GB2144727A - Crystalline silicates of the MTN-type and methods for their production - Google Patents

Abstract

Crystalline metallosilicates having catalytic and molecular sieving properties and of the MTN-type are prepared using as template a six-membered heterocyclic compound having at least two hetero atoms selected from oxygen and nitrogen. Specific templates are pyridazine, pyrimidine, pyrazine, morpholine and dioxane.

Description

SPECIFICATION Crystalline silicates of the MTN-type and methods for their production The present invention relates to methods of preparing metallo-silicates. More particularly, this invention relates to a method of producing crystalline metallosilicates having catalytic and molecular sieving properties for hydrocarbon and related conversion reactions and separations therewith.

Zeolites are well known natural and synthetic compositions. Many of them have been demonstrated to have catalytic and molecular sieving properties for various types of hydrocarbon and related reactions, or properties enabling the use thereof as molecular sieves in separation processes. Zeolites can be defined as ordered porous crystalline aluminosilicates having a framework structure sufficiently open to accommodate at least water molecules. Such structures generally contain a regular array of small voids interconnected by channels or pores. The dimensions of the voids and channels can range from those of water to those of quite large molecules. For a given framework structure, the dimensions of the voids and channels are limited to a small number of values, which can vary from structure to structure.Thus these structures are capable of sorbing molecules of certain dimensions while rejecting those of dimensions larger than a critical value which varies with structure. This has led to zeolites being used as molecular sieves. Zeolites are tectoaluminosilicates which comprise (in addition to zeolites) felspars and felspathoids. They can be defined as having a framework structure consisting of a rigid regular three dimensional network of SiG4 and Al04 tetrahedra in which the tetrahedra are cross-linked by sharing the oxygen atoms. All oxygen atoms are shared, thus the ratio of total aluminium and silicon atoms to oxygen atoms is 1:2.The inclusion of aluminium in the framework leads to a net negative charge which is balanced by the inclusion in the crystal of an electrochemical equivalence of cations, for example alkali metal, alkaline earth metal, hydrogen or ammonium cations or mixtures thereof. This can be expressed by a formula in which the ratio of Al to the number of the various cations such as Ca/2, Sri2, Na, K, Li or generally M/n (where n is the formal oxidation state of the cation) is equal to unity. Additionally in zeolites, but not in felspars and some felspathoids, the framework is sufficiently open to accommodate water molecules as well as cations. This enables these cations to be exchanged in their entirety or partially by other cations using ion-exchange techniques in a conventional manner.These materials can exhibit specific affinities for specific cations and can thus be used as selective ion-exchangers. By means of ion-exchange, it is possible to vary the size of the pores in a given crystalline zeolite material, modifying its molecular sieve properties. Also by means of ion-exchange the catalytic properties of these materials can be altered. In addition to the framework and charge-compensating cations, zeolites can contain other materials such as water and organic molecules, (hydrated) salts and oxides of eg Na, Al and Si introduced during synthesis, or formed or added during subsequent treatments.

Zeolites are best characterised according to framework structure type, ie on the topolyogy of the framework, irrespective of composition, distribution of different tetrahedral atoms, cell dimensions and symmetry. A code consisting of three capital letters has been adopted for each known structure type following the recommendations by IUPAC on zeolite nomenclature ("Chemical Nomenclature, and Formulation of Compositions, of Synthetic and Natural Zeolites," lUPACyellow booklet, 1978) and a compilation of 38 known zeolite structure types has been published by The Structure Commission of the International Zeolite Association ("Atlas of Zeolite Structure Types", by Meier, W. M. and Olsen, D. H. (1978), distributed by Polycrystal Book Service, Pittsburgh, Pa, USA). Three further structure types have subsequently been proposed (see Barrer, R.M., Hydrothermal Chemistry of Zeolites, Academic Press, 1982). They are the MTH, MTN and MEP structure types exemplified respectively by the zeolites ZSM-3, ZSM-39 and the porous silica melanophlogite. In addition to the groups classified by known structure type, there is a further group of crystalline zeolite materials whose X-ray diffraction patterns, sorption, ion-exchange and related properties indicate that they do not have known structure types but appear to have new, as yet undetermined structure types. An example of such a material is the novel porous crystalline aluminosilicate designated Theta-1 and described in our published copending European patent specification No 0057049.

