GB2322364A - Zirconium molecular sieve catalysts - Google Patents

Abstract

Isomorphous zirconium substituted molecular sieve catalysts may be prepared dissolving a silica-zirconia coprecipitate in a solvent to form a gel, seeding the gel with colloidal molecular sieve seeds, allowing the isomorphous zirconium molecular sieve to crystallise and separating it. The solvent may be an organic template eg a tetraalkyl ammonium hydroxide. The molecular sieve may be a zeolite or a silicalite.

Description

Zirconium Molecular Sieve Catalysts The present invention relates to zirconium containing molecular sieve catalysts and in particular to zirconium containing silicalite catalyst and their use particularly for the oxidation of and cracking of organic compounds. The catalysts are particularly useful in the selective oxidation of organic compounds particularly aliphatic compounds and alkyl aromatic compounds. A particularly useful feature is their performance as ring opening catalysts. In a preferred embodiment the invention is concerned their use in the oxidation of saturated hydrocarbon chains.

Metal containing molecular sieve catalysts such as silicalite are known. US Patent No's 3,329,480 and 3,329,481 mention Zirconium Silicalite (ZrS-1) as a possible metal containing molecular sieve materials amongst many others but do not describe its actual synthesis.

In "Synthesis and Catalytic reaction of [Zr] ZSM-5", Gui-Ru Wang et.al., a process is described for the synthesis of Zr containing ZSM-5. This synthesis requires the preparation of a silica source solution which contains sodium ions and a zirconium source solution prepared from the hydrolysis of zirconyl chloride with sulphuric acid.

The hydrolysed zirconyl chloride solution is added to the silica source solution to produce a white gelatinous precipitate which is used to produce the zirconium containing molecular sieve. The resultant molecular sieve is not isomorphous and contains polymeric ZrO2 species which are detrimental to the performance of the catalyst during oxidation reactions.

In " Synthesis Characterisation and Catalytic Properties of [Zr] ZSM-5", M. K.

Dongore et.al., Zeolite, 1991, Vol 11, p690, a process is described for the synthesis of Zr containing ZSM-5. In this process a zirconium isopropoxide solution in isopropylalcohol is added dropwise to an isopropylalcohol solution of tetraethylorthosilicate. The resultant mixed solution is hydrolysed in the presence of tetrapropylammonium hydroxide and water; this being a basic environment. The resultant mixture is used to prepare a molecular sieve. The applicants have repeated this procedure but were unable to produce a zirconium containing molecular sieve which was isomorphous; the resultant product did nl5t have zirconium incorporatèd into the molecular sieve structure.

US 5 399 336 describes in Example 4 a process for the production of a zirconium containing molecular sieve which process comprises the use of hydrofluoric acid without the preparation of a co-precipitate.

We have now discovered a process for the production of catalytically active zirconium containing molecular sieves e.g. ZrS-1 which differs from the known processes for preparing such molecular sieves. The process of the present invention provides a zirconium isomorphous substituted molecular sieve which is free of significant quantities of polymeric ZrO2 species.

Accordingly the invention provides a method for the synthesis of a zirconium isomorphous substituted molecular sieve which method comprises dissolving a silica zirconia coprecipitate, free of alkali metal and polymeric ZrO2 species, in an organic templating agent, seeding the resulting gel with colloidal molecular sieve and allowing the required zirconium isomorphous substituted molecular sieve to crystallise.

In our process the silica zirconia co-precipitate may be formed by mixing any suitable silica source together with a source of zirconium to form a co-solution which may then be subject to the appropriate conditions for co-precipitation. In a preferred embodiment the silica source such as for example tetra ethyl ortho silicate (TEOS) is hydrolysed in an acid environment preferably a nitric acid environment followed by addition of a solution of a zirconate such as for example tetrapropyl ortho zirconate (TPOZr) in a suitable solvent such as for example isopropanol. The zirconate is not hydrolysed before mixing with the silica source. The silica source solution is free of alkali metal ions such as K+ or Na+ and is either neutral (pH=7) or is acidic (pH < 7).

This careful control of the hydrolysis environment ensures that there are no significant amounts of polymeric ZrO2 species formed in the co-solution and in the resulting coprecipitate. The silica source solution and the zirconia source solution are free of organic templating agents.

