Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2002-12-10
2003-11-11
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S111010, C208S134000, C585S481000, C585S475000, C585S446000, C585S407000, C585S469000, C585S640000, C585S733000, C585S653000
Reexamination Certificate
active
06645370
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for preparing high silica zeolites that are bound by zeolite and the use of the zeolite-bound high silica zeolites as prepared by the process as a catalyst in hydrocarbon conversion.
BACKGROUND OF THE INVENTION
Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow for adsorption of molecules with certain dimensions while rejecting those of large dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves such as crystalline silicates, crystalline aluminosilicates crystalline silicoalumino phosphates, and crystalline aluminophosphates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term “zeolites” includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms “zeolite”, “zeolites” and “zeolite material”, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such aluminum, such as gallosilicates. The term “aluminosilicate zeolite”, as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
High silica zeolites, i.e., zeolites with a high molar silica content, are desirable because of their particular catalytic selectivity and their thermal stability. Thermal stability is particularly important if the zeolite when used as a catalyst or in adsorption procedures is exposed to high temperatures. High silica zeolites are intrinsically hydrophobic and remain stable at temperatures in excess of 500° C.
The silica to trivalent metal oxide, e.g., alumina, gallia, and the like, mole ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with a silica to alumina mole ratio of from 2 to 3; zeolite Y can be synthesized with a silica to alumina mole ratio from 3 to about 7, and zeolite L can be synthesized with a silica to alumina mole ratio from 4 to about 7. In some zeolites, the upper limit of the silica to trivalent metal oxide mole ratio is virtually unlimited. These zeolites are known in the art and include for example, frame work structure types such as MFI, e.g., ZSM-5, MEL, e.g., ZSM-11, MTW, e.g., ZSM-12, and TON, e.g., ZSM-22.
Synthetic zeolites are normally prepared by crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound.
The zeolite powder is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines, which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used for hydrocarbon conversion, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion. Furthermore, when the bound zeolite is used in hydrocarbon conversion, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions, which can result in the formation of undesirable products.
One procedure for making zeolite-bound zeolite involves converting the silica present in the silica binder of a silica-bound zeolite aggregate to a zeolite binder. The silica-bound zeolite aggregates can be made by extruding a paste containing silica and zeolite. This method comprises mixing a mixture of silica and zeolite with water and optionally an extrusion aid followed by mulling and extruding the paste to form a silica-bound zeolite extrudate, and subsequently drying and calcining the extrudate. When such an extrusion procedure is used to prepare silica-bound high silica zeolite extrudates, the extrusion paste usually does not have sufficient plasticity for extrusion of the paste in conventional extruding equipment. Thus, to prepare silica-bound zeolite aggregates suitable for conversion to zeolite bound high silica zeolite, other techniques must be used such as by mixing the silica and zeolite and squeezing the mixture together to form a shaped structure having minimal physical integrity. Such techniques are commercially inefficient and even if used, can result in silica-bound aggregates with less than desirable physical strength and/or physical integrity.
The present invention provides a process for preparing zeolite-bound high silica zeolites useful for hydrocarbon conversion that overcomes or at least mitigates the above-described problems.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a process for preparing high silica zeolites useful for hydrocarbon conversion that are bound by zeolite. The process of the present invention is carried out by forming by extrusion of a silica-bound aggregate containing high silica zeolite in the hydrogen form and then converting the silica to a zeolite binder such as by aging the silica-bound extrudate in an aqueous ionic solution containing a source of hydroxyl ions in an amount sufficient to convert the silica to the zeolite binder.
In another embodiment, the present invention provides a hydrocarbon conversion process for converting organic compounds by contacting the organic compounds under hydrocarbon conve
Berge Jannetje Maatje van der
Mohr Gary David
Verduijn Johannes P.
Berge Jannetje Maatje van der
Exxon Mobil Chemical Patents Inc.
Griffin Walter D.
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