WO1998006495A1

WO1998006495A1 - Method for preparing a composite catalyst

Info

Publication number: WO1998006495A1
Application number: PCT/NL1997/000461
Authority: WO
Inventors: Nelleke Van Der Puil; Hans Peter Alexander Calis; Otto Leonard Oudshoorn; Jacobus Cornelis Jansen; Herman Van Bekkum
Original assignee: Technische Universiteit Delft
Priority date: 1996-08-09
Filing date: 1997-08-11
Publication date: 1998-02-19
Also published as: NL1003778C2; AU3869797A

Abstract

The invention relates to the preparation of composite catalysts consisting of a support material and a molecular sieve crystal phase that is present on the surface of the support material. The molecular sieve crystals can, as desired, be provided both on the inside and on the outside of a porous support material. With the method according to the invention a faster preparation of a composite with a higher crystal load and a better accessibility of the individual zeolite crystals can be realized. Further, the invention relates to the use of template on a support material for inducing in situ nucleation.

Description

Title: Method for preparing a composite catalyst

The invention relates to a method for preparing a composite catalyst consisting of a support material on the surface of which an active catalyst is provided. More particularly, the invention relates to a method whereby on the surface of a support material a molecular sieve crystal phase, such as a zeolite crystal phase, is provided by contacting the support material with a solution of a template-containing synthesis mixture for molecular sieve crystals . Different methods are known for providing a layer of molecular sieve crystals, such as zeolite crystals, on a support which may or may not be shaped. In the majority of these known methods a support is covered with such crystals which, for a considerable part, come from a slurry. In U.S. Patent 5,266,542, for instance, a method is described whereby alumina substrates are immersed in a slurry containing zeolite crystals in addition to precursors of these zeolite crystals, whereafter the crystals, using the precursors mentioned, are baked onto the substrate. An important disadvantage of such methods is that the molecular sieve crystals used are provided randomly on the substrate. Neither in respect of the site of binding nor in respect of the molecular sieve crystal orientation can these known methods be directed or controlled. Moreover, many molecular sieve crystals are not bound to the substrate, but precipitate on the bottom and walls of the reactor vessel .

To solve this known problem a number of methods have been proposed whereby molecular sieve crystals and particularly zeolites are grown in si tu on support materials. In this connection, reference is made to international patent application WO-A-94/09902 , in which a composite catalyst is described, consisting of a structured support which is covered with a layer of molecular sieve crystals, which sieve crystals are chemically bound to that support. Such a catalyst can be obtained by thoroughly cleaning a support material, for instance a stainless steel wire, and subsequently keeping it in contact with an aqueous synthesis mixture at an elevated temperature for a number of hours. This synthesis mixture contains a silicon oxide source, a sodium ion source, an aluminum source and a template for the desired molecular sieve crystal. From this synthesis mixture molecular sieve crystals are grown directly onto the support surface, which crystals have substantially the same orientation relative to the support surface.

U.S. Patent 5,310,714 discloses a method for synthesizing a zeolite film consisting of zeolite crystals continuously grown together on a substrate or monolith surface. This method comprises preparing a chemical mixture containing the precursors and a template for the desired zeolite, whereby particular requirements are set in respect of the molar ratio Y0₂/X₂0₃ (wherein Y represents a tetravalent element and X a trivalent element) in combination with the molar ratio H₂0/Y0₂. When Y0₂/X₂0₃ is greater than 400, H₂0/Y0₂ must be at least 25; when Y0₂/X₂0₃ is between 150 and 400, H₂0/Y0₂ must be at least 35; and when Y0₂/X₂0₃ is less than 150, H₂0/Y0₂ must be at least 45. Under these conditions there is supposed to occur hardly any crystallization in the solution, but virtually exclusively growth on the support surface.

International patent application 96/01683 discloses structures comprising a support and a molecular sieve crystal phase. In the manufacture of these structures, molecular sieve crystals with a small particle size of maximally 1 μm are provided on a support or formed thereon, whereafter the thus obtained support is contacted with a molecular sieve crystal synthesis mixture. In this synthesis mixture, a template is optionally present.

International patent application 92/19574 describes a composite catalyst for use in a Fischer-Tropsch process. The catalyst comprises a zeolite on a support. In one of the three crystallization methods explicitly described, the support, for instance a support from silica, is first impregnated to incipient wetness with a solution containing the sieve crystal precursors sodium silicate and sodium hydroxide, and optionally the precursor sodium aluminate and the template tetrapropylammonium bromide.

