WO2022025834A1

WO2022025834A1 - Meso/microporous silica particles and a preparation method thereof

Info

Publication number: WO2022025834A1
Application number: PCT/TR2020/050673
Authority: WO
Inventors: Aatif IJAZ; Annamaria MIKO; Adem Levend DEMIREL
Original assignee: Koc Universitesi
Priority date: 2020-07-29
Filing date: 2020-07-29
Publication date: 2022-02-03
Also published as: EP4188878A1

Abstract

The present invention discloses and claims a novel synthesis method of meso/microporous silica particles having a hierarchical morphology. The present invention also relates to a single step and room temperature synthesis method which is performed by adjusting the composition of the catalyst, surfactant and the precursor of the silica to control the shape, size, specific surface area of mesoporous silica particles.

Description

MESO/MICROPOROUS SILICA PARTICLES AND A PREPARATION

METHOD THEREOF FIELD OF THE INVENTION

The present invention discloses and claims a novel synthesis method of meso- and microporous silica particles having a hierarchical morphology. BACKGROUND

Micron-sized spherical silica particles with porous structure have great potential in the fields of catalysis, sensing, adsorbents for various compounds and gases (C02), environmental pollution control, drug delivery and separation techniques especially ultra-high performance liquid chromatography, thermal insulation and many others due to ease of handling, high mechanical strength, stability, high specific surface area, pore size, biocompatibility and easy surface functionalization . While micrometer sized silica particles with micropores (pore size less than 2 nm) or mesopores (pore size 2 to 50 nm) have many potential applications, most of the synthesis processes of such particles are complex, tend to take several days under controlled environment and involve post treatment methods to improve pore structures which make them expensive for commercial applications.

Several studies can be found in the literature to achieve desired specific surface area and morphology; however none of them reports a simple synthesis method as described in this present invention. Mesoporous silica is usually synthesized by hydrolysis and condensation reaction of silica precursors in aqueous solution. The rate of reaction can be enhanced by using acid or base catalysts. The mesoporous silica structure formation and the effect of surfactant on the structure under highly acidic conditions are not well explored.

The template-directed synthesis method is the most common technique used for the synthesis of mesoporous silica with controlled structural features. The template is generally an ionic or non-ionic surfactant, which can self-assemble in solution to form ordered structures.

The current state of silica researches showed that to control the morphology and specific surface area of mesoporous silica, the synthesis parameters such as concentration of the pore forming agent and silica source, humidity, temperature, additives, stirring rate and aging have to be controlled. Several examples can be found below:

In one study, an emulsion process of silica synthesis in acidic conditions was presented. Silica precursor was dissolved in organic solvent and mixture was added slowly in surfactant containing aqueous acidic solution. It was found that the increase in stirring rate changed the silica morphology from fibers to spheres. They reported that the spheres were brittle in nature and can be broken with spatula (Schacht et ah, 1996).

In another study, the effect of synthesis temperature and duration, dilution and acidity on the morphology of mesoporous silica was reported. The spherical silica particles were synthesized at higher temperatures. The synthesis method depends on one heating cycle and two heating cycle systems. The mesoporous silica particles with spherical structures were synthesized under one heating cycle, low acidity, elevated temperature and low dilution conditions (Mesa et al., 2003). In a further study, the synthesis of mono-disperse silica particles was presented. Silica precursor with ionic surfactant was allowed to react in aqueous acidic solution for 7-10 days at 80 °C. The long synthesis time was not feasible for commercial applications (Yang et ah, 1998).

In another study, the synthesis of spherical silica particles of 3-8 pm size was reported in acidic solution. The synthesis of these spherical particles required 48- 72 hr (Boissiere et al., 2001). The purpose of this invention is to provide mono-dispersed micron sized particles as well as monoliths with high surface area within a short time (2 to 3 days) with very simple methodology and low energy consumption as strict control over the temperature is not needed during evaporation process and synthesis. The size and shape of mesoporous silica particles play an important role in determining their applications, since they determine the accessibility of the pores and diffusion of various molecules in them. Various techniques exist for synthesizing mesoporous silica particles in acidic solution. The United State patent document US6334988B1 discloses a method of synthesizing spherical mesoporous silica particles from a reaction mixture having an acidic catalyzer, surfactant, silica precursor and water. The stirring, sonication and heating of the reaction mixture from 110 °C to 210 °C is required to achieve more than 90% spherical particles.

