WO2019002045A1

WO2019002045A1 - Hierarchical zsm-5 zeolite with open pore structure

Info

Publication number: WO2019002045A1
Application number: PCT/EP2018/066407
Authority: WO
Inventors: Junzhong LIN; Junliang Sun
Original assignee: STOCKHOLMS UNIVERSITET HOLDING AB
Current assignee: STOCKHOLMS UNIVERSITET HOLDING AB
Priority date: 2017-06-27
Filing date: 2018-06-20
Publication date: 2019-01-03
Anticipated expiration: 2019-12-27

Abstract

The present invention relates to a hierarchical aluminosilicate ZSM-5 molecular sieve having a total pore volume of at least 0.85 cm3/g comprising mesopores having a diameter of 2-50 nm and micropores having a diameter of 0-2 nm. At least 50 percent of the mesopores have an open pore structure.5

Description

HIERARCHICAL ZSM-5 ZEOLITE WITH OPEN PORE STRUCTURE

Technical field

The present invention relates to the field of molecular sieves, more

specifically to the field of hierarchical aluminosilicate ZSM-5 molecular sieves and to a method for producing such a molecular sieve.

Background

Molecular sieves, and in particular zeolite molecular sieves are widely used in the chemical industry today, especially for applications such as adsorption, separation, ion-exchange and catalysis. Aluminosilicate zeolites, such as Zeolite Socony Mobil-5 (ZSM-5), are particularly useful as molecular sieves, owing mainly to their specific pore structure and large specific surface area. The ZSM-5 zeolite is an aluminosilicate zeolite belonging to the pentasil family of zeolites. ZSM-5 contains a three-dimensional 10MR crossing pore structure, and relatively high thermal stability and hydrothermal stability. Thus, ZSM-5 has a number of applications for catalysis in the petrochemical fields of paraffin cracking, isomerization of n-butylene, preparation of gasoline from synthesis gas and the like. ZSM-5 molecular sieves used in the art today typically exhibits a microporous morphology. Such molecular sieves suffer from a number of drawbacks.

Firstly, ZSM-5 molecular sieves are highly sensitive to deactivation and secondly, ZSM-5 molecular sieves used as catalysts are not well suited for the synthesis of bulky molecules.

Several attempts have been performed to introduce a different pore system into ZSM-5 molecular sieves. However, most proposed methods yield molecular sieves suffering from a non-uniform distribution of the pores, a low crystallinity and/or a high amount of closed pores present on the inside of the molecular sieve particles. Thus, there exists a need in the art of molecular sieves today to provide improved ZSM-5 molecular sieves as well as a need for improved methods for producing such a ZSM-5 molecular sieve. Summary of the invention

It is an object of the invention to at least alleviate some of the problems associated with the prior art. In particular it is an object of the invention to provide a molecular sieve having improved mass transport properties. It is a further object of the invention to provide a highly crystalline molecular sieve having a uniform distribution of mesopores throughout the crystals.

Yet another object of the present invention is to provide a method for producing a crystalline molecular sieve having a uniform mesopore

distribution throughout the crystals.

The above mentioned objects, as well as other objects apparent to a person skilled in the art, are each addressed by the aspects of the present invention. In first aspect thereof, the present invention provides a hierarchical

aluminosilicate ZSM-5 molecular sieve having a total pore volume of at least 0.85 cm³/g comprising mesopores having a diameter of 2-50 nm and micropores having a diameter of 0-2 nm. At least 50 percent of the

mesopores have an open pore structure.

The present invention is based on the realization that by introducing mesopores into a ZSM-5 molecular sieve, problems associated with conventional ZSM-5 molecular sieves can be alleviated. In particular, the molecular sieve according to the present invention exhibits a uniform mesopore distribution which allows better mass transport for bulky molecules, during use of the molecular sieve as e.g. a catalyst. Thus, the molecular sieve according to the invention will be less sensitive towards deactivation due to steric blockage by heavy secondary products. The presence of mesopores further allows the hierarchical molecular sieve of the invention to be used as a catalyst in the synthesis of bulky molecules, as the introduction of mesopores allows for improved mass transport of larger molecules.

