# Single-Layer Silica Nanosheets with In-Plane Porosity Enable High-Performance Nanolaminate Membranes for Organic Solvent Nanofiltration

Wensen Wang, Nadia Batool, Mathilde Moderne, Juan José Quintana

González, Eddy Petit, Valérie Flaud, William Chèvremont, Luc Lajaunie,

Tangi Aubert, Damien Voiry

► To cite this version:

Wensen Wang, Nadia Batool, Mathilde Moderne, Juan José Quintana González, Eddy Petit, et al.. Single-Layer Silica Nanosheets with In-Plane Porosity Enable High-Performance Nanolaminate Membranes for Organic Solvent Nanofiltration. Journal of the American Chemical Society, inPress, 10.1021/jacs.5c13454 . hal-05460776

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Single-Layer Silica Nanosheets with In-Plane Porosity Enable High-Performance Nanolaminate Membranes for Organic Solvent Nanofiltration

Wensen Wang¹†, Nadia Batool²†, Mathilde Moderne¹, Juan José Quintana González³, Eddy Petit¹, Valérie Flaud², William Chèvremont⁴, Luc Lajaunie³, Tangi Aubert²*, Damien Voiry¹*

¹Institut Européen des Membranes, IEM, UMR 5635, Université Montpellier, ENSCM, CNRS, Montpellier 34000, France

²ICGM, Université Montpellier, CNRS, ENSCM, 34000 Montpellier, France

³Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro S/N, Puerto Real, 11510, Cádiz, Spain

⁴ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France

TOC graphic

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Abstract

Organic solvent nanofiltration (OSN) membranes hold substantial promise for energy-efficient molecular separations in chemical and pharmaceutical industries, yet current technologies often face limitations in solvent permeation and precise sieving. To address these issues, we introduce novel nanolaminated membranes assembled from single-layer, two-dimensional (2D) mesoporous silica nanosheets with inherently uniform in-plane pores. These silica nanosheets, synthesized via a surfactant-templated soft approach, feature highly ordered and tunable hexagonal porosity with sub-10 nm thickness. By adjusting the surfactant carbon chain length, silica nanosheets with controllable pore dimensions were prepared and subsequently stacked into defect-minimized nanolaminate structures. The resulting membranes exhibited polarity-dependent solvent permeation, with permeance values up to 238 L m⁻² h⁻¹ bar⁻¹ [LMHB] for polar solvents. Molecular sieving evaluations further revealed robust size-selective performance, achieving molecular weight cut-offs as low as 678 Da with high solvent flux. Remarkably, these nanolaminated silica membranes surpassed existing state-of-the-art OSN membranes in both permeability and selectivity benchmarks. Our findings underscore the potential of
34

precisely engineered 2D mesoporous silica nanosheets as scalable and chemically robust building
blocks for next-generation OSN membranes tailored for advanced molecular separation applications.

35
## Introduction

Organic solvent nanofiltration (OSN) based on semipermeable membrane has emerged as a promising molecular separation technology, offering significant potential for reducing energy consumption in petrochemical, pharmaceutical, and fine chemical industries [1, 2]. Traditional polymer-based membranes typically exhibit low solvent permeation rates due to their disordered three-dimensional interconnected network and low porosity [3, 4]. In contrast, membranes with robust and well-defined nanopores/nanochannels allow for direct and continuous solvent flow, substantially enhancing transport rates [5-7]. Materials with intrinsic cavities or uniform pore structures have therefore attracted growing interest [8, 9]. For instance, membranes fabricated by polymerizing cyclodextrins exhibit aligned, through-channel pores that not only improve solvent permeability but also enhance molecular selectivity due to their uniform pore size [10, 11]. Similarly, covalent organic framework (COF) membranes, characterized by their highly ordered porous architecture, have demonstrated excellent performance in OSN [12-14]. However, many organic membranes suffer from swelling and solvent-induced degradation under harsh chemical conditions, limiting their long-term stability [15]. In this context, inorganic porous materials, which offer superior solvent resistance and stability, present a more suitable alternative for organic solvent nanofiltration applications [16].

Porous graphene and its derivatives, are among the most promising materials for OSN membranes due to their atomically thin structure, which provides an ultrashort transmembrane pathway and enables exceptionally high solvent permeation rates [17, 18]. A seminal study by Karnik et al. demonstrated the feasibility of nanoporous graphene for this application [19, 20]. However, large-scale implementation is hindered by challenges in controlling defects and achieving uniform pore size in single-layer graphene membranes. A practical solution is to assemble porous two-dimensional (2D) nanosheets into laminated membranes. This interstitial architecture not only mitigates defect formation—caused by misaligned nanosheet stacking—but also facilitates membrane fabrication over large surface areas. For instance, Kim et al. developed laminated graphene membranes by thermally activating pores in graphene oxide nanosheets and subsequently stacking them using vacuum filtration [21, 22]. These membranes exhibited exceptionally high solvent permeance along with excellent molecular sieving capabilities. Despite this progress, precise control over pore formation and size distribution in graphene nanosheets remains a significant challenge.

