Silicoaluminophosphate molecular sieves

Details Silicoaluminophosphate molecular sieves Silicoaluminophosphate molecular sieves

2002-02-13

2004-02-03

Sample, David (Department: 1755)

Chemistry of inorganic compounds

Phosphorus or compound thereof

Oxygen containing

C423SDIG003, C585S640000

Reexamination Certificate

active

06685905

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to European Patent Application No. EP 01310795.8, filed Dec. 21, 2001, which is fully incorporated herein by reference.
FIELD OF INVENTION
This invention relates to a process for the manufacture of silicoaluminophosphate molecular sieves.
BACKGROUND OF THE INVENTION
Silicoaluminophosphate (SAPO) molecular sieves are described in U.S. Pat. No. 4,440,871. These molecular sieves are generally classified as being microporous materials containing 8, 10 or 12-membered ring structures. These ring structures can have an average pore size ranging from about 3.5 Å to 15 Å. Small pore SAPO molecular sieves with an average pore size from about 3.5 Å to 5 Å are typical of molecular sieves containing 8-membered rings.
An important use of SAPO is the catalytic conversion of oxygenates, e.g., methanol, to olefins, especially the lighter olefins, e.g., ethylene and propylene. The global demand for ethylene and propylene is increasing and as the cost of petroleum feedstock, their traditional source, increases, the use of SAPO catalysts in oxygenate to olefin conversion has significant commercial value. Numerous references, including U.S. Pat. No. 4,499,327 and EP-A-541,915, describe the manufacture of olefins from methanol using various SAPO molecular sieve catalysts. The literature describes a number of different processes for the manufacture of SAPO's, in particular for the formation of the synthesis mixture which is to be subjected to the hydrothermal treatment typically used for the formation of the crystalline molecular sieves. In many such processes, the source of aluminium is added to an aqueous solution of the source of phosphorus, which is conveniently a strong phosphoric acid solution. The aluminium source is peptized by the phosphoric acid, a reaction that is hard to control, and results in a pasty composition difficult to handle, especially on a commercial scale.
Although the commercially most economic sources of aluminium are inorganic, many of these, for example hydrated aluminas, e.g., pseudoboehmite, have the disadvantage of not being readily soluble in water, tending to form a gel, which is also difficult to handle on a commercial scale. Although in principle these difficulties may be overcome, for example by the use of larger quantities of water, this dilutes the phosphoric acid and inhibits the process of digestion of the aluminium source.
If SAPO molecular sieves are to be used commercially, an economically acceptable way of preparing these materials in high yield and large quantities is needed. It is highly desirable that the structural and chemical properties of the product be consistent from one batch to another. The formation of an inconsistent process mixture or gel will interfere with preparing SAPO with consistent properties.
SUMMARY OF THE INVENTION
The present invention provides a process for the manufacture of a silicoaluminophosphate molecular sieve, which comprises forming a synthesis mixture by the steps of:
a) mixing an inorganic source of aluminium and water to form a mixture having a water to aluminium, measured as Al
2
O
3
, ratio within the range of 3:1 to 8:1 by weight;
b) mixing a source of phosphorus with the product of step a);
c) mixing a structure directing agent and a source of silicon with the product of step b); and
d) subjecting the synthesis mixture resulting from step c) to hydrothermal treatment under conditions effective to produce the desired silicoaluminophosphate molecular sieve.
Advantageously, the H
2
O:Al
2
O
3
weight ratio of the mixture formed in step a) is within the range 3.5:1 to 6:1.
The components of the synthesis mixture are typically those known in the art or as described in the literature for the production of the SAPO concerned.
The aluminium source may be, for example, an aluminium oxide (alumina), optionally hydrated, an aluminium salt, especially a phosphate, an aluminate, or a mixture thereof. A preferred source is a hydrated alumina, most preferably pseudoboehmite, which contains about 75% Al
2
O
3
and 25% H
2
O by weight.
Advantageously, the source of phosphorus is a phosphoric acid, especially orthophosphoric acid, but other sources, for example, organic phosphates, e.g., triethyl phosphate, and aluminophosphates may be used.
Advantageously, the source of silicon is silica, for example colloidal silica, famed silica, or an organic silicon source, e.g., a tetraalkyl orthosilicate, especially tetraethyl orthosilicate.
The synthesis mixture also contains a structure directing agent, or template. In general, these are organic bases, especially nitrogen-containing bases, more especially amines and quaternary ammonium compounds, used singly or in combinations. The template is chosen according to the SAPO being manufactured.
As templates there may be mentioned, for example, tetraethyl ammonium compounds, cyclopentylamine, aminomethyl cyclohexane, piperidine, triethylamine, cyclohexylamine, trimethylhydroxyethylamine, morpholine, dipropylamine (DPA), pyridine, isopropylamine and mixtures thereof. Preferred templates are triethylamine, cyclohexylamine, piperidine, pyridine, isopropylamine, tetraethyl ammonium compounds, dipropylamine, and mixtures thereof. The tetraethylammonium compounds include tetraethyl ammonium hydroxide (TEAOH), and tetraethyl ammonium phosphate, fluoride, bromide, chloride, and acetate. Preferred tetraethyl ammonium compounds are the hydroxide and the phosphate. The molecular sieve structure may be effectively controlled using combinations of templates.
Optionally, seeds of the desired SAPO or of a different molecular sieve, may be used to assist formation of the desired SAPO.
DETAILED DESCRIPTION OF THE INVENTION
The process of the invention is suitable for the manufacture of, inter alia, SAPO-5, -8, -11, -16, -17, -20, -31, -36, -37, -40, -41, -42 and -47, especially suitable for the manufacture of SAPO-35, -44 and -56, and more especially suitable for the manufacture of SAPO-18 and SAPO-34, or more generally materials of the CHA or AEI structure types. The two last-mentioned SAPO's may form as an intergrown material.
Substituted SAPOs may also be manufactured by the process of the invention. These materials are generally known as metal-containing silicoaluminophosphates or MeSAPOs. The metal may be an alkali metal (Group IA), alkaline earth metal (Group IIA), rare earth (Group IIIB, including the lanthanide elements) or the metals of Groups IB, IIB, IVB, VB, VIB, VIIB, and VIIIB. Preferably, Me represents Zn, Ni or Cu. These atoms may be incorporated into the tetrahedral framework through a MeO
2
tetrahedral unit, typically by adding the metal component during synthesis of the molecular sieve.
The product of step a) of the process, which is generally a slurry, is advantageously mixed with the source of phosphorus by slowly adding the phosphorus source to the aluminium-containing product. However, the product may instead be slowly added to the phosphorus source. In either case, the addition is carried out at a rate sufficiently slow to prevent the formation of a viscous gel.
Admixture of the silicon source and the template then follow, being carried out in any conventional manner.
In general, the hydrothermal treatment of the synthesis mixture to yield the desired crystalline molecular sieve is advantageously carried out under autogenous pressure, for example in an autoclave, for example a stainless steel autoclave which may, if desired, be ptfe-lined. The treatment may, for example, be carried out at a temperature within the range of from 50, advantageously from 90, especially 120, to 250° C., depending on the molecular sieve being made. The treatment may, for example, be carried out for a period within the range of from 20 to 200 hours, preferably up to 100 hours, again depending on the molecular sieve being formed. The procedure may include an ageing period, either at room temperature or, preferably, at a moderately elevated temperature, before the hydrothermal treatment at more elevated tem

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