WO1993015035A1

WO1993015035A1 - Improved oxidation of saturated hydrocarbon chains

Info

Publication number: WO1993015035A1
Application number: PCT/EP1993/000186
Authority: WO
Inventors: Philip Luc Buskens; Diane Renata Cornelia Huybrechts
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1992-01-29
Filing date: 1993-01-27
Publication date: 1993-08-05
Also published as: AU3494393A; GB9201872D0

Abstract

Saturated hydrocarbon chains are oxidised preferably in solutions, using a titanium silicalite catalyst having an infrared absorption band around 950 cm-1, the chains may be alkanes or alkyl groups of alkyl cyclic compounds.

Description

IMPROVED OXIDATION OF SATURATED HYDROCARBON CHAINS

The present invention relates to the oxidation of saturated hydrocarbon chains and in particular to the use of a certain catalyst system which has been found to enable the selective oxidation of aliphatic compounds and alkyl aromatic compounds.

Saturated organic compounds are particularly 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 is given in Catalysis Today Vol I No 5 of October 1987 relative to the selective catalytic oxidation of butane to maleic anhydride involving dehydrogenation and oxidation of the resulting intermediate define, 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.

Previously it was found that catalysts proposed for the epoxydation of olefins, the hydroxylation of aromatics and the oxidation of alcohols to ketones and aldehydes, as described in US-A-4,501; EP-A-200200, EP-B-100119, EP-A- 196109, EP-A-100118 and DE-A-3135559 can be used for the controlled oxidation of saturates under mild conditions enabling direct formation of alcohols and ketones without acid formation, as described in WO 90/05126. In particular it was found that a synthetic silicon zeolite containing titanium atoms is able to cause saturated hydrocarbon groups and hydrogen peroxide or organic peroxides to react in a heterogeneous catalytic reaction to yield selectively alcohols and ketones.

These catalysts are based on crystalline synthetic material comprising silicon and titanium oxides and are characterised by an Infra red absorption band at around 950 cm"¹. They are typically of the general formula : xTiO₂(l-x)SiO₂

where x is from 0.0001 to 0.04.

They are typically prepared from a mixture containing a source of silicon oxide, • a source of titanium oxide, a nitrogenated organic base and water as described in United Kingdom Patent 2071071 which is concerned with the catalysts themselves or by the dealumination of ZSM-5 and reaction with titanium tetrachloride vapour as described by B. Kraushaar and J.H.C. Van Hoof in Catalysis Letters 1 (1988) pages 81-84. The catalysts may contain small amounts of other metals such as aluminium, gallium and iron (as described in European Patent Application 0226258).

United States Patent 4854976 relates to the use of these types of catalysts for the epoxidation of defines with H₂O₂ and in this patent x may be in the range from about 0.0001 to about 0.04. United Kingdom Patents 2083816 and 2116974 relate to the use of similar catalysts for the introduction of hydroxyl groups into aromatic substrates by oxidation with H₂O₂. These patents are incorporated herein by reference for their descriptions of the Infra-red and x ray diffraction analyses of the catalysts, as stated the band intensity at approximately 950 cπr¹ increases as the quantity of titanium present increases.

We have now found that the process for the oxidation of saturated hydrocarbon chains is improved by using a titanium silicalite catalyst with titanium contents which are significantly higher than those used in the prior art. The synthesis of such titanium rich titanium silicalites has been described in J. Catal. 130 (1991) pages 1-8.

The invention therefore provides the use for the oxidation of saturated organic groups of a titanium containing silicalite catalyst having an infra red absorption band around 950 cm-¹.

Typically the catalyst is of the general formula:

xTi0₂(l-x)Si0₂ where x is 0.04 to 0.10.

These catalysts may be typically prepared by:

(i) heating a reaction mixture comprising:

(a) a silicon oxide source (Siθ₂),

(b) a titanium oxide source (ΗO₂),

(c) optionally an alkali metal source,

(d) a nitrogen containing organic base,

(e) optionally an organic solvent, and

(f) water,

(ii) separating the formed crystals from the reaction mixture; and

(iii) calcining the separated crystals to form the catalyst.

The catalyst may be agglomerated to form crystal clusters which are also active and readily recovered after the oxidation reaction.

The invention further provides a process for the oxidation of saturated organic groups by the treatment of the compound containing the saturated organic group with an oxidising agent in the presence of a titanium containing silicalite catalyst having an infra red absorption band around 950 cm ¹.

Typically the catalyst is of the general formula:

xTi0₂(l-x)Si0₂

where x is 0.04 to 0.10.