Nanolaminated membranes assembled from exfoliated two-dimensional (2D) materials have garnered significant interest for molecular separation. However, their performance is often limited by reliance on interlayer porosity, which introduces tortuous and inefficient transport pathways, resulting in moderate solvent permeance. Enhancing transport by incorporating intralayer porosity within the nanosheets is a promising strategy, yet achieving uniform, controllable pore formation over large areas remains a significant challenge. An attractive alternative is to utilize 2D inorganic nanosheets that possess intrinsic, uniformly distributed pores, thereby circumventing the need for post-synthetic pore engineering. In this context, mesoporous silica materials have attracted consistent attention as a versatile platform for a wide range of applications, including in separation, catalysis, and biotechnology. Their synthesis via soft-templating with surfactant micelles offers exquisite control over pore size and pore structure. Recent
advances in this field have led to the development of 2D mesoporous silica nanosheet featuring high porosity with long-range order [23]. These colloidal 2D materials consist of well-ordered cage-like structures formed around discrete micelles packed at a liquid-liquid interface. This approach enables precise control at the level of single layers of micelles, resulting in intrinsically uniform material thicknesses down to the sub-10 nm range. The cage-like structures form a network of pores that are open across the nanosheet while remaining interconnected within its plane. The resulting superstructure enables direct solvent flow and offers high chemical stability in organic solvents. Given these favorable characteristics, we sought to harness the untapped potential of 2D silica nanosheets as a new class of membrane-building units for high-performance OSN.

In this study, we utilized mesoporous silica nanosheets as building blocks for laminated OSN membranes. By changing the carbon chain length of the surfactant, we further synthesized 2D silica nanosheets with varying pore sizes. We systematically investigated the influence of pore size on solvent transport and molecular sieving performance. Finally, we demonstrated their effectiveness in separating realistic mixed-solute solutions, confirming their potential as robust and efficient OSN membranes.

## Results and discussion

**2D mesoporous silica nanosheets.** Large and highly ordered 2D mesoporous silica nanosheets were synthesized *via* a colloidal approach [23]. As illustrated in Fig. 1a, these low-dimensional nanostructures are synthesized through the confined growth of mesoporous silica at the surface of cyclohexane microdroplets dispersed in water with the help of a quaternary ammonium cationic surfactant. The addition of tetramethyl orthosilicate (TMOS) and (3-aminopropyl)trimethoxysilane (APTMS) as the silica precursors leads to the formation of large vesicles. During purification, these vesicles are disrupted by sonication in ethanol, resulting in flat 2D nanosheets of mesoporous silica (Fig. 1a). Atomic force microscopy (AFM, Fig. 1b and Supporting Information, Fig. S1) and transmission electron microscopy (TEM, Fig. 1c and Supporting Information Fig. S2) images show that these 2D silica nanosheets can extend up to several micrometers in lateral dimension with uniform thickness and display a well-ordered 2D hexagonal porosity as evidenced by the fast Fourier transform analyses of the TEM images (Fig. 1c and Supporting Information, Fig. S2). To fabricate membranes for size-selective separation, 2D nanosheets were synthesized using two different surfactants, namely hexadecyltrimethylammonium bromide ($C_{16}$TAB) and myristyltrimethylammonium bromide ($C_{14}$TAB), in order to modulate the intralayer pore size. The materials synthesized with either $C_{16}$TAB or $C_{14}$TAB surfactants are thereafter denoted as C16-silica and C14-silica, respectively. From the TEM images (Fig. 1c and supporting Information, Fig. S2), the apparent pore diameters of C16- and C14-silica nanosheets were estimated to 7.7 nm and 6.4 nm, respectively. This trend is consistent with the difference in alkyl-chain length of the two surfactants. Surfactants with longer hydrophobic tails typically form larger structure-directing micelles, which in turn lead to larger pore sizes. This relationship between surfactant chain length and pore size has already been established in our previous studies [23]. Small-angle X-ray scattering (SAXS) analyses were performed on colloidal solutions of these 2D mesoporous silica
nanosheets using synchrotron radiation (TRUSAXS instrument of ESRF [24]), in order to probe the nanoscale structure across a large fraction of the material. In Fig. 1d, the SAXS patterns exhibit a clear structure factor consistent with a 2D hexagonal (p6mm) symmetry, confirming the long-range and homogeneous in-plane periodic porosity in the nanosheets. In addition, the shift in the positions of the Bragg peak corresponding to the (10) and (11) reflections of the 2D hexagonal lattice between C16- and C14-silica nanosheets supports a reduction in pore size resulting from the use of a shorter surfactant.