All of these known methods, where in si tu crystal growth on a support is contemplated, utilize synthesis mixtures that contain at least sodium and silicon sources and normally a suitable template.

According to the present invention, it has now been found that if the surfaces of the support material are pretreated in a special way, a considerably faster and controllable in si tu growth of molecular sieve crystals, such as zeolites, on that support surface can occur. In addition, according to the invention highly inert surfaces, such as α-alumina surfaces, can be coated with molecular sieve crystals.

This special pretreatment step comprises contacting the support surface to be coated, with a solution of the template, before the support surface is contacted with the synthesis mixture for the molecular sieve crystallization.

More particularly, the present invention concerns a method for preparing a composite catalyst, comprising contacting a support material with a solution of a template, subsequently placing the so pretreated support material in a synthesis mixture for molecular sieve crystallization, for instance a zeolite synthesis mixture, for the purpose of obtaining a composite, and calcining the obtained composite to remove the template. Calcination is carried out in a known manner by controlled subjection to a heating step of the support material on which a layer has formed from the synthesis mixture. Conventionally, the product to be calcined is heated at a rate of l-2°C/minute, and subsequently held at a high temperature of, for instance, 450-600°C in an air atmosphere for some ten hours, followed by cooling at a controlled rate of again approximately l-2°C/min.

After calcining, the obtained composite catalyst product can optionally be activated in a known and conventional manner, for instance by exchanging at least a part of the sodium ions for ammonium ions or other activating ions, optionally followed by calcination again.

The molecular sieve crystal synthesis mixtures, the synthesis conditions and the templates needed are generally known. Suitable zeolites that can be synthesized on a support in accordance with the invention are - without the invention being limited thereto - zeolite beta (BEA) , ferrierite (FER) ,

ZSM-5 (MFI) , ZSM-11 (MEL) , mordenite (MOR) , zeolite Y (FAU) ,

ZSM-22 (TON) and ZSM-23 (MTT) . Depending on the type of zeolite contemplated, specifically the silica source and the template are determinative factors.

Synthesis mixtures that are used in the method according to the invention always include silicon sources, often aluminum sources, sodium sources and phosphate sources, and conventionally a template. Examples of such sources comprise silicates, aluminates, aluminosilicates, aluminophosphates, silicoaluminates, etc., as well as combinations thereof.

Suitable synthesis mixtures are described inter alia in all of the publications cited hereinabove. The term "template" is known to those skilled in the art of molecular sieve crystal formation; a template is an agent that determines the structure of the molecular sieve crystal, at least partly directs it. Known templates are, for instance, hydroxides or salts, and specifically halogenide salts, of the following cations: tetraalkylammonium, such as tetramethylammonium (TMA) , tetraethylammonium (TEA) , tetrapropylammonium (TPA) , and tetrabutylammonium (TBA) ; other ammonium cations substituted with four hydrocarbon groups, such as trimethylbenzylammonium and trimethylcetylammonium; tetra-substituted phosphonium, such as tetrabutylphosphonium and triphenylbenzylphosphonium; bispyrrolidinium; ethylpyridinium; diethylpiperidinium; substituted azoniabicyclooctane; crown ethers etc.

An essential step in the method according to the invention is the pretreatment of a support material with a solution which contains template but substantially no precursors of the molecular sieve crystals yet. The pretreatment according to the invention is preferably carried out with a template solution as concentrated as possible, for instance with a solution containing at least 20% by weight of template. With an increase of the concentration of template the in si tu nucleation and hence the in si tu crystal growth is promoted. Depending on the template used, different solvents and mixtures thereof can be used. Preferably, however, an aqueous solution is used. The template to be chosen depends on the desired molecular sieve crystal phase to be deposited.

It is supposed that the template binds physically to the support material or is adsorbed, and, as it were, impregnates, wets and/or decorates it and activates the support surface completely for nucleation.

The invention accordingly relates also to the use of template on a support material for inducing in si tu nucleation. In such use, the support material is contacted first with a template-containing medium and after this pretreatment with a molecular sieve crystal synthesis mixture .

The synthesis of the molecular sieve crystal phase takes place - in situ - in the presence of the pretreated support. To that end, the support material pretreated with template and not, at least not thoroughly, dried, is contacted with a synthesis mixture in a suitable reactor vessel, for instance an autoclave, whereupon molecular sieve crystals are formed on the support at a suitable synthesis temperature.

