Silicalite-1 crystals and process for synthesis thereof

Abstract

The present invention relates to individualised MFI silicalite-1 zeolite crystals having a three-dimensional morphology in which the cross section parallel to the greatest length conforms to the shape of a straight truncated disc (or ellipse). The invention also relates to the process for synthesizing said crystals, and to the uses thereof as zeolitic adsorbents.

Description

DESCRIPTION TITLE: SILICALITE-1 CRYSTALS AND THEIR SYNTHESIS PROCESS

[0001] The present invention relates to the field of zeolites, more precisely the field of zeolites of MFI structural type and in particular that of Silicalite-1. The present invention is particularly concerned with the synthesis of Silicalite-1 zeolite, as well as the process for synthesizing small Silicalite-1 crystals.

[0002] The term "zeolites" generally encompasses crystallized microporous aluminosilicates with different crystalline structures in which the proportion of silicon and aluminum, often referred to as the Si / Al atomic ratio, is highly variable. In particular, zeolites with the MFI crystalline structural type are called "ZSM-5" in the presence of aluminum in their framework, or called "Silicalite-1" when the framework is purely silicic. Due to the absence of aluminum in their network, the latter benefit from better thermal stability, increased hydrophobicity and organophilicity. Thus, Silicalite-1 zeolites are industrially used for various applications such as adsorption, catalysis and separation.

[0003] The synthesis routes for Silicalite-1 zeolites known today frequently use synthetic gels containing mineralizing agents, such as fluorinated mineralizing agents, as well as organic structuring agents, often used in high or very high quantities.

[0004] These currently known synthesis routes are also generally characterized by relatively long durations, for example from a few dozen hours to several days, or even ten days. In addition, these syntheses can require high crystallization temperatures, which most often requires operations to be carried out under pressure, which can represent a definite handicap, and additional costs in industrial installations.

[0005] Furthermore, the syntheses of Silicalite-1 zeolites known from the prior art often lead to crystals of relatively large sizes, for example of the order of a few tens of micrometers.

[0006] Other syntheses of silicalite-1 involve different reagents generating a certain operational complexity. Thus, syntheses of silicalite-1 with fluorinated mineralizing agent are for example described in document CN105858672B. The use of fluorinated agent complicates the operation of the synthesis due to the danger of the fluorinated molecules potentially formed (hydrofluoric acid for example). On the other hand, the document CN112607746A discloses a means of synthesizing silicalite-1 crystals with hierarchical porosity which necessarily involves, in addition to the structuring agent, an additional compound derived from guanidine to create a certain degree of mesoporosity.

[0007] Other syntheses of zeotypes, which are zeolites with similar crystalline structures but incorporating in the framework, in addition to silicon, one or more other elements, among which titanium and boron may be cited as non-limiting examples, are also described in the literature. Thus, patent applications US10766871 AA and US20220266236 A1 describe syntheses of silicalite zeotypes comprising titanium and boron, which is not sought here due to the complexity and cost of such syntheses and such final structures.

[0008] The article (Catalysts, 2019, 9, 13, doi:10.3390 / catal9010013) by Jianguang Zhang et al, proposes syntheses of silicalite-1 from different silica sources, at high temperature (453 K, or about 180°C), involving significant quantities of organic structuring agent, of the order of 30%. But none leads to individual crystals, i.e. without twinning, and with regular and homogeneous morphologies.

[0009] Patent CN112850740 proposes a synthesis leading to the formation of needle-shaped crystals of irregular dimensions and therefore difficult to implement in the fields of application where silicalite-1 is commonly used.

[0010] One of the objectives of the present invention is to provide individualized crystals of silicalite-1 having a very particular and regular morphology, and in particular having a morphology called a “straight truncated disc (or ellipse)” with a certain thickness. Such a morphology can be called by the English term “squircle”, combining the word “square” and the word “circle” to designate these three-dimensional shapes whose cross-section parallel to the largest dimension of the crystal is a disc (or an ellipse) truncated by at least two straight lines, parallel two by two. Examples of such morphologies are for example illustrated in Figure 1, attached to this description.