Once the co-solution is produced the silica and zirconia may be co-precipitated by removal of water and the solvent used to prepare the co-solution. The water and solvent may be removed by evaporation at ambient temperatures or may be removed by heating. We have found that heating to a temperature in the range of room temperature to 200"C is particularly suitable, preferably it is in the range 80 to 12000.

The silica zirconia coprecipitate may then be used for the production of zirconium isomorphous substituted molecular sieves, such as silicalite-1, by dissolution in an appropriate templating agent e.g. tetra propyl ammonium hydroxide (TPAOH). The resultant solution is then seeded with colloidal molecular sieve such as colloidal silicalite which may be prepared according to the procedure described in for example WO93/08125 the disclosure of which is incorporated by reference. Once seeded the zirconium containing molecular sieve e.g. silicalite-1 may be obtained by crystallisation whilst stirring at an appropriate temperature and over an appropriate period of time. For example we found 1 to 30 days preferably 1 to 10 days and most preferably is at least 3 days at 50 to 200 C preferably 130 to 180"C to be particularly suitable for the formation of isomorphous zirconium silicalite-1.

In the process of the present invention it has been found that the use of an intermediate silica zirconia co-precipitate in the synthesis of zeolite isomorphous substituted molecular sieves enables the use of impure templating agents. In particular it is possible to use templating agents which contain relatively high levels of alkali metal species such as Na+ and K+ cations. This is particularly advantageous as the normal practice in the synthesis of molecular sieve materials using organic templating agents is to ensure as far as possible the total absence of alkali metal cations. Typically the levels of sodium are 20 ppm or less and the levels of potassium are 5 ppm or less. In the present invention therefore the templating agents do not have to be alkali metal free and may have 20ppm or higher levels of sodium and/or 5 ppm or higher levels of potassium. Preferably the sodium and/or potassium levels are greater than 50 ppm, preferably greater than 100 ppm and most preferably greater than 130 ppm. The levels may be greater than 500 ppm.

A number of zirconium isomorphous substituted molecular sieves may be prepared by this method. Which molecular sieve is produced will depend on the templating agent composition and temperature used. It is envisaged that the process will be particularly suitable for the production of zirconium isomorphous substituted silicalite molecular sieves such as beta and MAMA1 molecular sieves.

The crystallised product obtained may be removed from the crystallisation medium by filtration and the washed.

When the templating agent used is tetrapropyl ammonium hydroxide the resultant zirconium containing zeolite was found to be have the following properties which are consistent with the formation of ZrS-1. XRD pattern showed that the structure obtained was the MFI structure. The SEM showed spherical crystals with crystal sizes variation from 0.75 to 1.5 pm The FTIR pattern shows a shoulder at 960 cm-l (which is indicative of the asymmetry in the silica framework caused by the Si-O-Me bond), in addition to other characteristic bands of the MFI structure. Its elemental analysis showed a molar ratio Si:Zr of 1:0.03 in the synthesised gel and a molar ratio of 1:0.011 in the crystalline product.

The present invention therefore further provides a zirconium isomorphous substituted silicalite-1 which has the above characteristics and is substantially free of ZrO2 polymeric species. Preferably the zirconium isomorphous substituted silicalite-1 has 1 mole% or greater of Zr most preferably greater than 2 mole % and preferably 1 to 10 0 mole % of Zr.

We have found that the zirconium containing silicalite-1 catalysts of the present invention are useful catalysts, particularly for hydrocarbon oxidation.

Saturated organic compounds are difficult to oxidise and despite attempts to develop methods and techniques for their controlled or selective oxidation techniques using mild conditions with relatively high yields are only known for the conversion of butane via butenes into maleic anhydride, furthermore the known processes use homogenous and sometimes hazardous catalysts requiring complex separation techniques. An example of such processes are given in Catalysis Today Vol. I Nos. 5 of October 1987 relative to the selective catalytic oxidation of butane to maleic anhydride involving dehydrogenation and oxidation of the resulting intermediate olefin, the article in Tetrahedron Vol. 31 pages 777-784 concerning the oxidation of cyclohexane with molecular oxygen and the article in the Journal of the CHEM. SOC.

