Synthesis of SAPO-44

Details Synthesis of SAPO-44 Synthesis of SAPO-44

1999-10-08

2001-11-20

Sample, David R. (Department: 1755)

Chemistry of inorganic compounds

Zeolite

Organic compound used to form zeolite

C423S306000, C423SDIG003, C502S214000

Reexamination Certificate

active

06319487

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of synthesizing a molecular sieve comprising silicoaluminophosphate 44 (SAPO-44).
BACKGROUND OF THE INVENTION
Light olefins (defined herein as ethylene, propylene, butenes and mixtures thereof) serve as feeds for the production of numerous chemicals and polymers. Light olefins traditionally are produced by petroleum cracking. Due to the escalating cost of crude petroleum, there are increasing efforts to develop light olefin production technologies based on alternative feedstocks. An important type of alternative feedstocks are oxygenates, such as alcohols, particularly methanol, dimethyl ether, dimethyl carbonate and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohols, alcohol derivatives, and other oxygenate have promise as an economical, non-petroleum source for olefin production.
Because light olefins are the most sought after products from the catalytic petroleum cracking and oxygenate conversion processes, a continuing need exists for new catalysts and/or new ways of making known catalysts to increase the yield of light olefin products and/or reduce the yield of unwanted products such as heavy hydrocarbons having molecular weights heavier than butane or low-valued by-products like methane.
Most catalysts that are used in the petroleum cracking and oxygenate conversion processes are molecular-sieve containing catalysts. A molecular sieve can be zeolitic—zeolites—or non-zeolitic. Typical examples of zeolitic molecular sieves are zeolite A, zeolite X zeolite Y, ZSM-5. ZSM-34, erionite, chabazite, and others. A number of non-zeolitic molecular sieves, particularly silicoaluminophosphates (SAPO's) have been synthesizcd and investigated as catalysts for converting oxygenates or cracking heavy hydrocarbons to light olefins.
SAPO's have a three-dimensional microporous crystalline framework of PO
2
+
, AlO
2
−
, and SiO
2
tetrahedral units. Because an aluminophosphate (AlPO
4
) framework is inherently neutral, the incorporation of silicon into the AIPO
4
framework by substitution generates add sites and acidity. Controlling the quanity and location of silicon atoms incorporated into an AIPO
4
framework is important in determining the catalytic properties of a particular SAPO molecular sieve. Properly adjusted acid strength and acid site density are the keys to a good petroleum cracking or oxygenate conversion catalyst.
The catalytic properties of a SAPO catalyst also can be modified after the SAPO molecular sieve has been synthesized. This type of “post-synthesis” modification is accomplished by treating the molecular sieve with metallic, semi-metallic or non-metallic materials comprising nickel, cobalt, manganese, magnesium, barium, strontium, lanthanides, actinides, fluorine, chlorine, chelating agents, and others. The modifiers may or may not become part of the final composition of the modified catalyst.
SAPO's suitable for converting tho oxygenates to light olefins include SAPO-17, SAPO-18, SAPO-34 and SAPO-44. These are small-pore molecular sieves with pore diameter smaller than about 5 Angtroms. Small pores are believed to favor light olefins production as a result of sieving effects. For the chabazite-like SAPO-34 and SAPO-44 molecular sieves, it may be possible to incorporate more silicon atoms into the tetrahedral positions of the framework to afford greater flexibility in adjusting their acidic properties.
A hydrothermal synthesis method for making SAPO-44 was described in USA 4,440,871. The materials were aqueous silica sol, aluminum isopropoxide, orthophosphoric acid, and an organic template, cyclohexylamine. The synthesis was performed at 200° C. for 52 hours. The SAPO-44 was obtained as the “major phase” in the product, but the product was impure. It contained unidentified materials. A similar method based on the same starting materials and added hydrofluoric acid (HF) was reported by U. Lohse et al, in
J. Chem. Soc. Faraday Trans
. 91, 1155 (1995). In the presence of HF, the reaction time was shortened to five hours at 200° C. It is not clear how pure the SAPO-44 products were. Among the five products reported in the paper, at least one of them contained SAPO-35 impurity.
Because SAPO-44 can be used as a catalyst for the hydrocarbon cracking and oxygenate conversion processes, it is desirble to produce molecluar sieve catalysts comprising SAP44 from cheaper starting materials and/or under less demanding reaction conditions. It is preferable to produce a molecular sieve product consisting essentially of SAPO-44. It is also preferable to avoid using highly corrosive reactants such as hydrofluoric acid (HF) in the molecular sieve synthesis reaction mixture.
SUMMARY OF THE INVENTION
The present invention provides a method which comprises forming a mixture comprising a silicon component; an organic template at a first molar ratio of greater than about 1 to an aluminum component; and a phosphorus component at a second molar ratio of less than about 1 to aid aluminum component, and subjecting said mixture to conditions effectove to produce molecular sieves comprising siliconaluminophosphate 44.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method of making a molecular sieve catalyst comprising silicoaluminophosphate 44 (SAPO-44). preferably in substantially pure form.
The method generally comprises forming a mixture comprising a silicon component, an aluminum component, a phosphorus component, and a template. This mixture then is allowed to go through an optional aging period before being subjected to a hydrothermal treatment at an elevated temperature. The product formed from the hydrothermal treatment may be calcined further at an elevated temperature. Either before or following the optional calcination, the molecular sieve product may be modified further using an ion-exchange method or other types of modifications such as steaming, to produce a molecular sieve comprising SAPO-44, preferably a molecular sieve consisting essentially of pure SAPO-44.
A number of silicon compounds and their mixtures may be used as the silicon component for the method of the present invention. The silicon compounds include, but are not limited to silica sol silica gel, colloidal silica, filmed silica, silicic acid, sodium silicate, tetraethyl silicate, tetramethyl silicate, and mixtures thereof. A preferred silicon component comprises a material selected from the group consisting of silica sol, silica gel, colloidal silica, filmed silica, silicie acid, and mixturs thereof The silicon compounds can be purchased from many commercial sources such as Aldrich Chemical Company, du Pont Company, Johnson Matthey Catalog Company, Merck Company and others.
Many aluminum compounds and their mixtures are suitable for use as the aluminum component in the present invention. The aluminum compounds include, but are not necessarily limited to aluminum oxide, boehmite, pseudo boehmite, aluminum hydroxy chloride, aluminum alkoxides such as aluminum tri-isopropoxide, aluminum tri-ethoxide, aluminum tri-n-butoxide and aluminum tri-isobutoxide, and mixtures thereof. A preferred aluminum component comprises a material selected from the group consisting of boehmite and pseudo boehmite. These compounds can be purchased from many companies such as Aldrich Chemical Company, Reheis Company, Aluminum Company of America, Vista Company, and others.
The phosphorus compounds suitable for use as the phosphorus component include but are not necessarily limited to orthophosphoric acid, phosphorus acid, trimethyl phosphate, triethyl phosphate, and mixtures thereof. A preferred phosphorus component comprises orthophosphoric acid (H
3
PO
4
). Another preferred phosphorus component comprises the commercially available 85 wt % phosphoric acid (in water). Alternately, phosphorus oxides (P
2
O
3
, P
2

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