[0011] Another objective of the present invention is to provide individualized crystals of silicalite-1 having a number average diameter, measured by scanning electron microscopy, relatively small, for example between 0.2 pm and 7 pm and a monomodal particle size distribution, also determined by SEM counting, narrow, typically with a peak width (2o) of less than 5.

[0012] Still other objects will become apparent in light of the following description. In the present invention, and unless otherwise indicated, all ranges of values ​​introduced by the expressions "from ... to ..." or "between .... and ...." or other similar expressions intended to frame two values, are understood to include limits, unless expressly indicated.

[0013] According to a first aspect, the present invention relates to zeolite crystals of type MFI Silicalite- 1: - whose number average diameter, measured by scanning electron microscopy, is between 0.2 pm and 7 pm, - whose width / length ratio, measured by observation with a scanning electron microscope, is between 0.1 and 1, and whose thickness / length ratio, measured by observation with a scanning electron microscope, is between 0.05 and 0.5, and - whose monomodal particle size distribution, determined by observation under a scanning electron microscope, has a peak width 2o equal to or less than 5.

[0014] According to a preferred embodiment, the number average diameter, measured by scanning electron microscopy, is between 0.2 pm and 6 pm, better still between 0.3 pm and 6.0 pm, advantageously between 0.3 pm and 5.0 pm, and most preferably between 0.3 pm and 4.0 pm.

[0015] According to another preferred embodiment of the present invention, the crystals have a width / length ratio of between 0.15 and 0.90, more preferably between 0.2 and 0.85. According to another embodiment, the thickness / length ratio of the crystals according to the present invention is preferably between 0.1 and 0.4. Unless otherwise indicated, all crystal sizes are measured by scanning electron microscope (SEM) observation, as explained further in the paragraphs relating to analytical techniques.

[0016] According to yet another preferred embodiment, the particle size distribution of the average diameter is a monomodal particle size distribution with a peak width 2o equal to or less than 4, advantageously equal to or less than 3, better still equal to or less than 2, and most preferably equal to or less than 1.

[0017] By "monomodal" distribution we mean a distribution that is not significantly different from the lognormal distribution at 1% risk, lognormal distribution applied to the statistical results from the x test 2 (Chi2) (software used “Statistica” from StatSoft France). More specifically, by distribution not significantly different from the log-normal distribution at 1% risk, we mean that the risk “p” of the x test 2 is greater than or equal to 1%, preferably greater than or equal to 5%, more preferably greater than or equal to 8%.

[0018] The MFI Silicalite-1 type zeolite crystals of the invention have a very particular three-dimensional morphology whose cross-section parallel to the greatest length takes the shape of a right truncated disc or ellipse, as defined above.

[0019] Furthermore, the MFI Silicalite-1 type zeolite crystals of the invention exhibit great regularity of shape and size, and more precisely each of the dimensional characteristics (length, width and thickness) of the crystals of the invention do not exhibit variations greater than 15%, preferably greater than 10%, when observed under a scanning electron microscope.

[0020] In yet another preferred aspect of the present invention, the Silicalite-1 crystals are pure, this purity being verified by the absence of parasitic phases identified and identifiable by X-ray diffraction (XRD).

[0021] Thus, the crystals obtained according to the process of the present invention are generally and most often characterized by a Dubinin volume equal to or greater than 0.10 cm 3 . g -1 , preferably equal to or greater than 0.14 cm 3 . g -1 , preferably equal to or greater than 0.16 cm3 . g -1 .

[0022] According to another aspect, the present invention relates to the process for synthesizing MFI Silicalite-1 zeolite crystals as just defined. The synthesis process of the invention comprises 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) raising the temperature of the synthesis medium to a value between 70°C and 170°C; c) crystallization at a temperature at least equal to the temperature of the previous step, at a temperature ranging from 70°C to 170°C; d) recovery of the silicalite-1 zeolite crystals, as previously defined.

