EP0975549A1

EP0975549A1 - Molecular sieve

Info

Publication number: EP0975549A1
Application number: EP98921478A
Authority: EP
Inventors: Johannes Petrus Verduijn; Jihad Dakka; Georges Marie Karel Mathys
Original assignee: Exxon Chemical Patents Inc
Current assignee: ExxonMobil Chemical Patents Inc
Priority date: 1997-04-21
Filing date: 1998-04-21
Publication date: 2000-02-02
Also published as: GB9707981D0; WO1998047816A1; CA2287519A1

Abstract

Molecular sieves with an enhanced content of active framework elements, e.g., titanium, are made by first depleting the concentration of the silica source in the synthesis mixture, e.g., by low temperature molecular sieve formation.

Description

"Molecular Sieve" This invention relates to molecular sieves, to processes for their manufacture, and to processes using the molecular sieves as catalysts, and to compositions useful in the manufacture of the molecular sieves and in other useful products- Molecular sieves, and more especially crystalline molecular sieves referred to commonly as zeolites, have various industrial applications among which may be mentioned by way of example separations of different molecular species and catalysis of chemical reactions. Zeolites of different structures have different applications, and is has been found advantageous to incorporate small proportions of "foreign" elements into the skeletal structure of many different zeolites to modify their characteristics.

In the case of particulate zeolites, the foreign element may be incorporated more or less uniformly throughout the particle. If, however, the foreign element is expensive or causes difficulties in synthesis, it may be incorporated only in the outer shell of each particle, over a core of a material which is usually of the same structure as the shell but without the foreign element; since in many instances the structure and chemical characteristics of the outer shell of each particle largely determine the characteristics of the zeolite a substantial saving in material or processing cost may result.

As examples of elements, especially metals or metalloids, that have been incorporated into zeolites based on silica only there may be mentioned Al, As, B, Be, Co, Cr, Fe, Ga, Mb, Mn, Ni, Ti, Pt, Pd, Re, Ru, V, W, Zr, or combinations of two or more such elements. As described in EP-A-55044, the disclosure of which is incorporated by reference herein, such elements may be incorporated into the shell surrounding a silicalite (or high silica content MFI zeolite) core. In many zeolites, aluminium is present at a given proportion or range of proportions as an element essential to the zeolite structure along with silicon. In such zeolites, a third element is present as foreign element in the framework.

Zeolites having such elements have numerous industrial uses. For example, the titanium-containing zeolite TS-1 is used commercially as a catalyst in hydroxylation of phenol by hydrogen peroxide, producing hydroquinone and catechol in tonnage quantities. A titanium-containing zeolite β for catalysing ring-opening oxidation of cyclohexane has been described in WO 94/02245 and WO 95/03249, the latter also discussing V-β and Zr-β. EP-A-55044 discloses the use of the gallium-containing silicalite-based cherry-type zeolites as isomerization catalysts. The use of Ti,Al-β as epoxidation catalyst is disclosed by Camblor, et al. Zeolites 13 (1193) 82, while V-containing zeolites β and silicalite have activity as oxidation catalysts using various oxidizing agents, including 0₂, N₂0 in the gas phase and t-butyl hydroperoxide (TBHP) and H₂0₂ in the liquid phase (Stud. Surf. Sci. Catal 49B (1989) 1243 and 69(1991)137, and J. Catal., 141 (1993)595). Cr- containing MFI type silicalites have been used as catalysts for H 0₂ oxidative cleavage of unsaturated compounds to aldehydes (JP-B-356439 and 358954), and CrAPO-5 has been used as a catalyst in liquid phase oxidations using TBHP and oxygen (WO 94/08932) . CoAPO-5 and -11 have been used in catalysing auto-oxidation of p_- cresol to p-hydroxybenzaldehyde (NL-A-9200968) .

