Nanometer-size zeolitic particles and method for the production thereof

Abstract

A particulate material and a process for the production thereof are provided, which particulate material comprises zeolitic particles having a crystalline structure, which contain as the main component a zeolite material having a zeolitic framework structure formed from Si, O and optionally Al, and/or a zeolite-like material having a zeolitic framework structure which is formed not only from Si, O and optionally Al, wherein the zeolitic particles are in the form of essentially spherical particles with nanometer dimensions.

Claims

1. A process for producing a particulate material which comprises zeolitic particles having a crystalline structure, wherein the particles contain 60 wt % or more of a zeolite material having a microporous zeolitic framework structure formed from Si, O and optionally Al, and/or a zeolite-like material having a microporous zeolitic framework structure which is formed not only from Si, O and optionally Al, wherein the zeolitic particles are in the form of particles with nanometer dimensions, and wherein at least 90% of all zeolitic particles, based on a number of particles, have a particle size of 20 to 500 nm, characterized in that the process comprises the following steps: a) providing a starting material comprising amorphous porous starting particles, which are composed of at least one oxide that can form a zeolite material having a zeolitic framework structure or a zeolite-like material having a zeolitic framework structure; b) introducing a solution or dispersion of an organic compound in a solvent, wherein the organic compound functions as a template for the synthesis of a zeolitic framework structure, into the pores of the amorphous porous starting particles, and subsequently fully or partially removing the solvent of the solution or dispersion, so that the organic compound remains in the pores throughout the amorphous porous starting particles and is also present in pores at the center of the porous starting particles; c) converting the material obtained in step b), which contains the amorphous porous starting particles with the organic compound in the pores, by heating the starting material in contact with steam so that the zeolitic particles are formed.

2. The process as claimed in claim 1, wherein the zeolitic particles are in the form of spherical particles with nanometer dimensions, wherein at least 90% of the amorphous porous starting particles of the starting material, expressed in terms of a number of particles, have a particle size of between 100 nm and 1000 nm.

3. The process as claimed in claim 1, wherein the pore volume of the pores with a diameter of 1 nm or more in the amorphous porous starting particles of the starting material lies in the range of from 0.2 to 2.0 ml/g, expressed in terms of the weight of the amorphous porous starting particles.

4. The process as claimed in claim 1, wherein the introduction of the solution or dispersion in step b) is carried out such that the solution or dispersion penetrates into all pores that are open toward the particle surface, with a diameter of 1 nm or more, of the amorphous porous starting particles of the starting material.

5. The process as claimed in claim 1, wherein a fill factor of the pores with a diameter of 1 nm or more of the amorphous porous starting particles obtained in step b) with the organic compound, defined as the ratio of the volume of the organic compound contained in the pores and the pore volume of mesopores of the particles, is from 50 to 100%.

6. The process as claimed in claim 1, wherein a metal compound is additionally introduced into the pores of the amorphous porous starting particles of the starting material in step b).

7. The process as claimed in claim 1, wherein the organic compound is a tetraorganoammonium cation or a tetraorganophosphonium cation.

Description

EXAMPLES

Example 1 (Comparative Example): Production of Nanozeolites According to an Established Method According to Van Grieken [R. Van Grieken et al. in Microporous and Mesoporous Materials, Vol. 39 (2000), 135-147]

(1) 37 g of distilled water and 94 g of tetrapropylammonium hydroxide solution (40 wt % TPAOH solution, Clariant) were stirred in a polypropylene bottle. 4.65 g of Al(NO.sub.3)3*H.sub.2O (98 wt % from Sigma Aldrich) were added thereto, and the mixture was cooled to 0° C. with ice water. At 0° C., 105 g of tetraethyl orthosilicate (TEOS, 98 wt % from Alfa Aesar) were added, and stirring was continued for several hours. The colloidal solution obtained was then concentrated at 80° C. in order to remove water and ethanol. The concentrated colloidal solution, with a molar composition of SiO.sub.2:0.011250 Al.sub.2O.sub.3:0.36 TPAOH:11H.sub.2O, was transferred into a plurality of 50 ml autoclaves. All the autoclaves were then closed and heated to 170° C. The subsequent crystallization was carried out at 170° C. for 48 h in a preheated circulating air oven. The autoclave was then cooled to room temperature with cold water and opened, and the synthesis product was separated from the supernatant solution by centrifugation, and then washed several times with distilled water (pH 8). The drying was carried out at 75° C. overnight.

(2) FIG. 3 shows by way of example an electron microscopy (SEM) image of the MFI crystals obtained. The nanoparticles obtained, with diameters of up to 50 nm, form agglomerates with diameters of up to 200 nm.

