WO2019121939A1

WO2019121939A1 - Process for the manufacture of hollow zsm-5 and ts-1 zeolites

Info

Publication number: WO2019121939A1
Application number: PCT/EP2018/085877
Authority: WO
Inventors: Junzhong LIN; Junliang Sun
Original assignee: Stockholms Universitet Holding Ab
Priority date: 2017-12-19
Filing date: 2018-12-19
Publication date: 2019-06-27
Also published as: CN111655370A

Abstract

The present invention relates to a process for producing a hollow MFI zeolite comprising the following steps: a) contacting a MFI zeolite, a structure directing agent and an aqueous phase to form a composition; b) reacting the composition at a first temperature T1 for a first period of time t1 to form a first reacted composition; c) reacting the first reacted composition at a second temperature T2 for a second period of time t2 to form a second reacted composition; and d) calcining the second reacted composition to form a hollow MFI zeolite. The first temperature T1 and the second temperature T2 are separated by at least 10 °C. The invention also relates to a zeolite produced by the process and a use of such a zeolite.

Description

PROCESS FOR THE MANUFACTURE OF HOLLOW ZSM-5 AND TS-1

ZEOLITES

Technical field

The present invention relates to a process for producing a hollow MFI zeolite, a hollow MFI zeolite produced by the process and use of a hollow MFI zeolite produced by the method.

Background

Molecular sieves, and in particular zeolites are widely used in the chemical industry today, especially for applications such as adsorption, separation, ion-exchange and catalysis. Aluminosilicate zeolites, in particular aluminosilicate zeolites having MFI framework are particularly useful as molecular sieves, owing mainly to their specific pore structure and large specific surface area. ZSM-5 and TS-1 zeolites are typical examples of zeolites exhibiting an MFI framework contains a three-dimensional pore structure, and relatively high thermal stability and hydrothermal stability. Thus, ZSM-5 has several applications for catalysis in the petrochemical fields of paraffin cracking, isomerization of n-butylene, preparation of gasoline from synthesis gas and the like.

The rate limiting mechanism for mass transport in MFI zeolites is diffusion. This means that the rate of the diffusion in the zeolite is the rate- determining step. When the MFI zeolites are used in industrial applications, in particular industrial applications involving bulky molecules, significant diffusion resistance leading to lower reaction rate is common. This diffusion resistance furthermore leads to that only the fraction of the zeolite near the outer volume can serve as active catalyst.

Therefore, there exist a need to alleviate at least some of the problems associated with the prior art. Summary

It is an object of the present invention to provide novel processes for producing MFI zeolites having improved diffusion properties.

It is a further object of the invention to provide a process for producing a MFI zeolite having an improved pore structure. In particular, there is an object of the invention to provide a process for producing a hollow MFI zeolite having macro- and/or mesopores on the crystal wall surfaces of the hollow MFI zeolite crystal particles.

Yet another object of the present invention to provide a process which is applicable in the production of both hollow ZSM-5 and hollow TS-1 zeolites.

Still further, there is an object of the invention to provide a process for producing hollow MFI zeolites, wherein the process does not require the use harmful chemical agents.

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.

Flence, in a first aspect the invention there is provided a process for producing a hollow MFI zeolite comprising the following steps:

a) contacting a MFI zeolite, a structure directing agent and an aqueous phase to form a composition;

b) reacting the composition at a first temperature T1 for a first period of time t1 to form a first reacted composition;

c) reacting the first reacted composition at a second temperature T2 for a second period of time t2 to form a second reacted composition; and

d) calcining the second reacted composition to form a hollow MFI zeolite; wherein

the first temperature T1 and the second temperature T2 are separated by at least 10 °C.

The present invention is based on the realisation that by post-treating a MFI zeolite with a structure directing agent in combination with a multi-step temperature treatment, problems associated with conventional MFI zeolite synthesis can be alleviated. In particular, MFI zeolites having hollow morphology can be produced. A hollow morphology is advantageous in that it provides a uniform distribution of hollow zeolite crystals which allows for improved mass transport for bulky molecules which improves the catalytic efficiency of the MFI zeolite. The process of the present invention furthermore provides hollow MFI zeolites having a uniform distribution of macropores and/or mesopores connecting the inner hollows with the external surface of the zeolite crystals, which further improves the mass transport in the zeolites yielding improved catalytic properties. By introducing macropores and/or mesopores on the surface of the crystal connected to the hollows, the mass transport in the zeolite can be vastly improved.

In the present disclosure, the term“hollow” denotes a MFI zeolites comprising zeolite crystals having a substantially hollow architecture, i.e. zeolite crystals that at least partially surround a void. Flollow zeolite architectures include, but are not limited to, hollow nanotubes and hollow spheres.

In some examples, the hollow MFI zeolite is a MFI zeolite nanotube.

The process of the present disclosure is furthermore advantageous in that it can create additional pores, such as mesopores and/or macropores on the surface of the zeolite crystals. These additional pores may be in connection with at least partial voids inside the hollow zeolite crystals. The combination of hollow crystals and additional pores on the surface of the hollow crystals can greatly improve the mass transport and diffusion properties in the MFI zeolite.