In contrast with many known methods, in the method according to the invention it is not necessary, and even disadvantageous, to allow the gel-like synthesis mixture to age.

In comparison with the in si tu growth methods, described hereinabove, in WO-A-94/09902 and US-A-5 , 310, 71 , a support surface is provided faster with molecular sieve crystals and with a higher crystal loading. Moreover, a better accessibility of the individual crystals is obtained than by means of the known composite catalyst preparation methods. All this is clearly illustrated by comparing the results in Example 1 (comparative example) and Example 2 (example according to the invention) below. In Example 1 an α-alumina substrate which has not been pretreated in accordance with the invention is contacted with a zeolite synthesis mixture. When the α-alumina is contacted with a zeolite synthesis mixture under the conditions described in WO-A- 94/09902 or US-A-5, 310, 714, it appears that hardly any zeolite crystals are formed on the support surface (see Fig. 1) . By contrast, Example 2 and the associated Fig. 2 show the effect of the pretreatment according to the invention: the surfaces of α- alumina are to a large extent covered with zeolite crystals. In view of the inertness and availability of this material, in a preferred embodiment of the method according to the invention, as support material α-alumina is used.

Analyses of the interfaces between support and molecular sieve crystal phase show that the support is not going to form part of the sieve crystal phase. The sieve crystals merely grow from the solution; the support does not dissolve. In other words, there is a very clear-cut transition between support surface and zeolite phase. The so-called "unition" of support and crystal phase is great and appears to be strongly promoted by the presence of template as a nucleation core on the support surface. Despite the observed sharp transition the molecular sieve crystal phase in the method according to the invention is strongly bound to the support. In particular, the sieve crystal phase is not dislodged upon a sonification treatment or under the influence of high liquid or gas flow rates.

The nature of the support material, for that matter, is not critical in the method according to the invention. In particular, a variety of ceramics, metals and metal oxides can be coated with a molecular sieve crystal layer, and in particular with a zeolite layer. Thus, suitable support materials include different types of alumina, titania, zirconia, silica, clays and mixed forms of the foregoing oxides, silicon carbides, metals and metal oxides, and zeolites, as well as all kinds of mixed forms thereof. The invention makes it possible, for instance, to deposit on zeolite crystals with relatively small pores a layer of a different type of zeolite, which different type of zeolite possesses greater pores.

Furthermore, the shape of the support is not critical. Possible forms of ceramic support materials include beads of a diameter of 10 μm to a few millimeters, extrusions of a diameter from approximately 1 millimeter, discs of a diameter of up to a few centimeters, tubes, monoliths, tubs or pellets. Possible forms of metals or metal oxidic supports comprise foils, gauze, balls consisting of filaments from approximately 1 μm, foam, sponge, packings and beads of preferably a diameter greater than 0.5 mm. The surface of the support material can be both smooth and porous: macro-, meso- and microporous. In a particular embodiment of the method according to the invention a micro- or meso-porous support is started from. The fact is, it has been found that these types of supports can be controllably coated with molecular sieve crystals, such as zeolite crystals. When such porous supports are used, the present invention makes it possible to provide the zeolite crystals on the outer surface alone, on the inner surface alone, or both on the outer and the inner surface. The term "inner surface" in this connection is understood to mean in the pores that are open, that is, pores in direct communication with the medium or the atmosphere outside the sieve crystals, while "outer surface" is understood to mean: substantially not in the pores.

When it is desired to cover the support with zeolite crystals mainly on the outside, it is not necessary to fill the pores in some way or other with a crystal growth preventing medium before the support is contacted with the synthesis mixture. It suffices to carry out the pretreatment in such a manner that substantially no template molecules end up in the pores. This can be done simply by carrying out the pretreatment step with template at a temperature not too high, for instance at a temperature lower than 80°C, preferably at a temperature between 0°C and 40°C, most preferably at room temperature. Another option is to allow the pretreatment to proceed only for a short time. Those skilled in the art can easily determine all this experimentally, depending on the porosity of the support and the template to be used.

In this embodiment the treatment with synthesis mixture is carried out under static conditions, which moreover can have an advantageous effect on the orientation of the zeolite crystals to be formed.