The term "hierarchical" as referred to herein, should generally be understood as a property related to the porosity of the molecular sieve. A hierarchical molecular sieve should be understood as a molecular sieve which comprises two or more different pore systems. The hierarchical molecular sieve according to the present invention generally consists of two different pore systems having different pore size distribution. The molecular sieve according to the present invention comprises mesopore type pores and micropore type pores. According to the definition provided by the International Union for Pure and Applied Chemistry (lUPAC) mesopores is defined as pores with intermediate size; i.e. those with widths in the range of 2-50 nm. The definition of micropores, also according to lUPAC, is pores with widths not exceeding about 2.0 nm.

In the context of the present invention, a "molecular sieve" is intended to denote a porous solid with pores of molecular dimensions which permit the passage of molecules below a certain size. Molecular sieves are typically used for drying gases and liquids and for separating molecules on the basis of their sizes and shapes. When two molecules are equally small and can enter the pores, separation is based on the polarity (charge separation) of the molecule, the more polar molecule being preferentially adsorbed. Other applications include catalytic applications. The molecular sieve of the present invention is a zeolite molecular sieve, in particular a ZSM-5 molecular sieve. The ZSM-5 (Zeolite Socony Mobil-5) zeolite is an aluminosilicate zeolite having the framework type MFI. ZSM-5 zeolites are generally composed of several pentasil units linked together by oxygen bridges. ZSM-5 zeolites comprise a three-dimensional 10MR crossing pores structure, and exhibit relatively high thermal and hydrothermal stability. ZSM-5 molecular sieves are porous materials. The term "total pore volume" as used herein denotes the total volume pore volume in the molecular sieve per unit mass molecular sieve, measured in the unit cm³/g. ZSM-5 molecular sieves are porous materials. The total pore volume can be determined by use of a nitrogen gas adsorption measurements. Nitrogen gas adsorption measurements are typically depicted as adsorption and desorption isotherms, wherein the quantity of absorbed nitrogen is plotted as a function of the relative pressure of nitrogen. The total pore volume may be determined at a partial pressure of nitrogen near the saturation pressure, such as P/Po=0.97. Near the saturation pressure, it can be assumed that the total pore volume has been filled by N2 molecules and that the gas volume adsorbed on the free surface is insignificant. The total adsorbed gas volume can therefore be used to estimate the total pore volume by converting it to a liquid volume.

According to the present invention, the total pore volume of the molecular sieve may be at least 0.85 cm³/g, such as at least 0.90 cm³/g, preferably about 0.91 cm³/g. For example, the total pore volume may be in the range of 0.85-1 .00 cm³/g, such as in the range of 0.85-0.95 cm³/g. The total pore volume may also be at least 0.70 cm³/g, such as in the range of 0.70-1 .10 cm³/g. In some examples, the total pore volume may be at least 0.80 cm³/g.

The hierarchical aluminosilicate ZSM-5 molecular sieves according to the invention comprises mesopores having a diameter of 2-50 nm and

micropores having a diameter of 0-2 nm. The volume of the micropores can also be determined using nitrogen gas adsorption. The volume of the mesopores may be calculated as the difference between the total pore volume and the volume of the micropores. The total pore volume may be determined using the method described above. The volume of the micropores can be calculated by applying the t-plot method to the nitrogen gas adsorption isotherm. The t-plot method is known to a person skilled in the art. The t-plot method uses the nitrogen adsorption isotherm data in the region near the vertical uptake at lower relative pressures, after which the uptake plateaus. The gas uptake volume at the plateau will be converted to a liquid volume which corresponds to the total volume of the micropores.

In some embodiments of the present invention at least 50 percent of the total pore volume is made up by mesopores. In an example at least 70 percent of the total pore volume is made up by mesopores, such as at least 75 percent of the total volume, preferably about 78 percent of the total volume. The mesopore volume may be below 90 percent of the total volume, such as below 80 percent of the total volume. Stated differently, the mesopore volume is preferably in the range of 0.40-0.80 cm³/g, such as in the range of 0.65- 0.75 cm³/g, preferably about 0.71 cm³/g.