<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Synthesis and characterization of 2D mesoporous silica nanosheets. **a**, Illustration of the synthesis of 2D mesoporous silica nanosheets. **b**, AFM image of a 2D mesoporous silica nanosheet. **c**, TEM image of a 2D mesoporous silica nanosheet synthesized using C₁₆TAB surfactant (inset in **c**: corresponding FFT). **d**, SAXS patterns of C16- and C14-silica nanosheets.</figcaption></figure>

*Amine screening.* Initial experiments revealed that using DMSO as the solvent for membrane fabrication resulted in excessive permeation performance. An ageing effect was also observed in solutions of 2D nanosheets stored in DMSO, which gradually turned from colorless to yellowish within a couple of months (Supporting Information, Fig. S3). This yellowish coloration could be consistent with the
formation of imine or azo compounds by coupling of aminopropyl groups present at the surface of
nanosheets, as DMSO is known to promote the oxidation of primary amines in the presence of dissolved
oxygen [25]. This is further supported by our XPS analyses of the N1s region in the aged nanosheets,
which confirmed the emergence of a peak at 402.1 eV corresponding to N=N bonds (Supporting
Information, Fig. S4). This ageing is also accompanied by the appearance of two additional bands at
2855 cm⁻¹ and 2925 cm⁻¹ observed by infrared spectroscopy, which are attributed to different
conformation of the C-H bonds in the yellow suspension (Supporting Information, Fig. S5). Membranes
fabricated using aged nanosheet solutions exhibited realistic solvent fluxes, whereas those prepared
with freshly synthesized nanosheets showed abnormally high fluxes, unsuitable for filtration
applications. Such unrealistic fluxes originate from the formation of large cavities or pinholes within
the membrane structure stacked by fresh nanosheets (Supporting Information, Fig. S6). We therefore
conclude that the native amine groups present on the freshly prepared nanosheets interfere with the
membrane fabrication process. During aging in DMSO, these reactive groups may become deactivated,
thereby enabling the formation of structurally sound membranes.

Previous work has shown that these amine groups, coming from the APTMS silica precursor, are
important for the synthesis of well-ordered mesoporous silica nanosheets as they facilitate anchoring of
silica at the liquid-liquid interface [23], and cannot entirely be avoided. As a result of this formation
mechanism, it was shown that the amine groups are found predominantly on one side of the nanosheet.
This asymmetric chemical nature of this 2D material could be the reason why fresh nanosheets produce
membranes with poor properties. Indeed, alkyl amine groups are known to interact electrostatically with
silica surfaces [26]. During membrane fabrication via pressure-assisted filtration of colloidal
suspensions, the gradually increasing concentration of nanosheets may promote specific interactions
between their amine-rich and amine-poor surfaces, thereby hindering homogeneous stacking on the
nylon substrate. On the other hand, ageing of the nanosheets in DMSO is a rather slow process, typically
requiring several months, which makes it an impractical solution for the reliable fabrication of
membranes. This observation of the ageing effect therefore motivated us to find a solution to screen the
amines intentionally post-synthesis. The amine groups were screened after synthesis by reacting them
with N-(9-fluorrenylmethoxycarbonyloxy) succinimide (Fmoc-OSu), thereby preventing their
detrimental effect on membrane formation. Fmoc-OSu is commonly used for the protection of primary
and secondary amine groups through the formation of a stable carbamate bond by straightforward click
chemistry (Supporting Information, Fig. S3) [27]. The passivation was achieved by adding Fmoc-OSu
directly to the colloidal solution of 2D nanosheets in ethanol following their synthesis. Effective
functionalization was confirmed by infrared spectroscopy that revealed characteristic peaks at 728 cm⁻¹
and 1447 cm⁻¹ in the spectrum of Fmoc-functionalized 2D nanosheets, attributed to the aromatic ring
structure of the Fmoc group (Supporting Information, Fig. S5) [27]. The organic fraction of the
nanosheets was quantified through thermogravimetric analyses before and after functionalization with
Fmoc (Supporting Information, Fig. S7). Assuming that the aminopropyl groups are not lost during this
functionalization step, the results indicate organic fractions of 26 wt% and 8 wt% coming from the
aminopropyl and Fmoc groups, respectively, for C16-silica nanosheets, and 23 wt% and 5 wt% for C14-
silica nanosheets. While these analyses confirm the effective functionalization of the nanosheets, they also show that only a small fraction of the amine groups (estimated to less than 10%) get conjugated with Fmoc moieties. The colloidal suspension of Fmoc-functionalized 2D nanosheets showed no signs of aging after transfer in DMSO, and the solution remained colorless (Supporting Information, Fig. S3). Most importantly, these Fmoc-functionalized nanosheets could be used directly after synthesis for the fabrication of high quality membranes showing homogeneous structures and realistic solvent flux (vide infra), without needing a long ageing process.

