Elucidating Ultrafast Molecular Permeation through Well‐Defined 2D Nanochannels of Lamellar Membranes

Abstract

Lamellar membranes with well‐defined 2D nanochannels show fast, selective permeation, but the underlying molecular transport mechanism is unexplored. Now, regular robust MXene Ti3C2Tx lamellar membranes are prepared, and the size and wettability of nanochannels are manipulated by chemically grafted hydrophilic (−NH2) or hydrophobic (−C6H5, −C12H25) groups. These nanochannels have a sharp difference in mass transfer behavior. Hydrophilic nanochannels, in which polar molecules form orderly aligned aggregates along channel walls, impart ultrahigh permeance (>3000 L m−2 h−1 bar−1), which is more than three times higher than that in hydrophobic nanochannels with disordered molecular configuration. In contrast, nonpolar molecules with disordered configuration in both hydrophilic and hydrophobic nanochannels have comparable permeance. Two phenomenological transport models correlate the permeance with the mass transport mechanism of molecules that display ordered and disordered configuration.

2D Material Based Advanced Membranes for Separations in Organic Solvents

“…Furthermore, fabricating MXene nanosheets into laminar membranes remains a challenging task, for reasons similar to those of 2D COFs. [12,20,23] Despite these issues, there are some studies, which have successfully applied MXene membranes for separation applications. Most of the early studies focused on separations in aqueous media.…”

“…Thus, such stable MXenes can be fabricated into membranes for separation in organic solvents. [12,20,23,39,103] Gogotsi et al studied the dispersion of MXenes in various polar organic solvents. [39] Ti 3 C 2 T x showed good dispersion stability in some organic solvents, such as DMF, NMP, dimethyl sulfoxide (DMSO), propylene carbonate (PC), and ethanol.…”

“…However, the rejection rates of different dyes were higher than 90%, which are among one of the best recently reported MXene membranes (Figure 10h,i). [12,20,23,97,104,105] Again, it was reported that organic solvent viscosity and dye solubility affected the permeance and selectivity of the hybrid membrane. Moreover, Figure 10j shows that the hybrid membrane has excellent stability under long-term operation.…”

“…All these features render filtration as a versatile method for fabricating a wide range of 2D material membranes, such as MXenes, BN, and some exfoliated dichalcogenides. [12,20,23,25,40,[56][57][58] Further, filtration is easily implementable and has a cost advantage over other fabrication methods discussed below.…”

“…[8,11] Compared with conventional polymeric materials, 2D materials can offer various unique advantages as a membrane material: 1) the distinct mono-to few-atoms-thick morphology of 2D nanosheets enable them to serve as desirable building blocks to achieve well-controlled thickness, which can be used to engineer the transport properties of membranes; [12][13][14][15] 2) the interlayer spacing between 2D material nanosheets is well-defined at the nanometric scale with a narrow size distribution, making it possible for achieving specific molecular selectivity; [15][16][17][18] 3) the ability of 2D materials in forming slit-type nanochannels can facilitate molecule transport under nanoscale spatial confinements, which sometimes present unexpected nanofluidic behaviors; [13,14,19] 4) the tunable physicochemical properties and interlayer spacings/nanopores of 2D materials offer greater versatility for membrane designs; [20][21][22] and last but not least 5) some 2D materials also possess outstanding chemical resistances in organic solvents and harsh chemical environments, making membrane fabrication and application possible in a multitude of separation processes. [16,17,23] For example, 2D boron nitride (BN) nanosheets [24,25] possess excellent chemical stability, which enables their utilization as membranes under harsh conditions, such as at high temperatures or in strongly acidic and alkaline media. These attributes, along with excellent mechanical robustness of certain 2D materials, enable 2D materials as promising material candidates to fabricate highperformance membranes for organic solvent recovery and separation applications.…”