The United State patent document US20050244322A1 discloses a hollow structured mesoporous silica material having a shell and pore channels perpendicular to the inner surface of the shell. These mesoporous materials were formed by using inorganic calcium carbonate templates with different shapes and then removing these inorganic templates to prepare mesoporous silica with different hollow shapes.

The European patent document EP3470371A1 discloses the method of controlling the morphology of mesoporous silica particles in a modified Stober procedure. The morphology of mesoporous silica particles was controlled by using Bile acid or an alkali metal cation salt, silica source, surfactant in an aqueous medium having ammonia as catalyzer. Further, the reaction mixture was stirred for 90 min, centrifuged, washed with ethanol and purified by ultracentrifugation process.

The United State patent document US20060118490A1 described the synthesis of mesoporous inorganic oxide particles from a solution having proton donor or an alcohol, fluoride source or a mineral acid, an inorganic acid and surfactant. The mesoporous structure was formed by heating the reaction mixture for 120 min at temperature > 35 °C. The United State patent document disclosed the spherical/elliptical mesoporous silica particles having an inner structure composed of nanotubes. These mesoporous structures were formed by reacting the aqueous solution of surfactant, an alcoholic cosolvent, silica precursor and a basic catalyzer. The United State patent document US6696258B1 explained a synthesis process of mesoporous materials from an aqueous solution of organometallic compounds, pore forming poly acids and an acid catalyzer. The Japanese patent document JP2019214505A disclosed the synthesis of connected mesoporous silica particles. These connected mesoporous silica particles are made of primary particles having diameter of 7 - 300 nm. The connected mesoporous silica particles are formed by stirring the reaction mixture of surfactant, basic catalyzer and silica source for 8 h, then filtering the solution and baking in an oven at 45 °C. The pore expanding agents and hydrothermal treatments were used to expand the pore size and surface area of connected mesoporous silica particles. The Japanese patent document JP5358570B2 described the synthesis of mesoporous silica microparticles from a solution having surfactant, silica source, co-solvent and a basic catalyzer. The porous microparticles were hydrothermally treated at 70 °C - 150 °C to increase the pore size. Another Japanese patent document JP4478766B2 described the synthesis of monodisperse mesoporous silica particles from an acidic solution. These particles were prepared by dissolving non-ionic surfactant in acidic solution and mix this solution with aqueous silica precursor under stirring for 2 h, filtering and washing with warm water of 60 °C and then finally sufficiently drying at 50 °C.

In conclusion, various studies have been done to control the morphology and specific surface area of silica particles both in acidic and alkaline solutions. The effect of different factors such as temperature, aging, stirring rate, and additives on the morphology and pore structures of mesoporous silica particles was investigated. The specific surface area of mesoporous structures is improved by aging the samples at elevated temperatures, using inorganic templates or treating them with oil/water emulsions. However, the control over size and specific surface area of mesoporous spherical silica particles is still very challenging due to the complexity of process and number of factors affecting the shape and size of particles. In the previous literature, no study has previously reported synthesis of mesoporous silica having hierarchical morphology, high specific surface area and controllable size and shape.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a method for synthesizing meso/microporous silica particles with hierarchical morphology. The other aspect of the present invention is to provide a method for synthesizing meso/microporous silica particles with high surface area in highly to mildly acidic solution.

Another aspect of the present invention is to provide a method for synthesizing meso/microporous silica particles that doesn’t need any stirring, aging or additional additives.

A further aspect of the present invention is to provide a method for synthesizing meso/microporous silica particles which is achieved in one single step by adjusting the composition of the catalyst, surfactant (or pore forming template) and the precursor of the silica to control the shape, size, specific surface area of the porous silica particles.

This present invention provides mono-dispersed micron sized particles as well as monoliths with high surface area within a short time (2 to 3 days) with very simple methodology and low energy consumption as strict control over the temperature is not needed during evaporation process and synthesis.

The method is based on the simultaneous production of porous silica nanoparticles and their assembly into spherical, polyhedral or hexagon shape or synthesis of porous silica nanoparticles and their growth to monolithic shape.

This object and other objects of this invention become apparent from the detailed discussion of the invention that follows.

Brief Description of Figures

The present invention is illustrated in the accompanying figures wherein; Figure 1 is schematic diagram of the method to produce porous silica with controlled morphology and specific surface area.