If it is desired that not only the outside of a porous support but also the open pores thereof be provided with a layer of zeolite crystals, then template must be introduced into the pores as well. This can be done simply by carrying out the pretreatment step with template at a temperature above the boiling point of the template solution. It is supposed that under these conditions the pores are rendered gas- free, so that the template solution can penetrate into the pores. Another option is to keep the support in contact with the template solution for a prolonged time. This option, however, is less practical. Finally, other methods can be used to make the pores gas-free, such as subjecting the support to a step whereby the pressure is strongly reduced before the support is contacted with the template solution. Optionally, the aforementioned methods can be combined.

The support can further be coated substantially exclusively in the pores, by carrying out the treatment with the zeolite synthesis mixture in a stirred reaction vessel, at any rate in a synthesis mixture which is intensively in motion. It is supposed that in such an embodiment the template cannot perform its nucleation promoting action, for instance because the template molecules or growing zeolite precursors are dislodged from the support surface.

Before the pretreatment with template is carried out, the support material should preferably be thoroughly cleaned, so that liquid, solid and/or gaseous contaminations on the surface and in the pores are removed. This cleaning step maximizes the number of sites where template can adhere. Thus, grease is to be removed, for instance by boiling in alcohol and/or carrying out a calcination step, so that the surface can be wetted maximally with the template solution and thus the template can optimally adsorb or otherwise adhere to the surface.

The method according to the invention makes it possible, over the known synthesis methods, to realize a shorter synthesis time, whereby an increased crystal loading of the support can be obtained at lower cost. Moreover, the method according to the invention makes it possible to controllably coat porous supports site-selectively with molecular sieve crystals. Further, the molecular sieve crystal layer can be thin and "smoothly" follow the contour of the support, so that the geometry of the support does not change and the contemplated behavior of the support in liquid and gas streams is not adversely affected.

A relatively low zeolite/support ratio gives the composite catalyst highly favorable heat transfer properties. Also, owing to the specific design of the composite catalyst and the zeolite deposited thereon, no mass transport limitation to the catalyst surface arises. The composite catalysts that are prepared according to the invention are suitable for known reactions, for instance acid-catalyzed reactions, in both gaseous phase and liquid phase. In particular when a porous composite catalyst is used in gas phase processes, it is advantageous to have zeolite present also in the pores.

The invention will presently be elaborated in more detail in and by the following non-limiting examples, where percentages are percentages by weight, unless otherwise specified. In these examples, a fast zeolite beta synthesis is described.

In the figures: Fig. 1 shows a scanning electron microscope (SEM) image (Acc.V 15.0 kV; magnification 3,000x) of the outer surface of α-alumina which has not been pre-treated according to the invention;

Fig. 2 shows an SEM image of a composite catalyst which has been obtained by contacting an α-alumina support pretreated with template, with a zeolite beta synthesis mixture under static conditions;

Fig. 3 shows an SEM photograph of a composite catalyst which has been obtained by contacting an α-alumina support pretreated with template, with a zeolite beta synthesis mixture under non-static conditions; Fig. 4 shows an SEM cross-section of α-alumina/zeolite beta; Fig. 5 is an FTIR spectrum of unsupported zeolite beta crystals; and

Fig. 6 is a DRIFT spectrum of composite catalyst particles according to the invention.

Example 1 (comparative example)

As support material, 1 gram of 1 mm beads consisting of individual particles of 10-20 μm α-Al₂0₃ (Norton) was used. This support material is free from eso- and micropores and in this example is added to a synthesis mixture without pretreatment according to the invention. The synthesis mixture is based on the synthesis mixture that is described in US-A-3 , 308, 069 for obtaining zeolite beta, with the following modifications: an increased concentration of sodium ions, a reduced concentration of template (tetraethylammonium hydroxide (TEAOH) ) . In addition, compared with US-A-3 , 308, 069, an increased synthesis temperature and an improved mixing of the reactants are employed.

More particularly, as synthesis mixture a gel was used with the following molar oxidic ratios:

23.6 Si0₂ : 1.0 Al₂0₃ : 1.87 Na₂0 : 1.94 TEA₂0 : 156 H₂0, which gel was obtained as follows.

Silica (Aerosil 200; Degussa) , tetraethylammonium hydroxide (template; 35%; Aldrich) , sodium hydroxide (p.A. ,- J.T. Baker) , in the just specified ratio, and half of the amount of distilled water were properly mixed with a turbine agitator for an hour at a high stirring speed. The remainder of the water and sodium aluminate (41% Na₂0 and 54% Al₂0₃ ; Riedel de Haen) were added dropwise to the stirred silicate solution. The total mixture was stirred for 4 hours.

