Silicalite-1 crystals and methods for synthesizing same

Abstract

The present invention relates to MFI-type silicalite-1 zeolite crystals, which are individual crystals having a three-dimensional morphology whose cross section parallel to its longest dimension corresponds to the shape of a straight-cut disk (or ellipse). The present invention also relates to a method for synthesizing said crystals and their use as zeolitic adsorbents.

Description

[Technical Field]

[0001] The present invention relates to the field of zeolites, more precisely to the field of MFI type zeolites, and in particular to the field of silicalite-1. The present invention relates in particular to the synthesis of silicalite-1 zeolite and to a method for synthesizing small crystals of silicalite-1. [Background technology]

[0002] The term "zeolite" generally encompasses crystalline microporous aluminosilicates that benefit from a variety of crystal structures with highly variable silicon and aluminum ratios, often referred to as the Si / Al atomic ratio. In particular, crystalline MFI-type zeolites are called "ZSM-5" when aluminum is present in their framework and "silicalite-1" when the framework is purely silica. The absence of aluminum in their network means that the latter benefit from better thermal stability, enhanced hydrophobicity, and improved organophilicity. Thus, silicalite-1 zeolites have been employed in industry for a variety of applications, including adsorption, catalysis, and separation.

[0003] Currently known synthetic routes to silicalite-1 zeolite frequently employ synthesis gels containing mineralizers, such as fluorinated mineralizers, as well as organic structuring agents, which are often used in large or very large amounts.

[0004] These currently known synthetic routes are furthermore generally characterized by a relatively long duration, for example from several tens of hours to several days or even about 10 days. In addition, these syntheses may require high crystallization temperatures, which means that in most cases the operations must be carried out under pressure, which not only entails additional expenditures for industrial plants but can also be somewhat prohibitive.

[0005] Furthermore, the synthesis of silicalite-1 zeolites known from the prior art often leads to crystals with a relatively large size, for example of the order of several tens of micrometers.

[0006] Other syntheses of silicalite-1 involve the use of various reagents, which pose certain operational challenges. Thus, the synthesis of silicalite-1 with fluorinated mineralizers is described, for example, in document CN105858672B. The use of fluorinating agents complicates the synthetic procedure due to the harmful effects of potentially formed fluorinated species (e.g., hydrofluoric acid). On the other hand, document CN112607746A presents a means for synthesizing silicalite-1 crystals with hierarchical porosity, which requires the use of an additional compound derived from guanidine in addition to the structuring agent to create a certain degree of mesoporosity.

[0007] The literature also describes the synthesis of other zeotypes, which are zeolites with a similar crystalline structure but which contain, in addition to silicon, one or more other elements incorporated into the framework, including, but not limited to, titanium and boron. Thus, patent applications US10766871AA and US20220266236A1 describe the synthesis of silicalite zeotypes containing titanium and boron, but given the complexity and cost of such synthesis and such final structures, they are not the focus of this article.

[0008] The paper Catalysts, 2019, 9, 13, doi:10.3390 / catal9010013 by Jianguang Zhang et al. provides the synthesis of silicalite-1 from various silica sources at high temperatures (453 K or about 180 °C) with large amounts of organic structuring agents, around 30%. However, none of them lead to discrete crystals, i.e., crystals with an untwinned, regular, and uniform morphology.

[0009] Patent CN112850740 provides a synthesis that results in the formation of needle-like crystals with irregular dimensions, making it difficult to use in the fields in which silicalite-1 is commonly employed. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Chinese Patent No. 105858672 [Patent Document 2] Chinese Patent Application Publication No. 112607746 [Patent Document 3] U.S. Patent No. 10,766,871AA [Patent Document 4] US Patent Application Publication No. 2022 / 0266236 [Patent Document 5] Chinese Patent No. 112850740 [Non-patent literature]

[0011] [Non-Patent Document 1] Catalysts,2019,9,13,doi:10.3390 / catal9010013 by Jianguang Zhang et al. Summary of the Invention [Means for solving the problem]

[0012] One of the objectives of the present invention is to provide individual crystals of silicalite-1 having a very unique and regular morphology, in particular a morphology called a "straight-cut disk (or ellipse)" having a certain thickness. This type of morphology is sometimes called a "squircle," a term combining the words "square" and "circle" to describe a three-dimensional shape that is a disk (or ellipse) cut by at least two parallel lines in a cross section parallel to the largest dimension of the crystal. An example of such a morphology is shown, for example, in Figure 1 attached hereto.

