Method for preparing a separating membrane with transition metal dichalcogenide nanoparticles

Abstract

The present invention relates to a method for preparing a separating membrane, which method comprises: a) a step of preparing nanoparticles of a sulphide of one or more transition metals chosen from Mo, V and W, comprising a step of the thermal decomposition of: - at least one organic or mineral transition metal precursor and at least one sulphur precursor; and / or - at least one organic transition metal-sulphur precursor; in the presence of at least one organic stabilising agent so as to obtain nanoparticles in the form of monolayers; b) a step of depositing the nanoparticles prepared in step a) in the form of a selective layer on a support membrane, in particular: - by direct filtration of a solution containing said nanoparticles on the support membrane; or - by immersion of the support membrane in a solution containing said nanoparticles.

Description

[0001] METHOD FOR PREPARING A SEPARATION MEMBRANE WITH TRANSITION METAL DICHALCOGENIDE NANOPARTICLES

[0002] technical field

[0003] The present invention relates to the preparation of membranes incorporating nanoparticles of (dis)sulfides of metal / transition metals, in particular M0S2.

[0004] It also relates to the use of these nanoparticle membranes in the field of filtration and selective separation.

[0005] Previous technique

[0006] In the fields of filtration and selective separation, many applications in pollution control, desalination, industrial effluent treatment for metal recovery, and energy production are currently limited by the performance of ion-exchange membranes. Existing polymer membranes cannot achieve the separation levels required for many water and energy technologies because they are constrained by the permeability-selectivity trade-off, whereby permeability tends to increase as selectivity tends to decrease. This limitation stems from the often insufficient control of membrane material properties at the (sub)nanometric scale.

[0007] Thin-film nanocomposite (TFN) polymeric membranes are generally manufactured using an interfacial polymerization process. This involves immersing a polymer-based support, such as polyethersulfone, in an aqueous diamine solution, like piperazine. After rinsing, the support is then contacted with an organic trimesoyl chloride solution. This results in the formation of a thin polyamide layer on the surface of the support, ranging in thickness from a few tens to a few hundred nanometers. [Dan Li and Huanting Wang J. Mater. Chem., 2010, 20, 4551-4566 “Recent developments in reverse osmosis desalination membranes” DOI: 10.1039 / b924553g] This type of process has significantly improved material performance, particularly due to the very thin film thicknesses that can be achieved.However, some applications, such as those related to water treatment or energy production technologies, require further improvement in these performances.

[0008] Improved performance of polymeric membranes has been achieved by adding an inorganic filler to the polymer matrix, particularly a filler in the form of two-dimensional nanoparticles. These can include nanoparticles of graphene or graphene derivatives, boron nitride, or transition metal chalcogenides, especially transition metal sulfides (TMS).

[0009] According to a known preparation method, a suspension (stable solution) of nanoparticles is filtered under vacuum through a polymeric membrane used as a support. The resulting thin, lamellar layer theoretically possesses solute retention properties through a molecular sieving process. Numerous studies have highlighted the benefits obtained with this type of material. For more details on this preparation method, see in particular the following publications: “MoS2-based lamellar membranes for mass transport applications: Challenges and opportunities” Journal of Environmental Chemical Engineering 11 (2023) 109329. Lucie Ries, Eddy Petit, Thierry Michel, Cristina Coelho Diogo, Christel Gervais, Chrystelle Salameh, Mikhael Bechelany, Sébastien Balme, Philippe Miele, Nicolas Onofrio and Damien Voiry, and “Enhanced sieving from exfoliated M0S2 membranes via covalent functionalization” Nature Materials VOL 18 1112 OCTOBER 2019 1112-1117.

[0010] Among the types of nanoparticles that could be considered, chalcogenides, and in particular molybdenum disulfide, are especially interesting because of their intrinsic properties that allow them to efficiently retain ions. Unlike the graphene family, they do not cause swelling of the nanoparticle layer upon contact with water, a swelling that tends to increase the space between the nanoparticle layers and, consequently, a significant loss of separation performance.

[0011] As described previously, the methods for preparing these thin films use particle dispersions, usually in aqueous phase, which are filtered onto a polymeric substrate. Obtaining these dispersions is a lengthy, complex process that is difficult to scale up.

[0012] Indeed, the nanoparticle synthesis step can be carried out via different routes, notably bottom-up or top-down, as is known in the field of nanomaterial manufacturing processes.

[0013] These particles can then be dispersed in an aqueous phase by a step called exfoliation, which aims to dissociate the platelet particles so that they do not precipitate in the liquid medium. This exfoliation step can be carried out physically (mechanosynthesis or ball milling, also known as "bail milling," or ultrasonication, for example) or chemically (using n-butyllithium, for example). A separation step is then generally performed to retain only the supernatant, for example, by centrifugation or dialysis.

[0014] Physical methods are very energy-intensive and do not allow for sufficiently concentrated nanoparticle dispersions, necessitating the use of large solution volumes. Furthermore, the dispersed state of the nanoparticles is not complete, as they are mostly found in a stacked form ranging from a few platelets to a few dozen platelets. Chemical methods, on the other hand, require working in a controlled atmosphere because n-butyllithium is a highly pyrophoric compound that hydrolyzes rapidly in air, significantly complicating the synthesis.

[0015] Recently, it has been shown that the more or less stacked state of TMS platelet nanoparticles (such as M0S2) plays a role in separation performance. For example, see the publication by Xinglin Lu, Uri R. Gabinet, Cody L. Ritt, Xunda Feng, Akshay Deshmukh, Kohsuke Kawabata, Masashi Kaneda, Sara M. Hashmi, Chinedum O. Osuji, and Menachem Elimelech in Environ. Sci. Technol. 2020, 54, 9640-9651 “Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (M0S2) Membranes to Nanosheet Stacking Behavior”. Indeed, during deposition on the polymeric substrate, stacked nanoplatelets are likely to generate large structural defects (notably in the form of microchannels), which can prove prohibitive for obtaining good performance.