CHEM. COMMUN. 1987 page 1487 and Journal of Molecular Catalysis 44 (1988) pages 73-83. The direct oxidation of saturates to introduce functional groups such as ketones and alcohols using a heterogeneous catalyst system would be extremely attractive.

The zirconium isomorphous substituted molecular sieve catalysts of the present invention and in particular zirconium silicalite-1 have been found to be an active oxidation catalyst especially for reactions involving hydrogen peroxide as oxidant.

The new catalysts may also be effective with organic hydroperoxide oxidants.

When aqueous hydrogen peroxide is used the solution contains from 10-100, preferably 10 to 70 wt % hydrogen peroxide for example diluted hydrogen peroxide (40% by weight in water). It is also preferred that a polar solvent be present when aqueous hydrogen peroxide is used to increase the solubility of the organic compound in the H202 aqueous phase. Examples of suitable solvents include acetone and methanol.

The oxidising agent may be an organic hydroperoxide, examples of suitable organic hydroperoxides include di-isopropyl benzene monohydroperoxide, cumene hyd roperoxide, tert. butyl hyd roperoxide, cyclohexylhyd roperoxide, ethylbenzene hydroperoxide, tert. amyl hydroperoxide, tetraline hydroperoxide and the compound containing the saturated organic group is liquid or in the dense phase at the conditions used for the reaction. It is also preferred that the reaction is carried out in the presence of a suitable solvent. When the oxidant is a an organic hydroperoxide then tertiary butyl hydroperoxide is particularly beneficial since the tertiary butyl alcohol produced can readily be converted to the valuable isobutylene molecule. The preferred oxidising agent is hydrogen peroxide.

It is possible to oxidise saturated aliphatic compounds including aliphatic substituents of aliphaticlaromatic compounds by the process of the invention. The saturated groups which may be oxidised by the process of this invention include long or short, branched or linear alkanes containing 3 or more, preferably 3 to 30, more preferably 3 to 12 carbon atoms, cyclic alkanes and mono- and poly- alkyl aromatics in which at least one of the alkyl groups contain at least two preferably at least three, more preferably 3 to 18, most preferably 3 to 12 carbon atoms and mono- and poly-alkyl cyclic alkanes. The process of the invention is equally applicable to the epoxidation of olefins, dienes, the production of ether glycols, diols, the oxidation of alcohols or ketones, aldehydes, to acids and the hydroxylation of aromatics. We have surprisingly found that by the selection of appropriate conditions saturated groups may be oxidised with high selectivity to alcohols and ketones under relatively mild conditions. One particularly useful application is in the oxidation of linear and branched paraffins to secondary alcohols and ketones. The process is especially useful in the lower carbon number range to enable use of low-cost propane and butane feedstock in the manufacture of isopropanol alcohol, acetone, secondary butyl alcohol and methyl ethyl ketone. The aliphatic substituent may be a part of a totally aliphatic compound, an aryl compound (alkyl aromatic) or an alkylnaphthene compound. Furthermore, said compound may contain other functional groups providing they do not prevent the desired oxidation reaction taking place. One particularly interesting reaction is the oxidation of propionic acid to lactic acid.

The reactivity sequence for the aliphatic compounds slows down from tertiary to secondary and to primary compounds.

Particular advantages of the present invention are that the process uses mild temperature and pressure conditions and the conversion and yield are high and byproduct formation is small. In particular the oxidant conversion is high. The optimum reaction temperature is between 50 and 1500C, preferably about 100"C when using hydrogen peroxide. The oxidation reaction may be in the liquid or dense phase or in the gaseous phase, preferably the reactions are in the liquid phase.

The reaction can be carried out at room temperature but higher reaction rates may be involved at higher temperatures, for example under reflux conditions. Through increase of the pressure either due to the autogeneous pressure created by the heated reactants or by use of a pressurised reactor still higher temperatures can be reached. Use of higher pressures in the range of 1 to 100 bars (105 to 107Pa) can increase the conversion and selectivity of the reaction.

The oxidation reaction can be carried out under batch conditions or in a fixed bed, and the use of the heterogeneous catalyst enables a continuous reaction in system.

The catalyst is stable under the reaction conditions, and can be totally recovered and reused.