MFI zeolites are zeolites having the framework type MFI (the name derived from ZSM-FIVE), that is having the same framework type as the ZSM-5 zeolite. MFI 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. Typical MFI zeolites include the

aluminosilicate zeolite ZSM-5 (Zeolite Socony Mobil-5) and the

titaniumsilicate zeolite TS-1. MFI zeolites are used in the chemical industry in large quantities today, in particular as catalysts for a wide range of chemical reactions. Although the term“zeolite” is generally used to denote

aluminosilicate species, in the present disclosure the term“zeolite” also includes other species, such as titaniumsilicate species.

The hollow MFI zeolites of the present invention are generally porous MFI zeolites. The pores may be micropores, mesopores and/or macropores. According to the definition provided by the International Union for Pure and Applied Chemistry (IUPAC) macropores is defined as pores having widths exceeding about 50 nm. IUPAC defines mesopores as pores with

intermediate size; i.e. those with widths in the range of 2-50 nm. The definition of micropores, also according to IUPAC, is pores with widths not exceeding about 2.0 nm. The hollow MFI zeolite may also be a hollow, hierarchical MFI zeolite. A hierarchical molecular sieve should be understood as a molecular sieve which comprises two or more different pore systems.

Conventional ZSM-5 zeolites typically comprise micropores distributed throughout the crystals. The process of the invention is advantageous in that it provides a MFI zeolite having larger pores distributed on the surface of the crystals. The larger pores are preferably mesopores and/or macropores.

The pore system of the present invention may be determined using a scanning electron microscope. In particular, the pores on the surface of the zeolite system can be determined using an electron microscope, such as a scanning electron microscope or a transmission electron microscope.

Preferably, the electron microscope may be used in conjunction with a software which allows measurements of the pore size such that the pore system can be determined.

The pore system may also be investigated using nitrogen gas adsorption. The t-plot method is known to a person skilled in the art. The t- plot method uses nitrogen adsorption isotherm data for the whole range the of adsorption branch by choosing an appropriate reference thickness curve.

The gas uptake volume at the plateau will be converted to a liquid volume which corresponds to the total volume of the micropores.

In the present disclosure, the hollow MFI zeolite is produced by a process which includes a two-step temperature post-treatment of a MFI zeolite. The post-treatment involves contacting the zeolite with a structure directing agent before exhibiting the mixture to a two-step temperature treatment, which includes reacting the zeolite at first temperature T1 for a period of time t1 , before reacting the zeolite at a second temperature T2 for a period of time t2. The first temperature T1 may be higher than the second temperature T2. Also, the second temperature T2 may be higher than the first temperature. The reaction time at the higher temperature is typically longer than the reaction time at the lower temperature.

The term“contacting” is in the present disclosure intended to denote a step where the reagents are allowed to physically contact each other in a container. The step of contacting preferably included stirring. In some examples, the step of“contacting” the reagents is a step of mixing the reagents.

The term“reacting" is generally intended to denote a reaction between at least two species. In particular, it is intended to denote a reaction wherein the reactants are crystallized from an aqueous solution at a relatively high temperature and at a high vapor pressure. The reactions of the present disclosure are 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 steps b) and c) 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 term“reacting” may be intended to denote reacting

hydrothermally. A hydrothermal reaction is generally performed in an autoclave at a relatively high temperature and a high vapor pressure.

Without wishing to be bound by any specific scientific theory, the inventors believe that by utilizing at least two different reaction temperatures the rates of dissolution and re-crystallization of the parent MFI zeolite can be controlled in order to create hollow MFI zeolite. The inventors have found that the reaction rate constant (k) for both the dissolution reaction and the re- crystallization reaction can be described using an Arrhenius equation wherein k=A*exp(E_a/RT). Thus, both the dissolution and crystallization reactions could have a lower rate at a lower temperature and a higher rate at high

temperatures.

By controlling the rates of the dissolution and re-crystallization of the parent zeolite, a hollow MFI zeolite can be produced. Preferably, the hollow MFI zeolite may furthermore comprise macropores and/or mesopores, such as macropores and/or mesopores on the surface of the hollow MFI zeolites. The macropores and/or mesopores may be connected to the voids in the hollow MFI zeolite.

The term“structure directing agent” is known to a person skilled in the art and denotes a compound used in order to guide the formation of particular types of pores and channels during the synthesis of zeolites. The structure directing agent is typically used as templates during the synthesis of zeolites.

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 0.05-0.5 M.

In some examples, the first temperature T1 and the second

temperature T2 are separated by at least 10 °C, such as by at least 15 °C, such as by at least 20 °C, preferably at least 30°C, more preferably by at least 40 °C, still more preferably by at least 50 °C, most preferably by at least 70 °C

In some examples, the step c) further comprises a step d ) separating a solid product from the reacted second mixture. The step of separating may be performed using means known to a person skilled in the art, such as by centrifuging or by filtration. In embodiments where step d ) is included, the calcining of step d) relates to calcining the solid product.

In some embodiments of the present invention, the MFI zeolite in step a) is ZSM-5. The ZSM-5 zeolite is an aluminosilicate zeolite having MFI framework. The ZSM-5 zeolite is widely used in the industry today, in particular as a catalyst, for example in the petroleum industry. ZSM-5 is commonly used a heterogeneous catalyst for hydrocarbon isomerization reactions. The ZSM-5 zeolite may be a conventional, off-the-shelf ZSM-5. If the MFI zeolite used in step a) is ZSM-5, then the hollow zeolite is a hollow ZSM-5 zeolite.