[0016] Composite membranes using TMS-type platelet nanoparticles thus have strong potential in terms of separation performance, but they are limited by the need to control the stacking state of the nanoparticles during synthesis: known preparation processes remain difficult to implement on a large scale, and do not allow reaching a sufficient exfoliated state corresponding to particles in the form of a monosheet.

[0017] As an illustration, one can refer to patent application CN103585896, which describes a method for exfoliating commercial M0S2 nanoparticles mixed for 24 to 48 hours in an n-BuLi solution in hexane under a controlled atmosphere. After filtration and washing with hexane, the solid is contacted with water, agitated, and subjected to ultrasound for 1 hour. The suspension is then centrifuged to remove large aggregates, and the supernatant is finally purified by dialysis for 4 days.

[0018] Reference can also be made to patent application CN116510541 A, which describes a method for synthesizing a composite membrane by interfacial polymerization: M0S2 nanoparticles, the synthesis of which is not specified, are functionalized with propane sulfone and mixed with the trimesoyl chloride organic phase after being centrifuged and washed several times with ethanol. This mixture is then contacted with the diamine (piperazine) during interfacial polymerization to form the thin film.

[0019] Patent application W02022010960A1, moreover, proposes the functionalization of M0S2 nanoparticles used to manufacture a selective membrane: Carboxylic acid functions covalently grafted to the surface of previously exfoliated M0S2 nanoparticles make it possible to obtain a thin layer which would be more homogeneous and which would have interlayer spaces favorable to the separation of ions.

[0020] The invention is therefore intended to remedy the aforementioned drawbacks.

[0021] Its aim is in particular to develop an improved method of preparing membranes containing or supporting transition metal disulfide nanoparticles, in particular molybdenum, vanadium or tungsten sulfide, a method of preparation which is, in particular, easier to implement, with better control of the quality and stacking state of the nanoparticles deposited on / integrated into the membrane.

[0022] It also aims to obtain separation membranes incorporating these nanoparticles that are more efficient.

[0023] Summary of the invention:

[0024] The invention is, firstly, a method for preparing a separating membrane, which includes

[0025] - a) a step of preparing transition metal(s) sulfide nanoparticles, in particular selected from Mo, V and W, comprising a thermal decomposition step

[0026] - of at least one organic or mineral precursor of the transition metal and at least one sulfur precursor

[0027] - and / or at least one organic precursor of both the transition metal and sulfur,

[0028] - in the presence of at least one organic stabilizing agent in order to obtain nanoparticles mainly composed of monolayers,

[0029] - b) a step of depositing said nanoparticles prepared in step a) to form a so-called selective thin layer on a support membrane, in particular

[0030] - by direct filtration, possibly under pressure higher or lower than atmospheric pressure, of a solution containing said nanoparticles onto said support membrane,

[0031] - or by immersing the support membrane in a solution containing said nanoparticles, said solution preferably containing a concentration of nanoparticles between 0.05 and 5 mg / ml, in particular between 0.05 and 3 mg / ml, or between 0.1 and 3 mg / ml or between 0.1 and 2 mg / ml or between 0.2 and 1 mg / ml, or between 0.4 and 0.8 mg / ml.

[0032] In cases where at least one organic precursor of both the transition metal and sulfur is used, a preferred embodiment is to choose a precursor having a bond between the transition metal and sulfur, for example between molybdenum Mo and sulfur S when the aim is to obtain molybdenum disulfide nanoparticles MoS2. This could be, for example, a transition metal dithiocarbamate, in particular molybdenum.

[0033] According to one embodiment, the organic precursor of both the transition metal and sulfur (also called organosoluble) is from the family of oxymolybdenum dithiocarbamates corresponding to the general formula (I) or from the family of oxymolybdenum dithiophosphates corresponding to the general formula (II),

[0034] (I) in which the radicals R1, R2, R3, R4 are independently selected from linear or branched alkyl groups, in C1 to C12, cycloalkyl groups in C6 to C12, and aryl or alkyl-aryl groups comprising in C6 to C12, preferably identical linear alkyl groups in C1 to C6 or identical branched alkyl groups in C1 to C12, preferably the first metallic precursor is molybdenum dibutyldithiocarbamate.

[0035] According to one embodiment, the organic precursor of both the transition metal and sulfur (also called organosoluble) is chosen from the list consisting of molybdenum dimethyldithiocarbamate, molybdenum diethyldithiocarbamate, molybdenum dipropyldithiocarbamate, molybdenum dibutyldithiocarbamate, molybdenum dipentyldithiocarbamate, molybdenum dihexyldithiocarbamate, molybdenum dimethylphosphorodithioate, molybdenum diethylphosphorodithioate, molybdenum dipropylphosphorodithioate, molybdenum dibutylphosphorodithioate, molybdenum dipentylphosphorodithioate, molybdenum dihexylphosphorodithioate, and molybdenum di(2-ethylhexyl)phosphorodithioate.

[0036] The process according to the invention preferably provides that step a) of nanoparticle preparation and / or step b) of nanoparticle deposition are preferably carried out in liquid phase.

[0037] Step a) of preparing the nanoparticles according to the invention therefore consists of carrying out a thermal decomposition of organic precursor(s) constituting the source(s) of transition metal, such as Mo, and the source(s) of sulfur in the presence of one or more stabilizing agents.

[0038] This step a) allows the synthesis of nanoparticles, particularly in suspension in a liquid phase as detailed later, using a bottom-up approach. A significant advantage is that these nanoparticles can be found primarily in the form of nanometric monosheets: a preliminary exfoliation and separation step is no longer necessary to select the predominantly monosheet nanoparticles before using them to prepare the separation membrane.

[0039] The synthesis according to the invention makes it possible to obtain this structure in isolated (non-stacked) monolayers through a combination of operating parameters including thermal decomposition at a relatively low temperature and the addition of a stabilizing agent, particularly in the form of oleylamine, which limits / controls particle growth by preventing aggregation. The isolated nanoparticles thus obtained assemble perfectly homogeneously upon deposition on the nanofiltration support membrane (for example, made of PES) without creating stacking defects that would lead to the loss of separation properties.

[0040] "Mostly" is understood to mean the usual meaning of this term, namely at least 50% of the nanoparticles, specifically more than 50% of the nanoparticles.

