METHOD OF SYNTHESIZING MOLECULAR SIEVES

FIELD OF THE INVENTION

[0001] This invention relates to a method of synthesizing molecular sieves and particularly, but not exclusively, silicoaluminophosphate molecular sieves, and to the use of the resultant molecular sieves as catalysts for the conversion of oxygenates, particularly methanol-to-olefins, particularly ethylene and propylene.

BACKGROUND OF THE INVENTION

[0002] Light olefins, such as ethylene, propylene, butylenes and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C ₂ -C ₄ light olefins are produced by cracking petroleum refinery streams, such as C ₃ + paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased. [0003] An important type of alternative feed for the production of light olefins is oxygenates, such as C ₁ -C ₄ alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.

[0004] The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Crystalline molecular sieves all have a three-dimensional, four- connected framework structure of corner-sharing [TO4] tetrahedra, where T is one or more tetrahedrally coordinated cations. Examples of well known molecular

sieves are silicates, which comprise [SiO4] tetrahedral units; aluminosilicates, which comprise [SiO4] and [A1O4] tetrahedral units; aluminophosphates, which comprise [A1O4] and [PO4] tetrahedral units; and silicoaluminophosphates, which comprise [SiO4], [A1O4], and [PO4] tetrahedral units.

[0005] Molecular sieves are typically described in terms of the size of the ring that defines a pore, where the size is based on the number of T atoms in the ring. Other framework-type characteristics include the arrangement of rings that form a cage, and when present, the dimension of channels, and the spaces between the cages. See van Bekkum, et al., Introduction to Zeolite Science and Practice, Second Completely Revised and Expanded Edition, Volume 137, pp. 1-67, Elsevier Science, B.V., Amsterdam, Netherlands (2001). Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, small pore silicoaluminophosphates (having a pore size less than 5A), such as SAPO-34, have shown particular promise. SAPO-34 belongs to the family of molecular sieves having the framework type of the zeolitic mineral chabazite (CHA). [0006J The synthesis of molecular sieves involves preparing a reaction mixture comprising a source of water, a source of at least one oxide of silicon, aluminum, and phosphorus, and at least one organic directing agent for directing the formation of said molecular sieve. The resultant mixture is then heated, normally with agitation, to a suitable crystallization temperature, typically between about 100 ⁰ C and about 300 ⁰ C, and then held at this temperature for a sufficient time, typically between about 1 hour and 20 days, for crystallization of the desired molecular sieve to occur. In most cases, it is critical that the synthesis conditions are closely controlled in order to avoid or minimize the production of impurity phases that can adversely affect the catalytic properties of the desired product. [0007] According to the present invention, it has now been found that the temperature homogeneity of the reaction mixture during heat-up and crystallization is one of the conditions that can significantly impact the success of the synthesis process, particularly with the synthesis of aluminophosphates and silicoaluminophosphates, such as SAPO-34. In particular, it is found that the heating should be controlled to avoid the production of hot spots in the r-eaction

mixture. For example, if the desired average temperature of the reaction mixture at a given time in the heat-up and crystallization process is T ⁰ C, then it has now been found that the heating should be controlled so that the temperature at any point in the reaction mixture is within the range (T ± 5)°C.

[0008J SAPO-34 and its synthesis in the presence of tetraethylammonium hydroxide as the directing agent are disclosed in U.S. Patent No. 4,440,871. Since then a large number of other patents, such as U.S. Patent Nos. 6,696,032; 6,838,586; and 7,014,827; have focused on different problems to be solved in the successful production of silicoaluminophosphates, such as SAPO-34. To date, however, we are aware of no published work on the importance of temperature homogeneity in synthesizing pure phase molecular sieves materials.

SUMMARY OF THE INVENTION

[0009J In one aspect, the invention resides in a method of synthesizing a molecular sieve, the method comprising:

(a) preparing a synthesis mixture comprising water, a source of at least one of silicon, aluminum, and phosphorus, and at least one organic directing agent for directing the formation of said molecular sieve;

(b) heating said synthesis mixture to a crystallization temperature between about 100 ⁰ C and about 350 ⁰ C,

(c) retaining said synthesis mixture at said crystallization temperature until crystals of said molecular sieve are produced;

(d) controlling the temperature of the synthesis mixture during (b) and (c) so that, if the average temperature of the synthesis mixture is T ⁰ C, the temperature at any point in the synthesis mixture is within the range (T ± 5)°C, preferably within the range (T ± 3)°C; and

(e) recovering said molecular sieve.