In some embodiments at least 50 percent of the mesopores have an open pore structure. For example, 50-90 percent of the mesopores may have an open pore structure, such as 50-80 percent, such as 50-70 percent. In an example, 50-60 percent of the mesopores may have an open pore structure. In some examples, at least 60 percent of the mesopores have an open pore structure, such as at least 70 percent, preferably at least 80 percent, more preferably at least 90 percent. In the present disclosure, the pores are classified into two categories based on their availability to an external fluid. Closed pores are defined as pores which are totally isolated from

neighbouring pores. Open pores are defined as pores which have a

continuous channel of communication with the external surface of the material. As used herein, the term open pores denote both pores which are open only at one end (generally denoted as e.g. blind pores, dead-end pores or saccate pores) and pores which are open at two ends (generally denoted as e.g. through pores). The number of open pores may be estimated from transmission electron microscopy (TEM) images. A molecular sieve according to the some embodiments of the present invention wherein at least 50 percent of the mesopores have an open pore structure is advantageous for several reasons. Open pores improve the mass transport of molecules in the molecular sieve compared to closed pores. It allows for a faster mass transport and which is advantageous in applications such as separation and catalysis. Furthermore, an open pore structure provides less steric hindrance for bulky molecules. The open mesopores of the present invention may be homogenously distributed both on the surface and in the molecular sieve particles.

In some embodiments of the present invention, at least 80 percent of the mesopores have a diameter in the range of 10-30 nm, such as about 15-25 nm. The molecular sieve according to the present invention comprises uniformly distributed mesopores of a defined size in the range of 10-30 nm, such as about 15-25 nm. In the present disclosure, terms such as "pore size" and "pore width" may be used interchangeably. Mesopore size distribution may be estimated from nitrogen gas adsorption and calculated with the Barret-Joyner-Halenda (BJH) method known to a person skilled in the art. The BJH method is based on the assumption that pores have a cylindrical shape and that pore radius is equal to the sum of the Kelvin radius and the thickness of the film adsorbed on the pore wall. The adsorption and desorption branches can both be used to calculate pore size distribution The amount of mesopores having a size in the range of 10-30 nm can also be estimated from TEM images. In an example, 80 to 95 percent of the mesopores may have a diameter in the range of 10-30 nm. Furthermore, 80- 90 percent of the mesopores may have a diameter in the range of 10-30 nm. In yet another example, at least 60 percent of the mesopores may have a diameter in the range of 10-30 nm

In some embodiments of the present invention the pores have an average wall thickness in the range of 20-50 nm, preferably in the range of 30-40 nm, such as about 34-36 nm. In the present disclosure, the wall size is defined as the average distance between any two pores in the material. The average wall size can be measured from TEM images using an appropriate software, such as Gatan DigitalMicrograph, known to a skilled person in the art. In some embodiments of the present invention, the specific surface area is at least 400 m²/g, such as at least 500 m²/g, preferably 500-520 m²/g. The specific surface area may be in the range of 450-550 m²/g. For example, the specific surface area may be in the range of 480-520 m²/g. In the present disclosure, the specific surface is defined as the accessible area of solid surface per unit mass. The specific surface can be derived from physisorption isotherm data (such as nitrogen adsorption data) by applying the Brunauer- Emmett-Teller (BET) method known to a skilled person in the art. A high specific surface area is preferred in applications such as catalysis. A high specific surface area is believed to increase the number of sites at which a catalyzed reaction may take place.

In some embodiments of the present invention the molecular sieve has a crystalline component of at least 70 weight percent as measured by X-ray diffraction. The amount of crystalline component can be determined from the X-ray diffractogram using the PANalytical HighScores software. The X-ray diffraction may be powder X-ray diffraction, such as powder X-ray diffraction using Panalytical Xpert Pro diffractometer, with a CuK_Q radiation (λ=1 .5406 A) source. Preferably, the crystalline component is at least 80 weight percent, such as at least 90 weight percent. The crystalline component is typically in the range of 90-100 weight percent. The high amount of crystalline

component in the molecular sieve is especially preferred in several industrial applications. The ZSM-5 molecular sieve of the present invention is preferably crystalline with a crystal size in the range of 1 -30 μιτι, such as in the range of 5-20 μιτι, preferably in the range of 5-15 μιτι. The crystals preferably have a MFI framework. The mesopores are well distributed both on the surface and inside of the ZSM-5 crystals.