[0013] Another object of the present invention is to provide discrete crystals of silicalite-1 having a relatively small number average diameter, e.g., 0.2 μm to 7 μm, as measured by scanning electron microscopy, and a narrow, monomodal particle size distribution, as determined by SEM counting, typically having a peak width (2σ) of less than 5.

[0014] Further objects will become apparent in light of the following description: In the present invention, unless otherwise specified, all ranges of values ​​introduced by the expressions "from" or "to" or other similar expressions intended to encompass two values ​​are understood to include the boundary values ​​unless otherwise specified.

[0015] According to a first aspect, the present invention provides MFI-type silicalite-1 zeolite crystals, comprising: - the number average diameter measured by scanning electron microscopy is 0.2 μm to 7 μm; - a width / length ratio measured by scanning electron microscopy of 0.1 to 1 and a thickness / length ratio measured by scanning electron microscopy of 0.05 to 0.5; and - A unimodal particle size distribution, as determined by scanning electron microscopy, with a peak width 2σ of 5 or less This relates to MFI-type silicalite-1 zeolite crystals.

[0016] According to a preferred embodiment, the number average diameter, measured by scanning electron microscopy, is between 0.2 μm and 6 μm, better still between 0.3 μm and 6.0 μm, advantageously between 0.3 μm and 5.0 μm, and most preferably between 0.3 μm and 4.0 μm.

[0017] According to another preferred embodiment of the present invention, the crystals have a width / length ratio of 0.15 to 0.90, more preferably 0.2 to 0.85. According to another embodiment, the thickness / length ratio of the crystals according to the present invention is preferably 0.1 to 0.4. Unless otherwise specified, all crystal sizes are measured by scanning electron microscopy (SEM) as outlined below in the section on analytical techniques.

[0018] According to yet another preferred embodiment, the particle size distribution of the mean diameter is monomodal with a peak width 2σ of less than or equal to 4, advantageously less than or equal to 3, better still less than or equal to 2, and most preferably less than or equal to 1.

[0019] A “unimodal” distribution is χ 2 This is understood to mean a distribution that is not significantly different at 1% risk from a log-normal distribution, which is the log-normal distribution applied to statistical results from the (chi-square) test (using the "Statistica" software from StatSoft France). More specifically, "a distribution that is not significantly different at 1% risk from a log-normal distribution" is understood to mean a distribution that is not significantly different at 1% risk from a log-normal distribution, which is the log-normal distribution applied to statistical results from the (chi-square) test (using the "Statistica" software from StatSoft France). 2 It is understood to mean that the risk "p" of the assay is 1% or more, preferably 5% or more, more preferably 8% or more.

[0020] The MFI-type silicalite-1 zeolite crystals of the present invention have a very unique three-dimensional morphology in which a cross section parallel to its longest dimension corresponds to the shape of a straight-line-truncated disk or ellipse, as defined above.

[0021] In addition, the MFI-type silicalite-1 zeolite crystals of the present invention exhibit a high degree of regularity in shape and size; more precisely, none of the dimensional characteristics (length, width and thickness) of the crystals of the present invention exhibits a variation of more than 15%, preferably more than 10%, upon examination by scanning electron microscopy.

[0022] In yet another preferred embodiment of the present invention, the silicalite-1 crystals are pure, as identified by X-ray diffraction (XRD) and verified by the absence of any identifiable parasitic phases.

[0023] Thus, the crystals obtained by the method of the present invention generally have a size of 0.10 cm in most cases. 3 .g -1 More than 0.14cm, preferably 3 .g -1 More than 0.16cm, preferably 0.16cm 3 .g -1 It is characterized by a Dubinin volume of 100 or more.