[0010] Conveniently, the heating (b) is conducted in a convection oven or in an autoclave heated by circulating heat transfer medium.

[0011] Conveniently, the synthesis mixture is agitated during the heating (b). [0012] Conveniently, the crystallization temperature is between about 125°C and about 270 ⁰ C.

[0013] Conveniently, the molecular sieve is an aluminophosphate or silicoaluminophosphate and preferably is SAPO-34.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is an X-ray diffraction pattern of the product of the process of

Example 1.

[0015] Figure 2 is an X-ray diffraction pattern of the product of the process of

Example 2.

[0016] Figures 3 A and 3B are X-ray diffraction patterns of the products from the bottom and top, respectively, of the autoclave used in the process of Example

3.

[0017] Figure 4 is an X-ray diffraction pattern of the product of the process of

Example 4.

[0018] Figures 5 A and 5B are X-ray diffraction patterns of the products from the bottom and top, respectively, of the autoclave used in the process of Example

5.

[0019] Figure 6 is an X-ray diffraction pattern of the product of the process of

Example 6.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is directed to a method of synthesizing molecular sieves and, in particular, silicoaluminophosphate and aluminophosphate molecular sieves useful in the conversion of an oxygenate-containing feedstock, such as methanol, to a product comprising olefins, such as ethylene and propylene. The present method is based on the unexpected finding that maintaining a homogeneous temperature across the synthesis mixture during heating of the synthesis mixture to the crystallization temperature and during subsequent crystallization of the molecular sieve assists in producing pure phase materials.

Molecular Sieves

[0021] Crystalline molecular sieves have a three-dimensional, four-connected framework structure of corner-sharing [TO ₄ ] tetrahedra, where T is any tetrahedrally coordinated cation. In the case of aluminophosphates (AlPOs), the

framework structure is composed of [A1O4] and [PO4] tetrahedral units, whereas in the case of silicoaluminophosphates (SAPOs), the framework structure is composed of [SiO ₄ ], [AlO ₄ ], and [PO ₄ ] corner sharing tetrahedral units. The molecular sieves produced by the present process are generally aluminophosphates and silicoaluminophosphates although additional metal oxide [MeO4] units can also be present, where, for example, Me is magnesium, zinc, iron, cobalt, nickel, manganese, chromium, and mixtures thereof.

[0022] Molecular sieves have been classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolite and zeolite-type molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001), which is fully incorporated herein by reference.

[0023] Non-limiting examples of the molecular sieves for which a structure has been established include the small pore molecular sieves of a framework type selected from the group consisting of AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, THO, and substituted forms thereof; the medium pore molecular sieves of a framework type selected from the group consisting of AFO, AEL, EUO, HEU, FER, MEL, MFI, MTW ₃ MTT, TON, and substituted forms thereof; and the large pore molecular sieves of a framework type selected from the group consisting of EMT, FAU, and substituted forms thereof. Other molecular sieves have a framework type selected from the group consisting of ANA, BEA, CFI, CLO, DON, GIS, LTL, MER, MOR, MWW, and SOD. Also known are molecular sieves that comprise intergrowths of two or more framework topologies.

[0024] Non-limiting examples of the preferred molecular sieves, particularly for converting an oxygenate-containing feedstock into olefin(s), include those having a framework type selected from the group consisting of AEL, AFY, BEA,

CHA, EDI, FAU, FER, GIS, LTA, LTL, MER, MFI, MOR, MTT, MWW, TAM and TON.

[0025] Molecular sieves are typically described in terms of the size of the ring that defines a pore, where the size is based on the number of T atoms in the ring. Small pore molecular sieves generally have up to 8-ring structures and an average pore size less than 5 A, whereas medium pore molecular sieves generally have 10- ring structures and an average pore size of about 5A to about 6A. Large pore molecular sieves generally have at least 12-ring structures and an average pore size greater than about 6A. Other framework-type characteristics include the arrangement of rings that form a cage, and when present, the dimension of channels, and the spaces between the cages. See van Bekkum, et al., Introduction to Zeolite Science and Practice, Second Completely Revised and Expanded Edition, Volume 137, pp. 1-67, Elsevier Science, B.V., Amsterdam, Netherlands (2001).

[0026] Conveniently, the molecular sieve produced by the method of the invention is a small pore silicoaluminophosphate material and preferably a molecular sieve comprising at least a CHA framework-type material, particularly SAPO-34.