In some embodiments, the molecular sieve may have an amorphous component. In a second aspect of the invention, there is provided a method for producing a hierarchical molecular sieve comprising the following steps:

a) mixing a ZSM-5 zeolite with a structure directing agent in an aqueous phase to form a mixture

b) reacting the mixture hydrothermally at a temperature of 120-180 °C for 1 -6 hours

c) separating a solid product from the reacted mixture

d) calcining the solid product at a temperature of 400-600 °C to form a hierarchical molecular sieve.

The method according to the second aspect is advantageous in that a hierarchical ZSM-5 molecular sieve having homogenously distributed mesopores throughout the structure can be produced by treating a ZSM-5 zeolite with a post-treatment step. The post-treatment step can be used to produce the molecular sieve disclosed in the first aspect of the invention. One advantage with a post-treatment step according to the present invention is that it may be used to introduce mesoporosity into the ZSM-5 zeolite. Thus, it can be used to produce a hierarchical molecular sieve. The post-treatment step according to the present invention furthermore provides a method for producing a molecular sieve which uses a mild hydrothermal re- crystallization, performed at a relatively low temperature for only a few hours. Furthermore, the method according to the present invention requires only a small amount of the alkali source. The hydrothermal recrystallization in step b) requires only a short period of time.

The method according to the second aspect of the invention is based on the realization that by post-treating a ZSM-5 molecular sieve with a structure directing agent, homogenously distributed mesopores can be introduced into the ZSM-5 molecular sieve. Thus, a hierarchical ZSM-5 molecular sieve comprising mesopores and micropores can be formed. Another advantage of the inventive method is that it may be used to produce a hierarchical molecular sieve having a high amount of crystalline material. An advantage of the method of the present invention is that it allows for the introduction of homogenously distributed mesopores on the surface and inside the ZSM-5 crystals.

The structure directing agent of step a) may be a quaternary ammonium compound, such as tetrapropylammonium bromide or tetrapropylammonium hydroxide. In step a), the structure directing agent is often added to the ZSM- 5 in the form of an aqueous solution. The solution may have a concentration in the range of 0.05-0.15 M, such as about 1 M.

The step b) involves a hydrothermal reaction. The term hydrothermal reaction is generally intended to denote a reaction wherein reactants are crystallized from aqueous solutions. The hydrothermal reaction is preferably performed at a high vapor pressure, such as in the range of

0.1 -2 MPa (1 -20 bar), such as about 1 MPa (10 bar). The step b) may be performed in an autoclave, preferably a Teflon lined autoclave. The autoclave may also be lined with any suitable material known to a person skilled in the art. The hydrothermal reaction of step b) may be performed at temperature of 120-200 °C, such as 160-185 °C, preferably 175-185 °C. The hydrothermal reaction is generally performed for 1 -6 hours, such as 1 -3 hours.

The step c) involves separating the solid product from the reacted mixture. During the hydrothermal reaction of step b) a solid product is formed. This solid product is separated from the reacted mixture by means known to a person skilled in the art, for example by filtering. The solid product is optionally dried after the step of separation. The step of drying may be performed at a moderate temperature in an oven.

The step d) involves a step of calcining the solid product. Calcining, or calcination, is supposed to denote a reaction performed at high temperature. Preferably, the reaction is performed in air or oxygen. The calcining step may be used to drive off water vapour from the solid product. Step d) produces a solid product, typically a powder, of the hierarchical molecular sieve. The hierarchical molecular sieve may be in accordance with any one of the embodiments discussed in the first aspect.

The ZSM-zeolite used in step a) of the inventive method may be a ZSM-5 zeolite produced according to methods known in the art. The aqueous phase may be water. It may also be a solution comprising water.

In some embodiments the method further comprises the following steps: a1 ) mixing an aluminum source, a silicon source, a structure directing agent and an alkali source to an aqueous phase to form a first mixture

a2) reacting the first mixture hydrothermally at a temperature of 140- 200 °C for 48-650 hours

a3) separating a solid product from the reacted mixture

a4) calcining the solid product at a temperature of 400-600 °C to form the ZSM-5 zeolite of step a).

