Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – From nonhydrocarbon feed
Reexamination Certificate
2000-03-07
2002-06-11
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
From nonhydrocarbon feed
C585S638000, C585S639000, C502S214000, C502S439000, C502S514000, C423SDIG003
Reexamination Certificate
active
06403855
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the synthesis of a crystalline silicoaluminophosphate (SAPO) molecular sieve. In particular, this invention relates to increasing the yield of a crystalline SAPO molecular sieve by stirring the starting materials while applying heat at a temperature and duration effective to form a crystalline silicoaluminophosphate molecular sieve, wherein stirring is applied for 20-95% of the duration that heat is applied.
BACKGROUND OF THE INVENTION
Molecular sieves generally have a microporous structure and are composed of either crystalline aluminosilicate, chemically similar to clays and feldspars and belonging to a class of materials known as zeolites, or crystalline aluminophosphates derived from mixtures containing an organic amine or quaternary ammonium salt, or crystalline silicoaluminophosphates which are made by hydrothermal crystallization from a reaction mixture comprising reactive sources of silica, alumina and phosphate. Molecular sieves have a variety of uses. They can be used to dry gases and liquids; for selective molecular separation based on size and polar properties; as ion-exchangers; as catalysts in cracking, hydrocracking, disproportionation, alkylation, isomerization, oxidation, and conversion of oxygenates to hydrocarbons; as chemical carriers; in gas chromatography; and in the petroleum industry to remove normal paraffins from distillates.
Silicoaluminophosphate (SAPO) molecular sieves have become of particular interest recently. These molecular sieves have the ability to convert oxygenates to olefins, aromatics and other compositions. They are especially effective in converting oxygenate compositions such as methanol and dimethyl ether to olefins such as ethylene and propylene.
Various methods of making SAPOs have been disclosed. For example, U.S. Pat. No. 4,440,871 discloses a method of making a SAPO molecular sieve. The method typically involves mixing together a phosphorus containing compound and an aluminum containing compound until a homogeneous mixture is obtained. To the homogeneous mixture is added a template and a silica containing composition, and this mixture is stirred until a final homogeneous mixture is obtained. This final homogeneous mixture is then heated to form a crystalline SAPO molecular sieve.
U.S. Pat. Nos. 4,943,424 and 5,087,347 disclose a method of making a SAPO-11 molecular sieve that is reported to exhibit unique and useful catalytic and shape selective properties. The molecular sieve is made by preparing an aqueous reaction mixture containing aluminum isopropoxide and phosphoric acid. Thereafter, the mixture is combined with silicon oxide. This mixture is then combined with an organic template to form the reaction mixture. The reaction mixture is then pH adjusted and heated to form the crystalline molecular sieve product. Crystallization is conducted in an autoclave and without stirring.
U.S. Pat. No. 5,663,471 discloses a method of making SAPO-34. The method includes mixing together an aluminum containing compound, a phosphorus containing compound, and an acid. This mixture is homogenized and a template material is added. The template added mixture is then homogenized, and the entire mixture is poured into a vessel. The vessel is shaken at room temperature. Then it is heated to form a crystalline molecular sieve product. The product is recovered and dried.
U.S. Pat. No. 5,324,493 discloses a method from crystallizing aluminophosphates and silicoaluminophosphates having an AEL structure using 1,2-bis-(4-pyridyl)-ethane. The method includes mixing together a reactive source of aluminum, phosphorous, and, optionally, silicon, with the addition of 1,2-bis-(4-pyridyl)-ethane. The mixture is heated under autogenous pressure in a closed system to form the crystalline product. The product is isolated after the reaction, washed and dried. Synthesis can be carried out either statically or with stirring.
The known processes for making crystalline silicoaluminophosphate molecular sieves are, unfortunately, very inefficient. In some cases, less than 50 wt % of the reaction components are reacted to form the final crystallized product. The uncrystallized reaction components are typically not recovered and disposed as waste. It is, therefore, desirable to find a reaction process which does not result in a substantial amount of waste.
SUMMARY OF THE INVENTION
In order to overcome the various problems associated with the manufacture of crystalline silicoaluminophosphate molecular sieves, this invention provides a novel method of making crystalline silicoaluminophosphate molecular sieves. The method comprises adding to a vessel a mixture comprising a silicon containing composition, an aluminum containing composition, a template, and a phosphorus containing composition, and stirring the mixture while applying heat at a temperature and duration effective to form a crystalline silicoaluminophosphate molecular sieve, wherein stirring is applied for 20-95% of the duration that heat is applied.
Desirably, heat is applied at a temperature of between 50° C. and 250° C. Heat can also be applied at a duration of between 10 minutes and 240 hours. Desirably, stirring is applied from 20-90% of the duration that heat is applied, more preferably continuous stirring is applied from 30-80% of the duration that heat is applied.
In another embodiment, the silicon containing composition, the aluminum containing composition, the template, and the phosphorus containing composition are added in amounts effective to provide a crystalline molecular sieve composition having a ratio of 0.30-0.34 SiO
2
/Al
2
O
3
/0.82-0.86 P
2
O
5
. Desirably, the template comprises tetraethyl ammonium hydroxide, more desirably, the template comprises tetraethyl ammonium hydroxide and dipropylamine.
In another embodiment, the invention provides a method of making an olefin product from an oxygenate composition. The method comprises providing a crystalline silicoaluminophosphate molecular sieve made by the method of this invention, calcining the molecular sieve, and contacting the calcined molecular sieve with an oxygenate composition under conditions effective to form an olefin product. Desirably, the oxygenate composition comprises a compound selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C
4
-C
20
alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof. The olefin product is desirably contacted with a polyolefin-forming catalyst under conditions effective to form a polyolefin.
DETAILED DESCRIPTION OF THE INVENTION
Silicoaluminophosphate (SAPO) molecular sieves serve as particularly desirable catalytic materials in converting oxygenate feedstocks to olefin compositions. They are particularly good catalysts for making olefins such as ethylene and propylene from oxygenate compounds.
Silicoaluminophosphate molecular sieves are generally classified as being microporous materials having 8, 10, or 12 membered ring structures. These ring structures can have an average pore size ranging from about 3.5-15 angstroms.
Preferred are the small pore SAPO molecular sieves having an average pore size ranging from about 3.5 to 5 angstroms, more preferably from 4.0 to 5.0 angstroms. These preferred pore sizes are typical of molecular sieves having 8 membered rings.
In general, silicoaluminophosphate molecular sieves comprise a molecular framework of corner-sharing [SiO
2
], [AlO
2
], and [PO
2
] tetrahedral units. This type of framework is effective in converting various oxygenates into olefin products.
The [PO
2
] tetrahedral units within the framework structure of the molecular sieve of this invention can be provided by a variety of compositions. Examples of these phosphorus-containing compositions include phosphoric acid, organic phosphates such as triethyl phosphate, and aluminophosphates. The phosphorous-containing compositions are mixed with reactive silicon and aluminum-cont
Exxon Mobil Chemical Patents Inc.
Griffin Walter D.
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