Figure 2 is schematic illustration of the formation of spherical meso/microporous silica particles with hierarchical morphology. Figure 3 is a) a SEM image of a spherical meso/microporous silica particles. The scale bar is 1 pm. b) Annular dark field scanning transmission electron (ADF- STEM) image of one of the silica particles showing hexagonal platelets.

Figure 4 is an illustration of the change of spherical secondary particle size as a function of acid concentration at constant surfactant and silica precursor concentration.

Figure 5 is an illustration of BET surface area of meso/microporous silica assemblies as function of acid concentration (specific surface area (SSA))

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, it is aimed to discover a new method for synthesizing meso/microporous silica particles with hierarchical morphology.

The invention presents the one step room temperature synthesis of porous silica with high specific surface area. The invention is novel as it combines the simultaneous formation of porous silica nanoparticles with their agglomeration to form various shapes. Specifically, the present invention explains the role of catalyst concentration and the precursor-surfactant ratio on the control of amorphous silica morphology, size of mono-dispersed spherical particles, and mesopore/micropore formation and specific surface area. According to the invention, a method is provided for synthesizing meso/microporous silica particles comprising the step of:

- mixing a catalyst, a surfactant and silica precursor, respectively, in an aqueous solution to form a gel through cooperative self-assembly process and create various morphologies

- evaporating the solvent to dry the gel under ambient condition, where ambient condition means the surrounding temperature range between 15 °C to 45 °C and relative humidity range of 30 % to 80%, more preferably in the temperature range of 20 °C to 30 °C and relative humidity range of 50 % to 70 %. - heating the silica gel to a temperature of 400°C to 450°C to calcine the silica and to form meso/microporous silica.

In the present invention, the synthesis process is performed at room temperature between 10-45 °C, preferably between 25-45 °C, more preferably between 30-40° C. According to the present invention, the catalyst used in the synthesis method is an acid chosen from the group of mineral acids. In a preferred embodiment the catalyst is HC1, HNO3, H2SO4, HBr, HI or mixtures of them, more preferably the catalyst is HC1.

In a preferred embodiment, the concentration of acid catalyst is between 10⁴ M - 12 M to form controlled size spherical meso/microporous silica particles and mesoporous monoliths (as shown in Fig.4). More preferably, in the present invention the concentration of acid catalyst is between 3 M - 8 M.

According to the present invention, the surfactant used in the synthesis method is selected from the group comprising ionic surfactant, non-ionic surfactant and the like and mixtures. In a preferred embodiment according to the present invention, said surfactant is a non-ionic surfactant. More preferably, the surfactant is a cetyltrimethylammonium bromide or -chloride, polyethylene oxide, polyethylene oxide-based block copolymer, such as polyoxyethylene based ether known under the trademark Brij, poloxamer block copolymers known under the trademark Pluronic. In a preferred embodiment according to the present invention, said surfactant is Pluronic block copolymers, more preferably Pluronic F127. Pluronic F127 (Sigma-Aldrich) having the formula poly(ethylene oxide)106- poly (propylene oxide)70-poly (ethylene oxide) 106 (PE0106PP070-PE0106) block copolymers consist of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly (propylene oxide) (PPO) blocks arranged in A-B-A triblock copolymer structure: PEO-PPO-PEO with end terminal hydroxyl groups. Pluronic FI 27 is a hydrophilic non-ionic surfactant.

In a preferred embodiment, the concentration of the surfactant is between 0.01 g/mL - 1.0 g/mL, more preferably 0.01 g/mL - 0.40 g/mL in the aqueous solution containing acid catalyst. According to the present invention, the silica precursor used in the synthesis method is selected from the group of silicon alkoxides, sodium silicate and the like and mixtures. In a preferred embodiment according to the present invention, said silica precursor is silicon alkoxide, such as tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS). More preferably, the said silica precursor is tetraethyl orthosilicate (TEOS).

In a preferred embodiment, the concentration of the silica precursor is between 200-3000 pL/mL, preferably 200-1000 pL/mL in the process.

In one embodiment of the invention, the meso/microporous silica particles are synthesized in a highly acidic aqueous solution at room temperature using HC1 as catalyst, Pluronic F127 as the surfactant, and TEOS as precursor, respectively. Unless specified otherwise, the term “hierarchical morphology” refers to hierarchical order morphology of the present invention wherein it is organized in a hierarchical manner in the three spatial dimensions.