The oxidation process of the present invention is preferably carried out in the presence of a solvent. Choice of solvent is important since it should dissolve the organic phase and the aqueous phase when hydrogen peroxide is used which is generally present due to the use of aqueous hydrogen peroxide as the oxidising agent, where organic hydroperoxides are used suitable organic solvents should be used. Polar compounds are preferred which are inert under reaction conditions, and examples of preferred solvents are alcohols, ketones and ethers, with a number of carbon atoms which is not too high, preferably less than or equal to 6. Methanol or tertiary butanol is the most preferred of the alcohols, acetone and butanone are the most preferred of the ketones. The amount of solvent is important and can influence the reaction product and the conversion, the choice of solvent and the amount depending on the material to be oxidised. We have found, for example, that when oxidising normal hexane with aqueous hydrogen peroxide yields are improved when the ratio of acetone to hexane is in the range 1:1 to 4:1. The solvent improves the miscibility of the hydrocarbon phase and the aqueous phase which is generally present due to the use of aqueous hydrogen peroxide as the oxidising agent. If, however, the peroxide is supplied as a solution, such as tertiary butyl hydroperoxide which is frequently dissolved in ditertiary butyl peroxide, and the substrate is soluble in the solvent then no additional solvent is required.

The invention will be described with further details including a preparation of the catalyst and several examples of oxidation reactions.

Example 1 A zirconium isomorphous substituted silicaiite-1 (ZrS-1) was prepared as follows. A commercially avaliable ZrO2-SiO2 co-precipitate (W.R.Grace, 5.7 wt % ZrO2, and particle size 50 micron) was used as a precursor for the synthesis. 142.59 of tetrapropyl ammonium hydroxide (TPAOH) 40% (containing 26 ppm K+, 630 ppm Na+) were diluted with 142.5g water, resulting in a TPAOH 20% solution. 59.739 of the ZrO2-SiO2 co-precipitate was dissolved in the TPAOH solution to produce a gel.

Subsequently, 59 colloidal silicalite seeds prepared by the process described in WO93/08125 (the disclosure of which is incorporated by reference), which is 0.16 wt %, was added and the mixture charged into Teflon - lined autoclave.

The mixture was stirred (350rpm) and heated to 1800C for 7 days. The mixture was cooled, centrifuged and washed with water several times. The solid material was dried at 1200C overnight and calcined in air at 550 C for 16 hrs. The catalyst was characterised by XRD, ICP, IR, UV-Vis. The XRD pattem showed that the structure obtained was the MFI structure. The SEM showed spherical crystals with crystal sizes variation from 0.75 to 1.5 clam. The FTIR pattern shows a shoulder at 960 cm-l (which is indicative of the asymmetry in the silica framework caused by the Si-O-Me bond), in addition to other characteristic bands of the MFI structure. Table 1 shows the results of elemental analysis.

Table 1

Synthesised gel Crystalline material MolarratioSi:Zr 1:0.03 1:0.011 Example 2 A ZrO2-SiO2 coprecipitate was made by hydrolysing a tetra ethyl ortho zirconate (TEOZr) solution in isopropanol with a 0.05 molar nitric acid solution of tetra ethyl ortho silicate (TEOS). The resulting clear solution was heated at 100 C under stirring until a white solid material formed. This material was then dried overnight at 1200C to remove the remaining water and alcohol to produce the co-precipitate. The coprecipitate was found to be amorphous by X-ray powder diffraction and was free of added alkali metal cations.

Typical procedure is as follows: 300g of TEOS (1.44mol) was added slowly to 14609 0.05M HNO3 (7.26g HNO3 65%). 16.22g TEOZr (0.048mol) was dissolved in 162g isopropanol and was added dropwise to the silicon solution (addition time 5h). The water and solvent from the resultant co-solution was then removed by heating and evaporation to provide a coprecipitate which had a Si:Zr molar ratio 1:0.03. This co-precipitate may be used with templating agents containing high levels of alkali metal to produce zirconium isomorphous substituted molecular sieve.

Example 3 The catalyst of Example 1 was tested in propionic acid oxidation with aqueous H202 30%. The reaction conditions are 20.4g propionic acid (0.204mol), 44.4g H202 30% (0.39mole), Ig catalyst, 71.1g acetone, 100"C, under magnetic stirring. A 50.8% conversion to the propionic was achieved with 23% selectivity for lactic acid.