Zeolites (and other tectoaluminosilicates) belong to a larger class of materials that can be termed tectometallosilicates which can be defined in the same way as tectoaluminosilicates except that the aluminium is replaced by a range of elements, which includes, it is claimed, Ti, Zr, V, Cr, Mo, Mn, Fe, Co, Rh, Ni, Zn, B, Al, Ga, Ge, Sn, As and Sb, but in some cases the claimed materials have not been well characterised. In some cases (where the element has the same formal oxidation state as Si ie +4) the resultant framework is electroneutral and the resultant materials resemble crystalline silicas. In other cases there is a resultant framework negative charge which must be compensated by cations as in tectoaluminosilicates.In some cases the materials have porous frameworks like those of zeolites or porous crystalline silicates which they therefore resemble.

ZSM-39 has been claimed in USP 4259306 as a useful zeolite catalyst. The crystal structure of ZSM-39 has been determined by Schenker, J. L. et al in Nature, 1981,294, pp 340-342 and shows that the ZSM-39 framework is topologically equivalent to that of the 17 Angstrom cubic gas hydrate. As this is a topologically distinct new zeolite structure type it has been suggested (Barrer, R. M. loc. cit.) that the structure type be classified as MTN.

According to US patent 4259306, ZSM-39 (or MTN-type (alumino)-silicate) is crystallised from an aqueous solution containing sources of an alkali metal oxide, silica, pyrrolidine and a tetraureacobalt (II) complex with orwithoutalumina. The US patentalso refers to the use oftetraslkylammonium hydroxide and n-propylamine as the templates to produce ZSM-39 (or MTN-type aluminosilicates).According to USP 4287166 MTN-type (alumino)silicates can also be prepared using templates which can be represented in the parent form as R2O wherein R2O is the oxide form of a compound of Group VA of the Periodic Table due to Mendeleef preferably N or P, containing at least one alkyl or aryl group having from 1 to 7 carbon atoms, preferably from 2 to 5 carbon atoms and preferably containing at least one ethyl group. Still more preferably, R2O is a quaternary ammonium compound. The Examples in USP 4287166 show use of mixtures of tetramethyl- and tetraethyl-ammonium hydroxides or of tetramethylammonium chloride and npropylamine. The templates referred to in these patents are relatively expensive and complex. Both USP 4259306 and USP 4287166 claim that MTN-type (alumino)silicate with a Al Si molar ratio up to 0.051 :1 can be prepared.However the Examples in USP 4259306 and USP 4287166 show only (alumino)silicates with Al:Si molar ratios up to 0.011:1. The work of Bibby, D. M. and Parker, L. M. reported in Zeolites, 1983, Vol 3, pp 11-12 is in agreement with a very low A1203 content for MTN-type (alumino)silicates. They found that the templates pyrrolidine,2-aminopropane or 2-amino-2-methyl propane would form MTN-type (alumino)silicates only if the Al:Si molar ratio of reactants was less than 0.004:1. If the Al:Si molar ratio of reactants was greater than 0.004:1, the aluminosilicate KZ-1 was formed instead.Amongst the MTN-type (alumino)silicates they produced the hig hest AI:Si molar ratio they observed was 0.005:1.

It has now been found that crystalline MTN-type silicates, and metallosilicates, especially aluminosilicates, with a metal to Si ratio, especially an Al:Si ratio, as high as 0.04:1 can be produced by crystallisation from a mixture containing a source of silica, optionally a source of the appropriate metal oxide, a source of alkali metal(s) and an organic template other than those used hitherto.

Accordingly, the present invention provides a method of producing crystalline (metallo)silicates of the MTN structure type having the following composition in terms of the mole ratios of the oxides (0.9 + 0.2) (4-t) M:TO2:xSiO2:yH2O:zQ n wherein M is at least one cation having a valence n, T is at least one metal having a valence t, xis at least 25, H2O is water of hydration additional to water notionally present when M is H, ylx is from 0 to 5, Q is a template used in the synthesis of MTN-type metallosilicates, and zix is 0-20, wherein the (metallo)silicate in the calcined hydrogen form has an X-ray diffraction pattern substantially as set forth in Table A of the specification, said (metallo)silicate being prepared by crystallisation from an aqueous solution comprising a source for each of an alkali metal oxide, silica, optionally the appropriate metal oxide, and an organic template, characterised in that the organic template is a six-membered heterocyclic compound having at least two hetero-atoms which may be the same or different and selected from nitrogen and oxygen.