According to the present invention, this room temperature synthesis method leads to form of monodisperse and micron-sized meso/microporous spherical silica particles which have two level of hierarchical order as hierarchical morphology. The spherical silica particles are formed by agglomeration of hexagonal platelets (that is, primary particles) and having unidirectional aligned cylindrical mesoporous channels. These meso/microporous silica particles are formed of densely packed distorted hexagonal platelets.

According the present invention, it is found that size and the specific surface area of the spherical particles can be controlled by the concentration of the acid catalyst (concentrations > 3 M) at a constant Pluronic F127/TEOS molar ratio. To achieve the objective of forming meso/microporous primary particles by cooperative self-assembly and obtaining micron-sized spherical secondary particles by agglomeration of these primary particles, the effect of HC1 catalyst concentration on the morphology of the synthesized silica structures are important.

Furthermore, the size and specific surface area of spherical meso/microporous silica particles is controlled by changing the concentration of acid catalyst between 12 M - 3 M. The concentration of acid catalyst also controls the morphology of the meso/microporous silica structures, resulting in monolith formation below 1 M concentration. Decreasing the acid concentration (below 3 M HC1) further increases the order of the cylindrical mesopores in the monoliths, while increasing ((above 3 M HC1) their specific surface area as well. These results enable us understand the evolution in structure and morphology of the meso-silica during the synthesis process, by tuning a simple parameter — the acid catalyst concentration — and to design new meso-silica materials with controlled morphology and high specific surface area.

It is an object of this invention to provide controlled size and specific surface area of the spherical particles which is achieved by the concentration of the acid catalyst at a constant Pluronic F127/TEOS molar ratio. According to the invention, the constant molar ratio of Pluronic F127/TEOS is 10³.

In a preferred embodiment according to the present invention, said meso- microporous silica particles have specific surface area of 200- 900 m²/g. In another embodiment, the meso/microporous silica particles are made of hexagonal platelets of 20-30 nm in diameter. The hexagonal platelets of meso/microporous silica particles have unidirectional pore diameter between 3-4 nm.

The pore volume of the particles is between 0.05- 0.07 m³/g. The size of meso/microporous silica particles is between 1-7 pm.

These examples are intended to representative of specific embodiments of the invention and are not intended as limiting the scope of the invention.

SPECIFIC EMBODIMENTS

In these embodiments, a preparation procedure was applied to provide the monodisperse and micron-sized meso/microporous spherical silica particles. After obtaining the candidate meso/microporous spherical silica particles by the preparation method illustrated also in Figures, experimental and characterization studies lead to show the relevant results related to the invention.

Examples Example 1 Experimental method

The meso/microporous silica samples were synthesized in aqueous solutions at room temperature using Pluronic F127 as the surfactant, TEOS as precursor and HC1 as catalyst. 0.01-0.30 mg/mL Pluronic F127 (poly (ethylene oxide)io_6- poly(propylene oxide)70-poly(ethylene oxide)io6 (REOioόRROgo-REOioό) (Sigma- Aldrich) was dissolved in the aqueous solution (10⁴ M - 8.0 M) of hydrochloric acid (HC1, Sigma- Aldrich). To this solution 200 - 1000 pL of TetraEthyl OrthoSilicate (TEOS, Merck) was added at once, while the sample was shaken vigorously for 30 s. Within 30 s to 6 min, a white precipitate was observed in the solution of 5.0 M-8.0 M HC1. At lower acid concentrations, precipitation took longer. All of the samples were cast on a Teflon plate within 15 min after TEOS addition, for evaporating the solvent (water) and the byproduct (ethanol) in ambient conditions for 24 h. The dried samples were calcined to remove the organic templating agent at 450 C degree (heating rate of l°C/min) for 8 h. The morphology of the resulting silica was analyzed by imaging in a field- emission scanning electron microscope (FE-SEM, Zeiss Ultra Plus). The average size of the silica spheres was determined from the SEM images using ImageJ program by measuring the diameter of at least 20 objects. The surface area of the samples was determined by nitrogen adsorption measurements (Micromeritics ASAP 2020 HD). The specific surface area (SSA) of the samples was determined by applying the Bmnauer-Emmett-Teller (BET) model to fit the nitrogen adsorption isotherm. The pore diameter (dp) and pore volume (Vp) were estimated by using the Barrett-Joyner-Halenda (BJH) method to fit the N2 isotherms in the 2-50 nm pore diameter range. The mesoporous microstmcture was analyzed by annular dark field (ADF) imaging in a scanning transmission electron microscope (STEM; JEOL JEM ARM200- CF) operated at 200 kV, using an ADF detector collection semi-angle of 31.5 mrad.

