Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By dehydrogenation
Reexamination Certificate
2000-08-03
2002-08-20
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By dehydrogenation
C585S661000, C585S662000, C585S906000
Reexamination Certificate
active
06437210
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to catalysts for use in hydrocarbon conversion, specifically to a modified mazzite supported catalyst impregnated with at least one metal selected from the group consisting of chromium, molybdenum, tungsten and oxides of these metals which can be used for the dehydrogenation of hydrocarbons.
Hydrocarbon conversion is especially important in the petroleum industry. In recent years, an increasingly competitive market for gasoline has developed, and profit margins on gasoline sales have steadily declined. As a result, there has been considerable interest among petroleum companies in developing processes for chemically converting gasoline into various compounds of higher economic value.
A Gasoline Conversion Unit (GCU) is a system which converts gasoline into olefins and aromatics of higher economic value such as ethylene, propylene, butene, benzene, toluene, and xylene. Propane is a major by-product from the original GCU process; however, propane from a GCU is a paraffin of low value. It is, therefore, desirable to convert the propane from the original GCU process into a more valuable olefin such as propylene.
Numerous catalytic processes for dehydrogenating propane into propylene are known today, however many of the existing techniques are ineffective under the conditions of a GCU. The majority of catalysts which are currently used for the dehydrogenation of hydrocarbons are supported noble-metal catalyst. It is well known that supported noble-metal catalyst are vulnerable to sulfur poisoning which results in irreversible catalytic deactivation. Because of the original gasoline feedstream, it is inevitable that the propane produced by the GCU process will contain a considerable amount of sulfur. Therefore, a supported noble-metal dehydrogenation catalyst, when used under GCU conditions, is likely to be ineffective because it will undergo catalytic deactivation due to sulfur poisoning.
In addition, many existing catalytic processes for dehydrogenation require the presence of steam as a diluent in the feedstream. This requirement for steam makes existing dehydrogenation processes very utility intensive and, thus, very expensive.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a catalyst and catalytic process for the dehydrogenation of paraffins to olefins which has equal or superior conversion and selectivity to prior processes and is less vulnerable to sulfur poisoning.
It is a further object of the present invention to provide a method for creating such a catalyst to be used in the dehydrogenation of paraffins to olefins.
The above objects are realized by making and using a catalyst comprising a modified mazzite zeolite impregnated with a metal from the group consisting of chromium, molybdenum, tungsten and oxides of these metals.
Prior to impregnation, the original mazzite zeolite is modified by a process comprising: performing an ion-exchange between the original mazzite zeolite and an aqueous solution of an ammonium salt, such that the alkali metal content of the original mazzite zeolite is reduced to less than 0.15% by weight; dealuminating the ion-exchanged mazzite zeolite by acid leaching with an organic acid; physically mixing the ion-exchanged and dealuminated mazzite zeolite with an inorganic binder and a peptizing agent to create a mixture which contains chromic oxide (in a preferred embodiment, chromic oxide is physically mixed with the other components) and is of a consistency and texture suitable for shaping by extrusion, pelletizing, tableting or other suitable means; shaping the mazzite zeolite mixture into extrudate, pellets, tablets or other suitable form; and calcining the shaped mazzite zeolite.
The process for impregnation of the modified mazzite zeolite with a metal from the group consisting of chromium, molybdenum, and tungsten or oxides of these metals comprises: contacting the entire outer surface of the modified mazzite zeolite with an aqueous solution containing a precursor of at least one metal from the group consisting of chromium, molybdenum, tungsten, oxides of these metals and mixtures of these metals and metal oxides; and calcining the impregnated modified mazzite zeolite.
The process for dehydrogenating hydrocarbons comprises contacting a mazzite zeolite catalyst, modified and impregnated according to the above process, using any suitable method known in the art, with a paraffin-containing feedstock under conditions effective to dehydrogenate the paraffins.
DETAILED DESCRIPTION OF THE INVENTION
The components used in the above-described catalyst preparation and the above processes will now be described in more detail. Except where otherwise indicated, the weight percentage given for each component is based upon the total weight of the components.
The starting zeolite used to prepare the novel composition of the present invention can be any zeolite from the family of synthetic mazzite zeolites.
The general formula of the synthetic zeolites of mazzite type in their form as synthesized can be written in terms of moles of oxides:
1±20.5(M{fraction (2+L
)}O+A{fraction (2+L /m)}O);Al
2
O
3
;3−20SiO
2
;0−20H
2
O
where M is an n-valent cation, in general sodium, and A is an organic reagent carrying m positive charges. Generally, the molar ratio of SiO
2
to Al
2
O
3
in the crystalline framework of the mazzite zeolite is at least about 2:1 and can range up to 250:1. Preferably the molar ratio of SiO
2
to Al
2
O
3
in the zeolitic framework is about 3:1 to about 100:1.
Preferred mazzite zeolites include Zeolite Omega (&OHgr;) and ZSM-4. The synthesis of the Zeolite Omega is described in U.S. Pat. No. 4,241,036 (Union Carbide). The synthesis of ZSM-4 is described in French Patent 2,074,007 (Mobil Oil) and British Patent 1,297,256 (Mobil Oil). The presently more preferred mazzite zeolite is Zeolite Omega.
A preferred embodiment of the present invention involves modifying a commercially available mazzite zeolite and impregnating the modified mazzite zeolite with at least one metal from the group consisting of chromium, molybdenum, and tungsten or oxides of these metals. The resulting composition is a superior catalyst for the dehydrogenation of hydrocarbons.
The first step in modifying the original mazzite zeolite is to decrease its alkali-metal content. The alkali-metal cations are removed by performing at least one, and preferably two or more, ion (cation) exchanges. A preferred method of performing such an ion exchange is to mix a mazzite zeolite powder with a solution of ionizable ammonium salt (nitrate, sulfate, chloride, etc . . . ) of molarity ranging from 0.1 up to saturation, preferably from 0.5 to 10. The mixture may then be continuously stirred for a period ranging from 0.5 hour to 48 hours, preferably from 8 hours to 24 hours, at a temperature ranging from 1° C. to 500° C., preferably from 50° C. to 150° C. The ion-exchanged zeolitic product may be decanted, washed with water, and dried in air at a temperature ranging from 60° C. to 300° C., preferably from 100° C. and 200° C., for a period ranging from 4 hours to 48 hours, preferably from 8 hours to 24 hours. The resulting ion-exchanged mazzite zeolite should contain less than 0.5% by weight of alkali metals, preferably less than 0.15% by weight of alkali metals.
The mazzite zeolite is dealuminated by acid leaching in order to create a more stable and effective zeolitic framework. A preferred method for dealumination is to mix the mazzite zeolite with an solution of an acid of molarity ranging from 0.1 up to saturation, preferably from 0.5 to 2. The acid can be an organic acid, such as oxalic acid, or an inorganic acid, such as hydrochloride or nitric acid. Preferably, the acid is an organic acid selected from the group consisting of acetic acid, citric acid, and oxalic acid, and is most preferably oxalic acid. Mixing of the mazzite zeolite with the aqueous solution of acid may occur under continuous stir conditions at a temperature ranging from 0° C. to 200° C., most preferabl
Drake Charles A.
Wu An-hsiang
Dang Thuan D.
Phillips Petroleum Company
Stewart Charles W.
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