Process for the microwave induced preparation of crystalline microporous titanium silicalite

The present invention relates to a process for the microwave induced preparation of crystalline, microporous titanium silicalite. More particularly, the present invention relates to a process for the preparation of crystalline microporous titanium silicalite of the formula: xTiO 2 :(1−x)SiO 2 wherein x is greater than 0.043 and less than or equal to 0.11. These silicalites, prepared under microwave irradiation have the same physico-chemical properties as titanium silicalite prepared by hydrothermal methods reported in the literature and are useful as oxidation catalysts in reactions such as oxidation of phenol to catechol and hydroquinone, benzene to phenol, hydrocarbons, alcohols and oxidation of aromatic heterocycles to heterocyclic N-Oxides.

More specifically, the invention relates to the application of microwave irradiation during the crystallization of the gel leading to faster formation of titanium silicates with the crystal structure of silicalite-1.

Titanium silicates are useful in catalyzing various oxidation reactions such as the conversion of benzene into phenol, phenol into hydroquinone (commercialized by Enichem, Italy) and catechol, cyclohexanone into cyclohexanone oxime, ethanol into acetaldehyde, isopropanol into acetone, heterocycles to corresponding heterocyclic N-oxides etc. In all the above applications, it has been found that the catalytic efficiency of titanium silicate material is directly proportional to the content of titanium in the titanium silicalite. That is, the more the amount of titanium contained in these materials, the higher the rate with which they catalyze the above mentioned conversion processes. Hence, any process of preparation that leads to TS-1 of formula xTiO 2 :(1−x)SiO 2 wherein x is higher than 0.04 would lead to material with improved catalytic activity in the above mentioned reactions.

U.S. Pat. No. 3,329,482 Young describes crystalline titanosilicates prepared from siliceous and inorganic titanium compounds in the absence of organic bases. The U.S. Pat. No. 4,410,501 describes a process for the preparation of titanium silicalite, TS-1 with the formula xTiO 2 :(1−x)SiO 2 where x lies between 0.005 and 0.04. GB Patent 2071071 B describes a process for the preparation of TS-1 with the structure of silicalite-1. Thangaraj et al in their papers published in Zeolites, (vol. 12, pp.943-950 (1992), November/December and Journal of Catalysis, vol. 130, No. 1, pp. 1-8 (1991), no month) describes a method of preparation of TS-1 which takes 1-6 days for crystalliztion. Rajiv Kumar et al. (Nature, vol.381, 298-300 (1996), May) improved the method of TS-1 synthesis, by the incorporation of promoter ions and reported that the crystallization time could be brought down to 5-6 hours.

Microwave irradiation has been applied extensively for the faster and cleaner synthesis of organic compounds, organometallics and inorganic compounds. Its application in catalyst synthesis and evaluation is gaining momentum. Microwave irradiation ig reported to offer faster process rates, uniform distribution of active species in the catalyst pellet and improvement in crushing strength of the pellet. Microwave heating successfully applied in the synthesis of ZSM-5, A Zeolites (Chu, P., et al EP 358 827 (1990)), Y (Van Bekkum et al, Zeolites, vol.13, 162-165(1993)) and AlPO sub-5 molecular sieves (J. Caro et al, Zeolites Vol. 15, 33-39(1995).

The process for the preparation of TS-1, in the prior art, as for example in the above mentioned U.S. Pat. No. 4,410,501 Example 2) comprises mixing the sources of silicon (preferably silicon tetraethoxide) and titanium (preferably titanium tetraethoxide) at low temperature (5° C.) before adding organic base (such as tetrapropyl ammonium hydroxide aqueous solution) precooled to 5° C. and water. Since the rate of hydrolysis of titanium tetraethoxide is much faster compared to that of silicon tetraethoxide the cooling of the reaction mixture is important to avoid the formation of insoluble or sparingly soluble titanium oxide/hydroxyoxide, which if formed hinders the formation of titanium silicate. However, the necessity to cool the reactants to 5° C. under industrial conditions is a limitation in the prior art process which entails significantly higher process energy costs. Further, in the above mentioned patent (U.S. Pat. No. 4,410,501), the crystallization time of TS-1 under hydrothermal conditions at autogeneous pressure ranges between six and thirty days.