It has been found that the activity of the zeolites in catalytic reactions increases with the proportion of sites in the framework that are occupied by the foreign elements, and there have been recent attempts to develop manufacturing processes that yield higher levels than earlier processes. One such process is described in EP- A-568336, in which the procedure of Thangaraj et al. J. Catal., 130 (1991) is said to be followed, and to yield a TS-1 product having up to 12.5 molar per cent titanium. In Catalysis Today, 18 (1993) 163, however, B. Notari suggests that the Thangaraj process yields a mixture of Ti0₂ and a zeolite containing titanium at a maximum molar level of 2.5% in the zeolite framework. The presence of Ti0₂ in the final product is disadvantageous in many catalysts in H₂0₂ oxidation reactions apparently since, as indicated by Notari, op.cit., at 166, Ti0 causes decomposition of H₂0₂ by a separate mechanism and reduces H 0₂ utilization.

There accordingly remains a need for a zeolite having a higher proportion of framework sites occupied by active metal species than is presently available, and preferably being substantially free of extra-framework impurities resulting from the active metal presence, and for a process by which such a material may be obtained.

The present invention provides a process for the manufacture of a molecular sieve containing in addition to silicon, or in addition to silicon and aluminium, a further framework metal, which comprises forming a synthesis mixture appropriate for the manufacture of the desired molecular sieve, and containing a source of each of the framework elements essential to the desired molecular sieve, selectively reducing the concentration of the silicon and optionally if present the aluminium source in the synthesis mixture, and hydrothermally treating the synthesis mixture having an enhanced relative concentration of the further framework metal source.

The further framework metal may be any of the usual elements, other than aluminium, employed in zeolite manufacture. As examples there may be mentioned B, Be, Co, Cr, Fe, Ga, Mo, Mn, Ni, Pt, Pd, Re, Rh, Ti, V, W, Zr, or combinations of any two or more such elements.

An important framework element is titanium. The present invention accordingly provides a substantially aluminium-free and substantially Ti0 -free titanium- containing molecular sieve, especially a titanium silicalite, containing at least 3, and advantageously from 4 to 12, molar per cent titanium, based on the titanium and silicon present.

By "substantially aluminium-free" is meant a material containing no more aluminium than is derivable from aluminium present as impurity in the silicon source.

The synthesis mixtures employed in and produced by the present invention are those appropriate to the formation of a zeolite of the structure being manufactured, together with a source of the further framework metal.

For example, for the manufacture of a titanium- containing silicalite, the starting synthesis mixture is that appropriate for silicalite synthesis, together with a source of titanium. As a typical titanium silicalite synthesis mixture, there may be mentioned that described in U.S. Patent No. 4410501, which contains a source of silicon, -e.g. , a tetraalkylorthosilicate, preferably a tetraethylorthosilicate, colloidal silica, or an alkali metal silicate, a source of titanium, e.g., a hydrolysable titanium compound, e.g., TiCl₄, TiOCl₂, or Ti(alkoxy)₄, preferably Ti(OC₂H₅)₄, a nitrogen-containing organic base e.g., a tetraalkylammonium hydroxide, especially tetrapropylammonium hydroxide, water, and optionally an inorganic base. Suitable molar reagent ratios are said to be within the ranges:

Reactants Advantageous Preferred sio₂ : τio₂ 5 to 200 : 1 35 to 65 1

OH" • Si0₂ 0.1 to 1.0 : 1 0.3 to 0.6 1

H₂0 • Si0₂ 20 to 200 : 1 60 to 100 1

Me • SiO-₅ 0.0 to 0.5 : 1 0 1

RN + Si0₂ 0.1 to 2.0 : 1 0.4 1 RN⁺ represents the cations of the organic base; Me represents the inorganic base if present. As indicated above, however, it is believed that contamination of the zeolite by Ti0₂ results at Si0₂:Ti0 ratios at the lower end of the broader range.