Example 2 Results Fig. 2 schematically illustrates the formation of spherical meso/microporous silica particles with hierarchical morphology. The formation mechanism consists of a succession of processes, starting with the hydrolysis and cluster formation of silicate species in the surfactant containing acidic aqueous solution (concentrations > 3.0 M HC1). As the number of clusters increase, they interact with the surfactant’s PEO corona and cooperatively self-assemble into cylindrical structures. At the same time, the interacting clusters in the assembled corona form hexagonal plate-like primary particles, whose size was determined to be around 20-30 nm. The unidirectional orientation of the cylindrical mesopores within primary particles is due to the cooperative self-assembly between silica and the surfactant. The primary particles having meso/microporous structure and “distorted hexagonal” shapes aggregate to form secondary spherical particles. The HC1 catalyst concentration increases the monosilicic acid polymerization rate, and therefore the formation rate of primary particles. An increase in the collision rate of primary particles increases the number of spherical secondary particles, while decreasing their sizes. The whole process simply provides a single parameter, the HC1 catalyst concentration, as an easy tool to control the size of the spherical secondary particles consisting of hexagonal mesoporous silica primary particles.

Annular dark field (ADF) STEM imaging revealed the hierarchical morphology at 2 distinct size scales in the micron- sized spherical silica particles (Fig. 3) Fig. 3a shows the SEM image of MS2 spherical particles -1200 pm in diameter. The spherical particles were in fact aggregates of mesoporous, hexagonal, platelet-like primary particles, as shown in Fig. 3b. The diameter of the hexagonal primary particles was in the range -20-30 nm. The diameter of the cylindrical pores in the primary particles was measured to be -3.3 ± 0.4 nm from the ADF-STEM images. The mesoporous channels were aligned in one direction along the long axis within a primary particle (Fig. 3b). The orientation of primary particles was random within the secondary spherical particles. Such hierarchical morphology of densely packed mesoporous silica primary particles forming the secondary spherical particles is being reported for the first time, according to the authors’ knowledge.

In Figure 4, the change of spherical secondary particle size as a function of acid concentration at 0.01 g/mL Pluronic F127 and 200 pL/mL TEOS concentration is shown. Fig. 4 shows the change in diameter of the spherical secondary particles as a function of HC1 concentration determined from SEM images as 4250 ± 500 nm at 3 M HC1 and 1000 ± 200 nm at 8 M HC1. The size of the spherical particles decreased exponentially with increasing acid concentration. It suggests that HC1 catalyst concentration increased exponentially the hydrolysis and the condensation rate (monosilicic acid polymerization rate) of silica and the rate of primary particle formation. The increase in the number of primary particles and thus their collision rate with increasing HC1 concentration resulted in the nucleation of more secondary particles leading to smaller sizes in secondary particles.

The effect of acid concentration on the morphology and the specific surface area of silica structures is presented in Fig. 5 at constant Pluronic F127 (0.01 g/mL) and TEOS (200 pL/mL) concentration. The acid concentration was changed in the range of 10⁴ M- 8 M. Regions showing the two distinct types of morphology, namely the spherical particles and monoliths are marked by circles in Fig. 5. Each morphology showed a different dependence of specific surface area on the acid concentration in the two distinct regions: i) in high HC1 concentrations (>3.0 M), in which spherical particles were observed, the specific surface area increased with increasing acid catalyst concentration; ii) in low HC1 concentrations (< 1.0 M), in which monoliths were observed, the specific surface area increased with decreasing acid catalyst concentration. Fig 5. Clearly demonstrates that both the morphology and the specific surface area of the synthesized silica structures can be controlled by the concentration of the catalyst. References

Schacht S, Huo Q, Voigt-Martin IG, Stucky GD, Schuth F. Oil-Water Interface Templating of Mesoporous Macroscale Structures. Science. 1996;273(5276):768-771. Mesa, M., Sierra, L., Lopez, B., Ramirez, A., Guth, J-L. Preparation of micron sized spherical particles of mesoporous silica from a triblock copolymer surfactant, usable as a stationary phase for liquid chromatography. Solid State Science. 2003; 5:1303-1308.