Another patent (U.S. Pat. No. 5,885,546) describes the preparation of titanium silicalite with the general formula, xTiO 2 :(1−x)SiO 2 where in x is equal to or less than 0.075. The microporous, crystalline titanium silicates having a silicalite-1 structure was prepared and a gel formed by mixing a source of (i) silicon oxide (such as: alkoxides, amorphous silica, colloidal silica etc.) and (ii) titanium oxide (like alkoxide-β diketone mixture), (iii) a nitrogen containing organic base having the formula R 4 N + where R represents propyl group and (iv) water at room temperature, heating the resultant gel in an autoclave at autogeneous pressure and a temperature between 140° C. to 200° C., filtering, washing, drying and calcining the resultant solid composite material at a temperature in the range of 300° C.-550° C. The crystallization time, as reported in the patent, was 1-5 days. In a publication by Kumar et al, ( Nature , vol.381, 298-300 (1996)) a process for the preparation of the titanium silicalite using some promoter ions was described. The authors claim that by using the promoters the crystallization time could be brought down to 1-5 hours. Depending upon the Si/Ti ratio it can be understood from the literature that if the Ti content is low, the crystalization is faster and if the Ti content increases the crystallization time takes one hour to several days. Thus it is advantageous if the crystallization time could be reduced further.

It is an objective of the present invention to develop an improved process for the preparation of titanium silicates using microwave irradiation.

Another objective of the present investigation is to prepare titanium silicalite with higher framework content of titanium.

It is a further objective of the present invention to develop a process wherein all the process operations are carried out at ambient or higher temperatures.

It is a further objective of the present invention to develop a faster process which provides crystalline titanium silicalite in less than one hour for the silicate-with higher Si/Ti ratio and about two hours when the Si/Ti approximate a value 10.

Accordingly, the present invention provides a process for the microwave induced preparation of crystalline, microporous titanium silicalite of the chemical formula: xTiO 2 :(1−x)SiO 2 wherein x is greater than or equal to 0.043 and less than or equal to 0.11 and characterized by the x-ray diffraction patterns as shown in Table 1.

and an infrared absorption spectra as shown in Table 2,

and wherein the Si/Ti ratio is in the range between 10 and 40, said process comprising heating a gel comprising a mixture containing (a) a source of silicon oxide, (b) a source of titanium oxide, (c) tetrapropyl ammonium hydroxide and (d) water by microwave irradiation at an autogenous pressure in the range of 10-15 bar and at a temperature in the range of 100° C. to 200° C. for a time period in the range of 30-120 minutes to obtain a solid composite material, filtering, washing and drying of the said solid composite material to form a dried composite material, and calcining the said dried solid composite material at a temperature greater than or equal to 300° C.

In one embodiment of the invention, x is at least 0.043.

In a further embodiment of the invention x is at least 0.050.

In another embodiment of the invention the gel further comprises an alcohol of the formula R—OH where R is an alkyl group having 2-5 carbon atoms.

In a frther embodiment of the invention, the alcohol is selected from the group consisting of ethyl, propyl, isopropyl, butyl, isobutyl and mixtures thereof

In one embodiment of the invention the source of titanium oxide is a titanium alkoxide.

In yet another embodiment of the invention the source of said silicon oxide is selected from the group consisting of alkoxides, amorphous silica and colloidal silica.

In a further embodiment of the invention the source of said silicon oxide is tetraalkyl orthosilicate.

In another embodiment of the ivention the gel has a molar composition of 0.043-0.11 TiO 2: 1 SiO 2: 0.3-0.8(C 3 H 7 ) 4 N + :2.0-5.0ROH:10-100H 2 O, wherein R is alkyl group having less than or equal to 5 carbon atoms.

In one embodiment of the invention, the gel is kept in a microwave oven and then irradiated.

In one embodiment of the invention, the gel is irradiated in a microwave oven by exposing to radiation with an applied power in the range 500 to1000W.

The process for the preparation of crystalline microporous titanium silicalite comprises forming a gel by mixing (a) a source of silicon (b) a source of titanium (c) tetrapropyl ammonium hydroxide and water at ambient temperatures, irradiating the resultant gel in a microwave oven at pressures up to 10-15 bar and temperatures between 130° C. to 200° C. for 30-120 minutes, filtering, washing, drying. and calcining the resultant solid composite material at a temperature above 300° C.