[0023] The synthesis medium is prepared by mixing a source of silicon, a structuring agent and seeds, said mixing being able to be carried out according to any method well known to those skilled in the art, and any type of apparatus also well known to those skilled in the art.

[0024] By source of silicon is meant any source well known to those skilled in the art and in particular a solution, preferably aqueous, of silicate, in particular of silicate or ortho-silicate of alkali or alkaline-earth metal, for example sodium, or of colloidal silica or even of tetraethyl ortho-silicate, to name only the main sources. most common and most generally used silicon for the preparation of zeolite crystals.

[0025] By structuring agent is meant any organic source well known to those skilled in the art allowing the obtaining of an MFI type structure, in particular a solution containing amines, preferably aqueous, such as a halide, preferably bromide, of tetrapropylammonium, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, and others, to cite only the best known and most used structuring agents.

[0026] According to a preferred embodiment of the invention, the synthesis medium comprises: - a source of silicon which is an aqueous solution of silicate or ortho-silicate of alkali or alkaline earth metal, for example and 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, and - seeds.

[0027] Optionally, the synthesis medium may also comprise a source of sodium, for example in the form of sodium hydroxide.

[0028] By seeds in the sense of, and in the synthesis process of the present invention, we mean all seeds well known to those skilled in the art and in particular those chosen from: - nucleating solutions, i.e. suspensions of nuclei obtained from a source of silicon, an organic structuring agent and optionally water and sodium hydroxide, - MFI type zeolite crystals, and in particular MFI ZSM-5 or MFI Silicalite-1 type zeolites, or MEL type zeolites, which zeolite crystals may optionally be previously ground or cryo-ground, preferably to a submicron size.

[0029] According to a highly preferred aspect of the method of the present invention, the seeds are introduced either as a mixture with the silicon source and / or the structuring agent, or after the introduction of the silicon source and the structuring agent into the synthesis reactor. The introduction of the seeds can be carried out according to any operating method well known to those skilled in the art, and quite preferably upstream of the crystallization step.

[0030] The percentage by weight of seeds relative to the total weight of the synthetic medium is generally and most often between 0.1% and 20%, preferably between 0.1% and 10%, more preferably between 0.1% and 5%.

[0031] The molar ratio H2O / SiO2 in the synthesis medium, before introduction of the seeds, is generally between 1 and 100, preferably between 5 and 90 and most preferably between 5 and 70. The molar ratio Na2O / SiO2 is between 0 and 0.5, preferably between 0 and 0.3, most preferably between 0 and 0.2, limits included.

[0032] The molar ratio R / SiO2 is between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, limits excluded, where R represents the organic structuring agent. The process of the present invention is thus characterized by the fact that at least one structuring agent is necessarily present in the synthesis medium, even if it is present in a very small quantity.

[0033] According to yet another preferred embodiment, the synthetic medium, before introduction of the seeds, has: - a molar ratio H2O / SiO2 of between 1 and 100, preferably between 5 and 90 and more preferably between 5 and 70, - a Na2O / SiO2 molar ratio of between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, and - a molar ratio R / SiO2 of between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, limits excluded, where R represents the organic structuring agent.

[0034] The method for synthesizing MFI Silicalite-1 type zeolite crystals of the present invention can be implemented in any type of reactor commonly used for the synthesis of zeolite crystals and well known to those skilled in the art. In particular, the reactor can be a batch reactor fed discontinuously or a tubular reactor fed continuously, for the latter, optionally preferably equipped with one or more stirring systems chosen from mechanical stirring and oscillation stirring system, and also combinations of one or more mechanical stirring systems with one or more oscillation stirring systems.

[0035] For the process of the present invention, reactors allowing batch or continuous syntheses are preferred, with one type of stirring system, and preferably a single type of stirring system, either mechanical or oscillatory.