In embodiments of the present invention, the process further comprises the steps

a1 ) contacting an aluminium source, a silicon source, a structure directing agent, an aqueous phase and optionally an alkali source in to form a composition

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

a3) separating a solid product from the reacted composition

a4) calcining the solid product to form the ZSM-5 of step a).

As stated above, the ZSM-5 zeolite used in step a) may be a

conventional ZSM-5. However, the ZSM-5 zeolite may preferably be produced using the process steps a1 )-a4). The inventors have surprisingly found that by using the steps a1 )-a4) to produce the ZSM-5 used in step a), a more well defined hollow ZSM-5 zeolite can be produced by the inventive process. The steps a1-a4 define a process which yields a ZSM-5 zeolite having a high crystallinity and a well-established framework. Another advantage of the steps a1 -a4 is that the use of an alkali source, such as NaOH, may is optional. In some examples, the step a1 comprises mixing an aluminium source, a silicon source and a structure directing agent to form a mixture.

The step a3) involves separating the solid product from the reacted mixture. During the reaction of step a2) 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 or by centrifuging. 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 a4) 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 remove the organic residues from the zeolite framework. The calcination may also drive off water vapor from the solid product. Step a4) produces a solid product, typically a powder, of the ZSM-5 used to be used in step a). The calcining steps disclosed in the present disclosure may be performed at a temperature of 200-800 °C, such as at a temperature of 300- 700°C, preferably at a temperature of 400-600 °C.

The aqueous phase may be water. It may also be a liquid comprising water.

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 aluminium source is preferably selected from the group consisting of sodium aluminate, aluminium silicate, aluminium isopropoxide, aluminium nitrate or aluminium chloride. In one embodiment is the silicon source tetraorthyl silicate and the aluminium source sodium aluminate. Any one of the silicon sources may be used in combination with any one of the aluminium sources.

In some embodiments the structure directing agent (SDA) is a quaternary ammonium compound, such as tetrapropylammonium bromide (TPAB) or tetrapropylammonium hydroxide (TPAOH). Preferably, the same type of structure directing agent is 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 examples, the molar ratios in step a1 ) is in the ranges of 0-0.1 NaOH : 1 Si : 0-0.1 Al : 0.1 -0.5 SDA : 15-2000 H2O. In an example the molar ratios are 0 NaOH : 1 Si : 0.016 Al : 0.3 TPAOH : 100 H₂0. In some embodiments of the present invention, the wherein the MFI zeolite in step a) is TS-1. TS-1 is a titanium silicate zeolite having the type of framework as ZSM-5, namely MFI. TS-1 zeolites may typically be used in catalysis applications. When the MFI zeolite in step a is a TS-1 zeolite, the process produces a hollow ZSM-5 zeolite. The TS-1 zeolite may be a conventional, off-the-shelf TS-1 zeolite.

In some embodiments, the process further comprises the steps a1 ) contacting a structure directing agent and an aqueous phase to form a composition

a2) adding a titanium source and a silicon source to the composition to form a second composition

a3) reacting the second composition at a temperature of 140-200 °C for 48-650 hours to form a reacted composition

a4) separating a solid product from the reacted composition

a5) calcining the solid product to form the TS-1 of step a).

As stated above, the TS-1 zeolite used in step a) may be a

conventional TS-1. Flowever, the TS-1 zeolite may preferably be produced using the process steps a1 )-a5). The inventors have surprisingly found that by using the steps a1 )-a5) to produce the TS-1 used in step a), a well-defined hollow TS-1 zeolite can be produced by the inventive process. The steps a1 - a5 define a process which yields a TS-1 zeolite having a high crystallinity and a well-established framework.

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.

In some embodiments, the structure directing agent is a quaternary ammonium compound, such as tetrapropylammonium bromide or

tetrapropylammonium hydroxide.

In some embodiments, the titanium source is selected from the group consisting of titanium (IV) butoxide, hexafluo-rotitanic acid, titanium (IV) oxide, TiCI₄, titanium butoxide, titanium (IV) isopropoxide. The titanium source is preferably provided as a powder. It may also be provided as a gel. In some examples, the molar ratios are in the ranges of 1 Si : 0-0.06 Ti : 0.1 -0.5 SDA : 15-2000 H₂0, such as 1 Si : 0.04Ti : 0.3 SDA : 40 H₂0.

In some embodiments of the present invention, the first temperature T1 is in the range of 70-140 °C, the first time period t1 is in the range of 1 -10 hours, the second temperature T2 is in the range of 150-220 °C and the second time period t2 is in the range of 10-28 hours.

In some examples, the first temperature T1 is in the range of 75-105 °C, such as in the range 80-100 °C, preferably in the range 85-95 °C. In other examples, the first temperature T1 is in the range of 105-135°C, such as in the range of 110-130 °C, preferably in the range of 115-125 °C.

In some examples, the first time period t1 is in the range of 1 -5 hours, such as in the range of 1 -3 hours, preferably in the range of 1.5-2.5 hours. In other examples the time first period t1 is in the range of 4-8 hours, such as in the range of 5-7 hours, preferably in the range of 5.5-6.5 hours.