[0041] Advantageously, these nanoparticles can be substantially all in the form of monosheets. They can thus be composed of at least 70%, and in particular at least 80, 90, or 95%, in the form of monosheets.

[0042] At the end of step a), the nanoparticles are generally in a liquid phase, which is an excess of stabilizing agent in liquid form or in a stabilizing agent / solvent mixture when a solvent has been used. The nanoparticles can then be deposited onto the support membrane in step b) by directly using these nanoparticles in the liquid phase, which is very advantageous (possibly after first diluting or concentrating the liquid solution containing the nanoparticles).

[0043] It is also possible to perform a nanoparticle separation step at the end of step a) (separation between the nanoparticles and the organic stabilizing agent or the organic stabilizing agent / organic solvent mixture), and possibly a washing and / or drying step for the separated nanoparticles. The separation step can be any liquid-solid separation method known to those skilled in the art, and is preferably carried out by centrifugation or filtration. They are then preferably returned to solution to proceed to step b).

[0044] In step a) of nanoparticle preparation, the organic precursor of the transition metal in the form of molybdenum and sulfur can be chosen from at least one of the following compounds:

[0045] - a compound A with formula [R2N-XS-S] y -Mo mOnSp in which R is an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms, with m between 1 and 2, with n between 0 and 2, with p between 0 and 4, with y between 2 and 4, with X a carbon atom or a phosphorus atom,

[0046] - a compound B from the molybdenum xanthate family with the formula (ROS2)4Mo.

[0047] In this embodiment, there can be only one precursor at a time of both the transition metal and sulfur.

[0048] - a compound C from the molybdenum dithiocarbamate family

[0049] - a compound D from the molybdenum dithiophosphate family

[0050] - a compound E from the molybdenum dithioimidophosphinate family

[0051] - a compound F from the family of molybdenum dithioimidophosphates.

[0052] In step a) of nanoparticle preparation, the mineral precursor of the transition metal vanadium, tungsten or molybdenum can be chosen from at least one of the following compounds: a molybdate, vanadate or tungstate, in particular of ammonium or sodium.

[0053] In step a) of nanoparticle preparation, the organic precursor of the transition metal in the form of molybdenum can also be chosen from at least one of the following compounds:

[0054] - a compound G of the molybdenum stearate family with the formula (RCC^Mo

[0055] - a compound H from the family of molybdenum nathenates with the formula (RCC^Mo. Note that the meaning / chemical formulation of the radicals R of compounds B, G and H is the same as for compound A, namely an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms.

[0056] In this embodiment, in step a) of nanoparticle preparation, the sulfur precursor can then be chosen

[0057] - in the form of an organic sulfur precursor, selected from at least one of the compounds of the alkyl thiourea family, in particular of formula (R2NHSNHR2), or of the thiourea family, in particular of formula H2NSNH2, or of the carbamide family, in particular of formula R1R2NSR3R4 or of the thioacetamide family, in particular of formula CH3SNH2 or of formula R1SNR2R3 with R2 and R2, R3 and R4 being alkyl or aryl groups of 1 to 18 carbon atoms,

[0058] - and / or in the form of a mineral sulfur precursor, in particular in the form of ammonium sulfide.

[0059] - in the form of an organic sulfur precursor, selected from at least one of the compounds of the alkyl thiourea family, in particular of formula (R2NHSNHR2), with R2 and R2' being alkyl or aryl groups of 1 to 18 carbon atoms, or a mineral sulfur precursor, or

[0060] - in the form of a mineral precursor, notably in the form of ammonium sulfide.

[0061] A precursor of the transition metal is then associated with (at least) such a precursor of sulfur.

[0062] In step a) of nanoparticle preparation, the organic stabilizing agent can be chosen from at least one of the following compounds:

[0063] - an alkylamine chosen from primary and secondary amines having a hydrocarbon chain preferably comprising between 4 and 34 carbon atoms,

[0064] - a primary, secondary, tertiary or quaternary alkylamine comprising at least one hydrocarbon chain preferably comprising between 12 and 18 carbon atoms, and optionally at least one unsaturation, in particular octylamine, dodecylamine, hexadecylamine, octadecylamine, oleylamine,

[0065] - a quaternary alkylamine selected from cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB),

[0066] - a cyclic polyamine, in particular polyvinylpyrrolidone (PVP),

[0067] - an alkali stearate or naphthenate, in particular sodium, in particular CigCC^Na,

[0068] - an alkylthiol comprising a hydrocarbon chain of 6 to 18 carbon atoms, including 1-hexanethiol, 1-octanethiol, nonanethiol, 1-dodecanethiol, 1-hexadecanethiol,

[0069] - a carboxylic acid, comprising in particular 6 to 18 carbon atoms, including citric acid, octanoic acid, decanoic acid, palmitic acid, oleic acid, - a phosphine, including tributylphosphine, trioocytylphosphine, detriooctylphosphine oxide, triphenylphosphine,

[0070] - an alkyl(s) sulfonate, in particular of formula RSO3R', with R and R' being alkyl or aryl groups of 1 to 18 carbon atoms.

[0071] In step a) of nanoparticle preparation, thermal decomposition is preferably carried out at a temperature between 100 and 350°C, in particular between 150 and 230°C or between 160 and 220°C, preferably at 200°C.

[0072] It is therefore observed that the thermal decomposition recommended in step a) according to the invention can be carried out at ambient or moderate temperature, in particular depending on the choice of precursor and stabilizer.

[0073] Note that the decomposition of the stabilizing agent(s) is not necessarily complete.

[0074] In step a) of nanoparticle preparation, thermal decomposition is preferably carried out for a period of 30 minutes to 24 hours, in particular for a period of one to three hours.

[0075] In step a) of nanoparticle preparation, the thermal decomposition of the organic molybdenum precursor(s) in the presence of the stabilizing agent(s) is preferably carried out in liquid phase in an aqueous or organic solvent, or in the absence of solvent (the metallic precursor and / or the stabilizing agent being in liquid form).