[0024] According to another aspect, the present invention relates to a method for synthesizing MFI-type silicalite-1 zeolite crystals as defined above, the synthesis method comprising at least the following steps a) to d): a) feeding a batch or tubular reactor with a synthesis medium comprising a silicon source, an organic structuring agent and seeds; b) increasing the temperature of the synthesis medium to 70°C to 170°C; c) crystallization at a temperature at least equal to the temperature of the previous step, in the range of 70°C to 170°C; d) collecting the silicalite-1 zeolite crystals as defined above. Includes:

[0025] The synthesis medium is prepared by mixing the silicon source, the structuring agent and the seeds, which can be carried out by any method known to those skilled in the art and using any type of equipment also known to those skilled in the art.

[0026] Silicon source is understood to mean any source known to those skilled in the art, in particular silicates, in particular silicates or orthosilicates of alkali metals or alkaline earth metals, for example sodium, or solutions, preferably aqueous solutions, of colloidal silica or tetraethyl orthosilicate, to name only the main sources of silicon that are most common and most widely used in the preparation of zeolite crystals.

[0027] Structuring agents are understood to mean any organic source known to those skilled in the art, in particular solutions containing amines, preferably aqueous solutions of tetrapropylammonium halides, preferably bromides, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide, to name only the most well-known and widely used structuring agents, which make it possible to obtain MFI type structures.

[0028] According to a preferred embodiment of the present invention, the synthetic medium comprises: a silicon source which is an aqueous solution of a silicate or orthosilicate of an alkali metal or alkaline earth metal, for example preferably sodium, or colloidal silica, an organic structuring agent which is an aqueous solution of tetrapropylammonium bromide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, in particular tetrapropylammonium hydroxide or tetrabutylammonium hydroxide, -Seed and Includes:

[0029] Optionally, the synthesis medium may also include a sodium source, for example in the form of sodium hydroxide.

[0030] In the context of the present invention and the synthesis method, the seed may be any seed known to those skilled in the art, in particular a nucleation solution, i.e. a suspension of nuclei obtained from a silicon source, an organic structuring agent and optionally water and sodium hydroxide, - crystals of MFI type zeolites, most particularly MFI type ZSM-5 zeolites or MFI type silicalite-1 zeolites or MEL type zeolites, which zeolite crystals can optionally be crushed beforehand, preferably to submicron size, or cryogenically crushed; is understood to mean a selection from

[0031] According to the most preferred embodiment of the method of the present invention, the seeds are introduced either as a mixture with the silicon source and / or structuring agent or after the silicon source and structuring agent are introduced into the synthesis reactor. The seeds can be introduced by any procedure known to those skilled in the art, most preferably upstream of the crystallization step.

[0032] The weight percentage of the seeds relative to the total weight of the synthetic medium is generally in most cases between 0.1% and 20%, preferably between 0.1% and 10%, more preferably between 0.1% and 5%.

[0033] The H2O / SiO2 molar ratio in the synthesis medium before the introduction of the seeds is generally between 1 and 100, preferably between 5 and 90, and most preferably between 5 and 70. The Na2O / SiO2 molar ratio is between 0 and 0.5, preferably between 0 and 0.3, and more preferably between 0 and 0.2.

[0034] The R / SiO2 molar ratio is between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, without limit, and R represents an organic structuring agent. The process of the present invention is therefore characterized by the essential presence of at least one structuring agent in the synthesis medium, even if only in very small amounts.

[0035] According to yet another preferred embodiment, the synthetic medium is, before the introduction of the seeds, - a HO / SiO molar ratio of 1 to 100, preferably 5 to 90, and more preferably 5 to 70; a Na2O / SiO2 molar ratio of 0 to 0.5, preferably 0 to 0.3, and more preferably 0 to 0.2; an R / SiO2 molar ratio between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, inclusive of the boundary values, where R represents an organic structuring agent; It has.

[0036] The method of the present invention for synthesizing MFI-type silicalite-1 zeolite crystals can be carried out in any type of reactor commonly used in the synthesis of zeolite crystals and known to those skilled in the art. In particular, the reactor can be a discontinuously fed batch reactor or a continuously fed tubular reactor, optionally preferably equipped with one or more stirring systems selected from mechanical stirring and vibration stirring systems and combinations of one or more mechanical stirring systems and one or more vibration stirring systems.