The thickness of the individual nanosheets after Fmoc functionalization was estimated by AFM to 8 nm in the case of C16-silica nanosheets (Supporting Information, Fig. S1), which is consistent with a 2D structure made of a single layer of silica cages. C14-silica nanosheets on the other hand exhibit an average thickness of 15 nm, suggesting that these nanosheets may consist of a bilayered cage structure. In addition to AFM analyses, the thickness of the nanosheets was also estimated through TEM observations of folds that occasionally form upon drying (Supporting Information, Fig. S2). When such folds stand vertically on the TEM grid, they provide a direct view corresponding to twice the nanosheet cross-section. Based on this method, the thickness of the C16- and C14-silica nanosheets was estimated to ca. 6 nm and 10 nm, respectively. However, this estimation does not account for the organic fraction (propylamine-Fmoc conjugate), and only a small portion of the nanosheet can be observed using this approach due to the limited field of view and the low frequency of fold formation. These limitations may explain why the thickness values obtained from our TEM observations are lower than those measured by AFM.

*Membrane fabrication.* To construct the nanolaminated membranes, these nanosheets were stacked on a porous nylon substrate via pressure-assisted filtration of the 2D mesoporous silica nanosheets in DMSO. As shown in Fig. 2a, the resulting membranes could be easily peeled off from the substrate upon drying, forming freestanding films, allowing for the characterization of their textural properties. Nevertheless, for solvent permeation and filtration experiments (vide infra), the membranes were not dried and typically kept in ethanol after fabrication (see Experimental section for details), in which case their remained firmly attached to the nylon substrate. Cross-sectional scanning electron microscopy (SEM) images reveal a compact structure, free of any large defect or crack (Fig. 2b). The thickness of the membranes could be precisely controlled from 0.5 µm to 5.7 µm by adjusting the amount of filtered solution (see Experimental Section for details). Transmission electron microscopy (TEM) images of ultramicrotome sections of the membrane further evidence the well-ordered laminated structure formed through tight nanosheet stacking (Fig. 2c). The higher-magnification TEM image in Fig. 2c also highlights the open porosity in the plane of the membrane. The large holes visible in the lower-magnification TEM image of Fig. 2c were likely caused by the ultramicrotome sectioning process, rather than being pre-existing defects in the membrane. The higher-magnification TEM image reveals the cross-sectional structure of the membrane, highlighting the tightly stacked arrangement of the nanosheets. The visible porosity in this image differs from the 2D hexagonal porosity observed in top-view images, such as in Fig. 1c. Instead, it corresponds to the intralayer porosity of the stacked
nanosheets, which is also open in the plane of these 2D nanostructures. This is the same porosity that can be seen from vertically standing folds on TEM images of single 2D nanosheets (Supporting Information, Fig. S2). The textural properties of the nanolaminated membranes were characterized through nitrogen sorption analyses and compared to those of air-dried 2D silica nanosheets (Fig. 2d). Both the nanosheets and membranes exhibit type IV isotherms that are characteristic of mesoporous materials. The membrane fabrication process, however, resulted in a substantial reduction of the specific surface area from 360 m² g⁻¹ to 170 m² g⁻¹ for C16-silica materials, and from 215 m² g⁻¹ to 45 m² g⁻¹ for C14-silica materials. This reduction in specific surface area is consistent with the tight stacking of the nanosheets within the membranes, which renders the outer surfaces of individual nanosheets inaccessible [28]. This is further supported by the high-magnification TEM image of the membrane cross-section (Fig. 2c), where the interlayer regions appear as dense, dark lines, while the individual nanosheets exhibit open intralayer porosity. Taken together, the TEM and nitrogen sorption analyses indicate that interlayer porosity in these membranes is virtually absent, and that the overall porosity of the system is primarily governed by the intralayer pores of the 2D silica nanosheets. The nanolaminated membranes exhibit total pore volumes of 0.2 cm³ g⁻¹ and 0.04 cm³ g⁻¹ for the C16- and C14-silica membranes, respectively, which further supports the lower porosity of C14-silica membranes. The typical pore sizes of the fabricated membrane were estimated using the Barrett-Joyner-Halenda (BJH) model on the adsorption branch of the membrane isotherms (Supporting Information, Fig. S8). The analysis suggested pore sizes centered around 4 nm for the C16-silica membrane and 3 nm for the C14-silica membrane. However, it is important to note that the BJH model relies on assumptions about pore geometry that may not accurately represent our system and is known to underestimate pore sizes below 10 nm. As a result, the pore size values derived from the BJH analysis should be interpreted with caution due to significant potential error. Nonetheless, the analysis suggests that, in line with the properties of the corresponding 2D silica nanosheets, the C14-silica membrane exhibits smaller pores than the C16-silica membrane.
<figure><img src="image_4.png" /><figcaption>**Fig. 2** | Structural characterization of nanolaminated silica membranes. **a**, Optical image of a free-standing silica membrane. **b**, Cross-sectional SEM image of a nanolaminated silica membrane. **c**, TEM images of a ultramicrotome section of a nanolaminated membrane at different magnifications. **d**, N₂ sorption isotherms of C16- and C14-silica nanosheets (air-dried from ethanol solution) and their corresponding nanolaminated membranes.</figcaption></figure>