In some examples, the temperature second T2 is in the range of 150- 180 °C, such as in the range of 150-170 °C, preferably in the range of 155- 165 °C. In other examples the second temperature T2 is in the range of 150- 190 °C, such as in the range of 160-185°C, preferably in the range of 170-180 °C.

In some examples, the second time period t2 is in the range of 10-16 hours, such as in the range of 10-14 hours, preferably in the range of 11 -13 hours. In other examples, the second time period t2 is in the range of 20-28 hours, such as in the range of 22-26 hours, preferably in the range of 23-25 hours.

In some embodiments of the present invention, the step c) is

performed after step b). By post-treating the zeolite with a multi-step temperature treatment wherein the first temperature treatment is performed at a lower temperature and at a shorter time than temperature and time used in the second step, unwanted extra framework species can be avoided. The term“extra framework species” is in the present disclosure intended to denote species having a different framework than the MFI framework. In some examples, wherein the hollow zeolite in step a) is ZSM-5 and step c) is performed after step b), the first temperature T1 is in the range of 80-100 °C, the first time t1 is in the range of 1 -3 hours, the second

temperature T2 is in the range of 150-170 °C and the second time t2 is in the range of 23-25 hours.

In some examples, wherein the hollow zeolite in step a) is ZSM-5 and step c) is performed after step b), the first temperature T1 is in the range of 110-130 °C, the first time t1 is in the range of 1 -6 hours, such as in the range of 1 -3 hours, the temperature second T2 is in the range of 150-170 °C and the second time t2 is in the range of 12-72 hours, such as in the range of 18- 30 hours, preferably in the range of 23-25 hours.

In some examples, wherein the hollow zeolite in step a) is TS-1 and step c) is performed after step b), the first temperature T1 is in the range of 110-130 °C, the first time t1 is in the range of 1 -6 hours, such as in the range of 1 -3 hours, the second temperature T2 is in the range of 155-185 °C and the second time t2 is in the range of 12-72 hours, such as in the range of 18- 30 hours, preferably in the range of 23-25 hours.

In some examples, wherein the hollow zeolite in step a) is TS-1 and step c) is performed after step b), the first temperature T1 is in the range of 80-100 °C, the first time t1 is in the range of 1 -3 hours, the second

temperature T2 is in the range of 150-190 °C, such as in the range of 155-165 °C or in the range of 170-180°C, and the second time t2 is in the range of 23- 25 hours.

In some embodiments of the present invention, the step b) is performed after step c).

In some examples, wherein the hollow zeolite in step a) is ZSM-5 and step b) is performed after step c), the first temperature T1 is in the range of 80-100 °C, the first time t1 is in the range of 1 -3 hours, the second

In some examples, wherein the hollow zeolite in step a) is ZSM-5 and step b) is performed after step c), the first temperature T1 is in the range of 110-130 °C, the first time t1 is in the range of 1 -3 hours, the second temperature T2 is in the range of 150-170 °C and the second time t2 is in the range of 23-25 hours.

In another aspect of the present invention, there is provided a hollow MFI zeolite produced by the process disclosed in the first aspect of the invention.

In yet another aspect of the present invention, there is provided a use of a hollow MFI zeolite produced by the process disclosed in the first aspect of the invention. The hollow MFI zeolite may for example be used in catalysis applications.

It is noted that the invention relates to all possible combinations of features recited in the claims.

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

Brief description of appended figures

The invention will hereinafter be described in detail by reference to exemplary embodiments as illustrated in the following drawings, in which:

Figure 1 a-b are X-ray diffractograms showing the X-ray diffractions patterns for of the investigated ZSM-5 and TS-1 samples, respectively.

Figure 2 is scanning electron micrograph of MFI zeolites before the inventive post-treatment. Figure 2a shows ZSM-5 and Figure 2b shows TS-1.

Figure 3a-b show scanning electron micrographs of hollow crystals of ZSM-5 and TS-1 , respectively, after the conventional post-treatment post- treatment.

Figure 3c-d show transmission electron micrographs of hollow crystals of ZSM-5 and TS-1 , respectively, after the conventional post-treatment post- treatment.

Figure 4a-c show adsorption/desorption isotherms at 77 K for a) ZSM-5 and TS-1 before and after the inventive post-treatment; b) hollow ZSM-5 after the inventive post-treatment; and c) hollow TS-1 after the inventive post- treatment. Figure 5a-f show scanning electron micrographs six different samples prepared according to the invention.

Figure 6 shows a 27AI Solid State NMR spectra of two hollow ZSM-5 zeolites prepared according to the invention compared to a conventional ZSM-5 zeolite.

Figure 7a-b show the catalytic efficiency of a) hollow ZSM-5

manufactured according to the invention by plotting the concentration of benzyl alcohol with time during a self-etherification of benzyl alcohol in liquid phase and b) hollow TS-1 manufactured according to the invention by plotting the conversion of cyclohexanone during ammoximation of cyclohexanone to time.