[0037] In the process of the present invention, it is preferred to use a reactor that allows batch or continuous synthesis and has one type of stirring system, preferably only one type of stirring system, either mechanical or vibrational.

[0038] The stirring means mentioned above may be of any type known to those skilled in the art, for example, but not limited to, if the reactor is a batch reactor, it may be a bladed impeller, a deflocculator, a turbine-type shear impeller, an Archimedes screw or an anchor type, and if the reactor is a tubular reactor, it is possible to simulate a suitable stirring system by providing restriction elements such as rings and baffles in the tubular reactor. Alternatively or additionally, the reactor may be equipped with one or more stirring systems, for example a stirring shaft provided with several stirring impellers, a cascade of agitators distributed along the reactor and / or one or more vibration or pulse systems generating a reciprocating motion in the reaction medium, such as pistons, diaphragms and head-to-tail pumps, or a combination of two or more of these techniques.

[0039] The reactor used in the method of the present invention further comprises at least one heating system for all or part of the reactor and, optionally, an insulation system for all or part of the reactor. The reactor may also comprise one or more ultrasound sources to promote crystallization and / or formation of mostly individual crystals, i.e., with few or no agglomerates.

[0040] The at least one heating system may be of any type known to those skilled in the art, and may for example be selected from steam injection systems, jacketed systems containing a heat transfer fluid, systems employing an additional microwave source, and systems combining one or more of the aforementioned means.

[0041] The heating system must allow the temperature to increase during step b), then increase to the crystallization temperature and maintain the temperature during step c), which is most often and advantageously equal to or higher than the temperature reached in step b).

[0042] The temperature increase in step b) is carried out to a temperature of 70°C to 170°C. Typically, but not limited to, the duration of the temperature increase is 0.1 to 10 hours. It does not depart from the scope of the present invention even if the temperature reached at the end of the increase is maintained for 0.1 to 20 hours.

[0043] According to the present invention, the crystallization step (step c)) is carried out at a temperature in the range of 70°C to 170°C, more preferably 70°C to 160°C, and most preferably 70°C to 150°C, for example 70°C to 130°C.

[0044] The duration of the crystallization step can vary widely, generally from a few minutes to a few hours, most often for periods varying from 30 minutes to 72 hours, preferably from 30 minutes to 48 hours, more preferably from 1 hour to 30 hours.

[0045] During the temperature increase (step b)) and crystallization (step c)) steps, the pressure is between atmospheric pressure and 1.5 MPa. Preferably, the pressure is equal to the autogenous pressure.

[0046] The silicalite-1 crystals are collected by any means known to those skilled in the art, for example, by any means or combination of means selected from continuous filtration, discontinuous filtration, centrifugation, and freeze-drying. The crystals are then generally washed, most often with any suitable means, for example, an aqueous solution. The crystals are then optionally calcined by conventional techniques known to those skilled in the art, for example, but not limited to, at 550° C. for 6 hours.

[0047] The synthesis method of the present invention offers many advantages, including, inter alia, the production of crystals of reduced size, reduced energy and raw material costs, and improved product quality and regularity.

[0048] Furthermore, the method of the present invention allows synthesis in a non-fluorinated medium, potentially at relatively low crystallization temperatures, and employing only small amounts of organic structuring agents, thus providing an economical and efficient synthesis method that is easy to industrialize.

[0049] As mentioned above, the process according to the invention makes it possible to obtain MFI-type silicalite-1 zeolite crystals with uniform particle size and morphology in an entirely simple and efficient manner.

[0050] In addition, the method of the present invention makes it possible to obtain well-crystallized crystals of impurity-free MFI-type silicalite-1 zeolite, characterized by small crystals with a regular morphology, preferably a "straight-cut disc (or ellipse)" morphology, with a number-average diameter of the crystals between 0.2 μm and 7 μm and a monomodal particle size distribution.

[0051] The particular properties of the zeolite crystals of the present invention, in particular their uniformity of size and their regular morphology of "straight-cut disks (or ellipses)", greatly facilitate their incorporation into various organic polymer matrices compared to crystals with a broader particle size distribution and / or less regular morphology.