In the preferred process the oxidising agent is hydrogen peroxide or an organic peroxide 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 solvent. The catalyst used in this invention is preferably prepared from a reaction mixture consisting of sources of silicon oxide, titanium oxide and possibly an alkaline oxide, a nitrogen containing organic base, possibly an organic solvent, and water, the composition in terms of the molar reagent ratios being as heretofore defined.

The silicon oxide source can be tetraalkylorthosilicate, preferably ' tetraethylorthosilicate, or simply a silicate in colloidal form, or again a silicate of an alkaline metal, preferably Na or K.

The titanium oxide source is a hydrolysable titanium compound preferably chosen from TiOC-U, TiOCl₂ and Ti(alkoxy)₄, preferably Ti(OC₄H₉)₄ or Ti(OC₂H₅) .

The organic base is a tetraalkylammonium hydroxide, and in particular tetrapropylammonium hydroxide.

The organic solvent is preferably an alcohol, in particular isopropanol.

In the preferred method to produce the catalyst the mixture of these reactants is subjected to hydrothermal treatment in an autoclave at a temperature of between 130 and 200°C under its own developed pressure, for a time of 1-30 preferably 6 to 30 days until the crystals of the catalyst precursor are formed. These are separated from the mother solution, carefully washed with water and dried. When in the anhydrous state they have the following composition:

xTiO₂.(l-x)Siθ₂.0.04(RN⁺)₂O.

The precursor crystals are then heated for between 1 and 72 hours in air at 550°C to eliminate the nitrogenated organic base.

The final catalyst has the following composition:

xTi0₂.(l-x)Si0₂

where x is as heretofore defined. The preferred molar ratio (MR) of the different reactants with regard to the silicon oxide source (Siθ₂) are mentioned in the following table:

The catalyst may also contain alkali metal cations M⁺ where M is sodium or potassium and in this situation it is preferred that the molar ratio of M⁺:Si0₂ is in the range 0.001 to 0.5.

It is possible to oxidise saturated aliphatic compounds including aliphatic substituents of aliphatic/aromatic 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 18, 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, more preferably 3 to 12 carbon atoms and mono- and poly- alkyl cyclic alkanes. We have surprisingly found that by the selection of appropriate conditions the 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 part of a totally aliphatic compound, an aryl compound (alkylaromatic) or an alkylnaphthene compound. Furthermore, said compound may contain other functional groups which have electron-repulsive properties and which, accordingly, are not reactive. The reactivity sequence for the aliphatic compounds slows down from tertiary to secondary and to primary compounds.

The oxidising agents used in the reaction may be organic peroxides, ozone or hydrogen peroxide, aqueous hydrogen peroxide being preferred. The aqueous • 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 for example acetone or methanol, this will increase the solubility of the organic compound in the H₂O₂ aqueous phase when aqueous hydrogen peroxide is used.

Particular advantages of the present invention are that the process uses mild temperature and pressure conditions and the conversion and yield are high and by-product formation is small. In particular the conversion of hydrogen peroxide is high. The optimum reaction temperature is between 50 and 150°C, preferably about 100°C. The pressure should be such that all materials are in the liquid or dense 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 10⁷ Pa) 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 a monophase or biphase system. The catalyst is stable under the reaction conditions, and can be totally recovered and reused.

The 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 which is generally present due to the use of aqueous hydrogen peroxide as the oxidising agent. Polar compounds are preferred and examples of preferred solvents are alcohols, ketones, ethers, glycols and acids, 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 the most preferred of the ketones, and acetic or propionic acid the most preferred acid. The amount of solvent is important and can influence the reaction product and conversion, the choice of solvent and the amount depending on the material to be oxidised. For example we have found 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.

The invention will be described with further details including a preparation of the catalyst and several examples of oxidation reactions. A comparison with a catalyst known in the prior art will be made.

Preparation of the Catalysts

11.45 g of a 40% solution of tetrapropylammonium-hydroxide (available from Alfa), diluted with 11.86 g distilled water, are added to a solution of 26.45 g tetraethylorthosilicate (available from Janssen Chimica) in 14.56 g dry isopropanol. The produced mixture is heated at 40°C during 5 minutes, and the resulting clear mixture is stirred during 1.5 hours. Thereafter, a solution of 3.37 g tetrabutoxyorthotitanate (available from Janssen Chimica) in 26.85 g isopropanol is slowly dropped into the mixture during a period of 1 hour. After 40 minutes stirring at 27°C, a solution of 11.56 g distilled water is added slowly. The mixture is stirred at room temperature during 15 hours and finally at 78°C during 3.5 hours. The synthesis gel is then transferred into an autoclave and maintained at 170°C during a time period of 6 days. The autoclave is thereafter cooled to room temperature and the formed crystals are separated from the mother-liquor and washed with distilled water by centrifugation. The product was finally dried and calcined at 550°C in air for 14 hours.