Detailed description

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

Examples

Experimental

Synthesis of conventional ZSM-5 (ZSM-5-C) and TS-1 (TS-1-C) zeolite

ZSM-5 and TS-1 to be used in the post-treatment were synthesized. For the synthesis of ZSM-5, 11.2 ml tetraethyl orthosilicate (TEOS) was added dropwise into 9 g of tetrapropylammonium hydroxide (TPAOFI, 1 M) under stirring. TPAOFI was used as the organic structure directing agent for the MFI framework. Then, 15ml water and 0.11 g of aluminum isopropoxide were added into the solution. After an aging process at 70 °C for 10 hours, a clear solution having the following molar ratios 1 S1O2 : 0.02 AI2O3 : 0.3 TPAOFI : 50 FI2O was obtained. The clear solution was transferred into a Teflon-lined stainless autoclave for the crystallization. The crystallization took place at 175 °C for 3 days. After that, the solid product was washed using ethanol for at least three times and separated by repeated centrifugation until the pFH of final product was around 8. Then, the product was dried at 100 °C overnight, and finally calcined at 550 °C for 10 hours to remove the structure directing agent from the framework and to form a conventional ZSM-5 (denoted herein as“ZSM-5-C”).

For the synthesis of TS-1 , 15.2 g of the structure directing agent tetrapropylammonium hydroxide (TPAOH) was mixed with 45 ml deionized water under stirring to form a mixture, then 10.4 ml tetraethyl orthosilicate (TEOS) and 0.68 g Titanium(IV) butoxide (TBOT) were added dropwise into the mixture under stirring. Then the temperature was heated up to 70 °C under stirring for aging and to remove the ethanol generated by the hydrolyzation of TEOS and TBOT; water was added in compensate of the mass loss due to the ethanol removal. Then, the obtained clear solution was transferred into a Teflon-lined stainless steel autoclave for hydrothermal crystallization. The hydrothermal crystallization took place at 175 °C for 3 days. After that, the solid product was washed and separated by repeated centrifugation until the pH of final product was around 8. The product was dried at 100 °C overnight, and finally calcined at 550 °C for 10 hours to remove the structure directing agent in the framework to form a conventional TS-1 (denoted herein as“TS-1 -C”).

Stepwise temperature post-treatment with TPAOH

The inventive post-treatment using a tetrapropylammonium

hydroxide (TPAOH) solution is similar with both ZSM-5 and TS-1 zeolite. Typically, 2 g of calcined zeolite was added to 20 ml of TPAOH (0.3 M) water solution, the obtained mixture was transferred to autoclaves. The autoclaves were first put into the oven, which is preheated to the first temperature T1 , for the first period of time t1 hours, and then transferred to another oven, with the second temperature T2, for the second period of time t2 hours. Afterwards, the autoclaves were cooled down by cold water and the solid product was washed using ethanol for at least three times and separated by repeated centrifugation. The pH of the product was around 8. Then, the product was dried at 100 °C overnight, and finally calcined at 550 °C for 10 hours with a ramp of 1 °C/min. The samples collected after the stepwise post-treatment were denoted as ZSM-5-T1 (t1 )-T2(t2) or TS-1-T1 (t1 )-T2(t2). The samples marked with an Ή” (Hollow) were not subjected to the second temperature treatment. The detailed information for the post-treatment experiment of other samples is outlined in Table 1. Table 1. Sample legend.

Catalytic evaluation

Benzyl alcohol self-etherification in presence of Di-tert-butyl peroxide (DTBP) (in internal micropores)

Reactions of benzyl alcohol self-etherification happen inside the internal micropores of ZSM-5 zeolite in the presence of DTBP. The DTBP molecules are typically adsorbed on the acid sites which are located on the external surface of the zeolite, due to the large molecular size of DTBP. In a typical catalytic run, 15ml of mesitylene was added to a three-necked round bottom flask (25ml) equipped with a reflux condenser and heated in a temperature controlled oil bath (70 °C) under atmospheric pressure and magnetic stirring. Then an appropriate amount of catalyst, which has been repeatedly ion exchanged by 1 M NH₄N0₃ solution and calcined in an air flow under 550 °C for 3 times, was added under stirring and kept at 70 °C for 1 hour. Then excess amount of DTBP (5 times of the total number of acid sites inside the catalyst) was added into the reaction solution, and kept stirring at 70 °C for 3 hours to ensure that the DTBP molecules completely covered the acid sites on the external surface of catalyst. Then 0.25 ml of benzyl alcohol was added into the solution. The start of the reaction was defined as the addition of benzyl alcohol. Liquid samples were withdrawn at a regular interval and analyzed by a gas chromatograph connected to a hydrogen flame ionization detector.

Cyclohexanone ammoximation on TS-1 zeolite.

The catalytic activity of TS-1 zeolite obtained above was evaluated in cyclohexanone ammoximation. The reaction was catalyzed by TS-1 using hydrogen peroxide (30 wt.% in aqueous solution) as oxidant and tert-butanol as solvent at the temperature of 75 °C. Typically, for 2 g TS-1 , the amount of cyclohexanone was 10 g, and the molar ratios of the reagents were 1 cyclohexanone : 1.2 hydrogen peroxide : 1.5 ammonia: 4.5 tert-butanol. After all reagents were charged into the flask, the reaction was started immediately by the addition of H2O2 at a constant flow rate for 0.5 h with a constant-flow pump. The cyclohexanone and oxime were analyzed by a gas chromatograph equipped with a flame ionization detector.