[0052] Another particular advantage is that the use of fluorinating agents is not required. In addition to the advantage of simplified implementation, the absence of fluorinating agents significantly reduces the environmental impact, as these agents are often very corrosive. In addition, the fluorinating agent-free process simplifies wastewater treatment steps and reduces the production costs of zeolites.

[0053] The particular morphology of the crystals of the present invention allows their use and easy dispersion in all types of matrices, whether liquid, pasty or powdery solid, compared to the morphologies typically obtained in the prior art (spherical, hexagonal, acicular), whether this is for adsorption, separation or catalytic applications.

[0054] The uniform size of the crystals (unimodal distribution) and their small average diameter allow for optimal control of, for example, good crystal flow and diffusion properties in many applications, further facilitating dispersion in all types of matrices.

[0055] The crystals of the present invention can also be advantageously pelletized by any conventional technique known to those skilled in the art, for example, using a pelletizing binder, such as a clay-type binder or an alumina-type binder. In particular, as a result of their shape, uniformity, and mostly individual nature, the crystals of the present invention exhibit little or no mesoporosity. In addition, when the crystals of the present invention are pelletized and shaped, a particularly regular porous network is formed within the pellet.

[0056] The crystals of the present invention, optionally but preferably in the form of activated powders or shaped pellets, therefore find some very interesting applications in many fields as zeolitic adsorbents, including in the fields of recycling, packaging, batteries, coatings (such as paints and varnishes), water treatment, electronics, medicine, food and chemical industries in general, as well as in fields that make use of the adsorption properties of zeolites, such as the adsorption of organic compounds and in particular volatile organic compounds in aqueous and / or organic media, the adsorption of odors, the separation of compounds in liquid or gaseous media or as supports for catalytic metals and the like, to name only the main fields of use. [Brief explanation of the drawings]

[0057] [Figure 1] Figures 1a, 1b, 1c and 1d show examples of crystal shapes according to the present invention that are "squircles" in cross section parallel to their largest dimension. [Figure 2] 1 shows the X-ray diffraction pattern of silicalite-1 crystals from Example 1. [Figure 3] FIG. 3 (divided into FIG. 3a and FIG. 3b) shows an SEM image of one of the silicalite-1 crystals from Example 1. [Figure 4] 1 shows the X-ray diffraction pattern of MFI-type silicalite-1 zeolite crystals from Example 2. [Example]

[0058] analysis technology Crystal size estimation The size of zeolite crystals is estimated by examination with a scanning electron microscope (SEM). To estimate the size of the zeolite crystals in a sample, a set of images is taken at a magnification of at least 5000x. The diameter of a straight cut disk is defined as the diameter of the circumscribed circle.

[0059] Then, using dedicated software, such as the Smile View software from the publisher LoGraMi, measure the length (corresponding to the diameter of the circumscribed circle) of at least 200 crystals, as well as the width and thickness. The number-average diameter is obtained by calculating the number-average of the crystal length. The accuracy is about 3%.

[0060] Qualitative analysis by X-ray diffraction The purity of the zeolite in the zeolite adsorbent material is assessed by X-ray diffraction analysis, known to those skilled in the art by the acronym XRD. This identification is carried out on a Bruker XRD instrument.

[0061] This analysis makes it possible to distinguish between the various zeolites present in the adsorbent material, as each zeolite structure has a unique diffraction pattern defined by the positions of the diffraction peaks and their relative intensities.

[0062] Before the measurement, the zeolite material is crushed and then spread flat on the sample holder by simple mechanical compression.

[0063] The conditions for acquiring diffraction patterns on a Bruker D5000 instrument are as follows: Cu tube used at 40kV-30mA, Slit size (divergence, scattering and analysis slits) = 0.6 mm, Filter: Ni, A sample device rotating at 15 rpm, Measurement range: 3°<2θ<50° Step: 0.02° ·Counting time per step: 2 seconds.

[0064] The resulting diffraction patterns are interpreted using EVA software and the 2011 release of the ICDD PDF-2 database is utilized to identify zeolites.