Example 2 (Production of Starting Particles): Production of Mesoporous SiO.SUB.2 .Particles as Starting Products for Zeolitic Nanoparticles According to the Invention

(3) 828 g of distilled water were initially charged into a polypropylene cup, and 6 g of hexadecyltrimethylammonium bromide (CTAB, 98%, Sigma Aldrich) were added while stirring. 2876 g of technical ethanol (96%) were added to this mixture, and stirring was continued until a clear solution was obtained. 144 g of ammonia solution (25 wt %) with then added while stirring, and stirring was continued for 1 hour. 20 g of tetraethyl orthosilicate (TEOS, 98 wt %, Alfa Aesar) were then added, and the resulting mixture was stirred for a further 2 hours. The resulting SiO.sub.2 particles were then separated from the synthesis mixture by centrifugation at 10,000 rpm and washed three times with distilled water. Lastly, the purified SiO.sub.2 particles were dried in air at 75° C. overnight and then calcined at 550° C. in an air atmosphere.

(4) The structure and the porosity of the SiO.sub.2 particles produced in this way were confirmed by X-ray analysis (FIG. 4) and N.sub.2 physisorption (FIG. 6), the particles having mesopores with a pore maximum in the distribution of about 3 nm. These particles furthermore had particle diameters of between 450 and 600 nm, as shown in the electron microscopy image in FIG. 5.

Example 3 (Production of Starting Particles): Production of Porous Al.SUB.2.O.SUB.3.—SiO.SUB.2 .Particles as Starting Products for Zeolitic Nanoparticles According to the Invention

(5) Porous Al.sub.2O.sub.3—SiO.sub.2 particles as starting products for the synthesis of zeolitic nanoparticles according to the invention were produced according to a modification of the method of Ahmed et. al. [Ahmed et. al., Industrial & Engineering Chemistry Research, 49 (2010) 602]. For a typical batch, 4 g of polyvinyl alcohol (PVA Mw of 31-50 k, 98 wt % from Sigma-Aldrich) were first dissolved in 105 g of deionized water at 80° C. in a beaker. After about 20 to 30 min, 0.12 g of sodium aluminate solution (53 wt % Al.sub.2O.sub.3 and 43 wt % Na.sub.2O from Chemiewerk Bad Köstritz GmbH) were added to the PVA solution at 80° C. while stirring. The mixture thereby obtained continue to be stirred until the sodium aluminate had completely dissolved. The solution was then cooled to room temperature and transferred into a 500 ml stirred reactor made of glass. 1.61 g of CTAB and 101 g of ethanol were subsequently added to the cooled mixture while stirring and heated to 40° C. Finally, 7.2 g of TEOS were added, and the synthesis mixture obtained, with a molar composition of 1 TEOS:0.006 Al.sub.2O.sub.3:2.9 NH.sub.3:0.12 CTAB:162H.sub.2O:58 ethanol:0.003 PVA at 40° C., was stirred further for about 40 h. The SiO.sub.2 particles obtained were separated from the synthesis mixture by centrifugation at 10,000 rpm and washed three times with deionized water. Finally, the purified Al.sub.2O.sub.3—SiO.sub.2 particles were dried in air at 75° C. overnight and then calcined at 550° C. in an air atmosphere.

(6) The structure and the porosity of the SiO.sub.2 particles produced in this way were analyzed by X-ray analysis (FIG. 8) and N.sub.2 physisorption, and it was confirmed that the particles have mesopores. Furthermore, these particles had particle diameters of between 550 and 700 nm as shown in the electron microscopy image in FIG. 7.

Example 4 (According to the Invention): Production of Aluminum-Free Zeolitic Nanoparticles

(7) 1 g of SiO.sub.2 particles (Example 2) were mixed in a Teflon vessel with 1.25 g of tetrapropylammonium hydroxide solution (TPAOH, 40 wt %, Clariant), and 5 g of deionized water were added and stirring was continued for 16 h at room temperature. The suspension obtained was then dried at 65° C. in an oven for 6 h. In order to determine the degree of loading of TPAOH in the mesopores, the dried powder was analyzed by means of thermogravimetry (TGA). This revealed a proportion of the organic compound (the organic template) of about 25 wt %, which corresponds to about 83 vol % of the total mesopore volume of the SiO.sub.2 particles. Furthermore, it could be demonstrated from the weight loss/temperature diagram (TGA profile) that the organic template (TPAOH) for the most part was located in the mesopores. Subsequently, the dried powder was finely ground in a mortar and transferred into 4 porcelain dishes, and these were then introduced into 23 ml Teflon vessels as represented in FIG. 2. In each Teflon container, there was 8 g of water. Care was taken that the water did not come in contact with the TPAOH-SiO.sub.2 particles. Subsequently, all the Teflon vessels were transferred into 4 stainless steel autoclaves and closed in a pressure-tight fashion. Finally, the autoclave was heated for 12 h at 110° C. After the time had elapsed, all the autoclaves were cooled to room temperature. The solid contained therein was separated from the synthesis mixture by centrifugation at 10,000 rpm, washed three times with distilled water, dried overnight at 75° C. and subsequently characterized.