[0036] The aforementioned stirring means may be of any type well known to those skilled in the art, and for example and in a non-limiting manner, when the reactor is a reactor batch, bladed mobile type, deflocculator, turbine type shearing mobile, Archimedes screw or anchor, and when the reactor is a tubular reactor, the suitable stirring systems can be simulated by restrictions, such as rings, baffles and others, present in said tubular reactor. Alternatively, or in addition, said reactor can be equipped with one or more stirring systems, such as for example a stirring shaft equipped with several stirring mobiles, a cascade of stirrers distributed along the reactor, and / or one or more oscillating or pulsating systems, which make it possible to generate a back-and-forth movement of the reaction medium by means for example of a piston, membrane, head-to-tail pumps, and others, as well as two or more of these combined techniques.

[0037] The reactor used for the process of the present invention further comprises at least one system for heating all or part of the reactor, as well as possibly a system for insulating all or part of the reactor. The reactor may also comprise one or more sources of ultrasound in order to promote crystallization and / or the formation of well-individualized crystals, i.e. without or with few aggregates.

[0038] Said at least one heating system may be of any type well known to those skilled in the art, and for example chosen from systems by steam injection, by heat transfer fluid in a double jacket, by addition of a microwave source, and by combination of one or more of the aforementioned means.

[0039] The heating system must allow a rise in temperature during step b), then a rise to the crystallization temperature and the maintenance of the temperature during step c). The crystallization temperature of step c) is most often and advantageously equal to or higher than the temperature reached in step b).

[0040] The temperature rise in step b) is carried out up to a temperature between 70°C and 170°C. Typically, but not exclusively, the temperature rise time is between 0.1 hour and 10 hours. It would not be outside the scope of the invention if the temperature reached at the end of the rise were maintained for a period between 0.1 hour and 20 hours.

[0041] According to the present invention, the crystallization step (step c)) is carried out at a temperature ranging from 70°C to 170°C, more preferably from 70°C to 160°C, and most preferably from 70°C to 150°C, for example from 70°C to 130°C.

[0042] The duration of the crystallization step can vary greatly and is generally between a few minutes and several hours, most often for a period varying from 30 minutes to 72 hours, preferably from 30 minutes to 48 hours, more preferably from 1 hour to 30 hours.

[0043] 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.

[0044] The recovery of the Silicalite-1 crystals is carried out by any means known to those skilled in the art, for example by any means or combination of means chosen from continuous filtration, discontinuous filtration, centrifugation and freeze-drying. The crystals are then generally and most often washed by any suitable means, for example by means of an aqueous solution. The crystals are then optionally calcined according to conventional techniques known to those skilled in the art, for example, and without limitation, at 550°C for 6 hours.

[0045] The synthesis process of the present invention provides numerous advantages, including the production of reduced-size crystals, a reduction in energy costs and costs related to raw materials, as well as increased production quality and improved production consistency.

[0046] Furthermore, the process of the present invention allows synthesis in a non-fluorinated medium, using small amounts of organic structuring agent, possibly at relatively low crystallization temperatures. Thus, this invention provides a synthesis process that is easily industrializable, economical and efficient.

[0047] As indicated previously, the process according to the present invention allows the obtaining, in a completely simple and efficient manner, of MFI Silicalite-1 type zeolite crystals having a homogeneous particle size and morphology.

[0048] Furthermore, the process of the present invention allows the production of well-crystallized, impurity-free MFI Silicalite-1 zeolite crystals, which are characterized by small crystals having a regular morphology, preferably a “right truncated disc (or ellipse)” morphology, having an average diameter in crystal number, between 0.2 pm and 7 pm and a monomodal particle size distribution.

[0049] Due to the particular properties of the zeolite crystals of the present invention, in particular its homogeneity in size and its regular morphology, of the “right truncated disc (or ellipse)” type, the incorporation of said crystals into various organic and polymer matrices is greatly facilitated compared to crystals having a wider particle size distribution and / or less regular morphologies.