The support material particles were mixed with this fresh synthesis mixture under gentle stirring, whereafter the whole was heated in a 35 ml teflon-coated autoclave at 155°C for 19 hours. In this autoclave, under static conditions, zeolite synthesis occurred. The autoclave was slowly cooled to room temperature in air.

The product obtained in the autoclave consisted of unsupported material and alumina particles with a coating; this product was separated from the residual solution by means of filtration. The product was then dried in air at

120°C. The catalyst particles, after drying of the unsupported zeolite, were separated by means of a 500 μm sieve .

Scanning electron microscope (SEM) photographs (Acc.V 15.0 kV; magnification 3,000x) show that the unsupported material consists of spherical crystals of a size between 0.2 and 0.5 μm. From a Fourier Transform Infra Red spectroscopy measurement in transmission it appears that this unsupported material shows the spectrum that is specific for zeolite beta, with Si-0 bands at 1214, 1170, 1080 and 779 cm¹, and bands belonging to the template (see Fig. 5) .

The support that was not pretreated according to the invention appears to contain only a few zeolite crystals at the outer surface (see Fig. 1) .

The supported catalyst particles were treated in 20 ml distilled water in an ultrasound bath for 2 hours. Then followed filtration and the residue was dried at 120°C.

Calcination of the composite particles occurred at 550°C for 10 hours, with a heating rate and a cooling rate of 2°C per minute. For the purpose of activation, an ion exchange was carried out in a solution of 0.1 M N₄C1 in water under static conditions for 24 hours, whereafter the calcination step was repeated.

Example 2 Example 1 was repeated, but now the support material was first pretreated with a 35% aqueous TEAOH solution at room temperature for 18 hours. The solution was decanted. The support was not dried before addition to the synthesis mixture. Zeolite synthesis occurred again under static conditions in the autoclave.

From SEM and FTIR photographs it appeared that the unsupported material again consisted of spherical zeolite beta crystals with a particle size between 0.2 and 0.5 μm.

The support which had been pretreated with a template solution in accordance with the invention was found to be covered on the outer surface thereof with a layer of zeolite crystals (see Fig. 2) .

From Diffuse Reflection Infra Red analysis of the ammonium-ion exchanged composite catalyst follows a DRIFT spectrum (Fig. 6) that is characteristic of NH.,-exchanged zeolite beta, with Si-0 vibrations at 1240, 1093, 888 and 785 cm¹, an NH₄-band at 1458 cm^"1 and two absorption bands between 2,000 and 1,800 cm¹.

Example 3 Example 2 was repeated, but now the pretreatment with the 35% TEAOH solution was carried out under reflux conditions. The results were similar to those of Example 2. Fig. 4 shows a cross-section of the composite catalyst.

Example 4

Example 3 was repeated, but now the zeolite synthesis was carried out under rotating conditions, in that the autoclave was mounted on a shaft which was rotated at a speed of 40 rpm. It appears from Fig. 4 that under these conditions a lesser amount of zeolite crystals was formed on the surface of the support material .

Claims

1. A method for preparing a composite catalyst, comprising contacting a support material with a solution of a template, subsequently placing the thus pretreated support material in a synthesis mixture for molecular sieve crystallization for the purpose of obtaining a composite, and calcining the obtained composite for removing the template.

2. A method according to claim 1, wherein after calcination, activation occurs.

3. A method according to claim 1 or 2 , wherein as support material α-alumina is used.

4. A method according to any one of the preceding claims, wherein a micro- or meso-porous support material is used.

5. A method according to claim 4, wherein the support material is covered on the outside with molecular sieve crystals, by carrying out the pretreatment step with template at a temperature lower than 80°C.

6. A method according to claim 4, wherein the support material in the pores is covered with molecular sieve crystals, by carrying out the pretreatment step with template at a temperature above the boiling point of the template solution.

7. A method according to claim 4 , wherein the support material is coated substantially exclusively in the pores, by carrying out the treatment with the synthesis mixture in a stirred reaction vessel.

8. A method according to any one of the preceding claims, wherein the step of contacting the support material with the solution of template is carried out with a solution containing at least 20% by weight of template.

9. A method according to claim 8, wherein an aqueous solution is used.

10. Use of template on a support material for inducing in si tu nucleation.