In some embodiments the silicon source is selected from the group consisting of silica, fumed silica, carbon white, silica gel, sodium silicate or tetraethyl orthosilicate. The silicon source may be provided as a powder. It may also be provided as a gel.

In some embodiments the aluminum source is preferably selected from the group consisting of sodium aluminate, aluminum silicate, aluminum

isopropoxide, aluminum nitrate or aluminum chloride. In one embodiment is the silicon source tetraorthyl silicate and the aluminum source sodium aluminate. Any one of the silicon sources may be used in combination with any one of the aluminum sources. In some embodiments the structure directing agent is a quaternary

ammonium compound, such as tetrapropylammonium bromide or

tetrapropylammonium hydroxide. Preferably, the same type of structure directing agent may be used in both step a1 ) and step a). Structure directing agents are common in the art of synthesizing zeolites and known to a skilled person in the art. The role of the structure directing agent in zeolite synthesis is generally to direct the aluminosilicate into the desired structure and framework. In some embodiments the alkali source is sodium hydroxide (NaOH). The alkali source may also be potassium hydroxide (KOH).

In some embodiments the molar ratio of Na⁺ to silica (S1O2) in the first mixture of step a1 ) is in the range of 0.01 -0.4, preferably in the range of 0.01 -0.3. The sodium ions are generally provided as part of the alkali source. An advantage of the present invention is that the method can be performed using only a small amount of alkali source. The reaction of the present method does not require an excess of alkali source. In some embodiments the molar ratio of S1O2 to AI2O3 in the first mixture of step a1 ) is in the range of 30-70, preferably 40-70. The molar ratio of S1O2 to AI2O3 is often used to describe zeolites.

In some embodiments the molar ratio of structure directing agent to S1O2 in the first mixture of step a1 ) is in the range of 0.1 -0.5, preferably 0.1 -0.4.

In some embodiments the molar ratio of water (H2O) to silica (S1O2) in step a) is in the range of 30-200, preferably 40-80. In some embodiments the mixing of step a2) is performed by agitation at a temperature of 20 - 50 °C, preferably about 30 - 40 °C. The mixing is typically performed for 1 -24 hours. In some embodiments the hydrothermal reaction of step a3) is performed at a temperature of 160-185 °C, such as 175-185 °C.

In some embodiments the hydrothermal reaction of step a3) is performed for 48-120 hours.

The steps a1 ) to a5) may be used to produce a microporous ZSM-5 zeolite. It is advantageous if the ZSM-5 zeolite has a well-defined, crystalline structure. It is furthermore an advantage if the ZSM-5 comprises micropores. In the present disclosure, it is advantageous if the ZSM-5 added in step a) has a large crystalline component.

In some embodiments the hierarchical molecular sieve produced by the method disclosed herein is a hierarchical aluminosilicate ZSM-5 molecular sieve having a total pore volume of at least 0.85 cm³/g comprising mesopores having a diameter of 2-50 nm and micropores having a diameter of 0-2 nm.

The above described and other features are exemplified by the following figures and detailed description.

Brief description of the drawings

Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike. Figure 1 is a scanning electron micrograph at low magnification of the hierarchical ZSM-5 molecular sieve prepared according to Example 1 of the present invention.

Figure 2 is a scanning electron micrograph at high magnification of hierarchical ZSM-5 molecular sieve prepared according to Example 1 of the present invention. Figure 3 is a transmission electron micrograph at low magnification of hierarchical ZSM-5 molecular sieve prepared according to Example 1 of the present invention. Figure 4 is a transmission electron micrograph at high magnification of hierarchical ZSM-5 molecular sieve prepared according to Example 1 of the present invention.

Figure 5 is a nitrogen adsorption and desorption isotherm of hierarchical ZSM-5 prepared according to Example 1 of the present invention.

Figure 6 is a powder X-ray diffraction pattern of the hierarchical ZSM-5 molecular sieve prepared according to Example 1 of the present invention. Detailed description

The embodiments and effects of the present invention will be studied below by way of examples.

