the most potential alternative.
In spite of great potential, desalination performance of zeolite nanosheets is still
currently limited in computer simulations with little experimental study. For example,
the structures and desalination performance of zeolite nanosheets were researched
by MD simulation [20]. It was believed that ultrathin-film nature and the versatile
pore structures enabled zeolite nanosheets to have a great opportunity in desalination. Influence of pore density, free energy barrier, and cages inclusion was studied.
It is indicated that desalination performance of zeolite nanosheets was derived from
their one-atom-thin thickness and unique water channels including one-dimensional
(1D), multi-dimensional, and cage-containing channels. Moreover, appropriate pore
diameter of 0.4–0.75 nm was also important. MD simulation results indicated that
zeolite nanosheets could exhibit salt rejection of 100% and water permeability one
time higher than that of currently used PA-TFC membranes. Water permeability
of zeolite nanosheet with one layer was as high as 40 L/(cm
2 ·day·MPa), similar
as that of other novel ultrathin membranes such as graphene. Even with thickness of 100 nm, zeolite nanosheets still demonstrated water permeability of 1.3
L/(cm
2 ·day·MPa), much higher compared to conventional membranes in the range
of 0.03–0.2 L/(cm
2 ·day·MPa).
Generally, heavy metal ions are always removed by adsorption. But membrane
filtration using nanosheets could also be used to realize heavy metal ions removal.
Potential of nanomembranes in removing heavy metal ions was Initially studied by
DFT [48]. The graphene structure was optimized to obtain the highest Cu
2+ and Hg
2+
removal. Moreover, they also investigate the ability of BN and graphene sheets in
removing Zn
2+ and indicated that pore structure played key roles [49]. Furthermore,
they found that only ions with energy barrier difference could be effectively separated. Besides the nanosheets above, MoS 2 nanosheets were also applied to separate