The significant feature of the present invention is that microwave irradiation of the gel enhances the solubility of the titanium species thereby preventing the precipitation of titanium as titanium oxide/hydroxide which process occurs readily in the normal method of hydrothermal preparation. Thus cooling the gel to temperatures to as low as 5° C. or the usage of additional promoters or complexing agents like the beta-diketone to increase the solubility are avoided. An feature of the present invention is that the increased solubility allows more amount of titanium to go into the frame work when compared to the hydrothermal conditions.

In the composite of formula xTiO 2 :(1−x)SiO 2 x is preferably equal to or less than 0.011, and is characterized by the x-ray diffraction patterns given in Table-1 and infrared absorption spectra as shown in Table-2. The starting gel is formed by mixing a source of (i) silicon oxide (such as: alkoxides, amorphous silica, colloidal silica etc.) and (ii) titanium oxide (like alkoxide), (iii) a nitrogen containing organic base having the formula R 4 N + where R represents propyl group and (iv) water at room temperature. To form the crystalline microporous titanium silicalite, the gel is heated in a microwave oven operating at powers between 500-1000W, at autogeneous pressure ranging 10-15 bar and a temperature between 140° C. to 200° C. for 30-120 minutes, the resultant solid composite material filtered, washed, dried and calcined at a temperature in the range of 300° C.-550° C.

The crystalline material of this invention was characterized by x-ray powder diffraction and infrared spectroscopic examination. The most significant lines of the x-ray powder diffraction pattern of the titanium rich titanium silicates is recorded over Philips 1051 using Ni-filter CuK α radiation (λ=1.5405 Å).

The IR bands in framework region exhibited by these titanium silicates are compiled in Table-2. They exhibit a characteristic IR absorption band at about 960 cm −1 which band is not present either in the spectrum of pure silicalite-1, or in the IR spectrum of titanium oxides (rutile, anatase) and alkaline titanates, The ratio of the intensity of 960 cm −1 band and that of 550 cm −1 band is found to have linear correlation with their titanium content as can be seen from Table-3.

The reaction mixture comprises the sources of SiO 2 , TiO 2 an organic base and water, and has the reagent molar composition as given above. The SiO 2 source is preferably a tetraalkyl orthosilicate (e.g. tetraethyl orthosilicate). The source of titanium is preferably a hydrolysable titanium compound e.g, Ti-alkoxides (e.g. Ti(OC 4 H 9 ) 4 . The organic base is a tetraalkyl ammonium hydroxide e.g. tetrapropyl ammonium hydroxide. The starting reagent mixture is subjected to microwave irradiation in a microwave oven at an operating pressure ranging 500-1000W, temperature of between 150° C. to 200° C. under autogenous pressure in the range of 10-15 bar, for a time of 5-55 minutes till the crystals of titanium silicalites are formed. The crystalline solid material so obtained is separated from mother liquor, washed thoroughly with deionized water and dried. The crystalline as synthesized and dried material is preferably calcined in an airflow at 500° C. -550° C. for 6-24 hours until the organic material is decomposed. The final organic free, anhydrous, titanium silicalites of the present invention have the composition: xTiO 2 :(1−x)SiO 2 where the value of x is equal or less than 0.11.

These crystalline material can be subjected to ion exchange treatment with an aqueous solution containing ammonium ions using conventional ion exchange procedures, filtration, washing with water, drying and calcining at 400° C.-500° C. for 16 hours. Though the catalyst composite material obtained herein before may be directly used as a catalyst, it is desirable in various industrial applications, to enhance the mechanical strength and ease of handling by admixing it with a suitable binder material and converting it into a suitable shape such as cylindrical extradates, spheres, etc. Silica, clay mineral such as bentonite, kaolinite or mixtures thereof are examples of suitable binder material which impart improved mechanical properties to the titanium silicalite.

The crystallization time of the material is substantially reduced to minutes when subjected to microwave irradiation, compared to material prepared by normal hydrothermal methods which takes several hours or days. Synthesis was carried out in lesser timings with highest titanium incorporation which is responsible for the catalytic activity of the TS-1. The material prepared by the process of the present invention are useful as oxidation catalysts in reactions like oxidation of phenol to hydroquinone, hydrocarbons, alcohols etc.

The crystalline titanium silicalite prepared according to the process of the invention is particularly useful in 1) oxidation/hydroxylation of hydrocarbons or derivatives of hydrocarbons, 2) ammoximation of ketones, aldehydes, and 3) oxidation of organic sulfides, amines.

The practice of the process of the present invention is further illustrated by the following examples which are, however, not to be construed to limit the scope of the present invention.