*Solvent Permeation in Nanolaminated Silica Membranes.* To evaluate the solvent permeation performance of the nanolaminated silica membranes, we tested 11 representative organic solvents—comprising both polar and nonpolar molecules—under pressure-driven filtration. The mass of permeated solvent was recorded and used to calculate solvent permeance, as shown in Fig. 3a & 3b. Interestingly, no detectable permeation was observed for nonpolar solvents such as hexane and cyclohexane, indicating that these molecules were unable to enter the nanopores of the silica nanosheets. This phenomenon is attributed to the weak interaction between nonpolar molecules and the polar surface of the SiO₂ nanopores, which inhibits their entry into the membrane [29-31]. In contrast, polar solvents readily permeated the membrane, exhibiting high permeance values ranging from 31 to 238 LMH bar⁻¹
for a 3.8 µm-thick membrane, depending on solvent type. To evaluate the membrane's ability to discriminate between polar and non-polar solvents, binary mixtures of DMSO/hexane and NMP/hexane were filtered (Fig. 3c and Fig. S9). In all cases, the polar component (DMSO or NMP) permeated efficiently, while hexane was effectively rejected – irrespective of the polar solvent's molecular weight – demonstrating robust and reproducible selectivity toward polar species. This selective transport behavior aligns well with our previous findings in nanolaminated clay membranes [32]. Moreover, the permeation rates of polar solvents closely followed an inverse relationship with solvent viscosity, demonstrating a pore-flow-dominated transport mechanism. This is in stark contrast to the phenomenological transport model typically observed in polymer membranes, where permeability is governed by a complex interplay of solubility, molecular size, and viscosity [33].

When comparing membrane variants, the monolayer-cage C16-silica membrane consistently exhibited higher solvent permeance than the bilayered-cage C14-silica membrane under identical conditions. The C14-silica membrane possessed smaller pores on individual nanosheets and a lower overall pore volume—which introduced greater entry resistance for solvent molecules. As a result, the proportionality constant (i.e., the slope) describing permeance–viscosity behavior in C14-silica membranes was approximately 30% lower than that of the C16-silica membrane, reflecting the influence of nanosheet architecture on mass transport. To further explore the influence of membrane thickness on solvent transport, we measured ethanol flux across C16-silica membranes with thicknesses ranging from 0.5 µm to 5.7 µm. As shown in Fig. 3d, ethanol permeance increased significantly up to >700 LMHB with decreasing membrane thickness, due to the shortened transmembrane pathway. Curve fitting revealed that ethanol permeance follows an inverse power-law relationship with thickness, with an exponent of ~1.26. This deviation from the ideal inverse proportionality (power of 1) suggests that thinning the membrane increases the contribution of structural imperfections or edge defects, which introduce additional transmembrane pathways and enhance solvent transport. We also measured ethanol flux through a 3.8 µm-thick C16-silica membrane under applied pressures ranging from 0.5 to 5 bar. As shown in Fig. 3e, the ethanol flux increases linearly with pressure up to 5 bar, maintaining a permeance of ~75 L m⁻² h⁻¹ bar⁻¹.
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<figure><img src="image_9.png" /><figcaption>**Fig. 3** | Solvent permeation behavior of nanolaminated silica membranes. **a**, **b**, Permeance of various polar organic solvents through the C16-silica (a) and C14-silica (b) membranes as a function of the inverse of their viscosities. Both membranes exhibit zero permeation for nonpolar solvents, indicating strong polarity-dependent selectivity. Red dashed lines represent linear fits to highlight the inverse viscosity relationship. **c**, Evolution of the solvent flux over time for DMSO/hexane mixture (20 mL DMSO + 10 mL hexane) for C16 silica membrane. **d**, Ethanol permeance through C16-silica membranes of varying thicknesses. The red line indicates the best-fit curve, showing an inverse power-law dependence on membrane thickness. Error bars represent standard deviations based on at least three independent measurements. **e**, Ethanol flux through 3.8 µm-thick C16-silica membrane under varying pressures. The red solid line is the best linear fit. The standard deviation originates from variations in flow rate during the pressure ramp-up and ramp-down sequence.</figcaption></figure>

