Chemistry of inorganic compounds – Zeolite – Seed used
Reexamination Certificate
2003-02-19
2003-12-23
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Seed used
C423SDIG002
Reexamination Certificate
active
06667023
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for synthesizing a crystalline aluminosilicate zeolite having an MFI structure.
BACKGROUND INFORMATION
The aluminosilicates known as zeolites are highly complex chemical structures that 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, one frequently used is the zeolite of faujasite structure, which after its synthesis may show a molar composition according to the formula below:
0.9±0.2M
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 that 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.
Zeolite ZSM-5 (also referred to as “MFI”) is a unique class of alumino silicates with a crystal structural framework. ZSM-5 compositions, in a preferred synthesized form, have the formula:
0.9±0.2M
2
O:Al
2
O
3
:5-100SiO
2
:z
H
2
O
M is selected from the group consisting of a mixture of alkali metal cations, especially sodium, and tetraalkylammonium cations, the alkyl groups of which preferably contain 2-5 carbon atoms.
The very high SiO
2
/Al
2
O
3
molar ratio of ZSM-5 as indicated in the above formula 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.
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.
The disadvantage of the use of organic templates is that they are expensive and that, as a result of decomposition of the organic template, environmentally harmful compounds are released upon heating of the so-prepared zeolites. Hence, preparation of ZSM-5 in the absence of organic template is desired.
However, seeding of the reaction mixture remains desired. The use of seeding material accelerates the crystallization process. If the same ratio of raw materials were used in an unseeded reaction, then the crystallinity of MFI material would be less than 40% in ten hours and typically would take greater than 48 hours to reach completion. Preferably, such seeding materials do not contain MFI-type zeolites, as this would make the process more expensive.
ZSM-5 zeolite, generally prepared by using an organic template agent, such as 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.
U.S. Pat. No. 4,606,900 (Kacirek et al.) discloses a method of preparing crystalline ZSM-5 zeolite using an X-ray amorphous aluminosilicate nucleating gel in the silica-alumina reaction batch with an SiO
2
to AlO
2
molar ratio ±10. This is equivalent to a SiO
2
to Al
2
O
3
molar ratio of ±20.
U.S. Pat. No. 4,166,099 (McDaniel et al.) discloses a method for preparing crystalline zeolites using amorphous aluminosilicate nucleation centers having an SiO
2
to Al
2
O
3
molar ratio of 2.3 to 2.7.
Objectives of the present invention include the preparation of MFI-type zeolite in very high yield, but without the use of an organic template with its associated expense and environmental problems, or the use of seeds comprising crystalline material that tends to require more process steps.
SUMMARY OF THE INVENTION
Accordingly, in one embodiment, the present invention relates to a process for synthesizing a crystalline aluminosilicate zeolite having an MFI structure. The process comprises crystallizing the zeolite from an alkaline aqueous reaction mixture that comprises SiO
2
and Al
2
O
3
or their hydrated derivatives, and an amorphous aluminosilicate nucleating gel with an SiO
2
/Al
2
O
3
ratio of from about 10 to less than 20. The reaction mixture does not contain an organic template.
It has been found that the yield of zeolite having MFI structure (e.g. ZSM-5) depends on the SiO
2
/Al
2
O
3
ratio of the amorphous aluminosilicate nucleating gel. When this ratio ranges from 10 to less than 20, higher MFI-yields are obtained then with higher or lower SiO
2
/Al
2
O
3
ratios.
Other embodiments of the invention relate to compositions of reaction mixtures, reaction conditions and optimum SiO
2
/Al
2
O
3
ratios in the nucleating seeds, each of which will be described in detail hereinbelow.
DETAILED DESCRIPTION OF THE INVENTION
Broadly speaking, the present invention as described and claimed in the present application relates to a process for preparing MFI-type zeolites such as ZSM-5, as detailed hereinbelow, in very high yield.
The preparation of zeolite having an MFI-type structure starts with the preparation of the reaction mixture comprising sources of silica and alumina and the seeds that comprise an amorphous aluminosilicate nucleating gel. The silica-alumina source can be obtained in several ways, by widely varying the silica-alumina ratio. For example, a silica-alumina can be obtained from the co-precipitation of a source of silica, such as sodium silicate, with a source of alumina, such as aluminum sulfate or sodium aluminate, as taught in GB 2166971, yielding a silica-alumina of SiO
2
/Al
2
O
3
(SAR) between 1 and 15 and containing of from 10 to 90 weight % SiO
2
, or according to EP 0129766, which shows a silica-alumina ratio of 10.
Suitable alumina sources are aluminum salts such as aluminum sulfate, sodium aluminate, aluminum nitrate, aluminum chlorohydrate, aluminum trihydrate such as gibbsite, BOC, and bayerite, calcined forms of aluminum trihydrate including flash calcined gibbsite. Also mixtures of the above-mentioned alumina sources may be used. If aluminum trihydrate or its thermally treated forms are used, it is preferred to age the silica source and alumina source in a slurry at a temperature below 100° C. to obtain the precursor gel. Suitable silica sources include sodium silicate and silica sol, for instance ammonium stabilized silica sol.
If desired, organic or inorganic acids and bases may be added, for example, to maintain the desired pH of the reaction mixture.
The amorphous aluminosilicate nucleating gel or seeds can be prepared according to several recipes to be found in the literature. Kasahara et al. in “Studies of Surface Science and Catalysis,”
Proceedings of the
7
th International Conference on Zeolites
1986, pp. 1
Akzo Nobel N.V.
Morris Louis A.
Sample David
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