Characterization methods

X-ray diffraction

Powder X-ray diffraction (XRD) patterns was acquired using

Panalytical Xpert Pro diffractometer, with a CuK_a radiation (l = 1.5406 A) source. The powder patterns were recorded over a 2Q range of 5°-50° (or 80°) with a step size of 0.0176°. Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100 LaB₆ microscope operating at 200 kV. The samples for TEM analysis were prepared by dipping carbon-coated copper grids into ethanol solutions of the samples and drying under ambient conditions.

Scanning electron microscopy

Scanning electron microscopy (SEM) images were taken on a JEOL JSM-7401 F at an accelerating voltage of 2.0 kV. Gentle-beam was used to further investigate the surface morphology of particles.

L/2 adsorption-desorption isotherm

The N2 adsorption-desorption isotherm was recorded at 77 K on a Micromeritics ASAP2020 analyzer. Prior to the measurement, the samples were degassed in low vacuum (R/R0 < 10-4) at 573 K for 6 h, followed in high vacuum (P/Po < 10 ⁷) at 573 K for 4 h. The Brunauer-Emmett-Teller (BET) method was applied to calculate the total surface area, while the t-plot method was used to distinguish between microporosity and mesoporosity.

Solid NMR

Nuclear magnetic resonance (NMR) data were collected on a Mercury 300 spectrometer.

Results

Characterization and Analysis

Figure 1 shows powder diffraction patterns of all the sample mentioned herein. It can be clearly seen that all the samples show a MFI framework with good crystallinity without any existence of other impurity phases.

Figures 2a and 2b the SEM and TEM images of the conventional ZSM- 5 (ZSM-5-C) and conventional TS-1 (TS-1-C) zeolite synthesized using the method described herein. ZSM-5-C shows a well-defined shape of and a smooth external surface. The crystal size is around 800 nm, and the thickness along the b-axis is close to 200 nm as estimated from SEM images. TS-1 -C gives an aggregated blackberry shape-like morphology. The size of TS-1 -C samples are around 200 nm and it is difficult to define the b-axis directly through the scanning electron image. Nitrogen adsorption-desorption isotherms clear show a significant uptake at relative pressure (P/Po) below 0.01 , which indicates the presence of micropores.

Figure 3a and b show the SEM image of hollow crystal of ZSM-5-H and TS-1 -H after a typical post-treatment by 0.3M TPAOH water solution for 24 hours. There is no obvious alternation of morphology on the external surface. Furthermore, from the TEM images of ZSM-5-FI and TS-1 -H, which are shown in Figure 3c and 3d, hollow structures can be observed. For ZSM-5-FI (Figure 3c), a single hollow void is present and the thickness of wall is around 40 nm, as is estimated from the TEM image. In the case of TS-1 -FI (Figure 3d), numbers of small voids are randomly generated inside the TS-1 crystal. The reason for the discrepancy of these two different morphologies after the TPAOFI post-treatment is probably due to that, the blackberry shape-like TS- 1 -C crystal contains much more defects, which can serve start point of dissolving process, in the crystal which could lead to numbers of small voids randomly formatted.

Figure 4a shows the N2 adsorption-desorption isotherms of normal and hollow samples for both ZSM-5 and TS-1 zeolite. For the ZSM-5-C and TS-1 - C, they show a type I isotherm, the uptake occurs at the low-pressure region (P/Po < 0.01 ) is due to the filling of 10-member-ring micropores of the MFI framework. When it comes to the hollow samples of ZSM-5-FI and TS-1 -FI, a presence of FI2 hysteresis loop can be observed for both, which contains a steep closure, due to the pore blocking effect or cavitation phenomenon, at around P/Po = 0.45. This feature can be observed if a pore or void has access to external surface only through a narrow neck, as in an ink-bottle pore structure. Thus, the pore or voids empties only when the pressure drops below a characteristic percolation threshold associated with the onset of a continuous cluster of pores open to the surface. To create additional pores on the wall of hollow structure, which would further enhance the mass transfer or diffusion in the catalytic reaction, a simple synthetic strategy could be designed by using a temperature controlled recrystallization post-treatment. In practice, the TPAOH post-treatment was first performed at a high temperature (T1 ) for a certain period (t1 ); and then the system was cooled down to a lower temperature (T2) for another period (t2) to give the zeolite product denoted as ZSM-5-T1 (t1 )-T2(t2) or TS-1- T1 (t1 )-T2(t2).

Figure 5a and 5b show the morphology of two samples denoted as ZSM-5-160(24h)-90(2h) and ZSM-5-160(24h)-120(2h), respectively, after a two step recrystallization. From the SEM image of ZSM-5-160(24h)-90(2h), no macro-pores is visible on the external wall of the hollow ZSM-5 crystal. Interestingly, the crystals tend to collapse inwards along in particular the b- axis. This phenomenon is due to that adsorbed TPA cations on the external surface of zeolite will protect the crystal from the dissolving process typically occuring in a highly alkaline environment. At the same time, the dissolution would take place inside the crystal into which the tetrapropylammonium (TPA) cations cannot diffuse because of its large size compared to the pore size of MFI framework. For the sample of ZSM-5-160(24h)-120(2h), which is shown in Figure 5b, macropores can be observed, and the inner hollow voids have direct access to the external surface through those macropores after the post- treatment. The mechanism of the formation of such morphology can be deduced from the control of temperature in the recrystallization process.