*Molecular Sieving in Nanolaminated Silica Membranes.* To maximize solvent permeance while minimizing defect-related leakage and ensuring effective molecular separation, we selected membranes with a thickness of 3.8 µm to assess molecular sieving performance. Six organic dye molecules with varying molecular sizes and charges (see Supporting Information, Table S1, for a detailed list of the dyes and their structure) were individually dissolved in ethanol at a concentration of 25 ppm. Each solution was filtered through the membranes, and the concentrations of permeated dyes were quantified using high-performance liquid chromatography (HPLC) coupled with ultraviolet-visible (UV-Vis) spectroscopy to determine rejection efficiency. As shown in Fig. 4a, membrane rejection decreased progressively with decreasing dye molecular weight, indicating that size exclusion is the dominant
separation mechanism. The zeta potential of the silica nanosheets after screening of the amines with Fmoc groups was measured in ethanol solution, resulting in rather similar values of +5.3 ± 0.4 mV for C16-silica nanosheets and +5.7 ± 4.6 mV for C14-silica nanosheets. Despite this mild positive surface charge of the silica nanosheets in ethanol, the rejection behavior of the derived membranes toward charged dye molecules was not significantly affected. This observation reinforces the understanding that, in the low-polarity environment of organic solvents, electrostatic interactions between solutes and membrane surfaces are substantially diminished. Unlike aqueous nanofiltration, where charge-based repulsion often governs selectivity, OSN is primarily driven by steric exclusion, underscoring the critical role of precise pore size control in achieving molecular sieving [34]. Moreover, both membranes fully rejected dyes with molecular weights exceeding 900 Da while maintaining ethanol fluxes above 60 LMH bar⁻¹, confirming their suitability for high-flux organic solvent purification. Specifically, the C14-silica membrane achieved a molecular weight cut-off (MWCO) of 678 Da, compared to 803 Da for the C16-silica membrane. This enhanced sieving capability of the bilayered-cage C14-silica membrane is attributed to its tighter pore structure—originating from the smaller pores of the nanosheets—effectively restricting larger molecules.

To further validate molecular separation under practical conditions, we performed a mixed-solute filtration test using a binary dye solution of Rhodamine B (RB, 443 Da) and Alcian Blue 8GX (AB8GX, 1157 Da). Post-filtration UV-Vis spectroscopy analyses (Fig. 4b) revealed that RB passed freely through the C16-silica membrane, with nearly identical concentrations in the permeate and feed, as visible from its absorption feature at 550 nm. In contrast, AB8GX was fully rejected as indicated by the absence of its characteristic absorption features above 600 nm in the permeate. These results confirm the membrane ability to selectively separate mixed solutes, offering a valuable route for recovery of high-value components from complex organic mixtures. To benchmark the nanoporous silica membranes performance, we plotted their ethanol permeance together with the corresponding MWCO values and compared it to that of existing porous OSN membranes. As depicted in Fig. 4c, nanolaminated silica membranes exhibit exceptional solvent transport rates and outstanding molecular selectivity. Their performance surpasses most reported 2D COF and hybrid COF membranes and approaches the upper bound of state-of-the-art OSN membranes, highlighting the strong potential of 2D mesoporous silica as a next-generation material platform for OSN application. The stability of the membrane was assessed during ethanol filtration and nanofiltration using Alcian Blue 8GX as a model solute (Fig. 5 & Fig. S10). Both solvent permeance and solute rejection were preserved during >7.5 h of continuous operation with a stable flux of ~ 17 L m⁻² h⁻¹ bar⁻¹ and nearly 100 % rejection, indicating stable performance of the silica nanosheet membranes (Fig. 5a). SEM analysis further confirmed minimal structural changes after testing (Fig. 5b).
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<figure><img src="image_12.png" /><figcaption>Fig. 4 | Organic solvent nanofiltration performance of laminated nanoporous silica membranes.</figcaption></figure>

a, Rejection of organic dye molecules in ethanol using C16- and C14-silica membranes (thickness: 3.8 µm), plotted as a function of molecular weight (inset: photographs of dye solutions before and after filtration, illustrating effective separation). Error bars represent the standard deviations of measurements obtained from at least three independently fabricated membranes. b, UV–Vis absorbance spectra of RB and AB8GX during binary dye mixture separation using the C16-silica membrane (inset: photographs of the feed, retentate, and permeate solutions, highlighting selective retention of AB8GX).

c, Comparison of molecular sieving performance in ethanol between nanoporous silica membranes and state-of-the-art OSN membranes. The red dotted line denotes the empirical upper bound of current membrane performance. Detailed data are provided in Supplementary Table S2.
<figure><img src="image_13.png" /><figcaption>**Fig. 5** | Long-term operation stability of laminated silica membranes. a, Stability of the C16 silica membrane during OSN testing using a 25 ppm Alcian Blue 8GX solution in ethanol. b, Cross-sectional SEM image of the silica membrane after dye filtration.</figcaption></figure>

## Conclusion

In summary, we have developed a high-performance organic solvent nanofiltration membrane based on nanolaminated architectures assembled from single layer 2D nanoporous silica nanosheets. By precisely controlling the carbon chain length of surfactant templates during synthesis, we engineered nanosheets with two distinct intrinsic pore sizes and integrated them into well-ordered laminated membranes. The resulting membranes exhibited exceptional solvent permeability and molecular sieving capabilities, outperforming the current upper bound of state-of-the-art OSN membranes. This work demonstrates the versatility of 2D porous silica nanosheets as tunable, chemically robust building blocks for next-generation membrane design. The ability to modulate both pore size and stacking architecture provides a powerful platform for tailoring membrane properties to specific separation demands. These findings open up promising avenues for scalable, energy-efficient molecular separations across diverse sectors including chemical processing, pharmaceuticals, and environmental remediation.