Yang, H., Vovk, G., Coombs, N., Sokolov, L, Ozin, GA. Synthesis of Mesoporous Spheres Under Quiescent Aqueous Acidic Conditions, Journal of Materials Chemistry, 1998, 8(3), 743-750.

Boissiere, C., Kiimmel, M., Persin, M., Larbot, A., Prouzet, E. Spherical MSU-1 Mesoporous Silica Particles Tuned for HPLC, Adv. Funct. Mater. 2001, 11, 129- 134.

Claims

1. A method for synthesizing meso/mircoporous silica particles comprising the steps of: - mixing a catalyst, a surfactant and silica precursor, respectively, in an aqueous solution to form a silica gel

- evaporating the solvent to dry the gel under ambient condition.

- heating the silica to a temperature of 400°C to 450 °C for calcination and to form meso/microporous silica particles.

2. A method for synthesizing meso/microporous silica particles according to claim 1, wherein the mixing step is performed at room temperature between 10-45 °C

3. A method for synthesizing meso/microporous silica particles according to claim 1, the mixing step is performed at room temperature between 25-45 °C.

4. A method for synthesizing meso/microporous silica particles according to claim 1, the mixing step is performed at room temperature between 30-40 °C.

5. A method for synthesizing meso/microporous silica particles according to the preceding claims, wherein the catalyst is a mineral acid selected from the group of HC1, HNO₃, H₂SO₄, HBr, HI and mixtures.

6. A method for synthesizing meso/microporous silica particles according to claim 5, wherein the catalyst is HC1.

7. A method for synthesizing meso/microporous silica particles according to claim 6, wherein the concentration of the catalyst is between 10⁴ M - 12 M, more preferably between 3 M-8 M to form controlled size spherical meso/microporous silica particles and mesoporous monoliths.

8. A method for synthesizing meso/microporous silica particles according to the preceding claims, wherein the surfactant is selected from the group of ionic surfactant, non-ionic surfactant and the like and mixtures.

9. A method for synthesizing meso/microporous silica particles according to claim 8, wherein the surfactant is a non-ionic surfactant.

10. A method for synthesizing meso/microporous silica particles according to claim 9, wherein the surfactant is a polyethylene oxide-based block copolymer.

11. A method for synthesizing meso/microporous silica particles according to claim 10, wherein the surfactant is Pluronic F127.

12. A method for synthesizing meso/microporous silica particles according to claim 11, wherein the concentration of surfactant is between 0.01 g/mL - 1.0 g/mL, more preferably 0.01 g/mL - 0.40 g/mL in the aqueous solution containing catalyst.

13. A method for synthesizing meso/microporous silica particles according to the preceding claims, wherein the silica precursor is selected from the group of silicon alkoxides, sodium silicate and the like and mixtures.

14. A method for synthesizing meso/microporous silica particles according to claim 13, wherein the silica precursor is silicon alkoxide selected from the group of tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS).

15. A method for synthesizing meso/microporous silica particles according to claim 14, wherein the silica precursor is tetraethyl orthosilicate (TEOS).

16. A method for synthesizing meso/microporous silica particles according to claim 15, wherein the concentration of silica precursor is between 200-3000 pL/mL, preferably 200-1000 pL/mL.

17. A method for synthesizing mesoporous silica particles according to the preceding claims, wherein the constant Pluronic F127/TEOS molar ratio 10³.

18. Meso/microporous silica particles obtained by the method of claim 1-17, wherein specific surface area is between 200- 900 m²/g.

19. Meso/microporous silica particles according to claim 18, wherein it is made of hexagonal platelets of 20-30 nm in diameter.

20. Meso/microporous silica particles according to claim 19, wherein hexagonal platelets have unidirectional pore diameter of 3-12 nm.

21. Meso/microporous silica particles according to claim 17 to 20, the pore volume of the particles is between 0.05- 0.07 m³/g.

22. Meso/microporous silica particles according to claim 17 to 21, the size of meso/microporous silica particles is in range of 1-7 pm.