During the first step, at a higher temperature(160 °C), the kinetic of

crystallization on the external surface is similar with the one of dissolving from the inner part of crystal. Thus, a closed hollow structure was formed during this step. Then, as the temperature dropped to 90°C or 120 °C, dissolving process becomes much faster than the crystallization process and dominated in this step. Finally, the macropores are created at the end of this second step, which connect the external surface and the internal voids directly, since at 120°C dissolving kinetics is faster than at 90°C. N2 adsorption isotherms of these two samples, shown in Figure 4b, further proved the presence of macro-pores in the wall of hollow ZSM-5. The adsorption/desorption isotherm of ZSM-5-160(24h)-90(2h) remains a H2 type hysteresis loop after the two-stages post treatment indicating that the hollow voids were well preserved. In contrast, the isotherms of ZSM-5-160(24h)- 120(2h) exhibits a trend where the hysteresis loop decreases which suggests that the inner voids can be connected to the outer surface by macro-pores, which is corroborated from the SEM images. Table 2 shows the texture properties, such as calculated surface area and pore volume of the samples investigated herein. A significant decrease of surface area and micro-pore volume occurred after the post-treatment, as the surface area as calculated by the BET method for ZSM-5-160(24h)-90(2h) and ZSM-5-160(24h)-120(2h) decreased to 297 m²/g and 164 m²/g, respectively. A similar trend is present for the micro-pore volume. The dissolution process at the lower temperature, will destroy the micropores in the crystalline wall to form unique additional macropores which will result in a decrease of micropores and surface area in the final product.

Table 2.

framework species, which would reduce the active sites and affect the catalytic properties of the zeolite. ²⁷AI MAS solid state NMR spectra of calcined ZSM-5 nanotubes are shown in Figure 5. The peak at ca 55ppm belongs to a tetra-coordinated species, which is the framework aluminum species, and the peak at ca. 0 ppm belongs to ta hexa-coordinated Al species. One can observe that, after the post-treatment, for the sample of ZSM-5-160(24h)-120(2h), extra-framework Al species formed, as indicated by the rise of peak at 0 ppm. It can reasonably be deducted that the decrease of micropores and the presence of extra-framework Al species could impair the catalytic activity.

This shortcoming seems to have been overcome without compromising the morphology of the products in the samples by a process wherein the first reaction temperature was lower than the second reaction temperature. The calcined zeolite was first treated by TPAOH solution in a lower temperature for a short time, and then the temperature was increase, to perform a recrystallization. At the lower temperature, the dissolving process inside the crystals is faster than the re-crystallization on the external surface. With the increase of temperature, the crystallization rate increases and reaches a balance with the dissolving rate.

The morphologies of ZSM-5-90(2h)-160(24h) and ZSM-5-120(2h)- 160(24h) shown in Figure 5c and 5d respectively. As can be seen in Figure 5c, for ZSM-5-90(2h)-160(24h), no obvious macro-pores can be observed in the SEM images. For the ZSM-5-120(2h)-160(24h), a nanotube-like morphology formed (see Figure 5d), which is similar to the morphology of sample ZSM-5-160(24h)-120(2h) discussed above. The nitrogen adsorption/desorption isotherms of corresponding samples are shown in Figure 4b, the uptake shows up at low relative pressure which indicates well preserved micropores after post-treatment compared with the previous samples, discussed in relation to Figure 4a. The specific texture properties are shown in table 2. Both samples shown in Figure 5c and 5d have a high surface area as calculated using the BET method, compared to conventional ZSM-5. ZSM-5-120(2h)-160(24h) show a less microporous surface area which could be explained by dissolution of the crystals in the first temperature step at 120 °C. The ZSM-5-120(2h)-160(24h) sample will destroy more micropores and create more defects as compared to the sample of ZSM-5- 90(2h)-160(24h) which has been treated at 90°C for the first step. The ²⁷AI Solid State NMR spectra shows the same trend in Figure 6, the characteristic peak of extra-framework Al species at around 0 ppm can hardly be seen, indicating that only a small amount of extra-framework Al species was present in the sample after the post-treatment described above, wherein the first reaction temperature was lower than the second reaction temperature.

A similar post-treatment strategy can be applied also on TS-1 zeolite, the morphology of TS-1 zeolite after the post-treatment are shown in Figures 5e, 5f. Flere, the second step in the post-treatment were chosen to be performed at a higher temperature due to its high sensitivity to the formation of extra-framework titanium species which depends on temperature. SEM image of TS-1 -90(2h)-160(24h) (Figure 5e) exhibits a few mesopores on the external surface, and the formation mechanism is similar as with hierarchical ZSM-5. The major difference between the TS-1 and ZSM-5 is that, after the post-treatment the TS-1 tends to form small voids dispersed throughout the crystal compared with the single hollow voids in regard of ZSM-5. This explains the different morphologies of hierarchical ZSM-5 and TS-1 in SEM images. The well dispersed voids in TS-1 crystal made the thickness of walls much smaller than in the hollow ZSM-5, explaning why some mesopores can be observed on the TS-1 samples for which for the first treatment temperature was 90°C, as compared to the ZSM-5 samples for which for the first treatment temperature was 90° C. The nitrogen adsorption/desorption isotherms are shown in Figure 4c, which indicate a well preserved micropore system after post-treatment, according to the high uptake at low pressure region.