[0050] Another advantage is that it eliminates the need for a fluorinated agent. In addition to the advantage of simplifying implementation, the absence of a fluorinated agent reduces significantly the impact on the environment, these agents being very often corrosive. In addition, the process without fluorinated agent simplifies the effluent treatment stage, thus allowing a reduction in zeolite production costs.

[0051] 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, in comparison with the morphologies typically obtained in the prior art (balls, hexagons, needles), whether for adsorption, separation or catalysis applications.

[0052] The homogeneous size of the crystals (monomodal distribution) as well as their small average diameter, allow for example good flowability of the crystals, optimal control of diffusion properties in many applications and further promote dispersion in all types of matrices.

[0053] The crystals of the invention may also, and advantageously, be agglomerated according to any conventional techniques well known to those skilled in the art, and for example using an agglomeration binder, for example of the clay or alumina type. Due in particular to their shape, their homogeneity, and the fact that they are well individualized, the crystals of the invention have little mesoporosity, or even no mesoporosity. In addition, when the crystals of the invention are agglomerated and shaped, they lead to the formation of a particularly regular porous network within the agglomerate.

[0054] Thus, the crystals of the invention, in powder form, optionally but preferably activated, or in the form of shaped agglomerates, find very interesting applications as zeolite adsorbents in numerous fields, among which may be cited the fields of recycling, packaging, electric batteries, coatings (such as paints and varnishes), water treatment, electronics, in the medical field, the food field and in the chemical industry in general, fields using the adsorption properties of zeolites, for example adsorption of organic compounds, in particular volatile organic compounds, in aqueous and / or organic medium, adsorption of odors, separation of chemical compounds in liquid or gaseous medium, or as a support for catalytic metals, and others, to name only the main fields of application. Brief description of the figures

[0055] Figures 1a, 1b, 1c and 1d represent diagrams illustrating examples of crystal shapes according to the invention where the cross-sections parallel to the largest dimension are "squircle".

[0056] Figure 2 shows an X-ray diffractogram of silicalite-1 crystals from Example 1.

[0057] Figure 3 (split into Figures 3a and 3b) shows SEM images of the silicalite-1 crystals from Example 1.

[0058] Figure 4 represents the X-ray diffractogram of the MFI Silicalite-1 type zeolite crystals from Example 2. Analytical techniques Estimating crystal size

[0059] The estimation of the size of zeolite crystals is carried out by observation under a scanning electron microscope (SEM). In order to estimate the size of zeolite crystals on the samples, a set of images is taken at a magnification of at least 5000. The diameter of a right truncated disk is defined as the diameter of the circumscribed circle.

[0060] The length (which is equivalent to the diameter of the circumscribed circle), as well as the width and thickness of at least 200 crystals are then measured using dedicated software, for example Smile View software from the publisher LoGraMi. The number-average diameter is obtained by calculating the number-average of the crystal lengths. The accuracy is around 3%. Qualitative analysis by X-ray diffraction

[0061] The purity of zeolites in zeolite adsorbent materials is assessed by X-ray diffraction analysis, known to those skilled in the art by the acronym DRX. This identification is carried out on a Bruker DRX device.

[0062] This analysis makes it possible to identify the different zeolites present in the adsorbent material since each zeolite structure has a unique diffractogram defined by the positioning of the diffraction peaks and by their relative intensities.

[0063] Prior to measurement, the zeolite materials are crushed then spread and smoothed on a sample holder by simple mechanical compression.

[0064] The conditions for acquiring the diffractogram produced on the Bruker D5000 device are as follows: Cu tube used at 40 kV - 30 mA; slit size (diverging, diffusion and analysis) = 0.6 mm; filter: Ni; rotating sample device: 15 rpm 1 ; measuring range: 3° < 20 < 50°; step: 0.02°; counting time per step: 2 seconds.