## Experimental section

*Materials.* Myristyltrimethylammonium bromide (C₁₄TAB, 99%), Hexadecyltrimethylammonium bromide (C₁₆TAB, 98%), cyclohexane (99%), ammonium hydroxide (30% in water), tetramethyl orthosilicate (TMOS, 98%), (3-aminopropyl)trimethoxysilane (APTMS, 97%), acetic acid (98%) and N-(9-fluorrenylmethoxycarbonyloxy) succinimide (Fmoc-OSu, 98%), all organic solvents and dye molecules were purchased from Sigma-Aldrich and used without further purification.

*Synthesis of 2D mesoporous silica nanosheets.* Colloidal 2D nanosheets of mesoporous silica were synthesized following a previously reported protocol [23]. The synthesis was carried out using either C₁₆TAB or C₁₄TAB surfactants in order to generate nanosheets with different pore sizes. First, either
250 mg of C₁₆TAB or 230 mg C₁₄TAB (equivalent to 0.68 mmol) was dissolved in 50 mL of water under gentle heating at ca. 60 °C. After cooling to room temperature, 125 µL of ammonium hydroxide and 50 mL of cyclohexane were added, and the mixture was vigorously stirred for 4 hours to maximize the dispersion of cyclohexane microdroplets within the water phase. Next, a mixture of 170 µL of TMOS (1.14 mmol) and 125 µL of APTMS (0.71 mmol) was added dropwise and the reaction was left to proceed overnight at room temperature. The reaction mixture was then centrifuged at 2000 g for 2 minutes, resulting in complete separation of the aqueous and oil phases, with the synthesized material gathered at the interface. The upper and bottom phases were carefully removed using a pipette and the material was dispersed in 100 mL of ethanol with the help of an ultrasonic probe sonicator. The resulting 2D nanosheets were first washed three times with ethanol, each time followed by sonication and centrifugation at 15000 g for 10 minutes. To ensure complete removal of the surfactant, the nanosheets were then dispersed in a mixture of 100 mL ethanol and 500 µL of acetic acid, and left to sit for 30 minutes. They were then washed three more times with ethanol and finally dispersed in 15 mL of ethanol.

*NH₂ screening.* The as-synthesized 2D nanosheets bear surface-exposed amino (-NH₂) groups derived from the APTMS silica precursor. To prevent these functional groups from interfering with the membrane fabrication and performances, these NH₂ groups were screened with N-(9-fluorrenylmethoxycarbonyloxy)succinimide (Fmoc-OSu). To this end, 240 mg of Fmoc-OSu (0.71 mmol) was added to the above described 15 mL dispersion of 2D silica nanosheets, and the mixture was left to stir overnight at room temperature. The nanosheets were then centrifuged, washed three times with ethanol, and finally dispersed in 15 mL of DMSO.

*Fabrication of nanolaminated silica membranes.* The Fmoc-functionalized 2D silica nanosheets solution was first diluted 10-fold with DMSO, followed by sonication for 30 minutes to obtain a uniform colloidal dispersion. This dispersion was then mildly centrifuged at 3000 rpm for 5 minutes to remove potential aggregates. The resulting supernatant was stored for subsequent nanolaminated silica membrane preparation via pressure-assisted filtration. Specifically, volumes ranging from 5 mL to 30 mL of the silica nanosheet solution were filtered through a nylon support membrane (pore size: 220 nm) under a constant pressure of 0.5 bar to achieve membrane thicknesses between 0.5 µm and 5.7 µm. Following membrane formation, 20 mL of ethanol was introduced to rinse away residual DMSO and accelerate the drying process. For subsequent solvent permeation tests and evaluation of their molecular sieving performances, the membranes were used as such without drying them. For characterization of their physico-chemical properties the membranes were dried in air after rinsing with ethanol.

*Solvent permeation through nanolaminated silica membranes.* The solvent permeation behavior of nanolaminated silica membranes was evaluated using eleven representative organic solvents including hexane, cyclohexane, 1-butanol, isopropanol (IPA), DMSO, 1-propanol, ethanol, methanol, tetrahydrofuran (THF), acetonitrile, and acetone. Permeation tests were conducted using 3.8 µm-thick C14- and C16-silica membranes in a dead-end reverse osmosis configuration. In each experiment, the
organic solvent was added to the feed side, and a constant pressure of 0.5 bar was applied to drive solvent transport across the membrane. The mass of permeated solvent was automatically recorded over time using an analytical balance. Solvent flux was determined from the collected mass, and permeance was calculated using equation (1):

$$P_s = \frac{W}{\Delta p A t \rho} \quad (1)$$

Where W is the mass of permeated solvent (kg), Δp is the applied pressure (bar), A is the effective membrane area (m²), t is the permeation time (h), and ρ is the solvent density (kg L⁻¹). All measurements were carried out using membranes with an effective filtration area of 13.8 cm².