Ranges preferred for the present invention are as follows:

Si0₂ τio₂ 26 to 37 : 1 OH^" sio₂ 0.3 to 0.6 : 1 H₂0 Si0₂ 30 to 60 : 1 RN⁺ Si0₂ 0.3 to 0.6 : 1 WO 94/02245 describes the manufacture of Ti-β, the synthesis mixture containing sources of silicon, and titanium, which may be as described above with reference to U.S. Patent No. 4410501, a source of aluminium, water, and a nitrogen-containing organic base, usually tetraethylammonium hydroxide, advantageously together with a peroxide source. In the manufacture of V-β and Zr-β, the further framework metal source may be, for example, vanadyl sulphate or zirconyl sulphate. The molar composition of the synthesis mixture may be Si0₂:l; Ti0₂, V0₂, Zr0₂: 0.0001 to 0.2 :A1₂0₃ : 0.0005 to 0.1;H₂0:10 to 100; nitrogen-containing organic base:0.01 to 1.

In WO 94/02245, contamination of the desired Ti-β by Ti0₂ is encountered when a high Ti:Si ratio is employed. Without wishing to be bound by any theory, it is believed that at high titanium source concentrations hydrolysis and subsequent condensation of the titanium source alone to yield Ti0₂ proceed more rapidly than do the desired hydrolysis and co-condensation of the titanium and silicon source to yield a Ti-containing zeolite precursor, this undesired process taking place both in Ti-silicalite and Ti-β manufacture at high Ti source concentrations.

Selective reduction of the concentration of the silicon source involves depleting the silicon source concentration without depleting the concentration of the framework metal source or depleting the latter only to an extent such that the concentration of framework metal source, relative to that of the silicon source, is enhanced. Advantageously, depletion of the initial synthesis mixture is carried out to give a synthesis mixture having a Si0₂:Ti0₂ molar ratio in the range of from 4:1 up to 20:1, advantageously up to 15:1, more advantageously up to 10:1, and preferably up to 7:1.

The present invention also provides a molecular sieve-forming synthesis mixture, the mixture being suitable for the manufacture of a molecular sieve containing, in addition to silicon, or in addition to silicon and aluminium, a further framework metal, the synthesis mixture being stable and containing a source of each of the frε-mework elements of the molecular sieve and the mixture being obtainable by reducing the concentration of the silicon source and optionally if present the concentration of the aluminium source in the synthesis mixture.

Reduction of the concentration of the silicon source concentration and, if present, the aluminium source concentration is advantageously achieved by an initial hydrothermal treatment, preferably at a relatively low temperature and if desired with seeding. It has been found that by this means a molecular sieve (referred to for simplicity in the description below as a zeolite) containing a relatively low proportion of the further framework element is precipitated, and a stable synthesis mixture having an enhanced relative proportion of the further framework element remains. This synthesis mixture may be employed in a number of different ways. It is suitable for the manufacture of aerogels, xerogels, co-precipitated gels, and for the manufacture of high further framework element content zeolites, by further thermal treatment of the synthesis mixture, advantageously at a temperature higher than that of the initial hydrothermal treatment, either alone or, if desired, with seeding using, for example, colloidal zeolite seeds, preferably of the same structural type as that which the synthesis mixture would yield on its own but, if desired, of a different composition.

In the manufacture of a high titanium content silicalite, by the preferred method of depleting the synthesis mixture by low temperature zeolite formation in a first stage, it has been found that advantageously a temperature within the range of from 50 to 110°C, more advantageously from 50 to 90°C, preferably about 75°C, may be used, advantageously with reaction times of from 2 to 15 days, lower temperatures corresponding to longer reaction times. Crystallization may be accelerated by seeding the synthesis mixture; it has been found that at a given first stage synthesis temperature a zeolite having a lower titanium proportion results from a seeded mixture than from an unseeded mixture.

The temperature at which low temperature thermal treatment takes place affects the crystal size of the zeolite product of the high temperature thermal treatment; a synthesis mixture prepared at the lower end of the temperature range yields a smaller particle size product.

In the second, high temperature step, a temperature within the range of from 130 to 200°C, more advantageously from 140 to 190°C, is advantageously used, advantageously with a crystallization time within the range of from 3 to 6 days, a lower temperature corresponding to a longer crystallization time. The synthesis mixture may be static or stirred, stirring the mixture producing crystals of a more uniform particle size.

If desired, zeolite crystals of reduced particle size may be prepared by diluting the synthesis mixture; the diluent may be, for example, water or ethanol. Particle size reduction may also be effected by adding further nitrogen-containing organic base to the synthesis mixture .