[0065] Microcrystallinity by Dubinin volume Dubinin volume (or micropore volume V mi) is determined by conventional methods well known to those skilled in the art, in particular from the measurement of the adsorption isotherms of gases such as nitrogen, argon and oxygen at their liquefaction temperatures. Nitrogen is preferably used. Prior to said adsorption measurements, the zeolite crystals of the invention are heated at 300°C to 450°C under reduced pressure (pressure <6.7°C) for a period ranging from 9 hours to 16 hours. -4 The adsorption isotherm is measured at 77 K with a Micromeritics ASAP 2020 instrument, with at least 35 measurement points at relative pressures with a P / P ratio between 0.002 and 1. The micropore volume is determined from the resulting isotherm according to the Dubinin-Raduskevitch equation, employing standard ISO 15901-3:2007. The micropore volume thus evaluated is expressed as cm of liquid adsorbent per gram of anhydrous adsorbent. 3 The measurement uncertainty is ±0.003 cm 3 .g -1 is.

[0066] [Example 1] (according to the present invention) For the batch synthesis of silicalite-1 zeolite, a solution of tetraethyl orthosilicate (Sigma-Aldrich, CAS 78-10-4), a solution of tetrapropylammonium hydroxide (Thermo-Fisher, CAS 4499-86-9), and seeds are introduced into a batch reactor as shown below. A tetrapropylammonium hydroxide solution with the composition 0.03 TPAOH (tetrapropylammonium hydroxide), 13.22 H2O is prepared. A silicate solution with the composition 1 SiO2, 7.78 H2O is prepared. The seeds consist of ZSM-5 (Alfa Aesar, CAS 1318-02-1) crystals in a proportion of 0.27% by weight relative to the weight of the synthesis medium.

[0067] The synthesis medium is prepared in a batch reactor by mixing a tetrapropylammonium hydroxide solution, a tetraethylorthosilicate solution, and seeds. The reactor is then heated to 98°C by means of a jacket. The residence time in the reactor is 26 hours.

[0068] At the end of this synthesis, pure silicalite-1 zeolite is obtained, i.e., one with a diffraction pattern strictly characteristic of MFI-type zeolites (see X-ray diffraction pattern in Figure 2), with a peak at 0.170 cm 3 .g -1 The crystals obtained are uniform in size, mostly individual (without twins), have a number-average size (length) of 0.5 μm as determined by scanning electron microscopy (SEM) (see paragraph "Analytical Techniques"), have a truncated ellipsoidal morphology (see Figures 3a and 3b), an average crystal thickness / length ratio of 0.3, and an average crystal width / length ratio of 0.7. The 2σ value is equal to 0.16 μm.

[0069] [Example 2] (according to the present invention) For the batch synthesis of silicalite-1 zeolite, a suspension of colloidal silica (Sigma Aldrich, CAS 7631-86-9), a solution of TPAOH (Thermo-Fisher, CAS 4499-86-9), and seeds are introduced into a batch reactor as detailed below. A tetrapropylammonium hydroxide solution with a composition of 0.03 TPAOH / 13.22 H2O is prepared. A silicate solution with a composition of 1 SiO2 / 7.78 H2O is prepared. The seeds consist of ZSM-5 (Alfa Aesar, CAS 1318-02-1) crystals in a proportion of 0.27% by weight relative to the weight of the synthesis medium.

[0070] The synthesis medium is prepared in a batch reactor by mixing a tetrapropylammonium hydroxide solution, a suspension of colloidal silica, and then seeds. The reactor is then heated to 98°C by means of a jacket. The residence time in the reactor is 26 hours.

[0071] At the end of this synthesis, pure MFI-type silicalite-1 zeolite is obtained, i.e., one with a diffraction pattern strictly characteristic of MFI-type zeolites (see X-ray diffraction pattern in Figure 4), with a peak density of 0.172 cm 3 .g -1The crystals obtained are mostly individual (untwinned), have a number-average size (length) of 0.52 μm, and have a truncated ellipsoidal morphology. The average crystal thickness / length ratio is 0.26, and the average crystal width / length ratio is 0.64. The 2σ value is equal to 0.18 μm.