[0076] In the case of decomposition carried out in a solvent, the latter may be chosen from at least one of the following organic solvents: an organic solvent with a boiling point above 100°C, preferably chosen from the list consisting of toluene, ethylbenzene, xylene, mesitylene, decane, and dodecane.

[0077] An active mixing, for example a mixing by magnetic stirring, mechanical stirring or any other means of stirring, can be implemented at the decomposition stage, in order to ensure good mixing of the compounds and good formation of the nanoparticles.

[0078] Advantageously, in step a) of nanoparticle preparation, the ratio r mThe mass ratio between the mass of molybdenum in the organic molybdenum precursor(s) and that of the stabilizing agent(s), particularly when carried out in an aqueous or organic solvent, can be between 1 / 50 and 1 / 400, specifically between 1 / 100 and 1 / 350 or between 1 / 150 and 1 / 300. It has been observed that by controlling various parameters of the thermal decomposition (nature and quantity of the precursor, nature and quantity of the stabilizing agent, temperature, duration, etc.) it is possible to control the size and stacking of the nanoparticles obtained.

[0079] In step a) of nanoparticle preparation, thermal decomposition can be continued until nanoparticles are obtained, in particular in liquid phase, including at least M0S2 nanoparticles in the form of isolated monosheets, said nanoparticles being in particular of average size between average size between 1 nm and 25 nm, preferably between 1 nm and 10 nm, preferably between 1 nm and 5 nm or between 2 and 6 nm, in particular between 3 and 5 nm.

[0080] According to the invention, the nanoparticles, after thermal decomposition, can be functionalized by grafting functional groups or organic molecules onto their surface, in particular selected from

[0081] - organic halides, particularly iodomethane or iodoacetamide,

[0082] - thiols, mercapto-alkylamines, cetyltrimethylammonium bromide (CTAB),

[0083] - sugars, particularly sucrose,

[0084] - tannic acids

[0085] - polyphenols,

[0086] - dyes, such as dyes known by their Anglo-Saxon names

[0087] “Crystal violet (CV)”, “sunset yellow (SY)”, and “neutral red (NR)”

[0088] - NMethylpyrrolidone,

[0089] - lithium butyl,

[0090] - polymers, such as Polylactic Acid (PLA) and Polyvinyl Alcohol (PVA)

[0091] - amino acids.

[0092] For example, one can refer to the publication "Desalination and Nanofiltration through Functionalized Laminar M0S2 Membranes" by Wisit Hirunpinyopas et al. published in ASCNANO on October 11, 2017 (https: / / doi.org / 10.1021 / acsnano.7b05124) for the description of membranes with MoS2 functionalized with dyes.

[0093] After thermal decomposition, nanoparticles can be functionalized by grafting.

[0094] - either during step a) of nanoparticle preparation,

[0095] - either during step b) of deposition of said nanoparticles prepared in step a) in the form of a so-called selective layer on a support membrane,

[0096] - either between said step a) of preparation and said step b) of deposit,

[0097] - either after step b) of deposition. In fact, the aim is to functionalize the sheet-like nanoparticles with a compound capable of binding to the surface of the sheets: the functional groups or organic molecules thus grafted onto the sheets, for example in M0S2, create a spacing between the sheets, thereby modifying the inter-sheet distance. Grafting is understood in a broad sense, including the adsorption of these functional groups / molecules onto the surface of the nanoparticles.

[0098] The process for preparing a separating membrane according to the invention may also include a step c) of depositing a layer comprising at least one polymer, in particular from the polyamide family, after step b) of depositing the nanoparticles or concomitant with step b) of depositing the nanoparticles.

[0099] The optional step c) of depositing a layer comprising at least one polymer can be carried out by interfacial polymerization, in particular from an organic diamine solution and an aqueous trimesoylchloride solution, with possible addition of nanoparticles in one and / or the other of said solutions in the case where steps b) and c) are concomitant.

[0100] Once steps a), b) and possibly c) have been carried out, a step d) of drying the resulting separating membrane can be carried out, in particular at room temperature or at a temperature of no more than 100°C, in particular in a flow of air or neutral gas.

[0101] According to one embodiment of the invention, it can be envisaged that step b) of deposition is carried out in several stages by several successive deposition operations.

[0102] According to another embodiment of the invention, the sequence of steps comprising at least step b), possibly step c) when provided, and step d) can be provided for, to be repeated at least once, for example several times.

[0103] These two embodiments aim to deposit the selective layer in several stages, increasing the thickness of the layer at each repetition of step b) until the desired thickness is reached, with or without intermediate treatment such as drying d) between each deposition step b).

[0104] The separator membrane obtained at the end of step b), c), or d) can be stored under humid or dry conditions. The choice between dry and humid storage conditions may depend, in particular, on the type of filtration technique that will use the membrane.

[0105] The invention also relates to a separating membrane, in particular obtained by the process described above, and which comprises:

[0106] - a support membrane, made of a material chosen from among flat organic microfiltration or ultrafiltration membranes or ceramic mineral membranes or in the form of hollow fibers, preferably with a thickness between 50 and 300 µm, in particular between 80 and 200 µm,

[0107] - a so-called selective layer on said membrane comprising transition metal(s) sulfide nanoparticles, in particular chosen from Mo, V and W, of a thickness preferably between 0.05 and 10 pm, and in particular between 1 and 5 pm.

[0108] Advantageously, the separating membrane in question has the so-called selective layer comprising the sulfide nanoparticles such that said nanoparticles are composed of at least 70%, in particular at least 80 or 90 or 95% of isolated monosheets, and preferably of average size between 1 and 25 nm or between 1 and 10 nm, in particular between 2 and 6 nm or between 3 and 5 nm.

[0109] The support membrane can therefore be chosen from microfiltration or ultrafiltration membranes; it can be in flat form or in the form of hollow fibers, or even from porous mineral membranes, particularly oxide(s) from the silica family, alumina, titanium dioxide). Among the organic membranes that may be suitable are polyamide (PA) membranes, nylon for example, polyvinyl fluoride, cellulose acetate, polysulfones, polyvinyl alcohol (PVA), polyethersulfone (PES), polyimide, polyacrylonitrile (PAN), polyether ether ketones (PEEK), polypropylene (PP), polydimethylsiloxane (PDMS).