Molecular Sieve Synthesis

[0027] Generally, molecular sieves are synthesized by the hydrothermal crystallization of a synthesis mixture comprising water, a source of at least one of silicon, aluminum, and phosphorus, and at least one organic directing agent for directing the formation of said molecular sieve. Typically, the synthesis mixture, optionally, together with seeds from another or the same framework-type molecular sieve, is placed in a sealed pressure vessel, optionally, lined with an inert plastic such as polytetrafluoroethylene, and heated, under a crystallization pressure and temperature, until a crystalline material is formed, and then recovered by filtration, centrifugation, and/or decanting.

[0028] Non-limiting examples of suitable silicon sources include silicates, fumed silica, for example, those sold under the trade names Ultrasil, Hisil, Aerosil-200 available from Degussa Inc., New York, New York, and CAB-O-SIL

M-5, precipitated silica, e.g., that sold under the name Baker's silica, organosilicon compounds such as tetraalkyl orthosilicates, for example, tetramethyl orthosilicate (TMOS) and tetraethylorthosilicate (TEOS), colloidal silicas or aqueous suspensions thereof, for example, Ludox As-40 and HS-40 sols available from E.I. du Pont de Nemours, Wilmington, Delaware, silicic acid or any combination thereof.

[0029] Non-limiting examples of suitable aluminum sources include organoaluminum compounds such as aluminum alkoxides, for example, aluminum isopropoxide, and inorganic aluminum sources, such as alumina, alumina hydrate, alumina sols, aluminum phosphate, aluminum hydroxide, sodium aluminate, aluminum trichloride, gibbsite, and pseudo-boehmite, e.g., that sold under the trade names Pural SB, Catapal, Disperal, and Versal, or any combination thereof- Preferred sources are inorganic aluminum compounds, such as hydrated aluminum oxides and particularly boehmite and pseudoboehmite.

[0030] Non-limiting examples of suitable phosphorus sources, which may also include aluminum-containing phosphorus compositions, include phosphoric acid, organic phosphates such as triethyl phosphate, and crystalline or amorphous aluminophosphates such as AlPO ₄ , phosphorus salts, or combinations thereof. A preferred source of phosphorus is phosphoric acid.

[0031] The organic directing agent(s) employed in the synthesis will depend on the particular framework-type molecular sieve to be produced. However, in the case of CHA framework-type materials, such as SAPO-34, suitable directing agents include adamantammonium compounds, such as N,N,N-trimethyl-l- adamantammonium compounds, N,N,N-trimethyl-2-adamantammonium compounds, and N^NjN-trimethylcyclohexylammonium compounds, N,N- dimethyl-3,3-dimethylpiperidinium compounds, N,N-methylethyl-3,3- dimethylpiperidinium compounds, N,N-dimethyl-2-methylpiperidinium compounds, l,3,3,6,6-pentamethyl-6-azonio-bicyclo(3.2.1)octane compounds, NjN-dimethylcyclohexylamine, and the bi- and tri-cyclic nitrogen containing organic compounds cited in (1) Zeolites and Related Microporσus Materials: State of the Art 1994, Studies of Surface Science and Catalysis, Vol. 84, pp. 29-36; (2)

Novel Materials in Heterogeneous Catalysis (ed. Terry K. Baker & Larry L. Murrell), Chapter 2, pp. 14 - 24, May 1990, (3) J. Am. Chem. Soc, 2000, 122, pp. 263-273, and (4) in U.S. Patent Nos. 4,544,538 and 6,709,644. [0032] Alternatively, the organic directing agent for producing CHA framework-type materials can be a compound having the formula:

R ¹ R ² N-R ³ wherein R ¹ and R ² are independently selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms and hydroxyalkyl groups having from 1 to 3 carbon atoms and R ³ is selected from the group consisting of 4- to 8-membered cycloalkyl groups, optionally, substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms; and 4- to 8-membered heterocyclic groups having from 1 to 3 heteroatoms, said heterocyclic groups being, optionally, substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms and the heteroatoms in said heterocyclic groups being selected from the group consisting of O, N, and S. Preferably, the directing agent is selected from N,N-dimethylcyclohexylamine, N,N- dimethylmethyl-cyclohexylamine, N,N-dimethylcyclopentylamine, N,N- dimethylmethyl-cyclopentylamine, N,N-dimethylcycloheptylamine, N,N- dimethylmethylcycloheptylamine, and most preferably is N,N- dimethylcyclohexylamine (DMCHA).