The synthesis mixture may be seeded, although normally this is not necessary since it will normally contain residual solid material of particle size too small to be removed after the low temperature hydrotherma treatment. Seeding may be advantageous, however, if silicon source depletion is achieved by means other than hydrothermal treatment or if a "cherry" type particle is desired. In the latter case, a particulate zeolite of the same structure as that produced by the synthesis mixture, e.g., silicalite in the case of a synthesis mixture yielding titanium silicalite, is incorporated in the synthesis mixture before its high temperature hydrothermal treatment to provide a core on which the desired high further framework metal content zeolite may grow. Advantageously, the core is free from the further framework metal or has a lower content thereof than produced by the synthesis mixture. The particle size of the core zeolite is advantageously in the range of from 0.1 μm to 2 μm, preferably about 1 μm.

The high-proportion framework element zeolite provided by the process described above may be ion- exchanged, calcined, and otherwise treated in the same way as prior art zeolites to yield a zeolite catalyst of utility dependent on the structure of the zeolite.

The present invention accordingly also provides a process for catalysing a chemical reaction which comprises contacting a feedstock with a molecular sieve in accordance with the invention or one made by a process in accordance with the invention, the molecular sieve being in active catalytic form, under catalytic conversion conditions and recovering a composition comprising at least one conversion product.

The ^{^}invention also provides a process for the separation of a fluid mixture which comprises contacting the mixture with a molecular sieve in accordance with the invention or one made by a process in accordance with the invention and recovering a component or mixture of components in a concentration different from its concentration or their concentrations in the mixture.

The following examples illustrate the invention:

Example 1

This example illustrates the preparation of a synthesis mixture suitable for the preparation of a titanium-containing silicalite.

156.48 parts (proportional to 0.756 mol) of tetraethyl orthosilicate (TEOS) were mixed with 6.232 parts (0.219 mol) of tetra.isopropyl orthotitanate (TPOT) with stirring at 35°C in a glass container for half an hour under nitrogen flow and the resulting mixture was then cooled to 0°C. Tetrapropylammonium hydroxide (TPAOH) was then added, initially dropwise at intervals between adding every few drops, with stirring and continued cooling until after addition of about 10 parts the addition rate is speeded up, a total of 306 parts (0.302 mol) being added. After all the TPAOH has been added, when a clear, colourless, solution resulted, the mixture was heated to 80 to 90°C and maintained for two hours. Water was then added to a total of 400 parts, and the mixture heated until all the ethanol resulting from the reaction has been distilled off. The resulting gel had a molar composition as follows:

Si : 1; Ti : 0.0289; OH~ : 0.4; H₂0 : 21.5 Example 2

This example illustrates a preferred method of reducing the silicon source concentration in a synthesis mixture relative to that of the titanium source.

75 parts of a synthesis mixture prepared as described in Example 1 were placed in a plastic flask and heated in an oil bath under reflux conditions (90°C) for three days. The resulting suspension of crystals was centrifuged at 12000 rp for 1 hour. The clear liquid above the crystals was retained for future use, and the crystals were washed several times with water, dried, and calcined. X-ray diffraction (XRD) , scanning electron microscopy (SEM) and elemental analysis indicated a product having an Si:Ti molar ratio of 146:1, with a crystal size of 70 ran, of a highly crystalline MFI structure. 4.86 parts were recovered, indicating a 51% conversion of the silica present in the synthesis mixture.

The product was evaluated as a catalyst in H₂0₂ oxidation of n-heptane under standard conditions. A heptane conversion of 7.4 percent was observed.

Example 3

Example 2 was repeated, except that 56 ppm based on total synthesis mixture of colloidal silicalite crystals were added as seeds. The silica conversion was slightly reduced, to 46.8%, and the Si:Ti molar ratio was increased, to 180:1. Example 4

This example illustrates the preparation of a high- titanium silicalite.

