Molecular sieves of faujasite structure

Details Molecular sieves of faujasite structure Molecular sieves of faujasite structure

2002-06-07

2004-06-29

Sampple, David (Department: 1755)

Chemistry of inorganic compounds

Zeolite

With change of synthesized zeolite morphology

C423S709000, C423SDIG002, C502S085000, C208S120100

Reexamination Certificate

active

06756029

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for preparing molecular sieves of faujasite structure and to the pre-shaped molecular sieve bodies prepared according to the present process. More specifically, the present invention relates to a process for preparing molecular sieves of faujasite structure which comprises contacting a precursor gel with steam, optionally followed by caustic washing, said process making it possible to form a zeolite of faujasite structure in pre-shaped bodies.
BACKGROUND INFORMATION
The aluminosilicates known as zeolites are highly complex chemical structures which present different crystalline structures as a function of their composition. Although they occur naturally, zeolites nowadays are mostly produced by industry aiming at various uses, the more important among which are application as adsorbents and as catalysts in the oil industry.
Among the various kinds of synthetic zeolites used in the oil industry, the one most frequently used is the zeolite of faujasite structure, which after its synthesis may show a molar composition according to the formula below:
0.9±0.2 M
2
O.Al
2
O
3
x SiO
2
.w H
2
O
wherein M represents a cation of an alkaline metal, x is a number between 2.5 and 6, and w is a number between 6 and 9.
When preparing faujasite zeolites, the molar ratio between the oxides of aluminum and silicon is a further parameter which may be adjusted. As found in natural environments, the faujasite SiO
2
/Al
2
O
3
molar ratio is between 2.5 and 4. Synthetic zeolites of faujasite structure of higher SiO
2
/Al
2
O
3
molar ratio have been prepared, since it was found that structures with a higher silica content are more resistant to temperature and acids, situations which are usually encountered in the application of zeolites in the oil industry.
A faujasite structure zeolite used mainly in industry, and especially in the oil industry, is the type Y zeolite, where the SiO
2
/Al
2
O
3
molar ratio is higher than 4.5. The molar ratio of zeolites for use in fluid catalytic cracking catalysts, for example, is at least 5.0. However, the preparation of zeolites having such higher SiO
2
/Al
2
O
3
molar ratio presents several drawbacks, since generally those zeolites are obtained through processes of extended periods of crystallization, which require huge crystallization vessels and a sharp control of the overall production process to avoid impurities, namely, different crystalline phases, which harm the end product.
In spite of these drawbacks, various industrial processes for obtaining type Y zeolites are presently in use, so crucial is the importance of this material in the production of more active and more selective catalysts for the oil industry. One such process which widely employs the type Y zeolite in the composition of catalysts is the fluid catalytic cracking process.
Brazilian Patent BR 8402808, for example, teaches a process for preparing a high-silica, faujasite zeolite to be used in fluid catalytic cracking, where sodium silicate and aluminum sulfate are contacted under controlled conditions of pH and temperature, the crystallization periods varying between 0.5 and 12 hours. The product is a Y zeolite of SiO
2
/Al
2
O
3
molar ratio 5.6.
More detailed research on the preparation of type Y zeolite will reveal that most processes still in use employ the same operation sequences detailed in the above Brazilian Patent, the only variations concerning the raw materials and the formulation of the recipes. One could even say that nothing novel is to be expected in this field.
It is therefore clear to the experts in the production of catalysts and adsorbents that there is an unfulfilled need to develop new processes for preparing a type Y zeolite, processes which are more selective in order to avoid forming impurities, more economical, and most of all, capable of producing Y zeolites having features not yet attained by the state-of-the-art preparation processes.
Experts in the field of zeolite preparation who keep up with the technical literature of the field will be aware of the technique of zeolite crystallization described by X. Wenyang et al. in
Journal of Chemical Communication
Vol. 10, 1990, pp. 755, where ZSM-5 type zeolites of high SiO
2
/Al
2
O
3
molar ratio were obtained through the contact of a precursor gel with vaporized organic compounds. According to this process, the crystallization of the precursor gel (amorphous to X-rays) occurs through the transport of the organic compounds (amines) in the vapor phase. The process thus developed ensures that the synthesis of these kinds of zeolites, which normally is effected using the precursors as an aqueous suspension and in the presence of a template agent, normally a quaternary ammonium salt or amines, is rendered simpler and more economical.
By applying the same technique, other authors effected the synthesis of zeolites of the ZSM family such as ZSM-21 and ZSM-35. Chinese Patent 1,051,334 teaches the preparation of various zeolites of the ZSM family.
M.-H. Kim, H.-X Li, and M. Davis in
Microporous Materials
Vol. 1, 1993, pp. 191-200, also comment on the preparation of ZSM-5 zeolite using the above-cited preparation process. In their work they compare the state-of-the-art technique in an aqueous medium with the new technique, including the use of steam, confirming that such technique can be used successfully for preparing that particular zeolite family.
In spite of the extensive work of Kim et al. exploring a wide range of compositions of the amorphous precursor, where the SiO
2
/Al
2
O
3
molar ratio is varied between 2.5 and 80, the synthesis of type Y zeolite could not be achieved.
One single published work describes how to obtain a faujasite structure zeolite. J. Dong and P. Dong in
Shiyou Huagong
Vol. 24, 1995, pp. 321-324, comment on a type X zeolite of low SiO
2
/Al
2
O
3
molar ratio. This product, however, is not attractive for use in the preparation of catalysts.
In order to make clear why the method proposed by X. Wenyang et al. leads more easily to the ZSM-5 zeolite than to the type Y zeolite, the differences between the two zeolites should be emphasized.
ZSM-5 zeolite, besides using a template agent—generally an ammonium quaternary salt—shows a much higher SiO
2
/Al
2
O
3
molar ratio than the molar ratios usually encountered for type Y zeolites. This causes the excess silica found after the crystallization of the ZSM-5 zeolite to be very low compared to that observed for the synthesis of type Y zeolite. And it is exactly the excess silica—that is, the amount of silica which is not incorporated into the crystalline network of Y zeolite during the crystallization step—that should be controlled during the process of transforming an amorphous solid by the crystallization method, which involves contact with steam or organic compounds.
Another important feature in obtaining new kinds of catalysts and adsorbents is the forming of a type Y zeolite in pre-shaped bodies. A feature of this kind of material is that the surface of the pre-shaped bodies is covered with zeolite crystals. This higher concentration of zeolite in the outer layers of the pre-shaped body favors adsorption and catalysis.
Examples and references on the preparation of faujasite zeolites as pre-shaped bodies are scarce.
M. Matsukata and E. Kikuchi in “Zeolitic Membrane Synthesis, Properties and Prospects,”
Bulletin of the Chemical Society of Japan
70, pp. 2341-2356 (1997) obtained membranes of pentasil-type zeolites with the technique of transferring organic amine vapor onto gels of silica-rich compositions.
U.S. Pat. Nos. 3,657,154, 4,235,753, and 4,493,902 teach the preparation of a type Y faujasite zeolite as pre-shaped microspheres. These patents teach the preparation of type Y zeolites from kaolin microspheres. According to the process, the microspheres are calcined at elevated temperature, leading to meta-kaolin.
After this step, the meta-kaolin microspheres are immersed in an aqueous solution which contains ingredients such as sodium silicate and soda. Next, the su

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