Meanwhile, the decrease of pore volume shown in Table 2 also verified the presence of mesopores on the surface crystal along with the decreasing size of the hysteresis loop, similar to what has been observed in the case of ZSM- 5. Compared to the ZSM-5 nanotubes, the isotherms of TS-1 all show a more significant hysteresis loop after the post-treatment.

Figure 5f shows an SEM image of the TS-1-120(2h)-160(24h).

Mesopores has been created on the suface of the crystal, which connects the voids inside the crystal with the external surface. Such a morphology enhances the diffusion process during the catalysis.

Catalytic properties

To investigate the effect of macropores and mesopores on their catalytic properties both for ZSM-5 and TS-1 zeolite, two reaction systems have been studied here. One is the self-etherification of benzyl alcohol (ZSM- 5), and the other one is the ammoxidation of cyclohexanone (TS-1 ).

The catalytic properties of ZSM-5 nanotubes, sample ZSM-5-120(2h)- 160(24h) (denoted“Open ZSM-5” in Figure 7a), conventional ZSM-5 (denoted “Normal ZSM-5” in Figure 7a) and hollow ZSM-5 (denoted“Flollow ZSM-5” in Figure 7a) were compared and studied in the reaction of self-etherification of benzyl alcohol in liquid phase. In this reaction, the DTBP molecules cannot diffuse into the micropores but is instead adsorbed on the external bronsted acid sites. As a result, the self-etherification reaction will only take place in the microporous pore system.

The catalytic result was shown in Figure 7a. The concentration of benzyl alcohol was plotted with time and the selectivity was close to 100%, due to the presence of DTBP molecules. The activity of conventional ZSM-5 is much lower compared to both of hollow ZSM-5 and ZSM-5 nanotubes because of the slow diffusion rate of benzyl alcohol inside micro-pores. The diffusion coefficient of benzyl alcohol is close to 6.25x1 O ²⁰ cm²/s and as a result only an extremely small fraction of zeolite crystal on the surface can be used as the during the catalytic reaction. A large fraction of zeolite crystal remains unused due to extremely low diffusion rate. Accordingly, the hollow ZSM-5 shows better activity since the core part of ZSM-5 crystal has been recrystallized onto the external surface to form a shell-like structure. The ZSM-5 nanotube shows an even better catalytic properties than the other samples. This is because that the presence of macro-pores enhanced the external diffusion process during the reaction causing the concentration of benzyl alcohol to become substantially equal both outside and inside the nanotube, which makes the diffusion path shorter for two times during the reaction.

The ammoximation of cyclohexanone has been used to test the catalytic properties of hierarchical TS-1 synthesized using the same strategy.

The TS-1 -120(2h)-160(24h) also shows superior catalytic activity compared to the conventional TS-1 and hollow ones, due to the presence of mesopores on the external surface, as is shown in Figure 7b. The selectivity is close to 1 for all TS-1 samples.

Claims

1. A process for producing a hollow MFI zeolite comprising the following steps:

2. The process according to claim 1 , wherein the first temperature T1 is in the range of 70-140 °C, the first time period t1 is in the range of 1-10 hours.

3. The process according to any one of claims 1 or 2, wherein the second temperature T2 is in the range of 150-220 °C and the second time period t2 is in the range of 10-28 hours.

4. The process according to any one of claims 1 to 3, wherein the MFI zeolite in step a) is ZSM-5.

5. The process according to claim 1 , wherein the MFI zeolite in step a) is

TS-1.

6. The process according to claim 4, wherein the process further comprises the steps

a1 ) contacting an aluminium source, a silicon source, a structure directing agent, an aqueous phase and optionally an alkali source in to form a composition; a2) reacting the composition at a temperature of 140-200 °C for 48- 650 hours to form a reacted composition;

a3) separating a solid product from the reacted composition; and a4) calcining the solid product to form the ZSM-5 of step a).

7. The process according to claim 6, wherein the aluminum source is selected from the group consisting of sodium aluminate, aluminum silicate, aluminum isopropoxide, aluminum nitrate or aluminum chloride.

8. The process according to claim 5, wherein the process further comprises the steps

a1 ) contacting an structure directing agent and an aqueous phase to form a composition;

a2) adding a titanium source and a silicon source to the composition to form a second composition;

a3) reacting the second composition at a temperature of 140-200 °C for 48-650 hours to form a reacted composition;

a4) separating a solid product from the reacted composition; and a5) calcining the solid product form the TS-1 of step a).

9. The process according to claim 8, wherein the titanium source is selected from the group consisting of consisting of titanium (IV) butoxide, hexafluorotitanic acid, titanium (IV) oxide, TiCI₄, titanium butoxide, titanium (IV) isopropoxide.

10. The process according to any one of the preceding claims, wherein the silicon source is selected from the group consisting of silica, fumed silica, carbon white, silica gel, sodium silicate or tetraethyl orthosilicate.

11. The process according to any one of the preceding claims, wherein the structure directing agent is a quaternary ammonium compound, such as tetrapropylammonium bromide or tetrapropylammonium hydroxide.

12. A hollow MFI zeolite produced by the process according to any one of the claims 1 to 12.

13. Use of a hollow MFI zeolite produced according to any one of claims 1 to 12 in a catalysis application.