The clear liquid remaining after centrifuging the product of low temperature synthesis as described in Example 2 was subjected to heat treatment in a stainless steel autoclave at 175 °C for 3 days. A suspension of crystals again resulted; this was centrifuged and the resulting solid treated as described in Example 2. 3.6 parts of crystals were removed, indicating a silica conversion of 49%. The product had a Si:Ti molar ratio of 9.56:1 (indicating a 9.5% molar titanium content), with varied crystallite sizes and morphology, of a highly crystalline MFI structure. UN inspection indicated no Ti0₂ in the product. In the heptane oxidation evaluation, a heptane conversion of 68.7% was observed.

Examples 5 to 7

This example illustrates the effect of varying the temperature at which the silicon source in a synthesis mixture, prepared as described in Example 1, is depleted. The synthesis mixture had a molar composition as follows:

Si:l; Ti: 0.0318; OH^": 0.4; H₂O:30

Samples of the synthesis mixture were hydrothermally treated generally as described in Example 2 but varied as set out in Table 1 below, which also shows the Si:Ti ratios of the crystals recovered. Example 5 was carried out under reflux, in a plastic vessel. Examples 6 and 7 were carried out in stainless steel autoclaves.

Table 1

Ex. Crystallization Time, Yield Si:Ti No. Temperature, °C Days Molar

Ratio

5 50 5 31.0 244.8

6 70 5 47.6 171.9

7 90 3 52.2 104.1

The results indicate that a temperature of about 75°C is optimum if a high Si:Ti ratio is required at an acceptable yield (i.e., an acceptable depletion of the silicon content of the mother liquor) .

Examples 8 to 13

In Examples 8 to 10, Example 4 was repeated, on the mother liquors resulting from the three low temperature crystallizations described in Examples 5 to 7. Examples 11 to 13 were carried out in a similar manner, but the synthesis mixtures were stirred at 360 r.p.m. The improvement in crystal size uniformity as a result of stirring is apparent. Table 2 shows the results whil^ Table 3 shows the results of n-heptane oxidation using 30% H₂0₂. Table 2

Example No. Stirring Yield Crystal Crystal Si:Ti & Low Temp. % Morphology Size,μm Molar Synthesis Ratio Temp,°C

8, 50 No 47.5 coffins, 1.25 15 spheres/cubes 0.08

9, 70 No 27.5 coffins/needles 0.7 7.8 spheres/cubes 0.11

10, 90 No 31.5 coffins/needles 0.8-2 19.1 spherees/cubes 0.1

11, 50 Yes 41.1 coffins 0.4 16.9 cubes 0.08

12, 70 Yes 47.5 coffins/needles 0.7 12.4

13, 90 Yes 42.0 coffins/needles 0.7-1.6 12.6

Table 3

Example Conversion Selectivity Selectivity H 0₂

No. % to Alcohols, % to Ketones, % Efficiency, %

8 24.5 3.1 96.1 78

9 21.2 5.3 94.1 38

10 18.7 11.3 88.0 46

11 10.7 15.7 84.3 20

12 17.3 15.9 76.1 26

13 21.8 2.6 97.4 45

Example 14

The procedure of Example 4 was repeated , but at a temperature of 150 °C and with a crystallization time of 5 days . The product had a Si :Ti molar ratio of 30. 1 : 1 , an MFI structure , with most crystals cubic , of size about 0.1 μm. In the n-heptane oxidation test, a conversion rate of only 14.2% was achieved, possibly because of the relatively low titanium content.

Examples 15 to 17 The procedure of Example 6 was followed, except that crystallization took place for 6 days at 70°C. The mother liquor was diluted with an equal mass of water (Examples 15 and 16) or ethanol (Example 17) . The water- diluted sample was divided into two batches, one being crystallized without stirring (Example 15) and the other with stirring (Example 16) . The ethanol-diluted sample was crystallized with stirring, all crystallizations being carried out at 180°C. For a comparative example, a sample was prepared without dilution and without stirring. The effects on crystal size and catalytic performance are shown in Table 4.

Table 4 Example No. 15 16 17 Comp.