(8) Electron microscopy images (FIG. 9a) showed that the solid product obtained consisted of very small crystals with nanometer dimensions, which have the spherical morphology of the mesoporous SiO.sub.2 particles used as starting product (FIG. 5), and in contrast to the nanozeolites which were produced by the known synthesis method (Example 1) were not agglomerated. X-ray diffraction (FIG. 9a) shows that the product is a zeolite of the MFI type with high crystallinity. Furthermore, the yield of zeolite nanoparticles was determined at more than 80 wt %, expressed in terms of the weight of the porous starting particles used. FIG. 10 shows that all the mesopores in the mesoporous SiO.sub.2 particles used as starting material had been converted into zeolite micropores.

(9) FIG. 15 shows the particle size distribution of the starting particles produced in Example 2 (“MSP”) and the nanoparticles obtained in Example 4 (with a conversion time in the autoclave of 6 and 12 h, respectively) measured by means of dynamic light scattering (DLS). In this case, the particles were suspended in water and subsequently dispersed by means of ultrasound treatment for about 2 h. Subsequently, the dispersion obtained was transferred into a cuvette and the particle size distribution was determined by means of DLS.

Example 5 (According to the Invention): Production of Aluminum-Containing Zeolitic Nanoparticles

(10) 1 g of Al.sub.2O.sub.3—SiO.sub.2 particles (Example 3) were mixed in a Teflon vessel with 1.25 g of tetrapropylammonium hydroxide solution (TPAOH, 40 wt %, Clariant), and 5 g of deionized water were added and stirring was continued for 16 h at room temperature. The suspension obtained was then dried at 65° C. in an oven for 6 h. Subsequently, the dried powder was finely ground in a mortar and transferred into porcelain dishes, and these were then introduced into 23 ml Teflon vessels as represented in FIG. 2. In each Teflon container, there was 8 g of water. Care was taken that the water did not come in contact with the TPAOH-Al.sub.2O.sub.3—SiO.sub.2 particles. Subsequently, the Teflon vessels were transferred into stainless steel autoclaves and closed in a pressure-tight fashion. Finally, the autoclave was heated for 12 h at 110° C. After the time had elapsed, all the autoclaves were cooled to room temperature. The solid contained therein was separated from the synthesis mixture by centrifugation at 10,000 rpm, washed three times with distilled water, dried overnight at 75° C. and subsequently characterized.

(11) Electron microscopy images (FIG. 12b) showed that the solid product obtained consisted of very small crystals with nanometer dimensions, which have the spherical morphology of the mesoporous Al.sub.2O.sub.3—SiO.sub.2 particles used as starting product (FIG. 12b), and in contrast to the nanozeolites which were produced by the known synthesis method (Example 1) were not agglomerated. X-ray diffraction (FIG. 11) shows that the product is a zeolite of the MFI type with high crystallinity. Furthermore, the yield of zeolite nanocrystals was determined at more than 80 wt %. Furthermore, an Si/Al ratio of 100 was measured by ICP-OES.

Example 6 (According to the Invention): Production of Platinum-Containing Zeolitic Nanoparticles (Pt/ZSM-5)