[0033] In some cases, more than one organic directing agent may be employed. Examples of the synthesis of aluminophosphates and silicoaluminophosphates using multiple directing agents can be found in, for example, U.S. Patent Nos. 4,440,871; 5,096,684; and 6,767,858. [0034] Typically, where the desired molecular sieve is SAPO-34, the synthesis mixture has a molar composition within the following ranges:

P ₂ O ₅ : Al ₂ O ₃ from about 0.5 to about 1.5;

SiO ₂ : Al ₂ O ₃ from 0 to about 0.7;

R : Al ₂ O ₃ from about 0.5 to about 2; and

H ₂ O : Al ₂ O ₃ from about 30 to about 300, where R is the organic directing agent or agents.

[00351 Crystallization of the desired molecular sieve is typically effected by sealing the synthesis mixture in an autoclave and heating the mixture, preferably under autogenous pressure, to a temperature in the range of from 100 ⁰ C to about 350 ⁰ C, for example, from about 125°C to about 270 ⁰ C, such as from about 150 ⁰ C to about 200 ⁰ C. The time required to form the crystalline product is usually dependent on the temperature and can vary from immediately up to several weeks. Typically the crystallization time is from about 30 minutes to around 2 weeks, such as from about 45 minutes to about 240 hours, for example, from about 1 hour to about 120 hours. The hydrothermal crystallization may be carried out without or, more preferably, with agitation.

[0036] According to the invention, the temperature of the synthesis mixture during the crystallization process, that is during heat-up to the crystallization temperature and during subsequent molecular sieve formation at the crystallization temperature, is controlled so that the temperature of the mixture during the process remains substantially constant across the mixture, without the formation of the hot spots that can readily arise where, for example, the mixture is in contact with a heated surface of the autoclave. Similarly, in some cases cool regions could arise if the, for example, the mixture is not adequately mixed. In particular, the heating of the synthesis mixture is controlled so that if the average temperature of the synthesis mixture at a given time during heat-up and crystallization is T ⁰ C then the temperature across the entire synthesis mixture is maintained within the range (T ± 5)°C, preferably within the range (T ± 3)°C or even within the range (T ±_2)°C. In other words, the heating is controlled so that no portion of the synthesis mixture is at a temperature greater than 5°C above or below the average temperature of the synthesis mixture. It is to be appreciated that this requirement does not mean that the temperature of the synthesis mixture must be maintained constant during the crystallization process. However, if the temperature is varied during the crystallization process, it should be varied consistently across the synthesis mixture.

[00371 The method used to achieve the required degree of temperature control is not narrowly defined and the skilled worker will be aware of a variety of

raethods that can be used to maintain the temperature of the synthesis mixture substantially homogeneous. In general, however, important factors in achieving the required temperature control include the use of heat sources that supply heat homogeneously rather than locally to the synthesis mixture, such as a convection oven or a heat transfer medium, such as oil, circulating over the external surface of the autoclave, and the provision of means for agitating the synthesis mixture. It may also be desirable to use relatively low heating rates, for example, less than 30 ⁰ C/ hour, such as less than 20 ⁰ C/ hour, to raise the temperature of the synthesis mixture to the desired crystallization temperature. In addition, it is to be appreciated that temperature control is important not only in large scale commercial syntheses, but also in small scale syntheses, 2 liters or less, such as 600 milliliters or less, or even 150 milliliters or less.

[0038J Once the crystalline molecular sieve product is formed, usually in a slurry state, it may be recovered by any standard technique well known in the art, for example, by centrifugation or filtration. The recovered crystalline product may then be washed, such as with water, and then dried, such as in air. [0039] As a result of the synthesis process, the recovered crystalline product contains within its pores at least a portion of the organic directing agent(s) used in the synthesis. In a preferred embodiment, activation is performed in such a manner that the organic directing agent(s) is(are) removed from the molecular sieve, leaving active catalytic sites within the microporous channels of the molecular sieve open for contact with a feedstock. The activation process is typically accomplished by calcining, or essentially heating the molecular sieve comprising the organic directing agent at a temperature of from about 200 ⁰ C to about 800 ⁰ C in the presence of an oxygen-containing gas. Li some cases, it may be desirable to heat the molecular sieve in an environment having a low or zero oxygen concentration. This type of process can be used for partial or complete removal of the organic directing agent(s) from the intracrystalline pore system of the molecular sieve.

