Process for the microwave induced preparation of crystalline...

Details Process for the microwave induced preparation of crystalline... Process for the microwave induced preparation of crystalline...

2001-03-26

2002-05-14

Sample, David (Department: 1755)

Chemistry of inorganic compounds

Zeolite

Organic compound used to form zeolite

C423S713000, C423S326000, C423SDIG002

Reexamination Certificate

active

06387349

ABSTRACT:

FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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-&bgr; 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.
OBJECTS OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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.
TABLE 1
2&thgr; ± 0.05
Relative Intensity
 7.82
vs
 8.75
ms
13.20
w
13.98
w
14.74
mw
15.62
w
15.98
w
16.45
w
17.72
w
17.64
w
17.80
w
19.22
w
20.34
mw
20.82
mw
23.12
vs
24.28
ms
24.51
ms
24.40
ms
26.05
w
26.99
w
28.04
w
29.26
w
29.99
mw
45.12
w
45.51
w
where
vs = very strong
s = strong
m = medium
w = weak.
and an infrared absorption spectra as shown in Table 2,
TABLE 2
cm
−1
Relative Intensity
456
S
544
MS
800
W
960
W
1120 
VS
1216 
S
where
VS = very strong
S = strong
M = medium
W = weak.
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

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