Example 4 To illustrate the catalytic performance of the catalyst prepared according to Example 1 in catalytic cracking, micropulse experiments were performed in the temperature range 575 to 625 C, using 1-heptene/heptane mixture (1/1) as a feed. The effective residence time was 30 msec. The results are listed in Table 2.

Table 2

Temp C7 C7= C2= C3= C4= Total C2=+C3 "C conv conv% yield yield yield Aromatic =+C4= yield 575 0 100 2.3 19.3 24 0.5 45.6 600 3.5 100 5.1 19.5 22.5 0.7 47.1 625 ~ 9.89 100 10.6 19.5 19.6 1.1 49.8 These results illustrate the effectiveness of the catalyst in producing significant amounts of ethylene, propylene and butenes whilst keeping to a minimum byproducts such as aromatics.

Claims (1)

1. A zirconium isomorphous substituted molecular sieve catalyst substantially free of ZrO2.

2. A catalyst as claimed in claim 1 wherein the molecular sieve is a zeolite or a silicalite.

3. A catalyst as claimed in either claim 1 or claim 2 which comprises at least 1 mole % of Zr.

4 A method for manufacturing a zirconium isomorphous substituted molecular sieve catalyst which method comprises; (a) dissolving a silica zirconia co-precipitate, which is substantially free of polymeric ZrO2 and alkali metal cations, in a solvent to form a gel, (b) seeding the resultant gel with colloidal molecular sieve seeds to form a crystallisation mixture, (c) allowing the zirconium isomorphous substituted molecular sieve to crystallise from the crystallisation mixture, and (d) separating the crystallised molecular sieve from the crystallisation mixture.

5. A method as claimed in claim 4 wherein the solvent used in step (b) is an organic templating agent.

6. A method as claimed in claim 5 wherein the organic templating agent is a tetraalkyl ammonium hydroxide.

7 A method as claimed in any of claims 4 to 6 wherein the molecular sieve is a zeolite or a silicalite.

8. A method as claimed in any of claims 4 to 7 wherein the silica zirconia co precipitate is made by the process of any of claims 9 to 15.

9. A method for manufacturing a silica zirconia co-precipitate which is substantially free of alkali metal cations which method comprises; (a) preparing a zirconia source solution substantially free of alkali metal cations and which comprises an organometallic zirconium precursor which is not hydrolysed, (b) preparing a silica source solution substantially free of alkali metal cations which solution comprises the hydrolysis product of an organometallic silica precursor, (c) addition of solution (a) to solution (b) to produce a co-solution, and (d) removing the solution solvent and any water from the co-solution to provide a co-precipitate which is substantially free of alkali metal cations.

10. A method as claimed in claim 9 wherein the organometallic zirconium precursor is a zirconate.

11. A method as claimed in either claim 9 or claim 10 wherein the co-precipitate is formed from an alcoholic co-solution.

12. A method as claimed in claim 9 wherein the silica source solution is at a pH of 7 or less.

13. A method as claimed in any of claims 9 to 12 wherein the co-solution is heated in the range 70"C to 250"C.

14. A method as claimed in any of claims 9 to 13 wherein the silica source solution is acidified with a mineral acid.

15. A method as claimed in claim 14 wherein the mineral acid is nitric acid.

16. A method for the oxidation of an aliphatic saturated or unsaturated hydrocarbon or an alkylaromatic hydrocarbon which method comprises oxidising the hydrocarbon in the presence of a zirconium isomorphous substituted molecular sieve.

17. A method as claimed in claim 16 wherein the molecular sieve is a zeolite or silicalite-1.

18. A method as claimed in claim 16 wherein the oxidation is undertaken in the presence of hydrogen peroxide or an organic hydroperoxide.

19. A method as claimed in any of claims 16 to 18 wherein the hydrocarbon is a saturated aliphatic hydrocarbon.

21. A method of cracking a hydrocarbon feedstock which comprises contacting the feed stock under cracking conditions with a zirconium isomorphous substituted molecular sieve.

22. A method as claimed in claim 21 wherein the molecular sieve is a zeolite or a silicalite-1.

23. A method as claimed in any of claims 16 to 22 wherein the molecular sieve is as claimed in any of claims 1 to 3 or is prepared by the process of any of claims 1 to 8.