Examples

Example 1

Preparation of a hierarchical molecular sieve

Tetraethyl orthosilicate, sodium aluminate, sodium hydroxide,

tetrapropylammonium bromide was added to water, and stirred at a temperature of 30 °C for 18 hours to form a mixture. The reactants were added in the following molar ratios:

Structure directing agent (tetrapropylammonium bromide)/SiO₂=0.3; H₂O/SiO₂=50; Na+/SiO₂=0.1 . The mixture was then transferred to a Teflon lined hydrothermal reactor. The mixture was reacted in the reactor for 24 hours at 180 °C. The reaction product was then removed from the reactor, cooled in cold deionized water, filtered and dried to obtain a white powder. The white powder was then calcined at a temperature of 550 °C for 10 hours to obtain a ZSM-5 zeolite powder.

The ZSM-5 zeolite powder was then post-treated as follows. The ZSM-5 zeolite powder was mixed with an aqueous solution of tetraprolylammonium bromide having a concentration of 0.1 M and stirred to form a mixture. The mixture was then transferred to a Teflon lined hydrothermal reactor. The mixture was reacted for 2 hours at 180 °C. The reaction product was then removed from the reactor, cooled in cold deionized water, filtered and dried to obtain a second white powder. The second white powder was then calcined at a temperature of 550 °C for 10 hours to obtain a hierarchical ZSM-5 molecular sieve (Sample 1 ).

Scanning electron microscopy

Figure 1 shows a scanning electron microscope (SEM) image at a low magnification of the Sample 1 . The SEM images herein was taken using a JSM-7401 F field emission scanning electron microscope using a cold cathode field emission gun, Gentle beam settings and low electron voltage to reveal more detail on the surface of the sample The crystal of product is well crystallized, no obvious amorphous phase can be observed on the surface of ZSM-5 crystal after the re-crystallization post-treatment process. The size of the crystal is in the range of 10-15 μιτι.

Figure 2 shows a SEM image of the surface of the surface of hierarchical ZSM-5 Sample 1 at a higher magnification. Mesopores can be clearly seen on the surface of crystal. A network-like mesoporosity is formed inside the ZSM- 5 crystal, and the mesopores are dispersed homogeneously inside the crystal as it is seen from Figure 2. Transmission electron microscopy

Transmission electron microscope (TEM) images of Sample 1 is shown in Figure 3 and 4. The TEM images was taken using a JEM 2100, manufactured by JEOL Ltd., Japan. The accelerating voltage used was 200 kV. Figure 3 shows an image of Sample 1 at low magnification. Figure 3 shows that the mesopores were not only formed on the surface of ZSM-5 crystal, but also on its inside, and the distribution of these mesopores are homogenous thorough the whole particle of ZSM-5 crystal.

Figure 4 shows a TEM image of Sample 1 at a higher magnification than in Figure 3. As can be seen in Figure 4, the framework of the zeolite remains crystalline. The lattice of ZSM-5 can be clearly observed, which proves that the wall of mesopores created during the post-treatment remains crystalline.

Nitrogen gas adsorption

In order to further study the porosity of Sample 1 , nitrogen gas adsorption was performed. Nitrogen gas adsorption is an important technique that can characterize the pore system inside zeolite crystal particles. Figure 5 shows a N2 adsorption/desorption isotherm for Sample 1 (upper curves) and for conventional ZSM-5 (lower curves). The insert in Figure 5 shows the pore size distribution of the mesopores, as calculated by the Barret-Joyner- Halenda method, for Sample 1 (upper curve) and conventional ZSM-5 (lower curve). As shown, the hierarchical ZSM-5 (Sample 1 ) shows a significant amount of mesopores. For the conventional ZSM-5, no obvious mesopores can be observed, neither on the isotherm nor the BJH pore size distribution plot. Using nitrogen gas adsorption and the Brunauer-Emmett-Teller (BET) method, the specific surface area of Sample 1 was calculated to 509 m²/g. The total pore volume was calculated at a partial pressure of nitrogen of P/Po=0.97, as the total volume of nitrogen gas adsorbed. The total pore volume was determined to be 0.91 cm³/g. The total mesopore volume of Sample 1 was measured to 0.71 cm³/g, determined as the difference between the total pore volume and the micropore volume. The micropore volume was calculated using the t-plot method. Powder X-ray diffraction