By "template" is meant throughout this specification a chemical agent which encourages crystallisation to proceed towards a particular framework structure or structures.

The organic template is suitably a heterocyclic compound of the formula:

wherein X and Y may be the same or different groups selected from NR and 0 in which R is H or an alkyl group having 1 to 4 carbon atoms.

Examples of the organic templates include pyridazine, pyrimidine, pyrazine, morpholine and the N-alkyl substituted derivatives thereof, and dioxane.

The content "z" of template "Q" i the metallo(silicate) will depend upon the conditions under which it is washed, calcined or subjected to further aqueous treatments or combinations thereof after synthesis, and also on the synthesis parameters of the (metallo)silicate, particularly the proportion of Q present in the original hydrogel. The template content is usually highest for the "parent" (metallo)silicate. Complete removal of the template is usually only possible by thermal or oxidative degradation or both.

By the "parent" (metallo)silicate is meant throughout this specification the product of synthesis and washing and optionally drying as hereinafter described.

The cation M in the (metallo)silicate may be selected from H, ammonium, alkali metal cations, alkaline earth metal cations, the template cations, aluminium cations, gallium cation and mixtures thereof.

The cations present in the (metallo)silicate may be replaced using conventional ion exchange techniques either wholly or partially by other cations e.g. hydrogen ions or metal cations.

The calcined hydrogen-form of the (metallo)silicate may be produced by known methods such as exchange with acidic or ammonium cations or a combination of the two followed by one or more calcination stages.

By the "calcined hydrogen-form" is meant throughout this specification that the (metallo)silicate in the calcined state and that the cation M is hydrogen.

T is at least one metal that has an amphoteric oxide for example aluminium, iron, zinc, gallium, boron, beryllium, molybdenum, vanadium, chromium, arsenic, antimony, manganese, copper, zirconium, titanium, or germanium. The metals are preferably aluminium, gallium, chromium, iron or zinc.

The H2O content "y" of the (metallo)silicate will also depend upon the conditions under which it is dried, calcined, subjected to further aqueous treatments or combinations thereof with synthesis.

The (metallo)silicates according to the present invention, designated herein as MTN-type (metallo)silicates, have an X-ray diffraction pattern shown in Table A below.

The specific values in the Tables were determined using copper K-alpha radiation and a computer step scan.

The peak heights, I, and their position as a function of 2-theta, where theta is the Bragg angle, were read from the spectrometer output. From this output the relative intensities 100 x l/lo, where lo is the intensity of the strongest peak, and d the interplanar spacing in Ä, corresponding to the recorded peaks were calculated.

It will be understood by those skilled in the art that the X-ray diffraction pattern of (metallo)silicates may vary in the values of l/lo and the d-spacing depending for example upon whether the sample being examined is calcined or uncalcined, upon the temperature of calcination, upon the nature of the cation present in the (metallo)silicate, the identities of the framework metal(s), the mole ratio(s) of framework metal(s) to silicon, and the particle size of the (metallo)silicate.

The (metallo)silicate is suitably produced from an initial mixture containing a source of silica, optionally a source of the appropriate metal oxide(s), a source of alkali metal(s), water and the organic template.

The ratio oftotal (framework) metal(s):Si in the initial mixture is equal to or less than 0.04:1, preferably from 0.004 to 0.03:1. The free alkali metal(s) hydroxide to water mole ratio, defined as: [(Number of moles of total alkali metal(s)) -(Number of moles of alkali metal(s) required to convert the framework metal oxides present to alkali metal metallates of type Motto21 Number of moles of water present where M = alkali metal, T = (framework) metal is suitably greater than 1 x106:1, preferably in the range 2x 10-4:1 to 8x 1 0: 1. Similarly the mole ratio of silica to free alkali metal(s) hydroxide may suitably be in the range 5:1 to 300:1, preferably from 5:1 to 50:1.The mole ratio of water silica is suitably below 200:1, preferably in the range 9:1 to 30:1.