[0110] It turned out that the selective nanoparticle-based layer deposited on the membrane was substantially free of monolayer stacking defects, in particular free of micro-channel type defects, which in fact led to a membrane with high separation performance, in terms of permeability and solute retention.

[0111] The membrane obtained can advantageously be used in many separation techniques, including, in particular, so-called frontal filtration, so-called tangential filtration, forward and reverse osmosis, pressure-retarded osmosis (known by the acronym PRO for the English term "Pressure Retarded Osmosis").

[0112] List of figures

[0113] Figure 1 shows a high-resolution transmission microscopy image of M0S2 nanoparticles obtained by the process according to the invention (according to all the examples of the invention described below). The nanoparticles are exclusively in the form of single slabs (i.e., not stacked on top of each other), with an average size of 4 + / - 1.8 nm. Figure 2 shows a histogram of the size distribution of M0S2 nanoparticles obtained by the process according to the invention (according to all the examples of the invention described below), with the x-axis representing the size of the nanoparticles in nm, the nanoparticles being in the form of M0S2 single slabs.

[0114] Figure 3 shows the X-ray diffraction (XRD) patterns of a membrane fabricated by exfoliating commercial M0S2 particles (curve C1) and of the membrane according to Example 2 of the invention described below (curve C2). The M0S2 peak at approximately 14.3° corresponding to (002) (spacing d = 6.2 Å between the M0S2 layers) is observed for the membrane fabricated using exfoliated commercial M0S2 (Example 7). This peak is absent, at least up to 10° (spacing d 8.82 Å), in the case of the membrane of the invention (Example 2), which confirms the monolayer profile of the particles deposited on the membrane. The homogeneity of this layer produces highly selective membranes. We also observed that the heterogeneous particles obtained by exfoliating commercial M0S2 produced a disordered deposited layer with voids that compromised selectivity. This was not the case with our invented membrane.

[0115] Figure 4 represents four images by High Resolution Transmission Microscopy, two images of a section of a membrane according to the invention (example 2) with and without magnification on an area of ​​the nanoparticle layer of the membrane, and of a section of a membrane according to comparative example 7, also with and without magnification.

[0116] Figure 5 shows a graph with the quantity of M0S2 (in mg) in the solution used to prepare the membrane on the x-axis, and the thickness (in microns) of the M0S2 layer obtained according to Example 6, detailed above, on the y-axis. This demonstrates that it is also possible to control the layer thickness, which can be used to highlight the invention. The black dots represent experimental (measured) thickness (y) vs. the quantity of M0S2 used (x). The line is the "fit" of this point, and the value R refers to Pearson's correlation coefficient, denoted R. It measures the strength and direction of the linear relationship between two variables on a scatter plot. R=1 indicates a perfect positive linear relationship

[0117] Description of embodiments The invention will be described in more detail below with the aid of figures and non-limiting examples of the process according to the invention for preparing molybdenum disulfide M0S2 nanoparticles.

[0118] It should be noted that the invention applies in a similar way to a sulfide of another transition metal, such as, for example, V or W.

[0119] The nanoparticle preparation process according to the invention may use only one metallic precursor and only one stabilizing agent, as illustrated in the following examples, or use different precursors of the same metal or of different metals, and / or use several different stabilizing agents.

[0120] Examples

[0121] Example 1: Synthesis of M0S2 nanoparticles

[0122] A solution composed of 1.2 g of molybdenum dibutyldithiocarbamate (the organic precursor of molybdenum) and 70 mL of hexadecylamine (a stabilizing agent) was heated under reflux at 200°C for 1 h with argon flow and continuous stirring. The resulting solution contains 1 mg Mo / mL and was diluted with 100 mL of isopropanol, reducing its concentration to 0.6 mg / mL. M0S2 nanoparticles are obtained in the form of monolayers suspended in a liquid phase of isopropanol and hexadecylamine. This suspension will be used to fabricate the selective layer of the sieving membrane, also known as the separation membrane.

[0123] The molybdenum dithiocarbamate precursor, a precursor of Mo and S in the same compound, decomposes at relatively low temperature, forming lamellar M0S2 nanoparticles. The presence of a stabilizing agent (an oleylamine-type amine) in the medium limits particle growth, resulting in nanosheets of approximately 4 nm, and also prevents their aggregation.

[0124] The isolated nanoparticles thus obtained assemble during deposition on the nanofiltration support membrane (PES) in a perfectly homogeneous manner without the creation of stacking defects, as shown later.

[0125] Example 2: Obtaining the selective layer membrane by vacuum filtration

[0126] The separation membrane was fabricated by filtering 15 mL of the diluted solution obtained in Example 1 through a porous substrate (polyethersulfone (PES), 100 µm thick, 30 nm pore size) under low vacuum conditions. The solution was filtered through the support at an initial AP = -0.1 bar for 2–3 min, then the vacuum was increased to AP = -0.5 bar. After all the solution had been filtered, isopropanol was added and filtered through the membrane to remove excess hexadecylamine. The resulting material was either dried at room temperature or stored under humid conditions. Wet membranes were immediately sealed in sample plastic bags containing water-soaked fabric to maintain high humidity. Dried membranes were simply stored at room temperature.The wet membranes did not exhibit monovalent salt rejection according to the test results, but showed very high retention rates in tartrazine nanofiltration. The resulting membranes were a few micrometers thick (1 to 5 µm depending on the volume of solution containing the M0S2 nanoparticles used).

[0127] Example 3: Obtaining the selective layer membrane by immersion ("dip coating")

[0128] The solution containing the synthesized M0S2 nanoparticles obtained in Example 1 was used to fabricate a membrane using the dip coating method: a porous PES substrate, similar to that used in Example 2, was immersed in 12 mL of the solution obtained in Example 1 for 4 min. The wet membrane was then placed in an oven at 50°C and heated for 40 min. The resulting membrane was rinsed with ethanol and allowed to dry at room temperature. The thickness of the resulting membrane, observed by scanning electron microscopy, ranged from 0.1 to 1 µm.