[0065] The interpretation of the diffractogram obtained is carried out with the EVA software with identification of the zeolites using the ICDD PDF-2 base, release 2011. Dubinin volume microcrystallinity

[0066] The volume of Dubinin (or microporous V mi) is determined in a conventional manner well known to those skilled in the art, in particular from the measurement of the adsorption isotherm of a gas at its liquefaction temperature, for example nitrogen, argon, oxygen, and others. Preferably, nitrogen is used. Prior to this adsorption measurement, the zeolite crystals of the invention are degassed between 300°C and 450°C for a period ranging from 9 hours to 16 hours, under vacuum (Pressure < 6.7.10' 4 Pa). For example, for a zeolite with an MFI structure such as Si lical ite-1, the measurement of the nitrogen adsorption isotherm at 77K is then carried out on a Micromeritics ASAP 2020 type device, taking at least 35 measurement points at relative pressures with a P / P0 ratio between 0.002 and 1. The micropore volume is determined according to the Dubinin and Raduskevitch equation from the isotherm obtained, applying the ISO 15901-3: 2007 standard. The micropore volume thus evaluated is expressed in cm 3of liquid adsorbent per gram of anhydrous adsorbent. The measurement uncertainty is ± 0.003 cm 3 g- 1 . Example 1 (according to the invention)

[0067] The batch synthesis of Silicalite-1 zeolite consists of introducing into a batch reactor a solution of tetraethyl orthosilicate (Sigma-Aldrich, CAS 78-10-4), a solution of tetrapropylammonium hydroxide (Thermo-Fisher, CAS 4499-86-9) and seeds, as indicated later. A tetrapropylammonium hydroxide solution of composition 0.03 TPAOH (tetrapropylammonium hydroxide), 13.22 H2O is prepared. A silicate solution of composition 1 SiO2 7.78 H2O is prepared. The seeds consist of ZSM-5 crystals (Alfa AESAR, CAS 1318-02-1) at a rate of 0.27% by weight relative to the weight of the synthesis medium.

[0068] The synthesis medium is prepared by mixing the tetrapropylammonium hydroxide solution, the tetraethyl orthosilicate solution, and then the seeds in the batch reactor. The reactor is then heated to 98°C by a double jacket. The residence time in the reactor is 26 hours.

[0069] At the end of this synthesis, a pure Silicalite-1 zeolite, i.e. one presenting a diffractogram strictly characteristic of an MFI type zeolite, is obtained (see X-ray diffractogram in Figure 2), and has a Dubinin volume of 0.170 cm 3 . g- 1The crystals obtained are of homogeneous size, well individualized (no twins) and have an average size in number (length) by observation under scanning electron microscope (SEM), see analytical techniques paragraph of 0.5 pm and a truncated ellipse morphology (see Figures 3a and 3b) and have an average crystal thickness / length ratio of 0.3 and an average crystal width / length ratio of 0.7. The 2o value is equal to 0.16 pm. Example 2 (according to the invention)

[0070] The batch synthesis of the Silicalite-1 zeolite consists of introducing into a batch reactor a suspension of colloidal silica (Sigma Aldrich, CAS 7631-86-9), a solution of TPAOH (Thermo-Fisher, CAS 4499-86-9) and seeds which are detailed later. A tetrapropylammonium hydroxide solution of composition 0.03 TPAOH 13.22 H2O is prepared. A silicate solution of composition 1 SiO2 7.78 H2O is prepared. The seeds consist of ZSM-5 crystals (Alfa AESAR, CAS 1318-02-1) at a rate of 0.27% by weight relative to the weight of the synthesis medium.

[0071] The synthesis medium is prepared by mixing the tetrapropylammonium hydroxide solution, the colloidal silica suspension, and then the seeds in the batch reactor. The reactor is then heated to 98°C by a double jacket. The residence time in the reactor is 26 hours.