Sample 1 was further analyzed with powder x-ray diffraction and the diffraction pattern is shown in Figure 6. The powder X-ray diffraction pattern was acquired using a Pananalytical Xpert Pro diffractometer with a CuK_Q radiation (λ = 1 .5406 A) source in the 2Θ range of 4-45°. As shown in Figure 6, the powder pattern clearly indicates the MFI framework of Sample 1 as well as a high amount of crystalline component. The crystallinity was further analyzed using the PANalytical HighScores software. The synthesized products were all well crystallized ZSM-5 molecular sieve crystals. This strengthens the observations from the SEM and TEM images with regard to the crystallinity of the product.

Claims

1 . A hierarchical aluminosilicate ZSM-5 molecular sieve having a total pore volume of at least 0.85 cm³/g comprising mesopores having a diameter of 2-50 nm and micropores having a diameter of 0-2 nm, wherein at least 50 percent of the mesopores have an open pore structure.

2. The molecular sieve according to claim 1 , wherein at least 50 percent of the pore volume is made up by mesopores.

3. The molecular sieve according to any one of the preceding claims, wherein at least 80 percent of the mesopores have a diameter in the range of 10-30 nm, such as 15-25 nm.

4. The molecular sieve according to any one of the preceding claims, wherein the pores have an average wall thickness in the range of 20-50 nm, preferably 30-40 nm, such as 34-36 nm.

5. The molecular sieve according to any one of the preceding claims, wherein the specific surface area is at least 400 m²/g, such as at least 500 m²/g, preferably in the range of 500-520 m²/g.

6. The molecular sieve according to any one of the preceding claims, wherein the molecular sieve has a crystalline component of at least 70 weight percent as measured by X-ray diffraction.

7. A method for producing a hierarchical molecular sieve comprising the following steps:

a) mixing a ZSM-5 zeolite with a structure directing agent in an aqueous phase to form a mixture b) reacting the mixture hydrothermally at a temperature of 120-180 °C for 1 -6 hours

c) separating a solid product from the reacted mixture

8. The method according to claim 7, wherein the method further comprises the following steps:

a1 ) mixing an aluminum source, a silicon source, a structure directing agent and an alkali source to an aqueous phase to form a mixture

a3) separating a solid product from the reacted mixture

9. The method according to claim 8, wherein the silicon source is selected from the group consisting of silica, fumed silica, carbon white, silica gel, sodium silicate or tetraethyl orthosilicate.

10. The method according to claim 8 or 9, wherein the aluminum source is selected from the group consisting of sodium aluminate, aluminum silicate, aluminum isopropoxide, aluminum nitrate or aluminum chloride.

1 1 . The method according to any one of claims 7-10, wherein the structure directing agent is a quaternary ammonium compound, such as

tetrapropylammonium bromide or tetrapropylammonium hydroxide.

12. The method according to any one of claims 8-1 1 , wherein the alkali source is sodium hydroxide (NaOH).

13. The method according to any one of claims 7-12, wherein the molar ratio of Na⁺ to silica (S1O2) in the mixture of step a1 ) is in the range of 0.01 - 0.4, preferably in the range of 0.01 -0.3.

14. The method according to any one of claims 8-13, wherein the molar ratio of S1O2 to AI2O3 the mixture of step a1 ) is in the range of 30-70, preferably 40-70.

15. The method according to any one of claims 8-14, wherein the molar ratio of structure directing agent to S1O2 in the mixture of step a1 ) is in the range of 0.1 -0.5, preferably 0.1 -0.4.

16. The method according to any one of claims 8-15, wherein the molar ratio of water (H2O) to silica (S1O2) in the mixture of step a1 ) is in the range of 30-200, preferably 40-80.

17. The method according to any one of claims 8-16, wherein the mixing of step a1 ) is performed by agitation at a temperature of 20 - 50 °C, preferably about 30 - 40 °C.

18. The method according to any one of claims 8-17, wherein the mixing of step a2) is performed for 1 -24 hours, preferably 10-20 hours.