(12) Pt/ZSM-5 was produced from Al.sub.2O.sub.3—SiO.sub.2 particles (Example 3) by a 2-step ion exchange process and impregnation. In the 1.sup.st of the ion exchange steps, 4 g of Al.sub.2O.sub.3—SiO.sub.2 particles were stirred with 100 g of 0.2 M NaCl at 60° C. for 3 h. This procedure was repeated two times. The Na.sup.+—Al.sub.2O.sub.3—SiO.sub.2 particles were then washed three times with deionized water and dried overnight at 75° C. In the 2.sup.nd of the ion exchange steps, Na was replaced with [Pt(NH.sub.3).sub.4].sup.2+. To this end, 1.75 g of Na.sup.+—Al.sub.2O.sub.3—SiO.sub.2 particles were stirred with 43 g of 1 mM [Pt(NH.sub.3).sub.4](NO.sub.3).sub.2 at 60° C. overnight. The Pt-containing Al.sub.2O.sub.3—SiO.sub.2 particles were then separated by centrifugation, washed several times (six times) with deionized water and dried at 75° C. overnight. 1 g of [Pt(NH.sub.3).sub.4].sup.2+—Al.sub.2O.sub.3—SiO.sub.2 particles was subsequently mixed in a Teflon vessel with 1.25 g of tetrapropylammonium hydroxide solution (TPAOH, 40 wt %, Clariant), and 5 g of deionized water were added and stirring was continued for 16 h at room temperature. The suspension obtained was dried at 65° C. in an oven for 6 h. Subsequently, the dried powder was finely ground in a mortar and transferred into 4 porcelain dishes and then into 4 different 23 ml Teflon vessels as represented in FIG. 2. In each Teflon container, there was 8 g of water. Care was taken that the water did not come in contact with the TPAOH-SiO.sub.2 particles. Subsequently, the Teflon vessels were transferred into stainless steel autoclaves and closed in a pressure-tight fashion. Finally, the autoclave was heated for 24 h at 110° C. After the time had elapsed, all the autoclaves were cooled to room temperature. The solid contained therein was separated from the synthesis mixture by centrifugation at 10,000 rpm, washed three times with distilled water, dried overnight at 75° C. and subsequently characterized.

(13) Electron microscopy images (FIGS. 14a and b) showed that the solid product obtained consisted of very small crystals with nanometer dimensions, which have the spherical morphology of the mesoporous Al.sub.2O.sub.3—SiO.sub.2 particles used as starting product, and in contrast to the nanozeolites which were produced by the established synthesis method (Example 1) were not aggregated. X-ray diffraction (FIG. 13) shows that the product is a zeolite of the MFI type with high crystallinity. Furthermore, the yield of zeolite nanoparticles was determined at more than 80 wt %. Furthermore, an Si/Al ratio of 100 and a platinum content of 0.59 wt % were measured by ICP-OES. By STEM, it was possible to show that the Pt nanoparticles (1-2.5 nm size) were embedded in the zeolite framework structure of the zeolite nanoparticles.

DESCRIPTION OF THE FIGURES

(14) FIG. 1 shows by way of example a schematic representation of the main steps in the production of zeolitic nanoparticles of the MFI type.

(15) FIG. 2 shows by way of example a schematic representation of the various steps and the experimental setup in the production of zeolitic nanoparticles of the MFI type.

(16) FIG. 3 shows a scanning electron microscope (SEM) image of the nanozeolites of the MFI type produced according to Example 1 (comparative example).

(17) FIG. 4 shows an X-ray diffractogram of the calcined mesoporous silicon dioxide particles of Example 2.

(18) FIG. 5 shows a scanning electron microscope image of the calcined mesoporous silicon dioxide particles of Example 2.

(19) FIG. 6 shows the nitrogen sorption isotherm (a) and DFT pore size distribution (b) of the calcined mesoporous silicon dioxide particles of Example 2.

(20) FIG. 7 shows a scanning electron microscope image of the calcined mesoporous silicon dioxide particles of Example 3.

(21) FIG. 8 shows an X-ray diffractogram of the calcined mesoporous silicon dioxide particles of Example 3.

(22) FIG. 9 shows a scanning electron microscope image (a) and an X-ray diffractogram (b) of the zeolite nanoparticles of the MFI type without aluminum according to Example 4 (according to the invention). As a comparison, an SEM image (FIG. 5) of a nanozeolite of the MFI type produced according to Example 1 (comparative example) is shown.

(23) FIG. 10 shows the nitrogen sorption isotherm of the calcined mesoporous silicon dioxide particles and the zeolite nanoparticles of the MFI type without aluminum according to Example 4.

(24) FIG. 11 shows an X-ray diffractogram of the aluminum-containing zeolitic nanoparticles of the MFI type of Example 5.

(25) FIG. 12 shows a scanning electron microscope image of the aluminum-containing mesoporous silicon dioxide particles of Example 3 (a) and of the zeolitic nanoparticles of the MFI type (b) of Example 5.

(26) FIG. 13 shows an X-ray diffractogram of Nano-Pt/ZSM-5 of Example 6.

(27) FIG. 14 shows a scanning electron microscope image of the aluminum-containing mesoporous silicon dioxide particles of Example 3 (a) and of the Nano-Pt/ZSM-5 particles (b) of Example 6.

(28) FIG. 15 shows the particle size distribution of the starting particles produced in Example 2 (“MSP”) and of the nanoparticles obtained in Example 4 (with a conversion time in the autoclave of 6 and 12 h, respectively).