*Molecular sieving performance evaluations.* The molecular sieving capabilities of the nanolaminated silica membranes were assessed using a pressure-stirred dead-end filtration cell. Six organic dye molecules with varying molecular weights and charges were selected as model solutes: Rhodamine B (RB), Acid Fuchsin sodium salt (AF), 5,10,15,20-(tetra-N-methyl-4-pyridyl)porphyrin tetrachloride (PP), Brilliant Blue R (BBR), nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NP), and Alcian Blue 8GX (AB8GX). Stock dye solutions (2500 ppm) were initially prepared in water and stored at 4 °C. Before testing, each dye solution was diluted to 25 ppm in ethanol. The diluted solutions were then filtered through 3.8 µm-thick silica membranes under a constant pressure of 0.5 bar. The rejection efficiency R(%) of the membranes was calculated according to equation (2):

$$R(\%) = \left( 1 - \frac{C_p}{C_f} \right) \times 100 \quad (2)$$

where Cₚ and Cₗ are the dye concentration in the permeate and feed solutions, respectively. The permeated dye concentrations were quantitatively analyzed by HPLC coupled with UV-Vis detection. For the binary dye separation test, equal volumes of 25 ppm RB and 25 ppm AB8GX ethanol solutions were mixed to prepare a 12.5 ppm solution of each dye, which was then used as the feed solution. The concentrations of dyes in the feed and permeate were qualitatively assessed based on visible color differences and further confirmed by UV-Vis spectroscopy. The polar/non-polar binary mixtures filtration and long-term operation stability were conducted on a ~6 µm thick C16-silica membrane.

*Characterization methods.* Transmission electron microscopy (TEM) images were acquired using a JEOL-1400 microscope operating at 100 kV from MEA platform, Université de Montpellier. Small-angle X-ray scattering (SAXS) measurements were performed at the Time Resolved Ultra Small X-ray Scattering instrument of the European Synchrotron Radiation Facility (ESRF, beamline ID02), with a photon beam of 12.230 keV and sample-to-detector distance of 1.5 m, resulting to a scattering vector (q=4π sin(Θ)/λ) range of 0.05-5 nm⁻¹. Colloidal samples of 2D silica nanosheets were placed in quartz capillaries of 1.4 mm in diameter and analyzed in forward scattering geometry. All normalizations of SAXS curves were performed by the beamline's standard processing pipeline, then divided by the sample thickness to get the scattering curve in absolute scaling. The scattering from the solvent was
also measured for background subtraction. Infrared spectra were acquired by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy using a Nicolet Nexus spectrometer. Scanning electron microscopy (SEM) images were acquired using a Hitachi S4800 microscope. AFM images were acquired with a Dimension Icon microscope from Bruker working in PeakForce mode. Nitrogen sorption analyses were conducted at 77.3 K using a Tristar II Plus (Micromeritics). The specific surface area was calculated using the Brunauer Emmett and Teller (BET) theory with five points.

## Acknowledgement

The authors acknowledge Franck Godiard and Véronique Viguier from the MEA platform, Université de Montpellier, for the TEM experiments and sample preparation. The authors acknowledge Michel Ramonda for the AFM analyses. The authors acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal numbers sc-5446 and ih-sc-1837. D. V. and W. W. acknowledge funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 804320). D. V. acknowledges support from the Agence Nationale de la Recherche (ANR, program 2D-MEMBA - ANR-21-CE09-0034-01). N. B. acknowledges scholarship from the LabUM Chimie of the University of Montpellier, supported by the Agence Nationale de la Recherche within Investissement d'Avenir program under reference ANR-16-IDEX-0006. L. L. acknowledges funding from the Spanish Ministerio de Economía y Competitividad, the Ministerio de Ciencia e Innovación, la Agencia Estatal de Investigación, MCIN/AEI/10.13039/501100011033/FEDER UE, and the European Union "NextGenerationEU"/PRTR (PID2022-140370NB-I00, RYC2021-033764-I, CPP2021-008986, CNS2024-154587).

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## Author Contributions

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‡ W.W. and N.B. contributed equally. The manuscript was written through the contributions of all authors.

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## Data availability

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578

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Data from ESRF can be found online at https://doi.esrf.fr/10.15151/ESRF-ES-1371496482 and https://doi.esrf.fr/10.15151/ESRF-ES-1612548491.

579
580 Competing financial interests

581 The authors declare no competing financial interests.

582

583 Additional Information

584 **Supplementary information** and chemical compound information are available in the online version
585 of the paper.

586 **Correspondence and requests for materials** should be addressed to T.A. and D.V.
587 (tangi.aubert@umontpellier.fr; damien.voiry@umontpellier.fr).

588

589 **Supporting Information:** Additional experimental details, materials, and methods; AFM, TEM, XPS, 590 FTIR, and TGA characterizations of silica nanosheets; SEM cross-sectional view of the silica 591 membranes; Supporting Tables of dyes properties and membranes performance comparison; and 592 references (PDF)