[0072] [Example 3] (Comparative Example) Example 1 of Patent CN112850740 was reproduced. As shown in this patent, MFI-type silicalite-1 zeolite crystals with lamellar morphology (Figure 2 of Patent CN112850740) were obtained, i.e., crystals with a completely different morphology from the crystals of the present invention. The obtained silicalite-1 crystals have a needle-like morphology with different dimensions. The needles of highly variable dimensions average the following approximate size: 20 nm × 200 nm × 2000 nm, thus having an average crystal size (length) of 2 μm, a calculated average thickness / length ratio of 0.01, and a calculated average width / length ratio of 0.1. The crystals have a very different morphology compared to the morphology of the crystals of the present invention, showing great heterogeneity.

Claims

1. MFI-type silicalite-1 zeolite crystals, - a number average diameter, measured by scanning electron microscopy, of between 0.2 μm and 7 μm; a width / length ratio, measured by scanning electron microscopy, of between 0.1 and 1 and a thickness / length ratio, measured by scanning electron microscopy, of between 0.05 and 0.5; and - a unimodal particle size distribution determined by scanning electron microscopy with a peak width 2σ of 5 or less MFI-type silicalite-1 zeolite crystals.

2. 2. Crystals according to claim 1, having a number average diameter, measured by scanning electron microscopy, of between 0.2 μm and 6 μm, better still between 0.3 μm and 6.0 μm, advantageously between 0.3 μm and 5.0 μm, and most preferably between 0.3 μm and 4.0 μm.

3. 3. The crystal according to claim 1 or 2, having a width / length ratio measured by scanning electron microscopy of 0.15 to 0.90, more preferably 0.2 to 0.

85.

4. 4. The crystal according to claim 1, wherein the thickness / length ratio measured by scanning electron microscopy is between 0.1 and 0.

4.

5. 5. Crystals according to any one of claims 1 to 4, in which the peak width 2σ of the monomodal particle size distribution of the number average diameter is 4 or less, advantageously 3 or less, better still 2 or less and most preferably 1 or less.

6. 6. A crystal according to any one of claims 1 to 5, which has a very unique three-dimensional morphology in which a cross section parallel to its longest dimension corresponds to the shape of a straight-cut disk or ellipse.

7. 7. A crystal according to any one of claims 1 to 6, wherein none of the dimensional characteristics (length, width and thickness) exhibits a variation of more than 15%, preferably more than 10%, upon examination by scanning electron microscopy.

8. A method for synthesizing the crystal according to any one of claims 1 to 7, comprising at least the following steps a) to d): a) feeding a batch or tubular reactor with a synthesis medium comprising a silicon source, an organic structuring agent and seeds; b) increasing the temperature of the synthesis medium to between 70°C and 170°C; c) crystallization at a temperature at least equal to that of the previous step, in the range of 70°C to 170°C; d) collecting the silicalite-1 zeolite crystals as defined above. A method comprising:

9. The synthetic medium a silicon source which is an aqueous solution of an alkali metal or alkaline earth metal silicate or orthosilicate or colloidal silica, an organic structuring agent which is an aqueous solution of tetrapropylammonium bromide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, in particular tetrapropylammonium hydroxide or tetrabutylammonium hydroxide, -Seed and The method of claim 8, comprising:

10. 10. The method of claim 8 or 9, wherein the synthesis medium further comprises a sodium source.

11. The method according to any one of claims 8 to 10, wherein the weight percentage of the seeds relative to the total weight of the synthetic medium is between 0.1% and 20%, preferably between 0.1% and 10%, more preferably between 0.1% and 5%.

12. R / SiO ２ 12. The method according to any one of claims 8 to 11, wherein the molar ratio is between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, without limits, and R represents an organic structuring agent.

13. 10. Use of the crystals according to any one of claims 1 to 7, optionally but preferably in the form of activated powder or shaped pellets, as zeolitic adsorbents in the fields of recycling, packaging, batteries, coatings (such as paints and varnishes), water treatment, electronics, and in the medical, food and chemical industries in general.

14. 14. Use according to claim 13 for the adsorption of organic compounds, for the adsorption of volatile organic compounds in aqueous and / or organic media, for the adsorption of odors, for the separation of compounds in liquid or gaseous media or as a support for catalytic metals.

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