[0072] At the end of this synthesis, a pure MFI Silicalite-1 type zeolite, i.e. presenting a diffractogram strictly characteristic of an MFI type zeolite, is obtained (see X-ray diffractogram, Figure 4), and has a Dubinin volume of 0.172 cm 3 . g -1 The crystals obtained are well individualized (no twins) and have a number average size (length) of 0.52 pm, a truncated ellipse morphology. The average thickness / length ratio of the crystals is 0.26 and the average width / length ratio of the crystals is 0.64. The 2o value is equal to 0.18 pm. Example 3 (comparative)

[0073] Example 1 of patent CN112850740 is reproduced. As indicated in this patent, zeolite crystals of the MFI Silicalite-1 type are obtained having a lamellar morphology (Figure 2 of patent CN112850740), that is to say crystals with a morphology completely different from that of the present invention. The silicalite-1 crystals obtained have needle morphologies of different dimensions. The needles of very different dimensions have on average approximately the following sizes: 20 nm x 200 nm x 2000 nm and therefore an average crystal size (length) of 2 pm, an average calculated thickness / length ratio of 0.01 and an average calculated width / length ratio of 0.1. The crystals have a very different morphology compared to the morphology of the crystals of the present invention and exhibit great heterogeneity.

Claims

CLAIMS 1. MFI Silicalite-1 type zeolite crystals: - whose number average diameter, measured by scanning electron microscopy, is between 0.2 pm and 7 pm, - whose width / length ratio, measured by observation with a scanning electron microscope, is between 0.1 and 1, and whose thickness / length ratio, measured by observation with a scanning electron microscope, is between 0.05 and 0.5, and - whose monomodal particle size distribution, determined by observation under a scanning electron microscope, has a peak width 2o equal to or less than 5.

2. Crystals according to claim 1, the number-average diameter of which, measured by scanning electron microscopy, is between 0.2 pm and 6 pm, better still between 0.3 pm and 6.0 pm, advantageously between 0.3 pm and 5.0 pm, and very particularly preferably between 0.3 pm and 4.0 pm.

3. Crystals according to claim 1 or claim 2, the width / length ratio of which is between 0.15 and 0.90, more preferably between 0.2 and 0.85, measured by observation under a scanning electron microscope.

4. Crystals according to any one of the preceding claims, the thickness / length ratio of which is between 0.1 and 0.4, measured by observation under a scanning electron microscope.

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

6. Crystals according to any one of the preceding claims having a very particular three-dimensional morphology whose cross-section parallel to the greatest length takes the shape of a right truncated disc or ellipse.

7. Crystals according to any one of the preceding claims, each of the dimensional characteristics (length, width and thickness) of which does not exhibit variations greater than 15%, preferably greater than 10%, by observation under a scanning electron microscope.

8. Process for synthesizing the crystals defined in any one of the preceding claims, comprising at least the following steps a) to d): a) feeding a batch or tubular reactor with a synthesis medium comprising a source of silicon, an organic structuring agent and seeds; b) raising the temperature of the synthesis medium to a value between 70°C and 170°C; c) crystallization at a temperature at least equal to the temperature of the previous step, at a temperature ranging from 70°C to 170°C; d) recovery of the silicalite-1 zeolite crystals, as defined above.

9. Method according to claim 8, in which the synthesis medium comprises: - a source of silicon which is an aqueous solution of alkali 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, and - seeds.

10. A method according to claim 8 or claim 9, wherein the synthesis medium further comprises a sodium source.

11. Method according to any one of claims 8 to 10, in which the percentage by weight of 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. Method according to any one of claims 8 to 11, in which the molar ratio R / SiO2 is between 0 and 0.5, preferably between 0 and 0.3, more preferably between 0 and 0.2, limits excluded, where R represents the organic structuring agent.

13. Use of the crystals defined in any one of claims 1 to 7, in powder form, optionally but preferably activated, or in the form of shaped agglomerates, as zeolite adsorbents in the fields of recycling, packaging, electric batteries, coatings (such as paints and varnishes), water treatment, electronics, in the medical field, the food field and in the chemical industry in general.

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

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