Diluent Water Water Ethanol None

Stirring No Yes Yes No

Crystal size 0.3 μm 0.33 μm 0.1 μm 1 μm

Si:Ti ratio 12:1 12.3:1 17.1:1

^H2°2 efficiency % 50.3 45.3 89.8 39.8

Heptane Conversion % 25 25 78.8 26.2

Claims

CLAIMS :

1. A process for the manufacture of a molecular sieve containing in addition to silicon, or in addition to silicon and aluminium, a further framework metal other than aluminium, which comprises forming a synthesis mixture appropriate for the manufacture of the desired molecular sieve, and containing a source of each of the framework elements essential to the desired molecular sieve, selectively reducing the concentration of the silicon and optionally if present the aluminium source in the synthesis mixture, and hydrothermally treating the synthesis mixture having an enhanced relative concentration of the further framework metal source.

2. A process as claimed in claim 1, wherein selective reduction of the silicon source concentration is effected by hydrotherma1 treatment.

3. A process as claimed in claim 2, wherein the hydrothermal treatment to effect selective reduction is carried out at a temperature lower than that carried out on the synthesis mixture having an enhanced further framework metal source concentration.

4. A process as claimed in claim 2 or claim 3, wherein the hydrothermal treatment to effect selective reductionΓÇöis carried out in the presence of seeds.

5. A process as claimed in claim 4, wherein the seeds are colloidal.

6. A process as claimed in any one of claims 1 to 5, wherein the further framework metal is titanium.

7. A process as claimed in claim 6, wherein the molecular sieve is titanium-silicalite.

8. A process as claimed in claim 7, wherein the initial synthesis mixture has a molar composition within the ranges:

Si0₂ : Ti0₂ 26 to 37 : 1 OH^" : Si0₂ 0.3 to 0.6 : 1 H₂0 : Si0₂ 30 to 60 : 1 RN⁺ : Si0₂ 0.3 to 0.6 : 1

9. A process as claimed in claim 7 or claim 8, wherein the initial synthesis mixture is depleted to give a Si0₂:Ti0₂ molar ratio in the range of from 4:1 to 20:1.

10. A process as claimed in any one of claims 7 to 9, wherein reduction of the silicon source concentration is carried out by hydrothermal treatment at a temperature in the range of from 50 to 110┬░C and hydrothermal treatment of the enhanced titanium concentration synthesis mixture is carried out at a temperature within the range of from 130 to 200┬░C.

11. A molecular sieve whenever produced by the process of. any one of claims 1 to 10.

12. A molecular sieve-forming synthesis mixture, the mixture being suitable for the manufacture of a molecular sieve containing, in addition to silicon, or in addition to silicon and aluminium, a further framework metal other than aluminium, the synthesis mixture being stable and containing a source of each of the framework elements of the molecular sieve and the mixture being obtainable by reducing the concentration of the silicon source and optionally if present the concentration of the aluminium source in the synthesis mixture.

13. A synthesis mixture as claimed in calim 12, which has a molar ratio of Si0₂:Ti0₂ in the range of from 4:1 to 20:1.

14. A synthesis mixture as claimed in claim 12 or claim 13, obtained by hydrothermal treatment of a synthesis mixture.

15. A substantially Ti0 -free and substantially aluminium-free titanium- and silicon-containing molecular sieve, the framework titanium content of which is at least 3 molar per cent, based on the total weight of titanium and silicon present.

16. A molecular sieve as claimed in claim 15, having a molar titanium content of from 4 to 12%.

17. A molecular sieve as claimed in claim 15 or claim 16 which is a titanium silicalite.

18. TS-1 containing at least 3 molar per cent framework^ titanium.

19. A process for catalysing a chemical reaction, which comprises contacting a feedstock with a molecular sieve as claimed in claim 11 or any one of claims 15 to 18, in active catalytic form under catalytic conversion conditions and recovering a composition comprising at least one conversion product.

20. A process as claimed in claim 19, wherein the molecular sieve is titanium silicalite and the reaction is oxidation of an organic feedstock.

21. A process for the separation of a fluid mixture which comprises contacting the mixture with a molecular sieve as claimed in claim 11 or any one of claims 15 to 18, and recovering a component or mixture of components in a concentration different from its concentration or their concentrations in the mixture.