[0129] Example 4: Synthesis of functionalized M0S2 nanoparticles

[0130] To modify the intercalated space between the M0S2 membrane layers, the nanoparticles from the solution obtained in Example 1 were functionalized with diiodomethane. Functionalization was achieved by adding an excess of diiodomethane to the M0S2 solution. The resulting solution was then stored in a closed environment for 48 hours before membrane fabrication.

[0131] Example 5: Obtaining the selective layer membrane by vacuum filtration with functionalized M0S2 nanoparticles

[0132] The operating procedure of example 2 with vacuum filtration (AP = -0.1 bar) was repeated with the functionalized nanoparticle solution obtained in example 4.

[0133] Example 6: Obtaining the selective layer membrane by vacuum filtration with variation of the nanoparticle layer thickness

[0134] The thickness of the M0S2 layer can be controlled by varying the amount of M0S2 nanoparticles used during the vacuum filtration step: several tests were carried out with M0S2 solutions of increasing concentration. A 5 mL solution containing 3 mg of M0S2 resulted in an M0S2 layer approximately 1 µm thick. A membrane 6 µm thick was obtained by increasing the amount of solution filtered (20 mL with 12 mg of M0S2).

[0135] Figure 5 shows a graph with the quantity of MoS2 (in mg) in the solution used to prepare the membrane on the x-axis, and the thickness (in microns) of the MoS2 layer obtained according to Example 6 on the y-axis. The points are the experimental (measured) points of thickness (y) versus the quantity of MoS2 in the solution (x). The line represents the "fit" of this point, and the R value refers to Pearson's correlation coefficient, denoted R. It measures the strength and direction of the linear relationship between two variables on a scatter plot. R=1 indicates a perfect positive linear relationship.

[0136] This graph illustrates the (quasi) linear relationship between the concentration of Mo in the solution and the thickness of the final thin layer of nanoparticles on the support membrane of example 6: This graph demonstrates that it is possible, with the process of the invention, to precisely control the thickness of the layer of nanoparticles deposited on the support membrane, which is very interesting technically, with a view to extrapolation on an industrial scale, to guarantee the reproducibility of the characteristics of the layers on the membrane.

[0137] Example 6': Synthesis of M0S2 nanoparticles

[0138] A solution composed of 4 g of molybdenum naphthenate (the organic precursor of molybdenum), 0.4 g of thiourea (the precursor of sulfur), 70 mL of hexadecylamine (a stabilizing agent) and 50 mL of isopropanol was heated under reflux at 200°C for 1 h under argon flow and with continuous stirring.

[0139] The resulting solution contains 0.5 mg Mo / mL and was diluted with 50 mL of isopropanol, reducing its concentration to 0.6 mg / mL.

[0140] M0S2 nanoparticles are obtained in the form of monosheets suspended in a liquid phase consisting of isopropanol and hexadecylamine.

[0141] This suspension is then used to manufacture the selective layer of the sieving membrane, also known as the separation membrane.

[0142] Example 7: Comparative Example

[0143] For comparison, a membrane using commercial M0S2 was fabricated using the same PES support as in Example 2. To exfoliate the M0S2, 400 mg of M0S2 was placed in a grinding jar with 0.5 ml of isopropanol. Grinding was performed for 30 min at a frequency of 25 s -1The paste was recovered in 100 mL of isopropanol. The solution was then subjected to ultrasonics using a commercially available Vibra-Cell™ VCX,750 device for 2 hours with a 20-second pulse and a 10-second pause (total duration 3 hours) and an amplitude of 40%, and then centrifuged to remove unexfoliated solids at 4000 rpm for 90 min. 160 mL of the supernatant were used to fabricate the membrane using the method described in Example 2. The resulting M0S2 membrane has a thickness of approximately 1 µm.

[0144] Figure 3 shows the X-ray diffraction (XRD) patterns of a membrane made by exfoliation of commercial MoS2 particles according to comparative example 7 (curve C1) and of the membrane according to example 2 of the invention (curve C2):

[0145] - The MoS2 peak at approximately 14.3° corresponding to (002) (spacing d = 6.2 Å between MoS2 sheets) is observed for the membrane made using commercial exfoliated MoS2 (curve C1)

[0146] - This peak is absent, at least up to 10° (spacing d 8.82 Â), in the case of the membrane according to the invention (curve C2), which confirms the monolayer profile of the particles deposited on the membrane according to the invention.

[0147] The homogeneity of this layer of nanoparticles obtained according to the invention produces highly selective membranes.

[0148] It has also been observed that the heterogeneous particles obtained by exfoliation of commercial M0S2 (comparative example 7) produce a deposited layer that is disordered, and which, in particular, has voids that compromise the selectivity of the membrane: this is represented in figure 4:

[0149] - In the upper left (image a), a cross-section of the membrane shown in Example 2 of the invention is shown. To the right of this image is a magnification of image a through the thickness of the nanoparticle layer. The layer can be seen to be homogeneous and continuous.

[0150] - In the lower left (image b), the cross-section of the membrane is shown in its thickness according to comparative example 7, and, on the right, a magnification of this image through the thickness of the nanoparticle layer. The layer is observed to be disordered and discontinuous, with gaps indicated by the white arrows.

[0151] Example 8: Performance evaluation of membranes in 1-day Direct Osmosis and Frontal Nanofiltration tests

[0152] The desalination performance of the membrane was tested using the osmotic pressure method with an H-cell configuration similar to studies described in the literature (See in particular the following publication:

[0001] RK Joshi, P. Carbone, FC Wang, VG Kravets, Y. Su, IV Grigorieva, HA Wu, AK Geim, RR Nair, Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes, Science 343 (2014) 752-754. https: / / doi.org / 10.1126 / science.1245711.

[0153] [2] W. Hirunpinyopas, E. Prestat, SD Worrall, SJ Haigh, RAW Dryfe, MA Bissett, Desalination and Nanofiltration through Functionalized Laminar M0S2 Membranes, ACS Nano 11 (2017) 1 1082-11090. https: / / doi.org / 10.1021 / acsnano.7b05124.