The thus obtained catalyst will be referred to as catalyst A.

For comparison, a catalyst used in the prior art is prepared as described in US 4,410,501 (Example 1). This catalyst was also dried and calcined at 550°C in air for 20 hours.

This comparative catalyst will be referred to as catalyst B.

Characterization of the catalysts

Chemical analysis was performed on catalysts A and B. It was found that catalyst A contains 1.6 m% Ti while catalyst B contains 6.5 m% Ti. The infra red spectra of the two catalysts is shown in Figure 1. Both catalysts contain an infrared absorption band at ± 950 cm-¹, but the intensity of the band is higher in catalyst A than in the comparative catalyst B.

Example 1: Oxidation of n-hexane

115 mmoles of n-hexane were oxidised in a stirred 300 ml PARR reactor with 228 mmoles of hydrogenperoxide at 100°C using 45 ml of acetone and 0.2 to 0.5 g of catalyst A. The products obtained after 1 to 3 hours reaction time and their selectivities are given in Table 1. For comparison, the same reaction was carried out using 0.5 g of catalyst B. The products obtained after 3 hours reaction are also shown in Table 1.

Table 1

The results show that the oxidation of n-hexane is improved when using catalyst A instead of the comparative catalyst B. Under the same reaction conditions, higher hexane conversions are obtained with catalyst A than with catalyst B and at the same conversion, catalyst A shows an improved selectivity towards ketones.

Example 2: Oxidation of Cyclohexane

139 mmoles of cyclohexane were oxidised in a stirred 300 ml PARR reactor using 228 mmoles of hydrogenperoxide and 0.5 g of catalyst A with 45 ml of acetone as solvent. The reaction was carried out for a period of 16 hours at a temperature of 100°C. The products obtained and their selectivity are shown in Table 2. For comparison, the experiment was repeated using catalyst B. The results are also shown in Table 2. The results show that the oxidation of cyclohexane is improved when using catalyst A instead of the comparative catalyst B. Under the same reaction conditions, higher cyclohexane conversions are obtained with Catalyst A than with catalyst B.

Table 2

Claims

CLAIMS:

1. The use for oxidation of saturated hydrocarbon chains of a titanium containing silicalite catalyst having an infra red absorption band around 950 cm-¹ of the formula:

xTi0₂(l-x)Si0₂

where x is 0.04 to 0.10.

2. The use according to claim 1 in which the saturated hydrocarbon is an alkane containing from 2 to 18 carbon atoms.

3. The use according to any one of the preceding claims in which the saturated hydrocarbon chain is an alkyl group containing at least two carbon atoms attached to a ring structure.

4. The use according to claim 3 in which the alkyl group contains at least three carbon atoms.

5. The use according to claim 3 or claim 4 in which the ring structure is aromatic.

6. A process for the oxidation of saturated hydrocarbon chains by the treatment of the compound containing the saturated organic group with an oxidising agent in the presence of a titanium containing silicalite catalyst having an infra red absorption band around 950 cm-¹ in which the catalyst is of the formula:

xTi0₂(l-x)Si0₂

where x is 0.04 to 0.10.

7. A process according to claim 6 in which the oxidising agent is hydrogen peroxide or an organic peroxide.

8. A process according to claim 6 or 7 in which the compound containing the saturated organic group is liquid or in the dense phase at the conditions used for the reaction.

9. A process according to any of claims 6 to 9 in which the saturated hydrocarbon chain is an alkane containing from 2 to 18 carbon atoms.

10. A process according to any of claims 6 to 9 in which the saturated hydrocarbon chain is an alkyl group containing at least two carbon atoms attached to a ring structure.

11. A process according to claim 10 in which the alkyl group contains at least 3 carbon atoms.

12. A process according to any of claims 6 to 11 in which the reaction is carried out in the presence of a solvent.

13. A process according to any of claims 6 to 12 in which the oxidising agent is aqueous hydrogen peroxide.

14. A process according to any of claims 6 to 13 in which the solvent is a polar solvent.

15. A process according to any of claims 6 to 14 carried out at a temperature between 50 and 150°C.