Molecular Sieve Catalyst Compositions

[0040] The molecular sieves and, in particular, the silicoaluminophosphate molecular sieves, produced by the synthesis method of the invention are particularly intended for use as organic conversion catalysts. Before use in catalysis, the molecular sieves will normally be formulated into catalyst compositions by combination with other materials, such as binders and/or matrix materials, which provide additional hardness or catalytic activity to the finished catalyst.

[0041] Materials which can be blended with the molecular sieve can be various inert or catalytically active materials. These materials include compositions such as kaolin and other clays, various forms of rare earth metals, other non-zeolite catalyst components, zeolite catalyst components, alumina or alumina sol, titania, zirconia, quartz, silica or silica sol, and mixtures thereof. These components are also effective in reducing overall catalyst cost, acting as a thermal sink to assist in heat shielding the catalyst during regeneration, densifying the catalyst and increasing catalyst strength. When blended with such components, the amount of molecular sieve contained in the final catalyst product ranges from 10 to 90 weight percent of the total catalyst, preferably 20 to 80 weight percent of the total catalyst composition.

Use of the Molecular Sieve

[0042] The molecular sieves and, in particular, the silicoaluminophosphate molecular sieves, produced by the method of the invention are useful as catalysts in a variety of processes including cracking of, for example, a naphtha feed to light olefin(s) or higher molecular weight (MW) hydrocarbons to lower MW hydrocarbons; hydrocracking of, for example, heavy petroleum and/or cyclic feedstock; isomerization of, for example, aromatics such as xylene; polymerization of, for example, one or more olefin(s) to produce a polymer product; reforming; hydrogenation; dehydrogenation; dewaxing of, for example, hydrocarbons to remove straight chain paraffins; absorption of, for example, alkyl aromatic compounds for separating out isomers thereof; alkylation of, for

ex ample, aromatic hydrocarbons such as benzene and alkyl benzene, optionally with propylene to produce cumene or with long chain olefins; transalkylation of, for example, a combination of aromatic and polyalkylaromatic hydrocarbons; dealkylation; dehydrocyclization; disproportionation of, for example, toluene to make benzene and paraxylene; oligomerization of, for example, straight and branched chain olefin(s); and the synthesis of monoalkylamines and dialkylamines from organic oxygenates, such as methanol.

[0043] Where the molecular sieve produced by the method of the invention is a small pore material (with a pore size less than 5A) and in particular is a CHA structure-type material, such as SAPO-34, the molecular sieve is particularly suitable as a catalyst for use in the conversion of oxygenates to olefins. As used herein, the term "oxygenates" is defined to include, but is not necessarily limited to, aliphatic alcohols, ethers, carbonyl compounds (aldehydes, ketones, carboxylic acids, carbonates, and the like), and also compounds containing hetero-atoms, such as, halides, mercaptans, sulfides, amines, and mixtures thereof. The aliphatic moiety will normally contain from about 1 to about 10 carbon atoms, such as from about 1 to about 4 carbon atoms.

J0044] Representative oxygenates include lower straight chain or branched aliphatic alcohols, their unsaturated counterparts, and their nitrogen, halogen, and sulfur analogues. Examples of suitable oxygenate compounds include methanol; ethanol; n-propanol; isopropanol; C ₄ - C _1O alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; methyl mercaptan; methyl sulfide; methyl amine; ethyl mercaptan; di-ethyl sulfide; di-ethyl amine; ethyl chloride; formaldehyde; di-methyl carbonate; di-methyl ketone; acetic acid; n-alkyl amines, n-alkyl halides, n-alkyl sulfides having n-alkyl groups of comprising the range of from about 3 to about 10 carbon atoms; and mixtures thereof. Particularly suitable oxygenate compounds are methanol, dimethyl ether, or mixtures thereof, most preferably methanol. As used herein, the term "oxygenate" designates only the organic material used as the feed. The total charge of feed to the reaction zone may contain additional compounds, such as diluents.