The mole ratio of the organic template to silica is suitably below 5:1, preferably 10-3:1 to 3:1, more preferably in the range of 0.2:1 to 1:1.

The MTN-type (metallo)silicate is suitably prepared by forming a mixture of all the reactants, by simply mixing them together while maintaining the mixture suitably at a temperature between 0 to 100C, preferably between 20 and 60"C, until a homogeneous gel is formed and crystallising the mixture so-formed at a temperature above 70"C, preferably between 100 and 220"C for a period of at least 2 hours, preferably for 6 to 240 hours. The optimum crystallisation period can vary and may depend upon such factors as the temperature, pH and gel composition. Preferably the source of silica is an amorphous silica sol which is diluted with water. It is preferred that the silica source is added to the other reagents in such a way as to commence gelation at a relatively high pH.

The product obtained in this manner contains cations which may be hydrogen, alkali metal(s), or other metal used as a reactant, or the organic template cations or any combination thereof.

The cations in the product may be converted to hydrogen to give rise to the hydrogen-form of the product.

This may be achieved by techniques known to those skilled in the art, e.g. (a) ammonia exchange followed by calcination, (b) acid exchange or a combination of (a) and (b).

The product or the hydrogen-form thereof may also be subjected to exchange or impregnation with a metal suitable for imparting a specific type of catalytic or fluid separation activity. The metal compounds which may be for ion-exchange and/or impregnation may be compounds of any one of the following metals or groups of metals, namely those belonging to Groups IB, IIB, IIIA, IVA, VA, VIB, VIIB and VIII according to the Periodic Table due to Mendeleef. Specifically, compounds of copper, silver, zinc, aluminium, gallium, indium, thallium, lead, antimony, bismuth, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, iridium, platinum and rhenium are preferred.

The (metallo)silicate products of the present invention may be bound in a suitable binding material before or after impregnation or after exchange with one of the aforementioned metal compounds to produce an attrition resistant catalyst or an adsorbent. The binderforthis purpose may be any one of the conventional alumina or silica binders.

The (metallo)silicates of the present invention may be used, whether or not impregnated and/or ion-exchanged, as adsorbents or as catalysts for any of the following reactions. Alkylation, dealkylation, dehydrocyclodimerisation, aromatisation, transalkylation, isomerisation, dehydrogenation, hydrogenation, cracking, cyclisation, oligomerisation, polymerisation, and dehydration reactions, the last named in particular with reference to dehydration of alcohols and ethers. The MTN-type (metallo)silicate may be used as such or as a component of a catalysts mixture containing other active or inactive components. The MTN-type (metallo)silicate may be used in admixture with other metallosilicates. The catalysts may be used in the form of fixed, fluidised or moving beds.

The present invention is further illustrated with reference to the following Examples.

Example 1 A solution was prepared from a mixture of sodium aluminate (1.359), sodium hydroxide (0.539) and water (759).

Morpholine (509) was added to the solution and the resultant solution ("A") was stirred and maintained at room temperature for 5 minutes with constant stirring.

759 of a commercial silica sol, 'Ludox AS40' (Reg. Trade Mark) which contains 40% by weight of silica, was added dropwise to solution "A" over a period of 10 minutes with vigorous stirring. Stirring was continued for a further 20 minutes and then the resultant gel which had the composition: 2.4 Na2O:109(C4HgNO):AI203:94 SiO2:1259 H2O was transferred to an oven and crystallised with agitation at 175"C for 72 hours in a revolving stainless steel pressure vessel.

The product was filtered, washed and dried at 90"C. It was found by its X-ray diffraction pattern to be good, crystalline MTN-type material. The SiO2/AI203 (or Al/Si) molar ratio of the product was 84 (or 0.024 respectively).

TABLE A 2-theta d-spacing Relative Intensity FAngstroms) 100 x 111o 7.91 11.18 23 12.94 6.84 47 15.19 5.83 100 15.87 5.59 49 18.34 4.84 20 20.00 4.44 16 22.51 3.95 16 23.90 3.72 46 26.05 3.42 18 27.27 3.27 46 27.71 3.22 5 29.19 3.06 8 Scanned up to 2-theta = 30 Example 2 A solution was prepared from a mixture of sodium aluminate (1.109); sodium hydroxide (0.359) and water (409).

