Chemistry of inorganic compounds – Zeolite – Organic compound used to form zeolite
Patent
1995-06-02
1998-05-26
Bell, Mary L.
Chemistry of inorganic compounds
Zeolite
Organic compound used to form zeolite
423716, 423DIG38, 502 73, 502 74, 502 77, C01B 3930
Patent
active
057560641
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to zeolite crystals with an offretite structure, which crystals contain a divalent metal cation and processes for producing such zeolites and a process for using the zeolites in the separation or conversion of hydrocarbons.
2. Description of Related Art
One of the zeolitic structures which occurs naturally is offretite. Naturally occurring offretite is rare, and it is known to produce offretite synthetically. For example, U.S. Pat. No. 4,259,174 describes a synthetic offretite which is a crystalline aluminosilicate comprising tetramethylammonium, sodium and potassium ions. U.S. Pat. No. 4,834,961 and GB2119352 have similar disclosures. Offretite may then be ion exchanged to introduce a metal cation from group II or Group VIII of the periodic table or manganese.
GB-A-2160188 describes the preparation of a synthetic offretite containing potassium ions and at least one ion selected from other alkali or alkaline earth metals. The use of a mixture of potassium and sodium or a mixture of potassium, sodium and lithium ions is preferred. If the zeolite is to be used in acid catalysis then it may be exchanged with cations such as magnesium. When the zeolite is to be used as a carrier of a hydrocarbon conversion catalyst, it is possible to carry out post-production alterations to the zeolite, e.g. to lower its potassium content or to introduce other metal cations into the zeolite using known methods.
SUMMARY OF THE INVENTION
The present applicants have surprisingly found that the incorporation of a divalent metal cation into the synthesis mixture which crystallises to give an offretite is advantageous. The inclusion of the divalent cation speeds the rate of formation of the crystals and results in smaller crystals compared with a mixture which is identical but for the lack of added divalent cation and which is treated under the same conditions of temperature, pressure and time.
Thus the present invention provides a zeolite with an offretite crystal structure and which comprises a non-exchangeable amount of a divalent metal cation.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows scanning electron micrographs of offretite crystals produced with and without the presence of divalent metal in the synthesis mixture according to Example 1.
DETAILED DESCRIPTION OF THE INVENTION
Preferably the divalent metal cation is an alkaline earth metal from group IIA of the periodic table, particularly magnesium, barium or cobalt.
The inclusion of a cation such as magnesium makes it possible to use a synthesis mixture which has a lower alkalinity, expressed as a molar ratio of M'.sub.2 O/SiO.sub.2 (where M' is as defined hereinafter), than could previously be used, and still obtain an offretite crystal structure. A synthesis mixture of low alkalinity may be preferred to produce a zeolite such as offretite since the product will be more siliceous and consequently is more stable when used, e.g. as the basis for a catalyst in a hydrocarbon conversion or separation process.
The offretite crystals of the present invention may be produced using an adaptation of standard procedures to produce zeolite crystals of an offretite structure. Thus the present invention provides a process for producing offretite crystals by crystallising a synthesis mixture comprising sources of M'.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2, a tetramethylammonium (TMA) ion and a divalent metal cation M.sup.2+, where M' is an alkali metal which is potassium or a mixture of potassium and sodium.
The source of alkali metal oxide M'.sub.2 O, is conveniently introduced into the synthesis mixture in the form of potassium hydroxide and optionally also sodium hydroxide e.g. as pellets.
The source of aluminum may be an alumina introduced into the reaction mixture as for example Al.sub.2 O.sub.3.3H.sub.2 O, previously dissolved in alkali. However, it is also possible to introduce aluminum in the form of the metal which is dissolved in alkali.
The source of SiO.sub.2 is generally silica, and t
REFERENCES:
patent: 3578398 (1971-05-01), Elmer
patent: 4259174 (1981-03-01), Chen
patent: 4521297 (1985-06-01), Anqevine et al.
patent: 4687653 (1987-08-01), Arika et al.
patent: 4834961 (1989-05-01), Fajula
Sheppard et al. May-Jun. 1969 "Chemical Compostion & Physical Properties of Relate Zeolines Offretite & Erionite" The American Minerologist vol. 54 pp. 875-886.
Barrer et al 1978 (No month) "Hydrothermal Chemistry of Silicates. Part 22. Hydrated Barium -Sodium Aluminosilicates" Journal of The Chemical Society No.6 pp. 598-601.
Materials Science and Engineering, An Introduction Callister, 1985 ( No Month) p. 278.
Bell Mary L.
Exxon Chemical Patents Inc.
Sample David
Sherer Edward F.
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