[0045] When used in an oxygenate conversion process, a catalyst comprising a molecular sieve produced by the present process is contacted with a feedstock comprising an organic oxygenate, optionally, with one or more diluents, in the vapor phase in a reaction zone at effective process conditions so as to produce the desired olefins. Alternatively, the process may be carried out in a liquid or a mixed vapor/liquid phase. When the process is carried out in the liquid phase or a mixed vapor/liquid phase, different conversion rates and selectivities of feedstock- to-product may result depending upon the catalyst and the reaction conditions. [0046] When present, the diluent(s) is(are) generally non-reactive to the feedstock or molecular sieve catalyst composition and is(are) typically used to reduce the concentration of the oxygenate in the feedstock. Non-limiting examples of suitable diluents include helium, argon, nitrogen, carbon monoxide, carbon dioxide, water, essentially non-reactive paraffins (especially alkanes such as methane, ethane, and propane), essentially non-reactive aromatic compounds, and mixtures thereof. The most preferred diluents are water and nitrogen, with water being particularly preferred. Diluent(s) may comprise from about 1 mol % to about 99 mol % of the total feed mixture.

[0047] The temperature employed in the oxygenate conversion process may vary over a wide range, such as from about 200 ⁰ C to about 1000 ⁰ C ₇ for example, from about 250 ⁰ C to about 800 ⁰ C, including from about 250 ⁰ C to about 750 ⁰ C, conveniently from about 300 ⁰ C to about 650 ⁰ C, typically from about 350 ⁰ C to about 600 ⁰ C, and particularly from about 400 ⁰ C to about 600 ⁰ C. [0048] Light olefin products will form, although not necessarily in optimum amounts, at a wide range of pressures, including but not limited to autogenous pressures and pressures in the range of from about 0.1 kPa to about 10 MPa. Conveniently, the pressure is in the range of from about 7 kPa to about 5 MPa, such as in the range of from about 50 kPa to about 1 MPa. The foregoing pressures are exclusive of diluent, if any is present, and refer to the partial pressure of the feedstock as it relates to oxygenate compounds and/or mixtures thereof. Lower and upper extremes of pressure may adversely affect selectivity,

conversion, coking rate, and/or reaction rate; however, light olefins such as ethylene still may form.

[0049] The process should be continued for a period of time sufficient to produce the desired olefin products. The reaction time may vary from tenths of seconds to a number of hours. The reaction time is largely determined by the reaction temperature, the pressure, the catalyst selected, the weight hourly space velocity, the phase (liquid or vapor) and the selected process design characteristics.

[0050] A wide range of weight hourly space velocities (WHSV) for the feedstock will function in the present process. WHSV is defined as weight of feed (excluding diluent) per hour per weight of a total reaction volume of molecular sieve catalyst (excluding inerts and/or fillers). The WHSV generally should be in the range of from about 0.01 hr ^'1 to about 500 hr ^'1 , such as in the range of from about 0.5 hr ^"1 to about 300 hr ^'1 , for example, in the range of from about 0.1 hr ^"1 to about 20O hT ^"1 .

[0051] A practical embodiment of a reactor system for the oxygenate conversion process is a circulating fluid bed reactor with continuous regeneration, similar to a modem fluid catalytic cracker. Because the catalyst must be regenerated frequently, the reactor should allow easy removal of a portion of the catalyst to a regenerator, where the catalyst is subjected to a regeneration medium, such as a gas comprising oxygen, for example, air, to bum off coke from the catalyst, which restores the catalyst activity. The conditions of temperature, oxygen partial pressure, and residence time in the regenerator should be selected to achieve a coke content on regenerated catalyst of less than about 1 wt %. At least a portion of the regenerated catalyst should be returned to the reactor. [0052] Using the various oxygenate feedstocks discussed above, particularly a feedstock containing methanol, a catalyst composition of the invention is effective to convert the feedstock primarily into one or more olefin(s). The olefin(s) produced typically have from 2 to 30 carbon atoms, preferably 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, still more preferably 2 to 4 carbons atoms, and most preferably are ethylene and/or propylene. The resultant olefins

can be separated from the oxygenate conversion product for sale or can be fed to a downstream process for converting the olefins to, for example, polymers. [0053] The invention will now be more particularly described with reference to the following Examples.

[0054] In the Examples, X-ray Powder Diffractograms were recorded on a

Siemens D500 diffractometer with voltage of 40 kV and current of 30 mA, using a Cu target and Ni-filter (A=0.154nm).