1,4-Dioxane (309) was added to the solution and the resultant solution ('B') was stirred and maintained at room temperature for 5 minutes with constant stirring.

Commercial silica sol, (509) 'Ludox AS40' (Regd. Trade Mark) which contained 40% by weight of silica, was added dropwise to solution B over a period of 10 minutes and then the resultant gel which had the composition: 2.0 Na2O:79 (C4HsO2) : Al203:77 SiO2:900 H2O was transferred to an oven and crystallised with agitation at 175"C for 72 hours in a revolving stainless steel pressure vessel.

The product was filtered, washed and dried at 90or. It was found by its X-ray diffraction pattern to be substantially MTN-type material with a small amount of Ferrierite.

Claims (10)

1. A method of producing crystalline (metallo)silicates of the MTN structure type having the following composition in terms of the mole ratios of the oxides (0.9 + 0.2)(4-t) M:TO2:xSiO2:yH2O:zQ n wherein M is at least one cation having a valence n, T is at least one metal having a valence t, x is at least 25, H2O is water of hydration additional to water notionally present when M is H, y/x is from 0 to 5, Q is a template used in the synthesis of MTN-type metallosilicates, and z/x is 0-20, wherein the (metallo)silicate in the calcined hydrogen form has an X-ray diffraction pattern substantially as set forth in Table A of the specification, said (metallo)silicate being prepared by crystallisation from an aqueous solution comprising a source for each of an alkali metal oxide, silica, optionally the appropriate metal oxide, and an organic template, characterised in that the organic template is a six-membered heterocyclic compound having at least two hetero-atoms which may be the same or different and selected from nitrogen and oxygen.

2. A method according to claim 1 wherein the organic template is a heterocyclic compound of the formula:

wherein X and Y are the same or different groups selected from NR and 0 in which R is H or an alkyl group having 1 to 4 carbon atoms.

3. A method according to any one of the preceding claims wherein the organic template is selected from pyridazine, pyrimidine, pyrazine, morpholine and the N-alkyl substituted derivatives thereof, and dioxane.

4. A method according to any one of the preceding claims wherein the cation M in the (metallo)silicate is selected from H+, ammonium, alkali metal cations, alkaline earth metal cations, the template cations, aluminium cations, gallium cation and mixtures thereof.

5. A method according to any one of the preceding claims wherein T is at least one metal that has an amphoteric oxide and is selected from aluminium, iron, zinc, gallium, boron, beryllium, molybdenum, vanadium, chromium, arsenic, antimony, manganese, copper, zirconium, titanium and germanium.

6. A method according to any one of the preceding claims wherein in the initial reaction mixture the ratio of total (framework) metal(s):Si is equal to or less than 0.04:1, the free alkali metal(s) hydroxide to water mole ratio, defined as: [(Number of moles of total alkali metal(s)) -(Number of moles of alkali metal(s) required to convert the framework metal oxides present to alkali metal metallates of type MtTO2] Number of moles of water present where M = alkali metal, T = (framework) metal is greater than lx 106:1, the mole ratio of silica to free alkali metal(s) hydroxide is in the range 5:1 to 300:1 and the mole ratio of water to silica is below 200:1.

7. A method according to any one of the preceding claims wherein the mole ratio of the organic template to silica is at least 10-3:1.

8. A method according to any one of the preceding claims wherein the MTN-type (metallo)silicate is prepared by forming a mixture of all the reactants, by mixing them together while maintaining the mixture at a temperature between 0 to 100"C, until a homogeneous gel is formed and crystallising the mixture so-formed at a temperature above 70"C.

9. A process for producing (metallo)silicates of the MTN-type as hereinbefore described with reference to the Examples in the specification.

10. (Metallo)silicates produced by a process claimed in any one of the preceding claims whenever used whether or not impregnated and/or ion-exchanged, as catalysts for any of the following reactions: alkylation; dealkylation; dehydrocyclodimerisation; aromatisation; transalkylation; isomerisation; dehydrogenation; hydrogenation; cracking; cyclisation; oligomerisation; polymerisation; and dehydration of alcohols and ethers; or, as adsorbents and/or absorbents in separation processes.