[0154] [3] L. Ries, E. Petit, T. Michel, CC Diogo, C. Gervais, C. Salameh, M. Bechelany, S. Balme, P. Miele, N. Onofrio, D. Voiry, Enhanced sieving from exfoliated M0S2 membranes via covalent functionalization, Nature Mater. 18 (2019) 11 12-1 117. https: / / doi.Org / 10.1038 / S41563-019-0464-7.

[0155] [4] B. Sapkota, W. Liang, A. Vahid Mohammadi, R. Karnik, A. Noy, M. Wanunu, High permeability sub-nanometer sieve composite M0S2 membranes, Nature Commun. 11 (2020) 2747. https: / / doi.Org / 10.1038 / S41467-020-16577-y.

[0156] The membrane was placed between two solutions (feed and withdrawal), and ion diffusion was monitored by measuring conductivity. Magnetic stirring was used to minimize concentration polarization. The active layer of the membrane faced the solution containing the salt, whether it was used for feeding or withdrawal. Rejection was calculated based on the conductivity measurements. The osmotic pressure for calculating permeability was determined using the van't Hoff equation.

[0157] Frontal nanofiltration was performed using a stainless steel microfiltration / microfiltration cell. The applied pressure was controlled by regulating the gas flow rate. The volume of collected permeate was measured over time to calculate the flow rate.

[0158] The membrane obtained in I', 2 (vacuum filtration deposition) was tested in a forward osmosis (FO for the English term "Forward Osmosis") configuration using an H-type cell, described later.

[0159] The extraction solution was 1 M salt and the feed was distilled water.

[0160] Ion diffusion was monitored by taking conductivity measurements.

[0161] The rejection rate was calculated after 1 day using the following formula: 100

[0162] Or

[0163] - Cf is the salt concentration in the diet

[0164] - CD is the salt concentration in the extraction solution. The membrane showed a rejection rate greater than 99% for monovalent salts (NaCl and KCl), while the PES support showed no rejection. The synthesized M0S2 solution described in Example 1 was used to fabricate highly permeable nanofiltration membranes by filtering a certain amount of the M0S2 solution through a PES support and storing the membrane under humid conditions. Performance was evaluated for nanofiltration using a front-end configuration. The membrane showed a pure water flow rate greater than 100 LMH / bar and complete removal of the organic dye (R < 100%) (aqueous tartrazine solution at 50 mg / L) under front-end filtration (AP = 1 bar). The PES support showed no rejection. The rejection rate was calculated as follows: R (%) = 100

[0165] Or

[0166] - Cf is the concentration in the charge

[0167] - C p is the concentration of the permeate.

[0168] The concentrations were measured using a UV-visible spectrometer.

[0169] The membrane obtained in Example 3 (immersion deposition) was tested for the nanofiltration of an organic dye (aqueous methyl orange (MO) solution at 40 mg / L) in a frontal configuration (AP = 3 bar). The membrane showed complete decolorization (>90% rejection) of the methyl orange solution with a permeability of 19 LMH / bar.

[0170] The membrane obtained in Example 5 (with functionalized nanoparticles) was tested to filter an aqueous solution containing an organic dye: 50 mg / L of rhodamine B (RhB). The filtrate showed almost complete removal of the dye (>90%). The uncoated membrane showed no rejection.

[0171] The results are summarized in Table 1 below:

[0172] [Table 1]

[0173]

[0174] From this table, we can draw the following conclusions: we can clearly observe the loss of selectivity of the membrane for the membrane made with commercial exfoliated M0S2 (comparative example 7), this loss of selectivity being due to the imperfect stacking of particles during deposition.

[0175] On the contrary, the membranes according to the invention have shown superior selectivity, thanks to the homogeneity of the small sheets of M0S2, composed only of monolayer nanosheets, which produce ordered deposits without "gap", without void.

Claims

Demands 1. A method for preparing a separating membrane, characterized in that it comprises - a) a step of preparing transition metal(s) sulfide nanoparticles, in particular selected from Mo, V and W, comprising a thermal decomposition step - of at least one organic or mineral precursor of the transition metal and at least one sulfur precursor - and / or at least one organic precursor of both the transition metal and sulfur, - in the presence of at least one organic stabilizing agent in order to obtain nanoparticles predominantly in the form of monolayers, - b) a step of depositing said nanoparticles prepared in step a) in the form of a so-called selective layer on a support membrane, in particular - by direct filtration, possibly under pressure higher or lower than atmospheric pressure, of a solution containing said nanoparticles onto said support membrane, - or by immersing the support membrane in a solution containing said nanoparticles, said solution preferably containing a concentration of nanoparticles between 0.05 and 5 mg / ml.

2. A method for preparing a separator membrane according to the preceding claim, characterized in that, in step a) of preparing the nanoparticles, the organic precursor of both the transition metal in the form of molybdenum and sulfur is chosen from at least one of the following compounds: - a compound A with formula [R2N-XS-S] y -Mo mOnSp in which R is an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms, with m between 1 and 2, with n between 0 and 2, with p between 0 and 4, with y between 2 and 4, with X a carbon atom or a phosphorus atom, - a compound B of the molybdenum xanthate family of formula (ROS2)4Mo in which R is an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms, - a compound C from the molybdenum dithiocarbamate family - a compound D from the molybdenum dithiophosphate family - a compound E from the molybdenum dithioimidophosphinate family - a compound F from the family of molybdenum dithioimidophosphates.

3. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of nanoparticle preparation, the organic precursor of the transition metal in the form of molybdenum is selected from among minus one of the following compounds: - a compound G of the molybdenum stearate family of formula (RCC^Mo in which R is an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms, - a compound H of the molybdenum naphenate family of formula (RCC^Mo in which R is an alkyl group comprising from 1 to 12 carbon atoms, preferably between 2 and 6 carbon atoms.

4. A method for preparing a separator membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the mineral precursor of the transition metal vanadium, tungsten or molybdenum is chosen from at least one of the following compounds: a molybdate, vanadate or tungstate, in particular of ammonium or sodium.

5. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of nanoparticle preparation, the sulfur precursor is selected - in the form of an organic sulfur precursor, selected from at least one of the compounds of the alkyl thiourea family, in particular of formula (R2NHSNHR2), or of the thiourea family, in particular of formula H2NSNH2, or of the carbamide family, in particular of formula R1R2NSR3R4, or of the thioacetamide family, in particular of formula CH3SNH2 or of formula R1SNR2R3, with R2 and R2, R3 and R4 being alkyl or aryl groups of 1 to 18 carbon atoms, - and / or in the form of a mineral sulfur precursor, in particular in the form of ammonium sulfide.

6. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the organic stabilizing agent is chosen from at least one of the following compounds: - an alkylamine chosen from primary and secondary amines having a hydrocarbon chain preferably comprising between 4 and 34 carbon atoms, - a primary, secondary, tertiary or quaternary alkylamine comprising at least one hydrocarbon chain preferably comprising between 12 and 18 carbon atoms, and optionally at least one unsaturation, in particular octylamine, dodecylamine, hexadecylamine, octadecylamine, oleylamine, - a quaternary alkylamine selected from cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB), - a cyclic polyamine, in particular polyvinylpyrrolidone (PVP), - an alkali stearate or naphthenate, in particular sodium, in particular CigCC^Na, - an alkylthiol comprising a hydrocarbon chain of 6 to 18 carbon atoms, including 1-hexanethiol, 1-octanethiol, nonanethiol, 1-dodecanethiol, 1-hexadecanethiol, - a carboxylic acid, comprising in particular 6 to 18 carbon atoms, including citric acid, octanoic acid, decanoic acid, palmitic acid, oleic acid, - a phosphine, in particular tributylphosphine, trioocytlphosphine, detriooctylphosphine oxide, triphenylphosphine, - an alkyl(s) sulfonate, in particular of formula RSO3R', with R and R' being alkyl or aryl groups of 1 to 18 carbon atoms.

7. A method for preparing a separator membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the thermal decomposition is carried out at a temperature between 100 and 350°C, in particular between 150 and 230°C, in particular between 160 and 220°C.

8. A method for preparing a separator membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the thermal decomposition is carried out for a period of 30 minutes to 24 hours, in particular for a period of one to three hours.

9. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the thermal decomposition of the organic precursor(s) of molybdenum in the presence of the stabilizing agent(s) is carried out in the liquid phase in an aqueous or organic solvent, in particular decane, or in the absence of solvent.

10. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of nanoparticle preparation, the ratio r mmass ratio between the mass of molybdenum in the organic precursor(s) of molybdenum and that of the stabilizing agent(s), in particular carried out in an aqueous or organic solvent, is between 1 / 50 and 1 / 400, in particular between 1 / 100 and 1 / 350 or between 1 / 150 and 1 / 300.

11. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that, in step a) of preparing the nanoparticles, the thermal decomposition is carried out until nanoparticles are obtained, in particular in liquid phase, of which at least M0S2 nanoparticles are in the form of isolated monosheets, said nanoparticles being in particular of average size between 1 and 25 nm or between 1 and 10 nm, in particular between 2 and 6 nm or between 3 and 5 nm.

12. A method for preparing a separation membrane according to any one of the preceding claims, characterized in that the nanoparticles, after thermal decomposition, are functionalized by grafting functional groups or organic molecules onto their surface, in particular selected from - organic halides, particularly iodomethane or iodoacetamide, - thiols, mercapto-alkylamines, cetyltrimethylammonium bromide (CTAB), - sugars, particularly sucrose, - tannic acids - polyphenols, - dyes, - NMethylpyrrolidone, - Lithium butyl, - polymers, such as Polylactic Acid (PLA) and Polyvinyl Alcohol (PVA) - amino acids.

13. A method for preparing a separation membrane according to the preceding claim, characterized in that the nanoparticles, after thermal decomposition, are functionalized by grafting. - either during step a) of nanoparticle preparation, - either during step b) of deposition of said nanoparticles prepared in step a) in the form of a so-called selective layer on a support membrane, - either between said step a) of preparation and said step b) of deposit, - either after step b) of filing.

14. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that it also comprises a step c) of depositing a layer comprising at least one polymer, in particular from the polyamide family, after step b) of depositing the nanoparticles or concomitant with step b) of depositing the nanoparticles.

15. A method for preparing a separating membrane according to the preceding claim, characterized in that step c) of depositing a layer comprising at least one polymer is carried out by interfacial polymerization, in particular from an organic diamine solution and an aqueous trimesoylchloride solution, with possible addition of nanoparticles in one and / or the other of said solutions in the case where steps b) and c) are concomitant.

16. A method for preparing a separation membrane according to any one of the preceding claims, characterized in that, once steps a), b) and optionally c) have been carried out, a step d) of drying the resulting separation membrane is performed, in particular at ambient temperature or at a temperature of no more than 100°C, particularly in airflow or neutral gas flow.

17. A method for preparing a separating membrane according to any one of the preceding claims, characterized in that - Step b) of the deposit is carried out in several stages through several successive deposit operations. - or the sequence of steps including at least step b), possibly step c) when provided for, and step d) is repeated at least once, for example several times.

18. A method for preparing a separation membrane according to any one of the preceding claims, characterized in that the separation membrane obtained at the end of step b) or c) or d) is stored under wet or dry conditions.

19. Separating membrane obtained by the process according to any one of the preceding claims, characterized in that it comprises: - a support membrane, made of a material chosen from flat microfiltration or ultrafiltration membranes or in the form of hollow fibers, preferably with a thickness between 50 and 300 µm, in particular between 80 and 200 µm, - a so-called selective layer on said membrane comprising transition metal(s) sulfide nanoparticles, in particular selected from Mo, V and W, including M0S2, preferably with a thickness between 0.05 and 10 pm, and in particular between 1 and 5 pm, the nanoparticles being composed of at least 70%, in particular at least 80 or 90 or 95% of isolated monosheets, and preferably being of average size between 1 and 25 nm or between 1 and 10 nm, in particular between 2 and 6 nm or between 3 and 5 nm.

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