EXAMPLES

Example 1

[0055] A mixture with the following molar composition was prepared:

0.3SiO ₂ : Al ₂ O ₃ : P ₂ O ₅ : TEAOH : 1.6DPA : 53 H ₂ O ₅ using Ludox AS40 silica (supplied by Grace), phosphoric acid (85% concentration supplied by Aldrich), Condea Pural SB alumina, tetraethyl ammonium hydroxide TEAOH (35% concentration supplied by Eastern Chemical), dipropylamine DPA (supplied by Aldrich) and de-ionized water. In particular, 142.88 grams of the Pural SB was combined with 209.88 grams of water and the resultant slurry was stirred for 5 minutes. To this slurry was then added slowly a solution containing 244.27 grams of the 85% phosphoric acid and 167.43 grams of additional water. This mixture was left static for about 10 minutes before it was mixed for another 7 minutes. After this mixing time, the mixture was homogeneous. To this mixture was then added 47.23 grams of Ludox and the resulting mixture was again stirred for 5 minutes. The beaker containing the silica source was rinsed with 17.48 grams of water and this was added to the synthesis mixture. Then 436.84 grams of the TEAOH solution was added and mixed for 5 minutes before 170.05 grams of DPA was added. Both beakers were rinsed with a total 113.56 grams of water and the final synthesis mixture was stirred for 2 hours at room temperature. [0056] An appropriate quantity of the final synthesis mixture was transferred to a 150 milliliter autoclave resulting in a ~75% filling of the reactor volume. The autoclave was then placed at room temperature in a convection oven equipped with a programmable temperature controller. The autoclave was held static and

the oven was heated in 2 hrs to 175°C and then maintained at this temperature for 60 hours. The temperature in the oven was uniform and never exceeded 175°C during the entire heating process. After crystallization, the product was recovered by centrifiiging and washing several times with de-ionized water. The product was then dried and an XRD pattern was recorded. The results are shown in Figure 1 and demonstrate the product to be pure SAPO-34.

Example 2 (Comparative)

[00571 The procedure of Example 1 was repeated but with the final synthesis mixture being crystallized in a 1 liter single-walled stainless steel Parr autoclave equipped with a thermocouple immersed in the synthesis gel for the duration of the hydrothermal treatment. The autoclave -was mounted in a heating mantle containing electrical resistance coils and was heated, without agitation, to 175°C at an overall heating rate of 18.8°C/hour by manually gradually increasing the power to the resistance coils. The temperature of the bulk of the gel served as indicating the temperature increase and it was estimated that the temperature close to the wall of the autoclave was significantly (>5°C) higher than the bulk due to the close presence of the heating coils.

[0058] After crystallization, the product was recovered as in Example 1 and an XRX) pattern was recorded. The results are shown in Figure 2 and demonstrate the product to be a mixture of SAPO-34 and SAPO- 18.

Examples 3 to 5 (Comparative)

[0059] The procedure of Example 2 was repeated but with increasing measures being taken to improve on the homogeneity of the temperature inside the gel. In Example 3, the gel was stirred at 60 rpm with a single propeller stirrer during heating of the gel at 18.8°C/hour to the crystallization temperature of 175°C, whereafter the gel was held static for the remainder of the crystallization. In Example 4, the gel was stirred at 60 rpm with a single propeller stirrer during the entire heating and crystallization process, although in this case the gel heating rate was increased to 25°C/hour. In Example 5, the gel was stirred at 120 rpm

with a two propeller stirrer during the entire heating and crystallization process, with the gel heating again being at 18.8°C/hour.

[0060] In each case, the products were recovered as in Example 1 and XRD patterns were recorded. The results for Example 3 are shown in Figures 3A and 3B (for products taken from the bottom and top parts, respectively, of the autoclave), for Example 4 are shown in Figure 4 and for Example 5 are shown in Figures 5 A and 5B (for products taken from the bottom and top parts, respectively, of the autoclave). In each case the XRD patterns demonstrate the product to be a mixture of SAPO-34 and SAPO-18.

Example 6

[0061] The procedure of Example 1 was repeated but with the final synthesis mixture being heated in a 1 liter double walled oil heated autoclave and being stirred at all times with a 2 propeller mixer rotating at 170 rpm. The temperature of the gel was controlled by the temperature of the oil circulating in the double wall of the autoclave. The use of a 2 propeller mixer ensured good contact at all times between the gel and the whole surface of the autoclave and the use of circulating oil to heat the autoclave prevented hot spots on the metal wall and hence resulted in a homogeneous temperature throughout the gel. After crystallization, the product was recovered as in Example 1 and an XRD pattern was recorded. The results are shown in Figure 6 and demonstrate the product to be pure SAPO-34.

[0062] While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.